GABA-mediated changes in inter-hemispheric beta frequency activity in early-stage Parkinson's disease.

In Parkinson's disease (PD), elevated beta (15-35Hz) power in subcortical motor networks is widely believed to promote aspects of PD symptomatology, moreover, a reduction in beta power and coherence accompanies symptomatic improvement following effective treatment with l-DOPA. Previous studies have reported symptomatic improvements that correlate with changes in cortical network activity following GABAA receptor modulation. In this study we have used whole-head magnetoencephalography to characterize neuronal network activity, at rest and during visually cued finger abductions, in unilaterally symptomatic PD and age-matched control participants. Recordings were then repeated following administration of sub-sedative doses of the hypnotic drug zolpidem (0.05mg/kg), which binds to the benzodiazepine site of the GABAA receptor. A beamforming based 'virtual electrode' approach was used to reconstruct oscillatory power in the primary motor cortex (M1), contralateral and ipsilateral to symptom presentation in PD patients or dominant hand in control participants. In PD patients, contralateral M1 showed significantly greater beta power than ipsilateral M1. Following zolpidem administration contralateral beta power was significantly reduced while ipsilateral beta power was significantly increased resulting in a hemispheric power ratio that approached parity. Furthermore, there was highly significant correlation between hemispheric beta power ratio and Unified Parkinson's Disease Rating Scale (UPDRS). The changes in contralateral and ipsilateral beta power were reflected in pre-movement beta desynchronization and the late post-movement beta rebound. However, the absolute level of movement-related beta desynchronization was not altered. These results show that low-dose zolpidem not only reduces contralateral beta but also increases ipsilateral beta, while rebalancing the dynamic range of M1 network oscillations between the two hemispheres. These changes appear to underlie the symptomatic improvements afforded by low-dose zolpidem.

Frequency selectivity and dopamine-dependence of plasticity at glutamatergic synapses in the subthalamic nucleus.

In Parkinson's disease, subthalamic nucleus (STN) neurons burst fire with increased periodicity and synchrony. This may entail abnormal release of glutamate, the major source of which in STN is cortical afferents. Indeed, the cortico-subthalamic pathway is implicated in the emergence of excessive oscillations, which are reduced, as are symptoms, by dopamine-replacement therapy or deep brain stimulation (DBS) targeted to STN. Here we hypothesize that glutamatergic synapses in the STN may be differentially modulated by low-frequency stimulation (LFS) and high-frequency stimulation (HFS), the latter mimicking deep brain stimulation. Recordings of evoked and spontaneous excitatory post synaptic currents (EPSCs) were made from STN neurons in brain slices obtained from dopamine-intact and chronically dopamine-depleted adult rats. HFS had no significant effect on evoked (e) EPSC amplitude in dopamine-intact slices (104.4±8.0%) but depressed eEPSCs in dopamine-depleted slices (67.8±6.2%). Conversely, LFS potentiated eEPSCs in dopamine-intact slices (126.4±8.1%) but not in dopamine-depleted slices (106.7±10.0%). Analyses of paired-pulse ratio, coefficient of variation, and spontaneous EPSCs suggest that the depression and potentiation have a presynaptic locus of expression. These results indicate that the synaptic efficacy in dopamine-intact tissue is enhanced by LFS. Furthermore, the synaptic efficacy in dopamine-depleted tissue is depressed by HFS. Therefore the therapeutic effects of DBS in Parkinson's disease appear mediated, in part, by glutamatergic cortico-subthalamic synaptic depression and implicate dopamine-dependent increases in the weight of glutamate synapses, which would facilitate the transfer of pathological oscillations from the cortex.

The role of GABAergic modulation in motor function related neuronal network activity.

At rest, the primary motor cortex (M1) exhibits spontaneous neuronal network oscillations in the beta (15-30 Hz) frequency range, mediated by inhibitory interneuron drive via GABA-A receptors. However, questions remain regarding the neuropharmacological basis of movement related oscillatory phenomena, such as movement related beta desynchronisation (MRBD), post-movement beta rebound (PMBR) and movement related gamma synchronisation (MRGS). To address this, we used magnetoencephalography (MEG) to study the movement related oscillatory changes in M1 cortex of eight healthy participants, following administration of the GABA-A modulator diazepam. Results demonstrate that, contrary to initial hypotheses, neither MRGS nor PMBR appear to be GABA-A dependent, whilst the MRBD is facilitated by increased GABAergic drive. These data demonstrate that while movement-related beta changes appear to be dependent upon spontaneous beta oscillations, they occur independently of one other. Crucially, MRBD is a GABA-A mediated process, offering a possible mechanism by which motor function may be modulated. However, in contrast, the transient increase in synchronous power observed in PMBR and MRGS appears to be generated by a non-GABA-A receptor mediated process; the elucidation of which may offer important insights into motor processes.

Differential localization of GABA(A) receptor subunits in relation to rat striatopallidal and pallidopallidal synapses.

As a central integrator of basal ganglia function, the external segment of the globus pallidus (GP) plays a critical role in the control of voluntary movement. The GP is composed of a network of inhibitory GABA-containing projection neurons which receive GABAergic input from axons of the striatum (Str) and local collaterals of GP neurons. Here, using electrophysiological techniques and immunofluorescent labeling we have investigated the differential cellular distribution of α1, α2 and α3 GABA(A) receptor subunits in relation to striatopallidal (Str-GP) and pallidopallidal (GP-GP) synapses. Electrophysiological investigations showed that zolpidem (100 nm; selective for the α1 subunit) increased the amplitude and the decay time of both Str-GP and GP-GP IPSCs, indicating the presence of the α1 subunits at both synapses. However, the application of drugs selective for the α2, α3 and α5 subunits (zolpidem at 400 nm, L-838,417 and TP003) revealed differential effects on amplitude and decay time of IPSCs, suggesting the nonuniform distribution of non-α1 subunits. Immunofluorescence revealed widespread distribution of the α1 subunit at both soma and dendrites, while double- and triple-immunofluorescent labeling for parvalbumin, enkephalin, gephyrin and the γ2 subunit indicated strong immunoreactivity for GABA(A) α3 subunits in perisomatic synapses, a region mainly targeted by local axon collaterals. In contrast, immunoreactivity for synaptic GABA(A) α2 subunits was observed in dendritic compartments where striatal synapses are preferentially located. Due to the kinetic properties which each GABA(A) α subunit confers, this distribution is likely to contribute differentially to both physiological and pathological patterns of activity.

Pharmacologically induced and stimulus evoked rhythmic neuronal oscillatory activity in the primary motor cortex in vitro.

Parkinson's disease (PD) is associated with enhanced synchronization of neuronal network activity in the beta (15-30 Hz) frequency band across several nuclei of the basal ganglia (BG). Deep brain stimulation of the subthalamic nucleus (STN) appears to reduce this pathological oscillation, thereby alleviating PD symptoms. However, direct stimulation of primary motor cortex (M1) has recently been shown to be effective in reducing symptoms in PD, suggesting a role for cortex in patterning pathological rhythms. Here, we examine the properties of M1 network oscillations in coronal slices taken from rat brain. Oscillations in the high beta frequency range (layer 5, 27.8+/-1.1 Hz, n=6) were elicited by co-application of the glutamate receptor agonist kainic acid (400 nM) and muscarinic receptor agonist carbachol (50 microM). Dual extracellular recordings, local application of tetrodotoxin and recordings in M1 micro-sections indicate that the activity originates within deep layers V/VI. Beta oscillations were unaffected by specific AMPA receptor blockade, abolished by the GABA type A receptor (GABA(A)R) antagonist picrotoxin and the gap-junction blocker carbenoxolone, and modulated by pentobarbital and zolpidem indicating dependence on networks of GABAergic interneurons and electrical coupling. High frequency stimulation (HFS) at 125 Hz in superficial layers, designed to mimic transdural/transcranial stimulation, generated gamma oscillations in layers II and V (incidence 95%, 69.2+/-7.3 Hz, n=17) with very fast oscillatory components (VFO; 100-250 Hz). Stimulation at 4 Hz, however, preferentially promoted theta activity (incidence 62.5%, 5.1+/-0.6 Hz, n=15) that effected strong amplitude modulation of ongoing beta activity. Stimulation at 20 Hz evoked mixed theta and gamma responses. These data suggest that within M1, evoked theta, gamma and fast oscillations may coexist with and in some cases modulate pharmacologically induced beta oscillations.

Subthalamic nucleus neurones in slices from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned mice show irregular, dopamine-reversible firing pattern changes, but without synchronous activity.

The loss of dopamine in idiopathic or animal models of Parkinson's disease induces synchronized low-frequency oscillatory burst-firing in subthalamic nucleus neurones. We sought to establish whether these firing patterns observed in vivo were preserved in slices taken from dopamine-depleted animals, thus establishing a role for the isolated subthalamic-globus pallidus complex in generating the pathological activity. Mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) showed significant reductions of over 90% in levels of dopamine as measured in striatum by high pressure liquid chromatography. Likewise, significant reductions in tyrosine hydroxylase immunostaining within the striatum (>90%) and tyrosine hydroxylase positive cell numbers (65%) in substantia nigra were observed. Compared with slices from intact mice, neurones in slices from MPTP-lesioned mice fired significantly more slowly (mean rate of 4.2 Hz, cf. 7.2 Hz in control) and more irregularly (mean coefficient of variation of inter-spike interval of 94.4%, cf. 37.9% in control). Application of ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2-amino-5-phosphonopentanoic acid (AP5) and the GABA(A) receptor antagonist picrotoxin caused no change in firing pattern. Bath application of dopamine significantly increased cell firing rate and regularized the pattern of activity in cells from slices from both MPTP-treated and control animals. Although the absolute change was more modest in control slices, the maximum dopamine effect in the two groups was comparable. Indeed, when taking into account the basal firing rate, no differences in the sensitivity to dopamine were observed between these two cohorts. Furthermore, pairs of subthalamic nucleus cells showed no correlated activity in slices from either control (21 pairs) or MPTP-treated animals (20 pairs). These results indicate that the isolated but interconnected subthalamic-globus pallidus network is not itself sufficient to generate the aberrant firing patterns in dopamine-depleted animals. More likely, inputs from other regions, such as the cortex, are needed to generate pathological oscillatory activity.

5-Hydroxytryptamine induced excitation and inhibition in the subthalamic nucleus: action at 5-HT(2C), 5-HT(4) and 5-HT(1A) receptors.

Extracellular single-unit recordings in mouse brain slices were used to determine the effect of exogenously applied 5-HT on STN neurones. Recordings were made from 74 STN cells which fired action potentials at a regular rate of 7.19+/-0.5 Hz. In 61 cells (82%), 5-HT application increased STN neurone firing rate (10 microM, 180+/-16.8%, n=35) with an estimated EC(50) of 5.4 microM. The non-specific 5-HT(2) receptor agonist alpha-methyl 5-HT (1-10 microM) mimicked 5-HT induced excitations (15 cells). These excitations were significantly reduced by pre-perfusion with the specific 5-HT(2C) receptor antagonist RS102221 (500 nM, 9 cells) and the 5HT(4) antagonist GR113808 (500 nM, 7 cells). In 6 cells (8%) 5-HT induced biphasic responses where excitation was followed by inhibition, while in 7 cells (9%) inhibition of firing rate was observed alone. Inhibitory responses were reduced by the 5-HT(1A) antagonist WAY100135 (1 microM, 4 cells). No inhibitory responses were observed following alpha-methyl 5-HT applications. Both the excitations and inhibitions were unaffected by picrotoxin (50 microM, n=5) and CNQX (10 microM, n=5) indicative of direct postsynaptic effects. Thus, in STN neurones, 5-HT elicits two distinct effects, at times on the same neurone, the first being an excitation which is mediated by 5-HT(2C) and 5-HT(4) receptors and the second an inhibition which is mediated by 5-HT(1A) receptors.

Functional interconnectivity between the globus pallidus and the subthalamic nucleus in the mouse brain slice.

In accordance with its central role in basal ganglia circuitry, changes in the rate of action potential firing and pattern of activity in the globus pallidus (GP)-subthalamic nucleus (STN) network are apparent in movement disorders. In this study we have developed a mouse brain slice preparation that maintains the functional connectivity between the GP and STN in order to assess its role in shaping and modulating bursting activity promoted by pharmacological manipulations. Fibre-tract tracing studies indicated that a parasagittal slice cut 20 deg to the midline best preserved connectivity between the GP and the STN. IPSCs and EPSCs elicited by electrical stimulation confirmed connectivity from GP to STN in 44/59 slices and from STN to GP in 22/33 slices, respectively. In control slices, 74/76 (97%) of STN cells fired tonically at a rate of 10.3 +/- 1.3 Hz. This rate and pattern of single spiking activity was unaffected by bath application of the GABA(A) antagonist picrotoxin (50 microM, n = 9) or the glutamate receptor antagonist (6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX) 10 microM, n = 8). Bursting activity in STN neurones could be induced pharmacologically by application of NMDA alone (20 microm, 3/18 cells, 17%) but was more robust if NMDA was applied in conjunction with apamin (20-100 nM, 34/77 cells, 44%). Once again, neither picrotoxin (50 microM, n = 5) nor CNQX (10 microM, n = 5) had any effect on the frequency or pattern of the STN neurone activity while paired STN and GP recordings of tonic and bursting activity show no evidence of coherent activity. Thus, in a mouse brain slice preparation where functional GP-STN connectivity is preserved, no regenerative synaptically mediated activity indicative of a dynamic network is evident, either in the resting state or when neuronal bursting in both the GP and STN is generated by application of NMDA/apamin. This difference from the brain in Parkinson's disease may be attributed either to insufficient preservation of cortico-striato-pallidal or cortico-subthalamic circuitry, and/or an essential requirement for adaptive changes resulting from dopamine depletion for the expression of network activity within this tissue complex.

Independent neuronal oscillators of the rat globus pallidus.

In vivo, neurons of the globus pallidus (GP) and subthalamic nucleus (STN) resonate independently around 70 Hz. However, on the loss of dopamine as in Parkinson's disease, there is a switch to a lower frequency of firing with increased bursting and synchronization of activity. In vitro, type A neurons of the GP, identified by the presence of I(h) and rebound depolarizations, fire at frequencies (

Excitation by dopamine of rat subthalamic nucleus neurones in vitro-a direct action with unconventional pharmacology.

Recent anatomical and physiological studies have pointed to a functional innervation of the subthalamic nucleus by dopamine. This nucleus has a pivotal role in basal ganglia function and voluntary movement control and the possibility that dopamine, and dopaminergic medication used in Parkinson's disease, might directly influence its activity is of considerable interest. We have evaluated electrophysiologically the action and pharmacology of dopamine on single subthalamic neurones in rat brain slices. Dopamine increased firing rate to up to a mean of 60% in 98% of the 261 neurones tested when examined using extracellular single-unit recording. This excitation was unaffected by the GABA antagonist picrotoxin, and the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, and persisted in a low Ca(2+)/raised Mg(2+) solution, indicative of a direct action, independent of synaptic transmission. Of the 33 cells examined using whole patch-clamp recording, only 13 showed measurable increases in firing rate and/or depolarisations in response to dopamine. Dopamine-responsive cells displayed significantly greater access resistance, suggesting that an unidentified cytoplamic constituent, removed by whole-cell dialysis, was required for the response. Using extracellular recording, the D2-like dopamine receptor agonists quinpirole and bromocryptine, but not the D1-like receptor agonist 1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol, also consistently caused an excitation. This was mimicked by the catecholamine releaser amphetamine in 60% of cells tested. However, the dopamine excitation was not significantly reduced either by the D1-like receptor antagonist 7-chloro8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine or the D2-like receptor antagonists (-)-sulpiride, eticlopride and (+)-butaclamol, and the quinpirole excitation was also unaffected by (-)-sulpiride. In contrast, (-)-sulpiride, eticlopride and (+)-butaclamol all abolished the D2-like receptor-mediated inhibition by dopamine of substantia nigra pars compacta neurones. The alpha-adrenoceptor antagonist phentolamine was a weak antagonist of dopamine excitations, but not of those caused by quinpirole. Dopamine excitations also showed weak sensitivity to the 5-HT(2) antagonist ritanserin, but were unaffected by the alpha(1)-adrenoceptor antagonist prazocin and the beta-adrenoceptor antagonist propranolol. The pharmacology of this dopamine excitation is inconsistent with an action on any known catecholamine receptor. However, the effect of amphetamine indicates that an unidentified monamine--possibly dopamine--can be released within the subthalamic nucleus to cause an excitation. The anomalies of its pharmacological characterisation do not strongly support a physiologically relevant direct action of dopamine in the rat subthalamic nucleus.

Calbindin D-28k positive projection neurones and calretinin positive interneurones of the rat globus pallidus.

Immunohistochemistry for three calcium-binding proteins calbindin D-28k, calretinin, and parvalbumin revealed neuronal heterogeneity within the GP. Each neurone appeared to express either a single type of calcium binding protein or none at all. The co-localisation of calcium binding proteins was not observed. Combined immunohistochemistry and retrograde tract tracing using colloidal gold particles injected into the projection fields, the substantia nigra or subthalamic nucleus, revealed that projection neurones could be labelled with either calbindin or parvalbumin. These cells were of medium size (22 x 12 microm), multipolar and moderate varicose dendritic trees. In contrast, calretinin-positive neurones were never retrogradely labelled, even in regions where neuronal colloidal gold deposits were numerous. This, combined with their rarity (<1%) and small size (11 x 9 microm), suggests that calretinin may be a neurochemical marker for putative rat globus pallidus interneurones. Calcium-binding proteins are known to have unique buffering characteristics that may confer specific functional properties upon pallidal neurones. Indeed, differential calcium binding protein expression may underlie the electrophysiological heterogeneity observed in the rat globus pallidus.

Pharmacological comparison of human homomeric 5-HT3A receptors versus heteromeric 5-HT3A/3B receptors.

The present study determined the detailed pharmacological profile of heterologously expressed human (h) homomeric 5-HT3A receptors in direct comparison to heteromeric h5-HT3A/3B receptors. The very minor differences in their respective pharmacological profiles indicates that the 5-HT3B receptor subunit alters, predominantly, the biophysical rather than the pharmacological properties of the 5-HT3 receptor.

Dopamine D2 receptor mediated presynaptic inhibition of striatopallidal GABA(A) IPSCs in vitro.

The modulation of GABA release within the globus pallidus (GP) by dopamine was studied using whole-cell patch clamp recordings from visually identified neurones. In sagittal slices, single shock electrical stimulation in the striatum evoked GABA(A) inhibitory postsynaptic currents (IPSCs), which were inhibited by dopamine in a dose-dependent manner (0.3-30 microM) with an IC(50) value of 0.7 microM. The inhibition was accompanied by an increase in paired pulse facilitation, indicative of a presynaptic effect. In coronal slices, stimulation within the GP adjacent to the recording site evoked GABA(A) IPSCs which were relatively unaffected by dopamine indicating the lack of modulation of GABA release from terminals of local GP axon collaterals. No consistent changes in holding current, membrane potential, firing rate or the frequency of spontaneous IPSCs was observed.Tetrodotoxin-resistant miniature (m)IPSCs were recorded in chloride-loaded cells. Dopamine (3-30 microM) reduced the frequency of mIPSCs, but was without effect on mIPSC amplitude, confirming a presynaptic effect. The addition of the "D2 like" agonist quinpirole (3 microM), but not the "D1 like" agonist SKF 38393 (10 microM), mimicked these effects. The "D2 like" antagonist sulpiride (10 microM), while having no effect alone, blocked the action of dopamine. In contrast the dopamine D4 selective antagonist L745, 870 (1 microM) or D1 antagonist SCH 23390 (10 microM) were without effect. These results indicate that dopamine acts on presynaptic D2 receptors on striatopallidal terminals to reduce the release of GABA in the GP. Attenuation of this mechanism following the depletion of dopamine may contribute to the changes in GP neuronal activity observed in animal models of Parkinson's disease.

Electrophysiological and morphological characteristics of three subtypes of rat globus pallidus neurone in vitro.

Neurones of the globus pallidus (GP) have been classified into three subgroups based on the visual inspection of current clamp electrophysiological properties and morphology of biocytin-filled neurones. Type A neurones (132/208; 63 %) were identified by the presence of the time- and voltage-dependent inward rectifier (Ih) and the low-threshold calcium current (It) giving rise to anodal break depolarisations. These cells were quiescent or fired regular spontaneous action potentials followed by biphasic AHPs. Current injection evoked regular activity up to maximum firing frequency of 350 Hz followed by moderate spike frequency adaptation. The somata of type A cells were variable in shape (20 x 12 micrometer) while their dendrites were highly varicose. Type B neurones (66/208; 32 %) exhibited neither Ih nor rebound depolarisations and only a fast monophasic AHP. These cells were spontaneously active while current injection induced irregular patterns of action potential firing up to a frequency of 440 Hz with weak spike frequency adaptation. Morphologically, these cells were the smallest encountered (15 x 10 micrometer), oval in shape with restricted varicose dendritic arborisations. Type C neurones were much rarer (10/208; 5 %). They were identified by the absence of Ih and rebound depolarisations, but did possess a prolonged biphasic AHP. They displayed large A-like potassium currents and ramp-like depolarisations in response to step current injections, which induced firing up to a maximum firing frequency of 310 Hz. These cells were the largest observed (27 x 15 micrometer) with extensive dendritic branching. These results confirm neuronal heterogeneity in the adult rodent GP. The driven activity and population percentage of the three subtypes correlates well with the in vivo studies (Kita & Kitai, 1991). Type A cells appear to correspond to type II neurones of Nambu & Llinas (1994, 1997) while the small diameter type B cells display morphological similarities with those described by Millhouse (1986). The rarely encountered type C cells may well be large cholinergic neurones. These findings provide a cellular basis for the study of intercellular communication and network interactions in the adult rat in vitro.

Presynaptic mu and delta opioid receptor modulation of GABAA IPSCs in the rat globus pallidus in vitro.

The role of enkephalin and the opioid receptors in modulating GABA release within the rat globus pallidus (GP) was investigated using whole-cell patch recordings made from visually identified neurons. Two major GP neuronal subtypes were classified on the basis of intrinsic membrane properties, action potential characteristics, the presence of the anomalous inward rectifier (Ih), and anode break depolarizations. The mu opioid receptor agonist [D-Ala2-N-Me-Phe4-Glycol5]-enkephalin (DAMGO) (1 microM) reduced GABAA receptor-mediated IPSCs evoked by stimulation within the striatum. DAMGO also increased paired-pulse facilitation, indicative of presynaptic mu opioid receptor modulation of striatopallidal input. In contrast, the delta opioid agonist D-Pen-[D-Pen2, 5]-enkephalin (DPDPE) (1 microM) was without effect. IPSCs evoked by stimulation within the GP were depressed by application of [methionine 5']-enkephalin (met-enkephalin) (30 microM). Met-enkephalin also reduced the frequency, but not the amplitude, of miniature IPSCs (mIPSCs) and increased paired-pulse facilitation of evoked IPSCs, indicative of a presynaptic action. Both DAMGO and DPDPE reduced evoked IPSCs and the frequency, but not amplitude, of mIPSCs. However, spontaneous action potential-driven IPSCs were reduced in frequency by met-enkephalin and DAMGO, whereas DPDPE was without effect. Overall, these results indicate that presynaptic mu opioid receptors are located on striatopallidal terminals and pallidopallidal terminals of spontaneously firing GP neurons, whereas presynaptic delta opioid receptors are preferentially located on terminals of quiescent GP cells. Enkephalin, acting at both of these receptor subtypes, serves to reduce GABA release in the GP and may therefore act as an adaptive mechanism, maintaining the inhibitory function of the GP in basal ganglia circuitry.

Limbic gamma rhythms. I. Phase-locked oscillations in hippocampal CA1 and subiculum.

Gamma oscillations (approximately 40 Hz) were induced in transverse hippocampal slices by tetanic stimulation of CA1 and/or subiculum. Tetanic stimulation of each site elicited population gamma oscillations in the surrounding tissue <400 micro(m) away. Stimulation of CA1 alone could evoke activity at both CA1 and subiculum. Subicular stimulation, however, did not transmit to CA1. When the rostral end of CA1 was stimulated, gamma oscillations transmitted across <1.5 mm of silent CA1 before reappearing in the subiculum. Tetanic stimulation of CA1 increased [K+]o to 8.2 +/- 1.5 mM (mean +/- SE). The location of the peak increase corresponded to the site of local gamma generation. Silent areas of CA1 experienced smaller [K+]o increases, to 4.9 +/- 0.7 mM. The subiculum, which generated gamma, remained at the baseline 3.0 mM. Although fluctuations in [K+]o may have an impact on the generation of gamma rhythms, they are not necessary for them. Gamma oscillations had similar frequencies in CA1 and subiculum (40.4 +/- 2.9 and 43.9 +/- 3.1 Hz, respectively). When present in both, the oscillations typically were phase locked with the subiculum lagging by 5.4 +/- 1.8 ms. When both CA1 and subiculum were stimulated the lag decreased by 28%. These delays approximate those expected for the conduction velocity of axons between the two regions, here estimated at 0.52 +/- 0.07 m/s. Transmission of gamma oscillations from CA1 to subiculum was blocked by the focal addition of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-receptor antagonist, 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione, to the subiculum. Oscillations induced in CA1 by local tetanic stimulation were blocked by focal application of the gamma-aminobutyric acid-A (GABAA) receptor antagonist, bicuculline, to CA1. Focal application of bicuculline to the subiculum blocked gamma due to subicular stimulation but not that due to CA1 stimulation. Bath-applied bicuculline disrupted subicular gamma evoked by subicular stimulation and led to a transient period of epileptiform responses before completely blocking responses. The further addition of the GABAB receptor antagonist, CGP 55845A, reversed this block, restoring the epileptic discharges evoked by tetanic stimulation. This suggests that the subiculum differs from hippocampal CA3 and neocortex, in having a powerful GABAB receptor-dependent mechanism to prevent epileptic discharges. The subiculum generates gamma rhythms both in response to local stimulation and to gamma rhythms evoked in CA1. Subicular gamma differs from that in CA1 in the presence of population spike doublets rather than singlets on many cycles. In both areas, generation of gamma by local stimulation depends on GABAA receptors, suggesting that the subiculum shares the interneuronal network mechanism we proposed for CA1.

Limbic gamma rhythms. II. Synaptic and intrinsic mechanisms underlying spike doublets in oscillating subicular neurons.

Gamma oscillations were evoked in the subiculum in rat transverse hippocampal slices by tetanic stimulation (200 ms/100 Hz) of either CA1 or subiculum. Gamma oscillations in the subiculum differed from those in CA1 in containing population spike doublets as well as singlets. The present study addresses the origin of this more complex form of gamma oscillation in the subiculum. Intracellular recordings from subicular neurons revealed that 63% of them fired double action potentials on cycles of the gamma oscillation that generated population spike doublets after tetanic stimulation of either CA1 or subiculum. The remaining cells produced excitatory postsynaptic potentials (EPSPs), and occasional single spikes, on each cycle. Neurons that fired occasional single action potentials during gamma rhythms were "regular spiking" cells. They did not produce burst discharges during depolarizing steps, had minimal membrane potential sags on hyperpolarizing steps, and responded to single afferent volleys with a single action potential on an EPSP followed by a large inhibitory postsynaptic potential complex. Fast spiking cells were observed too infrequently to be studied in detail. Neurons that fired doublets during gamma rhythms were "intrinsic burst" (IB) cells. They generated bursts of action potentials on step membrane depolarizations, had significant membrane potential sags on step hyperpolarizations with an anodal break potential on return to rest, and fired multiple action potentials in response to high-intensity single afferent volleys. IB neurons did not fire action potential doublets during 1-s membrane depolarizations. Double action potentials, however, were evoked in these cells by depolarizing pulses at 40 Hz from hyperpolarized membrane potentials (-100 mV). Computer simulations suggest that the hyperpolarization between the depolarizations was essential for action potential doublets. The results in this and the previous paper suggest the following: either CA1 or subiculum alone can generate gamma oscillations gated by local networks of interneurons, oscillations in CA1 project through pyramidal cell axons to subiculum with a time lag expected from axon conduction delays, and oscillating sequences of EPSPs and intrinsic and/or synaptic hyperpolarizing potentials in IB subicular neurons generate gamma frequency spike doublets, which depend on both the intrinsic properties of these neurons and their temporally patterned synaptic input. This phenomenon could amplify gamma output from CA1 and modify its coupling to gamma oscillations in the wider limbic system.

Recurrent excitatory postsynaptic potentials induced by synchronized fast cortical oscillations.

Gamma frequency (about 20-70 Hz) oscillations occur during novel sensory stimulation, with tight synchrony over distances of at least 7 mm. Synchronization in the visual system has been proposed to reflect coactivation of different parts of the visual field by a single spatially extended object. We have shown that intracortical mechanisms, including spike doublet firing by interneurons, can account for tight long-range synchrony. Here we show that synchronous gamma oscillations in two sites also can cause long-lasting (>1 hr) potentiation of recurrent excitatory synapses. Synchronous oscillations lasting >400 ms in hippocampal area CA1 are associated with an increase in both excitatory postsynaptic potential (EPSP) amplitude and action potential afterhyperpolarization size. The resulting EPSPs stabilize and synchronize a prolonged beta frequency (about 10-25 Hz) oscillation. The changes in EPSP size are not expressed during non-oscillatory behavior but reappear during subsequent gamma-oscillatory events. We propose that oscillation-induced EPSPs serve as a substrate for memory, whose expression either enhances or blocks synchronization of spatially separated sites. This phenomenon thus provides a dynamical mechanism for storage and retrieval of stimulus-specific neuronal assemblies.

Spatiotemporal patterns of gamma frequency oscillations tetanically induced in the rat hippocampal slice.

1. We used transverse and longitudinal rat hippocampal slices to study the synchronization of gamma frequency (> 20 Hz) oscillations, across distances of up to 4.5 mm. gamma oscillations were evoked in the CA1 region by tetanic stimulation at one or two sites simultaneously, and were associated with population spikes. Tetanic stimuli that were strong enough to induce oscillations were associated with depolarization of both pyramidal cells and interneurones, largely produced by activation of metabotropic glutamate receptors. 2. Computer simulations of gamma oscillations were also performed in a model with pyramidal cells and interneurones, arranged in a chain of five cell groups. This model had suggested previously that interneurone networks alone could generate synchronous gamma oscillations locally, but that pyramidal cell firing, by inducing spike doublets in interneurones, was necessary for the occurrence of highly correlated oscillations with small phase lag (< 2.5 ms), in a distributed network possessing long axon conduction delays. 3. In both experiment and model, pyramidal cell spikes occurred in phase with local population spikes, as did the first spike of the interneurone doublet. 4. The conductance of the interneurone alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-mediated conductance was manipulated in the model, while the relation between oscillations at opposite ends of the chain was examined. When the conductance was large enough for doublet firing to be synaptically induced in interneurones, oscillation phase lags were < 2.25 ms across the chain. As predicted, experimental blockade of AMPA receptors resulted in increased phase lags between two sites oscillating simultaneously, compared with control conditions. 5. Both in model and in experiment, when stimuli to the two ends of the network were slightly different, cross-network synchronization occurred with a shorter phase lag at high frequencies than at lower frequencies. 6. These data suggest that, while interneurone networks alone can generate locally synchronized gamma oscillations, firing of pyramidal cells, and the synaptically induced doublet firing in interneurones, contribute to the stability and tight synchrony of the oscillations in distributed networks.

Differential actions of serotonin, mediated by 5-HT1B and 5-HT2C receptors, on GABA-mediated synaptic input to rat substantia nigra pars reticulata neurons in vitro.

The ability of serotonin to modulate GABA-mediated synaptic input to substantia nigra pars reticulata (SNr) neurons was investigated with the use of whole-cell patch-clamp recording from slices of rat midbrain. Fast evoked GABA(A) receptor-mediated synaptic currents (IPSCs) were attenuated reversibly approximately 60% by serotonin, which also caused an inward current with reversal potential of -25 mV. This inward current was blocked by the 5-HT2 receptor antagonist ritanserin, whereas the IPSC depression was blocked by the 5-HT1B receptor antagonist pindolol. The amplitude ratio of IPSC pairs (50 msec interpulse interval) was enhanced by serotonin (in ritanserin) and also by the GABA(B) receptor agonist baclofen (which also depressed the IPSC), consistent with a presynaptic site of action in both cases. In contrast, spontaneous tetrodotoxin-sensitive GABA(A) synaptic currents (sIPSCs) were increased in frequency by serotonin (an action that was sensitive to ritanserin, but not pindolol) but reduced in frequency by baclofen. SNr neurons therefore receive inhibitory synaptic input mediated by GABA(A) receptors from at least two distinct sources. One, probably originating from the striatum, may be depressed via presynaptic 5-HT1B and GABA(B) receptors. The second is likely to arise from axon collaterals of SNr neurons themselves and is facilitated by an increase in firing via postsynaptic, somatodendritic 5-HT2C receptor activation, but it is depressed by GABA(B) receptor activation. Thus, serotonin can both depolarize and disinhibit SNr neurons via 5-HT2C and 5-HT1B receptors, respectively, but excitation may be limited by GABA released from axon collaterals.