Regular firing of a single output neuron reduces its own inhibition through endocannabinoids in substantia nigra pars reticulata of juvenile mice.

Depolarization-induced suppression of inhibition in substantia nigra pars reticulata suggests that burst-like activity but not regular firing suffices to activate presynaptic endocannabinoid CB1 receptors. To more closely determine the type of activity required, we applied gramicidin perforated patch recording under visual control to substantia nigra slices of juvenile mice. We found that evoked inhibitory postsynaptic currents (eIPSCs) were reduced in amplitude by the spontaneous firing of a neuron under study, whereas silencing this neuron enhanced inhibitory responses. Autonomous firing reduced eIPSCs to 78%+/-2% in a time- but not frequency-dependent manner. The phenomenon which we termed firing-induced suppression of inhibition was cannabinoid receptor subtype 1-dependent, whereas adenosine A1 receptors played only a minor role. Depletion of intracellular Ca(2+) stores abolished the firing-induced suppression of inhibition suggesting that Ca(2+) release from internal stores is necessary for the production of endocannabinoids during autonomous firing. We suggest that the Ca(2+) influx during autonomous activity of pars reticulata neurons suffices to selectively dampen incoming inhibition from striatal neurons because it is amplified by ryanodine receptor-mediated Ca(2+) release from intracellular stores.

Subcellular localization of type 1 cannabinoid receptors in the rat basal ganglia.

Endocannabinoids, acting via type 1 cannabinoid receptors (CB1), are known to be involved in short-term synaptic plasticity via retrograde signaling. Strong depolarization of the postsynaptic neurons is followed by the endocannabinoid-mediated activation of presynaptic CB1 receptors, which suppresses GABA and/or glutamate release. This phenomenon is termed depolarization-induced suppression of inhibition (DSI) or excitation (DSE), respectively. Although both phenomena have been reported to be present in the basal ganglia, the anatomical substrate for these actions has not been clearly identified. Here we investigate the high-resolution subcellular localization of CB1 receptors in the nucleus accumbens, striatum, globus pallidus and substantia nigra, as well as in the internal capsule, where the striato-nigral and pallido-nigral pathways are located. In all examined nuclei of the basal ganglia, we found that CB1 receptors were located on the membrane of axon terminals and preterminal axons. Electron microscopic examination revealed that the majority of these axon terminals were GABAergic, giving rise to mostly symmetrical synapses. Interestingly, preterminal axons showed far more intense staining for CB1, especially in the globus pallidus and substantia nigra, whereas their terminals were only faintly stained. Non-varicose, thin unmyelinated fibers in the internal capsule also showed strong CB1-labeling, and were embedded in bundles of myelinated CB1-negative axons. The majority of CB1 receptors labeled by immunogold particles were located in the axonal plasma membrane (92.3%), apparently capable of signaling cannabinoid actions. CB1 receptors in this location cannot directly modulate transmitter release, because the release sites are several hundred micrometers away. Interestingly, both the CB1 agonist, WIN55,212-2, as well as its antagonist, AM251, were able to block action potential generation, but via a CB1 independent mechanism, since the effects remained intact in CB1 knockout animals. Thus, our electrophysiological data suggest that these receptors are unable to influence action potential propagation, thus they may not be functional at these sites, but are likely being transported to the terminal fields. The present data are consistent with a role of endocannabinoids in the control of GABA, but not glutamate, release in the basal ganglia via presynaptic CB1 receptors, but also call the attention to possible non-CB1-mediated effects of widely used cannabinoid ligands on action potential generation.

Two pathways for the activation of small-conductance potassium channels in neurons of substantia nigra pars reticulata.

Neurons in substantia nigra pars reticulata express the messenger RNA for SK2 but not for SK3 subunits that form small-conductance, Ca2+-dependent K+ channels in dopamine neurons. To determine pathways for the activation of small-conductance, Ca2+-dependent K+ channels in substantia nigra pars reticulata neurons of rats and mice, we studied effects of the selective blocker of small-conductance, Ca2+-dependent K+ channels, apamin (0.01 or 0.3 microM). Apamin diminished the afterhyperpolarization following each action potential and induced burst discharges in substantia nigra pars reticulata neurons. Apamin had a robust effect already at a low (10 nM) concentration consistent with the expression of the SK2 subunit. Afterhyperpolarizations were also reduced by the Ca2+ channel blockers Ni2+ (100 microM) and omega-conotoxin GVIA (1 microM). Depletion of intracellular Ca2+ stores did not change the afterhyperpolarization. However, we observed outward current pulses that occurred independently from action potentials and were abrogated by apamin. Apart from a faster time course, they shared all properties with spontaneous hyperpolarizations or outward currents that ryanodine receptor-mediated Ca2+ release from intracellular stores induces in juvenile dopamine neurons. Sensitization of ryanodine receptors by caffeine silenced substantia nigra pars reticulata neurons. This effect was abolished by the depletion of intracellular Ca2+ stores. We conclude that SK2 channels in substantia nigra pars reticulata neurons are activated by Ca2+ influx through at least two types of Ca2+ channels in the membrane and by ryanodine receptor-mediated Ca2+ release from intracellular stores. Ryanodine receptors do not amplify small-conductance, Ca2+-dependent K+ channel activation by the Ca2+ influx during a single spike. Yet, ryanodine receptor-mediated Ca2+ release and, thereby, an activation of small-conductance, Ca2+-dependent K+ channels by intracellular Ca2+ are available for excitability modulation in these output neurons of the basal ganglia system.

Retrograde signaling changes the venue of postsynaptic inhibition in rat substantia nigra.

Both endocannabinoids through cannabinoid receptor type I (CB1) receptors and dopamine through dopamine receptor type D1 receptors modulate postsynaptic inhibition in substantia nigra by changing GABA release from striatonigral terminals. By recording from visually identified pars compacta and pars reticulata neurons we searched for a possible co-release and interaction of endocannabinoids and dopamine. Depolarization of a neuron in pars reticulata or in pars compacta transiently suppressed evoked synaptic currents which were blocked by GABA(A) receptor antagonists (inhibitory postsynaptic currents [IPSCs]). This depolarization-induced suppression of inhibition (DSI) was abrogated by the cannabinoid CB1 receptor antagonist AM251 (1 microM). A correlation existed between the degree of DSI and the degree of reduction of evoked IPSCs by the CB1 receptor agonist WIN55,212-2 (1 microM). The cholinergic receptor agonist carbachol (0.5-5 microM) enhanced DSI, but suppression of spontaneous IPSCs was barely detectable pointing to the existence of GABA release sites without CB1 receptors. In dopamine, but not in GABAergic neurons DSI was enhanced by the dopamine D1 receptor antagonist SCH23390 (3-10 microM). Both the antagonist for CB1 receptors and the antagonist for dopamine D1 receptors enhanced or reduced, respectively, the amplitudes of evoked IPSCs. This tonic influence persisted if the receptor for the other ligand was blocked. We conclude that endocannabinoids and dopamine can be co-released. Retrograde signaling through endocannabinoids and dopamine changes inhibition independently from each other. Activation of dopamine D1 receptors emphasizes extrinsic inhibition and activation of CB1 receptors promotes intrinsic inhibition.

Long-lasting enhancement of corticostriatal neurotransmission by taurine.

Taurine occurs at high concentrations in the forebrain and its distribution varies with (patho)physiological conditions; however, its role in neural function is poorly understood. We have now characterized its effects on corticostriatal synaptic transmission. Bath application of taurine (10 mm) to slices obtained from mice and rats exerted a biphasic action on corticostriatal field potentials. The fast and reversible inhibition by taurine was accompanied by a depolarization and conductance increase in medium spiny neurons and was sensitive to gamma-aminobutyric acid (GABA)A and glycine receptor (GlyR) antagonists. A long-lasting enhancement (LLETAU) of field potentials was recorded after taurine withdrawal. The LLETAU was not prevented by N-methyl-d-aspartate (NMDA)- or by GABAA receptor-antagonists, but was sensitive to the GlyR-antagonist strychnine and blocked by the competitive taurine uptake inhibitor guanidinoethylsulphonate (GES, 1 mm). GES at 10 mm evoked an enhancement of field potentials similar to LLETAU. LLETAU depended on protein kinase C activation as it was blocked by chelerythrine, but was unaffected by trifluoperazine, and thus independent of calmodulin. LLETAU was significantly smaller in juvenile than in mature rodents. Activation of GlyRs and the specific taurine transporter by taurine evoke a long-lasting enhancement of corticostriatal transmission.

Defective hippocampal mossy fiber long-term potentiation in endothelial nitric oxide synthase knockout mice.

Mossy fiber long-term potentiation (mfLTP) was compared in hippocampal slices prepared from wild-type mice and mice lacking functional endothelial nitric oxide synthase (eNOS(-/-) mice) using field potential recording. In the presence of D-2-amino-5-phosphonovaleric acid (AP5, 50 microM), the mfLTP induced by tetanic stimulation (100 Hz, 1 sec) was significantly reduced in knockouts (n = 8) in comparison with wild-type (n = 8). Similarly, potentiation induced by forskolin (30 microM) or 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP, 100 microM) was less pronounced in knockouts. However, in wild-types the mfLTP-induced in the presence of the nonselective pharmacological inhibitor of NOS (N-nitro-L-Arginine, 100 microM, n = 6) was not significantly different from control (n = 8). Thus, eNOS is not directly involved in mfLTP, but lack of eNOS during development leads to a deficit downstream of adenylyl cyclase.

Histamine H(3) receptors depress synaptic transmission in the corticostriatal pathway.

The effect of histamine on the main input to the striatum - the corticostriatal pathway - was studied using electrophysiological techniques in brain slices from rats and mice. Field potentials (FPs) were recorded in the striatum following stimulation at the border of the striatum and the cortex. Bath application of histamine caused a pronounced and long-lasting depression of FPs in rat slices with an IC(50) of 1.6 microM and a maximal depression of around 40%. In mouse slices histamine also depressed FPs, but to a lesser extent and more transiently. Further experiments in rat slices showed that histamine H(3) receptors were responsible for this depression since the selective H(3) receptor agonist R-alpha-methylhistamine (1 microM) mimicked the action of histamine whilst the selective H(3) receptor antagonist, thioperamide (10 microM) blocked the depression caused by histamine application. The histaminergic depression was probably not mediated indirectly through interneurons since blockade of GABA(A), GABA(B), nicotinic and muscarinic receptors or nitric oxide synthase did not prevent the histamine effect. Intracellular recordings from medium spiny neurons in the striatum revealed that histamine did not affect postsynaptic membrane properties but increased paired-pulse facilitation of excitatory synaptic responses indicating a presynaptic locus of action.

Long-term suppression of synaptic transmission by tetanization of a single pyramidal cell in the mouse hippocampus in vitro.

1. The consequences of stimulating a single pyramidal cell in the CA1 area of the hippocampus for synaptic transmission in the stratum radiatum were investigated. 2. Tetanic activation of single pyramids caused by depolarizing current injection, but not an equal number of distributed action potentials, reduced excitatory transmission by 20 %, with a delayed onset, for more than 1 h. 3. EPSPs in the tetanized pyramidal cells were increased for equally long periods but this was not the cause of the field EPSP reduction. Spontaneous somatic IPSPs were not affected; evoked IPSPs were decreased in the tetanized cell. 4. Paired pulse facilitation of the field EPSPs was unchanged. 5. The field EPSP reduction was markedly diminished by a knife cut along the base of pyramidal cells in CA1. 6. The addition of antagonists of GABA, NMDA and metabotropic glutamate receptors blocked or diminished the field EPSP slope reduction evoked by intracellular stimulation. 7. Simultaneous recordings revealed long-lasting excitations of interneurons located in the outer oriens layer as a result of single pyramid tetanization. 8. Intense firing of small numbers of pyramidal cells can thus persistently inhibit mass transmission through the hippocampus. This effect involves activation of interneurons by glutamate receptors.

Histamine increases the bursting activity of pyramidal cells in the CA3 region of mouse hippocampus.

An excitatory action of histamine was investigated by intracellular recording in the CA3 region of hippocampal slices. Bath application of histamine or impromidine, a H2 receptor agonist, had the following effects: (1) a depolarisation in 60% and no changes in membrane potential in 40% of the CA3 pyramids; (2) single cell firing and burst activity were evoked or more than doubled when spontaneously present; (3) the bursts were prolonged and often followed by afterdischarges instead of the normal afterhyperpolarisations (AHPs); (4) synaptic stimulation evoked large bursts instead of excitatory synaptic potentials (EPSPs) and primary burst responses became prolonged. CA3 bursts may play a decisive role in memory trace formation, their facilitation and potentiation is in keeping with a positive role of the histaminergic system in attention and learning.

Activation of interneurons at the stratum oriens/alveus border suppresses excitatory transmission to apical dendrites in the CA1 area of the mouse hippocampus.

The consequences of activation or inactivation of interneurons at the CA1 stratum oriens/ alveus border for signal transmission at the apical dendritic region of pyramidal cells were investigated in slices from mice submerged in a perfusion chamber. A characteristic subpopulation of interneurons with a horizontal dendritic tree in this region, which sends a GABAergic projection to the apical dendrites of CA1 pyramidal cells is strongly excited by metabotropic glutamate receptor activation and receives GABAergic input from vasoactive intestinal polypeptide-containing interneurons. Pressure ejection of glutamate or the metabotropic agonist 1s,3r-aminocyclopentane dicarboxylic acid from micropipettes onto the stratum oriens/alveus border caused a long lasting (more than 90 min) decrease of field-excitatory postsynaptic potentials in the strata radiatum and lacunosum-moleculare. The GABAB antagonist CGP 35348 (100 microM in the perfusion fluid) partially and reversibly blocked this effect. Vasoactive intestinal polypeptide- (0.1 microM in the bath) excited neurons with response and firing properties characteristic for interneurons at the oriens/alveus border. Local pressure application of vasoactive intestinal polypeptide (10 microM) to the alveus region led, after a brief (2 min) and small (10%) increase, to a longer lasting (30-50 min) decrease (by 20-30%) in the slope of the field-excitatory postsynaptic potential in strata radiatum and lacunosum-moleculare. This action was completely blocked by bath application of CGP 35348. Local application of tetrodotoxin in the stratum oriens/alveus region markedly increased the slope of evoked dendritic excitatory postsynaptic potentials, and caused multiple firing of pyramidal cells. Thus, stratum oriens/alveus interneurons have a profound inhibitory effect on signal transmission in the apical dendritic area of CA1, which is, at least in part, mediated by GABAB receptors. It appears that the GABAB receptor-mediated effect in stratum lacunosum-moleculare is produced by vasoactive intestinal polypeptide-sensitive interneurons.

Suppression of long-term potentiation in hippocampal slices by copper.

Cu(2+)-ions are known to interfere with gamma-aminobutyric acid (GABA)- and glutamate-operated ion channels from experiments with isolated neurons. Such actions are likely involved in the pathophysiology of Wilson's disease. We have now studied the effects of Cu2+ in the CA1 region of hippocampal slices. Field excitatory postsynaptic potential (EPSP) slopes in the CA1 region were unaffected by 1 microM Cu2+ but were depressed by 10 microM (to 85%) and 100 microM (to 50%). A paired-pulse test revealed no difference in facilitation in the presence or absence of Cu2+, indicating a postsynaptic action. A late component of intracellularly registered EPSPs in Mg(2+)-free solution was also reduced by Cu2+. The N-methyl-D-aspartate (NMDA) component of the field EPSP, isolated by adding CNQX and bicuculline in Mg(2+)-free solution, was reduced to 69% of control by 1 microM and to 50% of control by 10 microM Cu2+. Long-term potentiation, evoked by 3 x 50 pulses at 100 Hz, 20 s interval amounted to 132 +/- 11% 90 min after tetanization under control conditions but was absent in the presence of 1 microM Cu2+ in the bath. Thus low concentrations of copper can selectively reduce NMDA-mediated potentials and synaptic plasticity.

pH-dependent facilitation of synaptic transmission by histamine in the CA1 region of mouse hippocampus.

The action of histamine on excitatory synaptic transmission in the CA1 region of the mouse hippocampus was investigated. In a medium at pH 7.4 population spikes were increased for approximately 60 min after a brief (5 min) perfusion with histamine (5 microM) but excitatory postsynaptic potentials (EPSPs) were unaffected, as previously reported in the rat. At pH 7.0, however, a late component of extra- and intracellularly registered EPSPs was enhanced in two phases: a shorter (by 30%) and a longer lasting one (> 1 h by 10%). The NMDA antagonist amino-phosphonovalerate (60 microM) blocked this late component and prevented the EPSP broadening by histamine. In some cells histamine induced burst-firing and prolonged EPSPs. The actions on EPSPs were not mediated by any of the known histamine receptors as they were not mimicked by histamine H1, H2 and H3 agonists or blocked by H1, H2 or H3 antagonists. We conclude that histamine enhances a late (NMDA) component of hippocampal EPSPs when protonization is increased by a slight shift of the pH in the acidic direction. Such shifts occur during intense nervous discharges, e.g. in epileptic tissue or following tetanic stimulation, and in hypoxic conditions.

Differences of CA3 bursting in DBA/1 and DBA/2 inbred mouse strains with divergent shuttle box performance.

The CA3 bursting activity was compared in slices from two genetically closely related inbred mouse strains with divergent shuttle box performance (low level performing DBA/1 and high performing DBA/2 strains) and a control, behaviorally untested inbred strain, NMRI. Spontaneous population bursts of hippocampal CA3 pyramidal cells (measured extracellularly as field potentials) occurred more frequently in slices of the DBA/2 strain (in 62.5% of the slices in DBA/2 mice) than in the DBA/1 strain (in 33.3% of the slices in DBA/1 mice) and the control NMRI strain (in 33.3% of the slices), whereas the ratio of bursting and nonbursting cells was not different. The resting membrane potential of spontaneously bursting and nonbursting cells was hyperpolarized and the frequency of spontaneous cell bursts were higher in DBA/2 mice compared with both other strains. Slices from the high performing DBA/2 strain had significantly lower thresholds for population bursts evoked by mossy fiber (but not perforant path) stimulation. Electrophysiological properties and bursting patterns of CA3 pyramidal cells are shown to correlate with learning behavior in three different mouse strains. This result is in keeping with an important role of CA3 bursting in memory trace formation.