MAM-2201, a synthetic cannabinoid drug of abuse, suppresses the synaptic input to cerebellar Purkinje cells via activation of presynaptic CB1 receptors.

Herbal products containing synthetic cannabinoids-initially sold as legal alternatives to marijuana-have become major drugs of abuse. Among the synthetic cannabinoids, [1-(5-fluoropentyl)-1H-indol-3-yl](4-methyl-1-naphthalenyl)-methanone (MAM-2201) has been recently detected in herbal products and has psychoactive and intoxicating effects in humans, suggesting that MAM-2201 alters brain function. Nevertheless, the pharmacological actions of MAM-2201 on cannabinoid receptor type 1 (CB1R) and neuronal functions have not been elucidated. We found that MAM-2201 acted as an agonist of human CB1Rs expressed in AtT-20 cells. In whole-cell patch-clamp recordings made from Purkinje cells (PCs) in slice preparations of the mouse cerebellum, we also found that MAM-2201 inhibited glutamate release at parallel fiber-PC synapses via activation of presynaptic CB1Rs. MAM-2201 inhibited neurotransmitter release with an inhibitory concentration 50% of 0.36 µM. MAM-2201 caused greater inhibition of neurotransmitter release than Δ(9)-tetrahydrocannabinol within the range of 0.1-30 µM and JWH-018, one of the most popular and potent synthetic cannabinoids detected in the herbal products, within the range of 0.03-3 µM. MAM-2201 caused a concentration-dependent suppression of GABA release onto PCs. Furthermore, MAM-2201 induced suppression of glutamate release at climbing fiber-PC synapses, leading to reduced dendritic Ca(2+) transients in PCs. These results suggest that MAM-2201 is likely to suppress neurotransmitter release at CB1R-expressing synapses in humans. The reduction of neurotransmitter release from CB1R-containing synapses could contribute to some of the symptoms of synthetic cannabinoid intoxication including impairments in cerebellum-dependent motor coordination and motor learning.

Protein kinase G linked to dopamine D3 receptors in the dorsal striatum controls dopamine release, ΔFosB expression and locomotor activity after repeated cocaine administration.

Protein kinase G (PKG) has been implicated in a variety of physiological functions including synaptic plasticity in the brain. This study investigated the involvement of dopamine D3 (D3) receptors in PKG-regulated dopamine release, long-term changes in gene expression and behavioral sensitization after repeated cocaine administration. Repeated systemic injections of cocaine (20mg/kg), once a day for seven consecutive days, increased extracellular dopamine concentrations in the dorsal striatum. Inhibition of neuronal nitric oxide synthase, cGMP or PKG, stimulation of D3 receptors, and simultaneous inhibition of each of them with D3 receptor stimulation decreased the repeated cocaine-induced increase in dopamine concentrations and locomotor activity. Similarly, inhibition of PKG and simultaneous inhibition of PKG with D3 receptor stimulation decreased ΔFosB immunoreactivity elevated by repeated cocaine administration, however stimulation of D3 receptors alone did not. These findings suggest that activation of PKG after repeated cocaine administration is more sensitive to interact with D3 receptors in the dopamine terminals than those in medium spiny neurons. This interaction may result in the development of behavioral sensitization by the upregulation of dopamine releases in the dorsal striatum.

Presynaptic kainate receptor-mediated facilitation of glutamate release involves Ca2+-calmodulin and PKA in cerebrocortical synaptosomes.

We have explored the mechanisms involved in the facilitation of glutamate release mediated by the activation of kainate receptors (KARs) in the cortex using isolated nerve terminals (synaptosomes). Kainate (KA) produced an increase on glutamate release at 100 µM. The effect of KA was antagonized by NBQX (with AMPA receptors blocked by GYKI53655). This facilitation was suppressed by the inhibition of PKA activation by Rp-Br-cAMP and H-89. Moreover, the facilitation of glutamate release mediated by KAR requires the mobilization of intrasynaptosomal Ca(2+) stores and the formation of a Ca(2+)-calmodulin complex. We conclude that KARs present on presynaptic terminals in the neocortex mediate the facilitation of glutamate release through a mechanism involving an increase in cytosolic Ca(2+) to activate a Ca(2+)-calmodulin-AC/cAMP/PKA signaling cascade.

[Activation of presynaptic ionotropic glutamate receptors stimulates GABA release from hippocampal and cortical nerve terminals in rats].

One of the pathways implicated in a fine-tuning control of neurosecretory process is the activation of presynaptic receptors. The present study was focused on the role of presynaptic glutamate receptor activation in the regulation of inhibitory synaptic transmission in the rat hippocampus and cortex. We aimed to clarify what types of ionotropic glutamate receptors are involved in the modulation of GABA secretion, and what mechanism underlies this modulation. We have revealed that specific agonists of kainate and NMDA receptors, kainate and NMDA, like glutamate, induced the release of [3H]GABA from hippocampal and cortical nerve terminals suggesting the involvement of both types in the regulation of GABAergic transmission. Our results indicate preferential involvement of vesicular, but not cytosolic, pool in response to glutamate receptor activation. This is based on the finding that NO-711 (a specific inhibitor of plasma membrane GABA transporters), fails to attenuate [3H]GABA release. We have concluded that presynaptic glutamate receptor-induced modulation of the strength of synaptic response is due to increasing the release probability of synaptic vesicles.

Modulation of N-type calcium currents by presynaptic imidazoline receptor activation in rat superior cervical ganglion neurons.

Presynaptic imidazoline receptors (R(i-pre)) are found in the sympathetic axon terminals of animal and human cardiovascular systems, and they regulate blood pressure by modulating the release of peripheral noradrenaline (NA). The cellular mechanism of R(i-pre)-induced inhibition of NA release is unknown. We, therefore, investigated the effect of R(i-pre) activation on voltage-dependent Ca(2+) channels in rat superior cervical ganglion (SCG) neurons, using the conventional whole-cell patch-clamp method. Cirazoline (30 µM), an R(i-pre) agonist as well as an α-adrenoceptor (R(α)) agonist, decreased Ca(2+) currents (I(Ca)) by about 50% in a voltage-dependent manner with prepulse facilitation. In the presence of low-dose rauwolscine (3 µM), which blocks the α(2)-adrenoceptor (R(α2)), cirazoline still inhibited I(Ca) by about 30%, but prepulse facilitation was significantly attenuated. This inhibitory action of cirazoline was almost completely prevented by high-dose rauwolscine (30 µM), which blocks R(i-pre) as well as R(α2). In addition, pretreatment with LY320135 (10 µM), another R(i-pre) antagonist, in combination with low-dose rauwolscine (3 µM), also blocked the R(α2)-resistant effect of cirazoline. Addition of guanosine-5-O-(2-thiodiphosphate) (2 mm) to the internal solutions significantly attenuated the action of cirazoline. However, pertussis toxin (500 ng ml(1)) did not significantly influence the inhibitory effect of cirazoline. Moreover, cirazoline (30 µM) suppressed M current in SCG neurons cultured overnight. Finally, omega-conotoxin (omega-CgTx) GVIA (1 µM) obstructed cirazoline-induced current inhibition, and cirazoline (30 µM) significantly decreased the frequency of action potential firing in a partly reversible manner. This cirazoline-induced inhibition of action potential firing was almost completely occluded in the presence of omega-CgTx. Taken together, our results suggest that activation of R(i-pre) in SCG neurons reduced N-type I(Ca) in a pertussis toxin- and voltage-insensitive pathway, and this inhibition attenuated repetitive action potential firing in SCG neurons.

Noradrenaline release in the locus coeruleus modulates memory formation and consolidation; roles for α- and ß-adrenergic receptors.

Noradrenaline, essential for the modulation of memory, is released in various parts of the brain from nerve terminals controlled by the locus coeruleus (LoC). Noradrenaline release consequent upon input from higher brain areas also occurs within the LoC itself. We examined the effect of noradrenaline on adrenergic receptors in the LoC on memory processing, using colored bead discrimination learning in the young domestic chick. We have shown previously that the release of noradrenaline in the hippocampus and cortex (mesopallium) is essential for acquisition and consolidation of short-term to intermediate and to long-term memory. Noradrenaline release within the LoC is triggered by the glutamatergic input from the forebrain. Inhibition by LoC injection of NMDA or AMPA receptor antagonists is rescued by injection of ß2-and ß3-adrenoceptor (AR) agonists in the hippocampus. We show that inhibition of α2A-ARs by BRL44408 in the LoC up to 30 min post-training consolidates weakly-reinforced learning. Conversely activation of α2A-ARs in the LoC at the times of consolidation between short-term and intermediate and long-term memory caused memory loss, which is likely to be due to a decreased release of noradrenaline within these two time windows. The α2A-AR antagonist will block presynaptic inhibitory receptors leading to an increase in extracellular noradrenaline. This interpretation is supported by the actions of noradrenaline uptake blockers that produce the same memory outcome. BRL44408 in the mesopallium also caused memory enhancement. ß2-ARs are important in the first time window, whereas α1-, α2C-and ß3-ARs are important in the second time window. The results reveal that for successful memory formation noradrenaline release is necessary within the LoC as well as in other brain regions, at the time of consolidation of memory from short-term to intermediate and from intermediate to long-term memory.

Involvement of presynaptic 5-HT(1A) receptors in immunomodulation in conditions of psychoemotional tension.

The development of an immune reaction in CBA mice during activation and blockade of serotonin 1A (5-HT(1A)) autoreceptors with highly selective agents has characteristic features depending on the behavioral stereotype formed. The 5-HT(1A) receptor agonist 8-OH-DPAT (0.1 mg/kg 15 min before immunization) did not alter the number of IgM antibody-forming cells in the spleen at the peak of the immune reaction (day 4) in aggressive mice, but increased the number in submissive mice, in which the immune response without treatment was lower than that in aggressive animals. Blockade of the same receptor type with WAY-100635 (0.1 mg/kg 30 min before immunization) led to a decrease in the immune response in animals with aggressive behavior and an increase in submissive animals. The question of the influences of agents acting on presynaptic 5-HT(1A) somatodendritic autoreceptors on the immune response, depending on the functional state of 5-HT(1A) receptors and the balance of activities in the serotoninergic and dopaminergic systems as a whole, is discussed.

Endocannabinoids suppress excitatory synaptic transmission to dorsal raphe serotonin neurons through the activation of presynaptic CB1 receptors.

Endocannabinoid signaling in the dorsal raphe (DR) has recently been implicated in the regulation of anxiety and depression. However, the cellular mechanisms by which endocannabinoids (eCBs) regulate the excitability of DR 5-hydroxytryptamine (serotonin; 5-HT) neurons remain poorly understood. In the present study, using whole-cell recording from DR 5-HT neurons, we examined the effects of eCBs on glutamatergic synapses in the DR. We found that the eCB anandamide decreased the amplitude of evoked excitatory postsynaptic currents (eEPSCs). This effect was blocked by CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM 251) and mimicked by (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212-2), a CB(1) receptor agonist. The inhibition of eEPSC amplitude was associated with an increase in the paired-pulse ratio and coefficient of variance. Activation of CB(1) receptors also reduced the frequency, but not the amplitude, of miniature excitatory postsynaptic currents, indicating that eCBs inhibit glutamate release in the DR. In addition, we found that depolarization of DR 5-HT neurons induced a transient inhibition of the amplitude of eEPSCs, termed depolarization-induced suppression of excitation (DSE). The induction of DSE required an increase in postsynaptic intracellular calcium and was due to a decrease in glutamate release. Furthermore, pharmacological studies showed that blockade of CB(1) receptors with AM 251 abolished the DSE. In contrast, activation of CB(1) receptors with WIN 55,212-2 mimicked and occluded the DSE, indicating that depolarization of DR 5-HT neurons triggers eCB release, which in turn mediates the DSE. Together, these results indicate that eCBs play a role in modulating glutamatergic synaptic transmission to DR 5-HT neurons.

The contribution of serotonin 1A receptors to kappa opioid immunosuppression.

Activation of kappa opioid receptors (kappa-OR) with the selective agonist rimorphin (0.1 mg/kg) produced marked suppression of the immune response in CBA mice. This effect was not seen on administration of rimorphin on the background of a reduction in the activity of the serotoninergic (5-HTergic) system resulting from stimulation of presynaptic (8-OH-DPAT, 0.1 mg/kg) or blockade of postsynaptic (WAY-100635, 1.0 mg/kg) 5-HT1A receptors. These data led to the conclusion that 5-HTergic mechanisms involving preand postsynaptic 5-HT1A receptors have a role in kappa-opioid-mediated immunosuppression.

Suppression of GABA input by A1 adenosine receptor activation in rat cerebellar granule cells.

Synaptic transmission has been shown to be modulated by purinergic receptors. In the cerebellum, spontaneous inhibitory input to Purkinje neurons is enhanced by ATP via P2 receptors, while evoked excitatory input via the granule cell parallel fibers is reduced by presynaptic P1 (A1) adenosine receptors. We have now studied the modulation of the complex GABAergic input to granule cells by the purinergic receptor agonists ATP and adenosine in acute rat cerebellar tissue slices using the whole-cell patch-clamp technique. Our experiments indicate that ATP and adenosine substantially reduce the bicuculline- and gabazine-sensitive GABAergic input to granule cells. Both phasic and tonic inhibitory components were reduced leading to an increased excitability of granule cells. The effect of ATP and adenosine could be blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), but not by other P1 and P2 receptor antagonists, indicating that it was mediated by activation of A1 adenosine receptors. Our results suggest that, in the cerebellar network, A1 receptor activation, known to decrease the excitatory output of granule cells, also increases their excitability by reducing their complex GABAergic input. These findings extend our knowledge on purinergic receptors, mediating multiple modulations at both inhibitory and excitatory input and output sites in the cerebellar network.

[Involvement of presynaptic 5-HT1A receptors in immunomodulation under conditions of psychoemotional state].

The development of the immune reaction after the activation or blockade of presynaptic 5-HT1A receptors, respectively, with a selective agonist or antagonist depends on specific behavioral patterns. Agonist of 5-HT1A receptors 8-OH-DPAT (0.1 mg/kg 15 min prior to immunization) did not change the number of IgM plaque-forming cells in the spleen of aggressive mice at the peak of the immune response (the 4-th day after immunization) but increased their number in submissive animals, in which the baseline immune response was lower than in aggressive animals. Administration of WAY-100635 (0.1 mg/kg 30 min prior to immunization) resulted in a decrease in the immune reaction in aggressive mice and led to its increase in submissive animals. It is suggested that the immunomodulatory effects of drugs affecting the activity of presynaptic 5-HT1A receptors are related to changes in the functional state of these receptors and the interaction between serotonin- and dopaminergic systems.

Getting specialized: presynaptic and postsynaptic dopamine D2 receptors.

Dopamine (DA) signaling controls many physiological functions ranging from locomotion to hormone secretion, and plays a critical role in addiction. DA elevation, for instance in response to drugs of abuse, simultaneously activates neurons expressing different DA receptors; how responses from diverse neurons/receptors are orchestrated in the generation of behavioral and cellular outcomes, is still not completely defined. Signaling from D2 receptors (D2Rs) is a good example to illustrate this complexity. D2Rs have presynaptic and postsynaptic localization and functions, which are shared by two isoforms in vivo. Recent results from knockout mice are clarifying the role of site and D2 isoform-specific effects thereby increasing our understanding of how DA modulates neuronal physiology.

Desensitization of pre and post synaptic 5-HT-1A receptor responses following long term consumption of sugar rich diet: implications for sugar-induced obesity.

The present study concerns the effectiveness of a selective 5-hydroxytryptamine (5-HT)-1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) in long term sugar diet treated rats. Male albino wistar rats were divided into control and test groups. Test animals were given sugar (5 g/10 ml water) orally for three weeks. Food intakes and body weight of all rats were measured weekly. After three weeks control and test animals were further divided into two groups i.e. saline injected and drug injected. 8-OH-DPAT at a dose of 0.25mg/Kg was injected to a group of normal diet treated and another group of sugar diet treated rats. Other two groups were injected with saline. 5-HT syndrome and food intakes at 2h and 4h were monitored. Then animals were decapitated to collect brain samples for the estimation of 5-HT and 5-hydroxyindole acetic acid (5-HIAA) levels by HPLC-EC method. We observed that weekly cumulative food intakes increased and body weights decreased in sugar diet treated rats. 8-OH-DPAT produced hyperactivity syndrome in both control and sugar treated rats. But these values were smaller in sugar diet than normal diet treated rats. Hyperphagic effects of 8-OH-DPAT were greater in normal diet than sugar diet treated rats. 5-HT and 5-HIAA levels were not altered. The results suggesting a desensitization of pre as well as postsynaptic 5-HT-1A receptors in rats treated with sugar diet are discussed in the context of a role of sugar diet in the precipitation of obesity and other neuropsychiatric illnesses.

Serotonin(1A) receptor agonism in the expression of behavioral dopaminergic supersensitivity in subchronic haloperidol treated rats.

The idea that serotonin (5-hydroxytryptamine; 5-HT) is contributed in schizophrenia has long been advocated and alterations in 5-HT neurotransmission has been hypothesized to modulate both the therapeutic and extrapyramidal symptoms (EPS) liability of conventional neuroleptics. The 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a preferential 5-HT(1A) ligand, has been reported to attenuate EPS functions of haloperidol in animals. In view of a possible role of 5-HT(1A) receptors in the management of EPS functions of a neuroleptic drug, the present study was designed to investigate behavioral responses of 8-OH-DPAT at a challenge dose of 0.5mg/Kg in rats with subchronic haloperidol administration at a dose of 5mg/Kg twice daily for 5 days. The intensity of 5-HT syndrome provoked by 8-OH-DPAT was taken as a measure of postsynaptic responses. In the present study administration of haloperidol at a dose of 5mg/Kg twice daily for 5 days decreased locomotion significantly (p<0.01) in familiar (home cage) environment. Subchronic administration of haloperidol at the same dose elicited significant (p<0.01) cataleptic responses in rats when compared with saline treated rats. Results revealed that 8-OH-DPAT-induced hyperlocomotion (p<0.05) and forepaw treading (p<0.1) were significantly smaller in rats pre-treated with haloperidol for 5 days than repeatedly saline injected rats. Conversely, the other components of the syndrome i.e. flat body posture (p<0.001), hind limb abduction (p<0.001) and straub tail (p<0.01) were significantly greater in repeated haloperidol treated rats when compared with repeated saline injected rats. These findings help to demonstrate a causal link between the upregulation of DA-D(2) receptors and the decrease in the effectiveness of presynaptic 5-HT(1A) receptors following subchronic haloperidol administration and this may further help to yield an antipsychotic agent with an improved profile of efficacy to EPS, thereby widening its therapeutic window.

[Impact of serotonin 1A receptors in the kappa-opioid immunosuppression].

The data obtained indicate that rimorphin (0.1 mg/kg), a specific kappa-agonist, evoked a significant inhibition of the immune response in CBA mice. Pretreatment of the animals with 8-OH-DPAT (0.1 mg/kg), a selective serotonin (5-HT) agonist, activating presynaptic 5-HT(1A) receptors, or WAY-100635 (1.0 mg/kg), a selective 5-HT(1A) receptors blocker of postsynaptic 5-HTIA receptors, prevents kappa-opioid effect. The present data indicate that kappa-opioid-induced immunosuppression is due to the involvement of the 5-HT-ergic mechanisms that are modulated via pre- and postsynaptic 5-HT(1A) receptors.

In vivo modulation of dopaminergic nigrostriatal pathways by cytisine derivatives: implications for Parkinson's Disease.

Nicotinic acetylcholine receptor agonists are considered potential pharmacological agents for Parkinson's disease treatment, due to their ability to improve experimental Parkinson symptomatology, reduce 3,4-dihydroxy-L-phenylalanine-induced dyskinesias and stop the neurodegenerative process at an experimental level. In the present work, the ability of the nicotinic agonist cytisine and two halogenated derivatives (3-bromocytisine and 5-bromocytisine) to induce striatal dopamine release was characterized in vivo by microdialysis. Cytisine, 5-bromocytisine and nicotine were much more efficacious than 3-bromocytisine in eliciting dopamine release in response to their local application through the microdialysis probe. Moreover, the agonists were intermittently administered before and after an intranigral injection of 6-hydroxydopamine (6-OHDA), and striatal dopamine tissue levels were assessed 8 days after the lesion. Both cytisine and its 5-bromo derivative (but not the 3-bromo derivative) significantly prevented the decrease of striatal dopamine tissue levels induced by 6-OHDA. These results suggest that the efficacy of nicotinic agonists to stimulate dopamine release in vivo through presynaptic nicotinic receptors could be related to their potential to induce striatal protection.

Presynaptic alpha 7- and beta 2-containing nicotinic acetylcholine receptors modulate excitatory amino acid release from rat prefrontal cortex nerve terminals via distinct cellular mechanisms.

Nicotine can enhance working memory and attention. Activation of both alpha7 and beta2(*) nicotinic acetylcholine receptors (nAChRs) in the prefrontal cortex (PFC) has been implicated in these processes. The ability of presynaptic nAChRs to modulate neurotransmitter release, notably glutamate release, is postulated to contribute to nicotine's effects. We have examined the cellular mechanisms underlying alpha7 and beta2(*) nAChR-mediated [(3)H]d-aspartate release from the PFC in vitro. Using the alpha7 and beta2(*) nAChR-selective agonists (R)-N-(1-azabicyclo[2.2.2]-oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide) (compound A) and 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-iodo-A-85380), respectively, in conjunction with inhibitors of voltage-operated Ca(2+) channels (VOCCs) and intracellular Ca(2+) stores, we show that [(3)H]d-aspartate release evoked by activation of beta2(*) nAChRs occurs via VOCCs. In contrast, alpha7 nAChR-evoked release was unaffected by VOCC blockers but was abolished by modulators of Ca(2+) stores, including ryanodine. The alpha7 nAChR ligand alpha-bungarotoxin and ryanodine receptors were colocalized to a subpopulation of PFC synaptosomes. Compound A-evoked [(3)H]d-aspartate release was also blocked by mitogen-activated protein kinase kinase 1 inhibitors, implicating extracellular signal-regulated kinase (ERK)1/2 in alpha7 nAChR-evoked exocytosis. Western blotting confirmed that compound A, but not 5-iodo-A-85380, application increased ERK2 phosphorylation in PFC synaptosomes, and this was dependent on ryanodine-sensitive stores. Compound A also promoted synapsin-1 phosphorylation at ERK1/2-dependent sites, in a ryanodine-sensitive manner. Thus, beta2(*) and alpha7 nAChR subtypes in the PFC mediate [(3)H]d-aspartate release via distinct mechanisms as a result of their differential coupling to VOCCs and Ca(2+)-induced Ca(2+) release (CICR), respectively. The ability of alpha7 nAChRs to promote the phosphorylation of presynaptic ERK2 and synapsin-1, downstream of CICR, provides a potential mechanism for presynaptic facilitation in the PFC.

GABA(B) receptor-mediated presynaptic inhibition of glycinergic transmission onto substantia gelatinosa neurons in the rat spinal cord.

The GABA(B) receptor-mediated presynaptic inhibition of glycinergic transmission was studied from young rat substantia gelatinosa (SG) neurons using a conventional whole-cell patch clamp technique. Action potential-dependent glycinergic inhibitory postsynaptic currents (IPSCs) were recorded from SG neurons in the presence of 3 mM kynurenic acid and 10 microM SR95531. In these conditions, baclofen (30 microM), a selective GABA(B) receptor agonist, greatly reduced the amplitude of glycinergic IPSCs and increased the paired-pulse ratio. Such effects were completely blocked by 3 microM CGP55845, a selective GABA(B) receptor antagonist, indicating that the activation of presynaptic GABA(B) receptors decreases glycinergic synaptic transmission. Glycinergic IPSCs were largely dependent on Ca2+ influxes passing through presynaptic N- and P/Q-type Ca2+ channels, and these channels contributed equally to the baclofen-induced inhibition of glycinergic IPSCs. However, the baclofen-induced inhibition of glycinergic IPSCs was not affected by either 100 microM SQ22536, an adenylyl cyclase inhibitor, or 1 mM Ba2+, a G-protein coupled inwardly rectifying K+ channel blocker. During the train stimulation (10 pulses at 20 Hz), which caused a marked synaptic depression of glycinergic IPSCs, baclofen at a 30 microM concentration completely blocked glycinergic synaptic depression, but at a 3 microM concentration it largely preserved glycinergic synaptic depression. Such GABA(B) receptor-mediated dynamic changes in short-term synaptic plasticity of glycinergic transmission onto SG neurons might contribute to the central processing of sensory signals.

Activation of presynaptic kainate receptors suppresses GABAergic synaptic transmission in the rat globus pallidus.

The globus pallidus (GP) plays a central integrative role in the basal ganglia circuitry. It receives strong GABAergic inputs from the striatum (Str) and significant glutamatergic afferents from the subthalamic nucleus (STN). The change in firing rate and pattern of GP neurons is a cardinal feature of Parkinson's disease pathophysiology. Kainate receptor (KAR) GluR6/7 subunit immunoreactivity is expressed presynaptically in GABAergic striatopallidal terminals which provides a substrate for regulation of GABAergic transmission in GP. To test this hypothesis, we recorded GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) in the GP following electrical stimulation of the Str. Following blockade of AMPA and N-methyl-d-aspartate receptors with selective antagonists, bath application of kainate (KA) (0.3-3 microM) reduced significantly the amplitude of evoked IPSCs. This inhibition was associated with a significant increase in paired-pulse facilitation ratio and a reduction of the frequency, but not amplitude, of miniature inhibitory postsynaptic currents (mIPSCs), suggesting a presynaptic site of KA action. The KA effects on striatopallidal GABAergic transmission were blocked by the G-protein inhibitor, N-ethylmaleimide (NEM), or protein kinase C (PKC) inhibitor calphostin C. Our results demonstrate that KAR activation inhibits GABAergic transmission through a presynaptic G protein-coupled, PKC-dependent metabotropic mechanism in the rat GP. These findings open up the possibility for the development of KA-mediated pharmacotherapies aimed at decreasing the excessive and abnormally regulated inhibition of GP neurons in Parkinson's disease.

Modulation of ATP-mediated contractions of the rat vas deferens through presynaptic cannabinoid receptors.

The effect of R-(+)-[2,3-dihydro-5-methyl-3-[(morpholiny)methyl]pyrolol[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone mesylate (WIN 55,212-2; a cannabinoid receptor agonist) was investigated on contractions of the bisected (epididymal and prostatic portions) rat vas deferens to assess the role of cannabinoid receptors in sympathetic ATP neurotransmission. WIN 55,212-2 inhibited the electrically induced contractions in both portions of the rat vas deferens. In the presence of the alpha1-adrenoreceptor antagonist prazosin, electrical stimulation produces a contraction mediated exclusively by ATP. In this condition, WIN 55,212-2 in the prostatic portion elicited a concentration-dependent inhibition that was antagonized by N-piperidinyl-[8-chloro-1-(2,4-dichlorophenyl)-1,4,5,6-tetrahydrobenzo[6,7]cyclohepta[1,2-c]pyrazole-3-carboxamide] (NESS 0327), a selective cannabinoid CB1 receptor antagonist. NESS 0327 caused a parallel dextral displacement of the WIN 55,212-2 concentration-response curve. It is suggested that activation of pre-junctional cannabinoid receptors on sympathetic nerves of the vas deferens modulates ATP neurotransmission.