Modulation Of GABAergic Synaptic Transmission Research Articles

Modulation of GABAergic Synaptic Transmission by NMDA Receptors in the Dorsal Horn of the Spinal Cord.

The dorsal horn (DH) of the spinal cord is an important structure involved in the integration of nociceptive messages. Plastic changes in the properties of neuronal networks in the DH underlie the development of analgesia as well as of hyperalgesia and allodynia in acute and chronic pain states. Two key mechanisms are involved in these chronic pain states: increased electrical activities and glutamate release leading to the recruitment of NMDAr and plastic changes in the synaptic inhibition. Although: (1) the balance between excitation and inhibition is known to play a critical role in the spinal network; and (2) plastic changes in spinal excitation and inhibition have been studied separately, the relationship between these two mechanisms has not been investigated in detail. In the present work, we addressed the role of NMDA receptors in the modulation of GABAergic synaptic transmission in the DH network. Using tight-seal whole-cell recordings on adult mice DH neurons, we characterized the effect of NMDAr activation on inhibitory synaptic transmission and more especially on the GABAergic one. Our results show that, in a subset of neurons recorded in lamina II, NMDAr activation facilitates spontaneous and miniature GABAergic synaptic transmission with a target specificity on GABAergic interneurons. In contrast, NMDA reduced the mean amplitude of evoked GABAergic IPSCs. These results show that NMDAr modulate GABAergic transmission by a presynaptic mechanism of action. Using a pharmacological approach, we investigated the composition of NMDAr involved in this modulation of GABAergic synaptic transmission. We found that the NMDA-induced facilitation was mediated by the activation of NMDAr containing GluN2C/D subunits. Altogether, our results bring new insights on nociceptive information processing in the spinal cord network and plastic changes in synaptic inhibition that could underlie the development and maintenance of chronic pain.

Developmental Changes in Serotonergic Modulation of GABAergic Synaptic Transmission and Postsynaptic GABAA Receptor Composition in the Cerebellar Nuclei.

Outputs from the cerebellar nuclei (CN) are important for generating and controlling movement. The activity of CN neurons is controlled not only by excitatory inputs from mossy and climbing fibers and by γ-aminobutyric acid (GABA)-based inhibitory transmission from Purkinje cells in the cerebellar cortex but is also modulated by inputs from other brain regions, including serotonergic fibers that originate in the dorsal raphe nuclei. We examined the modulatory effects of serotonin (5-HT) on GABAergic synapses during development, using rat cerebellar slices. As previously reported, 5-HT presynaptically decreased the amplitudes of stimulation-evoked inhibitory postsynaptic currents (IPSCs) in CN neurons, with this effect being stronger in slices from younger animals (postnatal days [P] 11-13) than in slices from older animals (P19-21). GABA release probabilities accordingly exhibited significant decreases from P11-13 to P19-21. Although there was a strong correlation between the GABA release probability and the magnitude of 5-HT-induced inhibition, manipulating the release probability by changing extracellular Ca2+ concentrations failed to control the extent of 5-HT-induced inhibition. We also found that the IPSCs exhibited slower kinetics at P11-13 than at P19-21. Pharmacological and molecular biological tests revealed that IPSC kinetics were largely determined by the prevalence of α1 subunits within GABAA receptors. In summary, pre- and postsynaptic developmental changes in serotonergic modulation and GABAergic synaptic transmission occur during the second to third postnatal weeks and may significantly contribute to the formation of normal adult cerebellar function.

Alpha7 nicotinic acetylcholine receptor agonist promotes retinal ganglion cell function via modulating GABAergic presynaptic activity in a chronic glaucomatous model

Alpha-7 nicotinic acetylcholine receptor (α7-nAChR) agonists can prevent glutamate-induced excitotoxicity in cultured retinal ganglion cells (RGCs). However, the neuroprotective effects and the mechanism of action of PNU-282987, an α7-nAChR agonist, in a chronic in vivo rat glaucoma model are poorly understood. We found that elevated intraocular pressure (IOP) downregulated retinal α7-nAChR expression. Electroretinography revealed that the amplitude of the photopic negative response (PhNR) decreased in parallel with the loss of RGCs caused by elevated IOP. PNU-282987 enhanced RGC viability and function and decreased terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive signals in RGCs. Patch-clamp recordings revealed differences in the baseline frequencies and decay times of the miniature GABAergic inhibitory postsynaptic currents (mIPSCs) of RGCs between control and glaucomatous retinal slices. The results of western blotting and immunostaining showed that glutamic acid decarboxylase 65/67 and GABA deficits persisted in glaucomatous retinas and that these deficits were reversed by PNU-282987. Patch-clamp recordings also showed that PNU-282987 significantly increased the frequency and amplitude of the GABAergic mIPSCs of RGCs. The protective effects of PNU-292987 were blocked by intravitreal administration of selective GABAA receptor antagonists. The modulation of GABAergic synaptic transmission by PNU-282987 causes de-excitation of ganglion cell circuits and suppresses excitotoxic processes.

Endocannabinoids control vesicle release mode at midbrain periaqueductal grey inhibitory synapses.

The midbrain periaqueductal grey (PAG) forms part of an endogenous analgesic system which is tightly regulated by the neurotransmitter GABA. The role of endocannabinoids in regulating GABAergic control of this system was examined in rat PAG slices. Under basal conditions GABAergic neurotransmission onto PAG output neurons was multivesicular. Activation of the endocannabinoid system reduced GABAergic inhibition by reducing the probability of release and by shifting release to a univesicular mode. Blockade of endocannabinoid system unmasked a tonic control over the probability and mode of GABA release. These findings provides a mechanistic foundation for the control of the PAG analgesic system by disinhibition. The midbrain periaqueductal grey (PAG) has a crucial role in coordinating endogenous analgesic responses to physiological and psychological stressors. Endocannabinoids are thought to mediate a form of stress-induced analgesia within the PAG by relieving GABAergic inhibition of output neurons, a process known as disinhibition. This disinhibition is thought to be achieved by a presynaptic reduction in GABA release probability. We examined whether other mechanisms have a role in endocannabinoid modulation of GABAergic synaptic transmission within the rat PAG. The group I mGluR agonist DHPG ((R,S)-3,5-dihydroxyphenylglycine) inhibited evoked IPSCs and increased their paired pulse ratio in normal external Ca2+ , and when release probability was reduced by lowering Ca2+ . However, the effect of DHPG on the coefficient of variation and kinetics of evoked IPSCs differed between normal and low Ca2+ . Lowering external Ca2+ had a similar effect on evoked IPSCs to that observed for DHPG in normal external Ca2+ . The low affinity GABAA receptor antagonist TPMPA ((1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid) inhibited evoked IPSCs to a greater extent in low than in normal Ca2+ . Together these findings indicate that the normal mode of GABA release is multivesicular within the PAG, and that DHPG and lowering external Ca2+ switch this to a univesicular mode. The effects of DHPG were mediated by mGlu5 receptor engagement of the retrograde endocannabinoid system. Blockade of endocannabinoid breakdown produced a similar shift in the mode of release. We conclude that endocannabinoids control both the mode and the probability of GABA release within the PAG.

5-HT2 receptors mediate functional modulation of GABAa receptors and inhibitory synaptic transmissions in human iPS-derived neurons

Neural progenitors differentiated from induced pluripotent stem cells (iPS) hold potentials for treating neurological diseases. Serotonin has potent effects on neuronal functions through multiple receptors, underlying a variety of neural disorders. Glutamate and GABA receptors have been proven functional in neurons differentiated from iPS, however, little is known about 5-HT receptor-mediated modulation in such neuronal networks. In the present study, human iPS were differentiated into cells possessing featured physiological properties of cortical neurons. Whole-cell patch-clamp recording was used to examine the involvement of 5-HT2 receptors in functional modulation of GABAergic synaptic transmission. We found that serotonin and DOI (a selective agonist of 5-HT2A/C receptor) reversibly reduced GABA-activated currents, and this 5-HT2A/C receptor mediated inhibition required G protein, PLC, PKC, and Ca2+ signaling. Serotonin increased the frequency of miniature inhibitory postsynaptic currents (mIPSCs), which could be mimicked by α-methylserotonin, a 5-HT2 receptor agonist. In contrast, DOI reduced both frequency and amplitude of mIPSCs. These findings suggested that in iPS-derived human neurons serotonin postsynaptically reduced GABAa receptor function through 5-HT2A/C receptors, but presynaptically other 5-HT2 receptors counteracted the action of 5-HT2A/C receptors. Functional expression of serotonin receptors in human iPS-derived neurons provides a pre-requisite for their normal behaviors after grafting.

Essential roles of mGluR1 and inhibitory synaptic transmission in NMDA-independent long-term potentiation in the spinal trigeminal interpolaris.

Patterns of synaptic activity determine synaptic strengthening or weakening that is typically represented as long-term potentiation (LTP) and long-term depression (LTD), respectively. In the present study, we aim to test whether a conditioning stimulation of the spinal trigeminal subnucleus caudalis (Vc) induces LTP at excitatory synapses in the subnucleus interpolaris (Vi) and to characterize the LTP. Generally, a presynaptic high-frequency stimulation (HFS) protocol can induce LTP at excitatory synapses in the brain, including the spinal cord. Therefore, LTP in the Vi was induced by the HFS (3 tetani at 100 Hz) of Vc in the horizontal brainstem slices. By pretreating slices with antagonists for NMDA receptors, metabotropic glutamate receptor subtype 1 or 5 (mGluR1 or 5), GABAA receptors, glycine receptors and Ca(2+) chelator, the LTP was characterized. The HFS reliably but slowly induced LTP of excitatory synaptic transmission in the Vi. This LTP was not dependent on NMDA receptor activation; however, it did require the activation of mGluR1, but not mGluR5, and an intracellular Ca(2+) rise. Interestingly, this LTP induction required inhibitory synaptic transmission mediated by GABAA and glycine receptors, and coincided with the slow development of LTD at GABAergic synapses. The GABAergic LTD was mediated by mGluR1 and the intracellular Ca(2+) rise. These data suggest that the modulation of GABAergic synaptic transmission by conditioning synaptic activity contributes to the induction and expression of LTP at excitatory synapses in the Vi.

Compartment-specific modulation of GABAergic synaptic transmission by TRPV1 channels in the dentate gyrus.

The transient receptor potential TRPV1 or vanilloid receptor is a nonselective ligand-gated channel highly expressed in primary sensory neurons where it mediates nociception. TRPV1 is also expressed in the brain where its activation depresses excitatory synaptic transmission. Whether TRPV1 also regulates inhibitory synapses in the brain is unclear. Here, using a combination of pharmacology, electrophysiology, and an in vivo knockdown strategy, we report that TRPV1 activation by capsaicin or by the endocannabinoid anandamide depresses somatic, but not dendritic inhibitory transmission in both rat and mouse dentate gyrus. The effect on somatic inhibition was absent in TRPV1 knock-out mice and was also eliminated by two different TRPV1 shRNAs expressed in dentate granule cells, strongly supporting a functional role for TRPV1 in modulating GABAergic synaptic function. Moreover, TRPV1-mediated depression occurs independently of GABA release, requires postsynaptic Ca(2+) rise and activation of calcineurin, and is likely due to clathrin-dependent internalization of GABA receptors. Altogether, these findings reveal a novel form of compartment-specific regulation whereby TRPV1 channels can modify synaptic function in the brain.

Exposure to a high fat diet during the perinatal period alters vagal motoneurone excitability, even in the absence of obesity.

Obesity is recognized as being multifactorial in origin, involving both genetic and environmental factors. The perinatal period is known to be critically important in the development of neural circuits responsible for energy homeostasis and the integration of autonomic reflexes. Diet-induced obesity alters the biophysical, pharmacological and morphological properties of vagal neurocircuits regulating upper gastrointestinal tract functions, including satiety. Less information is available, however, regarding the effects of a high fat diet (HFD) itself on the properties of vagal neurocircuits. The present study was designed to test the hypothesis that exposure to a HFD during the perinatal period alters the electrophysiological, pharmacological and morphological properties of vagal efferent motoneurones innervating the stomach. Our data indicate that perinatal HFD decreases the excitability of gastric-projecting dorsal motor nucleus neurones and dysregulates neurotransmitter release from synaptic inputs and that these alterations occur prior to the development of obesity. These findings represent the first direct evidence that exposure to a HFD modulates the processing of central vagal neurocircuits even in the absence of obesity. The perinatal period is critically important to the development of autonomic neural circuits responsible for energy homeostasis. Vagal neurocircuits are vital to the regulation of upper gastrointestinal functions, including satiety. Diet-induced obesity modulates the excitability and responsiveness of both peripheral vagal afferents and central vagal efferents but less information is available regarding the effects of diet per se on vagal neurocircuit functions. The aims of this study were to investigate whether perinatal exposure to a high fat diet (HFD) dysregulated dorsal motor nucleus of the vagus (DMV) neurones, prior to the development of obesity. Whole cell patch clamp recordings were made from gastric-projecting DMV neurones in thin brainstem slices from rats that were exposed to either a control diet or HFD from pregnancy day 13. Our data demonstrate that following perinatal HFD: (i) DMV neurones had decreased excitability and input resistance with a reduced ability to fire action potentials; (ii) the proportion of DMV neurones excited by cholecystokinin (CCK) was unaltered but the proportion of neurones in which CCK increased excitatory glutamatergic synaptic inputs was reduced; (iii) the tonic activation of presynaptic group II metabotropic glutamate receptors on inhibitory nerve terminals was attenuated, allowing modulation of GABAergic synaptic transmission; and (iv) the size and dendritic arborization of gastric-projecting DMV neurones was increased. These results suggest that perinatal HFD exposure compromises the excitability and responsiveness of gastric-projecting DMV neurones, even in the absence of obesity, suggesting that attenuation of vago-vagal reflex signalling may precede the development of obesity.

Repeated morphine treatment alters cannabinoid modulation of GABAergic synaptic transmission within the rat periaqueductal grey.

Cannabinoids and opioids produce antinociception by modulating GABAergic synaptic transmission in a descending analgesic pathway from the midbrain periaqueductal grey (PAG). While chronic opioid treatment produces opioid tolerance, it has recently been shown to enhance cannabinoid-induced antinociception within the PAG. This study examined the effect of repeated opioid treatment on opioid and cannabinoid presynaptic modulation of GABAergic synaptic transmission in PAG. Midbrain PAG slices were prepared from untreated rats, and rats that had undergone repeated morphine or saline pretreatment. Whole-cell voltage-clamp recordings were made from neurons within the ventrolateral PAG. In slices from untreated animals, the cannabinoid receptor agonist WIN55212 and the μ receptor agonist DAMGO inhibited electrically evoked GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) IPSCs in PAG neurons, with IC50 s of 30 and 100 nM respectively. The inhibition of evoked IPSCs produced by WIN55212 (30 nM) and DAMGO (100 nM) was similar in PAG neurons from morphine- and saline-treated animals. The cannabinoid CB1 receptor antagonist AM251 increased the frequency of spontaneous miniature IPSCs in PAG neurons from repeated morphine-, but not saline-treated animals. DAMGO inhibition of evoked IPSCs was enhanced in the presence of AM251 in morphine-, but not saline-treated animals. These results indicate that the efficiency of agonist-induced inhibition of GABAergic synaptic transmission is enhanced by morphine treatment, although this is dampened by endocannabinoid-mediated tonic inhibition. Thus, endocannabinoid modulation of synaptic transmission could provide an alternative analgesic approach in a morphine-tolerant state. This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Pharmacological Activation of Kainate Receptors Drives Endocannabinoid Mobilization

Activation of both presynaptic metabotropic cannabinoid type 1 receptors (CB(1)s) and ionotropic kainate receptors (KARs) can efficiently modulate GABA release at many synapses of the CNS. The inhibitory effect of kainic acid (KA) has been ascribed to metabotropic actions, and KAR-induced release of secondary neuromodulatory agents may partly mediate these actions. Here, we investigated the involvement of the endocannabinoid system in the modulation of GABAergic synaptic transmission by pharmacological activation of KARs with KA in CA1 pyramidal neurons of the mouse hippocampus. We show that the depression of GABAergic synaptic transmission induced by KA (3 μm) is strongly inhibited by the simultaneous blockade of CB(1) and GABA(B) receptors with SR141716A (5 μm) and CGP55845 (5 μm), respectively. KA induces a calcium-dependent mobilization of the endocannabinoid anandamide (AEA) by activation of GluK2-containing KARs in postsynaptic pyramidal neurons. Consistently, the effect of KA is prolonged by the inhibitor of AEA degradation URB597 (1 μm) in a CB(1)-dependent manner, but it is not altered by blockade of degradation or synthesis of the other main endocannabinoid 2-arachidonoylglycerol (2AG). Hence, our work reveals that the pharmacological activation of KARs leads to the stimulation of secondary metabotropic signaling systems. In addition, these data further underline the profound mechanistic differences between exogenous and endogenous activation of KARs in the hippocampus.

Postsynaptic Cell Type–Dependent Cholinergic Regulation of GABAergic Synaptic Transmission in Rat Insular Cortex

The cerebral cortex consists of multiple neuron subtypes whose electrophysiological properties exhibit diverse modulation patterns in response to neurotransmitters, including noradrenaline and acetylcholine (ACh). We performed multiple whole cell patch-clamp recording from layer V GABAergic interneurons and pyramidal cells of rat insular cortex (IC) to examine whether cholinergic effects on unitary inhibitory postsynaptic currents (uIPSCs) are differentially regulated by ACh receptors, depending on their presynaptic and postsynaptic cell subtypes. In fast-spiking (FS) to pyramidal cell synapses, carbachol (10 μM) invariably decreased uIPSC amplitude by 51.0%, accompanied by increases in paired-pulse ratio (PPR) of the second to first uIPSC amplitude, coefficient of variation (CV) of the first uIPSC amplitude, and failure rate. Carbachol-induced uIPSC suppression was dose dependent and blocked by atropine, a muscarinic ACh receptor antagonist. Similar cholinergic suppression was observed in non-FS to pyramidal cell synapses. In contrast, FS to FS/non-FS cell synapses showed heterogeneous effects on uIPSC amplitude by carbachol. In roughly 40% of pairs, carbachol suppressed uIPSCs by 35.8%, whereas in a similar percentage of pairs uIPSCs were increased by 34.8%. Non-FS to FS/non-FS cell synapses also showed carbachol-induced uIPSC facilitation by 29.2% in about half of the pairs, whereas nearly 40% of pairs showed carbachol-induced suppression of uIPSCs by 40.3%. Carbachol tended to increase uIPSC amplitude in interneuron-to-interneuron synapses with higher PPR, suggesting that carbachol facilitates GABA release in interneuron synapses with lower release probability. These results suggest that carbachol-induced effects on uIPSCs are not homogeneous but preiotropic: i.e., cholinergic modulation of GABAergic synaptic transmission is differentially regulated depending on postsynaptic neuron subtypes.

Nitric Oxide Modulation of GABAergic Synaptic Transmission in Mechanically Isolated Rat Auditory Cortical Neurons

The auditory cortex (A1) encodes the acquired significance of sound for the perception and interpretation of sound. Nitric oxide (NO) is a gas molecule with free radical properties that functions as a transmitter molecule and can alter neural activity without direct synaptic connections. We used whole-cell recordings under voltage clamp to investigate the effect of NO on spontaneous GABAergic synaptic transmission in mechanically isolated rat auditory cortical neurons preserving functional presynaptic nerve terminals. GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) in the A1 were completely blocked by bicuculline. The NO donor, S-nitroso-N-acetylpenicillamine (SNAP), reduced the GABAergic sIPSC frequency without affecting the mean current amplitude. The SNAP-induced inhibition of sIPSC frequency was mimicked by 8-bromoguanosine cyclic 3',5'-monophosphate, a membrane permeable cyclic-GMP analogue, and blocked by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, a specific NO scavenger. Blockade of presynaptic K(+) channels by 4-aminopyridine, a K(+) channel blocker, increased the frequencies of GABAergic sIPSCs, but did not affect the inhibitory effects of SNAP. However, blocking of presynaptic Ca(2+) channels by Cd(2+), a general voltage-dependent Ca(2+) channel blocker, decreased the frequencies of GABAergic sIPSCs, and blocked SNAP-induced reduction of sIPSC frequency. These findings suggest that NO inhibits spontaneous GABA release by activation of cGMP-dependent signaling and inhibition of presynaptic Ca(2+) channels in the presynaptic nerve terminals of A1 neurons.

Compartment-Specific Modulation of GABAergic Synaptic Transmission by μ-Opioid Receptor in the Mouse Striatum with Green Fluorescent Protein-Expressing Dopamine Islands

The striatum is a heterogeneous mosaic of two neurochemically, developmentally, and functionally distinct compartments: the mu-opioid receptor (MOR)-enriched striosomes and the matrix. Preferential activation of the striosomes and persistent suppression of the matrix have recently been suggested to represent neural correlates of motor stereotypy. However, little is known concerning the physiological properties of the striosomes. We made patch-clamp recordings from medium spiny neurons in identified MOR-immunoreactive "dopamine islands" as striosomes in a slice preparation taken from transgenic mice expressing green fluorescent protein in tyrosine hydroxylase mRNA-containing neurons. Striosomal neurons differed electrophysiologically from cells in the matrix in having significantly less hyperpolarized resting membrane potentials and larger input resistances, suggesting developmental differences between the two types of cells. Moreover, corticostriatal EPSCs were inhibited by MOR activation to similar extents in the two compartments, although inhibition of IPSCs was observed only in the striosomes. This MOR-induced inhibition of IPSCs was presynaptically mediated, because MOR agonist invariably decreased IPSC amplitudes when postsynaptic G-protein was inactivated, significantly increased the paired-pulse ratio of the IPSCs, and decreased the frequency but not the amplitude of miniature IPSCs. These effects of MOR were mediated principally by 4-aminopyridine-sensitive K+ conductance via a cAMP-dependent pathway, which was further augmented by previous blockade of the protein kinase C cascade. These findings suggest that MOR activation by endogenous and/or exogenous MOR-selective opioid substances differentially regulates the activities of the striosome and matrix compartments and thus plays an important role in motivated behavior and learning.

Serotoninergic modulation of GABAergic synaptic transmission in developing rat CA3 pyramidal neurons

Serotoninergic modulation of GABAergic mIPSCs was investigated in immature (postnatal 12-16-days old) rat CA3 pyramidal neurons using a conventional whole-cell patch clamp technique. Serotonin or 5-hydroxytryptamine (5-HT) (10 micromol/L) transiently and explosively increased mIPSC frequency with a small increase in the current amplitude. However, 5-HT did not affect the GABA-induced postsynaptic currents, indicating that 5-HT acts presynaptically to facilitate the probability of spontaneous GABA release. The 5-HT action on GABAergic mIPSC frequency was completely blocked by 100 nmol/L MDL72222, a selective 5-HT(3) receptor antagonist, and mimicked by mCPBG, a selective 5-HT(3) receptor agonist. The 5-HT action on GABAergic mIPSC frequency was completely occluded either in the presence of 200 mumol/L Cd2+ or in the Na+-free external solution, suggesting that the 5-HT(3) receptor-mediated facilitation of mIPSC frequency requires a Ca2+ influx passing through voltage-dependent Ca2+ channels from the extracellular space, and that presynaptic 5-HT(3) receptors are less permeable to Ca2+. The 5-HT action on mIPSC frequency in the absence or presence of extracellular Na+ gradually increased with postnatal development. Such a developmental change in the 5-HT(3) receptor-mediated facilitation of GABAergic transmission would play important roles in the regulation of excitability as well as development in CA3 pyramidal neurons.

Pitx3 Deficiency in Mice Affects Cholinergic Modulation of GABAergic Synapses in the Nucleus Accumbens

We investigated to what extent Pitx3 deficiency, causing hyperdopaminergic transmission in the nucleus accumbens microcircuitry, may lead to developmental changes. First, spontaneous firing activity of cholinergic interneurons in the nucleus accumbens was recorded in vitro. Firing patterns in the Pitx3-deficient mice were more variable and intrinsically different from those observed in wild-type mice. Next, to test whether the irregular firing patterns observed in mutant mice affected the endogenous nicotinic modulation of the GABAergic input of medium spiny neurons, we recorded spontaneous GABAergic inputs to these cells before and after the application of the nicotinic receptor blocker mecamylamine. Effects of mecamylamine were found in slices of either genotype, but in a rather inconsistent manner. Possibly this was attributable to heterogeneity in firing of nearby cholinergic interneurons. Thus paired recordings of cholinergic interneurons and medium spiny neurons were performed to more precisely control the experimental conditions of the cholinergic modulation of GABAergic synaptic transmission. We found that controlling action potential firing in cholinergic neurons leads to a conditional increase in GABAergic input frequency in wild-type mice but not in Pitx3-deficient mice. We conclude that Pitx3-deficient mice have neural adaptations at the level of the nucleus accumbens microcircuitry that in turn may have behavioral consequences. It is discussed to what extent dopamine release in the nucleus accumbens may be a long-term gating mechanism leading to alterations in cholinergic transmission in the nucleus accumbens, in line with previously reported neural adaptations found as consequences of repeated drug treatment in rodents.

Modulation of GABAergic synaptic transmission by terminal nicotinic acetylcholine receptors in the central autonomic nucleus of the neonatal rat spinal cord

Using patch clamp recordings from an in vitro spinal cord slice preparation of neonatal rats (9–15 days old), we characterized the GABAergic synaptic transmission in sympathetic preganglionic neurones (SPN) of the central autonomic nucleus (CA) of lamina X. Local applications of isoguvacine (100 μM), a selective agonist at GABA A receptors, induced in all cells tested a chloride current which was abolished by bicuculline, a competitive antagonist at GABA A receptors. In addition, 25% of the recorded cells displayed spontaneous tetrodotoxin-insensitive and bicuculline-sensitive chloride miniature inhibitory postsynaptic currents (mIPSCs). Acetylcholine (100 μM) increased the frequency of GABAergic mIPSCs without affecting their amplitudes or their kinetic properties indicating a presynaptic site of action. The presynaptic effect of ACh was restricted to GABAergic neurones synapsing onto sympathetic preganglionic neurones. The facilitatory effect of ACh was abolished in the absence of external calcium or in the presence of 100 μM cadmium added to the bath solution. Choline 10 mM, an agonist at α7 nicotinic acetylcholine receptors (nAChRs) or muscarine (10 μM), a muscarinic receptor agonist, did not reproduce the presynaptic effect of ACh. The presynaptic effect of ACh was blocked by 1 μM of dihydro-β-erythroidine (DHβE), an antagonist of non-α7 nAChRs but was insensitive to α7 nAChRs antagonists (strychnine, α-bungarotoxin and methyllycaconitine) or to the muscarinic receptor antagonist atropine (10 μM). It was concluded that SPNs of the central autonomic nucleus displayed a functional GABAergic transmission which is facilitated by terminal non α7 nAChRs.

Kainate receptor activation potentiates GABAergic synaptic transmission in the nucleus accumbens core

Inhibitory synaptic transmission plays an important role in regulating the activity of medium spiny neurons (MSNs) in the nucleus accumbens (NAcc). The kainate (KA) subtype of ionotropic glutamate receptor has been shown to potently modulate GABAergic synaptic transmission in several brain regions. Although KA receptor subunits are expressed in the NAcc, KA receptor modulation of GABAergic synaptic transmission in this brain region has not been previously examined. In the current study, we sought to determine if KA receptor activation could alter inhibitory synaptic transmission in the NAcc as it has been shown to do in other brain regions. Using the whole cell patch-clamp technique, we demonstrate that KA receptor activation potentiates evoked GABAergic synaptic transmission and increases the frequency of spontaneous, but not miniature, GABA A-receptor-mediated IPSCs in the NAcc. In contrast, KA has no effect on currents evoked by exogenous application of GABA onto MSNs. Taken together, these data suggest that activation of KA receptors in the NAcc core potently facilitates action-potential-dependent GABAergic synaptic transmission, likely via an excitation of presynaptic GABAergic interneurons.

Nicotinic modulation of GABAergic synaptic transmission in the spinal cord dorsal horn

While the mechanisms underlying nicotinic acetylcholine receptor (nAChR)-mediated analgesia remain unresolved, one process that is almost certainly involved is the recently-described nicotinic enhancement of inhibitory synaptic transmission in the spinal cord dorsal horn. Despite these observations, the prototypical nicotinic analgesic (epibatidine) has not yet been shown to modulate inhibitory transmission in the spinal cord. Furthermore, while nAChRs have been implicated in short-term modulation, no studies have investigated the role of nAChRs in the modulation of long-term synaptic plasticity of inhibitory transmission in dorsal horn. Whole-cell patch clamp recordings from dorsal horn neurons of neonatal rat spinal cord slices were therefore conducted to investigate the short- and long-term effects of nicotinic agonists on GABAergic transmission. GABAergic synaptic transmission was enhanced in 86% of neurons during applications of 1 μM nicotine (mean increased spontaneous GABAergic inhibitory postsynaptic current (sIPSC) frequency was ∼500% of baseline). Epibatidine (100 nM) induced an increase to an average of ∼3000% of baseline, and this effect was concentration dependent (EC 50 = 43 nM). Nicotinic enhancement was inhibited by mecamylamine and DHβE, suggesting an important role for non-α7 nAChRs. Tetrodotoxin (TTX) did not alter the prevalence or magnitude of the effect of nicotine, but the responses had a shorter duration. Nicotine did not alter evoked GABAergic IPSC amplitude, yet the long-term depression (LTD) induced by strong stimulation of inhibitory inputs was reduced when paired with nicotine. These results provide support for a mechanism of nicotinic analgesia dependent on both short and long-term modulation of GABAergic synaptic transmission in the spinal cord dorsal horn.

Long‐lasting nicotinic modulation of GABAergic synaptic transmission in the rat nucleus accumbens associated with behavioural sensitization to amphetamine

A robust increase in dopaminergic transmission in the nucleus accumbens (NAc) shell has been reported to be consistently associated with the long-term expression of behavioural sensitization to drugs of abuse. However, little is known about how this affects the neuronal network of the NAc. We made cellular recordings in NAc slices of saline- and amphetamine-pretreated adult rats and found that expression of behavioural sensitization was associated with long-lasting changes in the basal firing pattern of cholinergic interneurons up to 3 weeks after the last drug injection. Consequently, upon amphetamine sensitization, an inhibiting effect of the nicotinic receptor blocker mecamylamine on the amplitudes of spontaneous GABAergic synaptic currents as well as on the failure rate of electrically evoked GABAergic currents was found that was not present under control conditions. Thus, behavioural sensitization to amphetamine is associated with an up-regulation of the endogenous activation of nicotinic receptors that, in turn, stimulate the GABAergic synaptic transmission within the NAc shell. This is a new mechanism by which drugs of abuse may induce alterations in the processing and integration of NAc inputs involved in psychomotor sensitization.

Progress in pursuit of therapeutic A2A antagonists: the adenosine A2A receptor selective antagonist KW6002: research and development toward a novel nondopaminergic therapy for Parkinson's disease.

Research and development of the adenosine A2A receptor selective antagonist KW6002 have focused on developing a novel nondopaminergic therapy for Parkinson's disease (PD). Salient pharmacologic features of KW6002 were investigated in several animal models of PD. In rodent and primate models, KW6002 provides symptomatic relief from parkinsonian motor deficits without provoking dyskinesia or exacerbating existing dyskinesias. The major target neurons of the A2A receptor antagonist were identified as GABAergic striatopallidal medium spiny neurons. A possible mechanism of A2A receptor antagonist action in PD has been proposed based on the involvement of striatal and pallidal presynaptic A2A receptors in the "dual" modulation of GABAergic synaptic transmission. Experiments with dopamine D2 receptor knockout mice showed that A2A receptors can function and anti-PD activities of A2A antagonists can occur independent of the dopaminergic system. Clinical studies of KW6002 in patients with advanced PD with L-dopa-related motor complications yielded promising results with regard to motor symptom relief without motor side effects. The development of KW6002 represents the first time that a concept gleaned from A2A biologic research has been applied successfully to "proof of concept" clinical studies. The selective A2A antagonist should provide a novel nondopaminergic approach to PD therapy.