Analysis of the function of GABA(B) receptors on inhibitory afferent neurons of Purkinje cells in the cerebellar cortex of the rat

Presynaptic Adrenoceptors

Presynaptic α2-adrenoceptors are localized on axon terminals of many noradrenergic and non-noradrenergic neurons in the peripheral and central nervous systems. Their activation by exogenous agonists leads to inhibition of the exocytotic release of noradrenaline and other transmitters from the neurons. Most often, the α2A-receptor subtype is involved in this inhibition. The chain of molecular events between receptor occupation and inhibition of the exocytotic release of transmitters has been determined. Physiologically released endogenous noradrenaline elicits retrograde autoinhibition of its own release. Some clonidine-like α2-receptor agonists have been used to treat hypertension. Dexmedetomidine is used for prolonged sedation in the intensive care; It also has a strong analgesic effect. The α2-receptor antagonist mirtazapine increases the noradrenaline concentration in the synaptic cleft by interrupting physiological autoinhibion of release. It belongs to the most effective antidepressive drugs. β2-Adrenoceptors are also localized on axon terminals in the peripheral and central nervous systems. Their activation leads to enhanced transmitter release, however, they are not activated by endogenous adrenaline.

Purine receptor-mediated endocannabinoid production and retrograde synaptic signalling in the cerebellar cortex

BACKGROUND AND PURPOSE

Presynaptic CB₁ cannabinoid receptors can be activated by endogenous cannabinoids (endocannabinoids) synthesized by postsynaptic neurones. The hypothesis of the present work was that activation of calcium-permeable transmitter-gated ion channels in postsynaptic neurones, specifically of P2X purine receptors, can lead to endocannabinoid production and retrograde synaptic signalling.

EXPERIMENTAL APPROACH

GABAergic inhibitory postsynaptic currents (IPSCs) were recorded with patch-clamp techniques in Purkinje cells in mouse cerebellar slices. Purine receptors on Purkinje cells were activated by pressure ejection of ATP from a pipette.

KEY RESULTS

ATP evoked an inward current in Purkinje cells, most likely due to P2X receptor activation. The ATP-evoked currents were accompanied by currents via voltage-gated calcium channels. ATP suppressed electrical stimulation-evoked IPSCs and miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin, and these effects were prevented by the CB₁ antagonist rimonabant and the calcium chelator BAPTA (applied into the Purkinje cell). ATP also suppressed mIPSCs when voltage-gated calcium channels were blocked by cadmium, and intracellular calcium stores were depleted by thapsigargin. However, ATP failed to suppress mIPSCs when the extracellular calcium concentration was zero.

CONCLUSIONS AND IMPLICATIONS

ATP elicits CB₁ receptor-dependent retrograde synaptic suppression, which is probably mediated by an endocannabinod released by the postsynaptic neurone. An increase in intracellular calcium concentration in the postsynaptic neurone is necessary for this retrograde signalling. We propose that ATP increases the calcium concentration by two mechanisms: calcium enters into the neurone via the P2X receptor ion channel and the ATP-evoked depolarization triggers voltage-gated calcium channels.

Depolarizing GABAergic synaptic input triggers endocannabinoid-mediated retrograde synaptic signaling

Endocannabinoids released by postsynaptic neurons inhibit neurotransmitter release from presynaptic axon terminals. One typical stimulus of endocannabinoid production is an increase of calcium concentration in postsynaptic neurons. The aim of the present study was to clarify whether depolarizing GABAergic synaptic input, by increasing calcium concentration in postsynaptic neurons, can trigger endocannabinoid production. Spontaneous GABAergic inhibitory postsynaptic currents (sIPSCs) were recorded in Purkinje cells in mouse cerebellar slices with patch-clamp pipettes containing 151 mM chloride (a usual recording mode). sIPSCs were depolarizing inward currents under this condition. Combined electrophysiological and fluorometric calcium imaging experiments indicated that sIPSCs frequently triggered calcium spikes. After the calcium spikes, a short-term suppression of sIPSCs occurred. This suppression was prevented by the CB(1) cannabinoid receptor antagonist rimonabant and the diacylglycerol lipase inhibitor orlistat, but not changed by URB597, an inhibitor of anandamide degradation. It is, therefore, likely that CB(1) receptors and 2-arachidonoylglycerol were involved. For testing the physiological significance of the above observation, we carried out experiments on brains of 3- to 5-day-old mice. The gramicidin-induced perforated patch-clamp mode was used for preserving the physiological intracellular chloride concentration of the neurons. Depolarizing GABAergic sIPSCs occurred under this condition, but at a very low rate. Rimonabant did not change the frequency of these sIPSCs, arguing against the persistence of an endocannabinoid tone. The results point to a new kind of trigger of endocannabinoid production: depolarizing GABAergic synaptic input can elicit endocannabinoid production in postsynaptic neurons by activating calcium channels. The produced endocannabinoid suppresses GABA release from presynaptic axon terminals.

GABA release from cerebellar stellate cells is developmentally regulated by presynaptic GABA(B) receptors in a target-cell-specific manner

Transmitter release from boutons along a common axon is often regulated depending on the postsynaptic target. Here, GABA release from cerebellar stellate cells onto Purkinje cells and other stellate cells was examined in acute cerebellar slices of 2- and 4-week-old mice. Consistent with previous findings on action potential-dependent GABA release, we found a developmental decrease in inhibitory inputs onto Purkinje cells but not onto stellate cells when recording miniature inhibitory postsynaptic currents (mIPSCs). Although amplitudes of mIPSCs were developmentally reduced in both cell types, mIPSC frequencies were decreased in Purkinje cells but were increased in stellate cells. Similarly, modulation of GABA release by presynaptic GABA(B) receptors changed during development in Purkinje cells but not in stellate cells, as demonstrated by the baclofen-mediated depression of mIPSC frequency and evoked IPSC (eIPSC) amplitudes. The selectively diminished baclofen effect in 4-week-old Purkinje cells correlated with a selective downregulation of presynaptic GABA(B) receptors at stellate cell-to-Purkinje cell synapses observed by immunoelectron microscopy analysis. Thus, expression of GABA(B) receptors in stellate cell axons and presynaptic modulation of GABA release appear to change during development in a target-cell-specific manner.

Ethanol enhances both action potential-dependent and action potential-independent GABAergic transmission onto cerebellar Purkinje cells

Ethanol (EtOH) modulates synaptic efficacy in various brain areas, including the cerebellum, which plays a role in motor coordination. Previous studies have shown that EtOH enhances tonic inhibition of cerebellar granule cells, which is one of the possible reasons for the alcohol-induced motor impairment. However, the effects of EtOH on molecular layer interneurons (MLIs) in the mouse cerebellum have remained unknown. Here we found that MLIs were depolarized by EtOH through enhancement of hyperpolarization-activated cationic currents (I(h)). Under physiological conditions, a low EtOH concentration (3-50 mM) caused a small increase in the firing rate of MLIs, whereas, in the presence of blockers for ionotropic glutamate and GABA receptors, EtOH (>or=10 mM) robustly enhanced MLI firing, suggesting that synaptic inputs, which seem to serve as the phasic inhibition, could suppress the EtOH-mediated excitation of MLIs and Purkinje cells (PCs). Even in the absence of synaptic blockers, a high EtOH concentration (100 mM) markedly increased the firing rate of MLIs to enhance GABAergic transmission. Furthermore, 100 mM EtOH-facilitated miniature IPSCs via a mechanism that depended on intracellular cyclic AMP, voltage-dependent Ca(2+) channels, and intracellular Ca(2+) stores, but was independent of I(h) or PKA. The two distinct effects of a high EtOH concentration (>or=100 mM), however, failed to attenuate the EtOH-induced strong depolarization of MLIs. These results suggest that acute exposure to a low EtOH concentration (<or=50 mM) enhanced GABAergic synaptic transmission, which suppressed the EtOH-evoked excitation of MLIs and PCs, thereby maintaining precise synaptic integration of PCs.

Tonic activation of GABAB receptors reduces release probability at inhibitory connections in the cerebellar glomerulus

In the cerebellum, granule cells are inhibited by Golgi cells through GABAergic synapses generating complex responses involving both phasic neurotransmitter release and the establishment of ambient gamma-aminobutyric acid (GABA) levels. Although at this synapse the mechanisms of postsynaptic integration have been clarified to a considerable extent, the mechanisms of neurotransmitter release remained largely unknown. Here we have investigated the quantal properties of release during repetitive neurotransmission, revealing that tonic GABA(B) receptor activation by ambient GABA regulates release probability. Blocking GABA(B) receptors with CGP55845 enhanced the first inhibitory postsynaptic current (IPSC) and short-term depression in a train while reducing trial-to-trial variability and failures. The changes caused by CGP55845 were similar to those caused by increasing extracellular Ca(2+) concentration, in agreement with a presynaptic GABA(B) receptor modulation of release probability. However, the slow tail following IPSC peak demonstrated a remarkable temporal summation and was not modified by CGP55845 or extracellular Ca(2+) increase. This result shows that tonic activation of presynaptic GABA(B) receptors by ambient GABA selectively regulates the onset of inhibition bearing potential consequences for the dynamic regulation of signal transmission through the mossy fiber-granule cell pathway of the cerebellum.

Analysis of the effects of cannabinoids on identified synaptic connections in the caudate-putamen by paired recordings in transgenic mice

CB(1) cannabinoid receptors are expressed in many neurons in the caudate-putamen. However, it is not known how the activation of these receptors influences synaptic transmission between different neuron classes. The aim was to establish a method for studying identified synaptic connections in the caudate-putamen, and to determine the effects of cannabinoids on these connections. Brain slices were prepared from transgenic mice expressing enhanced green fluorescent protein (EGFP) in parvalbumin-positive fast spiking interneurons (PV-FSNs). PV-FSNs were identified based on their fluorescence. Non-fluorescent medium-sized neurons were considered to be medium spiny neurons (MSNs). Synaptic transmission was studied by simultaneous patch-clamp recording from identified neuron pairs. In the case of PV-FSN --> MSN neurotransmission, the synthetic cannabinoid receptor agonist WIN55212-2 lowered the success rate of transmission and the amplitude of successful postsynaptic events. Analysis of miniature inhibitory postsynaptic currents indicated that WIN55212-2 inhibited synaptic transmission presynaptically. WIN55212-2 did not elicit somatodendritic effects in PV-FSNs: membrane potential, membrane current and evoked firing were not changed. WIN55212-2 also depressed the MSN --> MSN neurotransmission. The inhibitory synaptic input to MSNs was only weakly suppressed by endocannabinoids released by depolarized postsynaptic MSNs. The results show that the combined use of transgenic animals and paired-recording techniques allows the study of synaptic connections between rare neurons. Using these techniques, we showed that activation of CB(1) receptors on axon terminals of (i) PV-FSNs and (ii) MSNs leads to presynaptic inhibition of GABAergic synaptic transmission between these axons and their postsynaptic targets, the MSNs. The cannabinoids acted preferentially on axon terminals without effects on the somatodendritic region of the neurons.

The role of secretin in the cerebellum

There is increasing interest in the role played by secretin in the central nervous system. Recent evidence suggests that this peptide is widely expressed in the brain but some areas, notably the cerebellum, show a prominent expression of the peptide and its specific receptor. In this article we summarize our current understanding of the expression pattern and action of secretin in the cerebellum. We discuss the findings supporting the endogenous release of this peptide from Purkinje neurons and its role as a retrograde messenger modulating GABAergic synaptic transmission via multiple mechanisms. In addition, we would like to propose other possible, but still to be confirmed, functions of secretin in the cerebellum..

Miniature synaptic events elicited by presynaptic Ca2+ rise are selectively suppressed by cannabinoid receptor activation in cerebellar Purkinje cells

Activation of cannabinoid receptors suppresses neurotransmitter release in various brain regions. In cerebellar Purkinje cells (PCs), cannabinoid agonists suppress both EPSC and IPSC evoked by stimulating the corresponding inputs. However, cannabinoid agonists suppress miniature IPSC (mIPSC) but not miniature EPSC (mEPSC) at normal external Ca2+ concentration ([Ca2+]o). Therefore, cannabinoid agonists are thought to suppress release machinery for IPSCs but not that for EPSCs. Here we investigated the possible cause of this difference and found that cannabinoid agonists selectively suppressed Ca2+-enhanced miniature events. A cannabinoid agonist, WIN55,212-2 (5 microM), did not affect mEPSC frequency with 2 mM extracellular Ca2+ (Ca2+(o)). However, WIN55,212-2 became effective when mEPSC frequency was enhanced by elevation of presynaptic Ca2+ level by perfusion with 5 mM Ca2+(o) or bath application of A23187, a Ca2+ ionophore. In contrast, WIN55,212-2 suppressed mIPSC frequency with 2 mM Ca2+(o), but it became ineffective when the presynaptic Ca2+ level was lowered by perfusion with a Ca2+-free solution containing BAPTA-AM. Experiments with systematic [Ca2+]o changes revealed that mIPSC but not mEPSC regularly involved events elicited by presynaptic Ca2+ rise with 2 mM Ca2+(o). Importantly, Ca2+-enhancement of mEPSC and mIPSC was not attributable to activation of voltage-dependent Ca2+ channels. Activation of GABAB receptor or group III metabotropic glutamate receptor, which couple to G(i/o)-protein, also preferentially suppressed Ca2+-enhanced miniature events in PCs. These results suggest that the occurrence of Ca2+-enhanced miniature events at normal [Ca2+]o determines the sensitivity to the presynaptic depression mediated by cannabinoid receptors and other G(i/o)-coupled receptors in PCs.

Simultaneous NMDA-dependent long-term potentiation of EPSCs and long-term depression of IPSCs in cultured rat hippocampal neurons

A fundamental issue in understanding activity-dependent long-term plasticity of neuronal networks is the interplay between excitatory and inhibitory synaptic drives in the network. Using dual whole-cell recordings in cultured hippocampal neurons, we examined synaptic changes occurring as a result of a transient activation of NMDA receptors in the network. This enhanced transient activation led to a long-lasting increase in synchrony of spontaneous activity of neurons in the network. Simultaneous long-term potentiation of excitatory synaptic strength and a pronounced long-term depression of inhibitory synaptic currents (LTDi) were produced, which were independent of changes in postsynaptic potential and Ca2+ concentrations. Surprisingly, miniature inhibitory synaptic currents were not changed by the conditioning, whereas both frequency and amplitudes of miniature EPSCs were enhanced. LTDi was mediated by activation of a presynaptic GABAB receptor, because it was blocked by saclofen and CGP55845 [(2S)-3-{[(15)-1-(3, 4-dichlorophenyl)ethyl]amino-2-hydroxypropyl)(phenylmethyl)phosphinic acid]. The cAMP antagonist Rp-adenosine 3 ', 5 ' -cyclic monophosphothioate abolished all measured effects of NMDA-dependent conditioning, whereas a nitric oxide synthase inhibitor was ineffective. Finally, network-induced plasticity was not occluded by a previous spike-timing-induced plasticity, indicating that the two types of plasticity may not share the same mechanism. These results demonstrate that network plasticity involves opposite affects on inhibitory and excitatory neurotransmission.

GABAB receptors in the medial septum/diagonal band slice from 16-25 day rat

GABA(B) receptors are believed to play a role in rhythmic activity in the mammalian brain. The aim of our study was to examine the presynaptic and postsynaptic locations of these receptors in the medial septal diagonal band area (MS/DB), an area known to pace the hippocampus theta rhythm. Whole-cell patch recordings were made from parasagittal MS/DB slices obtained from the 16-25 day rat. Neurons were classified into GABAergic and cholinergic subtypes according to previous electrophysiological criteria. Bath application of the GABA(B) receptor agonist baclofen in the presence of tetrodotoxin, and brief tetanic fiber stimulation in the presence of ionotropic receptor antagonists, provided evidence for the presence of postsynaptic GABA(B) receptor transmission to GABAergic but not cholinergic neurons. Bath application of baclofen, at concentrations too low to elicit postsynaptic activity in MS/DB neurons, significantly reduced the amplitudes of stimulus-evoked ionotropic receptor inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs) and the paired pulse depression of these evoked potentials. Baclofen also significantly reduced the frequencies but not the amplitudes of miniature inhibitory postsynaptic currents (IPSCs) and excitatory postsynaptic currents (EPSCs), indicating the presence of presynaptic GABA(B) receptors on GABAergic and glutamatergic terminals in the MS/DB. Baclofen, also at a concentration too low to elicit postsynaptic activity, reduced the frequencies and amplitudes of spontaneous IPSCs and EPSCs recorded in the presence of 200-400 nM kainate. Rhythmic compound IPSCs at theta frequencies were recorded under these conditions in some neurons, and these rhythmic compound IPSCs were disrupted by the activation but not by the inhibition of GABA(B) receptors. These results suggest that GABA(B) receptors modulate rather than generate rhythmic activity in the MS/DB, and that this modulatory effect occurs via receptors located on presynaptic terminals.

Cannabinoids depress excitatory neurotransmission between the subthalamic nucleus and the globus pallidus

The globus pallidus receives its major glutamatergic input from the subthalamic nucleus and subthalamic nucleus neurons synthesize CB1 cannabinoid receptors. The hypothesis of the present work was that CB1 receptors are localized in terminals of subthalamo-pallidal glutamatergic axons and that their activation leads to presynaptic modulation of neurotransmission between these axons and globus pallidus neurons. Patch-clamp studies were carried out on oblique-sagittal mouse brain slices. The subthalamic nucleus was stimulated electrically and the resulting excitatory postsynaptic currents (EPSCs) were recorded in globus pallidus neurons. The mixed CB1/CB2 receptor agonist R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone mesylate (WIN55212-2; 3 x 10(-7) M) had no effect on EPSCs. WIN55212-2 (10(-5) M) decreased the amplitude of EPSCs by 44+/-8%. The inhibition by WIN55212-2 (10(-5) M) was prevented by the CB1 antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazolecarboxamide (10(-6) M). WIN55212-2 (10(-5) M) did not change the amplitude of spontaneous EPSCs (sEPSCs) recorded in globus pallidus neurons but lowered their frequency. Moreover, WIN55212-2 (10(-5) M) had no effect on currents elicited by direct activation of postsynaptic receptors on globus pallidus neurons by glutamate (10(-3) M) ejected from a pipette. In a final series of experiments, the firing of subthalamic nucleus neurons was recorded; WIN55212-2 (10(-5) M) did not change the firing of these neurons. The results show that activation of CB1 receptors inhibits glutamatergic neurotransmission between the subthalamic nucleus and the globus pallidus. Lack of effect of cannabinoids on the amplitude of sEPSCs and on currents evoked by direct stimulation of postsynaptic glutamate receptors indicates that the mechanism is presynaptic inhibition of glutamate release from axon terminals. Cannabinoids seem to act preferentially presynaptically: in contrast to their action on axon terminals, they have no effect on somadendritic receptors regulating firing rate. Cannabinoids elicit catalepsy in vivo. The observed inhibition of glutamatergic neurotransmission in the globus pallidus would favor catalepsy.

Binge ethanol exposure delays development of GABAergic miniature postsynaptic currents in septal neurons

Whole cell GABA(A)R currents of septal neurons isolated from rat pups increase rapidly during the first weeks of life when inhibitory synapses are forming. Early postnatal binge ethanol intubation on days 4-9 delays this maturational up-regulation in septal neurons isolated several days later suggesting inhibitory synapse formation could be disrupted [S.-H. Hsiao, J.L. Acevedo, D.W. DuBois, K.R. Smith, J.R. West, G.D. Frye, Early postnatal ethanol intubation blunts GABA(A) receptor upregulation and modifies 3alpha-hydroxy-5alpha-pregnan-20-one sensitivity in rat MS/DB neurons, Brain Res. Dev. Brain Res. 130 (2001) 25-40]. Surprisingly, whole cell GABA(A)R function does not increase rapidly when septal neurons are grown for the same period in vitro and is not blunted by comparable ethanol exposure of the cultures [S.-H. Hsiao, D.W. DuBois, R.C. Miranda, G.D. Frye, Critically timed ethanol exposure reduces GABA(A)R function on septal neurons developing in vivo but not in vitro, Brain Res Dev. Brain Res. 1008 (2004) 69-80]. Because GABAergic miniature postsynaptic currents (mPSCs) show parallel patterns of maturation whether cortical neurons are growing in vivo or in vitro [D.D. Dunning, C.L. Hoover, I. Soltesz, M.A. Smith, D.K. ODowd, GABA(A) receptor-mediated miniature postsynaptic currents and alpha-subunit expression in developing cortical neurons, J. Neurophysiol. 82 (1999) 3286-3297], we examined the impact of binge ethanol exposure on synaptic receptors activated by these currents in septal cultures. Binge ethanol treatment of embryonic septal neurons over 6-11 days in vitro (DIV) slightly reduced GABA(A)R-mediated mPSC amplitude and frequency, but also substantially slowed decay kinetics when mPSCs were recorded later on DIV 13-18. Decreased frequency and slowed mPSC decay kinetics after ethanol were consistent with parameters measured in immature neurons. Untreated septal neurons exhibited decreased mPSC amplitude and frequency with acute 30-100 mM ethanol, without changing decay kinetics suggesting a direct inhibition of postsynaptic receptors. Sustained inhibition of GABA(A)Rs with 100 microM picrotoxin on DIV 6-11 decreased mPSC amplitude and frequency and slowed decay kinetics similar to binge ethanol exposure. These results suggest that binge ethanol exposure delays mPSC maturation by interfering with trophic postsynaptic GABA(A)R signaling during the early development of septal neurons.

Mechanism of GABAB receptor-mediated inhibition of spontaneous GABA release onto cerebellar Purkinje cells

gamma-Aminobutyric acid (GABA)(B) receptor-mediated modulation of spontaneous GABA release onto Purkinje cells was investigated in cerebellar slices from 3- to 5-week-old mice. The GABA(B) receptor agonists baclofen and CGP 44533 each reduced the frequency of miniature inhibitory postsynaptic currents (mIPSCs), with no significant effect on mIPSC amplitude; together, consistent with a presynaptic site of action. The GABA(B) receptor antagonist CGP 55845 blocked baclofen-induced inhibition. The sulphydryl alkylating agent N-ethylmaleimide occluded baclofen effects, implicating G(i/o) subunits in mediating a GABA(B) G protein-coupled receptor pathway. Baclofen-induced inhibition persisted in the presence of Ba(2+), a blocker of K(+) channels, and Cd(2+), a blocker of Ca(2+) channel-mediated GABA release. Application of nominally Ca(2+)-free extracellular solutions reduced mIPSC frequency and amplitude; however, baclofen produced a significant inhibition in mIPSC frequency, further suggesting that this pathway was independent of Ca(2+) influx. Spontaneous GABA release was increased by the adenylate cyclase activator, forskolin, and the phorbol ester, phorbol 12,13-dibutyrate. However, baclofen-induced inhibition was not significantly changed in either condition. Baclofen action was also not affected by the adenylate cyclase inhibitor SQ 22536 or the protein kinase C inhibitor chelerythrine chloride. Baclofen still reduced mIPSC frequency in the presence of the polyvalent cation ruthenium red, which acts as a secretagogue here; however, baclofen-induced inhibition was reduced significantly. Furthermore, baclofen produced no clear inhibition during high-frequency mIPSCs bursts induced by the potent secretagogue alpha-Latrotoxin. Together, these results suggest that GABA(B) inhibition occurs downstream of Ca(2+) influx and may be mediated, in part, by an inhibition of the vesicular release mechanism.

Analysis of the Effects of Cannabinoids on Synaptic Transmission between Basket and Purkinje Cells in the Cerebellar Cortex of the Rat

The hypothesis of the present work was that activation of CB1 cannabinoid receptors inhibits GABAergic neurotransmission between basket and Purkinje cells in the cerebellar cortex. The aim was to test this hypothesis under near-physiological conditions. Action potentials of basket cells and spontaneous inhibitory postsynaptic currents (sIPSCs) in synaptically coupled Purkinje cells were recorded simultaneously in rat brain slices. The cannabinoid agonists (R)-(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl) methyl] pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate (WIN 55212-2) and (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)-phenyl]-trans-4-(3-hydroxy-propyl)-cyclohexanol (CP55940) decreased the amplitude of sIPSCs occurring simultaneously with basket cell action potentials and lowered the success rate of synaptic transmission. These effects were prevented by the CB1 receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichloro-phenyl)-4-methyl-3-pyrazole-carboxamide (SR141716). Depolarization of Purkinje cells also led to suppression of neurotransmission; prevention of this suppression by CP55940 and SR141716 indicates that endocannabinoids released from Purkinje cells were involved. WIN 55212-2 lowered the amplitude of autoreceptor currents recorded in basket cells (autoreceptor currents are due to the action of GABA released from axon terminals on GABAA autoreceptors of the same axon terminals); this is novel proof of the presynaptic action of cannabinoids. Autoreceptor current experiments also indicated that endogenous cannabinoids are not released by basket cell axon terminals. A presynaptic action is additionally supported by the observation that WIN 55212-2 lowered the frequency of miniature IPSCs recorded in the presence of tetrodotoxin and the calcium ionophore ionomycin. In conclusion, activation of CB1 receptors by exogenous cannabinoids and by endogenous cannabinoids released by Purkinje cells presynaptically inhibits GABAergic neurotransmission between basket and Purkinje cells. This was demonstrated under near-physiological conditions: transmitter release was elicited by action potentials generated by spontaneously firing intact presynaptic neurons.

Serotonin drives a novel GABAergic synaptic current recorded in rat cerebellar purkinje cells: a Lugaro cell to Purkinje cell synapse

We recorded a novel fast GABAergic synaptic current in cerebellar Purkinje cells in rat brain slices using patch-clamp techniques. Because of a relatively low sensitivity to bicuculline, these currents can be recorded under conditions in which basket and stellate cell inputs are blocked. The observations that the novel synaptic currents occur spontaneously only in the presence of serotonin, and the specific limited positions in the slice from which they can be electrically evoked, suggest that the presynaptic cell is the Lugaro cell. Cell-attached recordings confirm that the Lugaro cell is the only interneuron in the cerebellar cortex with firing behavior consistent with the spontaneous activity recorded in Purkinje cells. The input shows a strong presynaptic modulation mediated by GABA(A) receptors, resulting in a dynamic range from almost 0 to >90% release probability. Modeling GABA(A) receptor responses to different GABA transients suggests that the relatively low sensitivity of the synaptic currents to bicuculline, compared with the higher affinity GABA(A) receptor antagonist SR-95531 (2-(3-carboxypropyl)-3-amino-6-(4-methoxyphenyl) pyridazinium), is attributable to an unusually long GABA dwell time and/or high GABA concentration in the synaptic cleft. The significance of this novel input is discussed in relation to other GABAergic synapses impinging on Purkinje cells. We suggest that the release of GABA onto Purkinje cells from Lugaro cells would primarily occur during motor activity under conditions in which the activity of basket and stellate cells might be inhibited.