Bisquaternary Pyridinium Oximes as Presynaptic Agonists and Postsynaptic Antagonists of Muscarinic Receptors

A study of the effects of bisquaternary pyridinium oximes on calcium-dependent potassium-evoked [3H]acetylcholine release from rat brain slices revealed that at presynaptic autoreceptors these drugs function like muscarinic agonists, as they mimic the effects of acetylcholine in their inhibition of the evoked [3H]-acetylcholine release in an atropine-sensitive and dose-dependent manner. Since the bisquaternary pyridinium oximes are mild muscarinic antagonists at postsynaptic muscarinic receptors, they constitute a category of muscarinic ligands that are characterized by inverse dual activity at pre- and postsynaptic muscarinic receptors. These drugs may have dual function on cholinergic transmission by acting as presynaptic agonists and as postsynaptic antagonists. The most potent inhibitor of the evoked [3H]acetylcholine release was 1,1'-(4-hydroxyiminopyridinium)trimethylene (TMB-4) (I50 = 8 microM) and the weakest were 1-(2-hydroxyiminoethylpyridinium) 1-(3-cyclohexylcarboxypyridinium) dimethylether (HGG-42) and 1-(2-hydroxyiminoethylpyridinium) 1-(3-phenylcarboxypyridinium) dimethylether (HGG-12) (I50 = 150 microM). As postsynaptic antagonists, the latter drugs are more potent (K1 = 1.3-3.3 microM) than TMB-4 (K1 = 50 microM). Combined therapy with two drugs such as TMB-4 and HGG-12 might be effective in blocking severe hyperactivity of the cholinergic system.     

杀虫药剂的神经毒理学研究进展

大多数杀虫药剂都具有较强的神经毒性,它们对神经系统的作用靶标不同。有机磷类杀虫剂不仅抑制乙酰胆碱酯酶活性和乙酰胆碱受体功能,影响乙酰胆碱的释放,而且还具有非胆碱能毒性,有些有机磷杀虫剂还能引发迟发性神经毒性。新烟碱类杀虫剂作为烟碱型乙酰胆碱受体（nAChR）的激动剂,作用于该类受体的α亚基;它对昆虫的毒性比对哺乳动物的毒性大得多,乃是因为它对昆虫和哺乳动物nAChR的作用位点不同。拟除虫菊酯类杀虫剂主要作用于神经细胞钠通道,引起持续开放,导致传导阻滞;该类杀虫剂也可抑制钙通道。另外,这类杀虫剂还干扰谷氨酸递质和多巴胺神经元递质的释放。拟除虫菊酯类杀虫剂对昆虫的选择毒性很可能是因为昆虫神经元的钠通道结构与哺乳动物的不同。阿维菌素类杀虫剂主要作用于γ-氨基丁酸（GABA）受体,它能促进GABA的释放,增强GABA与GABA受体的结合,使氯离子内流增加,导致突触后膜超级化。由于这类杀虫剂难以穿透脊椎动物的血脑屏障而与中枢神经系统的GABA受体结合,故该类杀虫剂对脊椎动物的毒性远低于对昆虫的毒性。多杀菌素类杀虫剂可与中枢神经系统的nAChR作用,引起Ach长时间释放,此外,这类杀虫剂还可作用于昆虫的GABA受体,改变GABA门控氯通道的功能。     

The enigma of transmitter-selective receptor accumulation at developing inhibitory synapses

The control of synaptic inhibition is crucial for normal brain function. More than 20 years ago, glycine and gamma-aminobutyric acid (GABA) were shown to be the two major inhibitory neurotransmitters. They can be released independently from different terminals or co-released from the same terminal to activate postsynaptic glycine and GABA(A) receptors. The anchoring protein gephyrin is involved in the postsynaptic accumulation of both glycine and GABA(A) receptors. In lower brain regions, both receptors can be concentrated in synapses, whereas in higher brain regions, glycine receptors are mostly excluded from postsynaptic sites. The activation of glycine and/or GABA(A) receptors determines the strength and precise timing of inhibition. Therefore, tight regulation of postsynaptic glycine versus GABA(A) receptor localization is crucial for optimizing synaptic inhibition in neurons. This review focuses on recent findings and discusses questions concerning the specificity of postsynaptic inhibitory neurotransmitter receptor accumulation during inhibitory synapse formation and development.     

Physiological and pharmacological studies on annelid and nematode body wall muscle

1. This review covers the pharmacology and physiology of the body wall muscle systems of nematodes and annelids.2. Both acetylcholine and gamma-aminobutyric acid (GABA) play important roles in the control of body wall muscle in both phyla. In annelids and nematodes, acetylcholine is the excitatory neuromuscular transmitter while GABA is the inhibitory neuromuscular transmitter. In addition, 5-hydroxytryptamine (5-HT) has a modulatory role at annelid body wall muscle but little if any effect on nematode body wall muscle.3. The acetylcholine receptor of the body wall muscle can be classified as nicotinic-like in both phyla though the annelid receptor has not been analysed in detail. In nematodes, vertebrate ganglionic nicotinic agonists were the most effective of those so far examined while mecamylamine and benzoquinonium were the most effective antagonists. Both neuronal bungarotoxin and neosurugatoxin were potent antagonists of acetylcholine excitation at the nematode receptor.4. The GABA receptor of the body wall muscle exhibits similarities with the vertebrate GABA-A receptor in both phyla. Picrotoxin is a very weak or inactive antagonist at leech and nematode GABA receptors, while bicuculline methiodide blocks leech GABA receptors but is inactive on nematode GABA receptors. Picrotoxin does block GABA responses of earthworm body wall muscle. All these GABA responses are chloride mediated.5. Neuroactive peptides of the RFamide family occur m both phyla and FMRFamide has been identified in leeches. RFamides probably have an important role in heart regulation in leeches and in modulation of their body wall muscles. RFamides also modulate nematode body wall muscle activity with KNEFIRFamide raising muscle tone while SDPNFLRFamide relaxes the muscle. It is likely that this family and other neuroactivc peptides play an important role in the physiology of body wall muscle throughout both phyla.     

Metabotropic Glutamate Receptors Modulate Ischemia-Induced GABA Release in Mouse Hippocampal Slices

The involvement of glutamate receptors in GABA release in ischemia was investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice. For in vitro ischemia, the slices were superfused in glucose-free media under nitrogen. Ionotropic glutamate receptor agonists failed to affect the ischemia-induced basal GABA release at either age. The K(+)-stimulated release in the immature hippocampus was potentiated by N-methyl-D-aspartate receptors, whereas in adults this release was reduced by both kainate and 2-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate receptor activation. The group I metabotropic receptor agonist (1+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate enhanced the basal ischemic GABA release in a receptor-mediated manner in adults, this being concordant with the positive modulation of GABAergic neurotransmission by group I metabotropic glutamate receptors. (1 +/-)-1-Aminocyclopentane-trans-1,3-dicarboxylate and (S)-3,5-dihydroxyphenylglycine also enhanced the K(+)-stimulated release in the developing hippocampus in a receptor-mediated manner. Because group I receptors generally increase neuronal excitability, the enhanced GABA release may attenuate hyperexcitation or strengthen inhibition, being thus neuroprotective, particularly under ischemic conditions. Group III metabotropic glutamate receptors were not at all involved in ischemic GABA release in the immature mice, but in adults their activation by O-phospho-L-serine potentiated the basal release and reduced the K(+)-stimulated release. These opposite effects were abolished by the antagonist (RS)-2-cyclopropyl-4-phosphonophenylglycine. Metabotropic glutamate receptors, namely group I and III receptors, are able to modify the release of GABA from hippocampal slices under ischemic conditions, both positive and negative effects being discernible, depending on the age and type of receptor activated.     

Barium, glibenclamide and CGS21680 prevent adenosine A1 receptor changes of ES coupling and spike threshold

Activation of adenosine A1 receptors raised spike thresholds and induced a dissociation of excitatory postsynaptic potential (EPSP) spike coupling in hippocampal pyramidal neurones. This effect could be prevented by activation of A2A adenosine receptors. The A1 receptor agonist N6-cyclopentyladenosine caused a dissociation of the EPSP spike coupling recorded extracellularly and increased the threshold for spike generation measured intracellularly. These effects were prevented by the A2A receptor agonist CGS21680. A series of agents interfering with adenylate cyclase activity, protein kinase A or C, or nitric oxide synthase had no effect on these responses to N6-cyclopentyladenosine. Superfusion with barium or glibenclamide prevented both the dissociation of EPSP spike coupling and the increase of spike threshold. It is concluded that a barium- and glibenclamide-sensitive potassium current may be involved in the postsynaptic effects of A1 receptors on spike threshold, and it is suggested that a similar suppression of a potassium current by A2A receptors could underlie the inhibition of A1 receptor responses.     

Pretreatment of the cockroach cercal afferent/giant interneuron synapses with nicotinoids and neonicotinoids differently affects acetylcholine and nicotine-induced ganglionic depolarizations

We have recently demonstrated that neonicotinoid insecticides were able to act as agonists of postsynaptic nicotinic acetylcholine receptors (nAChRs) expressed at the synapse between the cercal nerve XI and the giant interneurons, in the sixth abdominal ganglion. In this work, we demonstrated that nicotinoids such as nornicotine acted as an agonist of nicotinic acetylcholine receptors expressed at cercal afferent/giant interneurons while cotinine was a poor agonist. Indeed, nornicotine induced a ganglionic depolarization which was blocked by the nicotinic antagonist mecamylamine. In addition, we found that pretreatment of the sixth abdominal ganglion with 1 and 10 μM nornicotine and cotinine had no significant effect on acetylcholine and nicotine-induced depolarization. But pretreatment with 1 and 10 μM acetamiprid and imidacloprid had a strong effect. 1 and 10 μM acetamiprid completely blocked acetylcholine-induced depolarization, whereas imidacloprid had a partial effect. The present work therefore suggests, in agreement with previous studies, that nornicotine and cotinine bind to distinct cockroach postsynaptic nAChRs, whereas acetamiprid and imidacloprid have competitive effects with acetylcholine and nicotine on ganglionic depolarization.     

A Search for Receptors Modulating the Release of γ-[3H]Aminobutyric Acid in Rabbit Caudate Nucleus Slices

Various putative striatal transmitters and related compounds were studied for their effects on the release of gamma-aminobutyric acid (GABA) from slices of the head of the rabbit caudate nucleus. The slices were preincubated with [3H]GABA and then superfused and stimulated electrically at 5 or 20 Hz. Aminooxyacetic acid was present throughout. The main changes observed were the following. The basal and, less consistently, the electrically evoked overflow of [3H]GABA were enhanced by 3,4-dihydroxyphenylethylamine (dopamine), an effect not blocked by cis-flupentixol or domperidone and not mimicked by apomorphine and D1-selective agonists. The electrically evoked overflow was diminished by 5-hydroxytryptamine (serotonin); the inhibition was prevented by methiothepin. The basal but not the electrically evoked overflow was enhanced by carbachol; acetylcholine and nicotine also accelerated the basal outflow whereas oxotremorine caused no consistent change; the effect of carbachol and acetylcholine were blocked by hexamethonium but not by atropine or by tetrodotoxin. These findings indicate that the GABA neurons in the caudate nucleus may be stimulated by dopamine, although the receptor type involved remains unclear; inhibited by serotonin; and stimulated by acetylcholine acting via a nicotine receptor. However, all drug effects observed were relatively small. No evidence was obtained for autoreceptors, alpha 2-adrenoceptors or receptors for opioids, adenosine or substance P at the GABA neurons.     

Muscarinic excitation in grasshopper song control circuits is limited by acetylcholinesterase activity

The species- and situation-specific sound production of grasshoppers can be stimulated by focal application of both nicotinic and muscarinic receptor agonists into the central body complex of the protocerebrum. Pressure injection of the intrinsic transmitter acetylcholine only elicits fast and short-lived responses related to nicotinic receptor-mediated excitation. Prolonged sound production that includes complex song patterns requires muscarinic receptor-mediated excitation. In addition, basal muscarinic excitation in the central body neuropil seems to determine the general motivation of a grasshopper to stridulate. To demonstrate that endogenous acetylcholinesterase limits the activation of muscarinic receptors by synaptically released acetylcholine in the central body of Chorthippus biguttulus, we investigated both its presence in the brain and effects on sound production resulting from inhibition of esterase activity. Acetylcholinesterase activity was detected in the upper and lower division of the central body. Both these neuropils known to be involved in the cephalic control of stridulation were also shown to contain muscarinic acetylcholine receptors expressed by columnar neurons suggested to serve as output neurons of the central complex. Pressure injection of the acetylcholinesterase inhibitor eserine into protocerebral control circuits of restrained male grasshoppers stimulated long-lasting stridulation that depended on scopolamine-sensitive muscarinic receptors. In restrained males, eserine released the typical response song by potentiating the stimulatory effect of the conspecific female song. Eserine-mediated inhibition of acetylcholinesterase in the central body prolongs the presence of synaptically released acetylcholine at its postsynaptic receptors and increases its potency to activate muscarinic receptor-initiated signaling pathways acting to promote grasshopper sound production.     

GABA maintains the proliferation of progenitors in the developing chick ciliary marginal zone and non-pigmented ciliary epithelium

GABA is more than the main inhibitory neurotransmitter found in the adult CNS. Several studies have shown that GABA regulates the proliferation of progenitor and stem cells. This work examined the effects of the GABA(A) receptor system on the proliferation of retinal progenitors and non-pigmented ciliary epithelial (NPE) cells. qRT-PCR and whole-cell patch-clamp electrophysiology were used to characterize the GABA(A) receptor system. To quantify the effects on proliferation by GABA(A) receptor agonists and antagonists, incorporation of thymidine analogues was used. The results showed that the NPE cells express functional extrasynaptic GABA(A) receptors with tonic properties and that low concentration of GABA is required for a baseline level of proliferation. Antagonists of the GABA(A) receptors decreased the proliferation of dissociated E12 NPE cells. Bicuculline also had effects on progenitor cell proliferation in intact E8 and E12 developing retina. The NPE cells had low levels of the Cl-transporter KCC2 compared to the mature retina, suggesting a depolarising role for the GABA(A) receptors. Treatment with KCl, which is known to depolarise membranes, prevented some of the decreased proliferation caused by inhibition of the GABA(A) receptors. This supported the depolarising role for the GABA(A) receptors. Inhibition of L-type voltage-gated Ca(2+) channels (VGCCs) reduced the proliferation in the same way as inhibition of the GABA(A) receptors. Inhibition of the channels increased the expression of the cyclin-dependent kinase inhibitor p27(KIP1), along with the reduced proliferation. These results are consistent with that when the membrane potential indirectly regulates cell proliferation with hyperpolarisation of the membrane potential resulting in decreased cell division. The increased expression of p27(KIP1) after inhibition of either the GABA(A) receptors or the L-type VGCCs suggests a link between the GABA(A) receptors, membrane potential, and intracellular Ca(2+) in regulating the cell cycle.     

Peripheral receptor populations involved in the regulation of gastrointestinal motility and the pharmacological actions of metoclopramide-like drugs

This minireview is concerned with a re-examination of the locus of action and the possible peripheral mechanisms involved in the gastrointestinal (GI) stimulant effects of metoclopramide. Such a re-evaluation is opportune given the increasing use of this drug in the therapy of certain GI tract disorders. To provide an orientation on this subject the location in the GI tract and function of several relevant receptor types have been reviewed. In the past metoclopramide has been reported to enhance contractions of a variety of GI preparations to electrical stimulation, acetylcholine, carbachol and ganglion stimulants, to inhibit responses to alpha 2-adrenoreceptor agonists and 5-hydroxytryptamine, as well as blocking those to dopamine. Also in such preparations metoclopramide facilitates the release of acetylcholine to transmural stimulation. One important question is whether this effect is mediated via a specific prejunctional receptor. In this respect 2 suggestions have been made. Firstly that the drug may act as a preferential, prejunctional muscarinic antagonist thus inhibiting the negative feedback inhibition of acetylcholine release and secondly that metoclopramide may be a prejunctional agonist (partial) at 5-hydroxy-tryptamine receptors. Although the latter possibility appears most tenable at present, the involvement of a specific receptor remains to be confirmed. The important finding that dopamine receptors are probably not involved in the local stimulant effects of metoclopramide has important implications for future research orientated towards the discovery of a new generation of GI drugs lacking the side effects associated with central dopamine receptor blockade. Several compounds (cinitapride, BRL 20627A and cisapride) are now in the early stages of clinical evaluation.     

Glutamate mediates a slow synaptic response in hippocampal slice cultures.

Glutamate (GLU) mediates its 'fast' excitatory transmitter action in the brain by directly gating cation-selective ion channels ('ionotropic' receptors). However, GLU can also activate another type of receptor, coupled to phospholipase C ('metabotropic' receptor). In hippocampal cells, stimulation of this metabotropic receptor by GLU, or by a racemic mixture of (1S-3R and 1R-3S) 1-aminocyclopentyl-1,3-dicarboxylate (ACPD), induces a slower excitation mediated by inhibition of K+ currents. We have assessed whether this slow form of metabotropic receptor excitation can contribute to the effects of synaptically released GLU in hippocampal slice cultures, by recording the responses of CA3 pyramidal cells to afferent mossy fibre stimulation. When the fast ionotropic response was blocked pharmacologically, mossy fibre stimulation produced a slow depolarizing postsynaptic potential associated with a decrease in membrane conductance, a depression of the slow after-hyperpolarization following a train of action potentials, and reduced accommodation during the action potential train. Under voltage-clamp, mossy fibre stimulation produced a slow voltage-dependent inward current which resembled that produced by application of exogenous ACPD or quisqualate (QUIS), and which was occluded by these metabotropic agonists. We therefore suggest that synaptically released GLU can induce two types of postsynaptic responses: a fast excitation through activation of ionotropic receptors and a slower excitation associated with inhibition of K+ conductances through activation of metabotropic receptors. This is analogous to the dual action of acetylcholine on ionotropic (nicotinic) and metabotropic (muscarinic) receptors.     

The possible site of action of 5-hydroxytryptamine, 6-hydroxytryptamine, tryptamine and dopamine on identified neurons in the central nervous system of the snail, Helix aspersa

Intracellular recordings were made from identified neurons in the right parietal ganglion of the snail, Helix aspersa. Cells F 4, 5 and 6 were excited by 5-hydroxytryptamine (5-HT) and inhibited by dopamine while cells in the F 30 area were inhibited by both compounds. Low doses of both tryptamine and 6-HT produced weak excitation of cells F 4, 5 and 6 while higher doses of both compounds inhibit the activity of these cells. In terms of the inhibitory responses, tryptamine and 6-HT are approximately equipotent but between 10 and 100 times less potent than dopamine. d-Tubocurarine reversibly antagonized the excitatory action of 5-HT on cells F 4, 5 and 6 and converted tryptamine and 6-HT excitation to inhibition. In the presence of the antagonist, ergometrine, the dopamine inhibitory response was almost completely abolished while the inhibitory responses to tryptamine and 6-HT were converted to weak excitation. All four agonists inhibited cells in the F 30 area with the following potency ratios: dopamine much greater than tryptamine/6-HT greater than 5-HT. Tubocurarine had no antagonist effects on these responses while ergometrine reduced or blocked all four, often irreversibly. In potassium-free Ringer the inhibitory responses to all four agonists were enhanced. It is concluded that on cells F 4, 5 and 6, low concentrations of tryptamine and 6-HT act on 5-HT receptors while higher concentrations of both agonists act on dopamine receptors. On cells in the F 30 area, 5-HT, 6-HT and tryptamine all act on a dopamine receptor.     

鸡视网膜谷氨酸受体和GABA受体在两栖类卵母细胞中的表达

本工作利用两栖类卵母细胞作为功能表达系统,对鸡视网膜中的谷氨酸受体和ＧＡＢＡ受体的类型和基本性质进行了研究。在注射鸡视网膜ｍＲＮＡ的卵母细胞上,谷氨酸受体有明显的表达。Ｌ－Ｇｌｕ及其类似物ＫＡ,ＡＭＰＡ,ＱＡ都毫无例外地能诱导卵母细胞产生快速平滑的去极化电流,而ＮＭＤＡ,Ｌ－ＡＰ４,ＡＣＰＤ以及天冬氨酸不能诱导明显的电流反应。并且ＡＭＰＡ,ＱＡ对ＫＡ反应存在一定的抑制作用,提示ＡＭＰＡ,ＱＡ可能与ＫＡ作用于同一受体。抑制性氨基酸ＧＡＢＡ的受体被证明大部分为ＧＡＢＡＡ亚型,但有小部分的ＧＡＢＡ反应不能为荷包牡丹碱（ｂｉｃｕｃｕｌｌｉｎｅ）所阻断。     

Cross-talk between P2X4 and gamma-aminobutyric acid, type A receptors determines synaptic efficacy at a central synapse

The essence of neuronal function is to generate outputs in response to synaptic potentials. Synaptic integration at postsynaptic sites determines neuronal outputs in the CNS. Using immunohistochemical and electrophysiological approaches, we first reveal that steroidogenic factor 1 (SF-1) green fluorescent protein (GFP)-positive neurons in the ventromedial nucleus of the hypothalamus express P2X4 subunits that are activated by exogenous ATP. Increased membrane expression of P2X4 channels by using a peptide competing with P2X4 intracellular endocytosis motif enhances neuronal excitability of SF-1 GFP-positive neurons. This increased excitability is inhibited by a P2X receptor antagonist. Furthermore, increased surface P2X4 receptor expression significantly decreases the frequency and the amplitude of GABAergic postsynaptic currents of SF-1 GFP-positive neurons. Co-immunopurification and pulldown assays reveal that P2X4 receptors complex with aminobutyric acid, type A (GABA(A)) receptors and demonstrate that two amino acids in the carboxyl tail of the P2X4 subunit are crucial for its physical association with GABA(A) receptors. Mutation of these two residues prevents the physical association, thereby blocking cross-inhibition between P2X4 and GABA(A) receptors. Moreover, disruption of the physical coupling using competitive peptides containing the identified motif abolishes current inhibition between P2X4 and GABA(A) receptors in recombinant system and P2X4 receptor-mediated GABAergic depression in SF-1 GFP-positive neurons. Our present work thus provides evidence for cross-talk between excitatory and inhibitory receptors that appears to be crucial in determining GABAergic synaptic strength at a central synapse.     

Activity of cytisine and its brominated isosteres on recombinant human alpha7, alpha4beta2 and alpha4beta4 nicotinic acetylcholine receptors

Effects of cytisine (cy), 3-bromocytisine (3-Br-cy), 5-bromocytisine (5-Br-cy) and 3,5-dibromocytisine (3,5-diBr-cy) on human (h) alpha7-, alpha4beta2- and alpha4beta4 nicotinic acetylcholine (nACh) receptors, expressed in Xenopus oocytes and cell lines, have been investigated. Cy and its bromo-isosteres fully inhibited binding of both [alpha-(125)I]bungarotoxin ([alpha-(125)I]BgTx) to halpha7- and [(3)H]cy to halpha4beta2- or halpha4beta4-nACh receptors. 3-Br-cy was the most potent inhibitor of both [alpha-(125)I]BgTx and [(3)H]cy binding. Cy was less potent than 3-Br-cy, but 5-Br-cy and 3,5-diBr-cy were the least potent inhibitors. Cy and 3-Br-cy were potent full agonists at halpha7-nACh receptors but behaved as partial agonists at halpha4beta2- and halpha4beta4-nACh receptors. 5-Br-cy and 3,5-diBr-cy had low potency and were partial agonists at halpha7- and halpha4beta4-nACh receptors, but they elicited no responses on halpha4beta2-nACh receptors. Cy and 3-Br-cy produced dual dose-response curves (DRC) at both halpha4beta2- and halpha4beta4-nACh receptors, but ACh produced dual DRC only at halpha4beta2-nACh receptors. Low concentrations of cy, 3-Br-cy and 5-Br-cy enhanced ACh responses of oocytes expressing halpha4beta2-nACh receptors, but at high concentrations they inhibited the responses. In contrast, 3,5-diBr-cy only inhibited, in a competitive manner, ACh responses of halpha4beta2-nACh receptors. It is concluded that bromination of the pyridone ring of cy produces marked changes in effects of cy that are manifest as nACh receptor subtype-specific differences in binding affinities and in functional potencies and efficacies.     

The inhibitory action of exo- and endocannabinoids on [³H]GABA release are mediated by both CB₁and CB₂receptors in the mouse hippocampus

Exogenous and endogenous cannabinoids play an important role in modulating the release of neurotransmitters in hippocampal excitatory and inhibitory networks, thus having profound effect on higher cognitive and emotional functions such as learning and memory. In this study we have studied the effect of cannabinoid agonists on the potassium depolarization-evoked [(3)H]GABA release from hippocampal synaptosomes in the wild-type (WT) and cannabinoid 1 receptor (CB(1)R)-null mutant mice. All tested cannabinoid agonists (WIN55,212-2, CP55,940, HU-210, 2-arachidonoyl-glycerol, 2-AG; delta-9-tetra-hydrocannabinol, THC) inhibited [(3)H]GABA release in WT mice with the following rank order of agonist potency: HU-210>CP55,490>WIN55,212-2>2-AG>THC. By contrast, 2-AG and THC displayed the greatest efficacy eliciting almost complete inhibition of evoked [(3)H]GABA efflux, whereas the maximal inhibition obtained by HU-210, CP55,490, and WIN55,212-2 were less, eliciting not more than 40% inhibition. The inhibitory effect of WIN55,212-2, THC and 2-AG on evoked [(3)H]GABA efflux was antagonized by the CB(1) receptor inverse agonist AM251 (0.5 μM) in the WT mice. In the CB(1)R knockout mice the inhibitory effects of all three agonists were attenuated. In these mice, AM251 did not antagonize, but further reduced the [(3)H]GABA release in the presence of the synthetic agonist WIN55,212-2. By contrast, the concentration-dependent inhibitory effects of THC and 2-AG were partially antagonized by AM251 in the absence of CB(1) receptors. Finally, the inhibition of evoked [(3)H]GABA efflux by THC and 2-AG was also partially attenuated by AM630 (1 μM), the CB(2) receptor-selective antagonist, both in WT and CB(1) knockout mice. Our data prove the involvement of CB(1) receptors in the effect of exo- and endocannabinoids on GABA efflux from hippocampal nerve terminals. In addition, in the effect of the exocannabinoid THC and the endocannabinoid 2-AG, non-CB(1), probably CB(2)-like receptors are also involved.     

Muscarinic modulation of GABAergic transmission to neurons in the rat subfornical organ

Cholinergic actions on subfornical organ (SFO) neurons in rat slice preparations were studied by using whole cell voltage- and current-clamp recordings. In the voltage-clamp recordings, carbachol and muscarine decreased the frequency of GABAergic inhibitory postsynaptic currents (IPSCs) in a dose-dependent manner, with no effect on the amplitudes or the time constants of miniature IPSCs. Meanwhile, carbachol did not influence the amplitude of the outward currents induced by GABA. Furthermore, carbachol and muscarine also elicited inward currents in a TTX-containing solution. From the current-voltage relationship, the reversal potential was estimated to be -7.1 mV. These carbachol-induced responses were antagonized by atropine. In the current-clamp recordings, carbachol depolarized the membrane with increased frequency of action potentials. These observations suggest that acetylcholine suppresses GABA release through muscarinic receptors located on the presynaptic terminals. Acetylcholine also directly affects the postsynaptic membrane through muscarinic receptors, by opening nonselective cation channels. A combination of these presynaptic and postsynaptic actions may enhance activation of SFO neurons by acetylcholine.     

The effect of various neurotransmitters and some of their agonists and antagonists on the crayfish abdominal positioning system

1. Crayfish abdominal nerve cords were perfused with selected transmitters or their agonists or antagonists. Motor activity underlying abdominal positioning behavior was monitored.2. All the neurotransmitters except glycine had a measurable effect on this system.3. Acetylcholine and its agonists were slightly stimulatory. Both muscarinic and nicotinic receptors were indicated.4. GABA was weakly inhibitory. Picrotoxin was strongly stimulatory, perhaps as a result of its known ability to block GABA and inhibitory acetylcholine receptors.5. Histamine was strongly inhibitory. Both H1 and H2 receptors were indicated.6. Glutamate was found to be slightly inhibitory while its agonist, NMDA, showed no effect.7. Finally, L-Dopa was stimulatory, but only at a high concentration.     

The effect of various neurotransmitters and some of their agonists and antagonists on the crayfish abdominal positioning system.

1. Crayfish abdominal nerve cords were perfused with selected transmitters or their agonists or antagonists. Motor activity underlying abdominal positioning behavior was monitored. 2. All the neurotransmitters except glycine had a measurable effect on this system. 3. Acetylcholine and its agonists were slightly stimulatory. Both muscarinic and nicotinic receptors were indicated. 4. GABA was weakly inhibitory. Picrotoxin was strongly stimulatory, perhaps as a result of its known ability to block GABA and inhibitory acetylcholine receptors. 5. Histamine was strongly inhibitory. Both H1 and H2 receptors were indicated. 6. Glutamate was found to be slightly inhibitory while its agonist, NMDA, showed no effect. 7. Finally, L-Dopa was stimulatory, but only at a high concentration.