Effect of cholinomimetics on the release and uptake of L-[3H] glutamic acid in rat neostriatum

Presynaptic kappa-opioid and muscarinic receptors inhibit the calcium-dependent component of evoked glutamate release from striatal synaptosomes

In addition to cytosolic efflux, reversal of excitatory amino acid (EAA) transporters evokes glutamate exocytosis from the striatum in vivo. Both kappa-opioid and muscarinic receptor agonists suppress this calcium-dependent response. These data led to the hypothesis that the calcium-independent efflux of striatal glutamate evoked by transporter reversal may activate a transsynaptic feedback loop that promotes glutamate exocytosis from thalamo- and/or corticostriatal terminals in vivo and that this activation is inhibited by presynaptic kappa and muscarinic receptors. Corollaries to this hypothesis are the predictions that agonists for these putative presynaptic receptors will selectively inhibit the calcium-dependent component of glutamate released from striatal synaptosomes, whereas the calcium-independent efflux evoked by an EAA transporter blocker, L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-PDC), will be insensitive to such receptor ligands. Here we report that a muscarinic agonist, oxotremorine (0.01-10 microM), and a kappa-opioid agonist, U-69593 (0.1-100 microM), suppressed the calcium-dependent release of glutamate that was evoked by exposing striatal synaptosomes to the potassium channel blocker 4-aminopyridine. The presynaptic inhibition produced by these ligands was concentration dependent, blocked by appropriate receptor antagonists, and not mimicked by the delta-opioid agonist [D-Pen2,5]-enkephalin. The finding that glutamate efflux evoked by L-trans-PDC from isolated striatal nerve endings was entirely calcium independent supports the notion that intact basal ganglia circuitry mediates the calcium-dependent effects of this agent on glutamate efflux in vivo. Furthermore, because muscarinic or kappa-opioid receptor activation inhibits calcium-dependent striatal glutamate release in vitro as it does in vivo, it is likely that both muscarinic and kappa receptors are inhibitory presynaptic heteroceptors expressed by striatal glutamatergic terminals.

M1 muscarinic receptor stimulation decreases aspartate release in the rat neostriatum

This study investigates the effects of different muscarinic receptor agonists on extracellular glutamate and aspartate concentrations in the rat neostriatum. In vivo intracerebral perfusions were undertaken in the conscious rat using a concentric push-pull cannulae system. Amino acid concentrations in samples were determined by HPLC with fluorometric detection. The intrastriatal perfusion of arecoline, a M1-M2 muscarinic receptor agonist, produced a significant decrease in extracellular [ASP] (45% of decrease) but not in extracellular [GLU]. These effects were blocked by scopolamine, a M1-M2 muscarinic receptor antagonist. McN-A-343, a M1 muscarinic receptor agonist, but not the M2 muscarinic receptor agonist, oxotremorine, produced a significant decrease in extracellular [ASP] (40% of decrease) but not in extracellular [GLU]. The effects of McN-A-343 on extracellular [ASP] were blocked by pirenzepine, a M1 muscarinic receptor antagonist. These results suggest that the decrease in extracellular [ASP] could be mediated, at least in part, by M1 muscarinic receptor activation in the rat neostriatum.

The muscarinic toxin 3 augments neuropeptide mRNA in rat striatum in vivo

The selective M4 muscarinic receptor toxin, MT3, was used in vivo to evaluate the role of M4 receptors in cholinergic inhibition of neuropeptide mRNA expression in striatonigral neurons. Unilateral injection of the muscarinic toxin 3 (0.04-4 nmol) into the dorsal striatum of chronically-cannulated rats elevated basal levels of preprodynorphin, substance P and preproenkephalin mRNAs in the ipsilateral dorsal striatum as revealed by quantitative in situ hybridization. Pretreatment with muscarinic toxin 3 also augmented amphetamine (2.5 mg/kg, i.p.)-stimulated preprodynorphin and substance P expression in the dorsal striatum in a manner similar to that observed after the muscarinic antagonist, scopolamine. Since muscarinic toxin 3 has a much greater affinity for muscarinic M4 receptors than for other subtypes, it is possible that muscarinic toxin 3, by interacting with the muscarinic M4 subtype, regulates basal and/or dopamine-stimulated striatal neuropeptide gene expression.

Effect of brain acetylcholine depletion on bicuculline-induced cardiovascular and locomotor responses

Both GABAergic and cholinergic systems are involved in central cardiovascular regulation. Previous studies have shown that GABAA receptor antagonists cause increases in blood pressure, heart rate and locomotor activity. In this study, we examined the role of the depletion of brain acetylcholine on the cardiovascular responses and locomotor activity induced by bicuculline methiodide in conscious Sprague-Dawley rats. The doses of 0.3 and 0.5 nmol of intracerebroventricular bicuculline methiodide produced increases in blood pressure, heart rate and locomotor activity. The dose of 18 nmol of hemicholinium-3 to deplete brain acetylcholine was given intracerebroventricularly one hour prior to bicuculline methiodide. The pressor responses to bicuculline methiodide in animals pretreated with the hemicholinium-3 were higher than those seen in saline-pretreated groups, but locomotor activity and heart rate responses to bicuculline methiodide remained unchanged in hemicholinium-3 pretreatment group. On the other hand, high dose of bicuculline methiodide (0.5 nmol) caused convulsions in some animals pretreated with hemicholinium-3 whereas bicuculline methiodide, alone, did not cause any seizure activity. In conclusion, it seems likely that endogenous brain acetylcholine could be a modulator of GABAA receptor-mediated blood pressure control.

The role of brain acetylcholine in GABAA receptor antagonist-induced blood-pressure changes in rat

Previous experimental studies have shown that intracerebroventricular (i.c.v.) injection of the GABAA receptor antagonist, bicuculline methiodide, results in marked increases in blood pressure due to an increase in sympathetic nervous system activity. It is well recognized that the central cholinergic system is also involved in the regulation of blood pressure. In the present study, we examined the role of brain acetylcholine in the pressor response induced by bicuculline methiodide in conscious Sprague-Dawley rats. I.c.v. (0.05, 0.3 and 0.5 nmol) and intrahypothalamic (40 pmol) administration of bicuculline methiodide produced blood-pressure increases in a dose-dependent manner. Hemicholinium-3 was given i.c.v. 1 h prior to bicuculline methiodide. The depletion of brain acetylcholine was demonstrated by the suppression of physostigmine-induced pressor responses, but blood pressure increases in response to carbachol remained unchanged. The pressor responses to bicuculline methiodide in animals pre-treated with hemicholinium-3 were significantly higher than those seen in saline-pre-treated groups. Likewise, bicuculline methiodide, at a dose that did not alter blood pressure alone, caused pressor responses in rats pre-treated with the nicotinic receptor antagonist, mecamylamine, whereas the muscarinic receptor antagonist, atropine, was ineffective in this respect. In conclusion, it seems likely that endogenous brain acetylcholine has a modulator role on GABAA receptor-mediated blood-pressure control via nicotinic receptors.

Protective effect of vinconate on ischemia-induced neuronal damage in the rat hippocampus

The protective effect of vinconate, a vinca alkaloid derivative, on ischemia-induced neuronal damage was investigated using a model of rat forebrain ischemia caused by occlusion of four vessels. Hippocampal cell loss was observed histologically and neurochemically 5 days after 10 min of ischemia. Treatment with vinconate (50 and 200 mg/kg i.p.) before cerebral ischemia significantly suppressed neuronal cell loss in the hippocampal CA1 region and the decrease in the content of neuroactive amino acids in the hippocampus. The release of neuroactive amino acids in the hippocampus was significantly increased by cerebral ischemia. Pretreatment with vinconate (50 and 200 mg/kg i.p.) significantly attenuated the increased release of glutamic acid and aspartic acid, but not the release of gamma-aminobutyric acid (GABA), taurine and glycine. This suppressive effect of vinconate was antagonized by scopolamine (10(-5) M). The addition of vinconate (10(-11)-10(-4) M) had no effect on the binding of [3H]MK-801. These results indicate that pretreatment with vinconate attenuates the ischemia-induced release of excitatory amino acids into the extracellular space of the hippocampus via the stimulation of presynaptic muscarinic acetylcholine receptors. The present results also suggest that this suppressive effect of vinconate on the release of excitatory amino acids (glutamic acid and aspartic acid) may play a crucial role in the protective action of this agent against ischemia-induced neuronal damage in the hippocampus.

Muscarinic inhibition of endogenous glutamate release from rat hippocampus synaptosomes

The effects of acetylcholine (ACh) on the depolarization-evoked release of endogenous glutamic acid (Glu) have been studied using synaptosomes prepared from rat hippocampus and depolarized in superfusion with 15 mM KCl. Acetylcholine inhibited Glu release in a concentration-dependent way. The natural agonist was particularly effective causing 50% inhibition of Glu release at 10 microM in the absence of acetylcholinesterase (AChE) inhibitors. The inhibitory effect of ACh on the K+-evoked release of Glu was antagonized by the selective muscarinic receptor antagonist atropine but not by the nicotinic receptor antagonist mecamylamine. The data represent the first demonstration that muscarinic receptors located on Glu axon terminals in rat hippocampus may modulate the release of Glu.