Studies on the indirect feedback inhibition of cholinergic neurons triggered by oxotremorine in striatum

Determination of endogenous acetylcholine release in freely moving rats by transstriatal dialysis coupled to a radioenzymatic assay: effect of drugs

The technique of intracerebral dialysis in combination with a sensitive and specific radioenzymatic method was used for recovery and quantification of endogenous extracellular acetylcholine from the striata of freely moving rats. A thin dialysis tube was inserted transversally through the caudate nuclei, and the tube was perfused with Ringer solution, pH 6.1, at a constant rate of 2 microliter min-1. The perfusates were collected at 10-min intervals. In the presence of 1 and 10 microM physostigmine, acetylcholine release was 4.5 +/- 0.02 and 7.3 +/- 0.3 pmol/10 min, respectively (not corrected for recovery). The latter concentration of the acetylcholinesterase inhibitor was used in all experiments. Under basal conditions, acetylcholine output was stable over at least 4 h. A depolarizing K+ concentration produced a sharp, reversible 87% increase in acetylcholine output. Both the basal and K+-stimulated release were Ca2+ dependent. The choline uptake inhibitor hemicholinium-3 (20 micrograms intracerebroventricularly) reduced striatal acetylcholine output to 35% of the basal value within 90 min. Scopolamine (0.34 mg/kg s.c.) provoked a sharp enhancement of acetylcholine release of approximately 63% over basal values, whereas oxotremorine (0.53 mg/kg i.p.) transiently reduced acetylcholine release by 54%. These results indicate the physiological and pharmacological suitability of transstriatal dialysis for monitoring endogenous acetylcholine release.

Qualitative differences in the effects of adenosine analogs on the cholinergic systems of rat striatum and hippocampus

The effect of the purinergic agonist, 2-chloroadenosine (2-CADO), on central cholinergic parameters was studied in the rat. The drug (20 micrograms, i.c.v.) increased acetylcholine (ACh) content (approximately 30%) and inhibited sodium dependent high affinity choline uptake (30%) in the hippocampus. In striatum, the increase of ACh content was less marked (approximately 15%) and was not associated with inhibition of choline uptake. In both areas, ACh accumulation was prevented by theophylline but not by atropine or oxotremorine pretreatments. Differences were noted in the purinergic control of cholinergic function in the hippocampus and striatum. In hippocampus, the selective degeneration of noradrenergic, serotonergic and glutamatergic afferent pathways or the destructions of intrinsic neurons did not prevent the rise in ACh content induced by 2-CADO. Differently, in striatum, the action of 2-CADO was potentiated both by raphe deafferentation and by inhibition of serotonin synthesis and was completely prevented by chronic unilateral decortication. The cholinergic effect of 2-CADO was unchanged after impairment of the noradrenergic or dopaminergic systems. In addition, the D- and L-isomers of phenylisopropyladenosine, which have different affinities for A1 purinergic receptors but equal affinity for the A2 purinergic subtype, differed in their ability to affect acetylcholine content in these two brain regions, suggesting that A1 purinergic receptor activation mediates the effect of 2-CADO in the hippocampus and A2 receptor activation mediates the drug's action in the striatum.

In vivo and in vitro studies on the regulation of cholinergic neurotransmission in striatum, hippocampus and cortex of aged rats

Young (3 months) and senescent (23 months) rats were challenged with oxotremorine both in vivo, to determine its effects on acetylcholine content in hemispheric regions, and in vitro, to assess its action on K+-evoked release of ACh from brain synaptosomes. The drug failed to inhibit KCl-induced [3H]ACh release from the P2 fraction of striatal and hippocampal homogenates of the senescent animals, whereas it was less efficient in increasing striatal ACh content. In contrast, oxotremorine was still able to stimulate an increase in ACh in the hippocampus and cerebral cortex of the aged rats to the same extent as it did in the young ones. The [3H]ACh output from striatal synaptosomes was lower in old rats with respect to young ones at low KCl depolarizing concentrations but was equal in the two groups at a high depolarizing concentration. In the hippocampus of the senescent rats, the release was significantly lower at each concentration of KCl used, resulting in a parallel downward-shift in the concentration-release plot. We also measured cholinergic muscarinic receptor binding in rat hemispheric regions using the radioligand [3H]dexetimide, a classical non-selective muscarinic receptor antagonist. It was found, in conformity with some of the literature, that receptor binding was decreased by about 32% in striatum of aged female rats as compared to younger rats. Changes were not observed in cortex and hippocampus. Analysis of the binding data indicated that the observed decrease in specific ligand binding was due to a decrease in the number of binding sites without a change in affinity. The results favor, once again, the cholinergic hypothesis for geriatric dysfunction.(ABSTRACT TRUNCATED AT 250 WORDS)

Frontal decortication and adaptive changes in striatal cholinergic neurons in the rat

Interruption of the corticostriatal pathway by undercutting the cortex resulted in a reduction of glutamate uptake by 55% and in a depression of acetylcholine (ACh) synthesis by 30% in striatum after two postlesion weeks without affecting the content of ACh and choline, the specific binding of [3H]dexetimide to muscarinic receptors, the activity of choline acetyltransferase and the levels of noradrenaline, serotonin, dopamine and 3,4-dihydroxyphenylacetic acid. The influence of this excitatory pathway on striatal cholinergic neuropharmacology was investigated. It was found that the effect of a number of agonists (R-apomorphine, bromocriptine, lisuride, quinpirole, JL-14389, 2-chloroadenosine, oxotremorine and methadone), capable of depressing cholinergic activity in the striatum through receptor-mediated responses--reflected as an increase in ACh content--is operative only when the corticostriatal pathway is intact. By contrast, antagonists capable of decreasing ACh content, i.e. the typical neuroleptics pimozide, haloperidol and the atypical ones clozapine, L-sulpiride, as well as the anti-muscarinic agent scopolamine, were not influenced by the lesion. The possibility that the lesion non-specifically damaged striatal cells on which the agonists, but not the antagonists acted was excluded by results showing, firstly, that the increase in striatal ACh elicited by the ACh precursor, choline, was not blocked by decortication, and secondly, that the degeneration of the corticostriatal neurons did not prevent the ACh-increasing effect of bromocriptine, a long-acting ergot alkaloid, when sufficient time was allowed for the drug to act. It was furthermore possible to restore the inhibitory action of apomorphine on cholinergic neurons either by short-term chemical lesion of the nigrostriatal dopaminergic input or by the administration of choline.(ABSTRACT TRUNCATED AT 250 WORDS)

Mode of action of gamma-butyrolactone on the central cholinergic system

Gamma-butylactone (GBL), a drug depressing the central nervous system, produced marked increases in acetylcholine contents in rat brain hemispheric regions (striatum, hippocampus, cortex) and in striatal choline content without modifying choline acetyltransferase or acetylcholinesterase activities. In the hippocampus GBL also strongly decreased the acetylcholine turnover rate and inhibited the high affinity uptake of choline. Its increase in acetylcholine content was prevented by an acute electrolytic lesion of the medial septum but not by a wide array of drug treatments designed to interfere with neurotransmission in various pathways. The results are taken to indicate that GBL directly depresses the cholinergic septal-hippocampal afferents by interrupting impulse flow. In the striatum, too, GBL markedly depressed the acetylcholine synthesis rate but had no effect on the high affinity choline uptake process. Such dissociation of the two phenomena had previously been observed using other drugs and may denote that acetylcholine synthesis in this region is regulated differently from that in the hippocampus. By comparison, gamma-hydroxybutyric acid (GHBA), an active metabolite which shares with GBL the capacity to produce a somnolent state and depress impulse flow in the dopaminergic nigroneostriatal pathway, had no effect on either striatal acetylcholine content or on hippocampal high affinity choline uptake. The results suggest that GBL can be distinguished from GHBA in its neuropharmacological central cholinergic effects.