Increase in striatal acetylcholine levels by GABA mimetic drugs: lack of involvement of the nigro-striatal dopaminergic neurons

The effects of unilateral destruction of fimbria-fornix and supracallosal pathways in the rat

Rats with unilateral lesions of either the supracallosal regions (including the dorsal cingulate cortex) and the fimbria-fornix either on the same (S) or the opposite (O) sides of the brain were studied in a 16-hole open field without pharmacologic intervention and, subsequently, after 0.1 and after 1.0 mg/kg scopolamine HBr. Their performances were compared with those of unoperated control animals subjected to the same testing regime. Certain of their behaviors were compared with those of a larger number of animals with bilateral hippocampal destruction (and their control groups) from prior studies. Unilateral lesions of fimbria-fornix and supracallosal afferents to the hippocampal formation produced a decrease in hole poking activity relative to control animals. A further decrease in hole-poking behavior, coupled with increased locomotion, was observed in rats with fimbria-fornix and cingulate cortex lesions on opposite sides of the brain (group O). The smaller dose of scopolamine accentuated these effects. Indeed, the behavior of group O after scopolamine treatment was similar to animals with large bilateral hippocampal lesions. The large dose of scopolamine induced stereotyped rearing or hole poking in the brain-damaged animals but not in the control group. These findings suggest that both the fimbria-fornix and the supracallosal pathway is necessary for normal hippocampal function and that the behavioral deficit is greater when these structures are damaged on the opposite sides of the brain.

Blockade of the diazepam-induced increase in rat striatal acetylcholine content by the specific benzodiazepine antagonists ethyl-beta-carboline-3-carboxylate and Ro 15-1788

Diazepam increased the acetylcholine content in the striatum and the hippocampus of the rat. This effect was antagonized in both brain areas by treatment with the specific central benzodiazepine blockers ethyl-beta-carboline-3-carboxylate and Ro 15-1788, whereas the peripheral antagonist Ro 5-4864 was ineffective. Pretreatment with picrotoxin, a known GABA antagonist did not interfere with the diazepam-induced acetylcholine increase. These results indicate a specific involvement of benzodiazepine receptors in the cholinergic action of diazepam and this effect appears to be independent of GABA receptor activation.

The modulation of cortical acetylcholine release by GABA, GABA-like drugs and benzodiazepines in freely moving guinea-pigs

In order to define the modulatory role played by gamma-aminobutyric acid (GABA) in corticopetal cholinergic projections, the effect of this amino acid and related drugs on gross behaviour, the EEG and the release of acetylcholine (ACh) from the cerebral cortex in freely moving guinea-pigs was studied. gamma Aminobutyric acid, injected intracerebroventricularly (20-50 mumol) induced a three-phase picture: first (5-15 min) behavioural activation and increased release of ACh, then (30-90 min) depression, EEG synchronization and reduced release of ACh, and finally "rebound" stimulation. Ethanolamine-O-sulphate (EOS) injected intraventricularly (28 mumol/kg) or intraperitoneally (14 mmol/kg) reproduced the first two phases of the effects of GABA (i.e. stimulation followed by inhibition), while diazepam (0.7 and 3.5 mumol/kg, i.p.) and flurazepam (32 mumol/kg, i.p.) caused, at first, only depression. Muscimol and 4,5,6,7-tetrahydroisoxazolo(4,5-c)pyridine-3-ol (THIP) injected intraventricularly (in the nmol range) or intraperitoneally (in the mumol range) produced behavioural activation and increased release of ACh; the depressant signs appeared only after very large, toxic doses. Picrotoxin and bicuculline, at sub-convulsive doses, reduced the symptomatology caused by GABA and antagonized the sedation produced by diazepam. Methysergide (8-16 mumol/kg, i.p.) prevented the behavioural activation and the increased release of ACh by GABA, unmasked the depression due to subthreshold doses of diazepam (i.c.v., 7-70 nmol) and reversed the stimulation induced by muscimol into sedation and reduced the outflow of ACh. Pretreatment with 5,7-HT also dampened and shortened the stimulation by muscimol.(ABSTRACT TRUNCATED AT 250 WORDS)

A type of aspiny neuron in the rat neostriatum accumulates [3H]gamma-aminobutyric acid: combination of Golgi-staining, autoradiography, and electron microscopy

Light microscopic autoradiography was used to identify cells in the neostriatum that became labelled after the local injection of [3H]gamma-aminobutyrate (GABA). The GABA-accumulating cells comprised up to 15% of the total population of neurons. Thirty-seven of these cells were examined in the electron microscope and it was found that they all had similar cytological characteristics, i.e., prominent nuclear indentations, a moderate volume of cytoplasm, rich in organelles, and sparse synaptic input to the perikaryon. Nine of the cells that had accumulated GABA were also impregnated following Golgi staining. These Golgi-impregnated neurons were of medium size and all had dendrites that were aspiny, often varicose, and that occasionally followed a recurving path. After gold toning, the Golgi-impregnated, GABA-accumulating neurons were examined in the electron microscope and were found to receive boutons forming symmetrical or asymmetrical synaptic contacts on their somata and dendrites; the symmetrical synapses were most common on the cell body and proximal dendrites, while the distal dendrites mainly received boutons forming asymmetrical contacts. We conclude that one type of GABAergic neuron in the neostriatum is a type of medium-sized aspiny neuron and that this neuron is likely to receive synaptic input both from neurons within the striatum and from neurons in distant brain regions. We suggest that this neuron is a local circuit neuron in the neostriatum since its morphological features are quite distinct from those of identified projecting neurons.

GABA induced changes in acetylcholine release from slices of guinea-pig brain

The effect of GABA on acetylcholine (ACh) release was investigated on superfused slices of guinea-pig cerebral cortex (CC), caudate nucleus (CN), tuberculum olfactorium and brain stem. GABA (1--6 x 10(-3) mol/l) increased the spontaneous and KCl-evoked ACh overflow in CC and CN, reduced the electrically-evoked release in all areas tested (most evidently in CC and CN) and lowered the threshold of electric stimulation-induced ACh release in CC. These effects were also caused by 3-amino-1-propane sulphonic acid (1 x 10(-3) mol/l) and ethanolamine-O-sulphate (2 x 10(-3) mol/l), were reduced by bicuculline (1 x 10(-4) mol/l) and fully antagonized by picrotoxin (8 x 10(-5) mol/l), but they were not influenced by phentolamine, methysergide, spiroperidol or strychnine. Tetrodotoxin (TTX) (5 x 10(-7) mol/l) blocked the facilitation of spontaneous ACh release by GABA only when the slices were perfused with normal Krebs solution, but not when perfused with a KCl-enriched medium. These results suggest that GABA affects the cholinergic transmitter release through bicuculline- and picrotoxin-sensitive receptors, showing low affinity toward the agonist. Moreover GABA modulation of resting ACh release requires action potentials only in normal [K+]0, but not in high [K+]0, suggesting that GABA-receptive sites are located at cholinergic terminals.

Selected ion monitoring determination of acetylcholine during methoxypyridoxine seizures

The neurotransmitter gamma-aminobutyric acid is known to decrease preictally after administration of the potent convulsant methyoxypyridoxine, a competitive inhibitor of glutamate decarboxylase. An attempt was made to determine the effect of this gamma-aminobutyric acid decrease on the cholinergic system. Rabbits were immobilized and artificially ventilated in order to avoid hypoxidosis. Seizures were induced by intravenous injection of 100 mg kg-1 methoxypyridoxine; 40 minutes later the animals were decapitated and discrete brain areas removed. Tissue contents of acetylcholine and choline were estimated by gas chromatographic mass spectrometric analysis of the beta-dimethylaminoethyl acetate and propionate derivatives using deuterated internal standards. Gas chromatographic column optimization resulted in a considerable sensitivity gain. Computerized selected ion monitoring was carried out on the dimethylmethyleneimmonium ions using voltage switching. The use of a computer controlled solvent dump valve was implemented to increase precision. No significant difference was observed in the concentration of acetylcholine in the frontal cortex, cerebellar cortex, septum, hippocampus, or caudate of seizure versus control animals; septal choline increased, however. This suggests that the acetylcholine turnover could be increased during seizure.

Increase in striatal acetylcholine levels by GABAergic agents: dependence on corticostriatal neurons

Surgical lesion of the corticostriatal projections almost totally reduced the ability of GABA agonist agents but not apomorphine to increase rat striatal ACh concentrations. This suggests that in intact rats, the GABAergic inhibition of striatal cholinergic neurons depends, at least in part, upon the corticostriatal (possibly glutamatergic) tract whereas the action of the dopaminergic pathway on cholinergic cells is independent of corticostriatal neurons.

Biphasic effects of direct, but not indirect, GABA mimetics and antagonists on haloperidol-induced catalepsy

At very low doses the GABA agonists SL 76002 and muscimol diminish haloperidol-induced catalepsy. At somewhat higher doses these compounds potentiate catalepsy. Biphasic effects on DA-receptor mediated functions have previously been noted with bicuculline and picrotoxinin. In contrast, manipulation of GABA levels by enzyme inhibition induced only a monophasic effect on dopamine-mediated behaviour. The potentiation of GABA levels by enzyme inhibition induced only a monophasic effect on dopamine-mediated behaviour. The potentiation of haloperidol-induced catalepsy by GABA mimetics is also observed with dipropylacetate, delta-aminovaleric acid and gamma-acetylenic GABA. This GABA-mimetic potentiation of catakepsy was blocked by the coadministration of bicuculline. These results confirm and extend the hypothesis that GABA-neurons influence DA neuron function. Furthermore they suggest that more than one group of GABA receptors influence directly and/or indirectly DA neuronal function, with different resultant effects.