R. Samanin

Reduced effect of morphine in midbrain raphe lesioned rats

Abstract The reaction of painful stimulation was studied in rats by means of three different techniques. Lesions of the midbrain raphe in rats, which reduced the levels of brain 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) decreased the analgesic activity of morphine. In some tests, an increase in sensitivity to painful stimulation was noted in lesioned animals. The results obtained support the view that a decrease of brain 5-HT concentration antagonizes morphine analgesia. The possible role of the other brain monoamines in the analgesic activity of morphine is also discussed.

The effects of selective lesioning of brain serotonin or catecholamine containing neurones on the anorectic activity of fenfluramine and amphetamine

Abstract The effects of fenfluramine and amphetamine on food intake were studied in midbrain raphe-lesioned or in 6-hydroxydopamine-treated rats. Lesion of the midbrain raphe, which lowers forebrain serotonin, antagonized the anorectic effect of fenfluramine but did not modify the action of amphetamine. An intraventricular injection of 6-hydroxydopamine, which produced a marked decrease of brain noradrenaline and dopamine, did not affect the actions of either amphetamine or fenfluramine. A possible role of brain serotonin and catecholamines on anorexia induced by amphetamine and fenfluramine is discussed.

8-OH-DPAT impairs spatial but not visual learning in a water maze by stimulating 5-HT1A receptors in the hippocampus

The effect of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, on spatial and non-spatial learning in a water maze was studied using two tasks of equal difficulty, with the same motor, motivational and reinforcement demands. Rats were examined for choice accuracy in a two-platform spatial discrimination task. Rats treated subcutaneously with 100 micrograms/kg 8-OH-DPAT were impaired in choice accuracy with no effect on latency. Treated rats made more errors of omission than controls only on days 1 and 2 of training. Infusion of 1 microgram/microliter spiroxatrine (SPX) or 5 micrograms/microliters of (+)WAY100135, two potent 5-HT1A receptor antagonists, in the CA1 region of the dorsal hippocampus antagonized the impairment in choice accuracy caused by 8-OH-DPAT. The effect on errors of omission on days 1 and 2 of training were not significantly modified by spiroxatrine or (+)WAY100135. Rats treated with 8-OH-DPAT were not impaired in their ability to learn a visual discrimination in a water maze. The results suggest that stimulation of 5-HT1A receptors in the CA1 region of the dorsal hippocampus impairs spatial but not visual discrimination in rats.

Cholinergic-dopaminergic interaction in the striatum: The effect of 6-hydroxydopamine or pimozide treatment on the increased striatal acetylcholine levels induced by apomorphine, piribedil andD-amphetamine

Apomorphine (1 and 2 mg/kg), piribedil (15 and 60 mg/kg) and d-amphetamine (5 and 10 mg/kg) increased rat striatal acetylcholine levels without affecting choline. Pretreatment with pimozide (0.5 mg/kg) completely antagonized the effect of apomorphine and piribedil and by itself markedly decreased striatal acetylcholine levels. d-Amphetamine signigicantly antagonized the effect of pimozide. Nine days after pretreatment with 6-hydroxydopamine plus pargyline, striatal dopamine was decreased by 78% while acetylcholine and choline levels remained unaltered. Under these conditions, the effect of d-amphetamine was completely abolished while apomorphine and piribedil were just as active as in the vehicle-treated group. The results suggest that d-amphetamine acted indirectly to increase striatal acetylcholine levels probably through the release of dopamine and/or noradrenaline, while apomorphine and piribedil acted directly at dopamine receptor sites.

Evidence of an interaction between serotoninergic and cholinergic neurons in the corpus striatum and hippocampus of the rat brain.

Abstract The existence of an interaction between serotoninergic and cholinergic neurons in the brain has been investigated by studying the effects of quipazine and d -fenfluramine on regional brain acetylcholine in various experimental conditions. Quipazine, at a dose of 10 mg/kg, i.p., significantly increased the levels of acetylcholine in the striatum and hippocampus but not in the telencephalon and brain stem. The striatal increase was not significantly modified by electrolytic lesions placed in the midbrain raphe nuclei, an important site of origin of serotonin-containing neurons in the brain. On the other hand, pretreatment with serotonin antagonists such as methergoline and cinanserin or with parachlorophenylalanine, a serotonin synthesis blocker, prevented the increase of striatal acetylcholine induced by quipazine. Impairment of nigrostriatal dopaminergic mechanisms by local application of 6-hydroxydopamine or by pretreatment with alpha-methylparatyrosine did not modify the effect of quipazine on acetylcholine. The quipazine-induced increase in hippocampal acetylcholine was instead completely blocked by an electrolytic lesion of the nucleus medianus raphe. d -Fenfluramine also significantly increased striatal acetycholine, this effect being completely prevented by parachlorophenylalanine pretreatment. These findings are compatible with the hypothesis that serotoninergic neurons originating in the raphe nuclei may normally serve to inhibit cholinergic neurons in two areas of the rat brain, i.e. the corpus striatum and the hippocampus.

Effect of midbrain raphe lesion on the antinociceptive action of morphine and other analgesics in rats.

The antinociceptive action of several analgesics was studied by two methods: the hot-plate and the tail compression tests.Lesions of the midbrain raphe, which produce a marked depletion of serotonin in the forebrain, antagonize the analgesic effect of morphine but not that of methadone, meperidine, codeine and propoxyphene.It is concluded that the serotonin involvement suggested for the analgesic action of morphine cannot be generalized to other analgesics.

(S)-WAY 100135, a 5-HT1A receptor antagonist, prevents the impairment of spatial learning caused by intrahippocampal scopolamine

Scopolamine, 3.75 micrograms/microliters infused bilaterally into the CA1 region of the dorsal hippocampus 10 min before each training session, impaired choice accuracy but had no effect on choice latency or errors of omission in rats trained in a two-platform spatial discrimination task. Administered subcutaneously at 3 and 10 mg/kg 30 min before each training session, N-tert-butyl-3-4-(2-methoxyphenyl)piperazin-1-yl-2-phenylpropanami de dihydrochloride ((S)-WAY 100135), a 5-HT1A receptor antagonist, prevented the impairment of choice accuracy induced by intrahippocampal scopolamine. No subcutaneous dose of (S)-WAY 100135 by itself modified the acquisition of spatial learning. Administered into the dorsal hippocampus 15 min before each training session, (S)-WAY 100135 at doses of 0.2, 1 and 5 micrograms/microliters did not affect the acquisition of spatial learning but dose dependently prevented the impairment of choice accuracy caused by scopolamine, 3.75 micrograms/microliters infused into the same area. These findings suggest that blockade of 5-HT1A receptors can compensate the loss of cholinergic excitatory input on pyramidal cells, probably by favouring the action of other excitatory transmitters.

Increased expression of GAP-43, somatostatin and neuropeptide Y mRNA in the hippocampus during development of hippocampal kindling in rats.

The expression and distribution of the mRNA coding for the growth‐associated protein‐43 (GAP‐43), a putative marker for neuritic growth, for preprosomatostatin and the preproneuropeptide Y (ppNPY) were analysed in the rat hippocampus during the development of hippocampal kindling by an in situ hybridization technique and computer‐assisted grain counting in the cell. The levels of GAP‐43 mRNA increased significantly in the CA3 pyramidal neurons and hilar polymorphic neurons of the dentate gyrus 2 days after stage 2 of kindling (preconvulsive stage) but not stage 5 (full seizure expression) in the stimulated hippocampus. The distribution of GAP‐43 mRNA was the same in the hippocampus of kindled rats as in sham‐stimulated animals. Preprosomatostatin mRNA and ppNPY mRNA contents rose significantly in the hilar polymorphic neurons of the dentate gyrus of the stimulated and contralateral hippocampus at both stages of kindling, with the greatest effect at stage 5. In addition, the number of ppNPY mRNA neurons in the fascia dentata was significantly higher in kindled rats than in controls, but there were no differences in the number of preprosomatostatin mRNA‐positive cells. Preprosomatostatin and ppNPY mRNAs were also increased in the neurons of the stratum oriens of the CA1 ‐ CA3 subfield of fully kindled animals, whereas at stage 2 only neurons of the CA1 stratum oriens showed a significant increase of preprosomatostatin mRNA. No changes in preprosomatostatin and ppNPY mRNA expression were observed in the various regions of the hippocampus after a single afterdischarge or 1 month after stage 5. These data show that synthesis of somatostatin and neuropeptide Y increases in certain neurons of the hippocampus during the development of hippocampal kindling, and support the suggestion that these peptides are involved in epileptogenesis. Moreover, the increased synthesis of GAP‐43 may contribute to the synaptic remodelling of certain hippocampal neurons during kindling.

Electrical kindling of the hippocampus is associated with functional activation of neuropeptide Y-containing neurons

The release of neuropeptide Y (NPY) was measured from hippocampal slices of rats at stage 2 (preconvulsive stage) and stage 5 (full seizure expression) of electrical kindling of the dorsal hippocampus (upper blade of the dentate gyrus). Spontaneous release in naive rats (9.0 ± 0.8 fmol/ml every 10 min) was independent of external Ca2+ but was reduced by 38 ± 3.6% (P < 0.05) during 20 min incubation with 5μM tetrodotoxin. Spontaneous efflux in naive rats did not differ from that in shams (implanted with electrodes but not stimulated) or in rats kindled to stage 2 and stage 5. Twenty‐five, 50 and 100 mM KCl induced a concentration‐dependent release of NPY (P < 0.05 and P < 0.01 at 25 and 50–100 mM respectively) from slices of shams. The effect of 100 mM KCl was reduced by 94 ± 1% (P < 0.01) in the absence of Ca2+. Two days after the last stage 2 stimulation and 1 week after the last stage 5 seizure, NPY release was significantly larger than in shams at all KCl concentrations in the stimulated and contralateral hippocampus (P < 0.05 and P < 0.01). Forty‐eight hours after one single after‐discharge and 1 month after the last stage 5 seizure, 50 mM KCl induced a significantly larger release of NPY in the stimulated and contralateral hippocampus (P < 0.01 and P < 0.05), although the effect was less than during kindling. The tissue concentration of NPY increased significantly in both hippocampi at stage 2 and 1 week after stage 5 (2.6 times on average, P < 0.01) but no significant differences were found 1 month after stage 5. The present results provide the first evidence of enhanced neuronal release of NPY during kindling, suggesting that this neuropeptide may have a potential role in epileptogenesis.

WAY 100635, a 5-HT1A receptor antagonist, prevents the impairment of spatial learning caused by intrahippocampal administration of scopolamine or 7-chloro-kynurenic acid

The effect of WAY 100635, a 5-HT1A receptor antagonist, on the impairment of spatial learning caused by intrahippocampal administration of scopolamine, a cholinergic muscarinic receptor antagonist, or 7-chloro-kynurenic acid, an antagonist at the glycine site associated with the NMDA receptor complex, was studied in a two-platform spatial discrimination task. Scopolamine (4 microg/microl) or 7-chloro-kynurenic acid (3 microg/microl), administered bilaterally into the CA1 region of the dorsal hippocampus 10 min before each training session, impaired choice accuracy with no effect on choice latency and errors of omission. Administered subcutaneously at 1 (but not at 0.3) mg/kg 30 min before each training session, WAY 100635 did not modify the acquisition of spatial learning, but prevented the impairment of choice accuracy caused by intrahippocampal scopolamine or 7-chloro-kynurenic acid. These findings suggest that blockade of 5-HT1A receptors can compensate the loss of cholinergic or NMDA-mediated excitatory input to pyramidal cells in the hippocampus. The mechanisms involved and the importance of these findings for the symptomatic treatment of memory disorders in man are discussed.