Mediation by serotonin of the antiaversive effect of zimelidine and propranolol injected into the dorsal midbrain central grey

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Abstract
Previously reported results indicate that serotonin (5-HT) inhibits the neural sub strate of aversion in the dorsal midbrain central grey (DCG) of the rat. In addition, the present results show that microinjection of the 5-HT uptake inhibitor zimelidine (100 nmol) into the DCG of rats with chronically implanted chemitrodes raised the threshold of aversive electrical stimulation. This antiaversive effect of zimelidine was antagonized by pretreatment with the 5-HT2 receptor blocker ritanserin (10 nmol), also microinjected into the DCG. In contrast, the antiaversive effect of the benzodiazepine agonist midazolam (40 nmol) was unaffected by ritanserin. Propranolol (2.2, 4.4 and 8.8 nmol) raised the aversive threshold in a dose-depen dent way following its injection into the DCG. The antiaversive effect of 4.4 nmol of propranolol was antagonized by previous administration of ritanserin (10 nmol). Moreover, combined administration of zimelidine (100 nmol) followed by propranolol (4.4 nmol) caused an anti aversive effect which was equivalent to the sum of the effect of each drug alone. These results indicate that the antiaversive effect of intracerebrally injected zimelidine and propranolol is mediated by endogenous 5-HT, through activation of 5-HT2 receptors.