Hypothalamic serotonin release and raised blood pressure after raphe nuclei stimulation in rats

Electrical stimulation of dorsal raphe (DR) nuclei, in addition to enhancing serotonin (5-HT) release in the hypothalamus, elicited proportional hypertension and tachycardia in anesthetized rats. This could be mimicked by microinjection of two excitatory amino acids, kainic acid and glutamate, into the DR of rat brain. Intrahypothalamic administration of DOI (a 5-HT2 agonist), but not 8-OH DPAT (a 5-HT1 agonist) or 2-methyl-serotonin (a 5-HT3 agonist), also produced both hypertension and tachycardia in rats. The DR stimulation-induced hypertension, tachycardia, or increased hypothalamic 5-HT release were attenuated by prior destruction of the ascending serotonergic system produced by ICV injection of 5,7-dihydroxytryptamine and by prior blockade of postsynaptic serotonergic receptors produced by intrahypothalamic injection of 5-HT2 antagonists, cyproheptadine and ketanserin. The DOI-induced hypertension and tachycardia were also reduced by prior blockade of 5-HT2 receptors with cyproheptadine or ketanserin. Thus, it appears that DR stimulation activates the 5-HT release in the hypothalamus, then activates the hypothalamic 5-HT2 receptors and results in both hypertension and tachycardia in rats.