α 2 -Adrenoceptor-ir neurons' density changes after single dose of clonidine and yohimbine administration in the hippocampus of male rat.

OBJECTIVE: Despite the important role of α2 -adrenoceptors in pain modulation processes, the impact of administration of α2 -adrenoceptor agonist and antagonist on the density of hippocampal α2 -adrenoceptor-immunoreactive neurons has not been investigated. Therefore, we aimed to determine the effect of single doses of clonidine and yohimbine on the density of α2 -adrenoceptor-immunoreactive neurons in rat hippocampus.

MATERIALS AND METHODS: Adult male Wistar rats received a single dose of clonidine (0.7 mg/kg) alone or 5 min after intraperitoneal (1 mg/kg) and/or intracerebroventricular (5 µg/kg) injection of yohimbine. After histological processing, neurons with α2 -adrenoceptor immunoreactivity were identified and counted through immunohistochemical analysis of hippocampal regions.

RESULTS: Clonidine slightly increased the number of α2 -adrenoceptor-immunoreactive neurons in the hippocampal subregions compared with the normal saline group. Intraperitoneal injection of yohimbine followed by injection of clonidine significantly increased the number of α2 -adrenoceptor-immunoreactive neurons in subregions cornu ammonis 1 (CA1) and cornu ammonis 3 (CA3). Intracerebroventricular injection of yohimbine after injection of clonidine significantly reduced the number of α2 -adrenoceptor-immunoreactive neurons in all hippocampal subregions.

CONCLUSION: The present study demonstrates that intraperitoneal administration of α2 -adrenoceptor agonist clonidine increases the density of α2 -adrenoceptor-immunoreactive neurons in rat hippocampus, while intracerebroventricular injection of yohimbine decreases the density of these neurons.