Central alpha 2-adrenergic stimulation increases neurointermediate lobe immunoreactive beta-endorphin in...

A possible influence of the central alpha 2-adrenergic system on beta-endorphin was examined in rat anterior pituitary, neurointermediate lobe, and plasma. The concentration of beta-endorphin in anterior pituitary, neurointermediate lobe, and plasma was determined by radioimmunoassay 15 minutes after subcutaneous injection of clonidine in 14-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Clonidine reduced the concentration of the plasma beta-endorphinlike immunoreactivity in SHR and to a lesser extent in WKY. No significant changes in the concentration of beta-endorphinlike immunoreactivity were observed in anterior pituitary. Clonidine increased the concentration of neurointermediate lobe beta-endorphinlike immunoreactivity in SHR in a dose-related manner but did not affect the concentration in WKY. Administration of yohimbine (1 mg/kg) completely blocked the clonidine-induced increase of neurointermediate lobe beta-endorphinlike immunoreactivity in SHR, while prazosin (1 mg/kg) had no effect. These data suggest that the central alpha 2-adrenergic activation increases the neurointermediate lobe concentration of beta-endorphinlike immunoreactivity in SHR by suppressing beta-endorphin release from the neurointermediate lobe into the circulation.