Effects of hypothalamic lesions and drugs interfering with dopaminergic transmission on pituitary MSH content of rats

The physiological significance of the dopaminergic innervation of the pars intermedia was studied in rats. Electrothermic lesions were made in the mediobasal hypothalamus in order to destroy all hypothalamo-hypophysial connections. Pituitary MSH content decreased to 25% of control values 8 h after lesioning. From the 1st day, however, pituitary MSH content gradually increased, reaching control levels after about 1 week. Haloperidol and pimozide (both neuroleptics) were used to block specifically dopamine (DA)-receptors. Single administration of these drugs induced a time and dose dependent reduction in pituitary MSH. A maximal decrease to about 60% was found 4-5 h after i.p. administration of 2.5 mg/kg haloperidol and 1.0 mg/kg pimozide. Apomorphine and 2-Br-alpha-ergocryptine (DA-receptor stimulating drugs), had no effect on pituitary MSH stores of intact rats, but completely prevented the lesion-induced fall in pituitary MSH content. The results show that DA-receptors located within the pituitary itself are involved in the control of MSH release indicating that the effect of hypothalamic lesions on pituitary MSH content is primarily caused by interruption of dopaminergic neurotransmission in the hypophysis. We conclude therefore that the dopaminergic arcuato-hypophysial neurones innervating the pars intermedia tonically inhibit the release of MSH in intact rats.