Effects of naloxone and neonatal treatment with monosodium-l-glutamate on growth hormone and prolactin release induced by electrical stimulation of the medial-basal hypothalamus in rats

Abstract

The role of nerve cells of the arcuate nucleus and endogenous opioid peptides in the regulation of GH and prolactin secretion has been investigated. Electrical stimulation of the medial-basal hypothalamus (MBH) for 10 min raised plasma levels of both hormones in male rats anaesthetized with pentobarbitone sodium. Plasma hormone levels increased within 5 min after the termination of the stimulus, while no marked changes were found during stimulation. The GH response to the electrical stimulus was substantially reduced in rats with arcuate lesions induced by neonatal treatment with monosodium-L-glutamate (MSG). By contrast, the size of the prolactin response was not altered by MSG treatment. The opiate receptor antagonist naloxone (10 mg/kg, i.v.) failed to influence GH secretion induced by electrical stimulation in either control or MSG-treated animals. The post-stimulus rise of plasma prolactin levels was attenuated by naloxone in control rats, while the same dose of the drug was ineffective in rats which had been exposed to MSG. We conclude that endogenous opioids participate in the increase of prolactin release upon electrical stimulation of the MBH but are not involved in the GH secretory response. Arcuate neurones are important in the maintenance of the GH response to electrical stimulation. By contrast, lesioning of the arcuate nucleus failed to affect the prolactin secretory response elicited by MBH stimulation. However, prolactin release in MSG-treated rats appeared less susceptible to the inhibitory action of naloxone, suggesting a possible supersensitivity towards endogenous opioids.