The involvement of nitric oxide in the secretion of β-endorphin from the pituitary intermediate lobe of the rat

Nitric oxide (NO) generated by the enzyme nitric oxide synthase (NOS) has been implicated in the regulation of a variety of endocrine functions. A number of biochemical and anatomical studies have demonstrated the presence of neuronal NOS (nNOS) in the neuroendocrine axis and have shown significant effects of NO on the release of hypothalamic and pituitary hormones. Using a C-terminal directed peptide antibody that is specific for nNOS we have found a predominance of nNOS in the neural lobe of the pituitary and in a single layer of epithelial cells, possibly a remnant of Rathke's pouch that form a border between the intermediate lobe and the anterior lobe. Furthermore, we have examined the effect of sodium nitroprusside (SNP), a donor of NO on the secretion of β-endorphin (β-EP) from the isolated neuro-intermediate lobe (NIL) of the rat and cultured rat melanotrophs. It was shown that in explant cultures of intact neuro-intermediate lobes, SNP (100 μM) was able to cause an inhibition of β-EP secretion. In the presence of sulpiride (10 μM), a dopamine D₂-receptor antagonist, there was a partial reversal of the SNP effect. On the other hand SNP did not affect β-EP secretion in primary cultures of melanotrophs that no longer possessed any innervation. Taken together these data suggest that NO has an indirect inhibitory effect on the secretion of β-EP by the intermediate lobe via the release of dopamine.