Stimulation of beta-endorphin and beta-lipotropin release from the anterior but not the neurointermediate pituitary lobe in the rat after acute administration of serotonin-acting drugs

The respective contribution of the anterior (AP) and the neuro-intermediate (NIL) lobes of the pituitary gland to changes occuring in plasma β-endorphin (β-EP) and β-lipotropin (β-LPH) titers has been evaluated in the rat after administration of serotonin (5-HT)-acting drugs. β-EP-like immunoreactivity (β-EP-LI) was concurrently evaluated in the mediobasal hypothalamus (MBH). The administration of 50 mg/kg DL 5-hydorxytryptophan (5-HTP) or 12.5 mg/kg fluvoxamine, a 5-HT reuptake blocker, decreased markedly β-EP-LI in the AP and induced a striking rise in plasma β-EP and β-LPH concentrations. Combined administration of fluvoxamine and 5-HTP failed to potentiate the effect of individual treatments. Similarly, administration of 5.0 and 10 mg/kg quipazine, a 5-HT receptor agonist, evoked a marked decrease in β-EP-LI in the AP and a concomitant rise in β-EP and β-LPH concentrations in the plasma, while administration of 1.0 and 5 mg/kg of chlorophenylpiperazine, a weak 5-HT stimulant drug, did not alter the above indices. None of these treatments altered significantly β-EP-LI in the NIL and only the higher dose of quipazine increased it in the MBH. We conclude that brain serotonin neurons exert a stimulatory influence on β-EP and β-LPH release from the AP but, likely, not from the NIL and that hypothalamic endorphins are not implicated in the secretory events occuring at AP level after acute activation of 5-HT neurotransmission.