Central catecholamine depletion: effects on physiological growth hormone and prolactin secretion

The effects of depletion of central nervous system (CNS) catecholamines (CA) upon growth hormone (GH) and prolactin (Prl) secretion were examined in unanaesthetized male rats. Animals were chronically implanted with cannulae for blood sampling and for intracerebroventricular (i.c.v.) injections. GH and Prl secretory profiles were obtained by radioimmunoassay of blood samples drawn every 15 min for a 6-hour period. Hormone profiles were obtained before, 1 day (day 1) and 7 days (day 7) after i.c.v. injection of 6-hydroxydopamine (6-OHDA, total dose 600 microgram). GH secretion on day 1 was almost completely abolished (mean GH concentrations 25.2 +/- 10.1 vs. baseline 177.2 +/- 34.1 ng/ml), but secretory patterns tended to return towards normal by day 7. Prl secretion was markedly increased at day 1 (18.8 +/- 5.1 vs. 6.7 +/- 1.9 ng/ml), but levels had fallen by day 7. Treatment of animals at day 8 with the norepinephrine antagonist phentolamine (7.5 mg/kg i.v.) suppressed GH secretion, but stimulated Prl secretion. The administration of the dopamine antagonist butaclamol (1 mg/kg i.v.) on day 9 had no effect on Gh secretion, but stimulated Prl release. Histofluorescence examination of tissue sections confirmed extensive disruption of CA structures throughout the CNS. These findings indicate that central CA neurons are facilitatory to GH secretory rhythms, but inhibitory to Prl secretion. Furthermore, it is apparent that the effects of i.c.v. 6-OHDA treatment are transient, recovery being due to re-establishment of functional CA mechanisms.