NEUROENDOCRINE CONTROL OF IMMUNOREACTIVE GROWTH-HORMONE AND BIOACTIVEPROLACTIN SECRETION IN NEONATAL RATS - ONTOGENY AND INTERACTIONS BETWEEN THE SEROTONERGIC, CHOLINERGIC AND ALPHA-2-ADRENERGIC SYSTEMS

The effects of the alpha2-agonist clonidine (CLO), the serotonin (5-HT ) precursor 5-hydroxy-L-tryptophan (5-HTP), the 5-HT2/histamine (H-1) antagonist cyproheptadine (CYPRO), the muscarinic cholinergic antagoni st atropine (ATR), and an affinity-purified polyclonal anti-rat growth hormone-releasing hormone (rGHRH) immunoglobulin on serum concentrati ons of growth hormone (GH) and prolactin (PRL) were tested in 2- and 1 0-day-old litter-mate rat pups. Serum levels of GH and PRL were detect ed in RIA and Nb2 lymphoma bioassay, respectively. The effects of two different drugs either alone or in combination with each other were ev aluated by two-factor analysis of variance. The data indicated that se cretion of GH and PRL was regulated by alpha2-adrenergic, serotonergic and cholinergic mechanisms; the pathways regulating the two hormones, however, were distinct. 5-HTP stimulated GH secretion as early as day 2 postpartum via cholinergic mechanisms not involving GHRH; this path way was also present in 10-day-old pups. An additional serotonergic pa thway was functional in 10-day-old pups which mediated CLO-induced rel ease of GH, and did not include cholinergic transmission. The alpha2-a drenergic regulation of GH secretion appeared to involve three distinc t mechanisms: (1) a sexually uniform GH-stimulating alpha2-adrenergic pathway was demonstrated with CLO in 2-day-old pups only after pretrea tment with ATR; (2) a sexually dimorphic CLO-induced secretion of GH w as observed that was mediated by mechanisms sensitive to CYPRO but not to ATR, and occurred by day 10; and (3) 5-HTP-induced GH secretion wa s counteracted by CLO in 10-day-old pups of both sexes indicating that a sexually uniform GH-inhibiting alpha2-adrenergic pathway was presen t. The concentration of PRL was not affected by 5-HTP up to day 10, an d was decreased by ATR in 10-day-old (but not in 2-day-old) pups. Secr etion of GH and PRL appeared to be stimulated by different sets of cho linergic neurons because (1) ATR inhibited GH secretion on day 2 but i nhibition of PRL secretion appeared later, and (2) CLO-induced PRL sec retion was diminished by ATR, whereas CLO-induced release of GH was no t affected in 10-day-old pups. CYPRO increased serum levels of PRL in 10-day-old pups; thus, PRL-inhibiting network was functional at this a ge. Whether the inhibition of this pathway was due to the serotonin, h istamine (H-1) or dopamine antagonist action of CYPRO requires further investigation.