Effects of photoperiod on lactotrophs and on dopaminergic and 5-hydroxytryptaminergic neurones in bull calves.
This study was conducted to determine whether photoperiod-induced changes in serum concentrations of prolactin in cattle were associated with changes in activity of dopamine or 5-hydroxytryptamine (5-HT) neurones in the infundibulum/pituitary stalk and the secretion rate and number of lactotrophs in the anterior pituitary gland. Sixteen prepubertal bull calves (approximately 8 weeks of age) were divided into two groups. One group of eight was maintained on a photoperiod of 8 h light: 16 h darkness (8L:16D) and the other group was exposed to 16L:8D for 4 weeks. At this time calves were injected with a decarboxylase inhibitor (m-hydroxybenzylhydrazine dihydrochloride, NSD 1015) which blocks the conversion of dihydroxyphenylalanine (DOPA) to dopamine and of 5-hydroxytryptophan (5-HTP) to 5-HT. Calves were killed with pentobarbital 15 min later. Accumulations of DOPA and 5-HTP in selected brain regions were used as indices of activity of dopamine and 5-HT neurones respectively. Secretory rate and number of prolactin-secreting lactotrophs were determined by reverse haemolytic plaque assay. Relative to calves exposed to 8L:16D, exposure to 16L:8D increased serum concentrations of prolactin by eightfold, anterior pituitary gland weight by 23%, release of prolactin from pituitary explants by 57% and the area of the plaque for prolactin-secreting lactotrophs by 70%. There was no difference in the rates of accumulation of DOPA and 5-HTP in the infundibulum/pituitary stalk of animals exposed to 4 weeks of 16L:8D or 8L:16D.(ABSTRACT TRUNCATED AT 250 WORDS)
Zinn
S A
SA
Department of Animal Science, Michigan State University, East Lansing 48824.
Chapin
L T
LT
Lookingland
K J
KJ
Moore
K E
KE
Tucker
H A
HA
D000818:Animals; D002417:Cattle; D004298:Dopamine / Q000502:physiology*; D006834:Hydrazines / Q000494:pharmacology; D008027:Light; D008297:Male; D009474:Neurons / Q000502:physiology*; D010902:Pituitary Gland / Q000502:physiology*; D010903:Pituitary Gland, Anterior / Q000378:metabolism; D011388:Prolactin / Q000096:biosynthesis / Q000097:blood*; D012701:Serotonin / Q000502:physiology*
D006834:Hydrazines; D012701:Serotonin; D011388:Prolactin; C004297:3-hydroxybenzylhydrazine; D004298:Dopamine