Exogenous opioids increase plasma prolactin in Holstein calves primarily via a dopaminergic mechanism.
A study was conducted to determine whether exogenous opioids increase prolactin (PRL) secretion in Holstein heifer calves via a dopaminergic mechanism. Twenty-four Holstein heifer calves ranging in age from 5 to 7 mo were assigned to one of four treatment groups (six/treatment): 1) injection of saline (SAL); 2) injection of a synthetic enkephalin (D-Ala2, N-Me-Phe4, Met(O)5-ol enkephalin; DAMME); 3) injection of DAMME after pretreatment with the long-acting dopamine agonist 2-bromo-alpha-ergocryptine; or 4) injection of thyrotropin-releasing hormone (TRH) after pretreatment with 2-bromo-alpha-ergocryptine. Calves were equipped with indwelling jugular cannulas on d 1, and baseline plasma PRL concentrations were established. Animals receiving 2-bromo-alpha-ergocryptine were injected s.c. 3 h after the last baseline sample was drawn on d 1. On d 2, calves assigned to receive SAL, DAMME, or TRH were injected 2 h after the start of sampling, and sampling was continued for an additional 4.5 h. Basal plasma PRL was lower (P less than .01) on d 2 in calves injected with 2-bromo-alpha-ergocryptine than baseline levels on d 1. Plasma PRL was higher (P less than .01) in calves not pretreated with 2-bromo-alpha-ergocryptine after DAMME injection on d 2 but was not different after DAMME injection in calves pretreated with 2-bromo-alpha-ergocryptine. In contrast, plasma PRL increased (P less than .01) after TRH injection on d 2 in calves pretreated with 2-bromo-alpha-ergocryptine.(ABSTRACT TRUNCATED AT 250 WORDS)
Johnson
D W
DW
Virginia Polytechnic Institute and State University, Blacksburg 24061.
Barnes
M A
MA
Akers
R M
RM
Pearson
R E
RE
D000818:Animals; D001971:Bromocriptine / Q000494:pharmacology; D002417:Cattle / Q000097:blood / Q000378:metabolism*; D005405:D-Ala(2),MePhe(4),Met(0)-ol-enkephalin / Q000494:pharmacology; D004298:Dopamine / Q000502:physiology*; D004723:Endorphins / Q000494:pharmacology*; D005260:Female; D011388:Prolactin / Q000097:blood*; D011897:Random Allocation; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology
D004723:Endorphins; D001971:Bromocriptine; D013973:Thyrotropin-Releasing Hormone; D005405:D-Ala(2),MePhe(4),Met(0)-ol-enkephalin; D011388:Prolactin; D004298:Dopamine