Gonadotroph-lactotroph associations and expression of prolactin receptors in the equine pituitary gland throughout the seasonal reproductive cycle

An interaction between gonadotroph and lactotroph cells of the pituitary gl and has long been recognized in several species. The current study was cond ucted to investigate whether an association between gonadotrophs and lactot rophs occurs in mares and whether prolactin receptors are expressed within the pituitary gland of this species. The effects of both reproductive state and season on these variables were examined in pituitary glands obtained f rom sexually active mares in July (breeding season), sexually active mares in November (non-breeding season) and anoestrous mares in November. Pituita ries were dissected out immediately after death and immunofluorescent stain ing was carried out on 6 mu m sections using specific antibodies to the LH beta subunit, FSH beta subunit, prolactin and prolactin receptor. Gonadotro phs were observed in both the pars distalis and pars tuberalis; although th ey appeared mostly as isolated cells, small groups of gonadotrophs were als o identified in the pars distalis. In contrast, lactotrophs were observed o nly as clusters of cells exclusively in the pars distalis of sexually activ e and anoestrous mares in November and in most of the sexually active mares in July. A specific gonadotroph-lactotroph association was identified only between large isolated gonadotrophs and lactotroph clusters. Double immuno fluorescent staining for FSH beta and prolactin revealed a similar gonadotr oph-lactotroph association to the one detected for LH gonadotrophs. No stat istical difference in the gonadotroph:lactotroph ratio was observed as a re sult of changes in reproductive status or season. However, a tendency for a simultaneous decrease in the number of gonadotrophs and an increase in the number of lactotrophs was detected in anoestrous animals. Prolactin recept or immunoreactivity was found in the pars distalis, but not in the pars tub eralis, of sexually active (July and November) and anoestrous animals for b oth long and short forms of the receptor. No prolactin receptor co-localiza tion for either form of the receptor was observed in LH or FSH gonadotrophs in either of the reproductive states examined during both summer and winte r seasons. Furthermore, no significant difference was apparent in the propo rtion of cells expressing prolactin receptors between mares of different re productive state or season. The specific anatomical association between gon adotroph and lactotroph cells and the expression of prolactin receptors in the equine pituitary gland indicate a potential role of prolactin in the re gulation of gonadotrophin secretion. However, the absence of evidence for c o-localization of prolactin receptors in LH or FSH cells does not support t he hypothesis of a direct effect of prolactin on the gonadotroph as reporte d in a short day breeder. The results raise the possibility that, in horses , an intermediate regulatory cell may mediate the action of prolactin on go nadotroph function.