Dopaminergic A14/A15 neurons are activated during estradiol negative feedback in anestrous, but not breeding season, ewes.

Abstract

A major factor responsible for seasonal anestrus in sheep is a striking increase in the ability of estradiol (E) to inhibit pulsatile GnRH and LH secretion. Previous studies suggest that dopaminergic neurons in the A14 and A15 groups of the ovine hypothalamus play a key role in conveying the inhibitory effects of E in anestrous ewes. The present study tested the hypothesis that A14/A15 neurons in anestrous ewes are activated in response to E, and that this activation is specifically related to seasonal changes in E negative feedback. Expression of the immediate early gene products, Fos and the Fos-related antigens (FRAs), was used as a marker of neuronal activation. Ovariectomized anestrous ewes received either blank implants (no E) or 0.5-cm long E implants sc and were killed 6 h later (E+6h) or 7 days later (E+7d and no E groups). During the breeding season, two additional groups of ovariectomized ewes were perfused 7 days after insertion of either blank or E implants. During anestrus, E completely suppressed LH pulses in the E+7d group, but had no effect in the E+6h group. In the E+7d anestrous group, there was also a significant increase in the mean percentage of tyrosine hydroxylase (TH)-positive cells that expressed nuclear Fos/FRAs in A14 and A15 areas compared to that in either the no E or E+6h group. By contrast, during the breeding season, E had no effect on LH pulse frequency, and there were relatively few TH-positive cells in A14 and A15 that coexpressed Fos/FRAs in either the no E or E+7d group. No significant steroidal or seasonal differences in Fos/FRA expression were seen in other hypothalamic dopaminergic cell groups (A12 and A13) or in the preoptic area-anterior hypothalamus or suprachiasmatic nucleus. Furthermore, E did not alter the total number of TH-positive neurons in A14/A15 or other cell groups. There were seasonal differences in the number of TH-positive neurons, with a significantly greater number of cells in the A13 and A15 of breeding season animals compared to anestrous ewes. Thus, E increased Fos/FRA expression in A14/A15 neurons only during anestrus at a time when it also inhibited LH pulse frequency. These findings are consistent with the view that activation of dopaminergic cells in A14 and A15 is a critical link in the chain of events leading to seasonal shifts in sensitivity to E negative feedback in the ewe.

Multimodal Influence of Estrogen upon Gonadotropin-Releasing Hormone Neurons

“…Dopamine D2 receptors are clearly involved in the estrogen-dependent suppression of LH pulse frequency in anestrous ewes (209 -212), and lesion studies suggest that this involves the A14 and/or A15 dopaminergic cell populations of the anterior hypothalamus (213,214). Furthermore, recent investigations have shown that both A14 and A15 cell populations are activated by estrogen during anestrus but not during the breeding season (215,216). Although it is known that the season-dependent inhibition of GnRH secretion by dopamine occurs at the level of the median eminence (199,217,218), the relationship of the A14 and A15 dopamine neurons to the relevant dopaminergic terminals in the median eminence remains obscure (216,219).…”

Does nitric oxide act in the ventromedial preoptic area to mediate oestrogen negative feedback in the seasonally anoestrous ewe?

“…The complete neural pathway by which the response to oestrogen negative feedback changes with season is not yet established, although a considerable body of evidence has implicated two cell groups: 1) ERacontaining neurones in the ventromedial preoptic area (vmPOA) near the organum vasculosum laminae terminalis (OVLT; Stefanovic et al 2000, Anderson et al 2001 and 2) dopaminergic cells of the A15 cell group located within retrochiasmatic area (RCh; Meyer & Goodman 1985, Thiery et al 1989, Havern et al 1994, Lehman et al 1996. Because A15 neurones do not contain ERs (Lehman & Karsch 1993, Skinner & Herbison 1997, oestrogen likely influences A15 neurones through afferent inputs from areas such as the vmPOA.…”

Estradiol Increases Multiunit Electrical Activity in the A15 Area of Ewes Exposed to Inhibitory Photoperiods1

“…Specifically, E 2 treatment during LD increased the in vivo bioactivity of tyrosine hydroxylase (TH) in the A15 region [11] and induced the expression of the early immediate gene product, Fos, in A15 DA perikarya [12]. During SD, the same E 2 treatment failed to increase Fos expression in these neurons [12], suggesting that there may be seasonal differences in their response to E 2 . In contrast, E 2 increased TH mRNA levels during SD but not during LD [13].…”