Fos Expression in Luteinizing Hormone-Releasing Hormone Neurons of Guinea Pigs, with Knife Cuts Separating the Preoptic Area and the Hypothalamus, Demonstrating Luteinizing Hormone Surges1

Abstract

LHRH neurons in guinea pigs, as in primates and other non-rodent species, are broadly distributed in the basal forebrain. In this study, knife cuts were made in the anterior hypothalamus, effectively separating more caudally positioned hypothalamic LHRH neurons from those in rostral preoptic areas. Guinea pigs with knife cuts displayed an LH surge in response to steroid administration. There was no significant difference in the number of LHRH neurons that expressed Fos in conjunction with an LH surge, although fewer total LHRH neurons were detected in the forebrain of knife-cut versus sham-cut animals. Knife-cut animals displayed a larger percentage of LHRH/Fos neurons in one region of the caudal hypothalamus than sham-cut animals. The area and perimeter of the LHRH reaction product within the cytoplasm of LHRH/Fos neurons were smaller than those of single-labeled LHRH neurons in sham-cut animals and in the caudal hypothalamus, but not the rostral preoptic area, of knife-cut animals. We conclude that caudal hypothalamic LHRH neurons separated from rostral preoptic regions are capable of sustaining an LH surge in guinea pigs. This finding is important, as LHRH neurons are present in the caudal hypothalamus, as well as in preoptic areas, of a large number of mammalian species, including humans.

Kisspeptin expression in guinea pig hypothalamus: Effects of 17β‐estradiol

“…In the guinea pig, positive feedback regulation of LH secretion can be mediated by the BH (Terasawa and Wiegand, 1978; King et al, 1998), which argues for the involvement of Arc kisspeptin neurons in positive feedback regulation of LH (GnRH) by E2. Therefore, we examined the effects of E2 on Kiss1 mRNA expression by in situ hybridization in our guinea pig model of negative and positive feedback at 24 hours and 42 hours, respectively.…”

“…These findings are consistent with data showing that the AVPV is necessary for E2 positive feedback on GnRH and luteinizing hormone (LH) secretion in these species (Wiegand et al, 1980; Petersen and Barraclough, 1989; Ma et al, 1990). In other species such as the guinea pig, sheep, and rhesus monkey the POA appears not to be the main region responsible for E2 positive feedback (Terasawa and Wiegand, 1978; Plant et al, 1978; Weick, 1981; King et al, 1998; Caraty et al, 1998). Thus, it appears that the basal hypothalamus may be sufficient for maintaining positive feedback regulation of GnRH and LH secretion in these species.…”

Kisspeptin Neurons from Mice to Men: Similarities and Differences

“…In guinea pigs, expression data in response to E 2 treatment suggest that the positive feedback actions may occur in both the POA and ARC populations (78), whereas in monkeys one report suggest an increase in Kiss1 mRNA/cell late in the follicular phase in both ARC and POA (49), but the former was not evident when mRNA was assessed by RT-PCR (108). It should also be noted that if kisspeptin neurons are required for the preovulatory surge in guinea pigs and monkeys, the ARC population is most likely involved because knife cuts around the MBH do not block the positive-feedback action of E 2 in these species (114,115).…”

Neuroendocrine Control of Gonadotropin Secretion

“…Thus, knife cuts that partially, or completely, disrupt input to the MBH have little or no effect on tonic gonadotropin secretion in intact or OVX rats, [157][158][159] sheep, 160,161 monkeys, 162 or guinea pigs. 163 Some rats with complete cuts around the MBH show constant diestrous vaginal smears indicating low circulating E 2 and possibly inadequate tonic gonadotropin secretion. In one study, 157 these rats did not respond to OVX with an increase in LH secretion, but in another study 158 they did; in any case, no obvious histological differences in the location of the knife cuts were evident between rats showing constant diestrus and those with similar knife cuts showing constant estrus smears, indicating elevated E 2 secretion.…”

“…The most dramatic of these is between rats and monkeys: in rats, knife cuts (either complete or partial) between the POA and MBH completely block the estrogen-induced LH surge 240 and ovulation ( Figure 33.3), [35][36][37] while in monkeys similar cuts have no effect on menstrual cycles or the ability of estrogen to induce an LH surge. 162 In sheep 160,161 and guinea pigs, 163 the effects of these knife cuts are intermediate between these two extremes in that they do not completely block the estrogen-induced LH surge, but suppress its amplitude (Figure 33.8).…”