A comparative analysis of the neuroendocrine mechanisms regulating ovulation, affected by a unilateral implant of atropine in the preoptic-anterior hypothalamic area, in intact and hemiovariectomized adult rats

Abstract

The effects were analysed of a unilateral implant of atropine on ovulation in intact and hemiovariectomized adult rats, together with the response of the atropine-implanted rats to hormone replacement. An outer cannula directed to the left or right preoptic (POA)-anterior-hypothalamic area (AHA) was implanted into cyclic adult rats. A group of animals in oestrus was hemiovariectomized and some were also implanted with a cannula. After two consecutive 4-day cycles, the hemiovariectomized animals were implanted with atropine (23 +/- 4 micrograms) or cholesterol (25 +/- 2 micrograms) on the day of oestrus. Atropine implanted into the left side of the POA-AHA blocked ovulation and compensatory ovarian hypertrophy, whilst implants in the right side had no effects. Administration of gonadotrophin-releasing hormone (GnRH; 3.7 micrograms/kg) at 13.00 h on the expected day of pro-oestrus induced ovulation in six out of seven treated animals. Of 19 rats with an implant of atropine in the left side of the POA-AHA, one ovulated after treatment with pregnant mare serum gonadotrophin (PMSG) on oestrus, or oestradiol benzoate or human chorionic gonadotrophin (hCG) on day 2 of dioestrus. The effects on ovulation of a unilateral implant of atropine into the POA-AHA of cyclic adult rats and the responses of such rats to GnRH, PMSG, hCG and oestradiol benzoate replacement were also studied. Ovulation was induced in rats with a unilateral implant of atropine and which had been treated with GnRH or hCG at 13.00 h on the expected day of pro-oestrus after the implant.(ABSTRACT TRUNCATED AT 250 WORDS)

The participation of the muscarinic receptors in the preoptic-anterior hypothalamic areas in the regulation of ovulation depends on the ovary

“…Classic cholinergic synapses have rarely been observed in GnRHergic neurons, suggesting the presence of a predominantly non-synaptic path in this communication of the cholinergic neuronal system [13]. The current results in rats with muscarinic temporal blockade in the right POA-AHA support previous hypothesis stating that muscarinic neuroendocrine mechanisms regulating ovulation are dependent on the neural information arising from the ovaries and reaching the POA-AHA [12]. It is also likely that a combination of two events is at play here: 1) the absence of estrogen α-receptor in POA-AHA secondary to the blockade of muscarinic receptors as previously shown [25]; and/or 2) a decrease in the number of high affinity sites in the POA and adenohypophysis as suggested [26] as a consequence of changes on hormonal input [23] caused by ULO.…”

“…Rats were randomly assigned to different groups ( n  = 8–10 animals per group) and microinjected as follows: 1) vehicle: saline solution 1 μl; 2) atropine, 62.5 ng/μl (Sigma-Aldrich, Mexico); and 3) intact: cyclically untreated rats. The dose of atropine was selected based on previous studies in our laboratory [6, 12]. All test solutions were injected at a rate of 1 μl/min.…”

“…Previous studies performed by our group showed that ULO rats with implants of atropine in the left POA-AHA during the estrus day did not ovulate, while the majority of rats with implants in the right POA-AHA did. Furthermore, the injection of EB restored ovulation in animals with atropine implants in the left POA-AHA [12]. These observations suggested that the cholinergic muscarinic system in the POA-AHA exerted an asymmetric regulating action on spontaneous ovulation through the modutation of GnRH secretion.…”

Muscarinic Receptors Types 1 and 2 in the Preoptic-Anterior Hypothalamic Areas Regulate Ovulation Unequally in the Rat Oestrous Cycle

“…Subsequently, a 29-gauge stainless steel needle was lowered into the left or right side of the POA-AHA. The POA-AHA was located using the bregma coordinates from the atlas as the reference (A-P, 0.679 to 0.628; lateral, 0.06; and vertical, 0.86) [16], following a previously described protocol [9–12]. The needle was connected to a 20  μ L Hamilton syringe placed on a microinjection pump (CMA/100; BAS, Stockholm, Sweden) with a Teflon tube (0.65 mm OD 9, 0.12 mm OI; Bioanalytical Systems Inc., West Lafayette, IN).…”

“…Atropine implants placed in the right side, but not in the left side, of the POA-AHA on the day of oestrous or dioestrous-1 blocked the positive feedback of oestradiol benzoate (EB) on the release of gonadotropins necessary for ovulation. These results suggest that activation of mAChRs in the right side of POA-AHA plays a role in the oestrogen-dependent regulation of gonadotropin-releasing hormone (GnRH) preovulatory secretion [12]. Additionally, a unilateral implant of atropine in the POA-AHA modifies ovarian follicular growth in an asymmetric manner [13].…”