Gonadotrophin secretion in the ovariectomized guinea‐pig: effects of electrical stimulation of the hypothalamus and of LH‐RH.

Abstract

SUMMARY1. Serial blood samples were collected from anaesthetized spayed guinea-pigs subjected to anterior hypothalamic stimulation or injected with luteinizing hormone-releasing hormone (LH-RH), and the effect on the secretion of follicle-stimulating hormone (FSH) and LH followed.2. Hypothalamic stimulation enhanced LH secretion, although the response waned during the course of the 1 hr period of stimulation; the secretion of FSH was not increased. After stimulation, FSH and LH levels fell below control values.3. The response to stimulation of the hypothalamus was greater 2 weeks after spaying than after 1 week.4. Since repeated periods of hypothalamic stimulation induced repeated surges of LH secretion, the decline in LH secretion during stimulation was not likely to have resulted from hypothalamic damage, while the observation that the plasma concentration of FSH and LH declined below control values after brain stimulation implies the operation of inhibitory mechanisms.5. Pre-treatment of spayed guinea-pigs with progesterone depressed the resting plasma concentration of LH but did not affect the the immediate response to hypothalamic stimulation. After hypothalamic stimulation the plasma gonadotrophin levels did not fall below control values.6. Pre-treatment with oestradiol benzoate depressed the resting plasma level of LH and slightly reduced the size of the LH surge produced by hypothalamic stimulation. However, the raised plasma LH concentration was sustained after stimulation ended.7. The response to hypothalamic stimulation after pre-treatment with

Luteinizing Hormone Release in the Anaesthetised Cat Following Electrical Stimulation of Limbic Structures

“…14 17, 26], rabbit [25], guinea-pig [12], ferret [II], pig [4] and monkey [38] has been shown to influ ence levels of plasma luteinising hormone (LH). We have been studying in the anaesthetised cat the neurophysiology of some of these limbic connections which are involved in the modulation of LH release [20,21].…”

Luteinizing Hormone Responsiveness to Luteinizing Hormone Releasing Hormone in the Adult Guinea-Pig: Direct Ovarian Involvement

“…Chronically ovariectomized guinea-pigs are considerably more responsive to a single injection of LH-RH than are the intact animals (Donovan, ter Haar & Parvizi, 1977). Progesterone appears to be of major importance in the control of the length of the guineapig cycle (Donovan & Lockhart, 1972), as it is in many domestic animals; in general, chronic treatment with progesterone tends to reduce the responsiveness of the pituitary gland to LH-RH in most species (for references see Pelletier, 1976), and thus may account for the progressive fall in responsiveness in the guinea-pig as the cycle progresses.…”

Effect of stimulation or lesioning of the hypothalamus upon gonadotrophin secretion in the guinea‐pig.

“…were frequently recorded in association with natural ovulation in conscious animals (Blatchley et al 1976). Anaesthesia might be regarded as having depressed the response, but larger quantities of LH have been released in other work with barbiturate-anaesthetized animals (Donovan et al 1977) while Kilpatrick & Donovan (1981) found that the mean levels of LH in anaesthetized spayed females were higher than in conscious guinea-pigs. The parameters adopted for brain stimulation are considered suitable for the excitation of unmyelinated nerve fibres in the rat (Harris, Manabe & Ruf, 1969), but their applicability to the guinea-pig has not undergone exhaustive tests.…”

“…The procedures for electrical stimulation of the hypothalamus and blood collection were employed previously by Donovan, ter Haar & Parvizi (1977). In that study, stimulation of the anterior hypothalamus raised the already elevated plasma concentration of LH in gonadectomized females by about 2-5 ng/ml., although the concentration of FSH was not altered.…”

“…Many years ago, Dey, Fisher, Berry & Ranson (1940) showed that lesions in the anterior hypothalamus caused constant vaginal oestrus associated with highly follicular ovaries, but the changes in gonadotrophin secretion producing this effect have not been explored. However, gonadotrophin secretion in the guinea-pig is resistant to change in that electrical stimulation of the hypothalamus does not readily induce ovulation (Booth & Donovan, 1974a), and did not markedly alter the secretion ofluteinizing hormone (LH) or alter follicle-stimulating hormone (FSH) release in the spayed female (Donovan, ter Haar & Parvizi, 1977). Pre-treatment with oestrogen, progesterone or both steroids modulated the response to brain stimulation, though not to a marked degree.…”