Transient effects of MK-801 administration on secretion of luteinizing hormone and prolactin in ovariectomized and estradiol-treated sheep.

The neuroendocrine role of endogenous ligands for the excitatory amino acid receptor subtype known as the NMDA receptor was investigated by administering the NMDA receptor antagonist MK-801 to ovariectomized (ovx) and estradiol-treated sheep. Repetitive administration of MK-801 at intravenous (iv) dosages of 0.1 mg/kg to untreated ovx ewes did not affect the episodic profiles of luteinizing hormone (LH) release, but each injection of MK-801 abruptly stimulated release of prolactin (PRL) demonstrating the effectiveness of the dosage. Injection of estradiol-17beta (50 microg/ewe) to ovx ewes produced the expected biphasic LH response; an initial suppression followed by a surge-like LH increase together with an elevated basal secretion of PRL. Injections of MK-801 occurring 9 and 11 h after estradiol-17beta injection rapidly and transiently increased serum LH in a very unexpected response. However, these same MK-801 injections also resulted in decreased serum LH 14-17 h after estradiol-17beta by delaying the onset of the surge-like release of LH. Estradiol-17beta administration itself elevated basal release of PRL to serum concentrations observed after repetitive MK-801, and further injection of MK-801 no longer transiently increased serum PRL as it had done prior to estradiol-17beta treatment. In summary, antagonizing the endogenous excitatory amino acid ligands of the NMDA receptor with MK-801 did not alter either the timing or magnitude of the putative episodic discharges of gonadotropin-releasing hormone (GnRH) which in turn cause the episodic releases of pituitary LH in ovx sheep. Estrogen administration created a transient neuroendocrine environment in which antagonism of these endogenous ligands was stimulatory to abrupt discharge of GnRH and thereby to acute release of LH. Antagonism of NMDA receptors in untreated ovx ewes stimulated release of PRL suggesting that the endogenous NMDA ligands were probably stimulatory to the release of a PRL-inhibiting neurohormone such as dopamine.

Serum concentrations of luteinizing hormone, growth hormone, and prolactin in untreated and estradiol-treated ovariectomized ewes after immunoneutralization of hypothalamic neuropeptide Y.

Ovine antiserum against a conjugate of porcine neuropeptide Y (NPY) with bovine thyroglobulin was infused repeatedly into one lateral ventricle of five ovariectomized (ovx) ewes before and after subcutaneous injection of estradiol-17 beta (E beta). Serum concentrations of LH, growth hormone (GH), and prolactin (PRL) were measured at 10-min intervals before E beta and at approximately hourly intervals 8 to 18 h after E beta injection. Control ewes were infused with ovine serum from animals immunized against bovine thyroglobulin. Basal episodic profiles (pre-E beta) of LH, GH, and PRL were similar in ovx ewes infused with control or anti-NPY serum. Injection of E beta induced a surge-like increase of serum LH, which began at 12.6 h after E beta in ewes infused with anti-NPY and at 14.4 h after E beta in ewes infused with control antiserum (P < .05). The magnitude of the E beta-induced surge of LH was not different between treatments. In addition to initiating a surge-like release of LH over the period 12 to 18 h after E beta, serum GH was transiently increased during the period between 10 and 15 h after E beta. In contrast, serum PRL was increased during the entire period between 8 and 18 h after E beta. Based on the effects of immunoneutralization, endogenous NPY in the brain of E beta-treated ovx ewes seems to restrain or delay the onset of the surge-like secretion of LH and probably GnRH, but endogenous NPY does not affect the episodic pulsatile releases of LH characteristic of ovx ewes.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracerebral methionine-enkephalin, serum cortisol, and serum beta-endorphin during acute exposure of sheep to physical or isolation stress.

In vivo microdialysis was used to estimate extracellular concentrations of methionine-enkephalin in 19 brain sites for 5 h on each of three consecutive days (trials) in six conscious ewes. Following control procedures on d 1, ewes were completely isolated from other sheep for 60 min on d 2 (psychological stress). Physical stress was imposed on d 3 and consisted of continuous pinching of the skin for 60 min during the middle of the 5-h experimental period. Imposition of both physical and psychological stress rapidly increased serum concentrations of cortisol, and the induced increase persisted for at least 30 min after termination of the stress. Psychological stress of isolation initially increased cortisol to a greater extent than the physical stress of skin pinch, but this difference disappeared after 30 min of stress exposure. Psychological stress also transiently increased serum concentrations of beta-endorphin/beta-lipotropin, whereas physical stress did not. Average concentrations of methionine-enkephalin in dialysate ranged between 1.52 and 1.85 ng/mL when the intracerebral probes were placed into the caudate nucleus, the preoptic area of the hypothalamus, or the thalamus. The concentration of methionine-enkephalin was consistently less than 1.0 ng/mL when probes were placed into major fiber tracts of the brain (corpus callosum, internal capsule). Potassium-induced depolarization around the probe tip located in the caudate nucleus increased dialysate concentrations of methionine-enkephalin by 2.7-fold. Imposition of physical or psychological stress did not consistently increase or decrease dialysate concentrations of methionine-enkephalin in any of the brain sites studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intracerebral administration of neuropeptide-Y on secretion of luteinizing hormone in ovariectomized sheep.

Ovariectomized ewes received unilateral infusions of 20 micrograms neuropeptide-Y (NPY) at a total of 13 intracerebral sites. Episodic secretion of luteinizing hormone (LH) was transiently suppressed on more than one occasion by daily infusions at a total of five intracerebral sites. Four of the effective sites were located within the third ventricle (two sites) and the rostral and ventral part of a lateral ventricle (two sites). The precise neural site of action of exogenous NPY cannot be determined from intraventricular administration, but it indicates a neural rather than pituitary site of NPY action to inhibit LH-releasing hormone (LHRH) in sheep. The only tissue infusion site (ventromedial nucleus) at which NPY also suppressed LH/LHRH also supports a neural action on LHRH, but this single result is insufficient to establish the neural area at which NPY acted. It is known from other work that the production of endogenous NPY in neural tissue of underfed animals is increased, and if endogenous NPY exerts effects on LH/LHRH similar to the suppression presently observed following exogenous NPY this neuropeptide might serve as one neuroendocrine factor that suppresses reproduction in underfed animals.

Ovine brain areas sensitive to naloxone-induced stimulation of luteinizing hormone release.

Previous work established that intravenous administration of the opioid receptor antagonist naloxone abruptly increased release of luteinizing hormone (LH) and decreased release of prolactin (PRL) in suckled anestrous ewes and also increased LH release in cyclic luteal ewes. The goal of the present research was to identify brain sites at which local unilateral infusions of naloxone would consistently duplicate the previously noted effects of intravenous naloxone. Intracerebral guide tubes were surgically implanted into the brain of 13 nonpregnant and 16 pregnant ewes at least 4 weeks prior to experimentation. Intracerebral infusion (20-40 microliters each through an inner cannula) was performed once daily during postpartum anestrus in suckled fall-lambing ewes and during recurring luteal states of the estrous cycle. Naloxone infusion (n = 142) usually consisted of 50 or 100 micrograms naloxone, although 5 ewes received 200 and 400 micrograms per infusion. Control infusions (n = 103) consisted of the vehicle for naloxone (i.e., 0.9% NaCl). Serum concentrations of LH and PRL were quantified at 10-min intervals from 90 min before to 100 min after infusion. Hormone data from individual ewes were grouped for least-squares analysis of variance based upon postmortem neuroanatomical identification of each infusion site. Unilateral intracerebral administration of naloxone consistently induced an increase in LH release within 20 min in the following two neuroanatomical groups:basal forebrain (n = 9 ewes) and chiasmatic area (n = 4 ewes).(ABSTRACT TRUNCATED AT 250 WORDS)