P.V. Malven

Prolactin Release Induced by Electrical Stimulation of the Hypothalamic Preoptic Area in Unanesthetized Sheep

The hypothalamic preoptic area (POA) was electrically stimulated through permanently implanted electrodes in 4 unanesthetized sheep. Initiation of POA stimulation was followed witin 10-20 min by large increases in plasma levels of prolactin (PRL). The stimulation-induced discharges of PRL were consistently observed in both untreated and estradiol-treated castrated animals using 0.1 to 0.7 mA of applied current. Since behavioral modifications were only sometimes associated with electrical stimulation of the POA, the response appears unrelated to the stress-induced release of PRL.

Relationship between plasma concentrations of immunoreactive beta-endorphin and food intake in rats☆

Periods of increased food intake in male rats were characterized by significant elevations in the plasma concentrations of immunoreactive beta-endorphin (beta-ep). Administration of 2-deoxy-D-glucose (400 mg/kg) produced rapid and concurrent increases in both food intake and plasma beta-ep. Administration of insulin (10 units/kg) produced large delayed increases in food intake but only modest delayed increases in plasma beta-ep. Spontaneous nocturnal feeding was associated with increased plasma beta-ep. Increases in daytime food intake in rats subjected to 24 hr of food deprivation were also characterized by elevated plasma beta-ep. In all cases examined, those feeding behaviors in male rats which were subject to inhibition by naloxone were characterized by elevated concentration of plasma beta-ep.

Direct opioid regulation of pituitary release of bovine luteinizing hormone

Although endogenous opioid peptides (EOP) are thought to alter pituitary release of luteinizing hormone (LH) by modifying the release of gonadotropin-releasing hormone (GnRH) from the brain, EOP may also directly affect the release of LH from pituitary cells. This hypothesis was tested using dispersed cells from the bovine anterior pituitary gland. Pituitaries were enzymatically dissociated, preincubated for 18 h and then cultured for either 2 or 24 h with GnRH, naloxone, methionine-enkephalin (Met-enk) or their combinations. Basal release of LH into media was 18.2 and 38.4 ng/100,000 cells after culture for 2 or 24 h, respectively. When cultured for 2 or 24 h with 10 nM GnRH, LH release was 296% and 131% of the basal release for each culture period. Cellular viability (75% vs 68%) and total (cells + medium) LH (128 vs 134 ng/100,000 cells) did not differ (P greater than .05) between cells cultured for 2 or 24 h. Naloxone (1 microM) increased (P less than .01) basal release of LH by 57% after 2 h of culture but not after 24 h of culture. Naloxone did not augment the amount of LH released in response to 10 nM GnRH. Addition of Met-enk (1 nM to 1 microM) suppressed (P less than .05) basal release of LH (23% to 62%) after 2 h of culture. Similar suppressive effects (8% to 49%) occurred in a dose-dependent manner (0.1 nM to 1 microM) after 24 h of culture. Met-enk (1 and 100 nM) antagonized (P less than .05) the stimulatory effect of naloxone and reduced (P less than .05) the amount of LH released in response to GnRH after 2 h of culture. In summary, the stimulatory effect of naloxone on the basal release of LH suggests that EOP may directly regulate pituitary cell function; the inhibitory effect of physiological concentrations of Met-enk on the basal in vitro release of LH suggests that EOP may directly affect the release of LH in vivo; the antagonism between the stimulatory effect of naloxone and the inhibitory effect of Met-enk is consistent with effects exerted through opioid receptors; and the stimulatory effect of GnRH may be partially reduced by Met-enk. These results are consistent with the hypothesis that opioids may directly modulate the release of LH at the pituitary level.

Plasma cortisol and beta-endorphin in horses subjected to electro-acupuncture for cutaneous analgesia

Electro-acupuncture (EA) treatment of horses to induce cutaneous analgesia also increased plasma concentrations of beta-endorphin (beta-EP) and cortisol. The magnitude of these increases did not relate consistently to the degree of EA-induced analgesia. Respiration and heart rates were also markedly increased during EA treatment. Intact female horses had higher packed cell volume and plasma beta-EP as well as lower plasma total protein than castrated male horses. Plasma cortisol, heart rate, and respiration rate did not differ significantly between sexes. None of the parameters measured before or during EA treatment provided an explanation for the differential cutaneous analgesia which depended on sex of subject and locus of stimulation as reported elsewhere.

Effects of Confinement Stress on Episodic Secretion of LH in Ovariectomized Sheep

Episodic secretion of luteinizing hormone (LH) in ovariectomized ewes was evaluated before and during acute confinement and during several days of habituation to the same confinement. Plasma concentrations of LH were quantified in samples collected at 5-min intervals via indwelling jugular vein cannulae. Peaks in each time series of plasma LH were identified by a computer program (Pulsar) developed by Merriam and Wachter. In the first experiment, 6 ewes which had been familiarized with the sampling environment and procedures were abruptly moved into a confinement chamber midway through a 4- to 5-hour sampling. Compared with preconfinement sampling, average plasma LH concentrations as well as the incidence (average frequency) and amplitude of identified peaks of LH decreased. In the second experiment, 10 ewes which had not previously been sampled were transferred from their home pens directly into the confinement chamber for a 3- to 4-hour period of blood sampling. This procedure was repeated with the same 10 ewes on the following 2 days to assess the effect of habituation. The average frequency and amplitude of identified peaks of LH increased (p less than 0.05) in habituated ewes, but average plasma LH concentrations did not change. In summary, episodic secretion of LH was inhibited by the stress of initial confinement, but several days of habituation to the same daily periods of confinement minimized this inhibition and restored the episodic discharges of LH.

Biphasic Effect of Estradiol on Mechanisms Regulating LH Release in Ovariectomized Sheep

Single injections of 50 mug estradiol-17beta (E2beta) into overiectomized sheep caused biphasic changes in plasma luteinizing hormone (LH). An initial 8-h period of LH inhibition was followed by a period (12-20 h after E2beta) of facilitated LH release. Pituitary LH responsiveness to small dosages of synthetic gonadotropin-releasing hormone (GnRH) was tested repeatedly at 2-h intervals during the time periods when plasma LH was inhibited and when it was facilitated. Reduced sensitivity to GnRH (91% decrease) characterized only the initial 4 h of the inhibitory period, suggesting that E2beta suppressed endogenous LH-releasing factor (LRF) during the latter part of the inhibitory period. Hypersensitivity (2-fold increase) to GnRH was briefly observed at the beginning of the period of E2beta-facilitated LH release. This transient and modest hypersensitivity does not completely account for the very large E2beta-induced increases in plasma LH. Therefore, E2beta probably increased endogenous LRF during the period of facilitated LH release.

intracerebral immunoneutralization of beta-endorphin and met-enkephalin disinhibits release of pituitary luteinizing hormone in sheep

Experiments were conducted to determine if endogenously produced beta-endorphin and met-enkephalin exert a physiological inhibition on luteinizing hormone-releasing hormone (LHRH) release in the central nervous system of sheep. Twenty-two mature ewes were implanted with unilateral guide tubes, through which matched infusion cannulae could be inserted without discomfort once daily for intracerebral (i.c.) infusion of three anti-opioid treatments: naloxone (50 micrograms in 20 microliters), sheep antisheep beta-endorphin (ABE; 20 microliters of 1:25) or sheep anti-met-enkephalin (AME; 20 microliters of 1:25) and of two control treatments: nonimmune sheep serum (20 microliters of 1:25) or sheep antiporcine thyroglobulin (20 microliters of 1:25). To detect abrupt disinhibition of LHRH release by anti-opioid treatments, serum luteinizing hormone (LH) was quantified at 10-min intervals for 1-2 h before and after each i.c. infusion. Complete trials consisted of 3-4 different anti-opioid or control i.c. infusions once daily at a single site over a period of 2-3 days during the luteal phase of recurring estrous cycles. Results were statistically evaluated within each ewe since complete trials were replicated 2-5 times within each ewe and because no 2 ewes could have i.c. infusions in identical locations. Anatomical generalizations were possible when LH responses to anti-opioid treatments were similar for several ewes with i.c. infusion sites in comparable brain regions. However, it was not possible to make such generalizations when infusion sites were not comparable in other ewes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dietary energy, body condition and calf removal on pituitary gonadotropins, gonadotropin-releasing hormone (GnRH) and hypothalamic opioids in beef cows ☆ ☆☆

Beef cows (n = 64) were slaughtered to evaluate effects of dietary energy and calf removal (CR) on hypothalamic and adenohypophysial endocrine characteristics. From d 190 of gestation until parturition, cows received maintenance (ME; n = 32) or low (LE; n = 32) energy diets (ME = 100%, LE = 70% NRC recommendations). After parturition, half (n = 16) of each prepartum diet group received low (LE; n = 32) or high (HE = 130% NRC; n = 32) energy diets. At 30 d postpartum, cows were slaughtered 0 or 48 hr after CR. Hypothalami [preoptic area (POA), hypothalamus (HYP), stalk-median eminence (SME)] and pituitaries were collected. Basal and K(+)-induced release of GnRH from SME, and pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) did not differ among groups (P greater than .05). Hypophyseal LH was correlated (P less than .01) with body condition score (BCS) at parturition and slaughter (r = .36 and .47, respectively). Prepartum LE diet increased (P less than .05) met-enkephalin in POA compared to prepartum ME (.59 +/- .05 vs. .44 +/- .04 pmol/mg) regardless of postpartum diet or suckling status. Concentrations of beta-endorphin in combined HYP + POA were decreased (P less than .05) by 48 hr CR (15.1 +/- 1.1 vs. 18.1 +/- 0.7 fmol/mg).(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Effect of Growth Retardation on Pituitary Luteinizing Hormone and Hypothalamic Neuropeptide Y in Ovariectomized Sheep

The possible role of neuropeptide Y (NPY) in mediating the relationship between pituitary LH secretion and growth retardation due to restricted feeding was examined in ovariectomized (OVX) prepubertal ewe lambs. One specific objective examined whether there was an inverse relationship between concentrations of NPY in four diencephalic brain regions and pituitary LH secretion in ewe lambs 29-30 weeks old which had been growth retarded since 8 weeks and OVX at 24 weeks. Through dietary restriction, body weight was held constant at 20.7 +/- 0.5 kg in 13 growth-retarded ewes as compared with 48.0 +/- 0.6 kg for 5 age-matched control ewes at 29-30 weeks of age. Episodic LH was quantified at 10-min intervals for 190 min/day on 3 of the 8 days immediately before euthanasia. Serum LH averaged 6.5 +/- 0.6 ng/ml in control ewes with a mean pulse frequency of 1.0 +/- 0.1 pulses/h. Serum LH in growth-retarded ewes was much less episodic (0.2 +/- 0.05 pulses/h) and averaged only 1.2 +/- 0.2 ng/ml. All ewes were euthanized during week 30, and the following brain regions were dissected: basal forebrain, preoptic area, median eminence and remainder of hypothalamus. Following extraction, NPY concentrations (pg/mg of original tissue) were quantified by radioimmunoassay. In each brain region, NPY concentrations were greater (p < 0.05) in 6 growth-retarded ewes than in 5 control ewes (median eminence: 5.2 vs. 0.6; remainder of hypothalamus: 3.3 vs. 0.8; preoptic area: 3.1 vs. 0.8, and basal forebrain: 2.2 vs. 1.2). A secondary objective examined whether the LH and NPY parameters were rapidly altered by transient changes in feed consumption.(ABSTRACT TRUNCATED AT 250 WORDS)