Involvement of serotonin in prolactin release induced by electrical stimulation of the hypothalamus of the turkey (Meleagris gallopavo)

In anesthetized female turkeys electrical stimulation of the ventromedial nucleus (VMN) for 30 min caused increases in plasma prolactin (Prl); with maximum increase above the prestimulation level being 33.6 +/- 5.7 ng/ml for laying hens and 768.1 +/- 187.6 ng/ml for incubating hens. The possibility that serotonin (5-HT) plays a role in electrical stimulation-induced Prl release was investigated after administration of methysergide, a 5-HT receptor blocker (20 mg/kg), and stimulation in either the VMN or the infundibular nuclear complex-median eminence (INF-ME) region. Electrical stimulation in both the VMN and INF-ME region caused increases (P less than 0.05) in plasma Prl. Pretreatment with methysergide prevented the increase in plasma Prl that follows electrical stimulation in the VMN but had no effect on electrical stimulation-induced Prl release in the INF-ME region. We conclude that Prl release in the female turkey requires the functional integrity of serotonergic neurons within the VMN.     

ACTH release after tuberal electrical stimulation in rats with various cuts around the medial basal hypothalamus.

After the combined treatment with dexamethasone, morphine and pentobarbitone, the electrical stimulation in the rat tuber cinereum induces a rise in plasma corticosterone (CS) level, probably by direct stimulation of the corticoliberin-producing neural elements and the consecutive ACTH release. We have attempted to locate these corticoliberin-containing neural elements by electrical stimulation in rats with various surgical cuts (deafferentations) around the medial basal hypothalamus (MBH). Seven to eight days after the complete circumsection of the MBH, the electrical stimulation of the isolated region did not affect plasma CS level. Also, the electrical excitation proved to be ineffective after antero-lateral or only lateral sections that interrupted the fibers running through the lateral region of the retrochiasmatic area. In contrast, a rise in CS level followed electrical stimulation of rats with anterolateral transections, provided the basal part of the lateral retrochiasmatic area stayed intact. We suggest that fibers traversing the basal region of the lateral retrochiasmatic area have a major role in the release of corticoliberin from the median eminence (ME).     

Activation of 5-HT 1 serotonin receptors in the medial basal hypothalamus stimulates prolactin secretion in the unanaesthetized rat

Prolactin concentrations were measured in plasma in unanaesthetized male rats chronically prepared with venous and intracerebral cannulae, before and after treatment with bilateral intracerebral injections of serotonin and fenfluramine. Serotonin 1, 5, and 10 nmol injected in the medial basal hypothalamus caused dose-related rises in prolactin concentrations. The secretion of prolactin was blocked by metergoline (2.5 mg/kg i.p.) and only partially by ketanserin (2.0 mg/kg i.v.). The 5-HT 1A agonist 8-OH-DPAT potently stimulated prolactin at doses of 1 and 5 nmol. Fenfluramine 10 and 100 nmol also caused increases in plasma prolactin when injected in the basal hypothalamus. Prolactin secretion was also evoked by serotonin injections in the preoptic/anterior hypothalamic area, but the response was not blocked by serotonin receptor antagonists. It is concluded that activation of 5-HT 1A receptors on or near prolactin-regulating neurons in the arcuate nucleus causes secretion of prolactin. The effectiveness of fenfluramine in increasing plasma prolactin suggests that endogenous serotonin released from terminals in the basal hypothalamus may mediate prolactin secretion physiologically.     

Further evidence that serotonin is a neurotransmitter involved in the control of prolactin secretion.

Administration of a new specific serotonin uptake inhibitor, fluoxetine, depressed the firing rate of raphe neurons. A highly significant increase in serum prolactin levels was observed after ip injection of 30 mg/kg of 5-hydroxytryptophan (5-HTP) in male or female rats pretreated with 10 mg/kg (ip) of fluoxetine. Neither 5-HTP nor fluoxetine given separately had any effect on serum prolactin levels. In animals pretreated with methysergide the combination of fluoxetine and 5-HTP did not increase significantly serum levels of prolactin. In addition, the serotonin agonist quipazine elevated significantly serum prolactin levels in male and female rats. The results of this study strengthen the idea that 5-HTP is acting via serotonin-containing neurons that influence anterior pituitary prolactin release, and that serotonin receptor activation leads to prolactin release.     

Surgical disconnection of the medial basal hypothalamus and pituitary function in the rhesus monkey. II. GH and cortisol secretion.

Eighteen female rhesus monkeys subjected to complete or anterior disconnection of the medial basal hypothalamus (MBH) were studied to assess the effects of these deafferentation procedures on GH and cortisol secretion. Basal serum levels of GH were not disturbed or were slightly elevated following complete or anterior MBH disconnection. GH secretion in response to vasopressin administration or insulin hypoglycemia, however, was abolished by complete isolation of the MBH. In contrast, the elevations in serum cortisol concentrations observed in response to these noxious stimuli were not noticeably affected. The normal diurnal rhythm in cortisol secretion remained fully evident following anterior deafferentation, but was severely attenuated or abolished when all neuronal inputs to the MBH were transected. Such observations suggest that the central components of the neuroendocrine systems which regulate basal GH secretion and which subserve stress-induced elevations in cortisol secretion are resident within the MBH-hypophysial unit. In addition, these data indicate that the mechanisms underlying the diurnal rhythm in cortisol secretion, as well as those mediating the discharges of GH in response to vasopressin administration and insulin hypoglycemia, are dependent on the integrity of neuronal connections between the MBH and other hypothalamic and/or extrahypothalamic areas.     

Involvement of dopamine in prolactin release induced by electrical stimulation of the hypothalamus of the female turkey (Meleagris gallopavo).

Controversy exists regarding the role of dopamine (DA) in the regulation of avian prolactin (PRL) secretion. Consequently, we injected apomorphine, a DA agonist, and pimozide, a DA receptor blocker, into laying and nest-deprived incubating turkeys and studied their effect on PRL secretion before (-20, -10, 0 min), during (5, 10, 20, 30 min), and after (5, 15, 30 min) electrical stimulation in the ventromedial nucleus of the hypothalamus. Apomorphine (10 mg/kg, ip) completely abolished the electrical stimulation-induced PRL increase in both laying and nest-deprived incubating hens. Pimozide (2 mg/kg, ip) potentiated electrical stimulation-induced PRL secretion in laying hens. In the two pimozide experiments, peak responses were 10.9-fold for the pimozide-treated group vs 2.9-fold for the control group, and 5.4-fold for the pimozide-treated group vs 2.6-fold for the control group. In nest-deprived incubating hens, PRL response to electrical stimulation was unaffected by pimozide treatment. These data support the concept that DA is inhibitory to the neuroendocrine system which stimulates PRL secretion in laying hens. In incubating hens, the dopaminergic inhibition is diminished, allowing for the increased PRL level observed during incubation.     

Effects of estradiol-17beta on the induction of gonadotropin release by electrical stimulation of the hypothalamus in rhesus monkeys.

Serum LH and FSH were measured at 60, 30, and 0 min before, at 5, 15, and 30 min during, and at 10, 45, and 90 min after bilateral electrical stimulation (ES) of various hypothalamic regions in 12 unanesthetized ovariectomized rhesus monkeys. ES of the arcuate-ventromedial nuclei (medial basal hypothalamus; MBH) induced a prompt increase in serum LH that persisted throughout stimulation and returned to basal levels within 90 min thereafter. FSH was also released, but the release was slower and less dramatic than that of LH. Sham stimulation (0muA) caused no change in serum gonadotropins. The amount of LH released after MBH-ES depended upon current strength (1.0 mA greater than 0,5 or 0.7 mA). Three sequential 30-min MBH-ES trials at 90-min intervals induced comparable LH responses and 3 h of continuous MBH-ES maintained elevated serum LH levels throughout the stimulation period, suggesting that these stimulation period, suggesting that these stimulation parameters did not completely deplete pituitary stores of releasable LH. The character of the LH response was similar in individual monkeys through 3 to 24 trials during 4 to 18 months. Comparisons were made of the effects of estradiol-17beta (E2) treatment at different doses and for different intervals of time before MBH-ES. ES-induced LH release was not affected by low levels (25 and 55 pg/ml) ofE2 for 48 h, but was reduced by higher E2 concentrations (100 or 230 pg/ml). E2 concentrations of 100 pg/ml had no effect at 24 h, but reduced MBH-ES-activated LH release at 48 to 96 h; the degree of depression was time-related (48 h less than 72 h less than 96 h). ES of the preoptic-suprachiasmatic region (rostral hypothalamus; RH) in non-E2-treated monkeys also released LH, but this increase was less than after MBH-ES. FSH release was not measurable after RH-ES. In contrast to the depressed LH response to MBH-ES after 48 h of E2 (100 pg/ml), the response to RH-ES was not inhibited by this E2 regimen. These data suggest that ES of an area extending caudally from the rostral hypothalamus to the arcurate-median eminence region will evoke LH release in rhesus monkeys. This electrically induced gonadotropin release was affected by administration of physiological levels of E2 but the nature of effect depended on the specific region stimulated: distinct inhibition of the gonadotropic response to MBH-ES and slight facilitation of the response to RH-ES.     

Hypothalamic regulation of mating-induced prolactin release. Effect of electrical stimulation of the medial preoptic area in conscious female rats

Lesions of the medial preoptic area (MPOA) induce nocturnal prolactin surges similar to those initiated by cervical stimulation (CS). These same lesions can abolish diurnal prolactin surges previously initiated by CS. Based on these results the MPOA has been suggested to contain two functionally dissimilar sets of neurons, one inhibitory for the nocturnal surge and the other stimulatory for the diurnal surge. The present study sought to demonstrate the existence of these neural elements by electrically stimulating the MPOA of conscious ovariectomized female rats during those times of day when these neurons would be most active. Serial blood samples were collected via cannula before, during and after the stimulation. Stimulation of the MPOA (01.00-05.00 h) on day 2 after CS inhibited the nocturnal surge of prolactin while sham MPOA stimulation of CS females did not disturb the nocturnal surge. MPOA stimulation in non-CS females had no effect upon prolactin secretion. Application of MPOA stimulation (15.00-19.00 h) to CS females also suppressed the diurnal surge of prolactin. Sham-stimulated CS females, however, secreted a diurnal surge peaking at 17.00 h. Basal prolactin levels were unaffected by MPOA stimulation (15.00-19.00 h) in non-CS females. The results from these experiments suggest that the MPOA contains neurons inhibitory for both the nocturnal and diurnal prolactin surges. In a further attempt to show a stimulatory role for the MPOA in prolactin regulation, MPOA stimulation was applied (15.00-19.00 h) to pentobarbital anesthetized non-CS females. Pentobarbital treatment allowed the MPOA stimulation to trigger two prolactin peaks, one at 16.00 h and the other at 19.00 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reversal of the delta-9-tetrahydrocannabinol inhibitory effect on prolactin secretion by rostral deafferentation of the medial basal hypothalamus

The effect of rostral deafferentation of the medial basal hypothalamus (MBH) on delta-9-tetrahydrocannabinol (THC)-induced changes in serum prolactin (PRL) concentrations was investigated in female rats having retrochiasmatic frontal cuts that transected the rostral hypothalamus. Cuts dorsal to the hypothalamus were produced in the same plane in other animals in order to control for possible effects of the surgical procedure or dorsal brain damage. All animals were ovariectomized 28-35 days after stereotaxic surgery to obviate potential confounding effects of differences in ovarian function between groups. Unlesioned rats were ovariectomized to provide a positive control group for THC inhibitory activity. At least 4 weeks after ovariectomy, animals were treated intravenously with THC (0.5 or 1.0 mg/kg body weight) or vehicle at the midpoint of a 110-min experimental period during which blood samples were obtained at 10-min intervals via indwelling atrial cannulae. Serum PRL concentrations were determined by radioimmunoassay and cut locations were confirmed histologically. When administered to ovariectomized animals without brain lesions, THC suppressed serum PRL concentrations from the average treatment level within 30 min (p less than 0.05), and PRL levels remained suppressed for the remainder of the posttreatment sampling period. Treatment with the vehicle alone was without effect. Animals with retrochiasmatic plane cuts that did not transect the rostral hypothalamus similarly displayed PRL suppression in response to THC administration (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that specific dopamine-1 receptor activation is involved in dopamine-induced renin release

Direct effects of dopamine on renin release were examined using static incubations and perifusions of rat renal cortical slices. Dopamine (10(-5)M) significantly stimulated renin release compared with control. To determine which receptors are involved in dopamine-elicited renin release, studies were performed with specific dopamine-1 and dopamine-2 receptor agonists and antagonists, as well as with alpha- and beta-adrenergic antagonists. Fenoldopam, a dopamine-1 receptor agonist, dose dependently stimulated renin secretion both in static incubations and perifusions; whereas quinpirole (10(-7)-10(-5)M), a dopamine-2 receptor agonist, was ineffective. Phentolamine (10(-4)M), an alpha-adrenergic antagonist, did not alter dopamine- or fenoldopam-induced renin release. Similarly, propranolol, a beta-blocker, did not interfere with the renin stimulation of dopamine (10(-5)M) or fenoldopam (10(-6)M) incubations or perifusion experiments; whereas propranolol significantly blocked isoproterenol action. SCH 23390 (10(-5)M), a specific dopamine-1 antagonist, blocked dopamine- and fenoldopam-induced renin. In contrast, pimozide, a dopamine-2 receptor antagonist, was ineffective. These studies indicate that dopamine is a direct renin secretogogue, and its effects seem to be mediated by specific dopamine-1 receptor activation, as neither alpha- nor beta-adrenergic blockers nor dopamine-2 receptor antagonists altered dopamine actions. The results suggest that dopamine produced locally in the kidney may stimulate renin secretion directly by dopamine-1 receptor activation.     

Hypothalamic localization of multiunit electrical activity associated with pulsatile LH release in the rhesus monkey

Abrupt increases in electrical activity can be recorded from the medial basal hypothalamus of the rhesus monkey and these bursts are correlated with a pulse of LH release from the anterior pituitary. In this study we have localized the tips of the electrodes from which such electrical activity can be recorded and have attempted to correlate the placement with the presence of immunoreactive LHRH neurons and axons. Electrodes associated with bursting activity were found as far rostral as the suprachiasmatic nucleus and as far caudal as the premammillary nuclei. The majority were found in the medial basal hypothalamus including the region of the arcuate nucleus, the retrochiasmatic zone and the dorsal aspect of the median eminence. The tips of the electrodes were associated with either LHRH neurons or axon bundles. Negative electrodes (those not associated with bursting electrical activity) which did not exit through the ventral surface of the brain nor enter the ventricle were found in the same regions of the hypothalamus as positive electrodes. Although it is tempting to associate the bursting electrical activity with the LHRH neurosecretory system, such a correlation cannot yet be made.     

Regulation of the second (estrous) release of follicle-stimulating hormone in hamsters by the medial basal hypothalamus1,2.

In femal proestrous hamsters (1800 h), bilateral electrocoagulative lesions in the arcuate-median eminence (Arc-ME) region blocked the pituitary FSH release that normally would have begun in late proestrus and continued until the afternoon of estrus. This second (estrous) FSH elevation, which is not accompanied by LH release, was unaffected by neural disconnection of the medial preoptic area (MPOA) from the medial basal hypothalamus (MBH) or by production of MBH-pituitary islands. After gonadotropin-releasing hormone was injected into Arc-ME-lesioned hamsters, LH was released. These results suggest that the Arc-ME does not require the MPOA for initiation of the estrous release of FSH.     

Evidence that the hypothalamus mediates endotoxin stimulation of adrenocorticotropic hormone secretion.

The site of action of Escherichia coli endotoxin in inducing ACTH secretion was studied in vivo and in vitro. Hypophysectomized rats, bearing two to three transplanted pituitaries under the kidney capsule and "primed" with exogenous ACTH, responded to 2.0-7.5 microgram/100 g BW ip or iv endotoxin with a several-fold increase of plasma corticosterone. This response was markedly reduced by hypothalamic lesions and completely abolished by removing the entire forebrain. Endotoxin added directly to cultured rat adenohypophyseal cells in a concentration up to 10 microgram/ml did not induce significant ACTH secretion. We conclude that endotoxin-induced ACTH secretion from heterotopically transplanted pituitaries is mediated primarily by the hypothalamus, presumably through hypothalamic CRF that reaches the transplanted pituitaries via the systemic circulation.     

Serotonergic stimulation of prolactin and corticosterone secretion is mediated by different pathways from the mediobasal hypothalamus

In previous studies we obtained evidence that serotonin release by p-chloroamphetamine (PCA) causes an increase in corticosterone secretion but that this effect is not mediated via the raphe nuclei in the midbrain. In contrast, PCA-induced prolactin secretion was abolished by dorsal raphe lesions. In the present study, posterolateral cuts which interrupted caudal inputs to the hypothalamus attenuated the effect of PCA on plasma prolactin but did not block the PCA-induced increase in plasma corticosterone levels. Large lesions of the mediobasal hypothalamus produced a significant reduction of plasma corticosterone concentration but did not completely prevent the effect of PCA on corticosterone secretion. Hypophysectomy performed 24 h before sacrifice caused a marked decrease in plasma corticosterone levels but did not completely abolish the effect of PCA. These results suggest that PCA also stimulates corticosterone secretion via a direct action on the adrenal gland. The lesions in the mediobasal hypothalamus caused an increase in plasma prolactin concentration, and in these rats, PCA suppressed rather than stimulated prolactin secretion. This suggests that the known weak dopamine agonist activity of PCA is exposed when the effects of serotonin release in the brain are eliminated.     

Neuropeptide Y stimulates the release of luteinizing hormone-releasing hormone from medial basal hypothalamus in vitro: modulation by ovarian hormones

These studies investigated the effects of neuropeptide Y (NPY) on in vitro release of luteinizing hormone-releasing hormone (LHRH) from the medial basal hypothalmus (MBH) and tested whether ovarian steroids modulate the LHRH response to NPY. Ovariectomized rats were implanted with 20-mm-long Silastic capsules containing a low concentration of estradiol (E2) (150 micrograms/ml oil), a high concentration of E2 (250 micrograms/ml oil), or sesame oil vehicle. Additional animals received high-dose E2 capsules plus an injection of progesterone (15 mg) concomitantly. Two days later, individual MBH fragments were incubated in medium alone for a 30-min period to obtain the basal rate of LHRH release, followed by a second 30-min period in medium containing NPY or saline. Exposure to NPY (10(-6) M) increased the release of LHRH from MBH of ovarian hormone-treated, but not from hormonally untreated rats. The LHRH response was most pronounced from the MBH of rats treated with either high-dose E2 or E2 plus progesterone. The increase in LHRH release was also elicited by 10(-7) M, but not by 10(-8) M NPY concentrations, using MBH from E2 plus progesterone-treated rats. In addition, NPY markedly potentiated the KCl-evoked release of LHRH from MBH of ovariectomized, hormonally untreated or low-dose E2-treated rats, under conditions when there was little or no effect of NPY on the basal LHRH release. Further, the release of LHRH stimulated by NPY was not accompanied by increase in the release of norepinephrine or of dopamine.(ABSTRACT TRUNCATED AT 250 WORDS)