Involvement of cholinergic nicotine-like receptors as modulators of amine turnover in various types of hypothalamic dopamine and noradrenaline nerve terminal systems and of prolactin, LH, FSH and TSH secretion in the castrated male rat

The effects of high repeated subcutaneous doses (4 X 2 mg/kg) of nicotine have been evaluated on dopamine (DA) and noradrenaline (NA) levels and turnover in the long-term castrated male rat using catecholamine (CA) fluorescence histochemistry in combination with quantitative microfluorometry. The CA turnover was evaluated by studying the decline of the CA stores following tyrosine hydroxylase inhibition using alpha-methyltyrosine methyl ester (H 44/68). In the same experiments trunk blood was collected for the determination of serum prolactin, LH, FSH and TSH levels using standard radioimmunoassay procedures. The nicotine treatment produced a significant depletion of CA stores and an increase of CA turnover in DA and NA nerve terminals of the median eminence and in peri- and paraventricular NA systems. These effects were significantly counteracted by pretreatment with mecamylamine. Nicotine significantly reduced serum prolactin and TSH levels, and after H 44/68 it also reduced LH and FSH serum levels. These actions were counteracted by mecamylamine pretreatment, except the effects on serum TSH levels after H 44/68, which were even enhanced by pretreatment with mecamylamine. Overall intraindividual correlations showed a significant correlation between reduced CA turnover in several hypothalamic areas and increased serum LH and FSH levels, increased NA turnover in the paraventricular hypothalamic nucleus and increased serum TSH levels, and reduced DA turnover in the median eminence and increased serum LH levels. It is suggested that in the castrated male rat nicotine can activate cholinergic nicotine-like receptors facilitating DA and NA turnover and release in various hypothalamic CA nerve terminal systems including those inhibiting the secretion of prolactin and LH (DA terminals in medial and lateral palisade zone, respectively) and facilitating secretion of TSH (NA terminals in the parvocellular part of the paraventricular hypothalamic nucleus).     

Differential effects of mecamylamine on the nicotine induced changes in amine levels and turnover in hypothalamic dopamine and noradrenaline nerve terminal systems and in the secretion of adenohypophyseal hormones in the castrated female rat. Evidence for involvement of cholinergic nicotine-like receptors

The effects of mecamylamine on the nicotine induced changes in hypothalamic catecholamine (CA) levels and turnover in female rats ovariectomized for one month have been evaluated using a quantitative microfluorimetric approach to measure CA levels in sections of brains treated according to the Falck-Hillarp procedure for the cellular demonstration of CA. In the same group of animals the serum prolactin, LH, FSH, TSH, GH and corticosterone levels were measured using radioimmunoassay procedures. The nicotine treatment induced a significant depletion of amine stores and an increase of amine turnover in dopamine (DA) and noradrenaline (NA) nerve terminals of the median eminence and of the peri- and paraventricular and dorsomedial NA systems of the hypothalamus using the tyrosine hydroxylase (TH) inhibition model. Mecamylamine (2 X 1 mg/kg) partly counteracted the nicotine induced reduction of amine stores in peri- (anterior part) and paraventricular NA nerve terminal systems as well as the nicotine induced increase of NA turnover in these systems, but not the action of nicotine on the CA systems of the median eminence. Nicotine (4 X 2 mg/kg) significantly and markedly reduced prolactin, LH, TSH, and GH secretion increased corticosterone secretin but did not influence FSH secretion. These effects were partly counteracted by mecamylamine (2 X 1 mg/kg) in the case of prolactin, LH and TSH secretion but not in the case of GH and corticosterone secretion. Taken together the results show that mecamylamine treatment (2 X 1 mg/kg) differentially counteract nicotine induced changes of amine levels and turnover in peri- (anterior part) and paraventricular NA nerve terminal systems indicating that the cholinergic nicotine-like receptors located in peri- (anterior part) and paraventricular areas may be more susceptible to the blocking activity of mecamylamine than those located in the median eminence area. Furthermore, the inhibitory effects of nicotine on prolactin, LH and TSH secretion are differentially counteracted by mecamylamine. In conclusion, other inhibitory systems than the tuberoinfundibular DA neurons in the MPZ and LPZ must also be involved in mediating the inhibitory effects of nicotine on prolactin, LH and TSH secretion and different types of cholinergic nicotine-like receptors may exist.     

Nicotine-induced increases of noradrenaline turnover in discrete noradrenaline nerve terminal systems of the hypothalamus and the median eminence of the rat and their relationship to changes in the secretion of adenohypophyseal hormones

The effects of acute single doses (0.3 and 1 mg/kg) of nicotine on various hypothalamic catecholamine nerve terminal systems and on the secretion of adenohypophyseal hormones in the rat were studied. Nicotine, in a dose of 1.0 mg/kg, increased noradrenaline turnover in the median eminence and in the peri- and paraventricular hypothalamic regions. The dopamine and noradrenaline nerve terminal systems in the median eminence and the dorsomedial hypothalamic nucleus respectively were unaffected. Serum GH levels were decreased and serum prolactin levels increased after a dose of 1 mg/kg. In the presence of tyrosine hydroxylase inhibition, nicotine in a dose of 1 mg/kg, instead increased GH and also LH secretion. It is suggested that the preferential increases of noradrenaline turnover in various hypothalamic noradrenaline nerve terminal systems by nicotine may be partly responsible for the nicotine induced increases of serum prolactin, GH and LH levels observed.     

Mecamylamine pretreatment counteracts cigarette smoke induced changes in hypothalamic catecholamine neuron systems and in anterior pituitary function

We have studied the effects of acute, intermittent exposure to tobacco smoke on discrete hypothalamic CA nerve terminal networks and on neuroendocrine function by means of quantitative histofluorimetrical determinations of catecholamine (CA) fluorescence in sections of rat brain and by radioimmunoassay procedures for hormones. Acute intermittent exposure to cigarette smoke induced a lowering of NA levels and increased NA turnover in discrete hypothalamic nerve terminal regions. This exposure also induced increases in DA turnover in the median eminence. The cigarette smoke lowered TSH, prolactin, LH and FSH serum levels, but induced an increase in serum corticosterone concentrations. To determine if the above mentioned changes in neuroendocrine function were nicotine mediated, a cholinergic nicotine-like blocking agent, mecamylamine, was administered prior to exposure to cigarette smoke. Pretreatment with mecamylamine (1.0 mg kg-1) counteracted the cigarette smoke induced changes in CA levels and turnover in all hypothalamic CA nerve terminal regions as well as the changes in serum levels of the pituitary hormones and corticosterone. It is suggested that acute intermittent exposure to cigarette smoke, via its nicotine component, lowers TSH, prolactin, LH and FSH secretion at least in part through activation of the tubero-infundibular DA neurons. Furthermore, the nicotine component of the cigarette smoke is suggested to induce the increase in corticosterone serum levels via increasing NA turnover in the paraventricular hypothalamic nucleus.     

Intravenous injections of nicotine induce very rapid and discrete reductions of hypothalamic catecholamine levels associated with increases of ACTH, vasopressin and prolactin secretion

The present findings show that nicotine given i.v. can within minutes induce increases of ACTH, vasopressin and prolactin secretion in the male rat, giving further evidence for the existence of nicotine-like cholinergic receptors involved in the regulation of these hormones. These changes were associated with rapid reductions of NA levels in the subependymal layer of the median eminence, in the nuc, dorsomedialis hypothalami and in the anterior and posterior periventricular hypothalamic area as revealed by quantitative microfluorimetrical measurements of CA fluorescence. Intraindividual correlations indicate the involvement of an inhibitory noradrenergic mechanism in the subependymal layer of the median eminence in the regulation of ACTH secretion, the involvement of noradrenergic mechanisms in the posterior periventricular area in the regulation of prolactin secretion and the involvement of dopaminergic mechanisms in the medial palisade zone of the median eminence in the regulation of prolactin secretion. A rapid rise of prolactin secretion seems to be associated mainly with a reduction of NA levels in the posterior periventricular area indicating the existence of a possible facilitatory noradrenergic mechanism in this region regulating prolactin secretion.     

Regression analysis of catecholamine utilization in discrete hypothalamic and forebrain regions of the male rat: effects of thyroidectomy

The effects of thyroidectomy (4 weeks) on dopamine (DA) and noradrenaline (NA) turnover rates were determined by means of regression analysis. The disappearance of catecholamine (CA) fluorescence (using quantitative histofluorimetry) after tyrosine hydroxylase inhibition (alpha-methyl-DL-p-tyrosine methyl ester) has been investigated in discrete hypothalamic and forebrain DA and NA nerve terminal systems of the male rat. A time-dependent monophasic CA fluorescence disappearance was observed in all CA nerve terminal systems of the sham-operated and thyroidectomized rats. In the thyroidectomized rat, DA turnover in the anterior nucleus accumbens and in the medial and lateral palisade zones of the median eminence (ME) was reduced while DA turnover in the posterior nucleus accumbens was increased as compared to control rats. Furthermore, NA turnover was increased in the paraventricular hypothalamic nucleus (PA) and reduced in the dorsomedial hypothalamic nucleus (DM) and in the 'border zone' (lateral hypothalamus). Radioimmunoassay of hormones in serum demonstrated marked increases in TSH levels and reduced concentrations of GH, prolactin, corticosterone, triiodothyronine and thyroxine. The reduced DA turnover in the external layer of the ME and the increased NA turnover in the PA may indicate an inhibitory dopaminergic mechanism in the ME and a facilitatory noradrenergic mechanism in the PA in the regulation of TSH secretion. These mechanisms seem to interact with thyroid hormones. The reduced NA turnover demonstrated in the DM and in the border zone may be related to the lowering of growth hormone levels and pulsatility caused by thyroidectomy. Finally, the DA nerve terminal systems in the anterior and posterior parts of the nucleus accumbens are differently regulated by changes in the brain-pituitary-thyroid axis.     

Effects of acute intermittent exposure to cigarette smoke on catecholamine levels and turnover in various types of hypothalamic DA and NA nerve terminal systems as well as on the secretion of adenohypophyseal hormones and corticosterone

Male rats were exposed to cigarette smoke (Walton Horizontal Smoking Machine) from one to four cigarettes (Kentucky reference IR-1 type). Catecholamines in the diencephalon were measured by quantitative histofluorimetry in discrete dopamine (DA) and noradrenaline (NA) nerve terminal systems. Blood TSH, prolactin, LH, FSH, ACTH, vasopressin and corticosterone levels were determined by radioimmunoassay procedures. Exposure to unfiltered, but not to filtered (Cambridge glass fibre filters) cigarette smoke resulted in dose-dependent reductions of NA levels in the various hypothalamic NA nerve terminal systems. Evidence was obtained that exposure to unfiltered but not to filtered cigarette smoke resulted in dose-dependent increases of amine turnover (alpha MT-induced CA disappearance experiments) in the various DA and NA nerve terminal systems in the hypothalamus. The lowering of TSH, LH and prolactin secretion induced by unfiltered smoke were probably induced by nicotine and were independent of tyrosine hydroxylase inhibition. Furthermore, unfiltered cigarette smoke produced a dose-related increase in corticosterone secretion. The inhibitory effects of TSH, LH and prolactin secretion were probably in part related to the ability of unfiltered smoke via its nicotine component to activate the lateral and medial tubero-infundibular DA neurons. The increases in corticosterone secretion may at least in part be related to a smoke induced increase in the facilitatory influence of paraventricular NA nerve terminals on CRF activity.     

Effects of acute haloperidol treatment on regional catecholamine levels and utilization in rats exposed to toluene

The aim of the present investigation was to evaluate whether the responses of central catecholamine (CA) neurons to CA receptor blockade by haloperidol are altered upon toluene exposure. Male rats were exposed to air or toluene (80 ppm) for 5 and 4 days, 6 h day^-1. CA levels and utilization were determined in discrete regions of the forebrain and hypothalamus as well as in the substantia nigra (SN) and anteromedial frontal cortex (AMFC). Serum levels of corticosterone, thyroid stimulating hormone, luteinizing hormone and prolactin were determined by radioimmunoassay procedures. Toluene exposure led to increased dopamine (DA) utilization in the AMFC and increased CA utilization in the paraventricular hypothalamic nuclei. In air-exposed rats haloperidol (1 mg kg^-1, i.p., 2 h before killing) increased DA utilization in the marginal part of the nucleus caudatus putamen (CAUD). In toluene-exposed rats, haloperidol induced significant depletions of DA stores in the SN and in the medial and central parts of the CAUD. In the posterior nucleus accumbens (ACC) DA utilization was significantly increased. Combined haloperidol and toluene treatment selectively decreased DA levels in the ACC and SN, and significantly increased DA utilization in the CAUD, as compared with the air-exposed control group. Furthermore, after combined treatment, there was a specific increase in noradrenaline (NA) utilization in the SN and in CA utilization in the medial palisade zone of the median eminence. Serum prolactin levels were substantially raised in both the air and toluene groups after the haloperidol treatment. In conclusion, acute haloperidol treatment preferentially reduces DA levels and increases DA and NA utilization in the SN and in discrete tel- and diencephalic areas in rats exposed to toluene.     

Effects of intraventricular injections of galanin on neuroendocrine functions in the male rat. Possible involvement of hypothalamic catecholamine neuronal systems

Galanin-catecholamine interactions have been analysed within the hypothalamus and the anteromedial frontal cortex of male rats by means of quantitative histofluorimetrical and biochemical measurements of catecholamine fluorescence in discrete catecholamine nerve terminal systems and measurements of serum levels of adenohypophyseal hormones and corticosterone using radio-immunoassay determinations. 125I-galanin binding sites were analysed and related to the distribution of galanin-immunoreactive neuronal structures in the median eminence and paraventricular hypothalamic nucleus. The results show that intraventricular injections of galanin in the awake and unrestrained male rat produce rapid increases of prolactin and growth hormone secretion but no effects on serum luteinizing hormone, thyroid stimulating hormone or on corticosterone levels. These changes in neuroendocrine function were associated with a selective reduction of the catecholamine stores in the medial palisade zone of the median eminence at the 20 min time interval. 125I-galanin binding sites were found throughout the hypothalamus including the median eminence and the magnocellular part of the paraventricular hypothalamic nucleus with a good correspondence with galanin immuno-reactivity. It is suggested that the enhancement of prolactin secretion induced by galanin involves an interaction between galanin and dopamine in the medial palisade zone leading to a reduced synthesis and/or release of dopamine and thus to a reduced prolactin inhibitory activity and to increases in prolactin secretion. A possible involvement of hypothalamic catecholamines in the galanin-induced changes of growth hormone secretion remains to be established.     

Determinations of catecholamine half-lives and turnover rates in discrete catecholamine nerve terminal systems of the hypothalamus, the preoptic region and the forebrain by quantitative histofluorimetry

To determine whether amine pool sizes, half-lives and turnover rates could be measured in discrete hypothalamic, preoptic and forebrain catecholamine (CA) nerve terminal networks by quantitative histofluorimetry, the CA fluorescence disappearance was studied at different time intervals after tyrosine hydroxylase inhibition. For comparison, the depletion of DA and NA following tyrosine hydroxylase inhibition in discrete brain regions was determined by high pressure liquid chromatography (HPLC). Following tyrosine hydroxylase inhibition using alpha-methyl-DL-p-tyrosine methyl ester, an apparently monophasic decline of the CA stores was demonstrated in all brain regions analysed both histochemically and by HPLC. A multiphasic DA disappearance was measured by HPLC in the peri- and paraventricular hypothalamic area. The DA nerve terminal networks generally had shorter half-lives than the NA nerve terminal networks. The shortest half-life (99 min) of the regions demonstrating a monophasic decline of CA stores was found in the CA nerve terminal system in the medial palisade zone of the median eminence. By the use of CA standards in the histochemically prepared sections, it was possible to convert the measured CA fluorescence into absolute amounts of catecholamines expressed in nmol X g-1 of tissue wet weight. It was shown that the CA stores and the turnover rates measured by quantitative histofluorimetry were 20-30 times greater than those measured using HPLC. The difference has been related to amine dilution with amine-poor areas in the specimens analysed by HPLC. By studying the accumulation of catecholamines after monoamine oxidase inhibition, it could be demonstrated that no concentration-dependent quenching of CA fluorescence occurred.     

Effects of a postnatal exposure to cigarette smoke on hypothalamic catecholamine nerve terminal systems and on neuroendocrine function in the postnatal and adult male rat. Evidence for long-term modulation of anterior pituitary function.

The purpose of this paper was to study the possible long-term effects of postnatal exposure to cigarette smoke. Male Sprague-Dawley rats were exposed to the smoke from 2 cigarettes (Kentucky reference IR-1 type) every morning from day 1 after birth for a period of 5, 10 or 20 days. The rats were decapitated 24 hours (5, 10 and 20 days of exposure), 1 week (20 days of exposure) or 7 months (20 days of exposure) after the last exposure. Using the Falck-Hillarp methodology in combination with quantitative histofluorimetry catecholamine levels and changes in catecholamine utilization (alpha MT-induced CA fluorescence disappearance) in discrete hypothalamic catecholamine nerve terminal systems were analysed. Serum prolactin, LH, TSH and corticosterone levels were determined by means of radioimmunoassay procedures. In the postnatal period serum LH levels were significantly increased 24 hours after a 10 and 20 day exposure to cigarette smoke. In adult life after a 20-day postnatal exposure to cigarette smoke a highly significant increase was observed in serum prolactin levels, which were unaltered by this exposure when measured in the postnatal period. Twenty-four hours following a 20-day postnatal exposure, catecholamine utilization was increased in the medial palisade zone of the median eminence and substantially reduced in the parvocellular and magnocellular parts of the paraventricular hypothalamic nucleus. One week and 7 months following a 20-day postnatal exposure to cigarette smoke no alterations were observed in catecholamine levels or utilization in various hypothalamic areas including the median eminence. All the above changes were observed in the presence of an unaltered development of body weight. The results indicate that marked but temporary increases in LH secretion occur 24 hours after a postnatal exposure to cigarette smoke, while increase in prolactin secretion only develop in adult life, when the maturational processes of the brain and/or the anterior pituitary gland are completed. Changes in catecholamine levels and utilization are found in discrete hypothalamic nerve terminal networks but do not play a major role in mediating the above changes in anterior pituitary function and are probably the result of a withdrawal phenomenon.     

Effect of acute ether or restraint stress on plasma corticotropin-releasing hormone, vasopressin and oxytocin levels in the rat

Ether and restraint stress-induced peripheral plasma corticotropin releasing hormone (CRH), arginine vasopressin (AVP), oxytocin (OXY) and adrenocorticotropin (ACTH) levels were measured by radioimmunoassays. Plasma CRH, AVP, OXY and ACTH rose to approximately twice the level of control rats 2 min after the onset of a 1-min exposure to ether. Plasma CRH rose further 5 min after the onset of ether stress, while plasma AVP and OXY returned to the baseline levels at 5 min. Plasma CRH, OXY and ACTH showed significant elevation 2 min after the onset of restraint stress, while plasma AVP did not show a significant change. Plasma OXY and ACTH rose further 5 min after the onset of restraint stress, whereas plasma CRH returned to baseline levels. CRH and OXY concentrations in the hypothalamic median eminence decreased 5 min after the onset of ether exposure and restraint, while the AVP concentration did not differ from control levels. The results, including the discrepancy between plasma CRH and ACTH 5 min after stress, suggest that CRH in the peripheral plasma is derived from both hypothalamic and extrahypothalamic tissues. The levels of stress-induced CRH in the peripheral plasma were sufficient to stimulate ACTH release. These results suggest that ether and restraint stress elevate plasma CRH shortly after the onset of the stress, and that this elevation in the plasma CRH level is at least partly responsible for stress-induced ACTH secretion.     

Temporal changes in medial basal hypothalamic catecholamines in male Syrian hamsters exposed to short photoperiod

Summary The gonadal and accessory organ atrophy following transfer of male hamsters from long (LP) to short photoperiod (SP) is preceded by reduced prolactin secretion and involves reductions in hypothalamic LHRH release and catecholamine turnover. These experiments examined the temporal aspects of changes in medial basal hypothalamic/ median eminence (MBH/ME) catecholamine turnover rates in male hamsters undergoing SP-induced gonadal atrophy. Hamsters were sacrificed at three, six, nine and twelve weeks of SP exposure. MBH/ME catecholamines and indoleamines were determined by high performance liquid chromatography coupled with electrochemical detection. Reductions in serum prolactin (PRL) levels and increased MBH/ME dopamine (DA) turnover rates were observed at three and six weeks of SP exposure. Both steady state concentrations and turnover rates of norepinephrine (NE) and DA were depressed after nine and twelve weeks of SP exposure, at which time testicular and accessory organ atrophy had occurred. Serotonin (5-HT) and 5-hydroxy-3-indoleacetic acid (5-HIAA) concentrations were insignificantly changed during the period of SP treatment but the 5-HIAA/5-HT ratio was significantly increased after six weeks of SP exposure. It was concluded that increased MBH/ME DA turnover represents an initial, SP-induced neuroendocrine event. This increase in DA turnover probably contributes to the reduced PRL secretion which precedes, and may play a role in the ensuing gonadal and accessory organ atrophy.     

Effects of a postnatal exposure to cigarette smoke on hypothalamic catecholamine nerve terminal systems and on neuroendocrine function in the postnatal and adult male rat. Evidence for long-term modulation of anterior pituitary function

Jansson , A., Anderson , K., Bjelke , B., Fuxe , K. & Eneroth , P. 1991. Effects of a postnatal exposure to cigarette smoke on hypothalamic catecholamine nerve terminal systems and on neuroendocrine function in the postnatal and adult male rat. Evidence for long-term modulation of anterior pituitary function. Acta Physiol Scand. 144 , 453–462. Received 10 August 1 991 , accepted 11 August 1991. ISSN 0001–6772. Department of Histology and Neurobiology, Karolinska Instituter, Stockholm, Sweden, Department of Internal Medicine and Unit for Applied Biochemistry, Huddinge Hospital, Huddinge, Sweden. The purpose of this paper was to study the possible long-term effects of postnatal exposure to cigarette smoke. Male Sprague-Dawley rats were exposed to the smoke from 2 cigarettes (Kentucky reference IR-I type) every morning from day 1 after birth for a period of 5 , 10 or 20 days. The rats were decapitated 24 hours ( 5 , 10 and 20 days of exposure), 1 week (20 days of exposure) or 7 months (20 days of exposure) after the last exposure. Using the Falck-Hillarp methodology in combination with quantitative histofluorimetry catecholamine levels and changes in catecholamine utilization (aMT-induced CA fluorescence disappearance) in discrete hypothalamic catecholamine nerve terminal systems were analysed. Serum prolactin, LH, TSH and corticosterone levels were determined by means of radioimmunoassay procedures. In the postnatal period serum LH levels were significantly increased 24 hours after a 10 and 20 day exposure to cigarette smoke. In adult life after a 20–day postnatal exposure to cigarette smoke a highly significant increase was observed in serum prolactin levels, which were unaltered by this exposure when measured in the postnatal period. Twenty-four hours following a 20–day postnatal exposure, catecholamine utilization was increased in the medial palisade zone of the median eminence and substantially reduced in the parvocellular and magnocellular parts of the paraventricular hypothalamic nucleus. One week and 7 months following a 20–day postnatal exposure to cigarette smoke no alterations were observed in catecholamine levels or utilization in various hypothalamic areas including the median eminence. All the above changes were observed in the presence of an unaltered development of body weight. The results indicate that marked but temporary increases in LH secretion occur 24 hours after a postnatal exposure to cigarette smoke, while increase in prolactin secretion only develop in adult life, when the maturational processes of the brain and/or the anterior pituitary gland are completed. Changes in catecholamine levels and utilization are found in discrete hypothalamic nerve terminal networks but do not play a major role in mediating the above changes in anterior pituitary function and are probably the result of a withdrawal phenomenon.     

Effects of morphine on hypothalamic corticotropin-releasing factor (CRF), norepinephrine and dopamine in non-stressed and stressed rats

The effects of morphine on the hypothalamic corticotropin-releasing factor (CRF), norepinephrine (NE) and dopamine (DA) concentrations were investigated in non-stressed and stressed rats. Acutely administered morphine stimulated both the synthesis and release of CRF in the hypothalamus, thereby activating the pituitary-adrenocortical system in non-stressed rats, but inhibited the stress-induced CRF synthesis and ACTH-corticosterone secretion. Either a morphine or ether-laparotomy stress reduced NE and DA concentrations in the hypothalamus. A pretreatment with morphine inhibited the stress-induced reduction in the hypothalamic NE and DA concentrations, and induced a significant increase in the DA concentration. These observations suggest that hypothalamic NE and DA are involved in morphine-induced changes in hypothalamo-pituitary-adrenocortical (HPA) activity and that endogenous opiates have a role in regulating CRF secretion by interacting with hypothalamic biogenic amines.     

Radioautographic identification of lactogen binding sites in rat median eminence using 125I-human growth hormone: Evidence for a prolactin “short-loop” feedback site

Summary The binding characteristics of human growth hormone were exploited to identify radioautographically lactogen binding sites in the rat median eminence. Following systemic injection ¹²⁵I-human growth hormone bound preferentially to the lateral palisade zone, a region of median eminence rich in dopamine and LHRH. Coinjection of ¹²⁵I-human growth hormone with an excess of unlabeled human growth hormone or ovine prolactin, but not bovine growth hormone, competitively blocked ¹²⁵I-human growth hormone binding to the external median eminence. These observations provide direct evidence of recognition sites for lactogenic hormones in a discrete region of the median eminence associated with hypothalamic regulation of hypophyseal prolactin and luteinizing hormone secretion. Median eminence lactogen binding sites may mediate presumed direct effects of lactogenic hormones on the reproductive functions of the hypophysiotropic hypothalamus.A preliminary report of these findings was presented at the Annual Meeting of the Canadian Society for Clinical Investigation, February 5, 1979, Montreal, and appeared in Clinical Research 26, 874A (1979)     

The influence of salsolinol on dopaminergic system activity within the mediobasal hypothalamus of anestrous sheep: a model for studies on the salsolinol-dopamine relationship

Salsolinol with its derivatives has been considered as a potential neurotoxin for the dopaminergic system in the human and rat brain. Investigating a sheep model for studies on the action of salsolinol within the central nervous system we examined whether this compound is able to affect the hypothalamic neuroendocrine dopaminergic (NEDA) system during its high seasonal activity, when sheep entered to anestrus under the long day conditions. Therefore, salsolinol was infused into the third ventricle of the brain in combination with the in vivo push-pull perfusion of the mediobasal hypothalamus/median eminence (MBH/ME). The effects of this drug on either perfusate noradrenaline (NA) or plasma prolactin concentration were also studied. The infusion of salsolinol resulted in rapid and permanent diminution in dopamine (DA) release into the extracellular spaces of the MBH/ME up to an undetectable level and in the 57% decrease in DA metabolite 3,4-dihydroxyphenylacetic acid concentration, compared to the control. This effect of salsolinol was accompanied by the significant enhancement of the pituitary prolactin release into circulation. The concentration of other DA metabolite, homovanillic acid, as well as NA in the MBH/ME was not affected. Thus, our results in the anestrous sheep underline the role played by salsolinol as a neuromodulator for the hypothalamic NEDA system and as a signal transmitter for the pituitary prolactin release. We suggest that the hypothalamic NEDA system of anestrous sheep during its high secretory activity may be set as a model for studies on the salsolinol-dopamine relationship.     

Age related changes in the control of prolactin secretion in the female rat

The purpose of this investigation was to evaluate the effects of advancing age on the control of pituitary prolactin secretion. The effects of dopaminergic inhibition and estrogen stimulation of pituitary prolactin secretion were tested both in vivo and in vitro. Estrogen stimulated prolactin secretion in both old and young animals, and elevated estrogen levels in old rats may be partially responsible for elevated prolactin levels. Oral L-DOPA administration induced cycles in old rats but had no effect on prolactin levels in either old or young rats. Injections of L-DOPA lowered prolactin in young but not in old rats, while apomorphine reduced prolactin levels in both groups. The pituitaries of young rats secrete more prolactin in vitro than old pituitaries, further supporting a decrease in hypothalamic DA turnover as a cause for elevated prolactin levels in old rats.     

Hypo-osmolarity stimulates and high sodium concentration inhibits thyrotropin-releasing hormone secretion from rat hypothalamus.

The hypothalamic paraventricular nucleus, representing cell bodies in which thyrotropin-releasing hormone is synthesized, and the median eminence, representing nerve terminals, were incubated in vitro. Various hypo- and hyperosmotic solutions were tested to determine osmotic sensitivity of thyrotropin-releasing hormone secretion. High KCl (56 mM) causing membrane depolarization was used as a non-specific control stimulus to induce thyrotropin-releasing hormone secretion. A 30% decrease of medium osmolarity (from 288 to 202 mOsmol/l) increased thyrotropin-releasing hormone secretion from both the paraventricular nucleus and median eminence. A 30% decrease of medium NaCl content by its replacement with choline chloride did not affect basal thyrotropin-releasing hormone secretion. Increasing medium osmolarity with biologically inactive L-glucose did not affect basal or KCl-induced thyrotropin-releasing hormone secretion from either structure. Medium made hyperosmotic (350-450 mOsmol/l) by increasing the NaCl concentration resulted in a dose-dependent decrease of basal thyrotropin-releasing hormone secretion and abolished KCl-induced thyrotropin-releasing hormone secretion. If an osmotically equivalent amount of choline chloride was substituted for NaCl, there was no effect on thyrotropin-releasing hormone secretion, indicating a specific action of Na+. This study indicates a specific sensitivity to high concentrations of Na+ ions of both thyrotropin-releasing hormone-producing parvocellular paraventricular neurons and thyrotropin-releasing hormone-containing nerve terminals in the median eminence.     

Superior cervical ganglionectomy differentially modifies median eminence and anterior and mediobasal hypothalamic GABA content in male rats: effects of hyperprolactinemia

This work was undertaken to analyze the changes in GABA concentrations in the anterior and mediobasal hypothalamus and in the median eminence after acute or chronic superior cervical ganglionectomy (SCGx), and whether high prolactin levels interfere with the effects of SCGx on GABA content. Acute but not chronic SCGx increased GABA content in all the areas studied, as compared to controls. The presence of a pituitary graft abolished the effects of acute SCGx in the median eminence and anterior hypothalamus, as compared to controls, but potentiated its effects in the mediobasal hypothalamus. Chronic SCGx increased GABA content in the mediobasal and anterior hypothalami, as compared to pituitary grafted controls. Acute SCGx decreased plasma prolactin and GH levels, but chronic surgery did not modify these hormone plasma levels. Acute SCGx increased plasma ACTH levels, whereas chronic SCGx did not modify them. Pituitary grafting increased circulating values of prolactin, ACTH and GH, as compared to controls. Acute SCGx did not modify plasma prolactin levels in grafted animals, although it increased plasma GH levels and decreased those of ACTH in this experimental group. Chronic SCGx further increased both plasma prolactin and GH levels, without modifying plasma ACTH levels. These results suggest that SCGx differentially modifies GABA content within the hypothalamus and median eminence. Induction of hyperprolactinemia in the neonatal age interferes with SCGx effects on both GABA content within the hypothalamus and median eminence and the secretory patterns of the pituitary hormones studied.