Opioid-salsolinol relationship in the control of prolactin release during lactation

Endogenous opioid peptides (EOP) and dopamine (DA)-derived salsolinol are implicated in the suckling-induced prolactin surge. The aim of this study was to investigate the relationship between the opioidergic and salsolinergic activity in the mediobasal hypothalamus of nursing sheep. The sheep were infused intracerebroventricularly with opioid receptors antagonists: naloxone (all types of receptors, n=6); naloxonazine (μ receptor, n=6) or the vehicle (control, n=6) in a series of five 30-min infusions (60 μg/60 μl) from 10:00 to 15:00, at 30-min intervals. The period of the experiment included the non-suckling (10:00–12:30) and suckling (12:30–15:00) periods. Simultaneously, a push–pull perfusion of the infundibular nucleus/median eminence was performed in every sheep to study the dopaminergic system activity. Blood samples were also collected at 10-minute intervals to determine plasma prolactin concentration. Both the mean perfusate salsolinol and plasma prolactin concentrations were higher during the suckling vs. non-suckling (P<0.001) period in the control. The perfusate DA concentration was below the detection limit in this group. Treatment with either naloxone or naloxonazine significantly (P<0.01) diminished plasma prolactin concentration, as compared with the controls and blocked the prolactin surge during suckling. In drug-infused sheep, the perfusate salsolinol concentration was below the detection limit but the increased DA and its metabolite 3,4-dihydroxyphenylacetic acid concentrations were observed. In conclusion, the stimulatory action of EOP on prolactin secretion in nursing females is mediated, at least in part, by salsolinol, and the ligands for μ opioid receptor may be the primary factors of this relationship, especially with respect to the suckling-induced prolactin surge.     

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.     

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.     

Daily GnRH and LH secretion in ewes is not modified by exogenous melatonin during seasonal anestrus

The effect of central, short-term melatonin administration on daily GnRH and LH secretion was studied in ewes during seasonal anestrus. Melatonin, in a total dose of 32 micrograms and the vehicle were perfused for 4 hours into the mediobasal hypothalamus/median eminence (MBH/ME). The mean GnRH concentration during perfusion with melatonin decreased significantly (P < 0.05), as compared to the concentration during the preceding perfusion with the vehicle only. This change resulted from high variations in GnRH concentration noted during the initial phase of perfusion rather than from an action of melatonin. Melatonin perfused into the MBH/ME did not significantly affect LH secretion. A higher dose of melatonin and vehicle were also infused intracerebroventricularly (icv.) in either intact (300 micrograms for 3 hours) or ovariectomized (OVX) ewes (400 micrograms for 4 hours, 100 micrograms/100 microliters/h). In the intact animals, melatonin did not significantly affect LH secretion. Interestingly, melatonin significantly decreased (P < 0.05) the number of LH peaks in OVX ewes. These results demonstrate that melatonin delivered for a few hours directly into the central nervous system did not affect either daily hypothalamic GnRH release or pituitary LH secretion in intact ewes during seasonal anestrus, but did modify pulsatile LH secretion in ewes deprived of the negative feedback of estradiol.     

Comparisons between brain dopaminergic neurons of juvenile and aged rats: sex-related differences.

It is known that several aspects of dopaminergic neurotransmission deteriorate with advanced age. In the present report, we have studied the possible existence of sexual differences in these aging-induced changes. Thus, we measured several pre- and postsynaptic biochemical parameters, indicative of the activity of dopaminergic neurons, in striatum, limbic forebrain and hypothalamic-anterior pituitary area of aged (24-26 months) and young (2 months) rats of both sexes. Tyrosine hydroxylase (TH) activity, as well as the number of D2-dopaminergic receptors, decreased in the striatum of aged rats, especially in the males in which the decrease in the number of receptors was associated with an increase in their affinity. In addition, the ratio between dopamine (DA) and its intraneuronal metabolite, L-3,4-dihydroxyphenyl-acetic acid (DOPAC), which can be used as an index of neurotransmitter turnover, was increased in aged females in parallel with a decreased DA content. In the limbic forebrain, TH activity was also decreased during aging, but only in males, whereas the DOPAC/DA ratio was increased in females, although in parallel with an increased DOPAC production. Finally, in the hypothalamic-anterior pituitary area, aging only affected the females, in which increased plasma prolactin levels were observed. This effect might be the result of a low responsiveness of pituitary lactotrophs to DA released from hypothalamic neurons, in spite of high prolactin levels producing a constant, although ineffective, stimulation of the activity of these neurons, as reflected by the high DOPAC content and DOPAC/DA ratio observed in the medial basal hypothalamus. In summary, these data allow us to suggest that the activity of brain dopaminergic neurons is modified with aging and there are significant differences as a function of sex and brain area.     

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).     

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.     

Increase in pituitary dopaminergic receptors after monosodium glutamate treatment

We investigated whether a decrease in arcuate nucleus dopamine (DA) levels resulting from neonatal treatment with monosodium glutamate (MSG) affects the anterior pituitary DA receptors in adult male rats. MSG treatment resulted in a significant reduction in medial basal hypothalamic (MBH) DA levels, no change in its norepinephrine (NE) and epinephrine (E) concentrations, and a marked increase in circulating prolactin (PRL). Scatchard analyses of DA binding characteristics to anterior pituitary membranes using [3H]spiperone revealed linear plots, suggesting a single class of high-affinity, low-capacity binding sites. The DA binding capacity was significantly higher in MSG-treated rats than in controls with no change in affinity. The data indicate that anterior pituitary DA receptors change in accordance with altered physiological conditions. The increase in the number of DA receptors following destruction of the arcuate nucleus is probably a direct effect of reduced DA levels reaching the anterior pituitary gland.     

Regional changes in monoamine metabolism in the aging constant estrous rat

Studies were undertaken to determine levels of monoamines and their metabolites in brain regions in young (3–4 months) normally cycling and old (25–26 month) constant estrous female rats. Dopamine (DA) concentrations were reduced in old rats in the median eminence (ME), medial basal hypothalamus (MBH), preoptic area-anterior hypothalamus (POA-AH) and the striatum. Similarly, concentrations of dihydroxyphenylacetic acid (DOPAC), the major acid metabolite of DA, were reduced significantly in all 4 regions. In the ME, a strong positive correlation was observed between DA and DOPAC concentrations in both young and old rats. Concentrations of norepinephrine (NE) were reduced in old rats in the MBH and POA-AH but not in the ME or striatum. Concentrations of serotonin (5HT) and its major metabolite, 5-hydroxyindoleacetic acid (5HIAA) were generally unchanged with age in all of the regions examined. These studies indicate the age-related regional alterations in DA and 5HT metabolism can be monitored by methods which quantitate monoamines and their metabolites.     

Regional changes in monoamine metabolism in the aging constant estrous rat

Studies were undertaken to determine levels of monoamines and their metabolites in brain regions in young (3–4 months) normally cycling and old (25–26 month) constant estrous female rats. Dopamine (DA) concentrations were reduced in old rats in the median eminence (ME), medial basal hypothalamus (MBH), preoptic area-anterior hypothalamus (POA-AH) and the striatum. Similarly, concentrations of dihydroxyphenylacetic acid (DOPAC), the major acid metabolite of DA, were reduced significantly in all 4 regions. In the ME, a strong positive correlation was observed between DA and DOPAC concentrations in both young and old rats. Concentrations of norepinephrine (NE) were reduced in old rats in the MBH and POA-AH but not in the ME or striatum. Concentrations of serotonin (5HT) and its major metabolite, 5-hydroxyindoleacetic acid (5HIAA) were generally unchanged with age in all of the regions examined. These studies indicate the age-related regional alterations in DA and 5HT metabolism can be monitored by methods which quantitate monoamines and their metabolites.     

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.     

Immobilization stress-induced changes in discrete hypothalamic catecholamine levels and turnover, their modulation by nicotine and relationship to neuroendocrine function

Immobilization stress (1 h) induced discrete reductions in noradrenaline (NA) levels in the posterior periventricular hypothalamic region and in the paraventricular hypothalamic nucleus, and a decrease in dopamine (DA) turnover in the medial palisade zone (MPZ) of the median eminence, but failed to induce regional increases of hypothalamic NA turnover. Stress also stimulated the secretion of ACTH, corticosterone and prolactin, while vasopressin, LH and FSH serum levels were unaffected. The stress induced reduction of DA turnover in MPZ may mediate the stress induced increase of prolactin secretion. Nicotine (0.3 mg/kg, s.c., 1 h) did not by itself significantly influence catecholamine (CA) turnover in the various CA nerve terminal systems analyzed in the hypothalamus, but reduced NA levels in the subependymal layer (SEL) of the median eminence. Nicotine administration did not affect the serum levels of any of the hormones evaluated. Nicotine counteracted to a minor degree the immobilization stress-induced reduction in NA levels, and also the stress-induced secretion of ACTH, but not of prolactin suggesting the involvement of noradrenergic mechanisms possibly in the paraventricular nucleus in the nicotine modulation of stress induced increases of ACTH secretion. The nicotine-induced reduction of NA levels in SEL was blocked by stress as well as the tendency for nicotine induced increases of dopamine (DA) turnover in the medial and lateral palisade zones of the median eminence indicating opposing influences of immobilization stress and nicotine on at least some hypothalamic CA systems.     

Effects of clonidine and naloxone on the dopamine levels in the rat mediobasal hypothalamus measured by in vivo voltammetry

We examined the effect of alpha 2-adrenergic stimulation on dopamine (DA) levels in the mediobasal hypothalamus (MBH) by means of in vivo voltammetry in the rat. Intravenous (i.v.) injection of an alpha 2-agonist, clonidine (15 micrograms/100 g b.wt.) caused a decrease of MBH DA levels (76 +/- 3% of basal levels). Pretreatment with phenoxybenzamine (500 micrograms/100 g b.wt.i.v.), an alpha-antagonist, blunted the decrease of MBH DA levels induced by clonidine. Naloxone (125 micrograms/100 g b.wt.i.v.), an opioid antagonist, blunted the decrease of MBH DA levels induced by clonidine. These results suggest that alpha 2-adrenergic mechanisms may inhibit the tuberoinfundibular dopaminergic (TIDA) system through the activation of the endogenous opioid system.     

Pituitary modulation of sulpiride action in the central nervous system of the rat

To verify whether the stimulation by sulpiride of hypothalamic adenylate cyclase was direct or mediated by release of pituitary hormones, the effect of sulpiride on female hypophysectomized rats was studied. In these animals sulpiride does not substantially modify hypothalamic adenylate cyclase, brain 3,4-dihydroxyphenylacetic acid (DOPAC) levels and serum prolactin concentrations. Chlorpromazine on the contrary inhibits hypothalamic adenylate cyclase activity and increases DOPAC levels both in intact and in hypophysectomized rats.

As sulpiride hardly crosses the blood-brain barrier, its action on pituitary with prolactin release seems to be essential to start the neurochemical phenomena in the central nervous system.     

K+-induced thyrotropin-releasing hormone release from superfused mediobasal hypothalami in rats. Inhibition by somatostatin

Somatostatin (SRIF), in concentration of 10(-6) M, significantly inhibited the depolarization-induced release of immunoreactive thyrotropin-releasing hormone (IR-TRH) from superfused mediobasal hypothalami (MBH) containing mainly the median eminence (ME), without affecting the basal release of TRH. The total amount of K+-induced TRH release was 0.24 +/- 0.02 and 0.61 +/- 0.08 pg/MBH/min, respectively, in the presence and absence of SRIF in the medium. The data are consistent with a role of SRIF as a neuromodulator on TRH release from the ME. In contrast, superfusion with Locke medium containing triiodothyronine (10(-6) M) had no effect on basal and K+-induced IR-TRH release in our system.     

Comparative study of dopamine- and noradrenaline-immunoreactive terminals in the paraventricular and supraoptic nuclei of the rat

The distribution of dopaminergic and noradrenergic terminal fields of the paraventricular (PVN) and supraoptic (SON) nuclei of the rat was investigated at the optic and electron microscopical level using antibodies directed against dopamine (DA) and noradrenaline (NA). The DA innervation was uniform among these nuclei, although more important in the PVN than in the SON. NA fibers were preferentially distributed in the parvocellular parts of the PVN and in areas of the magnocellular nuclei where vasopressinergic neurons were mainly located. Both DA and NA terminals synaptically contacted magnocellular neurons on their cell body or dendrites. This study thus provides morphological evidence for a double and independent catecholaminergic control, by DA and NA, on neuroendocrine mechanisms at the hypothalamic level.     

Ageing and melatonin influence on in vitro gonadotropins and prolactin secretion from pituitary and median eminence

The effect of ageing and/or melatonin (MEL) on in vitro gonadotropins, luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) release and tissue content from pituitary and median eminence (ME) were investigated. Gonadotropins and PRL basal release (I-1) from hemipituitaries of young cyclic-rats was decreased by MEL to levels shown in old acyclic rats. Pituitary tissue content of LH and PRL were not affected by ageing or MEL treatment. However, pituitary FSH tissue content was decreased by ageing and MEL, suggesting a different regulatory mechanism. MEL inhibitory influence on pituitary hormones is mainly exerted on the secretory process. This effect is only exerted in young rats. ME LH and PRL release and content were significantly lower than in pituitary. However, FSH release and content in ME showed values similar to those found in the pituitary. This study confirms that the functional capacities of pituitary gland and ME are maintained during reproductive senescence.     

Different times of withdrawal from cocaine administration cause changes in muscarinic and dopaminergic receptors in rat premotor cortex

Concentration of dopamine (DA) and its metabolite, 3,4-dihydroxyphenylacetic acid is quantified by high-pressure liquid chromatography with a coulometric detection system in the eye of Xenopus laevis through ontogeny and in adults at two times during photocycle (midday and midnight). Ocular dopaminergic activity remains low during pre- and prometamorphosis and significantly rises in postmetamorphic froglets. This increase is more pronounced at midnight than at midday. The dualism of DA content versus DA release in Xenopus ocular tissue is studied in an eyecup culture system. On a 24-h cycle of DA release from adult Xenopus eyecups the highest DA release by eyecups is produced during daytime, and significantly decreases in darkness. From these results it can be concluded that in spite of the early development of the retinal dopaminergic system in the ontogeny of Xenopus, the final maturation must occur during the metamorphic climax. Endogenous DA release is significantly inhibited by light offset, which explains the higher ocular DA content found at midnight as compared to midday in postmetamorphic froglets and adults.     

Plasma noradrenaline and dopamine in renin-mediated hypertension

Noradrenaline (NA) and dopamine (DA) have opposite effects on the kidney; NA causes vasoconstriction and increased sodium reabsorption while DA promotes vasodilation and natriuresis. In 15 patients investigated for renin-mediated hypertension measurements of plasma renin activity (PRA), NA and DA concentrations were made in arterial and renal venous blood from both kidneys before and after acute stimulation of renin release by i.v. dihydralazine. Nine patients had unilateral renin secretion and were classified as renin-positive, while the remaining six patients were renin-negative. Renin-positive patients had higher arterial and renal venous PRA, NA and DA levels than the negative ones. In the renin-positive group V-A differences for NA and DA were present on both sides despite unilateral secretion of renin. NA but not DA levels were higher in the renin-secreting kidney, which can partly be explained by the reduced plasma flow to the involved kidney. After dihydralazine the arterial NA and DA rose similarly in renin-positive and renin-negative patients, while PRA rose only in the renin-positive cases. In the renin-positive patients where stimulation of renin secretion caused a marked increase of the PRA gradient on the affected side only, renal gradients for NA and DA increased bilaterally. The increase in DA was more pronounced than that of NA yielding a rise in DA/NA ratio on the affected side. Arterial PRA was positively correlated to the plasma concentrations of NA and DA. V-A differences for PRA and NA or DA were positively correlated on the involved renin-secreting side. In summary, patients with renin-dependent hypertension have elevated plasma NA and DA concentrations. Stimulation of renin release by dihydralazine increases the DA/NA ratio in arterial and renal venous blood indicating release of 'precursor dopamine' from noradrenergic fibres and/or activation of dopaminergic nerves. There seems to be a relationship between renal nerve activity and renin release in renin-dependent hypertension.     

Influence of GnRH agonist and neural antagonists on stress-blockade of LH and prolactin surges induced by 17beta-estradiol in ovariectomized rats

In our previous study, we demonstrated that immobilization stress blocked estrogen-induced luteinizing hormone (LH) surge possibly by inhibiting the synthesis and release of gonadotropin-releasing hormone (GnRH) at the hypothalamic level and by blocking estrogen-induced prolactin (PRL) surge by increasing the synthesis of dopamine receptor at the pituitary level in ovariectomized rats. The present study was performed to determine whether immobilization stress affects pituitary LH responsiveness to GnRH, and whether endogenous opioid peptide (EOP) and dopamine systems are involved in blocking LH and PRL surges during immobilization stress. Immobilization stress was found to inhibit basal LH release and to completely abolish LH surge. However, the intravenous application of GnRH agonist completely restored immobilization-blocked LH surge and basal LH release. Treatment with naloxone did not exert any effect on immobilization-blocked LH surge but increased basal LH release during immobilization stress. Pimozide did not affect immobilization-blocked LH surge or basal LH release. Naloxone also decreased immobilization-induced basal PRL release, but had no effect on immobilization-blocked PRL surge. Immobilization-increased basal PRL levels were augmented by pimozide treatment and immobilization-blocked PRL surge was dramatically restored by pimozide. We conclude that immobilization stress does not impair pituitary LH response to GnRH, and that the immobilization stress-induced blockage of LH surge is probably not mediated by either the opioidergic or the dopaminergic system. However, immobilization-blockade of PRL surge may be partly mediated by the dopaminergic system.