Catecholamine mechanisms in medio-basal hypothalamus influence prolactin but not growth hormone secretion

Cervical stimulation activates A1 and locus coeruleus neurons that project to the paraventricular nucleus of the hypothalamus

In female rats, stimulation of the uterine cervix during mating induces two daily surges of prolactin. Inhibition of hypothalamic dopamine release and stimulation of oxytocin neurons in the paraventricular nucleus (PVN) are required for prolactin secretion. We aim to better understand how stimulation of the uterine cervix is translated into two daily prolactin surges. We hypothesize that noradrenergic neurons in the A1, A2, and locus coeruleus (LC) are responsible for conveying the peripheral stimulus to the PVN. In order to determine whether projections from these neurons to the PVN are activated by cervical stimulation (CS), we injected a retrograde tracer, Fluoro-Gold (FG), into the PVN of ovariectomized rats. Fourteen days after injection, animals were submitted to artificial CS or handling and perfused with a fixative solution. Brains were removed and sectioned from the A1, A2, and LC for c-Fos, tyrosine hydroxylase (TH), and FG triple-labeling using immunohistochemistry. CS increased the percentage of TH/FG+ double-labeled neurons expressing c-Fos in the A1 and LC. CS also increased the percentage of TH+ neurons expressing c-Fos within the A1 and A2, independent of their projections to the PVN. Our data reinforce the significant contributions of the A1 and A2 to carry sensory information during mating, and provide evidence of a functional pathway in which CS activates A1 and LC neurons projecting to the PVN, which is potentially involved in the translation of CS into two daily prolactin surges.

Noradrenaline release in the medial preoptic area during the rat oestrous cycle: temporal relationship with plasma secretory surges of prolactin and luteinising hormone

During the rat oestrous cycle, the afternoon of pro-oestrous is characterised by preovulatory surges of luteinising hormone (LH) and prolactin. On the afternoon of oestrous, a secretory surge of prolactin has also been reported. Because the medial preoptic area (MPOA) is known to regulate prolactin and LH secretory surges and noradrenaline has been demonstrated to stimulate these hormones release, we evaluated whether noradrenaline release in the MPOA was temporally associated with plasma prolactin and LH surges in cycling rats. During the 4 days of oestrous cycle, noradrenaline concentrations were determined in microdialysates from the MPOA, collected at 30-min intervals from 10.30 h to 19.00 h. Plasma prolactin and LH levels were measured in blood samples withdrawn hourly from 14.00 h to 19.00 h on pro-oestrous and from 13.00 h to 18.00 h on the other days of the cycle. On the afternoons of both pro-oestrous and oestrous, noradrenaline levels increased at 14.00 h and remained elevated until 16.30 h. Conversely, they were low and constant throughout metoestrous and dioestrous. Correlating with noradrenaline release in the MPOA, plasma prolactin surges occurred during the afternoons of both pro-oestrous and oestrous. On pro-oestrous, the afternoon LH surge was also preceded by the increase in MPOA noradrenaline whereas, during oestrous, LH secretion was low and unaltered. A temporal association between noradrenaline release and prolactin secretion suggests that noradrenergic neurotransmission in the MPOA regulates prolactin surges in female rats. Moreover, our data also suggest that MPOA noradrenaline requires specific conditions to physiologically regulate LH secretion, which seems to occur during the afternoon of pro-oestrous.

Locus coeruleus norepinephrine regulates the surge of prolactin during oestrus

A secondary surge of prolactin has been recently characterised on the afternoon of oestrus. Because the noradrenergic nucleus locus coeruleus participates in the genesis of the pro-oestrous and steroid-induced surges of prolactin, the aim of the present study was to investigate the importance of locus coeruleus norepinephrine in the generation of the prolactin surge of oestrus. For this purpose, we initially re-evaluated the profile of prolactin secretion during the oestrous cycle to verify whether this surge of prolactin was physiological and specific to the day of oestrus. Thereafter, the following were evaluated: (i) the effect of locus coeruleus lesion on the secondary surge of prolactin and on norepinephrine concentration in the medial preoptic area (MPOA), medial basal hypothalamus (MBH) and paraventricular nucleus (PVN) during the day of oestrus and (ii) locus coeruleus neurones activity during the same day by Fos immunoreactivity. Locus coeruleus lesion completely blocked the prolactin surge of oestrus in all rats studied and also significantly reduced norepinephrine concentration in the MPOA, MBH and PVN during the day of oestrus. The number of double-labelled tyrosine hydroxylase/Fos immunoreactive neurones in locus coeruleus was significantly higher at 14.00 h of oestrus, suggesting an increase in its activity preceding the prolactin surge that generally occurs at 15.00 h. Therefore, the increase in locus coeruleus activity on the afternoon of oestrus supports the data obtained with bilateral lesion of this nucleus, suggesting a stimulatory role of locus coeruleus norepinephrine in the genesis of the secondary surge of prolactin.

Role of the locus coeruleus in the prolactin secretion of female rats

Since locus coeruleus (LC) lesion blocks preovulatory prolactin surge, the aim of this study was to determine if this lesion would also block prolactin surges induced by steroids in ovariectomized rats and would modify basal prolactin secretion. To determine the time of the steroid-induced prolactin surges, ovariectomized rats treated with estradiol (OVE) or estradiol and progesterone (OVEP) were cannulated at 08:00 h and blood samples were collected hourly between 14:00 and 18:00 h. Ovariectomized rats treated with oil (OV-Oil) were used as control. Prolactin peaked at 16:00 h in OVE rats and at 15:00 h in OVEP. In a second experiment, male rats, cycling rats, OVE, OVEP, and OV-Oil groups were cannulated at 08:00 h, followed by LC lesion or sham-surgery. Blood samples were withdrawn at times of basal and peak prolactin levels. LC lesion blocked afternoon prolactin surges of OVE, OVEP and proestrus rats. However, the low levels observed at 16:00 h in OV-Oil, diestrus and male rats as well as at 11:00 h in OVE, OVEP, estrus, and proestrus rats were not modified by LC lesion. The high prolactin levels observed on estrus afternoon were dramatically reduced by LC lesion. Data suggest that LC neurons are important for steroid-induced prolactin surge genesis, but not for prolactin basal secretion.

Effect of cadmium on 24-h variations in hypothalamic dopamine and serotonin metabolism in adult male rats

This study was designed to analyze the possible cadmium effects on time-of-day variations of anterior, mediobasal, and posterior hypothalamic contents of dopamine (DA), serotonin (5-HT), and norepinephrine (NE) content in adult male rats. Also DA and 5-HT metabolism, as expressed by the ratio 3,4-dihydroxyphenyl acetic acid (DOPAC) to DA and 5-hydroxyindoleacetic acid (5-HIAA) to 5-HT, respectively, were studied. Adult male rats were given cadmium at a dose of 25 ppm of cadmium chloride in drinking water for 1 month. Age-matched rats having access to cadmium-free water were used as controls. Weight gain for the whole period was not changed by cadmium exposure. The metal accumulated in the hypothalamus of rats. In the three hypothalamic regions, significant 24-h variations of NE and 5-HT concentration were found in controls, while DA content changed rhythmically in mediobasal hypothalamus only. Mean content of NE, 5-HT, and DA of anterior, mediobasal, and posterior hypothalamus decreased after cadmium exposure. After cadmium the 24-h pattern of NE changed only in mediobasal hypothalamus, whereas the metal changed significantly the pattern of 5-HT in all regions. DOPAC to DA and 5-HIAA to 5-HT ratios decreased and were differentially changed in all hypothalamic regions analyzed in cadmium-treated rats. There was a statistically significant relationship between time of administration of metal and time that the change took place in biogenic amines in the hypothalamus. These results indicate that cadmium may depress hypothalamic biogenic amine release.

Physiological role of salsolinol: its hypophysiotrophic function in the regulation of pituitary prolactin secretion

We have recently observed that 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol) produced by hypothalamic neurons can selectively release prolactin from the anterior lobe (AL) of the pituitary gland. Moreover, high affinity binding sites for SAL have been detected in areas, like median eminence (ME) and the neuro-intermediate lobe (NIL) that are known terminal fields of the tuberoinfundibular DAergic (TIDA) and tuberohypophysial (THDA)/periventricular (PHDA) DAergic systems of the hypothalamus, respectively. However, the in situ biosynthesis and the mechanism of action of SAL are still enigmatic, these observations clearly suggest that sites other than the AL might be targets of SAL action. Based on our recent observations it may be relevant to postulate that an "autosynaptocrine" regulatory mechanism functioning at the level of the DAergic terminals localized in both the ME and NIL, may play a role in the hypophyseotrophic regulation of PRL secretion. Furthermore, SAL may be a key player in these processes. The complete and precise mapping of these intra-terminal mechanisms should help us to understand the tonic DAerg regulation of PRL secretion. Moreover, it may also give insight into the role of pre-synaptic processes that most likely have distinct and significant functional as well as pathological roles in other brain areas using DAergic neurotransmission, like striatonigral and mesolimbic systems.

Thyrotropin-releasing hormone: inhibitory function on growth hormone through both somatostatin and growth hormone-releasing factor neurons

Double labelling immunohistochemistry using antibodies to thyrotropin releasing hormone (TRH) and somatostatin (SS) was undertaken in the anterior hypothalamus in 6 rats. Light microscopic quantitation revealed that 94.5% of SS immunopositive perikarya in the preoptic anterior hypothalamic area (PO/AHA) and 97.5% in the paraventricular nucleus appeared to be contacted by one or more TRH immunopositive terminals. In the chronically cannulated unanaesthetised male rat, unilateral microinjections of a range of doses of TRH were made in the PO/AHA, where SS neurons are located, or in the medial basal hypothalamus, where growth hormone (GH)-releasing factor (GRF) neurons are located. Transient reductions in GH plasma levels occurred only after injections of the highest (10 nmol) dose of TRH in both sites. The function of TRH inputs to both somatostatin and GRF neurons appears to be inhibitory for GH. The physiological conditions in which these inputs function remain to be defined.

Alpha 2 and beta adrenoceptors in the mediobasal hypothalamus and alpha 2 adrenoceptors in the preoptic-anterior hypothalamus stimulate prolactin secretion in the conscious male rat

Plasma prolactin concentrations were measured in unanaesthetized male rats before and after stereotaxic microinjection of adrenergic agents into the mediobasal and preoptic-anterior hypothalamus. In the mediobasal hypothalamus injection of the alpha 2 agonist clonidine produced a dose-dependent increase in prolactin secretion over the dose range 0.1 to 10 nmoles, the stimulation due to 1 nmole being blocked by idazoxan (alpha 2 antagonist). Stimulation of prolactin release was also caused by isoprenaline (beta agonist) and was significantly reduced by the beta antagonist propranolol. The beta 2 agonist salbutamol was also effective in stimulating prolactin secretion. However, the adrenergic agonists, noradrenaline (mixed alpha and beta), phenylephrine (alpha 1) and tyramine (sympathomimetic) failed to affect prolactin secretion. In the preoptic-anterior hypothalamus clonidine caused a dose-dependent increase in prolactin secretion over the dose range 0.001 to 10 nmoles, the stimulation due to 0.1 nmole being abolished by idazoxan. While prolactin levels were significantly elevated by noradrenaline and tyramine, phenylephrine was ineffective. We conclude that the activation of alpha 2 and beta 2 adrenoceptors in the mediobasal hypothalamus and of alpha 2 adrenoceptors in the preoptic-anterior hypothalamus, on or near prolactin-regulating neurons, results in increased prolactin secretion. An alpha 1 inhibitory action in the mediobasal hypothalamus has however not been ruled out. Adrenergic inputs in the preoptic-anterior hypothalamus appear to exert a predominant facilitatory effect on prolactin secretion.

Drug-induced changes in prolactin secretion. Clinical implications

Prolactin secretion is affected by various diseases as well as by many drugs in humans and animals. While marked hyperprolactinaemia suggests the presence of a pituitary tumor, moderate changes may also occur in various endocrine or non-endocrine disorders. Drugs can interfere with prolactin regulation via complex mechanisms at the hypothalamus or at the pituitary site, but possible changes in prolactin metabolism are poorly understood as yet. This survey of the literature up to June 1986 covers the influence of various groups of drugs and agents on the plasma prolactin level under various conditions. It contains information that will facilitate evaluation of whether hyper- or hypoprolactinaemia may result from therapeutic intervention or must be related to an underlying disease. It is obvious that more subtle changes can be revealed by the use of dynamic tests either to stimulate or to suppress prolactin secretion.

Body temperature and plasma prolactin and norepinephrine relationships during exercise in a warm environment: effect of dehydration

The effects of euhydration (Eh) and light (Dh1) and moderate (Dh2) dehydrations on plasma prolactin (PRL) levels were studied in 5 young male volunteers at rest and during exercise to exhaustion (50% of VO2max) in a warm environment (Tdb = 35 degrees C, rh = 20-30%). Light and moderate dehydrations (loss of 1.1 and 1.8% body respectively) were obtained before exercise by controlled hyperthermia. Compared to Eh, time for exhaustion was reduced in Dh1 and Dh2 (p less than 0.01) and rectal temperature (Tre) rose faster in Dh2 (p less than 0.05). Both venous plasma PRL and norepinephrine (NE) increased during exercise at any hydration level (p less than 0.05). Plasma PRL reached higher values after 40 and 60 min in Dh2 and Dh1 (p less than 0.05). Plasma NE values were higher in Dh2 at rest and at the 40th min during exercise (p less than 0.05). Plasma PRL was linearly correlated to Tre and plasma NE (p less than 0.001) but unrelated to plasma volume variation and osmolality. Our results provide further evidence for the major effect of body temperature in exercise-induced PRL changes. Moreover, the plasma PRL-NE relationship suggests that these changes may result from central noradrenergic activation.

Stimulation of prolactin secretion by L-3,4-dihydroxyphenyl-serine (L-DOPS) via central norepinephrine in the rat

Intracerebroventricular (icv) injection of L-3,4-dihydroxyphenylserine (L-DOPS) (50 and 250 micrograms/rat) raised in a dose-related manner both plasma prolactin (PRL) and CSF norepinephrine (NE) in urethane-anesthetized male rats. Intravenous (iv) injection of larger doses of L-DOPS (5 and 10 mg/100 g BW) slightly but significantly increased plasma PRL and CSF NE. L-DOPS injection (50 micrograms/rat, icv or 5 mg/100 g BW, iv) also raised plasma PRL in conscious rats. There was a good correlation (r = 0.74) between CSF NE and peak plasma PRL in the anesthetized animals. Propranolol (100 micrograms/100 g BW, iv) inhibited plasma PRL responses to L-DOPS (50 micrograms/rat, icv) and NE injection (1 microgram/rat, icv) raised plasma PRL in anesthetized animals. These findings indicate that L-DOPS stimulates PRL secretion via central noradrenergic mechanisms in the rat.

Adrenoceptors in the preoptic-anterior hypothalamic area stimulate secretion of prolactin but not growth hormone in the male rat

Plasma growth hormone (GH) and prolactin concentrations were measured by radioimmunoassay in unanesthetized male rats after stereotaxic microinjection of adrenergic agents and 6-hydroxydopamine into the preoptic-anterior hypothalamic area (PO/AHA). Norepinephrine, epinephrine, isoprenaline and clonidine failed to stimulate GH, moreover, 16 nanomoles norepinephrine produced a decrease. However, these agents stimulated prolactin secretion and the mixed alpha antagonist phentolamine, administered systemically, inhibited the stimulatory action of epinephrine on prolactin secretion. GH and prolactin secretory patterns were not affected by 6-hydroxydopamine disruption of catecholamine terminals in the PO/AHA. GH responses to adrenergic agonists and the failure of 6-hydroxydopamine to affect GH secretory patterns indicate that PO/AHA norepinephrine afferents do not facilitate GH secretion. Taken in conjunction with previous studies, the results suggest that there must be an extra-hypothalamic site at which norepinephrine is stimulatory for GH. Prolactin responses suggest that alpha adrenoceptors in the PO/AHA may participate in prolactin secretion.

The distribution of growth-hormone-releasing factor (GRF) immunoreactivity in the central nervous system of the rat: an immunohistochemical study using antisera directed against rat hypothalamic GRF

Immunohistochemical methods have been used to chart the distribution of rat hypothalamic growth-hormone-releasing factor (rhGRF) immunoreactivity in the brains of normal and colchicine-treated adult albino rats. The results suggest the existence of at least two distinct rhGRF-containing systems: one responsible for delivery of the peptide to portal vessels in the median eminence, and one whose relationship, if any, to hypophysiotropic function is less direct. A dense plexus of rhGRF-stained fibers was found throughout the external lamina of the median eminence that is the route by which the peptide is delivered to the anterior pituitary. This projection appears to arise primarily from a group of rhGRF-immunoreactive neurons centered in the arcuate nucleus. Some 1,000-1,500 rhGRF-positive neurons were counted on each side of the brain in rats pretreated with colchicine. Colocalization studies, using a sequential double staining technique, indicated that a subset of rhGRF-immunoreactive neurons in the arcuate region contain neurotensin immunoreactivity. No evidence was obtained for colocalization of rhGRF with either of two pro-opiomelanocortin-derived peptides (alpha-melanocyte-stimulating hormone, adrenocorticotropic hormone (1-24)) in individual neurons in the arcuate nucleus. Much smaller groups of neurons were localized in the parvicellular division of the paraventricular nucleus of the hypothalamus and in the dorsomedial nucleus, and it is unclear whether they contribute to the plexus of rhGRF-stained fibers in the median eminence. The only other region in the rat brain in which rhGRF-stained cells were found reliably was in the area that roughly encapsulates the caudal aspect of the ventromedial nucleus of the hypothalamus. Because cells in this region are not known to project to the median eminence, they may be assumed to contribute to the extrahypophysiotropic rhGRF-stained projections outlined below. From the level of the arcuate and ventromedial nuclei, rhGRF-immunoreactive fibers could be traced along the base of the brain and through the periventricular system to discrete terminal fields limited almost exclusively to the hypothalamus and adjoining parts of the basal telencephalon. All parts of the periventricular region of the hypothalamus receive an input, including the preoptic and anterior parts in which somatostatin-containing neurons that project to the median eminence are clustered. Other prominent terminal fields were localized in discrete parts of the dorsomedial, paraventricular, suprachiasmatic, and premammillary nuclei, and in the medial preoptic and lateral hypothalamic areas.(ABSTRACT TRUNCATED AT 400 WORDS)