Brain structures mediating the suckling stimulus-induced release of prolactin.

Suckling-induced prolactin release is a widely studied neuroendocrine reflex, comprising a neural afferent and a humoral efferent component. The information on the brain structures involved in this reflex is fairly limited. The present studies focused on this question. The following hypothalamic interventions were made in lactating rats and the dams were tested for the suckling-induced prolactin response: (i) unilateral or (ii) bilateral frontal cuts at the level of the anterior and posterior hypothalamus; (iii) administration of 5,7-dihydroxytryptamine or (iv) 6-hydroxydopamine into the hypothalamic paraventricular nucleus (PVN) to destroy serotonergic and catecholaminergic innervation of the cell group, respectively; (v) lesion of the medial subdivision of the PVN; and (vi) horizontal knife cuts below the PVN. Bilateral posterior and bilateral or unilateral anterior frontal cuts caused blockade of the suckling-induced release of prolactin. Likewise, most dams receiving 5,7-dihydroxytryptamine in the PVN did not respond to the suckling stimulus. Immunocytochemistry revealed that, in those rats which did not show a rise in plasma prolactin, there were almost no serotonergic fibres and terminals in the PVN, while in dams which exhibited a response, numerous serotonergic elements were evident. 6-Hydroxydopamine treatment did not cause significant alteration in the prolactin response. Lesion of the medial, largely parvocellular subdivision of the PVN, or horizontal knife cuts below this cell group, blocked the hormone response. The findings demonstrate for the first time that: (i) interruption of the connections between the brain stem and the hypothalamus interferes with the prolactin response to the suckling stimulus; (ii) serotonergic fibres terminating in the hypothalamic PVN are involved in the mediation of the suckling stimulus; and (iii) within the PVN, neurones in the medial, largely parvocellular subdivision of the cell group take part in the transfer of the neural signal, eventually inducing prolactin release.

Effect of adrenalectomy and dexamethasone treatment on prolactin secretion of lactating rats.

The contribution of corticosteroids to the control of prolactin secretion in lactating rats was investigated. The prolactin response to domperidone (20 microg/kg b.w., i.v.), a dopamine receptor antagonist and to domperidone plus formalin stress was tested in adrenalectomized and/or dexamethasone-treated continuously nursing rats. Animals were adrenalectomized on the 3rd day of lactation and tested on the 7th day of lactation. Dexamethasone was injected s.c. 24 h before testing (400 microg/kg b.w.) and on the day of testing (200 microg/kg b.w.). Domperidone caused a significant rise in plasma prolactin levels. The prolactin response to domperidone was twice as high in solely adrenalectomized dams and in solely dexamethasone-treated rats compared to controls. In adrenalectomized animals treated with dexamethasone, the prolactin response to domperidone was like in controls. Formalin injection to either adrenalectomized plus domperidone-treated animals or to animals injected with dexamethasone plus domperidone, resulted in a statistically significant depletion of plasma prolactin. In controls and in adrenalectomized animals receiving dexamethasone and domperidone, the prolactin response to formalin was very similar, i.e., plasma prolactin levels did not change after the administration of formalin. The present findings suggest that in lactating rats, corticosteroids are involved in the prolactin response to domperidone and to formalin stress.

Dual role of glucocorticoids in suckling-induced prolactin secretion.

The exact contribution of corticosteroids to the control of prolactin secretion in lactating rats is poorly understood. Therefore, the present studies were focused on the effect of adrenalectomy and dexamethasone treatment on the suckling-induced prolactin release. Animals were adrenalectomized on the 3rd day of lactation and tested on the 7th day of lactation. In adrenalectomized animals, the suckling stimulus failed to induce the characteristic increase in plasma prolactin levels. Dexamethasone pretreatment (400 microg/kg b.w. s.c. 24, 48, 72 h before testing) of adrenalectomized rats restored this prolactin response. The same treatment with dexamethasone given to control animals attenuated the suckling stimulus induced prolactin response. The present findings indicate that corticosteroids are essential for a basic prolactin response of lactating rats.

Analysis of pituitary prolactin and adrenocortical response to ether, formalin or restraint in lactating rats: rise in corticosterone, but no increase in plasma prolactin levels after exposure to stress.

It is well established that stress causes a rise in plasma prolactin (PRL) levels of male or cycling female rats. In lactating animals, the pituitary PRL response to stress is not well understood. Therefore, the purpose of the present study was to analyze this question in lactating rats having low or elevated prestress plasma PRL levels. The animals were exposed to ether, formalin or restraint, and plasma PRL and corticosterone levels were determined. In mothers continually together with their pups, plasma PRL levels decreased significantly after exposure to ether vapor or injection of formalin under the skin. At the same time, both agents caused a significant rise in blood corticosterone concentrations. Lactating rats isolated for 4 h had very low levels of PRL before application of stress. However, neither formalin nor restraint caused any elevation in their plasma PRL levels although both interventions increased blood corticosterone concentrations. Lactating mothers receiving formalin after a 30-min suckling stimulus preceded by 4 h isolation did not show appreciable changes in pituitary PRL secretion following the administration of formalin. For information on the mechanism of the effect of stress on PRL, lactating rats were pretreated with the dopamine receptor antagonist domperidone (injecting 80 micrograms/kg body weight) or were adrenalectomized 7 days prior to exposure to stress. The very high levels of PRL caused by domperidone decreased markedly in animals subjected to restraint stress. Administration of formalin to adrenalectomized lactating rats continually together with their litter caused a slight immediate decrease, followed by a transitory elevation and a subsequent small second decrease in blood PRL concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of various partial separations of the litters from their mother on plasma prolactin levels of lactating rats.

Removal of the pups results in an abrupt and marked depression in plasma prolactin (PRL) level of the lactating mother. The present studies were undertaken to investigate what kind of sensory input (smell, sound, visual, touch etc.) from the pups is essential for the mother to avoid the pituitary PRL response to pup-removal. Therefore, various partial separations were made and their effect on plasma PRL levels tested: a. The pups were placed into a small glass having holes on its cover; b. they were put into a long measuring tube not covered; c. the pups were placed into the feeding trough made of a wireframe; d. a dividing wall made of glass or metal was slowly let down when the mother spontaneously went away from her pups; e. the nipples were covered by a cotton plaster. Pituitary PRL responses were almost identical after all these separations and similar to that one obtained after removal of the pups from the cage. In addition, separation of the mother resulted in a rise in plasma corticosterone concentrations. The findings suggest that the pup-removal induced inhibition of PRL secretion is a very complex event for the mother and cannot be prevented by partial separations when the mother can see, smell her pups, or hear them or even can touch them with her nose. We assume that separation of the pups is a stress for the mother and cannot simply be due to the lack of just one kind of sensory input from the pups. This assumption is in line with our recent observations indicating that in lactating rat stress causes a decrease in plasma PRL level.

Dehydration attenuates plasma prolactin response to suckling through a dopaminergic mechanism.

The tuberoinfundibular dopaminergic neurons projecting to the median eminence are well accepted as a major physiological regulator of adenohypophyseal PRL secretion. However, recent evidence has shown that dopamine (DA) in the neurointermediate lobe also has an inhibitory effect on PRL secretion by anterior pituitary. Since the neurointermediate is innervated by the tuberohypophyseal dopaminergic (THDA) neurons, which is known to be selectively activated by dehydration of the animal, the aim of this study was to investigate the physiological role of the THDA system in PRL release during lactation. On the day of the experiments, the litters were separated from the mothers for 4 h before initiation of the suckling stimulus. The suckling-induced PRL surge was detected on three consecutive days. On the first day the normal response was tested; then immediately after taking the last blood samples, drinking solutions were changed to the high salt (2.5% saline) containing bottles or were taken away. Suckling-induced PRL response was significantly decreased after 24 h and almost completely blocked 48 h later in dehydrated mothers. This effect could be prevented by haloperidol (a DA receptor antagonist) pretreatment (0.1 mg/kg BW sc), and it was only transient because rehydration of the mothers reestablished basal as well as suckling-induced PRL response. In addition, the effect of an acute osmotic stimulus on the plasma PRL levels (injecting 0.5 ml 10% saline solution iv) was also tested. There was a marked and immediate decrease in PRL concentration within 15 min of injection. Domperidone, another DA receptor blocker (20 micrograms/rat iv) completely abolished the depletion of plasma PRL in response to 10% saline injection. These results support our assumption that the dopaminergic regulation of PRL secretion during lactation involves the THDA system. Furthermore, these data underline the importance of an interaction between regulation of PRL secretion and water and sodium homeostasis.

Further studies on circadian hormone rhythms after local pharmacological destruction of the serotoninergic innervation of the rat suprachiasmatic region before the onset of the corticosterone rhythm.

In previous studies we observed that local pharmacological destruction of the serotoninergic innervation of the hypothalamic suprachiasmatic nucleus before the onset of the circadian plasma corticosterone rhythm interferes with the appearance of the corticosterone rhythm up to the age of two months. In the present investigations we studied other hormone rhythms in such rats and tested animals for corticosterone rhythm when they were older than two months. We found normal circadian fluctuations in plasma testosterone and prolactin levels, variations in growth hormone concentrations, but no changes in plasma corticosterone levels of 63-day-old rats receiving 5,7-dihydroxytryptamine, a neurotoxin selectively destroying the serotoninergic structures, into the suprachiasmatic nucleus at the age of 16 days. Rats did show circadian variations in plasma corticosterone concentrations when tested 3 months after treatment with the neurotoxin. In these latter animals, a significant amount of serotonin-immunoreactive fibers and terminals were evident in the suprachiasmatic nucleus. Only a very few of such elements were seen in rats with a shorter postoperative survival. Our data support the view that serotoninergic innervation of the suprachiasmatic nucleus is essential for the onset of the circadian fluctuations of plasma corticosterone concentrations.

Circadian corticosterone rhythm did not develop in rats seven weeks after destruction with 5,7-dihydroxytryptamine of the serotoninergic nerve terminals in the suprachiasmatic nucleus at the age of 16 days.

In order to study the role of the serotoninergic innervation of the hypothalamic suprachiasmatic nucleus (SCN) in the development of the circadian rhythm of corticosterone 5,7-dihydroxytryptamine (5,7-DHT) selectively destroying the serotoninergic structures was injected into the cell group of 16-day-old male rats prior to the appearance of the corticosterone rhythm. Rats treated with 5,7-DHT did not show circadian fluctuations in plasma corticosterone concentrations 3, 5 and 7 weeks after the injection of the neurotoxin. Their hypothalamo-pituitary-adrenal system responded to ether stress. Immunocytochemistry revealed that only a very few serotonin immunoreactive elements were visible in the SCN of the 5,7-DHT-treated rats. The results indicate that serotoninergic innervation of the SCN is essential for the development of the corticosterone rhythm.

Rapid decrease of plasma prolactin levels in lactating rats separated from their pups.

The time course of the effect of separation of the pups on plasma prolactin (PRL) of the mother and the mechanism of this effect were investigated in conscious primiparous lactating rats. Within the first 15 min the separation of the mother from her litter caused a very rapid decrease in the concentration of plasma PRL. There was already a very marked depression by the end of the first 3 min period. During the first 3 min the disappearance rate of PRL from the blood was identical to that occurring after hypophysectomy and thus similar to the biological half-life of plasma PRL in lactating rats. The observed marked decrease in PRL could be completely blocked by alpha-methyl-p-tyrosine pretreatment (250 mg kg-1 i.p.) or by haloperidol (2 mg kg-1 i.p.) at 90 min prior to the start of separation. It was demonstrated that the marked depression in plasma PRL concentration develops within the first hour and particularly in the first few minutes. Thus, the separation of lactating rats apparently results in an immediate complete blockade of PRL release from the pituitary. Finally, the data suggest that a dopaminergic mechanism plays an important role in the inhibition of PRL release induced by separation of the mother.

Reversed effect of chlorpromazine on hypothalamic serotonin levels of rats with complete deafferentation of the medial basal hypothalamus.

The effect of chlorpromazine (CPZ) treatment, the complete deafferentation of the medial basal hypothalamus (MBH) and their combination on 5-HT, DA and NE levels of the MBH, hypothalamic structures outside the MBH (RH), mesencephalon and amygdala were studied. Monoamines were determined by fluorimetry. CPZ reduced 5-HT in the hypothalamus (MBH, RH), DA in the amygdala and NE concentration in the RH. Complete deafferentation of the MBH decreased 5-HT in the MBH, DA in the amygdala and NE in the hypothalamus. CPZ + deafferentation of the MBH increased 5-HT concentrations in the hypothalamus (MBH and RH) and mesencephalon, and depressed DA levels in the amygdala and NE in the hypothalamus. The data suggest that the interruption of the neural connections of the MBH results in an inverse response of the hypothalamic serotoninergic elements to CPZ.