The metabolism of dopamine in the median eminence reflects the activity of tuberoinfundibular neurons

Locus coeruleus noradrenaline depletion and its differential impact on CO2-induced panic and hyperventilation in male and female mice

CO2 exposure has been used to investigate the panicogenic response in patients with panic disorder. These patients are more sensitive to CO2, and more likely to experience the "false suffocation alarm" which triggers panic attacks. Imbalances in locus coeruleus noradrenergic (LC-NA) neurotransmission are responsible for psychiatric disorders, including panic disorder. These neurons are sensitive to changes in CO2/pH. Therefore, we investigated if LC-NA neurons are differentially activated after severe hypercapnia in mice. Further, we evaluated the participation of LC-NA neurons in ventilatory and panic-like escape responses induced by 20% CO2 in male and female wild type mice and two mouse models of altered LC-NA synthesis. Hypercapnia activates the LC-NA neurons, with males presenting a heightened level of activation. Mutant males lacking or with reduced LC-NA synthesis showed hypoventilation, while animals lacking LC noradrenaline present an increased metabolic rate compared to wild type in normocapnia. When exposed to CO2, males lacking LC noradrenaline showed a lower respiratory frequency compared to control animals. On the other hand, females lacking LC noradrenaline presented a higher tidal volume. Nevertheless, no change in ventilation was observed in either sex. CO2 evoked an active escape response. Mice lacking LC noradrenaline had a blunted jumping response and an increased freezing duration compared to the other groups. They also presented fewer racing episodes compared to wild type animals, but not different from mice with reduced LC noradrenaline. These findings suggest that LC-NA has an important role in ventilatory and panic-like escape responses elicited by CO2 exposure in mice.

Seasonality of prolactin in birds and mammals

In most animals, annual rhythms in environmental cues and internal programs regulate seasonal physiology and behavior. Prolactin, an evolutionarily ancient hormone, serves as a molecular correlate of seasonal timing in most species. Prolactin is highly pleiotropic with a wide variety of well-documented physiological effects; in a seasonal context prolactin is known to regulate annual changes in pelage and molt. While short-term homeostatic variation of prolactin secretion is under the control of the hypothalamus, long-term seasonal rhythms of prolactin are programmed by endogenous timers that reside in the pituitary gland. The molecular basis of these rhythms is generally understood to be melatonin dependent in mammals. Prolactin rhythmicity persists for several years in many species, in the absence of hypothalamic signaling. Such evidence in mammals has supported the hypothesis that seasonal rhythms in prolactin derive from an endogenous timer within the pituitary gland that is entrained by external photoperiod. In this review, we describe the conserved nature of prolactin signaling in birds and mammals and highlight its role in regulating multiple diverse physiological systems. The review will cover the current understanding of the molecular control of prolactin seasonality and propose a mechanism by which long-term rhythms may be generated in amniotes.

Embryonic Thermal Manipulation Affects Ventilation, Metabolism, Thermal Control and Central Dopamine in Newly Hatched and Juvenile Chicks

The first third of incubation is critical for embryonic development, and environmental changes during this phase can affect the physiology and survival of the embryos. We evaluated the effects of low (LT), control (CT), and high (HT) temperatures during the first 5 days of incubation on ventilation (V._E), body temperature (Tb), oxygen consumption (V.O₂), respiratory equivalent (V._E/V.O₂), and brain monoamines on 3-days-old (3d) and 14-days-old (14d) male and female chickens. The body mass of LT animals of both ages and sexes was higher compared to HT and CT animals (except for 3d males). The heart mass of 14d HT animals was higher than that of CT animals. Thermal manipulation did not affect V._E, V.O₂ or V._E/V.O₂ of 3d animals in normoxia, except for 3d LT males V._E, which was lower than CT. Regarding 14d animals, the HT females showed a decrease in V._E and V.O₂ compared to CT and LT groups, while the HT males displayed a lower V.O₂ compared to CT males, but no changes in V._E/V.O₂. Both sexes of 14d HT chickens presented a greater Tb compared to CT animals. Thermal manipulations increased the dopamine turnover in the brainstem of 3d females. No differences were observed in ventilatory and metabolic parameters in the 3d animals of either sexes, and 14d males under 7% CO₂. The hypercapnic hyperventilation was attenuated in the 14d HT females due to changes in V.O₂, without alterations in V._E. The 14d LT males showed a lower V._E, during hypercapnia, compared to CT, without changes in V.O₂, resulting in an attenuation in V._E/V.O₂. During hypoxia, 3d LT females showed an attenuated hyperventilation, modulated by a higher V.O₂. In 14d LT and HT females, the increase in V._E was greater and the hypometabolic response was attenuated, compared to CT females, which resulted in no change in the V._E/V.O₂. In conclusion, thermal manipulations affect hypercapnia-induced hyperventilation more so than hypoxic challenge, and at both ages, females are more affected by thermal manipulation than males.

Reduced dopaminergic tone during lactation is permissive to the hypothalamic stimulus for suckling-induced prolactin release

Dopamine from tuberoinfundibular dopaminergic (TIDA) neurones tonically inhibits prolactin (PRL) secretion. Lactational hyperprolactinaemia is associated with a reduced activity of TIDA neurones. However, it remains controversial whether the suckling-induced PRL surge is driven by an additional decrease in dopamine release or by stimulation from a PRL-releasing factor. In the present study, we further investigated the role of dopamine in the PRL response to suckling. Non-lactating (N-Lac), lactating 4 hour apart from pups (Lac), Lac with pups return and suckling (Lac+S), and post-lactating (P-Lac) rats were evaluated. PRL levels were elevated in Lac rats and increased linearly within 30 minutes of suckling in Lac+S rats. During the rise in PRL levels, dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the median eminence (ME) and neurointermediate lobe of the pituitary did not differ between Lac+S and Lac rats. However, dopamine and DOPAC were equally decreased in Lac and Lac+S compared to N-Lac and P-Lac rats. Suckling, in turn, reduced phosphorylation of tyrosine hydroxylase in the ME of Lac+S. Domperidone and bromocriptine were used to block and activate pituitary dopamine D2 receptors, respectively. Domperidone increased PRL secretion in both N-Lac and Lac rats, and suckling elicited a robust surge of PRL over the high basal levels in domperidone-treated Lac+S rats. Conversely, bromocriptine blocked the PRL response to suckling. The findings obtained in the present study provide evidence that dopamine synthesis and release are tonically reduced during lactation, whereas dopamine is still functional with respect to inhibiting PRL secretion. However, there appears to be no further reduction in dopamine release associated with the suckling-induced rise in PRL. Instead, the lower dopaminergic tone during lactation appears to be required to sensitise the pituitary to a suckling-induced PRL-releasing factor.

Pro-neurogenic effect of fluoxetine in the olfactory bulb is concomitant to improvements in social memory and depressive-like behavior of socially isolated mice

Although loneliness is a human experience, it can be estimated in laboratory animals deprived from physical contact with conspecifics. Rodents under social isolation (SI) tend to develop emotional distress and cognitive impairment. However, it is still to be determined whether those conditions present a common neural mechanism. Here, we conducted a series of behavioral, morphological, and neurochemical analyses in adult mice that underwent to 1 week of SI. We observed that SI mice display a depressive-like state that can be prevented by enriched environment, and the antidepressants fluoxetine (FLX) and desipramine (DES). Interestingly, chronic administration of FLX, but not DES, was able to counteract the deleterious effect of SI on social memory. We also analyzed cell proliferation, neurogenesis, and astrogenesis after the treatment with antidepressants. Our results showed that the olfactory bulb (OB) was the neurogenic niche with the highest increase in neurogenesis after the treatment with FLX. Considering that after FLX treatment social memory was rescued and depressive-like behavior decreased, we propose neurogenesis in the OB as a possible mechanism to unify the FLX ability to counteract the deleterious effect of SI.

Kisspeptin Stimulation of Prolactin Secretion Requires Kiss1 Receptor but Not in Tuberoinfundibular Dopaminergic Neurons

Kisspeptin has been shown to stimulate prolactin secretion. We investigated whether kisspeptin acts through the Kiss1 receptor (Kiss1r) to regulate dopamine and prolactin. Initially, we evaluated prolactin response in a Kiss1r-deficient mouse line, in which Kiss1r had been knocked into GnRH neurons (Kiss1r-/-R). Intracerebroventricular kisspeptin-10 (Kp-10) increased prolactin release in wild-type but not in Kiss1r-/-R female mice. In ovariectomized, estradiol-treated rats, the Kiss1r antagonist kisspeptin-234 abolished the Kp-10-induced increase in prolactin release but failed to prevent the concomitant reduction in the activity of tuberoinfundibular dopaminergic (TIDA) neurons, as determined by the 3,4-dihydroxyphenylacetic acid/dopamine ratio in the median eminence. Using whole-cell patch clamp recordings in juvenile male rats, we found no direct effect of Kp-10 on the electrical activity of TIDA neurons. In addition, dual-label in situ hybridization in the hypothalamus of female rats showed that Kiss1r is expressed in the periventricular nucleus of the hypothalamus (Pe) and arcuate nucleus of the hypothalamus (ARC) but not in tyrosine hydroxylase (Th)-expressing neurons. Kisspeptin also has affinity for the neuropeptide FF receptor 1 (Npffr1), which was expressed in the majority of Pe dopaminergic neurons but only in a low proportion of TIDA neurons in the ARC. Our findings demonstrate that Kiss1r is necessary to the effect of kisspeptin on prolactin secretion, although TIDA neurons lack Kiss1r and are electrically unresponsive to kisspeptin. Thus, kisspeptin is likely to stimulate prolactin secretion via Kiss1r in nondopaminergic neurons, whereas the colocalization of Npffr1 and Th suggests that Pe dopaminergic neurons may play a role in the kisspeptin-induced inhibition of dopamine release.

Dopamine Release Dynamics in the Tuberoinfundibular Dopamine System

The relationship between neuronal impulse activity and neurotransmitter release remains elusive. This issue is especially poorly understood in the neuroendocrine system, with its particular demands on periodically voluminous release of neurohormones at the interface of axon terminals and vasculature. A shortage of techniques with sufficient temporal resolution has hindered real-time monitoring of the secretion of the peptides that dominate among the neurohormones. The lactotropic axis provides an important exception in neurochemical identity, however, as pituitary prolactin secretion is primarily under monoaminergic control, via tuberoinfundibular dopamine (TIDA) neurons projecting to the median eminence (ME). Here, we combined electrical or optogenetic stimulation and fast-scan cyclic voltammetry to address dopamine release dynamics in the male mouse TIDA system. Imposing different discharge frequencies during brief (3 s) stimulation of TIDA terminals in the ME revealed that dopamine output is maximal at 10 Hz, which was found to parallel the TIDA neuron action potential frequency distribution during phasic discharge. Over more sustained stimulation periods (150 s), maximal output occurred at 5 Hz, similar to the average action potential firing frequency of tonically active TIDA neurons. Application of the dopamine transporter blocker, methylphenidate, significantly increased dopamine levels in the ME, supporting a functional role of the transporter at the neurons' terminals. Lastly, TIDA neuron stimulation at the cell body yielded perisomatic release of dopamine, which may contribute to an ultrafast negative feedback mechanism to constrain TIDA electrical activity. Together, these data shed light on how spiking patterns in the neuroendocrine system translate to vesicular release toward the pituitary and identify how dopamine dynamics are controlled in the TIDA system at different cellular compartments.SIGNIFICANCE STATEMENT A central question in neuroscience is the complex relationship between neuronal discharge activity and transmitter release. By combining optogenetic stimulation and voltammetry, we address this issue in dopamine neurons of the neuroendocrine system, which faces particular spatiotemporal demands on exocytotic release; large amounts of neurohormone need to be secreted into the portal capillaries with precise timing to adapt to physiological requirements. Our data show that release is maximal around the neurons' default firing frequency. We further provide support for functional dopamine transport at the neurovascular terminals, shedding light on a long-standing controversy about the existence of neuroendocrine transmitter reuptake. Finally, we show that dopamine release occurs also at the somatodendritic level, providing a substrate for an ultrashort autoregulatory feedback loop.

Rats with higher intrinsic exercise capacities exhibit greater preoptic dopamine levels and greater mechanical and thermoregulatory efficiencies while running

The present study investigated whether intrinsic exercise capacity affects the changes in thermoregulation, metabolism and central dopamine (DA) induced by treadmill running. Male Wistar rats were subjected to three incremental exercises and ranked as low-performance (LP), standard-performance (SP), and high-performance (HP) rats. In the first experiment, abdominal (TABD) and tail (TTAIL) temperatures were registered in these rats during submaximal exercise (SE) at 60% of maximal speed. Immediately after SE, rats were decapitated and concentrations of DA and 3,4-dihydroxyphenylacetic acid (DOPAC) were determined in the preoptic area (POA). In the second experiment, oxygen consumption was measured and mechanical efficiency (ME) was calculated in these rats during an incremental exercise. HP rats ran for longer periods and were fatigued with higher TABD values, with no difference in TTAIL. Nevertheless, thermoregulatory efficiency was higher in HP rats, compared with other groups. DA and DOPAC concentrations in the POA were increased by SE, with higher levels in HP compared with LP and SP rats. V̇o2 also differed between groups, with HP rats displaying a lower consumption throughout the incremental exercise but a higher V̇o2 at fatigue. ME, in turn, was consistently higher in HP than in LP and SP rats. Thus, our results show that HP rats have greater TABD values at fatigue, which seem to be related to a higher dopaminergic activity in the POA. Moreover, HP rats exhibited a greater thermoregulatory efficiency during exercise, which can be attributed to a lower V̇o2, but not to changes in tail heat loss mechanisms.

NEW & NOTEWORTHY Our findings reveal that rats with higher intrinsic exercise capacities have greater thermoregulatory efficiencies and increased dopaminergic activity in the preoptic area, a key brain area in thermoregulatory control, while exercising. Moreover, higher intrinsic exercise capacities are associated with decreased oxygen consumption for a given exercise intensity, which indicates greater mechanical efficiencies. Collectively, these findings help to advance our knowledge of why some rats of a given strain can exercise for longer periods than others.

Auditory stimulation by exposure to melodic music increases dopamine and serotonin activities in rat forebrain areas linked to reward and motor control

Listening to melodic music is regarded as a non-pharmacological intervention that ameliorates various disease symptoms, likely by changing the activity of brain monoaminergic systems. Here, we investigated the effects of exposure to melodic music on the concentrations of dopamine (DA), serotonin (5-HT) and their respective metabolites in the caudate-putamen (CPu) and nucleus accumbens (NAcc), areas linked to reward and motor control. Male adult Wistar rats were randomly assigned to a control group or a group exposed to music. The music group was submitted to 8 music sessions [Mozart's sonata for two pianos (K. 488) at an average sound pressure of 65 dB]. The control rats were handled in the same way but were not exposed to music. Immediately after the last exposure or control session, the rats were euthanized, and their brains were quickly removed to analyze the concentrations of 5-HT, DA, 5-hydroxyindoleacetic acid (5-HIAA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the CPu and NAcc. Auditory stimuli affected the monoaminergic system in these two brain structures. In the CPu, auditory stimuli increased the concentrations of DA and 5-HIAA but did not change the DOPAC or 5-HT levels. In the NAcc, music markedly increased the DOPAC/DA ratio, suggesting an increase in DA turnover. Our data indicate that auditory stimuli, such as exposure to melodic music, increase DA levels and the release of 5-HT in the CPu as well as DA turnover in the NAcc, suggesting that the music had a direct impact on monoamine activity in these brain areas.

Intrinsic exercise capacity in rats influences dopamine neuroplasticity induced by physical training

The study evaluates whether the intrinsic capacity for physical exercise influences dopamine neuroplasticity induced by physical training. Male rats were submitted to three progressive tests until fatigue. Based on the maximal time of exercise (TE), rats were considered as low performance (LP), standard performance (SP) or high performance (HP) to exercise. Eight animals from each group (LP, SP, and HP) were randomly subdivided in sedentary (SED) or trained (TR). Physical training was performed for 6 wk. After that, concentrations of dopamine (DA), serotonin (5-HT), and their metabolites and mRNA levels of D1 receptor ( Drd1), D2 receptor ( Drd2), dopamine transporter ( Dat), tyrosine hydroxylase ( Th), glia cell line neurotrophic factor ( Gdnf), and brain-derived neurotrophic factor ( Bdnf) were determined in the caudate-putamen (CPu). TE was increased with training in all performance groups. However, the relative increase was markedly higher in LP rats, and this was associated with a training-induced increase in dopaminergic activity in the CPu, which was determined by the 3,4-dihydroxyphenylacetic acid (DOPAC)/DA ratio. An opposite monoamine response was found in HP-TR rats, in which physical training decreased the DOPAC/DA ratio in the CPu. Moreover, LP-SED rats displayed higher levels of Drd2 in the CPu compared with the other SED groups, and this higher expression was decreased by physical training. Physical training also decreased Dat and increased Gdnf in the CPu of LP rats. Physical training decreased Bdnf in the CPu only in HP rats. Thus, we provide evidence that the intrinsic capacity to exercise affects the neuroplasticity of the dopaminergic system in response to physical training. NEW & NOTEWORTHY The findings reported reveal that dopaminergic neuroplasticity in caudate-putamen induced by physical training is influenced by the intrinsic capacity to exercise in rats. To evaluate the dopaminergic neuroplasticity, we analyzed mRNA levels of D1 receptor, D2 receptor, dopamine transporter, tyrosine hydroxylase, glia cell line neurotrophic factor, and brain-derived neurotrophic factor as well as concentrations of dopamine, serotonin, and their metabolites. These results expand our knowledge about the interrelationship between genetic background, physical training, and dopaminergic neuroplasticity.

Restraint stress increases prolactin-mediated phosphorylation of signal transducer and activator of transcription 5 in the hypothalamus and adrenal cortex in the male mouse

Prolactin is a pleiotropic peptide hormone produced by the lactotrophs in the anterior pituitary. Its rate of secretion is primarily regulated by a negative-feedback mechanism where prolactin stimulates the activity of the tuberoinfundibular dopaminergic (TIDA) neurones, increasing their release of dopamine, which accesses the pituitary via the median eminence to suppress further prolactin secretion. In addition to its well established role in lactation, circulating prolactin is secreted in response to stress, although the mechanism by which this is achieved or its cellular targets remains unknown. In the present study, we show that 15 minutes of restraint stress causes an approximately seven-fold increase in circulating prolactin concentration in male mice. Monitoring prolactin receptor activation, using immunohistochemistry to determine the level and distribution of tyrosine phosphorylated signal transducer and activator of transcription 5 (pSTAT5), we show that this stress-induced increase in prolactin interacts with both central and peripheral targets. Restraint stress for 15 minutes significantly increased pSTAT5 staining in the arcuate nucleus, median eminence and the zona fasciculata of the adrenal cortex. In each case, this response was prevented by pretreating the animals with bromocriptine to block prolactin secretion from the pituitary. Interestingly, in contrast to many cells in the arcuate nucleus, stress reduced pSTAT5 staining of the TIDA neurones (identified by dual-labelling for tyrosine hydroxylase). This suggests that there is reduced prolactin signalling in these cells and thus potentially a decline in their inhibitory influence on prolactin secretion. These results provide evidence that prolactin secreted in response to acute stress is sufficient to activate prolactin receptors in selected target tissues known to be involved in the physiological adaptation to stress.

α-Estrogen and Progesterone Receptors Modulate Kisspeptin Effects on Prolactin: Role in Estradiol-Induced Prolactin Surge in Female Rats

Kisspeptin (Kp) regulates prolactin (PRL) in an estradiol-dependent manner. We investigated the interaction between ovarian steroid receptors and Kp in the control of PRL secretion. Intracerebroventricular injections of Kp-10 or Kp-234 were performed in ovariectomized (OVX) rats under different hormonal treatments. Kp-10 increased PRL release and decreased 3,4-dihydroxyphenylacetic acid levels in the median eminence (ME) of OVX rats treated with estradiol (OVX+E), which was prevented by tamoxifen. Whereas these effects of Kp-10 were absent in OVX rats, they were replicated in OVX rats treated with selective agonist of estrogen receptor (ER)α, propylpyrazole triol, but not of ERβ, diarylpropionitrile. Furthermore, the Kp-10-induced increase in PRL was two times higher in OVX+E rats also treated with progesterone (OVX+EP), which was associated with a reduced expression of both tyrosine hydroxylase (TH) and Ser40-phosphorylated TH in the ME. Kp-10 also reduced dopamine levels in the ME of OVX+EP rats, an effect blocked by the progesterone receptor (PR) antagonist RU486. We also determined the effect of Kp antagonism with Kp-234 on the estradiol-induced surges of PRL and luteinizing hormone (LH), using tail-tip blood sampling combined with ultrasensitive enzyme-linked immunosorbent assay. Kp-234 impaired the early phase of the PRL surge and prevented the LH surge in OVX+E rats. Thus, we provide evidence that Kp stimulation of PRL release requires ERα and is potentiated by progesterone via PR activation. Moreover, alongside its essential role in the LH surge, Kp seems to play a role in the peak phase of the estradiol-induced PRL surge.

Influence of dynorphin on estradiol- and cervical stimulation-induced prolactin surges in ovariectomized rats

Prolactin is an anterior pituitary hormone necessary for fertility, pregnancy maintenance, lactation, and aspects of maternal behavior. In rodents, there is a surge of prolactin on the afternoon of proestrus, and a semi-circadian pattern of prolactin surges during early pregnancy, with a diurnal and nocturnal surge every day. Both of these patterns can be replicated in ovariectomized rats. A prior study demonstrated that central antagonism of κ-opioid receptors, the target of dynorphin, largely abolished the nocturnal prolactin surge in pregnant rats. We build on this to determine whether dynorphin, perhaps from the arcuate population that co-express kisspeptin, neurokinin B, and dynorphin (KNDy neurons), also contributes to the estradiol- or cervical stimulation-induced surges in ovariectomized rats. Ovariectomized rats were treated with either estradiol or cervical stimulation to induce prolactin surge(s). Blood samples were taken around the expected surge time to determine the effect of either acute κ-opioid receptor antagonism or previous chemical ablation of the KNDy population on prolactin levels. Dynorphin antagonism does significantly disrupt the nocturnal prolactin surge, but it does not contribute to the estradiol-induced surge. Chemical ablation of KNDy neurons had opposite effects; ablation of 40 % of the KNDy neurons had no impact on the nocturnal prolactin surge, while a somewhat larger ablation significantly reduced the size of the estradiol-induced surge. We conclude that dynorphin is likely a controlling factor for the nocturnal surge induced by cervical stimulation, and that other KNDy neuron products must play a role in the estradiol-induced surge.

Dopamine Autoreceptor Regulation of a Hypothalamic Dopaminergic Network

How autoreceptors contribute to maintaining a stable output of rhythmically active neuronal circuits is poorly understood. Here, we examine this issue in a dopamine population, spontaneously oscillating hypothalamic rat (TIDA) neurons, that underlie neuroendocrine control of reproduction and neuroleptic side effects. Activation of dopamine receptors of the type 2 family (D2Rs) at the cell-body level slowed TIDA oscillations through two mechanisms. First, they prolonged the depolarizing phase through a combination of presynaptic increases in inhibition and postsynaptic hyperpolarization. Second, they extended the discharge phase through presynaptic attenuation of calcium currents and decreased synaptic inhibition. Dopamine reuptake blockade similarly reconfigured the oscillation, indicating that ambient somatodendritic transmitter concentration determines electrical behavior. In the absence of D2R feedback, however, discharge was abolished by depolarization block. These results indicate the existence of an ultra-short feedback loop whereby neuroendocrine dopamine neurons tune network behavior to echoes of their own activity, reflected in ambient somatodendritic dopamine, and also suggest a mechanism for antipsychotic side effects.

60 YEARS OF NEUROENDOCRINOLOGY: The hypothalamo-prolactin axis

The hypothalamic control of prolactin secretion is different from other anterior pituitary hormones, in that it is predominantly inhibitory, by means of dopamine from the tuberoinfundibular dopamine neurons. In addition, prolactin does not have an endocrine target tissue, and therefore lacks the classical feedback pathway to regulate its secretion. Instead, it is regulated by short loop feedback, whereby prolactin itself acts in the brain to stimulate production of dopamine and thereby inhibit its own secretion. Finally, despite its relatively simple name, prolactin has a broad range of functions in the body, in addition to its defining role in promoting lactation. As such, the hypothalamo-prolactin axis has many characteristics that are quite distinct from other hypothalamo-pituitary systems. This review will provide a brief overview of our current understanding of the neuroendocrine control of prolactin secretion, in particular focusing on the plasticity evident in this system, which keeps prolactin secretion at low levels most of the time, but enables extended periods of hyperprolactinemia when necessary for lactation. Key prolactin functions beyond milk production will be discussed, particularly focusing on the role of prolactin in inducing adaptive responses in multiple different systems to facilitate lactation, and the consequences if prolactin action is impaired. A feature of this pleiotropic activity is that functions that may be adaptive in the lactating state might be maladaptive if prolactin levels are elevated inappropriately. Overall, my goal is to give a flavour of both the history and current state of the field of prolactin neuroendocrinology, and identify some exciting new areas of research development.

Kisspeptin regulates tuberoinfundibular dopaminergic neurones and prolactin secretion in an oestradiol-dependent manner in male and female rats

Prolactin (PRL) secretion is inhibited by hypothalamic dopamine. Kisspeptin controls luteinising hormone (LH) secretion and is also involved in PRL regulation. We further investigated the effect of kisspeptin-10 (Kp-10) on the activity of tuberoinfundibular dopaminergic (TIDA) neurones and the role of oestradiol (E2 ) in this mechanism. Female and male rats were injected with i.c.v. Kp-10 and evaluated for PRL release and the activity of dopamine terminals in the median eminence (ME) and neurointermediate lobe of the pituitary (NIL). Kp-10 at the doses of 0.6 and 3 nmol increased plasma PRL and decreased 4-dihydroxyphenylacetic acid (DOPAC) levels in the ME and NIL of ovariectomised (OVX), E2 -treated rats but had no effect in OVX. In gonad-intact males, 3 nmol Kp-10 increased PRL secretion and decreased DOPAC levels in the ME but not in the NIL. Castrated males treated with either testosterone or E2 also displayed increased PRL secretion and reduced ME DOPAC in response to Kp-10, whereas castrated rats receiving oil or dihydrotestosterone were unresponsive. By contrast, the LH response to Kp-10 was not E2 -dependent in either females or males. Additionally, immunohistochemical double-labelling demonstrated that TIDA neurones of male rats contain oestrogen receptor (ER)-α, with a higher proportion of neurones expressing ERα than in dioestrous females. The dopaminergic neurones of periventricular hypothalamic nucleus displayed much lower ERα expression. Thus, TIDA neurones express ERα in male and female rats, and kisspeptin increases PRL secretion through inhibition of TIDA neurones in an E2 -dependent manner in both sexes. These findings provide new evidence about the role of kisspeptin in the regulation of dopamine and PRL.

TIDAL WAVES: Network mechanisms in the neuroendocrine control of prolactin release

Neuroendocrine tuberoinfundibular dopamine (TIDA) neurons tonically inhibit pituitary release of the hormone, prolactin. Through the powerful actions of prolactin in promoting lactation and maternal behaviour while suppressing sexual drive and fertility, TIDA neurons play a key role in reproduction. We summarize insights from recent in vitro studies into the membrane properties and network behaviour of TIDA neurons including the observations that TIDA neurons exhibit a robust oscillation that is synchronized between cells and depends on intact gap junction communication. Comparisons are made with phasic firing patterns in other neuronal populations. Modulators involved in the control of lactation - including serotonin, thyrotropin-releasing hormone and prolactin itself - have been shown to change the electrical behaviour of TIDA cells. We propose that TIDA discharge mode may play a central role in tuning the amount of dopamine delivered to the pituitary and hence circulating prolactin concentrations in different reproductive states and pathological conditions.

Stress in Neonatal Rats with Different Maternal Care Backgrounds: Monoaminergic and Hormonal Responses

The first 2 weeks of life in rats are known as the stress hyporesponsive period because stress responses in pups are diminished as compared to adult animals. However, it is considered a critical period in development in which infant rats are susceptible to environmental events, such as stressful stimuli and quality of maternal care received. These early life events have long-lasting effects, shaping a variety of outcomes, such as stress responsivity. This study investigated the effects of maternal care and sex differences on the response to an aversive stimulus in rat pups from high (HL) and low licking (LL) mothers. Plasma corticosterone, oxytocin, and central monoaminergic activity in 13-day-old rats submitted to cold stress were analyzed. Stress increased plasma corticosterone and marginally decreased hypothalamic dihydroxyphenylacetic acid/dopamine (DOPAC/DA) ratio. HL pups showed higher levels of plasma oxytocin than LL pups. The maternal effect was also detected in the hippocampus, in which 5-hydroxyindole-3-acetic acid/serotonin (5-HIAA/5-HT) ratio was increased in HL pups, independently of the sex and stress. Investigating the early life events is useful not only into understand the neurobiological and hormonal mechanisms underlying maternal and stressful influences on infant development into a healthy or psychopathological adult phenotype, but also to unveil the immediate outcomes on infancy.

Olfactory bulb monoamine concentrations vary with time of day

The olfactory bulb (OB) has been recently identified as a circadian oscillator capable of operating independently of the master circadian pacemaker, the suprachiasmatic nuclei of the hypothalamus. OB oscillations manifest as rhythms in clock genes, electrical activity, and odor sensitivity. Dopamine, norepinephrine, and serotonin have been shown to modulate olfactory information processing by the OB and may be part of the mechanism that underlies diurnal changes in olfactory sensitivity. Rhythmic release of these neurotransmitters could generate OB rhythms in electrical activity and olfactory sensitivity. We hypothesized that these monoamines were rhythmically released in the OB. To test our hypotheses, we examined monoamine levels in the OB, over the course of a day, by high-performance liquid chromatography coupled to electrochemical detection. We observed that dopamine and its metabolite, 3-4-dihydroxyphenylacetic acid, rhythmically fluctuate over the day. In contrast, norepinephrine is arrhythmic. Serotonin and its metabolite hydroxyindoleacetic acid appear to rhythmically fluctuate. Each of these monoamines has been shown to alter OB circuit behavior and influence odor processing. Rhythmic release of serotonin may be a mechanism by which the suprachiasmatic nuclei communicate, indirectly, with the OB.

An endogenous dopaminergic tone acting on dopamine D3 receptors may be involved in diurnal changes of tuberoinfundibular dopaminergic neuron activity and prolactin secretion in estrogen-primed ovariectomized rats

The diurnal rhythm of tuberoinfundibular dopaminergic (TIDA) neuron activity, i.e., high in the morning and low in the afternoon, is prerequisite for the afternoon prolactin (PRL) surge in proestrous and estrogen-primed ovariectomized (OVX) female rats. Whether dopamine acts via D(3) receptors in regulating the rhythmic TIDA neuron activity and PRL secretion in estrogen-primed OVX (OVX+E(2)) rats is the focus of this study. Intracerebroventricular (icv) injection of a D(3) receptor agonist, PD128907 (0.1-10 μg/3 μl), in the morning significantly reduced the basal activity of TIDA neurons and increased plasma PRL level. The effects of PD128907 were reversed by co-administration of U99194A, a D(3) receptor antagonist, but not by raclopride, a D(2) receptor antagonist. To determine whether endogenous dopamine acts on D(3) receptors involved in the diurnal changes of the activities, we used both U99194A, a D(3) receptor antagonist, and an antisense oligodeoxynucleotide (ODN) against D(3) receptor mRNA in the study. U99194A (0.1 μg/3 μl, icv) given at 1200 h significantly reversed the lowered TIDA neuron activity and the afternoon PRL surge at 1500 h. Moreover, OVX+E(2) rats pretreated with the antisense ODN (10 μg/3 μl, icv) for 2 days had the same effects as the D(3) receptor antagonist on TIDA neuron activity and the PRL surge. The same treatment with sense ODN had no effect. In conclusion, an endogenous DA tone may act on D(3) receptors to inhibit TIDA neuron activity and in turn stimulate the PRL surge in the afternoon of OVX+E(2) rats.

Differential changes in the hypothalamic-pituitary-adrenal axis and prolactin responses to stress in early pregnant mice

Stress can cause pregnancy failure but it is unclear how the mother's neuroendocrine system responds to stress to impair mechanisms establishing implantation. We analysed stress-evoked hypothalamic-pituitary-adrenal (HPA) axis responses in early pregnant mice. HPA axis secretory responses to immune stress in early-mid pregnancy were strong and similar to that in virgins, although activation of hypothalamic vasopressin neurones, rather than corticotrophin-releasing hormone neurones, may be more important in the stress response in pregnancy. The site and mode of detrimental glucocorticoid action in pregnancy is not established. Because circulating prolactin is important for progesterone secretion and pregnancy establishment, we also hypothesised that stress negatively impacts on prolactin and its neuroendocrine control systems in early pregnant mice. Basal prolactin secretion was profoundly inhibited by either immune or fasting stress in early pregnancy. Prolactin release is inhibited by tonic dopamine release from tuberoinfundibular (TIDA) neurones. However, immune stress did not increase TIDA neurone activity in the median eminence in pregnant mice [measured by 3,4-dihydroxyphenylacetic acid (DOPAC) content and the DOPAC:dopamine ratio]. By contrast, both immune stress and fasting caused weak induction of Fos in TIDA neurones. However, Fos induction does not always reflect dopamine secretion. Taken together, the data suggest that the stress-evoked profound reduction in prolactin secretion does not involve substantially increased dopamine activity as anticipated. In pregnancy, there was also attenuated recruitment of parvocellular paraventricular nucleus neurones and increased activation of brainstem noradrenergic nuclei after immune stress, indicating that other mechanisms may be involved in the suppression of prolactin secretion. In summary, low prolactin and increased circulating glucocorticoids together may partly explain how a mother's endocrine system mediates stress-induced pregnancy failure.

Nitric oxide synthase inhibitors improve prepulse inhibition responses of Wistar rats

INTRODUCTION

Cognitive and attentional deficits in schizophrenia include impairment of the sensorimotor filter as measured by prepulse inhibition (PPI). In this way, the study of animals that naturally present low PPI responses could be a useful approach for screening new antipsychotic drugs. Several pieces of evidence suggest that dopamine and nitric oxide (NO) can modulate PPI but their role in those animals is unknown.

OBJECTIVES

The aim of this study was to investigate the role of dopamine and NO in Wistar rats with naturally low PPI response.

METHODS

Male Wistar rats with low PPI responses received an i.p. injection of the antipsychotics haloperidol (0.1, 0.3 or 1mg/kg) or clozapine (0.5, 1.5 or 5mg/kg), the anxiolytic diazepam (1 or 3mg/kg) or the NO synthase (NOS) inhibitors, N(G)- nitro-l-arginine (l-NOARG; 40mg/kg, acutely or sub-chronically) or 7-Nitroindazole (7-NI; 3, 10 or 30mg/kg). All animals were submitted to the PPI test 1h after injection. Striatal and cortical dopamine, DOPAC, and noradrenaline levels of rats with low PPI responses were compared to rats with normal PPI responses.

RESULTS

We found increased levels of catecholamines on the striatum and prefrontal cortex of Wistar rats with low PPI. In these animals, both antipsychotics, typical and atypical, and NOS inhibitors significantly increased PPI.

CONCLUSION

Taken together, our findings suggest that the low PPI phenotype may be driven by an overactive catecholamine system. Additionally, our results corroborate the hypothesis of dopamine and NO interaction on PPI modulation and suggest that Wistar rats with low PPI may represent an interesting non-pharmacological model to evaluate new potential antipsychotics.

Activity of hypothalamic dopaminergic neurones during the day of oestrus: involvement in prolactin secretion

A secretory surge of prolactin occurs on the afternoon of oestrus in cycling rats. Pituitary prolactin is inhibited by dopamine. We evaluated the activity of the neuroendocrine dopaminergic neurones during oestrus and dioestrus, as determined by dopaminergic activity in the median eminence and neurointermediate lobe of the pituitary, as well as Fos-related antigen expression in tyrosine hydroxylase (TH)-immunoreactive (ir) neurones of the arcuate nucleus (ARC) and periventricular nucleus (Pe). During oestrus, the 4-dihydroxyphenylacetic acid/dopamine ratio in the median eminence decreased at 16.00 h, coinciding with the increase in plasma prolactin levels. Similarly, the expression of Fos-related antigen in TH-ir neurones of Pe and rostral-, dorsomedial- and caudal-ARC also decreased at 16.00 h. On dioestrus, 4-dihydroxyphenylacetic acid/dopamine ratio in the median eminence and Fos-related antigen expression in TH-ir neurones of Pe and rostral-ARC decreased at 18.00 h, whereas prolactin levels were unaltered. No variation in dopaminergic activity was found in the neurointermediate lobe of the pituitary on either oestrus or dioestrus. The number of TH-ir neurones in the ARC and parameters of dopaminergic activity were found to be generally lower on oestrus compared to dioestrus. The transitory decrease in the activity of neuroendocrine dopaminergic neurones temporally associated with the prolactin surge on the afternoon of oestrus suggests a role for dopamine in the generation of the oestrous prolactin surge.

A tale of two rhythms: the emerging roles of oxytocin in rhythmic prolactin release

Hormone secretion often occurs in a pulsatile manner. In this review, we discuss two rhythms of in vivo prolactin release in female rats and the ongoing research that we and others have performed aiming to understand the mechanisms underlying them. The peptide hormone oxytocin appears to play an important role in both rhythms. One rhythm occurs during the first half of pregnancy, but can also be induced in ovariectomised rats. This is characterised by a circadian pattern with two prolactin surges per day. Two methods for triggering this rhythm are discussed, each utilising a unique physiological pathway that includes oxytocin action, presumably on pituitary lactotrophs. The second rhythm occurs during the oestrous cycle and is characterised by a surge of prolactin on the afternoon of pro-oestrus. We discuss recent findings that oxytocin is more effective at stimulating prolactin release from lactotrophs taken from animals on the afternoon of pro-oestrus than from those of animals on the morning of dioestrus 1, raising the possibility that this hormone plays a physiological role in the regulation of prolactin secretion during the oestrous cycle.

Kisspeptin regulates prolactin release through hypothalamic dopaminergic neurons

Prolactin (PRL) is tonically inhibited by dopamine (DA) released from neurons in the arcuate and periventricular nuclei. Kisspeptin plays a pivotal role in LH regulation. In rodents, kisspeptin neurons are found mostly in the anteroventral periventricular and arcuate nuclei, but the physiology of arcuate kisspeptin neurons is not completely understood. We investigated the role of kisspeptin in the control of hypothalamic DA and pituitary PRL secretion in adult rats. Intracerebroventricular kisspeptin-10 (Kp-10) elicited PRL release in a dose-dependent manner in estradiol (E2)-treated ovariectomized rats (OVX+E2), whereas no effect was found in oil-treated ovariectomized rats (OVX). Kp-10 increased PRL release in males and proestrous but not diestrous females. Associated with the increase in PRL release, intracerebroventricular Kp-10 reduced Fos-related antigen expression in tyrosine hydroxylase-immunoreactive (ir) neurons of arcuate and periventricular nuclei in OVX+E2 rats, with no effect in OVX rats. Kp-10 also decreased 3,4-dihydroxyphenylacetic acid concentration and 3,4-dihydroxyphenylacetic acid-DA ratio in the median eminence but not striatum in OVX+E2 rats. Double-label immunofluorescence combined with confocal microscopy revealed kisspeptin-ir fibers in close apposition to and in contact with tyrosine hydroxylase-ir perikarya in the arcuate. In addition, Kp-10 was not found to alter PRL release from anterior pituitary cell cultures regardless of E2 treatment. We provide herein evidence that kisspeptin regulates PRL release through inhibition of hypothalamic dopaminergic neurons, and that this mechanism is E2 dependent in females. These findings suggest a new role for central kisspeptin with possible implications for reproductive physiology.

Hormonal regulation of suppressors of cytokine signaling (SOCS) messenger ribonucleic acid in the arcuate nucleus during late pregnancy

Prolactin stimulates tuberoinfundibular dopamine neurons in the arcuate nucleus of the hypothalamus, mediated by signal transducer and activator of transcription 5b (STAT5b). During late pregnancy, these neurons become unresponsive to prolactin, with a loss of prolactin-induced activation of STAT5b and decreased dopamine secretion. Suppressors of cytokine signaling (SOCS) proteins inhibit STAT-mediated signaling, and SOCS mRNAs are specifically elevated in the arcuate nucleus during late pregnancy. We hypothesized that changes in circulating ovarian steroids during late pregnancy might induce expression of SOCS mRNAs, thus disrupting STAT5b-mediated prolactin signaling. Rats were ovariectomized on d 18 of pregnancy and treated with ovarian steroids to simulate an advanced, normal, or delayed decline in progesterone. Early progesterone withdrawal caused an early increase in prolactin secretion, and increased SOCS-1 and -3 and cytokine-inducible SH2-containing protein (CIS) mRNA levels in the arcuate nucleus. Prolonged progesterone treatment prevented these changes. To determine whether ovarian steroids directly alter SOCS mRNA levels, estrogen- and/or progesterone-treated ovariectomized nonpregnant rats were acutely injected with prolactin (300 microg sc) or vehicle. SOCS-1 and -3 and CIS mRNA levels in the arcuate nucleus were significantly increased by estrogen or prolactin, whereas progesterone treatment reversed the effect of estrogen. Results demonstrate that estrogen and prolactin can independently induce SOCS mRNA in the arcuate nucleus and that this effect is negatively regulated by progesterone. This is consistent with the hypothesis that declining progesterone and high levels of estrogen during late pregnancy induce SOCS in the tuberoinfundibular dopamine neurons, thus contributing to their insensitivity to prolactin at this time.

Oxytocin action at the lactotroph is required for prolactin surges in cervically stimulated ovariectomized rats

Cervical stimulation induces two daily rhythmic prolactin surges, nocturnal and diurnal, which persist for several days. We have shown that a bolus injection of oxytocin initiates a similar prolactin rhythm, which persists despite low levels of oxytocin after injection. This suggests that oxytocin may trigger the cervical stimulation-induced rhythmic prolactin surges. To investigate this hypothesis, we infused an oxytocin antagonist that does not cross the blood-brain barrier for 24 h before and after cervical stimulation and measured serum prolactin. We also measured dopaminergic neuronal activity because mathematical modeling predicted that this activity would be low in the presence of the oxytocin antagonist. We thus tested this hypothesis by measuring dopaminergic neuronal activity in the tuberoinfundibular, periventricular hypophyseal, and tuberohypophyseal dopaminergic neurons. Infusion of oxytocin antagonist before cervical stimulation abolished prolactin surges, and infusion of oxytocin antagonist after cervical stimulation abolished the diurnal and significantly decreased the nocturnal surges of prolactin. The rhythmic prolactin surges returned after the clearance of the oxytocin antagonist. Hypothalamic dopaminergic activity was elevated in antiphase with prolactin surges, and the antiphase elevation was abolished by the oxytocin antagonist in the tuberoinfundibular and tuberohypophyseal dopaminergic neurons, consistent with the mathematical model. These findings suggest that oxytocin is a physiologically relevant prolactin-releasing factor. However, the cervical stimulation-induced prolactin surges are maintained even in the absence of oxytocin actions at the lactotroph, which strongly suggests the maintenance of prolactin surges are not dependent upon oxytocin actions at the pituitary gland.

Prolactin secretory surge during estrus coincides with increased dopamine activity in the hypothalamus and preoptic area and is not altered by ovariectomy on proestrus

Prolactin (PRL) secretory surges have been reported on the afternoons of both proestrus and estrous in cycling rats. As neuroendocrine regulation of estrous PRL surge is poorly understood, the present study aimed to investigate the involvement of hypothalamic dopamine and serotonin as well as of plasma ovarian steroids in this hormonal surge generation. For that, we determined the concentrations of dopamine, serotonin and their respective metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindole-3-acetic acid (5-HIAA) in the mediobasal hypothalamus (MBH) and medial preoptic area (MPOA) throughout the day of estrus and correlated them with plasma PRL levels. In a second study, we evaluated the effect of ovariectomy on the morning of proestrus on PRL surges of both proestrus and estrus. Dopamine turnover, as determined by DOPAC/dopamine ratio, increased in both the MBH and MPOA coinciding with the afternoon PRL surge on estrus. In contrast, both the concentration and turnover (5-HIAA/serotonin) of serotonin within these areas were unaltered during estrus. In addition, ovariectomy reduced plasma estradiol and progesterone levels but did not alter the PRL surges on proestrus and estrus. Considering that dopamine is the main inhibitor of PRL release and that PRL auto-regulates its secretion through a short-loop feedback mechanism, our present results suggest that PRL may suppress its own secretion during the estrus surge through the activation of the dopaminergic neurons in the MBH and MPOA. In addition, the PRL surge on estrus seems do not depend on either the activity of hypothalamic serotonin or the increased secretion of ovarian steroids on proestrus.

Suppression of prolactin-induced signal transducer and activator of transcription 5b signaling and induction of suppressors of cytokine signaling messenger ribonucleic acid in the hypothalamic arcuate nucleus of the rat during late pregnancy and lactation

During late pregnancy and lactation, the tuberoinfundibular dopamine (TIDA) neurons that regulate prolactin secretion by negative feedback become less able to produce dopamine in response to prolactin, leading to hyperprolactinemia. Because prolactin-induced activation of dopamine synthesis in these neurons requires the Janus kinase/signal transducer and activator of transcription 5b (STAT5b) signaling pathway, we investigated whether prolactin-induced STAT5b signaling is reduced during lactation and whether induction of suppressors of cytokine signaling (SOCS) mRNAs occur at this time and in late pregnancy. During lactation, the ability of exogenous prolactin to induce STAT5 phosphorylation and STAT5b nuclear translocation was markedly reduced when compared with diestrous rats. In nonpregnant female rats, acute treatment with ovine prolactin markedly increased levels of SOCS-1 and -3 and cytokine-inducible SH2-containing protein mRNA in arcuate nucleus micropunches. On gestation d 22, SOCS-1 and SOCS-3 mRNA levels were 10-fold that on G20. SOCS-1 and -3 and cytokine-inducible SH2-containing protein mRNA levels were also elevated on lactation d 7. At these times, dopaminergic activity was decreased and the rats were hyperprolactinemic. The high levels of SOCS mRNA were prevented by bromocriptine pretreatment (gestation d 22) or pup removal (lactation d 7), which suppressed circulating prolactin to basal levels. These results demonstrate that around the end of pregnancy, prolactin loses the ability to activate STAT5b, associated with an increase in SOCS mRNAs. The loss of this stimulating pathway may underlie the reduced tuberoinfundibular dopamine neuron dopamine output and hyperprolactinemia that characterizes late pregnancy and lactation. The high maternal levels of SOCS mRNAs appear to be dependent on prolactin, presumably acting through an alternative signaling pathway to STAT5b.

Perinatal glucocorticoid treatment disrupts the hypothalamo-lactotroph axis in adult female, but not male, rats

This study aimed to test the hypothesis that the tuberoinfundibular dopaminergic neurons of the arcuate nucleus and/or the lactotroph cells of the anterior pituitary gland are key targets for the programming effects of perinatal glucocorticoids (GCs). Dexamethasone was administered noninvasively to fetal or neonatal rats via the mothers' drinking water (1 mug/ml) on embryonic d 16-19 or neonatal d 1-7, and control animals received normal drinking water. At 68 d of age, the numbers of tyrosine hydroxylase-positive (TH+) cells in the arcuate nucleus and morphometric parameters of pituitary lactotrophs were analyzed. In control animals, striking sex differences in TH+ cell numbers, lactotroph cell size, and pituitary prolactin content were observed. Both pre- and neonatal GC treatment regimens were without effect in adult male rats, but in females, the overriding effect was to abolish the sex differences by reducing arcuate TH+ cell numbers (pre- and neonatal treatments) and reducing lactotroph cell size and pituitary prolactin content (prenatal treatment only) without changing lactotroph cell numbers. Changes in circulating prolactin levels represented a net effect of hypothalamic and pituitary alterations that exhibited independent critical windows of susceptibility to perinatal GC treatments. The dopaminergic neurons of the hypothalamic periventricular nucleus and the pituitary somatotroph populations were not significantly affected by either treatment regimen in either sex. These data show that the adult female hypothalamo-lactotroph axis is profoundly affected by perinatal exposure to GCs, which disrupts the tonic inhibitory tuberoinfundibular dopaminergic pathway and changes lactotroph morphology and prolactin levels in the pituitary and circulation. These findings provide new evidence for a long-term disruption in prolactin-dependent homeostasis in females, but not males, after inappropriate GC exposure in perinatal life.

Ovarian steroid hormones modulate circadian rhythms of neuroendocrine dopaminergic neuronal activity

Dopamine (DA) is the primary inhibitor of prolactin (PRL) secretion. Three populations of neuroendocrine dopaminergic neurons (NDNs) designated tuberoinfundibular (TIDA), tuberohypophyseal (THDA) and periventricular hypophyseal DAergic (PHDA) neurons regulate PRL secretion. Given that ovarian steroids modulate both DA release and PRL secretion independently, we characterized the role of steroid hormones in coupling rhythmic NDN activity and PRL secretion. OVX rats under a standard 12:12 L:D cycle (L:D), constant dark (DD), or a 6-h phase-delayed L:D cycle (pdL:D) were treated with Estradiol-17beta (E) or E and Progesterone (E+P). NDN activity, defined by DA:DOPAC ratio in nerve terminals, was determined by HPLC-EC. E or E+P stimulated PRL surges in L:D that persisted under DD. In TIDA neurons, E or E+P treatment reduced the amount of DA released under L:D and DD and advanced the rhythm of DA turnover. E and E+P treatment reduced THDA and PHDA neuron activity under L:D, but did not affect these rhythms under DD. Circadian rhythms of PRL, corticosterone and DA turnover in NDN terminals from steroid treated rats entrained to a pdL:D cycle within 7 days. Therefore, ovarian steroids differentially adjust the timing and magnitude of NDN activity to facilitate coupling of DA release and PRL secretion.

Opioid receptor subtypes involved in the regulation of prolactin secretion during pregnancy and lactation

Afferent endogenous opioid neuronal systems facilitate prolactin secretion in a number of physiological conditions including pregnancy and lactation, by decreasing tuberoinfundibular dopamine (TIDA) inhibitory tone. The aim of this study was to investigate the opioid receptor subtypes involved in regulating TIDA neuronal activity and therefore facilitating prolactin secretion during early pregnancy, late pregnancy and lactation in rats. Selective opioid receptor antagonists nor-binaltorphimine (kappa-receptor antagonist, 15 micro g/5 micro l), beta funaltrexamine (mu-receptor antagonist, 5 microg/5 microl) and naltrindole (delta-receptor antagonist, 5 microg/5 microl) or saline were administered intracerebroventricularly (i.c.v.) on day 8 of pregnancy during a nocturnal prolactin surge, on day 21 of pregnancy during the ante partum prolactin surge or on day 7 of lactation before the onset of a suckling stimulus. Serial blood samples were collected at regular time intervals, via chronic indwelling jugular cannulae, before and after drug administration and plasma prolactin was determined by radioimmunoassay. TIDA neuronal activity was measured using the 3,4-dihydroxyphenylacetic acid (DOPAC) : dopamine ratio in the median eminence 2 h 30 min after i.c.v. drug injection. In each experimental condition, plasma prolactin was significantly inhibited by both kappa- and mu-receptor antagonists, whereas the delta-receptor antagonist had no effect compared to saline-injected controls. Similarly, nor-binaltorphimine and beta funaltrexamine significantly increased the median eminence DOPAC : dopamine ratio during early and late pregnancy, and lactation whereas naltrindole had no effect compared to saline-injected controls. These data suggest that TIDA neuronal activity, and subsequent prolactin secretion, is regulated by endogenous opioid peptides acting at both kappa- and mu-opioid receptors during prolactin surges of early pregnancy, late pregnancy and lactation.

Circadian rhythms of neuroendocrine dopaminergic neuronal activity in ovariectomized rats

Prolactin (PRL) secretion is inhibited by dopamine (DA) released from hypothalamic neuroendocrine neurons designated tuberoinfundibular (TIDA), tuberohypophyseal (THDA) and periventricular hypophyseal (PHDA) dopaminergic (DAergic) neurons. Since PRL is secreted in many physiological states with a circadian rhythm, the purpose of these experiments was to determine if patterns of neuroendocrine DAergic neuronal activity in rats are also circadian. The activity of neuroendocrine DAergic neurons, defined as DA turnover rate in nerve terminals and quantitated as the ratio of DOPAC (a primary DA metabolite) to DA content, was measured by high-performance liquid chromatography with electrochemical detection (HPLC-EC) in these populations of DA neurons of OVX rats. TIDA neurons exhibit a rhythm of activity in a light:dark cycle which free-runs in constant dark (DD) and is entrained by light, indicating that TIDA neuronal activity is circadian. THDA and PHDA neurons also display daily rhythms entrained to a photoperiod and PHDA neuronal activity free-runs in DD with a period of approximately 24 h. However, a significant rhythm of THDA neuronal activity was not detected under DD. In the OVX rat, the activities of TIDA and PHDA neurons, but not THDA neurons, describe all the characteristics of a circadian rhythm as they are both entrained by light, but only TIDA and PHDA neurons maintain a significant rhythm of activity under DD.

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.

Fasting stimulates tuberoinfundibular dopaminergic neuronal activity and inhibits prolactin secretion in oestrogen-primed ovariectomized rats: involvement of orexin A and neuropeptide Y

Fasting up-regulates central orexigenic systems including orexin A and neuropeptide Y (NPY) and it also inhibits the secretion of prolactin. We hypothesized that fasting may act through orexin A and NPY to influence tuberoinfundibular dopaminergic (TIDA) neurones, the major regulator of prolactin secretion. The effects of orexin A and NPY on TIDA neuronal activity and prolactin secretion were determined in oestrogen-primed ovariectomized rats, and the effects of fasting and the involvement of orexin A and NPY were tested. Orexin A, NPY and its analogs were administered through preimplanted intracerebroventricular (i.c.v.) cannulae. TIDA neuronal activity was determined by measuring concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) or 3,4-dihydroxyphenylalanine in the median eminence. i.c.v. injection of NPY (10 microg) or orexin A (1 microg) concomitantly increased median eminence DOPAC and decreased serum prolactin concentrations. The effect of NPY was mimicked by a Y1 receptor agonist at lower doses (0.1 and 1 microg) and no additive effect was observed when orexin A and the Y1 agaonist were coadministered. Moreover, a Y1 receptor antagonist, BIBP, not only blocked the effect of Y1 agaonist, but also that of orexin A. Treatment with BIBP alone decreased median eminence DOPAC and increased serum prolactin concentrations, indicating that endogenous NPY may play a role. Moreover, fasting for 48 h significantly increased TIDA neuronal activity, both in the morning and afternoon, and the effect was reversed by treatment with either BIBP or an antibody against orexin A. The findings support our hypothesis that fasting stimulates TIDA neuronal activity and inhibits prolactin secretion through up-regulated central orexin A and NPY systems.

Sensitization to the neuroendocrine, central monoamine and behavioural effects of murine tumor necrosis factor-alpha: peripheral and central mechanisms

Systemic administration of murine tumour necrosis factor-alpha (mTNF-alpha; 0.1-2.0 microg, i.p.) dose-dependently increased plasma corticosterone and augmented monoamine utilization within the paraventricular nucleus of the hypothalamus (PVN), locus coeruleus, medial prefrontal cortex (PFC), central and medial amygdala. A time-dependent sensitization was induced in mice, wherein reexposure to mTNF-alpha 28 days (but not 1 day) following the initial cytokine treatment provoked marked signs of illness (diminished activity, ptosis, piloerection) and increased plasma corticosterone levels. Serotonin (5-HT) activity was augmented upon mTNF-alpha reexposure at the 1- or 28-day intervals in the PFC and medial amygdala, respectively. Intracerebroventricular (i.c.v.; 1-500 ng) mTNF-alpha did not promote illness, but modestly increased plasma corticosterone levels. Neither the illness nor the corticosterone changes were subject to a sensitization upon i.c.v. cytokine reexposure. Acute i.c.v. mTNF-alpha increased norepinephrine (NE), 5-HT and dopamine (DA) activity within the PVN and median eminence/arcuate nucleus complex (ME/ARC), and NE utilization within the central amygdala. Subsequent i.c.v. mTNF-alpha further enhanced the hypothalamic monoamine variations. Finally, systemic (i.p.) mTNF-alpha pretreatment did not proactively influence sickness or corticosterone responses upon later i.c.v. cytokine challenge, but augmented locus coeruleus NE activity and 5-HT and DA utilization within the ME/ARC. It is suggested that the sensitization with respect to sickness and corticosterone activity in response to mTNF-alpha reflect the involvement of peripheral mechanisms. Moreover, it appears that mTNF-alpha promotes central neurochemical plasticity through independent central and peripheral mechanisms.

Dissociation of prolactin secretion from tuberoinfundibular dopamine activity in late pregnant rats

This study investigated whether the PRL surge that precedes parturition is accompanied by a decrease in activity of hypothalamic tuberoinfundibular dopamine (TIDA) neurons, as occurs during the PRL surges of early pregnancy. Serial blood samples were collected at regular intervals during early and late pregnancy via chronic indwelling jugular cannulae, and concentrations of plasma PRL were determined by RIA. In addition, pregnant rats were killed at either 1200 and 0300 h on different days throughout pregnancy. Levels of TIDA neuronal activity were estimated using concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence as an index of dopamine metabolism. During early pregnancy, plasma PRL concentrations showed characteristic diurnal and nocturnal surges peaking at 1700 and 0300 h, respectively, whereas during late pregnancy, there was a broad nocturnal surge throughout the night preceding parturition. During early pregnancy, DOPAC was elevated at 1200 h, associated with suppressed plasma PRL, whereas at 0300 h, during the nocturnal PRL surge, DOPAC was significantly reduced (P < 0.05). On the last day of pregnancy DOPAC levels were significantly reduced at both 1200 and 0300 h compared with those at 1200 h in early pregnancy regardless of the PRL concentration. This experiment was repeated with additional groups to further characterize the timing of the fall in TIDA activity during late pregnancy. DOPAC concentrations were elevated throughout the second half of pregnancy, then fell significantly between 0300-1200 h on day 21, approximately 36 h before parturition. As in the previous experiment, the timing of changes in DOPAC concentrations in the median eminence was dissociated from the antepartum PRL surge. These data indicate that the regulation of PRL secretion during late pregnancy is different from that of early pregnancy. Despite the prolonged reduction in activity of TIDA neurons during late pregnancy, PRL secretion still occurs as a nocturnal surge, suggesting that dopamine is not the only regulator of PRL secretion at this time.

Dopaminergic input to the ventromedial hypothalamus facilitates the oestrogen-induced luteinizing hormone surge in ewes

In this study we examined the release of dopamine and noradrenaline in the ventromedial hypothalamus (VMH) of ovariectomized ewes during the oestrogen-induced luteinizing hormone (LH) surge by measuring their respective metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and methoxyhydroxyphenylglycol (MHPG) using microdialysis. Further we investigated whether inhibition of catecholamine synthesis in the VMH by bilateral reverse dialysis of alpha-methyl-p-tyrosine (alpha-MPT) would block the oestrogen-induced LH and/or prolactin surges. Oestradiol treatment (50 microg oestradiol benzoate) of ovariectomized ewes resulted in a biphasic LH response, significantly (p < 0.05) decreasing LH concentrations from 2.5 to 10.5 h after injection, followed by an LH surge beginning at 16 h. Prolactin concentrations were also significantly (p < 0.05) increased in oestradiol-treated ewes from 13 h. VMH DOPAC concentrations in oil-vehicle-treated animals were at the level of detection (0.02 ng/ml) in most samples over the 24-hour sampling period. In oestradiol-treated ewes, VMH DOPAC levels were initially low before and up to 8 h after oestradiol injection but then increased significantly (p < 0.05) at 10-12 h and remained elevated up to 20 h after injection. In contrast, oestradiol injection had no effect on MHPG concentrations in the VMH. Bilateral reverse microdialysis of alpha-MPT into the VMH significantly (p < 0.05) delayed the time from oestradiol injection to the onset of the LH surge, the time to peak LH concentration and attenuated the LH surge compared with reverse dialysis of Ringer solution alone. In contrast, alpha-MPT treatment had no effect upon the oestradiol-induced increase in prolactin concentrations. This study provides evidence that the VMH is an important hypothalamic site in the neuro-endocrine control of the LH surge in ewes. The results suggest that dopaminergic neurons with terminals in the VMH are part of a neuronal pathway mediating the positive feedback effects of oestradiol on gonadotropin-releasing hormone secretion and the LH surge.

Ovarian steroids influence the activity of neuroendocrine dopaminergic neurons

The secretion of prolactin (PRL) from the anterior lobe (AL) of the pituitary gland is tonically inhibited by dopamine (DA) of hypothalamic origin. While ovarian steroids play a role in the regulation of the secretion of PRL, their effect on all three populations of hypothalamic neuroendocrine dopaminergic neurons is not fully understood. In this study we describe the effects of ovarian steroids on regulation of the release of DA from tuberoinfundibular dopaminergic (TIDA), tuberohypophyseal dopaminergic (THDA) and periventricular-hypophyseal dopaminergic (PHDA) neurons. Adult female rats were bilaterally ovariectomized (OVX) and, 10 days following ovariectomy (day 0), injected with corn oil (vehicle), estrogen, or estrogen plus progesterone (day 1). Animals were sacrificed every 2 h from 09.00 to 21.00 h by rapid decapitation. Trunk blood was collected and the concentration of PRL in serum was determined by radioimmunoassay. The median eminence (ME) and the AL, intermediate (IL) and neural (NL) lobes of the pituitary gland were dissected and the concentration of DA and DOPAC in each was measured by HPLC-EC. OVX rats presented small but significant increases in the secretion of PRL at 15.00 and 17.00 h. Replacement of estrogen or estrogen plus progesterone increased the basal concentration of PRL. Moreover, injection of estrogen only, or estrogen plus progesterone increased the concentration of PRL in serum at 15.00 h through 19.00 h, respectively, followed by a decrease to baseline thereafter. The turnover of DA in the ME and NL of OVX rats increased at 13.00 and returned to low levels. Turnover of DA in the IL of OVX rats increased in the morning by 11.00 h and remained elevated before decreasing by 17.00 h. The turnover of DA in the ME, IL and NL of OVX rats increased by 19.00 h. Injection of estrogen advanced the increase of TIDA activity by 2 h in the ME compared to OVX rats. Moreover, administration of estrogen suppressed the activity of THDA and PHDA neurons in the afternoon compared to OVX rats. In estrogen plus progesterone-treated rats, the activity of hypothalamic neuroendocrine dopaminergic neurons terminating in the ME, IL, and NL was inhibited prior to the increase in the secretion of PRL. The concentration of DA in the AL diminished prior to the estrogen-induced increase of PRL. Administration of progesterone, in concert with estrogen, delayed the increase of PRL in serum and the decrease of DA in the AL, compared to estrogen-treated rats, by 4 h. These data suggest a major role for ovarian steroids in controlling increases in the secretion of PRL by not only stimulating PRL release from lactotrophs, but also by inhibiting the activity of all three populations of hypothalamic neuroendocrine DAergic neurons.

An endogenous serotonergic rhythm acting on 5-HT(2A) receptors may be involved in the diurnal changes in tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in female rats

The central serotonergic system has long been known to have a stimulatory role on the secretion of prolactin (PRL). The integrity of serotonergic neurotransmission is essential for the expression of the estrogen-induced afternoon PRL surge. Whether its effect on PRL involves change in the activity of tuberoinfundibular dopaminergic (TIDA) neurons has not been ascertained. In adult ovariectomized rats treated with estrogen, depletion of central serotonin (5-HT) by 5,7-dihydroxytryptamine (5,7-DHT, 200 microg/rat, i.c.v.) effectively prevented the afternoon fall in TIDA neuronal activity (using the levels of 3,4-dihydroxyphenylalanine and 3, 4-dihydroxyphenylacetic acid (DOPAC), and the ratio of DOPAC/dopamine in the median eminence as indices), and blunted the afternoon PRL surge. A single injection of a 5-HT(2A) receptor antagonist, ketanserin (5 mg/kg, i.p. at 12.00 h), also had the same effects on the diurnal changes in TIDA neuronal activity and PRL secretion as the treatment with 5,7-DHT did. Intracerebroventricular injection of a 5-HT(2) receptor agonist 2, 5-dimethoxy-4-iodoamphetamine (DOI) in the morning inhibited the TIDA neuronal activity and stimulated PRL secretion in a dose-dependent manner; while injection of a 5-HT(1) agonist, 8-hydroxy-dipropylaminotetralin, was without effect. Injection of DOI in 5,7-DHT-pretreated rats at 14.30 h also lowered the TIDA neuronal activity and reinstated the PRL surge. In all, endogenous 5-HT, acting through the 5-HT(2A) receptor, appears to exhibit an inhibitory effect on TIDA neuronal activity during the afternoon, which is essential for the PRL surge.

Immunoneutralization of prolactin prevents stimulatory feedback of prolactin on hypothalamic neuroendocrine dopaminergic neurons

We have found that exogenous prolactin (PRL) stimulates all three populations of hypothalamic neuroendocrine dopaminergic neurons. In this study, we investigated the effects of immunoneutralization of endogenous PRL on the activity of these neurons. Injection of 17beta-estradiol (E2) (20 microg subcutaneously) 10 d after ovariectomy induced a proestrus-like increase in PRL in peripheral plasma the following afternoon. At 1000 h the day after E2 injection, rats received either rabbit antirat PRL antiserum (PRL-AS) (200 microL) or normal rabbit serum (NRS, 200 microL, controls) intraperitoneally. Groups of rats were then decapitated every 2 h from 1100 h to 2100 h. Trunk blood was collected and serum extracted with protein A to remove the PRL-AS/PRL complex, and the remaining free PRL was measured by radioimmunoassay. Sites of neuroendocrine dopaminergic nerve terminals, the median eminence (ME), and intermediate and neural lobes of the pituitary gland were excised and stored for determination of dopamine (DA) and 3,4-dihydroxyphenyl acetic acid (DOPAC) concentrations by high-performance liquid chromatography electrochemical detection (EC). In addition, the anterior lobe of the pituitary gland, the locus of DA action, was collected. The concentration of PRL in NRS-treated animals increased by 1500 h, peaked by 1700 h, and returned to low levels by 2100 h. PRL-AS prevented the increase in PRL secretion in response to E2. The turnover of DA (DOPAC:DA ratio; an index of dopaminergic neuronal activity) in the ME of NRS-treated animals increased at 1500 h and rapidly returned to basal levels. Treatment with PRL-AS prevented the increase in DA turnover in the ME. DA turnover in the intermediate lobe increased coincident with the peak of PRL in serum of NRS-treated rats. PRL-AS administration prevented increased DA turnover in the intermediate lobe. The turnover of DA in the neural lobe increased by 1300 h and decreased steadily through 2100 h. However, administration of PRL-AS minimally suppressed the turnover of DA in the neural lobe. Moreover, administration of PRL-AS attenuated the rise of DA in the anterior lobe associated with the waning phase of the E2-induced PRL surge. These results clearly indicate that endogenous PRL regulates its own secretion by activating hypothalamic neuroendocrine dopaminergic neurons.