Effects of immobilization stress on tuberoinfundibular dopaminergic (TIDA) neuronal activity and prolactin levels in lactating and non-lactating female rats

Hypothalamus and Post-Traumatic Stress Disorder: A Review

Humans have lived in a dynamic environment fraught with potential dangers for thousands of years. While fear and stress were crucial for the survival of our ancestors, today, they are mostly considered harmful factors, threatening both our physical and mental health. Trauma is a highly stressful, often life-threatening event or a series of events, such as sexual assault, war, natural disasters, burns, and car accidents. Trauma can cause pathological metaplasticity, leading to long-lasting behavioral changes and impairing an individual's ability to cope with future challenges. If an individual is vulnerable, a tremendously traumatic event may result in post-traumatic stress disorder (PTSD). The hypothalamus is critical in initiating hormonal responses to stressful stimuli via the hypothalamic-pituitary-adrenal (HPA) axis. Linked to the prefrontal cortex and limbic structures, especially the amygdala and hippocampus, the hypothalamus acts as a central hub, integrating physiological aspects of the stress response. Consequently, the hypothalamic functions have been attributed to the pathophysiology of PTSD. However, apart from the well-known role of the HPA axis, the hypothalamus may also play different roles in the development of PTSD through other pathways, including the hypothalamic-pituitary-thyroid (HPT) and hypothalamic-pituitary-gonadal (HPG) axes, as well as by secreting growth hormone, prolactin, dopamine, and oxytocin. This review aims to summarize the current evidence regarding the neuroendocrine functions of the hypothalamus, which are correlated with the development of PTSD. A better understanding of the role of the hypothalamus in PTSD could help develop better treatments for this debilitating condition.

Sensory Neurons, Neuroimmunity, and Pain Modulation by Sex Hormones

The inclusion of women in preclinical pain studies has become more commonplace in the last decade as the National Institutes of Health (NIH) released its "Sex as a Biological Variable" mandate. Presumably, basic researchers have not had a comprehensive understanding about neuroimmune interactions in half of the population and how hormones play a role in this. To date, we have learned that sex hormones contribute to sexual differentiation of the nervous system and sex differences in behavior throughout the lifespan; however, the cycling of sex hormones does not always explain these differences. Here, we highlight recent advances in our understanding of sex differences and how hormones and immune interactions influence sensory neuron activity to contribute to physiology and pain. Neuroimmune mechanisms may be mediated by different cell types in each sex, as the actions of immune cells are sexually dimorphic. Unfortunately, the majority of studies assessing neuronal contributions to immune function have been limited to males, so it is unclear if the mechanisms are similar in females. Finally, pathways that control cellular metabolism, like nuclear receptors, have been shown to play a regulatory role both in pain and inflammation. Overall, communication between the neuroimmune and endocrine systems modulate pain signaling in a sex-dependent manner, but more research is needed to reveal nuances of these mechanisms.

Neuroendocrine regulation of lactation and milk production

Prolactin (PRL) released from lactotrophs of the anterior pituitary gland in response to the suckling by the offspring is the major hormonal signal responsible for stimulation of milk synthesis in the mammary glands. PRL secretion is under chronic inhibition exerted by dopamine (DA), which is released from neurons of the arcuate nucleus of the hypothalamus into the hypophyseal portal vasculature. Suckling by the young activates ascending systems that decrease the release of DA from this system, resulting in enhanced responsiveness to one or more PRL-releasing hormones, such as thyrotropin-releasing hormone. The neuropeptide oxytocin (OT), synthesized in magnocellular neurons of the hypothalamic supraoptic, paraventricular, and several accessory nuclei, is responsible for contracting the myoepithelial cells of the mammary gland to produce milk ejection. Electrophysiological recordings demonstrate that shortly before each milk ejection, the entire neurosecretory OT population fires a synchronized burst of action potentials (the milk ejection burst), resulting in release of OT from nerve terminals in the neurohypophysis. Both of these neuroendocrine systems undergo alterations in late gestation that prepare them for the secretory demands of lactation, and that reduce their responsiveness to stimuli other than suckling, especially physical stressors. The demands of milk synthesis and release produce a condition of negative energy balance in the suckled mother, and, in laboratory rodents, are accompanied by a dramatic hyperphagia. The reduction in secretion of the adipocyte hormone, leptin, a hallmark of negative energy balance, may be an important endocrine signal to hypothalamic systems that integrate lactation-associated food intake with neuroendocrine systems.

L-DOPA attenuates prolactin secretion in response to isolation stress in Holstein steers

To clarify endocrine responses to psychological stressors in cattle, the effects of isolation from familiar peers on plasma prolactin (PRL) and cortisol (CORT) concentrations, and the effect of 3,4-dihydroxy-L-phenylalanine (L-DOPA), a precursor of dopamine (DA), on stress-induced PRL secretion were determined in Holstein steers. First, the potency of peripheral L-DOPA administration on attenuation of central DA levels was confirmed. Cerebrospinal fluid (CSF) collected from a chronic cannula in the third ventricle and plasma were sampled 1 h before and 3 h after intravenous injection of L-DOPA (100 mg/head). DA concentrations in CSF increased just after L-DOPA injection with subsequent decrease in PRL secretion. Injection of L-DOPA increased CORT secretion. Second, one experimental steer was isolated in its stall by removing its peers for 2 h with or without- pre-injection of L-DOPA. The concentration of PRL was elevated by isolation treatment, whereas the effect of isolation on CORT concentration could not be detected. The increase in PRL concentration after isolation was abolished by pre-injection of L-DOPA. These results suggest that PRL responds to isolation and that DA neurons in the central nervous system may regulate stress-induced PRL secretion in steers.

Comparison of the temporal programs regulating tyrosine hydroxylase and enkephalin expressions in TIDA neurons of lactating rats following pup removal and then pup return

Dopamine (DA) and enkephalin (ENK) release from the tuberoinfundibular dopaminergic neurons (TIDA) into the hypophysial portal circulation is fundamentally different under non-lactating and lactating conditions. The aim of this experiment was to compare the effect of a brief interruption then resumption of suckling on the temporal program of tyrosine hydroxylase (TH; rate-limiting enzyme of dopamine synthesis) and ENK regulation in dams. On post partum day 10, pups were removed for a 4-h period from a group of the dams then returned for 4- and 24-h periods. It was examined whether such a brief interruption of suckling provokes full up-regulation of TH and down-regulation of ENK, and whether reinitiation of suckling limits the extent to which TH up- and ENK down-regulate. At the end of experiment, the animals were decapitated. In situ hybridization was used to examine the expression of TH and ENK mRNA in the arcuate nucleus where TIDA neurons reside. The results showed that, on one hand, the removal of pups induced TH up-regulation, on the other hand, ENK expression also increased 8 h after removal of pups and then started to slowly decline. In dams whose sucklings were reinitiated both TH and ENK mRNAs were up-regulated at least for a day. ENK expression responded more slowly to the removal of pups than expression of TH, and after reinitiation of suckling, the temporal program of regulation of both TH and ENK expressions ran parallel in the first 24 h.

In vivo correlation between c-Fos expression and corticotroph stimulation by adrenocorticotrophic hormone secretagogues in rat anterior pituitary gland

In the anterior pituitary gland, c-Fos expression is evoked by various stimuli. However, whether c-Fos expression is directly related to the stimulation of anterior pituitary cells by hypothalamic secretagogues is unclear. To confirm whether the reception of hormone-releasing stimuli evokes c-Fos expression in anterior pituitary cells, we have examined c-Fos expression of anterior pituitary glands in rats administered with synthetic corticotrophin-releasing hormone (CRH) intravenously or subjected to restraint stress. Single intravenous administration of CRH increases the number of c-Fos-expressing cells, and this number does not change even if the dose is increased. Double-immunostaining has revealed that most of the c-Fos-expressing cells contain adrenocorticotrophic hormone (ACTH); corticotrophs that do not express c-Fos in response to CRH have also been found. However, restraint stress evokes c-Fos expression in most of the corticotrophs and in a partial population of lactotrophs. These results suggest that c-Fos expression increases in corticotrophs stimulated by ACTH secretagogues, including CRH. Furthermore, we have found restricted numbers of corticotrophs expressing c-Fos in response to CRH. Although the mechanism underlying the different responses to CRH is not apparent, c-Fos is probably a useful immunohistochemical marker for corticotrophs stimulated by ACTH secretagogues.

Primary and secondary mediators' influence on milk output in lactating mothers of preterm and term infants

This study examined potential primary mediators, such as intended length to breastfeed, maternal education, income, and infant gestation, and secondary mediators, such as early frequency of breast stimulation, early milk output, and supplementation with artificial milks that may influence milk output in mothers of preterm and term infants the first 6 weeks postpartum. Analysis suggested that for mothers of a preterm infant (n = 95), the primary mediators, income and infant gestation, and the secondary mediators, early milk output/d and early frequency/d, accounted for 53.5% of the variance in milk output/d at week 6. For mothers of a term infant (n = 98), the primary mediator, income, and secondary mediators, early milk output/d and supplementation, accounted for 48.4% of the variance in milk output/d at week 6. Further research is needed to determine what early interventions may improve milk output in mothers at risk for lactation failure.

Pivotal Role of an Endogenous Tetrahydroisoquinoline, Salsolinol, in Stress- and Suckling-Induced Release of Prolactin

In mammals, the role of a prolactin-releasing factor (PRF) in the acute changes of prolactin (PRL) secretion that usually occur after challenges (e.g., suckling stimulus or stress) of homeostasis has been suspected for a long time. We have recently observed that 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, salsolinol (SAL), produced by the hypothalamus and the neuro-intermediate lobe (NIL) of the pituitary gland, can selectively release PRL from the anterior lobe (AL). Moreover, binding sites for SAL have been detected in areas like median eminence, NIL, and AL. It has been proposed that SAL is a putative endogenous PRF. We have also found that a structural analogue of SAL, 1-methyl-3,4-dihydroisoquinoline (1MeDIQ), is able to block dose-dependently SAL-, suckling-, and immobilization (IMO) stress-induced release of PRL without having any influence on alpha-methyl-p-tyrosine (alphaMpT)-induced PRL responses. Neither SAL nor 1MeDIQ has any effect on alpha-melanocyte-stimulating hormone (alphaMSH), adrenocorticotrophic hormone (ACTH), beta-endorphin (beta-END) and arginine-vasopressin (AVP) secretion. Moreover, SAL-induced PRL response was attenuated in male rats pretreated with dexamethasone (DEX). These results strongly suggest that SAL has an important role in the regulation of PRL release induced by physiologic and environmental stimuli; therefore, it can be considered as the strongest candidate for being the PRF in the hypothalamo-hypophysial system. Our findings also indicate that the adrenal steroids may play an inhibitory feedback role in SAL-mediated PRL response.

[Phe(1)Psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2) does not antagonize orphaninFQ/nociceptin-induced prolactin release

The specificity of the orphaninFQ (OFQ)/nociceptin (N)-induced prolactin increase was determined in male and female rats by pretreating animals with different doses of [Phe(1)Psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2), a compound originally reported to be a specific OFQ/N antagonist. In addition, the effect of naloxone pretreatment on OFQ/N-induced prolactin release was examined to determine if OFQ/N's effects were mediated by opiate receptors. Furthermore, dose response studies using [Phe(1)Psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2) only were performed to determine potential agonist activity of this drug. Finally, growth hormone (GH) levels were determined as an index of specificity of the prolactin response. Our results confirm previous findings that OFQ/N potently stimulates prolactin release and that a gender difference exists in the magnitude of the response, with females showing a much greater response than male rats. The endocrine response is specific because OFQ/N potently stimulated prolactin, but not GH secretion. The prolactin response is not mediated by actions at opiate receptors because naloxone did not inhibit OFQ/N's effects on prolactin release. However, [Phe(1)Psi(CH(2)-NH) Gly2]NC(1-13) NH(2) did not antagonize OFQ/N's effects on prolactin release. Indeed, this drug acted as a potent agonist. Demonstrating pharmacological specificity of OFQ/N's effects on prolactin release awaits the development of more selective, specific antagonists.

Suckling stimulus suppresses messenger RNA for tyrosine hydroxylase in arcuate neurons during lactation

Tyrosine hydroxylase (TH) mRNA in tuberoinfundibular dopamine (TIDA) neurons is suppressed during lactation but rebounds upon pup removal. A time course of TH mRNA changes after pup removal revealed three phases: (1) a nuclear phase (evident 1.5 hours after pup removal, maximal at 3 hours) with TH mRNA appearing in 1 or 2 nuclear loci with little or no change in cytoplasmic mRNA; (2) a cytoplasmic phase (noted 6 hours after pup removal, peaked 12-24 hours) with a significant increase in total TH mRNA levels mainly in the cytoplasm; and (3) a stabilization phase (24-48 hours after pup removal) when nuclear signals were low and cytoplasmic RNA showed a slight decline with extension of RNA clusters into the cell dendrites. In rats whose pups could suckle only on one side, TH was up-regulated only on the side contralateral to nipple blockade. These data indicate that after suckling terminates, TH up-regulation is evident at 1.5 hours, but 6 hours is needed before the cells transport sufficient mRNA into the cytoplasm. The rapid signaling of TH up-regulation stems from the fact that the TIDA neurons respond to neural signals from termination of suckling.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Immunoneutralization of endogenous opioid peptides prevents the suckling-induced prolactin increase and the inhibition of tuberoinfundibular dopaminergic neurons

Previous studies have shown that the endogenous opioid peptides, acting at specific opiate receptor subtypes, are involved in the suckling-induced prolactin secretory response. The prolactin increase elicited by suckling is due, at least in part, to an inhibition of tuberoinfundibular dopaminergic (TIDA) neurons in the hypothalamus. We investigated the effects of immunoneutralization of dynorphin, leu-enkephalin and met-enkephalin on the suckling-induced prolactin increase and on the activity of the TIDA neurons in lactating female rats between days 7 and 12 postpartum. Rats were injected into the right lateral ventricle with antiserum specific for one of these three peptides. Control rats were administered equal amounts of immunoglobulin proteins. Suckling produced a profound and significant increase in prolactin levels, as well as a decrease in DOPA accumulation in the median eminence of lactating rats. Administration of immunoglobulin concentrations of up to 3.6 microg did not inhibit the prolactin secretory response to the suckling stimulus and did not prevent the suckling-induced inhibition of TIDA neurons. Antisera to all three endogenous opioid peptides abolished the suckling-induced prolactin increase and prevented the inhibition in DOPA accumulation in the median eminence. Thus, the endogenous opioid peptides, dynorphin, leu-enkephalin and met-enkephalin, are essential for the prolactin secretory response to suckling and inhibition of TIDA neuronal activity is at least one of the mechanisms of action utilized by these peptides.

NMDA receptor antagonists block stress-induced prolactin release in female rats at estrus

In order to evaluate the role of glutamate in prolactin secretion, we examined the effects of N-methyl-D,L-aspartic acid (NMDA) receptor antagonists on serum prolactin levels at both resting and restraint-stress conditions in female rats at estrus. NMDA increased basal serum prolactin levels. Administration of the selective NMDA receptor antagonist, cis-4-phosphonomethyl-2-piperidine carboxylic acid (CGS 19755) (5 and 10 mg/kg i.p.), to rats under resting conditions enhanced basal prolactin levels. A low dose of CGS 19755 (3 mg/kg) was unable to modify the hormone serum level. Under stress conditions the pretreatment with CGS 19755 (3 and 5 mg/kg) prevented the increase in serum prolactin levels. This effect was reversed by NMDA (60 mg/kg s.c.). The NMDA receptor antagonist (5 mg/kg) decreased the median eminence concentration of the dopamine metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), without modifying dopamine content. To examine the probable link between serotonin (5-HT) and glutamate in prolactin release, the 5-HT2A/5-HT2C receptor antagonist, ritanserin, was used. Under resting conditions, a dose of 5 mg/kg s.c. blocked the NMDA-induced prolactin release. In rats submitted to restraint, ritanserin decreased the prolactin response and NMDA was unable to correct the stress serum prolactin levels. The 5-HT1A receptor agonist, 8-hidroxypropyl-amino tetralin (8-OH-DPAT) (3 mg/kg s.c.), increased basal serum prolactin levels and restored serum prolactin in stressed animals pretreated with CGS 19755 (5 mg/kg). The present data strongly suggest that the glutamatergic system participates in the regulation of prolactin secretion. A stimulation tone seems to be exerted via the tuberoinfundibular dopaminergic system, and the prolactin release evoked by restraint apparently involves glutamate/NMDA receptors linked to a serotoninergic pathway.

Immunoneutralization of beta-endorphin blocks prolactin release during suckling without affecting tuberoinfundibular dopaminergic neural activity

The effect of immunoneutralization of beta-endorphin on the suckling-induced prolactin increase and on the activity of the tuberoinfundibular dopaminergic (TIDA) neurons was determined in lactating female rats between days 8 - 12 post-partum. Two antisera were used in the immunoneutralization studies. Both were specific for beta-endorphin, exhibiting little cross reactivity with met- or leu-enkephalin or dynorphin. Antisera to beta-endorphin completely abolished the suckling-induced prolactin increase indicating that this endogenous opioid peptide is involved in this response. Suckling significantly inhibited DOPA accumulation in the median eminence and antiserum to beta-endorphin did not prevent this inhibition. Additionally, 5-endorphin antiserum significantly reduced TIDA neural activity even in pup-deprived dams. These results indicate that beta-endorphin is involved in the prolactin secretory response to suckling but that inhibition of TIDA neuronal activity is not its mechanism of action. Other possible mechanisms are discussed.

Reduced response of the hypothalamo-pituitary-adrenal axis to alpha1-agonist stimulation during lactation

To determine whether altered noradrenergic activation of the hypothalamo-pituitary-adrenal (HPA) axis contributes to the attenuated neuroendocrine response to stress observed during lactation, the effect of intracerebroventricular injection of the alpha1-agonist methoxamine (100 microg) was compared between virgin and lactating rats. Virgin rats showed significant increases in plasma corticosterone after methoxamine, reaching 317 +/- 44 ng/ml at 10 min and remaining significantly elevated for more than 120 min, but lactating rats showed no significant increase in corticosterone levels. Furthermore, methoxamine induced an increase in paraventricular nucleus (PVN) CRF messenger RNA expression in virgin, but not lactating, animals. Both groups of rats exhibited comparable elevations in plasma PRL after methoxamine treatment. Arginine vasopressin messenger RNA expression within the parvocellular PVN was greater in the lactating animals than in the virgin controls, but methoxamine injection was without further effect. Studies performed on ovariectomized virgin rats and ovariectomized rats receiving estradiol or progesterone replacement failed to reproduce the attenuated HPA responses seen after methoxamine treatment, although methoxamine-induced PRL levels were greatly increased by estradiol, probably arising from an effect on hormone synthesis. In vitro electrophysiological recordings of PVN neurons in hypothalamic slices from proestrous virgin and lactating rats showed that 45-52% of neurons in both groups exhibited excitatory responses to 10(-4) M methoxamine, but there was a differential response to 10(-5) M methoxamine, with PVN neurons from lactating animals failing to show a response. These data show a selective down-regulation of alpha1-mediated activation of the HPA axis in lactating animals. This may contribute to the attenuated stress-induced activation of the HPA axis during lactation.

Morphine withdrawal alters anterior pituitary hormone secretion, brain endopeptidase activity and brain monoamine metabolism in the rat

Rats were made tolerant to morphine by a 5-day regimen with increasing doses. The time course of changes in serum anterior pituitary hormone levels, brain endo- and exopeptidase activity, levels of brain biogenic amines and body weight were studied during abrupt morphine withdrawal. Cold stimulated secretion of thyrotropin and the secretion of growth hormone were both decreased whereas that of prolactin was increased. In the hypothalamus both prolyl endopeptidase and dipeptidyl peptidase IV activities were concomitantly increased. The hypothalamic 5 hydroxyindole acetic acid levels were also increased. Changes in hormone secretion, peptidase activity and monoamine turnover had returned to baseline levels by 92 hr. Our results indicate that morphine withdrawal and the associated stress produce alterations in anterior pituitary thyrotropin and growth hormone secretion. Concomitant increases in hypothalamic prolyl endopeptidase and dipeptidyl peptidase activities may contribute to these changes.

Morphine induced analgesia is attenuated in post-partum lactating rats

The analgesic effects of morphine administration were determined in post-partum, lactating female rats, as well as in intact, cycling females during the diestrous stage of the estrous cycle. All doses of morphine (2.5, 5 and 10 mg/kg, iv) produced a significant analgesic response in both post-partum and diestrous females using the hot water tail immersion latency test. However, the analgesic response in the post-partum females was significantly less than during diestrus at all doses tested. In addition, pretreatment with the mu 1 specific antagonist, Naloxonazine, significantly blunted the analgesic response in diestrous females, but did not significantly affect analgesia in post-partum females. These results indicate that morphine is less effective in producing analgesia in post-partum females. The mu 1 opiate receptor site does not appear to be involved in the analgesia produced during the post-partum period.

The role of the mu(1) opioid receptor subtype in the regulation of prolactin and growth hormone secretion by Beta-endorphin in female rats: studies with naloxonazine

The μ opioid receptor subtype has been reported to mediate the prolactin secretory response to opioids. This receptor subtype has been implicated in the morphine-induced prolactin increase, as well as the prolactin response to μ-specific opioid peptides. Subtypes of the μ receptor have been proposed and the μ(1) , site has been postulated as the receptor subtype involved in the morphine-induced prolactin secretory response. However, the role of this receptor subtype in mediating the endocrine effects of the endogenous opioid peptides has not been characterized. In order to determine the physiological significance of this receptor subtype, animals were pretreated with saline, WIN 44,441-3 (a μ, δ and κ antagonist) or naloxonazine (a μ(1) antagonist) followed by a stimulatory dose of morphine or β-endorphin. A dose response study for β-endorphin was conducted to determine the minimal stimulatory dose of β-endorphin on the prolactin and growth hormone (GH) secretory response. The dose response study indicated that β-endorphin is a more potent stimulus for prolactin release than for GH. A dose as low as 25 ng increased prolactin levels as much as 100-fold in both lactating and diestrous female rats. In contrast, 2.5 μg β-endorphin was required in order to consistently and significantly increase circulating levels of GH by 2- to 3-fold. WIN 44,441-3 antagonized the stimulatory effects of β-endorphin on both prolactin and GH secretion. Naloxonazine pretreatment abolished the morphine-induced prolactin secretory response, without affecting the GH increase in diestrous females. Naloxonazine also antagonized the prolactin response to β-endorphin in both lactating and diestrous females. In addition, it attenuated the GH secretory response but did not totally abolish it. These data indicate that β-endorphin elicits an increase in prolactin release through an opioid specific receptor which appears to be the μ(1) opioid receptor subtype. They further suggest that β-endorphin may increase GH levels, at least partially, via its action at this μ(1) site.