Effect of suckling on NADPH-diaphorase (Nitric oxide synthase, NOS) reactivity and NOS gene expression in the paraventricular and supraoptic nuclei of lactating rats

Lactation induces increased IPSC bursting in oxytocinergic neurons

Hypothalamic magnocellular neurosecretory cells (MNCs) undergo dramatic structural reorganization during lactation in female rats that is thought to contribute to the pulsatile secretion of oxytocin critical for milk ejection. MNCs from male rats generate robust bursts of GABAergic synaptic currents, a subset of which are onset-synchronized between MNC pairs, but the functional role of the IPSC bursts is not known. To determine the physiological relevance of IPSC bursts, we compared MNCs from lactating and non-lactating female rats using whole-cell recordings in brain slices. We recorded a sixfold increase in the incidence of IPSC bursts in oxytocin (OT)-MNCs from lactating rats compared to non-lactating rats, whereas there was no change in IPSC bursts in vasopressin (VP)-MNCs. Synchronized bursts of IPSCs were observed in pairs of MNCs in slices from lactating rats. Our data indicate, therefore, that IPSC bursts are upregulated specifically in OT-MNCs during lactation, and may, therefore, contribute via rebound depolarization to the spike trains in OT neurons that lead to reflex milk ejection.

NADPH-Diaphorase Colocalizes with GPER and Is Modulated by the GPER Agonist G1 in the Supraoptic and Paraventricular Nuclei of Ovariectomized Female Rats

Nitric oxide is produced in the brain by the neuronal nitric oxide synthase (nNOS) and carries out a wide range of functions by acting as a neurotransmitter-like molecule. Gonadal hormones are involved in the regulation of the brain nitrergic system. We have previously demonstrated that estradiol, via classical estrogen receptors (ERs), regulates NOS activity in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus, acting through both ERα and ERβ. Magnocellular and parvocellular neurons in the SON and PVN also express the G protein-coupled ER (GPER). In this study, we have assessed whether GPER is also involved in the regulation of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase in the SON and PVN. Adult female ovariectomized rats were treated with G1, a selective GPER agonist, or with G1 in combination with G15, a selective GPER antagonist. G1 treatment decreased NADPH-diaphorase expression in the SON and in all PVN subnuclei. The treatment with G1 + G15 effectively rescued the G1-dependent decrease in NADPH-diaphorase expression in both brain regions. In addition, the activation of extracellular signal-regulated kinase (ERK) 1/2, one of the kinases involved in the GPER-dependent intracellular signaling pathway and in NOS phosphorylation, was assessed in the same brain nuclei. Treatment with G1 significantly decreased the number of p-ERK 1/2-positive cells in the SON and PVN, while the treatment with G1 + G15 significantly recovered its number to control values. These findings suggest that the activation of GPER in the SON and PVN inhibits the phosphorylation of ERK 1/2, which induces a decrease in NADPH-diaphorase expression.

Hormonal influences on neuroimmune responses in the CNS of females

Particular reproductive stages such as lactation impose demands on the female. To cope with these demands, her physiology goes through numerous adaptations, for example, attenuation of immune and stress responses. Hormonal fluctuation during lactation exerts a strong influence, inducing neuroplasticity in the hypothalamus and extrahypothalamic regions, and diminishing the stress and inflammatory responses. Thus, hormones confer decreased vulnerability to the female brain. This mini-review focuses on the adaptations of the immune and stress response during maternity, and on the neuroprotective actions of progesterone and prolactin and their effects on inflammation. The importance of pregnancy and lactation as experimental models to study immune responses and disease is also highlighted.

Nitric oxide has a role in attenuating the neuroendocrine response to anaphylactoid stress during lactation

Stress increases nitric oxide (NO) production in the paraventricular nucleus of the hypothalamus (PVH). Lactation diminishes the response to stress and increases basal NO production markers in the PVH of the dam. This study investigated whether lactation modified the anaphylactic reaction to egg white (EW) injection, and if nitric oxide regulates the neuroendocrine response to this stressor. The activational response of PVH to EW was assessed by c-Fos immunohistochemistry, and NO production was determined by histological staining of NADPH-diaphorase and neuronal nitric oxide synthase (nNOS) and by measuring the concentration of total nitrates and nitrites (NOx) in the hypothalamus of lactating and diestrus rats. EW injection significantly increased the number of Fos-positive neurons in the parvocellular subdivision of the PVH in diestrus, but not in lactating rats. Similarly, EW injection increased the number of NADPH-diaphorase- and nNOS-positive cells in the PVH of diestrus rats, but it did not alter the already increased basal number of NO-positive cells in lactating rats. Furthermore, the total concentration of NOx in the hypothalamus, the circulating level of corticosterone and interleukin-6 increased significantly after EW in diestrus, but not in lactating rats, compared to their corresponding controls. Intracerebral administration of L-NAME, a general NOS inhibitor, reversed the attenuation of the activational response to EW in the PVH of lactating rats. The present results show that lactation diminishes the anaphylactoid reaction to EW compared to that in diestrus rats. This attenuation was absent after L-NAME treatment, suggesting that sustained NO production in the PVH during lactation may limit the neuroendocrine response to stress.

Prolactin promotes oxytocin and vasopressin release by activating neuronal nitric oxide synthase in the supraoptic and paraventricular nuclei

Prolactin (PRL) stimulates the secretion of oxytocin (OXT) and arginine AVP as part of the maternal adaptations facilitating parturition and lactation. Both neurohormones are under the regulation of nitric oxide. Here, we investigate whether the activation of neuronal nitric oxide synthase (nNOS) in the hypothalamo-neurohypophyseal system mediates the effect of PRL on OXT and AVP release and whether these effects operate in males. Plasma levels of OXT and AVP were measured in male rats after the intracerebroventricular injection of PRL or after inducing hyperprolactinemia by placing two anterior pituitary glands under the kidney capsule. NOS activity was evaluated in the paraventricular (PVN) and supraoptic (SON) hypothalamic nuclei by NADPH-diaphorase histochemistry and in hypothalamic extracts by the phosphorylation/inactivation of nNOS at Ser847. Elevated central and systemic PRL correlated with increased NOS activity in the PVN and SON and with higher OXT and AVP circulating levels. Notably, treatment with 7-nitroindazole, a selective inhibitor of nNOS, prevented PRL-induced stimulation of the release of both neurohormones. Also, phosphorylation of nNOS was reduced in hyperprolactinemic rats, and treatment with bromocriptine, an inhibitor of anterior pituitary PRL secretion, suppressed this effect. These findings suggest that PRL enhances nNOS activity in the PVN and SON, thereby contributing to the regulation of OXT and AVP release. This mechanism likely contributes to the regulation of processes beyond those of female reproduction.

Endothelin-1 as a neuropeptide: neurotransmitter or neurovascular effects?

Endothelin-1 (ET-1) is an endothelium-derived peptide that also possesses potent mitogenic activity. There is also a suggestion the ET-1 is a neuropeptide, based mainly on its histological identification in both the central and peripheral nervous system in a number of species, including man. A neuropeptide role for ET-1 is supported by studies showing a variety of effects caused following its administration into different regions of the brain and by application to peripheral nerves. In addition there are studies proposing that ET-1 is implicated in a number of neural circuits where its transmitter affects range from a role in pain and temperature control to its action on the hypothalamo-neurosecretory system. While the effect of ET-1 on nerve tissue is beyond doubt, its action on nerve blood flow is often ignored. Here, we review data generated in a number of species and using a variety of experimental models. Studies range from those showing the distribution of ET-1 and its receptors in nerve tissue to those describing numerous neurally-mediated effects of ET-1.

Changes in c-Fos and NOS expression in the PVH of lactating rats in response to excitotoxicity and stress

This study investigated how lactation modified the expression patterns of Fos and nitric oxide synthase in the hypothalamic paraventricular nucleus (PVH) induced by excitotoxicity and stress. Kainic acid or egg white treatment weakly activated Fos expression in the PVH of lactating in comparison to diestrus or ovariectomized (OVX) rats. Labels for NADPH-diaphorase and nNOS revealed a different distribution pattern in the PVH depending on the physiological condition and challenge. The present results confirm that lactation attenuates the PVH activational response to stress and excitotoxicity, and both stimuli induced nitric oxide expression in the PVH of diestrus, lactating, and OVX rats.

Central nitric oxide synthase inhibition disrupts maternal behavior in the rat

Blocking nitric oxide (NO) production, by 3rd ventricle administration of a nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME; 250 microg/5 microl, postpartum [pp]) decreased milk ejections in Day 10 pp rats. On Day 4 pp, L-NAME treatment eliminated pup retrieval and at both stages of lactation suppressed maternal aggression. Fewer rats treated with L-NAME on Day 10 pp retrieved 4-day-old pups than controls, although all nursed older litters. Following exposure to a mobile intruder, Fos expression was lower in the medial preoptic area and the bed nucleus of the stria terminalis in L-NAME-treated rats than in controls but was lower in the medial amygdala only following exposure to an anaesthetized intruder. Thus, the elevated levels of NO observed in lactation may contribute to the mechanism(s) that mediate maternal behavior and aggression.

Prolactin and oxytocin interaction in the paraventricular and supraoptic nuclei: effects on oxytocin mRNA and nitric oxide synthase

We investigated the contribution of prolactin and oxytocin to the increase in staining for NADPH-d and oxytocin mRNA in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) observed at the end of pregnancy, or following a steroid-priming regimen that mimics the hormonal profile of late pregnant females. Ovariectomized rats received chronic implants of silastic capsules containing oestrogen and progesterone followed by progesterone removal. In experiment 1, oxytocin antagonist (OTA) was administered to rats to investigate whether intranuclear oxytocin release was necessary for NADPH-d staining. In experiments 2a and b, rats received concurrent treatment with bromocryptine (0.5 mg/day) to suppress endogenous prolactin release, and either systemic prolactin (0.5 mg once daily), or prolactin (2 micro g/ micro l), or vehicle infused twice a day into the third ventricle, or chronic oxytocin infusion (24 ng/day) for 3 days following progesterone removal. Brains were then processed for NADPH-d histochemistry. In experiment 3, the interaction of prolactin and oxytocin on oxytocin mRNA within the SON and PVN was examined. NADPH-d staining in the SON and PVN was reduced by the highest dose of the OTA, and by bromocryptine treatment. Central prolactin and oxytocin replacement completely restored NADPH-d staining in bromocryptine-treated rats. Finally, both bromocryptine and the OTA suppressed oxytocin mRNA expression and prolactin replacement restored expression levels to that of controls. Together, these data suggest that the increased capacity to produce nitric oxide in the SON and PVN during late pregnancy is dependent on prolactin stimulating oxytocin gene mRNA and hence intranuclear oxytocin release.

Co-localisation of nitric oxide synthase and endothelin in the rat supraoptic nucleus

The co-localisation of neuronal nitric oxide synthase and endothelin-1 was studied in the rat supraoptic nucleus at the electron microscopy level. Double pre-embedding immunocytochemistry was performed using ExtrAvidin-horseradish peroxidase and immunogold-silver techniques. Immunoreactivities to neuronal nitric oxide synthase and endothelin-1 were co-localised in sub-populations of endocrine neurones (cell bodies) and dendrites. Double-labelled axon terminals making asymmetrical synapses on unlabelled dendrites were also observed. The findings are discussed in terms of the possible role and significance of nitric oxide and endothlin-1 in the hypothalamo-neurohypophysial system.

Role of the supraoptic nucleus in regulation of parturition and milk ejection revisited

This review will focus on the activity of oxytocin neurons in the supraoptic nucleus (SON) and some factors that regulate their function during parturition and milk ejection in the rat. The level of oxytocin increases in the blood during parturition following a regression of the corpus luteum. The increase in oxytocin secretion is presumably a consequence of releasing the oxytocin neurons from restraining inhibitory influences of endogenous opioids-, nitric oxide-, and GABA-containing neurons following declining blood levels of progesterone on the one hand and increasing levels of estrogen on the other during late pregnancy. However, the principal stimulus for the increased oxytocin release is believed to originate, at least in part, from mechanical stimulation to the uterine cervix by fetuses near term, the resultant uterine contractile activity, and the fetal expulsion reflex. Hence, the contractile activity of the uterus acts through positive feedback mechanisms during parturition to stimulate oxytocin neurons as well, and this further increases the secretion of oxytocin. During suckling in lactating rats, somatosensory stimuli from the pups induce intermittent synchronized burst firing of oxytocin neurons, resulting in pulsatile increases in blood oxytocin concentrations to cause milk ejection. The oxytocin neurons appear to have an intrinsic capability to fire in a bursting fashion as determined by observation of this phenomenon in brain slice or tissue culture preparations. The release of oxytocin within the microenvironment of the SON and paraventricular nucleus coupled with morphological reorganization in these nuclei play important roles in the bursting activity of each oxytocin neuron and synchronization in vivo. However, the mechanism responsible for the synchronization of electrical activity in oxytocin neurons in the four discrete hypothalamic nuclei remains an interesting unanswered question.