Vaginocervical stimulation-induced release of classical neurotransmitters and nitric oxide in the nucleus of the solitary tract varies as a function of the oestrus cycle

Brainstem neuronal responses to transcutaneous auricular and cervical vagus nerve stimulation in rats

Transcutaneous auricular vagus nerve stimulation (taVNS) targets subcutaneous axons in the auricular branch of the vagus nerve at the outer ear. Its non-invasive nature makes it a potential treatment for various disorders. taVNS induces neuromodulatory effects within the nucleus of the solitary tract (NTS), and due to its widespread connectivity, the NTS acts as a gateway to elicit neuromodulation in both higher-order brain regions and other brainstem nuclei (e.g. spinal trigeminal nucleus; Sp5). Our objective was to examine stimulation parameters on single-neuron electrophysiological responses in α-chloralose-anaesthetized Sprague-Dawley rats within NTS and Sp5. taVNS was also compared to traditional cervical VNS (cVNS) on single neuronal activation. Specifically, electrophysiological extracellular recordings were evaluated for a range of frequency and intensity parameters (20-250 Hz, 0.5-1.0 mA). Neurons were classified as positive, negative or non-responders based on increased activity, decreased activity or no response during stimulation, respectively. Frequency-dependent analysis showed that 20 and 100 Hz generated the highest proportion of positive responders in NTS and Sp5 with 1.0 mA intensities eliciting the greatest magnitude of response. Comparisons between taVNS and cVNS revealed similar parameter-specific activation for caudal NTS neuronal populations; however, individual neurons showed different activation profiles. The latter suggests that cVNS and taVNS send afferent input to NTS via different neuronal pathways. This study demonstrates differential parameter-specific taVNS responses and begins an investigation of the mechanisms responsible for taVNS modulation. Understanding the neuronal pathways responsible for eliciting neuromodulatory effects will enable more tailored taVNS treatments in various clinical disorders. KEY POINTS: Transcutaneous auricular vagus nerve stimulation (taVNS) offers a non-invasive alternative to invasive cervical vagus nerve stimulation (cVNS) by activating vagal afferents in the ear to induce neuromodulation. Our study evaluated taVNS effects on neuronal firing patterns in the nucleus of the solitary tract (NTS) and spinal trigeminal nucleus (Sp5) and found that 20 and 100 Hz notably increased neuronal activity during stimulation in both nuclei. Increasing taVNS intensity not only increased the number of neurons responding in Sp5 but also increased the magnitude of response, suggesting a heightened sensitivity to taVNS compared to NTS. Comparisons between cVNS and taVNS revealed similar overall activation but different responses on individual neurons, indicating distinct neural pathways. These results show parameter-specific and nuclei-specific responses to taVNS and confirm that taVNS can elicit responses comparable to cVNS at the neuronal level, but it does so through different neuronal pathways.

The role of mating in oviduct biology

The oviduct connects the ovary to the uterus, and is subject to changes that influence gamete transport, fertilization, and early embryo development. The ovarian steroids estradiol and progesterone are largely responsible for regulating oviduct function, although mating signals also affect the female reproductive tract, both indirectly, through sensory stimulation, and directly, through contact with seminal plasma or spermatozoa. The resulting alterations in gene and protein expression help establish a microenvironment that is appropriate for sperm storage and selection, embryo development, and gamete transport. Mating may also induce the switch from a non-genomic to a genomic pathway of estradiol-accelerated oviduct egg transport, reflecting a novel example of the functional plasticity in well-differentiated cells. This review highlights the physiological relevance of various aspects of mating to oviduct biology and reproductive success. Expanding our knowledge of the mating-associated molecular and cellular events in oviduct cells would undoubtedly facilitate new therapeutic strategies to treat infertility attributable to oviduct pathologies. Mol. Reprod. Dev. 83: 875-883, 2016 © 2016 Wiley Periodicals, Inc.

Increased TRH and TRH-like peptide release in rat brain and peripheral tissues during proestrus/estrus

Women are at greater risk for major depression, PTSD, and other anxiety disorders. ERβ-selective agonists for the treatment of these disorders are the focus of pharmacologic development and clinical testing. Estradiol and its metabolites contribute to the neuroprotective effects of this steroid class, particularly in men, due to local conversion of testosterone to estiradiol in key brain regions which are predisposed to neurodegenerative diseases. We have used young adult female Sprague-Dawley rats to assess the role of TRH and TRH-like peptides, with the general structure pGlu-X-Pro-NH2 where "X" can be any amino acid residue, as mediators of the neurobiochemical effects of estradiol. The neuroprotective TRH and TRH-like peptides are coreleased with excitotoxic glutamate by glutamatergic neurons which contribute importantly to the regulation of the estrus cycle. The levels of TRH and TRH-like peptides during proestrus and/or estrus in the 12 brain regions analyzed were significantly decreased (due to accelerated release) 106 times but increased only 25 times when compared to the corresponding levels during diestrus days 1 and 2. These changes, listed by brain region in the order of decreasing number of significant decreases (↓) and/or increases (↑), were: striatum (20↓,1↑), medulla oblongata (16↓,2↑), amygdala (14↓,1↑), cerebellum (13↓,1↑), hypothalamus (12↓,1↑), entorhinal cortex (6↓,6↑), posterior cingulate (10↓,1↑), frontal cortex (3↓,5↑), nucleus accumbens (5↓,3↑), hippocampus (5↓,2↑), anterior cingulate (2↓,1↑), and piriform cortex (1↑). In peripheral tissues the corresponding changes were: ovaries (23↓), uterus (16↓,1↑), adrenals (11↓,3↑), and pancreas (1↓,6↑). We conclude that these peptides may be downstream mediators of some of the therapeutic effects of estrogen.

Social investigation and long-term recognition memory performance in 129S1/SvImJ and C57BL/6JOlaHsd mice and their hybrids

When tested for their behavioural performance, the mixed genetic background of transgenic mice is a critical, but often ignored, issue. Such issues can arise because of the significant differences in defined behavioural parameters between embryonic stem cell donor and recipient strains. In this context, the commonly used stem cell donor strain '129' shows 'deficits' in different paradigms for learning and long-term memory. We investigated the long-term social recognition memory performance and the investigative behaviour in commercially available 129S1/SvImJ and C57BL/6JOlaHsd mice and two F1-hybrids (129S1/SvImJ×C57BL/6JOlaHsd) by using the social discrimination procedure and its modification, the volatile fraction cage (VFC). Our data revealed an unimpaired olfactory long-term recognition memory not only in female and male 129S1/SvImJ and C57BL/6JOlaHsd mice but also in the two hybrid lines (129S1/SvImJxC57BL/6JOlaHsd) when the full 'olfactory signature' of the 'to-be-recognized' conspecific was presented. Under these conditions we also failed to detect differences in the long-term recognition memory between male and female mice of the tested strains and revealed that the oestrus cycle did not affect the performance in this memory task. The performance in the VFC, based only on the volatile components of the 'olfactory signature' of the 'to-be-recognized' conspecific, was similar to that observed under direct exposure except that females of one F1 hybrid group failed to show an intact long-term memory. Thus, the social discrimination procedure allowing direct access between the experimental subject and the stimulus animal(s) is highly suitable to investigate the impact of genetic manipulations on long-term memory in male and female mice of the strain 129S1/SvImJ, C57BL/6JOlaHsd and 129S1/SvImJxC57BL/6JOlaHsd hybrids.

Reduced aging defects in estrogen receptive brainstem nuclei in the female hamster

The nucleus pararetroambiguus (NPRA) and the commissural nucleus of the solitary tract (NTScom) show estrogen nuclear receptor-α immunoreactivity (nuclear ER-α-IR). Both cell groups are involved in estrous cycle related adaptations. We examined in normally cycling aged hamsters the occurrence/amount/frequency of age-related degenerative changes in NPRA and NTScom during estrus and diestrus. In 2640 electron microscopy photomicrographs plasticity reflected in the ratio of axon terminal surface/dendrite surface (t/d) was morphometrically analyzed. Medial tegmental field (mtf, nuclear ER-α-IR poor), served as control. In aged animals, irrespective of nuclear ER-α-IR+ or nuclear ER-α-IR- related cell groups, extensive diffuse degenerative structural aberrations were observed. The hormonal state had a strong influence on t/d ratios in NPRA and NTScom, but not in mtf. In NPRA and NTScom, diestrous hamsters had significantly smaller t/d ratios (NPRA, 0.750 ± 0.050; NTScom, 0.900 ± 0.039) than the estrous hamsters (NPRA, 1.083 ± 0.075; NTScom, 1.204 ± 0.076). Aging affected axodendritic ratios only in mtf (p < 0.001).

IN CONCLUSION

in the female hamster brain, estrous cycle-induced structural plasticity is preserved in NPRA and NTScom during aging despite the presence of diffuse age-related neurodegenerative changes.

Differential effects of progesterone and genital stimulation on sequential inhibition of estrous behavior and progesterone receptor expression in the rat brain

The effect of genital stimulation, either by vaginocervical stimulation (VCS) using a calibrated vaginal probe combined with manual flank stimulation (FS), or by mounts performed by the male, on the hypothalamus and preoptic area concentration of the progesterone receptors A (PR-A) and B (PR-B) was assessed in ovariectomized (ovx) estrogen-primed rats. VCS/FS or stimulation provided by male mounts, even without intromission, significantly decreased PR-B concentration in the hypoythalamus. Down regulation of PR produced by genital stimulation was quantitatively similar to that elicited by progesterone (P) administration. Bilateral or unilateral transection of the pelvic or the pudendal nerves prevented down regulation elicited by VCS/FS. Repeated VCS/FS elicited lordosis behavior in most ovx estrogen primed rats, but the lordosis intensity was lower than that observed in response to P. P administered to ovx estrogen primed rats, induced sequential inhibition, i.e., failure to display estrous behavior in response to a second P injection (24h after the initial P injection). VCS/FS failed to elicit sequential inhibition, since rats responded with normal estrous behavior to the second injection of P. This suggests that down regulation by VCS, by contrast with P, failed to inhibit the subpopulation of PR involved in the facilitation of estrous behavior by P.

Conditioned place preference for mating is preserved in rats with pelvic nerve transection

Female rats exhibit a conditioned place preference (CPP) for a context paired with mating. The present experiment tested the hypothesis that the activation of the pelvic nerve mediates the reinforcing effects of mating for female rats. Rats underwent bilateral pelvic nerve or sham transection and then received paced mating, nonpaced mating, or the control treatment during a CPP procedure. Pelvic nerve transection did not affect the CPP for paced or nonpaced mating. In tests of paced mating behavior, contact-return latencies following intromissions were significantly shorter in rats with pelvic nerve transection than they were in rats with sham transections. These results show that the pathway conveying the reinforcing effects of mating stimulation does not depend on the integrity of the pelvic nerve, but that activation of the pelvic nerve contributes to the display of paced mating behavior.

Cervix remodeling and parturition in the rat: lack of a role for hypogastric innervation

The hypogastric nerve is a major pathway innervating the uterine cervix, yet its contribution to the processes of cervical ripening and parturition is not known. The main objective of this study was to determine the effect of hypogastric nerve transection on remodeling of the cervix and timing of birth. As an initial goal, processes associated with remodeling of the peripartum cervix were studied. The cervix was obtained from time-dated pregnant rats on days 15, 19, 21, and 21.5 of pregnancy, and post partum on the day of birth (day 22). The cervix was excised, post-fixed overnight, and sections stained to evaluate collagen content and structure or processed by immunohistochemistry to identify macrophages or nerve fibers. The census of macrophages and density of nerve fibers in the cervix peaked on day 21, the day before birth, and then declined post partum. These results replicate in time course and magnitude previous studies in mice. To address the main objective, the hypogastric nerve was bilaterally transected on day 15 post-breeding; sham-operated rats served as controls. Pups were born in both groups at normal term. Transection of the hypogastric nerves did not affect remodeling of collagen or the census of macrophages or the density of nerve fibers in the cervix. These findings support the contention that enhanced innervation and immigration of immune cells are associated with remodeling of the cervix and parturition, but that a neural pathway other than the hypogastric nerve may participate in the process of cervical ripening.

Antagonists of the protein kinase A and mitogen-activated protein kinase systems and of the progestin receptor block the ability of vaginocervical/flank-perineal stimulation to induce female rat sexual behaviour

Brief vaginocervical stimulation using a glass rod (VCS) combined with manual flank-perineal stimulation (FS) rapidly (within 5 min) induced both receptive and proceptive behavioural responses to males in ovariectomised, oestrogen-primed rats. This receptive-proceptive response to males, resulting from a single brief (5-s duration) instance of manual VCS + FS, declined markedly within 4 h. However, the decline was prevented if the females were mounted by males immediately after the manual VCS + FS and 2 h later. We tested the participation of the cAMP-dependent protein kinase A system and the mitogen-activated protein kinase (MAPK) system in the response to VCS + FS by infusing either 100 ng of Rp-adenosine 3',5'-cyclic monophosphorothiate triethylamonium salt (a protein kinase A blocker) or 3.3 microg of PD98059 (a MAPK blocker) i.c.v. 15 min prior to VCS + FS. Both inhibitors blocked the ability of VCS + FS to induce the proceptive-receptive responses to males at all testing intervals. In experiment 2, systemic administration of 5 mg of RU486 1 h before VCS + FS also blocked the ability of VCS + FS to induce the proceptive-receptive responses to males. The present findings suggest that both VCS + FS and mating stimuli provided by males release neurotransmitters and neuromodulators that trigger the protein kinase A and the MAPK signalling systems, which interact with the progestin receptor to rapidly (within 5 min) induce proceptive-receptive behaviour in females.

Facilitation of estrous behavior by vaginal cervical stimulation in female rats involves alpha1-adrenergic receptor activation of the nitric oxide pathway

In estrogen-primed female rats, vaginal cervical stimulation (VCS) provided by male intromissions or by an experimenter enhances estrous behaviors exhibited by females during subsequent mating with a male. We tested the hypothesis that alpha(1)-adrenergic receptors, acting via the nitric oxide-cGMP-protein kinase G pathway, mediate VCS-induced facilitation of female reproductive behaviors. Ovariectomized, estradiol-primed rats received intracerebroventricular (icv) infusions of vehicle or pharmacological antagonists 15 or 60min before VCS. Estrous behaviors (lordosis and proceptivity) in the presence of a male were recorded immediately (0min), and 120min following VCS. First we verified that VCS, but not manual flank stimulation alone, enhanced estrous behaviors when females received icv infusion of the vehicles used to administer drugs. Increased estrous behavior was apparent immediately following VCS and persisted for 120min. We then infused prazosin, phenoxybenzamine (alpha(1)-adrenergic receptor antagonists), yohimbine, idaxozan (alpha(2)-adrenergic receptor antagonists), or propranolol (beta-adrenergic receptor antagonist) 15min prior to the application of VCS in females primed with 5mug estradiol benzoate. Only alpha(1)-adrenergic antagonists inhibited VCS facilitation of estrous behavior, apparent 120min after VCS. Finally, we administered specific inhibitors of soluble guanylyl cyclase, nitric oxide synthase or protein kinase G icv 15 or 60min before VCS. All three agents significantly attenuated VCS facilitation of estrous behavior. These data support the hypothesis that endogenously released norepinephrine, acting via alpha(1)-adrenergic receptors, mediates the facilitation of lordosis by VCS, and are consistent with a mechanism involving alpha(1)-adrenergic activation of the nitric oxide/cGMP/protein kinase G pathway.

Medullary noradrenergic neurons release norepinephrine in the medial amygdala in females in response to mating stimulation sufficient for pseudopregnancy

In the female rat, stimuli from the uterine cervix and vagina are carried to the brain areas involved in the mating-induced pseudopregnancy (PSP) response via the ventral noradrenergic bundle. Noradrenergic neurons projecting through this tract synapse in many forebrain areas including the amygdala, and neurons in the posterodorsal medial amygdala (MePD) are activated following mating. The goal of this experiment was to investigate whether norepinephrine (NE) is released into the MePD after mating using microdialysis and to determine the origin of this release. Ovariectomized estrogen- and progesterone-treated rats were implanted unilaterally with guide cannulae aimed at the MePD. Females were placed with males until they received 15 intromissions (15I), 5 intromissions (5I) or 15 mounts-without-intromission (MO). Dialysate samples collected every 20 min for 2 h before to 3 h after mating were analyzed for NE using HPLC with electrochemical detection. A significant increase in mean NE release in the MePD was seen at 80 min after mating onset in females receiving 15I, and no increase was seen in animals receiving 5I or MO. The time of peak NE release varied in 15I animals from 60 to 160 min after mating. Mean baseline levels of NE did not differ between groups. The retrograde tracer FluoroGold (FG), administered through the probe after cessation of dialysis sampling, was observed within identified noradrenergic cells primarily within the A1 and A2 cell groups. Infusion of anti-dopamine-beta-hydroxylase-saporin (DBH-SAP) into the MePD lesioned noradrenergic neurons located in the A1 and A2 cell groups. Because high levels of NE release occurred in the MePD only after the females received a number of intromissions sufficient to induce PSP, these results suggest that NE release within the MePD may be important for the establishment of PSP.

Ascending spinal pathways from sexual organs: effects of chronic spinal lesions

A recent survey of paraplegics indicates that regaining sexual function is of the highest priority for both males and females (Anderson, K.D. (2004) Targeting recovery: priorities of the spinal cord-injured population J. Newrotrauma, 21: 1371-1383). Our understanding of the neural pathways and mechanisms underlying sexual behavior and function is limited at the present time. More studies are obviously needed to direct experiments geared toward developing effective therapeutic interventions. In this chapter, a review of studies on the processing of sensory inputs from the male and female reproductive organs is presented with a review of what is known about the location of ascending spinal pathways conveying this information. The effect of spinal cord injury on sexual function and the problems that ensue are discussed.

Nitric oxide increases endocervical secretion at the ovulatory phase in the female

BACKGROUND

Uterine cervical mucus is crucial for reproduction, facilitating sperm transport and survival in certain mammals. Cholinergic autonomic nervous secretory innervation has been established, and modulation of secretion by prostaglandins and non-steroidal anti-inflammatory drugs has been postulated. It has been suggested that glandular nitric oxide (NO) production is a prerequisite for the autonomic cholinergic nervous modulation of cervical, endometrial, and the seminal vesicle secretion in the guinea pig. Most secretory genital tract cells, female as well as male, seem to display NO synthase activity.

METHODS

Cervical secretion at ovulation time was studied in 10 women with regular menstruation. In an in vivo model with repeated collection of mucus samples during four 60-min periods, the amount of mucus was estimated in a control experimental series and in an experimental series following sublingual administration of the NO donor nitroglycerin.

RESULTS

This nitroglycerin administration markedly increased cervical secretion, while no changes in cervical secretion were seen in the control experimental series.

CONCLUSIONS

The results suggest that glandular NO production increases cervical secretion. Thus, cervical secretion may, apart from hormonal regulation, be influenced by the autonomic nervous system, and in addition, NO may be a prerequisite for this influence. This in turn may have implications on fertilization and fertility regulation.

Pattern of sensory innervation of the perineal skin in the female rat

Here we describe the nerves innervating the perineal skin together with their sensory fields in the adult female rat. Electrophysiological recording showed that the lumbosacral and L6-S1 trunks, in part by way of the sacral plexus, transmit sensory information from the perineal skin via four nerves: the viscerocutaneous branch of the pelvic nerve innervating the skin at the midline between the vaginal opening and anus, the sensory branch of the pudendal nerve innervating the clitoral sheath, the distal perineal branch of the pudendal nerve innervating a broad area of skin adjacent to the vaginal opening and anus, and the proximal perineal branch of the sacral plexus innervating a broad area of skin adjacent to the clitoris and vaginal opening. The sensory fields of three of these nerves overlapped to some degree: the viscerocutaneous branch of the pelvic and the distal perineal branch of the pudendal nerves at the midline skin between the vaginal opening and the anus, and the distal perineal branch of the pudendal nerve and the proximal perineal branch of the sacral plexus at the skin lateral to the vaginal opening. Such overlap might provide a safeguard helping to ensure that somatosensory input from the perineal region important for triggering reproductive and nonreproductive reflexes reaches the CNS.

Biphasic role of dopamine on female sexual behaviour via D2 receptors in the mediobasal hypothalamus

Dopamine has been implicated in the control of sexual behaviour, but its role seems quite complex and controversial. The aim of the present experiments was to investigate the effects of dopamine (DA) acting on D2 receptors in the mediobasal hypothalamus (MBH) on sexual behaviour in female sheep. To achieve this, the D2 agonist, quinpirole, was administered bilaterally via microdialysis probes into the MBH of ovariectomized ewes either before or after oestradiol (E2) administration. Quinpirole (100 ng/ml) infused for 6 h just before E2 hastened the onset of oestrus behaviour and the luteinizing hormone surge, whereas the same treatment given 6-12 h or 18-21 h after E2 decreased the intensity of sexual receptivity without affecting LH or prolactin secretion. We then tested the hypothesis that E2 stimulates the onset of oestrus partly by decreasing DA activation of D2 receptors. In this case the D2 antagonists pimozide or spiperone (100 ng/ml) were infused into the MBH via microdialysis probes for 11 h in the absence of E2 administration. A significant number of ewes showed induction of receptivity with both antagonists, although its intensity was significantly lower than that induced by E2. These treatments generally did not significantly alter extracellular concentrations of monoamines or aminoacids although quinpirole modulated the ability of sexual interactions to increase noradrenaline release. These experiments show that DA acts via D2 receptors in the MBH to control female sexual behaviour in a biphasic manner: the onset of sexual motivation and receptivity requiring an initial increase in activation followed by a decrease. This dual action could explain some of the controversies concerning DA action on sexual behaviour.

The magnocellular oxytocin system, the fount of maternity: adaptations in pregnancy

Oxytocin secretion from the posterior pituitary gland is increased during parturition, stimulated by the uterine contractions that forcefully expel the fetuses. Since oxytocin stimulates further contractions of the uterus, which is exquisitely sensitive to oxytocin at the end of pregnancy, a positive feedback loop is activated. The neural pathway that drives oxytocin neurons via a brainstem relay has been partially characterised, and involves A2 noradrenergic cells in the brainstem. Until close to term the responsiveness of oxytocin neurons is restrained by neuroactive steroid metabolites of progesterone that potentiate GABA inhibitory mechanisms. As parturition approaches, and this inhibition fades as progesterone secretion collapses, a central opioid inhibitory mechanism is activated that restrains the excitation of oxytocin cells by brainstem inputs. This opioid restraint is the predominant damper of oxytocin cells before and during parturition, limiting stimulation by extraneous stimuli, and perhaps facilitating optimal spacing of births and economical use of the store of oxytocin accumulated during pregnancy. During parturition, oxytocin cells increase their basal activity, and hence oxytocin secretion increases. In addition, the oxytocin cells discharge a burst of action potentials as each fetus passes through the birth canal. Each burst causes the secretion of a pulse of oxytocin, which sharply increases uterine tone; these bursts depend upon auto-stimulation by oxytocin released from the dendrites of the magnocellular neurons in the supraoptic and paraventricular nuclei. With the exception of the opioid mechanism that emerges to restrain oxytocin cell responsiveness, the behavior of oxytocin cells and their inputs in pregnancy and parturition is explicable from the effects of hormones of pregnancy (relaxin, estrogen, progesterone) on pre-existing mechanisms, leading through relative quiescence at term inter alia to net increase in oxytocin storage, and reduced auto-inhibition by nitric oxide generation. Cyto-architectonic changes in parturition, involving evident retraction of glial processes between oxytocin cells so they get closer together, are probably a response to oxytocin neuron activation rather than being essential for their patterns of firing in parturition.