Evidence for a ventral non-strial pathway from the amygdala to the bed nucleus of the stria terminalis in the male golden hamster

Male hamsters in which the stria terminalis (ST) had been interrupted either by electrolytic lesions or knife cuts, or normal control males, received iontophoretic injections of horseradish peroxidase in either the bed nucleus of the stria terminalis (BNST) or the medial preoptic-anterior hypothalamic area (MPOAH). Comparison of intact and ST-lesioned brains revealed the existence of a ventral non-strial pathway, from cells in the medial amygdaloid nucleus (M) to the preoptic portion of the BNST but not to the MPOAH. Since bilateral lesions of M completely eliminate male hamster mating behavior, but ST lesions do not, we suggest that the ventral pathway to the BNST may be an important route by which M influences male copulatory behavior.     

MPOA and BNST lesions in male Syrian hamsters: differential effects on copulatory and chemoinvestigatory behaviors

Electrolytic lesions were made in the medial preoptic area (MPOA) and bed nucleus of the stria terminalis (BNST) to evaluate their participation in the neural regulation of copulatory and chemoinvestigatory behaviors in male hamsters. Damage to either the MPOA or the BNST caused severe deficits in copulatory performance in a subset of the animals in each group. In the MPOA group all males displaying severe deficits had lesions which included a small central region of the caudal MPOA. In the BNST group, animals with severe copulatory deficits all had large lesions which covered most of both the medial and lateral parts of the nucleus. In contrast, MPOA and BNST lesions differentially affected chemoinvestigatory behaviors. MPOA lesions did not affect any of the males' anogenital investigation rates or attraction to female odors, even though some of these hamsters had stopped mating completely. Males with BNST lesions, on the other hand, all displayed significant reductions in their chemoinvestigatory responding even though the majority of them continued to mate normally. We suggest that the MPOA and BNST may in part regulate male sexual behavior by differentially responding to 'attractant' and 'mounting' substances within female hamster vaginal secretion.     

Pubertal development of penile reflexes and copulation in male rats

(1) Three groups of male rats were tested from 25 to 55 days of age for copulatory behavior, penile reflexes, or plasma testosterone (T). (2) Penile reflexes and copulatory behavior developed prior to the time of maximum T, but not before T had reached appreciable levels. (3) Penile reflex potential developed concurrently with copulation: the ages at which mounts, intromissions, and ejaculations first appeared were matched by the ages of appearance of penile erections, flips and cups respectively. (4) Of the behavioral measures, only penile cups and the ejaculatory pattern did not appear before preputial separation. (5) To the extent that laboratory rats are unique in having parallel development of copulatory behavior and penile reflexes, their value as a mammalian model of behavioral puberty is restricted.     

Chemosignals and hormones in the neural control of mammalian sexual behavior

Males and females of most mammalian species depend on chemosignals to find, attract and evaluate mates and, in most cases, these appetitive sexual behaviors are strongly modulated by activational and organizational effects of sex steroids. The neural circuit underlying chemosensory-mediated pre- and peri-copulatory behavior involves the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), medial preoptic area (MPOA) and ventromedial hypothalamus (VMH), each area being subdivided into interconnected chemoreceptive and hormone-sensitive zones. For males, MA–BNST connections mediate chemoinvestigation whereas the MA–MPOA pathway regulates copulatory initiation. For females, MA–MPOA/BNST connections also control aspects of precopulatory behavior whereas MA–VMH projections control both precopulatory and copulatory behavior. Significant gaps in understanding remain, including the role of VMH in male behavior and MPOA in female appetitive behavior, the function of cortical amygdala, the underlying chemical architecture of this circuit and sex differences in hormonal and neurochemical regulation of precopulatory behavior.     

The bed nucleus of the stria terminalis in the rat: regional sex differences controlled by gonadal steroids early after birth

The effects of postnatal (on day 1 (D1) after birth) male orchidectomy and female androgenization on the bed nucleus of the stria terminalis (BNST) were studied. The volume of the medial posterior region of the BNST was greater in the control males than in the control females. Postnatal treatment reversed these differences. Sex differences were also found in the medial anterior region of the BNST where female rats always showed a greater volume than did the males. Female androgenization on D1 did not affect the volume of the BNST medial anterior region. However, D1 male orchidectomy did increase significantly the volume of this region. No sex differences were found in lateral, ventral or intermediate BNST divisions.     

Lesions of the posterior bed nucleus of the stria terminalis eliminate opposite‐sex odor preference and delay copulation in male Syrian hamsters: role of odor volatility and sexual experience

In Syrian hamsters (Mesocricetus auratus), the expression of reproductive behavior requires the perception of social odors. The behavioral response to these odors is mediated by a network of ventral forebrain nuclei, including the posterior bed nucleus of the stria terminalis (pBNST). Previous studies have tested the role of the pBNST in reproductive behavior, but the use of large, fiber‐damaging lesions in these studies make it difficult to attribute post‐lesion deficits to the pBNST specifically. Thus, the current study used discrete, excitotoxic lesions of the pBNST to test the role of the pBNST in opposite‐sex odor preference and copulatory behavior in both sexually‐naive and sexually‐experienced males. Lesions of the pBNST decreased sexually‐naive males’ investigation of volatile female odors, resulting in an elimination of opposite‐sex odor preference. This elimination of preference was not due to a sensory deficit, as males with pBNST lesions were able to discriminate between odors. When, however, subjects were given sexual experience prior to pBNST lesions, their preference for volatile opposite‐sex odors remained intact post‐lesion. Similarly, when sexually‐naive or sexually‐experienced subjects were allowed to contact the social odors during the preference test, lesions of the pBNST decreased males’ investigation of female odors but did not eliminate preference for opposite‐sex odors, regardless of sexual experience. Finally, lesions of the pBNST delayed the copulatory sequence in sexually‐naive, but not sexually‐experienced, males such that they took longer to mount, intromit, ejaculate and display long intromissions. Together, these results demonstrate that the pBNST plays a unique and critical role in both appetitive and consummatory aspects of male reproductive behaviors.     

The possible involvement of the nonstrial pathway of the amygdala in neural control of sexual behavior in male rats

The effect of medial amygdala lesions on male sexual behavior in male rats with stria terminalis cut was examined. First, castrated male rats received bilateral transactions of the stria terminalis (STC) or sham cut (SC). Most STC males showed no ejaculation, but displayed mount and intromission, although the frequencies were not high compared to those of males with SC. Next, bilateral lesions of the medial amygdala (MAL) or sham lesion (SL) were performed in males with STC or SC. The MAL caused severe loss of all aspects of copulatory behavior in males with STC as well as in males with SC. The suppressive effect of the MAL on copulatory activity was stronger than that of the STC. These results indicate that a neural pathway other than the stria terminalis is involved in the regulation of male sexual behavior by the amygdala in male rats.     

Lesion of the bed nucleus of the stria terminalis enhances learned despair

The bed nucleus of the stria terminalis (BNST) was lesioned in adult male Wistar rats (n = 9) and its involvement in coping behavior during forced swim stress examined. Rats remain immobile longer in the second of two swim tests, a phenomenon known as learned despair. Results revealed that, compared to sham-operated controls (n = 8), BNST-lesioned animals displayed immobility significantly earlier and for longer durations in the second swim test. Rats with BNST lesions also showed significantly reduced escape behavior in the form of fewer numbers of jumps and dives compared to controls. Mobility deficits were not due to general motor impairment as revealed by an open field test. Results suggest that the BNST may modulate coping behavior especially during uncontrollable stress.     

BNST lesions aggravate behavioral despair but do not impair navigational learning in rats

The bed nucleus of the stria terminalis (BNST) is a basal forebrain structure involved in many motivational processes closely linked to stress regulation. The present study investigated the effect of bilateral lesions of the BNST in male Wistar rats on behavioral despair and navigational learning in the Morris water maze both of which present stressful challenges. Compared to controls, BNST-lesioned animals displayed longer duration of immobility in the second of two forced swim tests used to assess behavioral despair but performed similarly in the water maze task. The present results indicate strongly that the BNST is involved in the modulation of behavioral despair. Experimentally induced depression by BNST lesions does not impair learning and memory in the water maze suggesting a possible dissociation between BNST-mediated depression and cognitive performance.     

Paced mating behavior in the female rat following lesions of three regions responsive to vaginocervical stimulation

The present study examines the effects of ibotenic acid lesions of the medial amygdala, the bed nucleus of the stria terminalis and the medial preoptic area on the display of paced mating behavior in female rats. Lesions of either the medial amygdala or the bed nucleus of the stria terminalis have no effect on the display of paced mating behaviors in ovariectomized, hormone-primed rats. In contrast, lesions of the medial preoptic area significantly lengthen contact-return latencies following intromissions and ejaculations and increase withdrawal from the male following intromissions. The present study demonstrates that the medial amygdala and the bed nucleus of the stria terminalis are not involved in the behavioral responses accompanying paced mating behavior, whereas the medial preoptic area is a critical component of the neural circuit mediating paced mating behavior as well as other appetitive aspects of mating.     

Direct retinal input to the limbic system of the rat

Retinal projections in rats were studied using anterograde horseradish peroxidase (HRP) tracing and electron microscopic (EM) degeneration. HRP-labeled axons were observed to leave the optic tract in the vicinity of the lateral geniculate nucleus and enter the stria terminalis where they coursed anteriorly to the bed nucleus of the stria terminalis (BNST), and the anterodorsal (TAD) and anteroventral (TAV) thalamic nuclei. After enucleation, selected areas studied with EM confirmed that retinal presynaptic terminals are located in the BNST and TAV. These results support the finding of Conrad and Stumpf that the retina has a direct link to the limbic system².     

Evidence that preimmunization with a heat-killed preparation of Mycobacterium vaccae reduces corticotropin-releasing hormone mRNA expression in the extended amygdala in a fear-potentiated startle paradigm

Posttraumatic stress disorder (PTSD) is a trauma and stressor-related disorder that is characterized by dysregulation of glucocorticoid signaling, chronic low-grade inflammation, and impairment in the ability to extinguish learned fear. Corticotropin-releasing hormone (Crh) is a stress- and immune-responsive neuropeptide secreted from the paraventricular nucleus of the hypothalamus (PVN) to stimulate the hypothalamic-pituitary-adrenal (HPA) axis; however, extra-hypothalamic sources of Crh from the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST) govern specific fear- and anxiety-related defensive behavioral responses. We previously reported that preimmunization with a heat-killed preparation of the immunoregulatory environmental bacterium Mycobacterium vaccae NCTC 11659 enhances fear extinction in a fear-potentiated startle (FPS) paradigm. In this follow-up study, we utilized an in situ hybridization histochemistry technique to investigate Crh, Crhr1, and Crhr2 mRNA expression in the CeA, BNST, and PVN of the same rats from the original study [Fox et al., 2017, Brain, Behavior, and Immunity, 66: 70–84]. Here, we demonstrate that preimmunization with M. vaccae NCTC 11659 decreases Crh mRNA expression in the CeA and BNST of rats exposed to the FPS paradigm, and, further, that Crh mRNA expression in these regions is correlated with fear behavior during extinction training. These data are consistent with the hypothesis that M. vaccae promotes stress-resilience by attenuating Crh production in fear- and anxiety-related circuits. These data suggest that immunization with M. vaccae may be an effective strategy for prevention of fear- and anxiety-related disorders.     

Increased Fos Expression in Oxytocin Neurons Following Masculine Sexual Behavior

Induction of the c-fos protein product (Fos) was used to immunocytochemically identify oxytocin (OT) neurons that may be activated during copulatory interactions. Fos induction was quantified in sexually-experienced male rats after either (a) exposure to a testing arena recently vacated by an estrous female, (b) copulatory interactions such as mounting and intromission without ejaculation, or (c) mounting and intromissions culminating in ejaculation. In the parvocellular regions of the paraventricular nucleus of the hypothalamus (PVN), the number of neurons expressing Fos increased following either intromission (53%) or ejaculation (124%). Significant, but less striking, increases in the number of cells expressing Fos were noted in magnocellular regions of the PVN where intromission resulted in a 13% increase and ejaculation in a 49% increase in Fos. The number of perikarya immunoreactive for OT and AVP did not differ as a function of increasing sexual contacts. In control (novel arena) males, 33–73% of the Fos labeling occurred in OT cells. Sexual interactions did not enhance the number of double-labeled cells in most parvocellular regions. However, in lateral parvocellular regions located in the most caudal aspects of the PVN, 31% of the Fos-positive cells occurred in OT neurons in ejaculated males, while in control males none of the OT cells were double-labeled. This PVN subdivision is known to consist of neurons that project to the brain stem and spinal cord at lumbar levels which contain motor neurons that regulate penile reflexes. The present data suggest a possible neurochemical circuit which incorporates oxytocinergic neurons in the mediation of masculine sexual responses.     

Acute reversible inactivation of the bed nucleus of stria terminalis induces antidepressant-like effect in the rat forced swimming test

Background  

The bed nucleus of stria terminalis (BNST) is a limbic forebrain structure involved in hypothalamo-pituitary-adrenal axis regulation and stress adaptation. Inappropriate adaptation to stress is thought to compromise the organism's coping mechanisms, which have been implicated in the neurobiology of depression. However, the studies aimed at investigating BNST involvement in depression pathophysiology have yielded contradictory results. Therefore, the objective of the present study was to investigate the effects of temporary acute inactivation of synaptic transmission in the BNST by local microinjection of cobalt chloride (CoCl₂) in rats subjected to the forced swimming test (FST).     

CRF1 and CRF2 receptors in the bed nucleus of stria terminalis differently modulate the baroreflex function in unanesthetized rats

The baroreflex is an important blood pressure regulating mechanism. The bed nucleus of stria terminalis (BNST) modulates the baroreflex function. However, the local BNST neurochemical mechanisms involved in control of baroreflex responses are not completely understood. Therefore, in this study, we investigated the involvement of corticotropin‐releasing factor (CRF) receptors within the BNST in baroreflex control of heart rate in unanesthetized rats. For this, we evaluated effects of bilateral microinjection into the BNST of either the selective CRF₁ receptor antagonist CP376395 (5 nmol/100 nL) or the selective CRF₂ receptor antagonist antisauvagine‐30 (5 nmol/100 nL) in bradycardiac response evoked by blood pressure increases caused by intravenous infusion of phenylephrine as well as tachycardiac response to blood pressure decrease caused by intravenous infusion of sodium nitroprusside. Bilateral microinjection of CP376395 into the BNST decreased the baroreflex bradycardiac response without affecting the reflex tachycardia. Conversely, BNST treatment with antisauvagine‐30 decreased heart rate response during blood pressure drop without affecting the reflex bradycardia. Overall, these findings provide evidence of an involvement of CRF neurotransmission within the BNST in baroreflex activity. Nevertheless, data indicate that local CRF₁ and CRF₂ receptors differently modulate the baroreflex control of heart rate.     

Involvement of the Bed Nucleus of the Stria Terminalis in Tonic Regulation of Paraventricular Hypothalamic CRH and AVP mRNA Expression

The bed nucleus of the stria terminalis (BNST) occupies a central position in pathways regulating hypothalamo-pituitary-adrenocortical (HPA) stress regulation. The potential role of the BNST in tonic neural control of HPA function was assessed by examining effects of selective BNST lesions on expression of ACTH secretagogues in HPA-integrative neurons of the medial parvocellular paraventricular nucleus. Anterior BNST lesions (ABN) involved major portions of the anteromedial, anterolateral, ventromedial, ventrolateral, dorsolateral and juxtacapsular subnuclei. These lesions resulted in significant (30%) decreases in corticotropin-releasing hormone (CRH) mRNA expression across the rostrocaudal extent of the medial parvocellular PVN, with no accompanying changes in basal arginine vasopressin (AVP) mRNA levels. Posterior BNST (PBN) lesions involved large but subtotal damage to the posterior intermediate, posterior medial, posterior lateral and preoptic subnuclei; these lesions resulted in small but significant changes in CRH mRNA and slight increases in number of AVP mRNA-producing parvocellular neurons. PBN effects on CRH mRNA expression were most pronounced at the caudal extent of the medial parvocellular zone, suggesting a topographic input from the posterior BNST to the PVN that is only partially compromised by PBN lesions. Analysis of individual cases revealed a correlation between damage of the anterolateral BNST and decreased CRH mRNA levels, and damage of the posterior intermediate and/or posterior medial BNST and increased CRH mRNA levels. The results suggest differential BNST input into HPA regulation, perhaps reflecting the diversity of limbic input into the BNST region.     

Sex differences in subregions of the medial nucleus of the amygdala and the bed nucleus of the stria terminalis of the rat

Sex differences are described in subregions of two nuclei of the rat brain: the medical nucleus of the amygdala (MA) and the bed nucleus of the stria terminalis (BNST). The volume of the posterodorsal region of the medial nucleus of the amygdala (MApd) is approximately 85% greater and the volume of the encapsulated region of the bed nucleus of the stria terminalis (BNSTenc) is approximately 97% greater in males than in females. The MApd and BNSTenc are distinct subregions of the MA and BNST. They exhibit intense uptake of gonadal hormones and are anatomically connected to each other and to other sexually dimorphic nuclei. The MA and BNST in general are involved in regulation of several sexually dimorphic functions, including aggression, sexual behavior, gonadotropin secretion and integration of olfactory information. Precise localization of sex differences in subregions of the MA and BNST, such as the MApd and BNSTenc, may facilitate understanding of the neural basis of such functions.     

Involvement of the bed nucleus of the stria terminalis in hippocampal cholinergic system-mediated activation of the hypothalamo–pituitary–adrenocortical axis in rats

This study was designed to determine the effect of the bed nucleus of the stria terminalis (BNST) in hippocampal cholinergic system-mediated activation of the hypothalamo–pituitary–adrenocortical (HPA) axis in the rat. Neurons in the BNST were lesioned by bilateral injection of the cell-selective neurotoxin, ibotenic acid (1.5 μg/μl of solution per side). Two weeks later, neostigmine was microinjected into the rats’ hippocampus. Rats in which ibotenic acid had been injected into the BNST showed attenuated expression of c-Fos in the hypothalamic paraventricular nucleus (PVN) and blunted elevation of plasma adrenocorticotropic hormone (ACTH) after microinjection of neostigmine into the hippocampus compared with rats in which saline had been injected into the BNST. The results of this study indicate that the BNST relays signals of hippocampal cholinergic system-mediated activation of the HPA axis in rats.     

Sexual performance and precontact 50-kHz ultrasonic vocalizations in WAG/Rij rats: Effects of opioid receptor treatment

WAG/Rij rats are genetically selected animals that model absence epilepsy in rats. Ultrasonic vocalizations and sexual behavior – both ethologically relevant markers of reward system functioning – are poorly described in this strain. The aim of our experiment was to investigate reward-dependent precontact 50-kHz vocalizations (PVs) and copulatory behavior as well as the effects of opioid receptor treatment on such behaviors in sexually experienced WAG/Rij males and rats from two control strains: Sprague–Dawley and Crl: Han Wistar. We analyzed the effects of the opioid receptor antagonist naltrexone (3 mg/kg) and the agonist morphine (1 mg/kg) administration. Additionally, we analyzed the initiation of copulation in sexually naïve males before drug treatment. A significantly lower number of sexually naïve WAG/Rij rats initiated copulation. Sexually experienced WAG/Rij males differed at the control session (after physiological saline treatment) compared with Sprague–Dawley rats: WAG/Rij rats displayed more 50-kHz precontact vocalizations and had longer mount and intromission latencies, longer ejaculation latency, longer postejaculatory latency to exploration, longer 22-kHz vocalization duration after ejaculation, and longer postejaculatory intromission latency. Compared with Crl: Han Wistar rats, WAG/Rij males displayed longer mount latency and shorter 22-kHz vocalization duration. Neither naltrexone nor morphine affected PVs in all groups. On the other hand, opioid receptor treatment differently influenced the number of intromissions required to achieve ejaculation and 22-kHz postejaculatory vocalization duration in WAG/Rij rats than in both control groups. This suggests functional differences in the opioid system in this strain. As a result of the number of males that initiated copulation as well as the number of intromissions to ejaculation and 22-kHz postejaculatory vocalizations which all depend on D1 receptor activation, we suggest that the proportion of opioid receptor to D1 receptors in WAG/Rij rats is different when compared with the control strains. The reward system of Wag/Rij rats with absence epilepsy is sensitive to social rewards (high level of precontact 50-kHz ultrasounds) although this strain displays a lower level of sexual motivation (longer mount latency) compared with other control strains. A lower number of sexually naïve rats initiating copulation and longer mount latency in sexually experienced males could suggest a moderate depressive-like syndrome in this strain of rats.     

Role of the striatum and nucleus accumbens in paced copulatory behavior in the female rat

Female rats engage in a series of approach and avoidance behaviors (pacing behavior) directed at the male in order to achieve a preferred rate of intromissions that make pregnancy more likely to occur with insemination. The striatum and nucleus accumbens have been implicated in the modulation of pacing behavior. It is unclear, however, whether these areas of the brain are necessary for the display of pacing behavior. To address this question, ovariectomized female rats received either bilateral quinolinic acid lesions of the striatum or nucleus accumbens or sham surgeries. After hormone priming, rats were allowed to engage in mating behaviors in an apparatus in which they could pace the rate of the copulatory bout. There was a significant reduction in pacing efficiency after striatal lesions, in that females were less likely to leave the male's side of the chamber after a contact. Animals with lesions of the nucleus accumbens that included the shell were more likely to avoid sexual interaction altogether than animals with control lesions. Therefore, it is concluded that the striatum and nucleus accumbens modulate specific aspects of pacing behavior in the female rat.