A role for the periaqueductal grey in opioidergic inhibition of maternal behaviour

Opiates are known to be involved in the regulation of various events surrounding parturition and lactation, such as maternal behaviour in rats. The onset of this behaviour has been closely linked to opiate action in the medial pre-optic area, where administration of morphine disrupts maternal behaviour during lactation. By combining the use of Fos protein immunohistochemical detection and pharmacological manipulations, in the present paper we show that the periaqueductal grey (PAG) is another region critically involved in the opioidergic blockade of maternal behaviour. According to our observations, a critical level of morphine-induced activation of the rostral lateral PAG appears to be required to inhibit maternal behaviour in lactating rats. This hypothesis was further confirmed in experiments showing that morphine's inhibitory effect on maternal responsiveness was blocked by unilateral naloxone injection into the rostral PAG, but not into nearby regions of the mesencephalic reticular nucleus. Therefore, only a partial inhibition of the opiate's effect on the rostral PAG was needed to block the inhibitory effect of morphine on maternal behaviour. Further studies are needed to ascertain whether the rostral lateral PAG plays a role in the natural onset of maternal behaviour, playing a complementary role to the medial pre-optic area, or merely inhibits maternal behaviour in response to this specific pharmacological challenge. Conversely, the present findings may well reflect a more general role of the PAG, seemingly providing an important piece of information for proposing a hitherto unexplored concept of the PAG as an important centre for the selection of adaptive behavioural responses.     

Central V1b Receptor Antagonism in Lactating Rats: Impairment of Maternal Care But Not of Maternal Aggression

Maternal behaviour in rodents is mediated by the central oxytocin and vasopressin systems, amongst others. The role of vasopressin, acting via the V1a receptor (V1aR), on maternal care and maternal aggression has recently been described. However, a potential involvement of the V1b receptor (V1bR) in maternal behaviour has only been demonstrated in knockout mice. The present study aimed to examine the effects of central pharmacological manipulation of the V1bR on maternal behaviour in lactating Wistar rats. On pregnancy day 18, female rats were implanted with a guide cannula targeting the lateral ventricle. After parturition, dams received an acute central infusion of a specific V1bR agonist (d[Leu4,Lys8]VP) or V1bR antagonist (SSR149415) once daily, followed by observations of maternal care [lactation day (LD) 1], maternal motivation in the pup retrieval test (LD 2), anxiety‐related behaviour on the elevated plus‐maze (LD 3) and maternal aggression in the maternal defence test followed by maternal care monitoring (LD 4). Our data demonstrate that, under nonstress conditions, the V1bR antagonist decreased the occurrence of both nursing and mother–pup interaction, whereas the V1bR agonist did not affect either parameter. Under stress conditions (i.e. after the maternal defence test), mother–pup interaction was decreased by infusion of the V1bR antagonist. During the maternal defence test, neither treatment affected aggressive or non‐aggressive behaviour. Finally, neither treatment altered maternal motivation or anxiety. In conclusion, central V1bR antagonism modulates aspects of maternal care but not of maternal aggression or maternal motivation in lactating rats. These findings further extend our knowledge on the vasopressin system as a vital mediator of maternal behaviour.     

Maternal social environment affects offspring cognition through behavioural and immune pathways in rats

The social environment of lactation is a key etiological factor for the occurrence of postpartum disorders affecting women and their children. Postpartum depression and anxiety disorders are highly prevalent in new mothers and negatively affect offspring’s cognitive development through mechanisms which are still unclear. Here, using a rat model, we manipulated the maternal social environment during lactation and explored the pathways through which social isolation (vs. the opportunity for limited social interaction with another lactating female, from 1 day before parturition to postpartum day 16) and chronic social conflict (daily exposure to a male intruder from postpartum day 2 to day 16) affect offspring learning and memory, measured at 40 to 60 days of age. We specifically explored the consequences of these social treatments on two main hypothesized mediators likely to affect offspring neurophysiological development: the quality of maternal care and maternal inflammation factors (brain‐derived neurotrophic factor, granulocyte‐macrophage colony‐stimulating factor, intercellular adhesion molecule 1, tissue inhibitor of metalloproteinases 1 and vascular endothelial growth factor) likely to influence offspring development through lactation. Maternal rats which had the opportunity to interact with another lactating female spent more time with their pups which, in turn, displayed improved working and reference memory. Social stress affected maternal plasma levels of cytokines that were associated with cognitive deficits in their offspring. However, females subjected to social stress were protected from these stress‐induced immune changes and associated offspring cognitive impairment by increased social affiliation. These results underscore the effects of social interaction for new mothers and their offspring and can be used to inform the development of clinical preventative measures and interventions.     

Disruption of maternal behaviour in virgin and postparturient rats following sagittal plane knife cuts in the preoptic area-hypothalamus

Parasagittal knife cuts along the medial preoptic area-medial anterior hypothalamus were placed at either the medial (near lateral or NL cuts) or lateral (far lateral or FL cuts) border of the medial forebrain bundle in separate groups of virgin (Experiment 1) and postparturient (Experiment 2) rats. FL cuts were placed so as to spare preoptic-hypothalamic efferent connections with the medial forebrain bundle. NL and FL knife cuts were equally effective in preventing the induction of maternal behaviour produced by repeated exposure of virgin rats to foster pups. Both types of cuts also reduced nest building in virgins. In postparturient rats, NL and FL cuts both abolished pup retrieval and reduced nursing behaviour. However, only NL cuts disrupted lactation and nest building. NL cuts also produced chronic hyperthermia. The deficits in maternal behaviour and nest building may be independent of this hyperthermia, as shaving the fur in animals with NL cuts reduced body temperatures to control levels but did not restore these behaviours. These findings indicate that while preoptic/hypothalamic connections through the medial forebrain bundle are important for nest building and possibly lactation, other lateral connections must also be important for pup retrieval and nursing behaviour. The identity of these connections remains to be determined.     

Hypocretin-1 dose-dependently modulates maternal behaviour in mice

Increases in neuronal activity of hypocretin (HCRT), a peptide involved in arousal, and in HCRT-1 receptor mRNA expression have recently been identified in association with lactation. HCRT is released within brain regions regulating maternal behaviour and it is possible that increased HCRT neurotransmission during lactation supports maternal care. The present study examined for the first time the behavioural effects of HCRT on lactating mice. At intermediate doses, intracerebroventricular (i.c.v.) injections of HCRT-1 (0.06 and 0.1 microg) elevated levels of licking and grooming of pups (but not self-grooming) and number of nursing bouts without affecting other behaviours. At the highest dose, HCRT-1 (0.3 microg, i.c.v) delayed latency to nurse, decreased nursing, increased time off nest, and decreased maternal aggression. Intraperitoneal injections of the HCRT-1 receptor antagonist, SB-334867, exhibited a general trend towards increasing time spent low-arched back nursing (P = 0.053) and decreasing licking and grooming of pups while high-arched back nursing (P = 0.052). This suggests that the endogenous release of HCRT, working independently or dependently with other neuromodulators, may be necessary for full maternal behaviour expression. Possible sites of HCRT action in enhancing and impairing maternal care were identified via examinations of c-Fos immunoreactivity in association with i.c.v. HCRT injections. Together, these finding support the idea of HCRT modulating maternal behaviour, with intermediate levels (0.06 and 0.1 microg) supporting (even augmenting) some behaviours, but with levels that are too high (0.3 microg HCRT, i.c.v.), maternal behaviour and aggression are suppressed.     

Oxytocin Messenger Ribonucleic Acid Levels in the Medial Preoptic Area are Increased During Lactation

The peptide hormone oxytocin has an important role in parturition, lactation and maternal behavior. The present study employed in situ hybridization histochemistry to determine whether oxytocin messenger ribonucleic acid (mRNA) levels in cells in the medial preoptic area, a brain area known to control maternal behavior, change during pregnancy and lactation in the rat. Female rats were perfused on either Day 18 or 22 of pregnancy or Day 5 of lactation. Ovariectomized female rats were included as an additional control group. Cells expressing oxytocin mRNA were detected by in situ hybridization using an [¹²⁵ l]-labeled 38 base synthetic oligodeoxynucleotide probe complementary to the C-terminal coding region of the preprooxytocin. Relative differences in oxytocin mRNA levels were determined by silver grain counting of labeled cells. A group of oxytocin neurons in the dorsal medial preoptic area, called the lateral subcommissural nucleus, showed elevated oxytocin mRNA levels in lactating animals relative to ail other groups. Oxytocin mRNA levels in the neurons of the periventricular nucleus of the preoptic area did not change across pregnancy and lactation. This result extends the findings of others showing elevated oxytocin mRNA levels in magnocellular nuclei of lactating animals. The results are discussed in terms of the possible role of oxytocin cells in the medial preoptic area in the expression of maternal behavior.     

Effects of psycho-social stress during pregnancy on neuroendocrine and behavioural parameters in lactation depend on the genetically determined stress vulnerability

The neuroendocrine consequences of repeated exposure of the pregnant mother to relevant stressors have been studied in the offspring, but not in the mothers. As these stress effects might depend on the genetically determined stress susceptibility of the dams, here, we investigated the effects of daily exposure to psycho-social stressors (maternal defeat by an aggressive lactating resident and restraint) between pregnancy days 4 and 18 in female rats selectively and bidirectionally bred for high (HAB) or low (LAB) anxiety-related behaviour. ACTH and corticosterone secretory responses to a mild stressor were found to be low in unstressed lactating HAB and LAB dams (day 8 of lactation) indicating an intact physiological attenuation of the HPA axis at this time. Pregnancy stress significantly increased the reactivity of the hypothalamo-pituitary-adrenal (HPA) axis in lactating HAB, but not LAB rats, reflecting impaired attenuation of the HPA axis selectively in pregnancy-stressed HAB dams. The high and low anxiety phenotypes were consistent in lactation and not significantly altered by pregnancy stress, despite an elevated level of arousal in pregnancy-stressed HAB dams. In general, HAB dams showed signs of a more protective maternal behaviour compared to LAB dams: (i) in the home cage, HAB dams spent more time in direct pup contact (day 1 of lactation), (ii) during two forms of the pup retrieval test, differing in the level of challenging the dam, HAB dams retrieved the pups faster, and (iii) during the maternal defence test, they were more aggressive towards a virgin intruder compared to LAB and NAB dams. Pregnancy stress did not alter any of these behavioural measures, except an increase in the speed of pup collection in a novel environment in HAB dams and increased maternal aggression in LAB dams. The results indicate a robust behavioural phenotype of HAB and LAB dams with respect to anxiety and maternal behaviour which was found to be almost unchanged by exposure to pregnancy stress. However, the finding of differential effects of pregnancy stress on the attenuation of the reactivity of the HPA axis in lactation makes HAB and LAB rats a potential animal model for studying genetically determined differences in stress vulnerability and stress-induced maladaptation of the HPA axis post-partum.     

Immunoreactive Prolactin in the Cerebrospinal Fluid of Estrogen-Treated and Lactating Rats as Determined by Push-pull Perfusion of the Lateral Ventricles

Immunoreactive (ir) prolactin levels in the cerebrospinal fluid were measured in push-pull perfusion samples collected from the lateral cerebral ventricles in freely-moving ovariectomized rats prior to and after estrogen treatment, and in rats during pregnancy and lactation. Ir-prolactin levels in cerebrospinal fluid were elevated throughout the perfusion period in ovariectomized animals given estrogen and in Day 8 lactating females following the onset of nursing by their young. Cerebrospinal fluid perfusates collected from ovariectomized animals prior to estradiol treatment and from females on Days 18 and 22 of gestation contained levels of ir-prolactin that were below the limit of detectability of the prolactin radioimmunoassay. Data from previous studies in our laboratory suggest that pituitary prolactin is involved in the mediation of maternal responsiveness. Where and how prolactin exerts its actions in this regard remains to be determined. Results of the present study indicate that detectable levels of ir-prolactin are present in the cerebrospinal fluid of nulliparous females after exposure to an estradiol regimen known to shorten their latency to display maternal responsiveness and in lactating females actively exhibiting maternal behavior. Once in the cerebrospinal fluid, prolactin should have access to neural target sites behind the blood-brain barrier. The presence of ir-prolactin in the cerebrospinal fluid of females in the present study is consistent with a role for this protein hormone, acting at the level of the central nervous system, in the facilitation of maternal responsiveness as well as in the regulation of other neural and neuroendocrine processes.     

Suckling induces a daily rhythm in the preoptic area and lateral septum but not in the bed nucleus of the stria terminalis in lactating rabbit does

Maternal behavior in the rabbit is restricted to a brief nursing period every day. Previously, we demonstrated that this event induces daily rhythms of Period1 (PER1) protein, the product of the clock gene Per1, in oxytocinergic and dopaminergic populations in the hypothalamus of lactating rabbit does. This is significant for the periodic production and ejection of milk, but the activation of other areas of the brain has not been explored. Here, we hypothesised that daily suckling would induce a rhythm in the preoptic area, lateral septum, and bed nucleus of the stria terminalis, which are important areas for the expression of maternal behavior in mammals, including the rabbit. To this end, we analysed PER1 expression in those areas through a complete 24‐h cycle at lactation day 7. Does were scheduled to nurse during either the day at 10:00 h [zeitgeber time (ZT)03] or the night at 02:00 h (ZT19). Non‐pregnant, non‐lactating females were used as controls. In contrast to control females, lactating does showed a clear, significant rhythm of PER1 that shifted in parallel with the timing of nursing in the preoptic area and lateral septum. We determined that the maximal expression of PER1 at 8 h after scheduled nursing decreased significantly at 24 and 48 h after the absence of suckling. This effect was more pronounced in the lateral septum than in the preoptic area. We conclude that daily suckling is a powerful stimulus inducing rhythmic activity in brain structures in the rabbit that appear to form part of a maternal entrainable circuit.     

Site and behavioral specificity of periaqueductal gray lesions on postpartum sexual, maternal, and aggressive behaviors in rats

Bilateral electrolytic lesions of the lateral and ventrolateral caudal periaqueductal gray (cPAG_l,vl) of lactating rats are known to severely reduce suckling-induced kyphosis (upright crouched nursing), which is necessary for maximal litter weight gains, and impair sexual behavior during the postpartum estrous, while heightening nursing in other postures and attacks on unfamiliar adult male intruders. In the present report, the site specificity of the cPAG with respect to the control of these behaviors was determined by comparing lesions of the cPAG_l,vl with similarly sized lesions within the rostral PAG (rPAG) and surrounding mesencephalon. The previously seen effects of prepartum cPAG_l,vl lesions on kyphotic nursing, sexual proceptivity and receptivity, maternal aggression, and daily litter weight gains were replicated. Additionally, the post-lesion facilitation of aggression was found to be behaviorally specific, first by being directed toward an adult, but not to a nonthreatening juvenile male rat, and second, by requiring the recent presence of the pups, being eliminated or decreased 24 h after removal of the litter. Damage to the rPAG did not affect nursing or sexual behaviors, and had only a minimal effect on maternal aggression. Lesions of the rPAG, however, greatly impaired the dams' ability to rapidly release pups held in the mouth, but not to pick them up or carry them directly to the nest during retrieval. Separate regions of the PAG, therefore, are differentially involved in the control of specific components of behaviors in lactating rats.     

Reduced brain corticotropin‐releasing factor receptor activation is required for adequate maternal care and maternal aggression in lactating rats

The brain corticotropin‐releasing factor (CRF) system triggers a variety of neuroendocrine and behavioural responses to stress. Whether maternal behaviour and emotionality in lactation are modulated by CRF has rarely been investigated. In the present study, we measured CRF mRNA expression within the parvocellular part of the paraventricular nucleus in virgin and lactating Wistar rats bred for high (HAB) and low (LAB) anxiety‐related behaviour or non‐selected for anxiety (NAB). Further, we intracerebroventricularly infused synthetic CRF or the CRF receptor (CRF‐R) antagonist D‐Phe to manipulate CRF‐R1/2 non‐specifically in lactating HAB, LAB, and NAB dams, and monitored maternal care, maternal motivation, maternal aggression, and anxiety. The CRF mRNA expression in the parvocellular part of the paraventricular nucleus was higher in HAB vs. LAB rats independent of reproductive status. The lactation‐specific decrease of CRF mRNA was confirmed in LAB and NAB dams but was absent in HAB dams. Intracerebroventricular CRF decreased maternal care under basal conditions in the home cage in all breeding lines and reduced attack behaviour in HAB and LAB dams during maternal defence. In contrast, D‐Phe rescued maternal care after exposure to maternal defence in the home cage without influencing maternal aggression. Furthermore, D‐Phe decreased and CRF tended to increase anxiety in HAB/NAB and LAB dams, respectively, suggesting an anxiogenic effect of CRF in lactating females. In conclusion, low CRF‐R activation during lactation is an essential prerequisite for the adequate occurrence of maternal behaviour.     

Effect of Hypothalamic Deafferentation on the Pulsatile Secretion of Luteinizing Hormone in Ovariectomized Lactating Rats*

The pulsatile luteinizing hormone (LH) secretion in ovariectomized lactating rats bearing complete (CD), anterior (AD), anterolateral (ALD), posterior (PD), or roof (RD) deafferentation of the hypothalamus was determined. All lactating rats were ovariectomized on Day 2 of lactation (Day 0, day of parturition). The deafferentation of nerve fibres to the mediobasal hypothalamus was performed on Day 6 or 7 of lactation. Twenty-four h after the surgery, blood samples were taken through the indwelling atrial catheter every 6 min for 3 h. Plasma concentrations of LH and prolactin (PRL) were measured by radioimmunoassay. The loss of LH pulses associated with lactation was still apparent following AD, PD and sham-deafferentation (SD); pulsatile LH secretion was, however, present in rats with CD, ALD and RD despite continued suckling. The only significant difference in plasma PRL concentrations among the various groups was a reduction in the PRL level in rats with RD in comparison to those with SD. We conclude that the neural signal responsible for the inhibition of pulsatile LH release by suckling is conveyed through the dorsal part of the hypothalamus and PRL does not mediate the suppression of LH pulses in mid-lactation.     

Metabolic mapping of the brain in pregnant, parturient and lactating rats using Fos immunohistochemistry

The present study was designed to investigate Fos-positive neurons of the female rat brain at various reproductive states in order to analyze the metabolic map connected with pregnancy, parturition and lactation. The number of Fos-positive neurons in each brain nucleus was analyzed with a quantitative immunohistochemical method in virgin, pregnant, parturient, lactating and arrested lactating rats. In parturient rats, a significant number of Fos-positive neurons was observed as compared to virgin or pregnant females in the following brain regions; the bed nucleus of the stria terminalis (BST), lateral septal nucleus (LS), medial preoptic area (MPA), periventricular hypothalamic nucleus (Pe), parvocellular paraventricular hypothalamic nucleus (PaPVN), magnocellular paraventricular hypothalamic nucleus (MaPVN), supraoptic nucleus (SON), paraventricular thalamic nucleus (PV), anterior hypothalamic area (AHA), lateral hypothalamic area (LH), amygdaloid nucleus (AM), supramammillary nucleus (SuM), substantia nigra (SN), central grey (CG), microcellular tegmental nucleus (MiTg), subparafascicular thalamic nucleus (SPF), posterior hypothalamic area (PH), dorsal raphe nucleus (DR), locus coeruleus (LC), dorsal parabrachial nucleus (DPB), nucleus of solitary tract (Sol), and ventrolateral medulla (VLM). Significant differences were found in the number of Fos-positive neurons between parturient and lactating females, although localization of Fos-positive neurons in lactating females was quite similar to parturient ones. Between parturient and lactating rats: (1) In the MPA, PaPVN, AHA, arcuate hypothalamic nucleus (Arc), ventromedial hypothalamic nucleus (VMH), mesencephalic lateral tegmentum (MLT), and genual nucleus (Ge), the number of Fos-positive neurons of lactating females were significantly higher than those of parturient ones; (2) In the LS, Pe, PV, LH, AM, SuM, CG, MiTg, SPF, PH, DR, LC, and VLM, there was no significant differences in the number of Fos-positive neurons; (3) In the BST, MaPVN, SON, SN, DPB and Sol, the number of Fos-positive neurons of lactating rats were significantly lower than those of parturient ones. These different patterns of Fos expression among many brain regions may be owing to the functional differences in each region. Fos expression in lactating rats was apparently induced by suckling stimulation because the removal of their litters immediately after parturition completely eliminated expression of Fos protein in each nucleus. These results suggest that the localization of Fos-positive neurons in a number of neural populations throughout the brain may be revealing the neural circuits in response to parturition or lactation.     

Stimulation of Maternal Behaviour in Rats with Cholecystokinin Octapeptide

Research on the neuroendocrine control of maternal behaviour has concentrated on the role of ovarian and pituitary hormones (1). It is known that the gastrointestinal tract plays an important role in synchronizing mother-young interactions (2), but the possible contribution of gastrointestinal secretions to maternal behaviour has not been investigated. We show here that treatment with oestradiol benzoate (OB) in combination with cholecystokinin octapeptide (CCK-8), a duodenal peptide (3), stimulates maternal behaviour within 4 h of exposure to newborn pups in ovariectomized rats. The elevated concentrations of CCK-8 which are found in the plasma of lactating rats may, therefore, contribute to the development and maintenance of mother-young interactions during lactation.     

Fos expression induced by changes in arterial pressure is localized in distinct,longitudinally organized columns of neurons in the rat midbrain periaqueductal gray

The distribution of neurons expressing Fos within the periaqueductal gray (FAG) following pharmacologically induced high or low blood pressure was examined to determine (1) if PAG neurons are responsive to changes in arterial pressure (AP) and (2) the relationship of these cells to the functionally defined hypertensive and hypotensive columns in PAG. Changes in AP differentially induced robust Fos expression in neurons confined to discrete, longitudinally organized columns within PAG. Increased AP produced extensive Fos-like immunoreactivity within the lateral PAG, beginning at the level of the oculomotor nucleus. At the level of the dorsal raphe, Fos expression induced by increased AP shifted dorsally, into the dorsolateral division of PAG; this pattern of Fos labeling was maintained throughout the caudal one-third of PAG. Double-labeling for Fos and nicotinamide adenine dinucleotide phosphate diaphorase confirmed that Fos-positive cells induced by increased AP were located in the dorsolateral division of PAG at these caudal levels. Fos positive cells were codistributed, but not colocalized, with nicotinamide adenine dinucleotide phosphate diaphorase-positive cells. Decreased AP evoked a completely different pattern of Fos expression. Fos-positive cells were predominantly located within the ventrolateral PAG region, extending from the level of the trochlear nucleus through the level of the caudal dorsal raphe. Double-labeling studies for Fos and serotonin indicated that only 1–2 double-labeled cells per section were present. Saline infusion resulted in very few Foslike immunoreactive cells, indicating that volume receptor activation does not account for Fos expression in PAG evoked by changes in AP. These results indicate that (1) substantial numbers of PAG neurons are excited by pharmacologically induced changes in AP and (2) excitatory barosensitive PAG neurons are anatomically segregated based on their responsiveness to a specific directional change in AP. © 1995 Wiley-Liss, Inc.     

Changes in neuropeptide Y immunoreactivity in the arcuate nucleus during and after food restriction in lactating rats

Previous studies have demonstrated that food restriction during lactation extends the length of lactational infertility in rats. In order to determine whether Neuropeptide Y (NPY) plays a role in the increased length of lactational infertility seen in food-restricted rats, NPY immunoreactivity in the arcuate nucleus of lactating rats that were ad lib fed or food restricted from Day 8 to 14 postpartum was compared. Food-restricted rats showed higher numbers of NPY-stained cells at the end of the food-restriction period (Day 15 postpartum) than did ad lib-fed rats at the same stage of lactation. This difference persisted until Day 25 postpartum although all animals were fed ad libitum from Day 15 onwards. Switching litters between ad lib-fed and food-restricted females from Day 15 until Day 20 postpartum did not eliminate the difference in NPY immunoreactivity between the two diet conditions. Given that food restriction during lactation leads to a prolonged suppression of LH release that also persists after refeeding and is not affected by the nutritional status of the litter, these data are consistent with a role for NPY in the prolonged suppression of reproductive function seen in food restricted lactating rats.     

Predatory hunting and exposure to a live predator induce opposite patterns of Fos immunoreactivity in the PAG

Considering the periaqueductal gray's (PAG) general roles in mediating motivational responses, in the present study, we compared the Fos expression pattern in the PAG induced by innate behaviors underlain by opposite motivational drivers, in rats, namely, insect predation and defensive behavior evoked by the confrontation with a live predator (a cat). Exposure to the predator was associated with a striking Fos expression in the PAG, where, at rostral levels, an intense Fos expression was found largely distributed in the dorsomedial and dorsolateral regions, whereas, at caudal levels, Fos-labeled cells tended to be mostly found in the lateral and ventrolateral columns, as well as in the dorsal raphe nucleus. Quite the opposite, insect predation was associated with increased Fos expression predominantly in the rostral two thirds of the lateral PAG, where the majority of the Fos-immunoreactive cells were found at the oculomotor nucleus levels. Remarkably, both exposure to the cat and insect predation upregulated Fos expression in the supraoculomotor region and the laterodorsal tegmental nucleus. Overall, the present results clearly suggest that the PAG activation pattern appears to reflect, at least partly, the animal's motivational status. It is well established that the PAG is critical for the expression of defensive responses, and, considering the present findings, it will be important to investigate how the PAG contributes to the expression of the predatory behavior, as well.     

Localization of the neurons active during paradoxical (REM) sleep and projecting to the locus coeruleus noradrenergic neurons in the rat

Locus coeruleus (LC) noradrenergic neurons are active during wakefulness, slow their discharge rate during slow wave sleep, and stop firing during paradoxical sleep (PS). A large body of data indicates that their inactivation during PS is due to a tonic GABAergic inhibition. To localize the neurons responsible for such inhibition, we first examined the distribution of retrogradely and Fos double-immunostained neurons following cholera toxin b subunit (CTb) injection in the LC of control rats, rats selectively deprived of PS for 3 days, and rats allowed to recover for 3 hours from such deprivation. We found a significant number of CTb/Fos double-labeled cells only in the recovery group. The largest number of CTb/Fos double-labeled cells was found in the dorsal paragigantocellular reticular nucleus (DPGi). It indeed contained 19% of the CTb/Fos double-labeled neurons, whereas the ventrolateral periaqueductal gray (vlPAG) contained 18.3% of these neurons, the lateral paragigantocellular reticular nucleus (LPGi) 15%, the lateral hypothalamic area 9%, the lateral PAG 6.7%, and the rostral PAG 6%. In addition, CTb/Fos double-labeled cells constituted 43% of all the singly CTb-labeled cells counted in the DPGi compared with 29% for the LPGi, 18% for the rostral PAG, and 10% or less for the other structures. Although all these populations of CTb/Fos double-labeled neurons could be GABAergic and tonically inhibit LC neurons during PS, our results indicate that neurons from the DPGi constitute the best candidate for this role.     

A functional neuroanatomical investigation of the role of the medial preoptic area in neural circuits regulating maternal behavior

The medial preoptic area (MPOA) is essential for normal maternal behavior in the rat. Hormone stimulation of the MPOA facilitates the behavior and lesions of the MPOA and the adjoining ventral part of the bed nucleus of the stria terminalis (vBST) disrupt the behavior. The MPOA/vBST also show increases in Fos protein expression during maternal behavior. The present study examines the larger neural circuitry within which the MPOA/vBST might operate to influence maternal behavior. Combining Fos immunocytochemistry with unilateral excitotoxic amino acid lesions or lateral knife cuts of the MPOA/vBST, we sought to identify brain regions which might be under the influence of Fos expressing neurons in the MPOA/vBST. Two brain regions, the shell of the nucleus accumbens (NAs), and the intermediate part of the lateral septum (LSi) were identified. Both the NAs and LSi exhibited elevated Fos expression during maternal behavior, while unilateral MPOA/vBST damage resulted in an ipsilateral reduction of maternal behavior-induced Fos expression in each area, suggesting that MPOA/vBST neurons modulate Fos expression and associated neural activity in both of these structures during maternal behavior. Importantly, these unilateral preoptic lesions also depressed maternal behavior-induced Fos expression in the ipsilateral MPOA and vBST. The effects of these lesions on Fos expression in the periaqueductal gray (PAG) and other brain regions are also presented.     

Maternal responsiveness to suckling is modulated by time post‐nursing: A behavioural and c‐Fos/oxytocin immunocytochemistry study in rabbits

Doe rabbits nurse once/day for approximately 3 minutes, with circadian periodicity, inside the nest box. The amount of suckling received at each bout regulates this behaviour because reducing the litter size to four kits or less disrupts nursing circadian periodicity and increases suckling bout duration. Additionally, the likelihood that does will nurse kits at a given time of day depends on the time elapsed since the last suckling episode and the litter size nursed then. We hypothesised that the time elapsed since the last nursing would impact the number of c‐Fos immunoreactive (IR) cells observed after suckling five kits and also that observed before the next nursing (“no kits”). Suckling significantly increased, relative to “no kits”, the number of c‐Fos‐IR cells in the medial preoptic area and lateral septum but not in the bed nucleus of the stria terminalis (BNST), suprachiasmatic nucleus or ventromedial hypothalamus in does nursing at 18 or 24 hours after the previous bout. No effects of suckling were observed in mothers nursing at 6 hours. Does given kits at 3 hours post last suckling refused to nurse but, in the remaining three groups, all does nursed normally. At “no kits”, more c‐Fos‐IR cells were seen (in all regions except the BNST) in does given kits at 24 hours after the last nursing and killed 1 hour later (ie, 4 hours after lights on) than in those killed earlier. The percentage of oxytocinergic (OT) cells co‐expressing c‐Fos was not modified by nursing in the paraventricular or supraoptic nuclei but, in the latter, the largest number of total OT‐IR cells occurred at 18 and 24 hours post‐last nursing. In conclusion, the responsiveness of particular forebrain regions involved in regulating circadian rhythms, lactation, and maternal behaviour is modulated by suckling and time of day.