Valerie Bishop

Changes in the Intensity of Maternal Aggression and Central Oxytocin and Vasopressin V1a Receptors Across the Peripartum Period in the Rat

Maternal aggressive behaviour, which protects the offspring from harm, is one component of maternal behaviour. Not only maternal aggression, but also maternal care and social behaviour in general, is regulated by the brain oxytocin (OXT) and vasopressin (AVP) systems. In the present study, we quantified the intensity of maternal aggression using the maternal defence test at key time points throughout pregnancy, parturition and lactation. Furthermore, we quantified changes in central OXT and arginine AVP V1a receptor (V1a‐R) binding in brain regions known to be important in regulating maternal aggression, aiming to investigate whether central changes coincide with the intensity of this behaviour. The intensity of aggression was found to dramatically change over the peripartum period, with its first appearance on the day before parturition. Aggression intensity fell immediately after parturition, although it increased during days 4–7 of lactation, before almost disappearing at weaning. OXT receptor (OTR) and V1a‐R binding also showed changes through the peripartum period. OTR binding was highest at parturition within the bed nucleus of the stria terminalis and medial preoptic area and on days 4–7 of lactation in the lateral septum (LS) compared to any other time point during the peripartum period. OTR binding positively correlated with the peak of maternal aggression, suggesting that OXT may act in the LS to facilitate the expression of aggressive behaviour. At parturition, V1a‐R binding was at its highest levels in the paraventricular nucleus and central amygdala (CeA) and, in the LS, V1a‐R binding positively correlated with aggressive behaviour. V1a‐R mRNA expression was also increased within the CeA at parturition. Taken together, the observed fluctuations in OTR and V1a‐R binding in the neural circuitry important for regulating maternal behaviour may ensure that maternal aggression is expressed at the correct time during the peripartum period.

Progesterone receptor expression in the pregnant and parturient rat hypothalamus and brainstem

Oxytocin is synthesized by magnocellular neurons in the supraoptic and paraventricular nuclei (SON and PVN) and during pregnancy progesterone prevents premature activation of oxytocin neurons. Progesterone receptors (PR) are not detectable in SON oxytocin neurons of non-pregnant rats, so we sought to determine whether they are expressed during pregnancy and parturition. In addition, we examined PR expression in brainstem and hypothalamic regions that have known direct projections to the SON. Neuronal immunoreactive PR (irPR)-labeled nuclei were counted in sections from proestrous virgin, late pregnant (day 21) and parturient rats (90 min from birth onset). IrPR nuclei were not evident in the SON at any stage but irPR expression in the medial preoptic nucleus (MPA) significantly increased in pregnancy and parturition (159% and 189% of proestrous controls, respectively). Other hypothalamic areas did not exhibit a significant change in irPR expression. In the nucleus tractus solitarius (NTS) in the brainstem, there was no significant change in irPR in late pregnancy, but there was a significant reduction in irPR expression at parturition (22% of proestrous controls). Very few NTS neurons immunoreactive for tyrosine hydroxylase (irTH), and thus putatively noradrenergic, contained irPR. These findings taken with evidence that brainstem irTH neurons projecting to the SON are stimulated at parturition, whereas MPA cells projecting to the SON are not, suggest that any direct actions of progesterone or progesterone withdrawal on NTS or SON neurons are not mediated through the classical PR. Upregulation of PR expression in the MPA during pregnancy and parturition may relate to the onset of maternal behavior and/or regulation of GnRH neuronal activity.

Decreases in Mineralocorticoid but not Glucocorticoid Receptor mRNA Expression During the Short Arctic Breeding Season in Free-Living Gambel's White-Crowned Sparrow (Zonotrichia leucophrys gambelii)

The acute stress response in vertebrates is a highly adaptive suite of physiological and behavioural mechanisms that promote survival in the face of deleterious stimuli from the environment. Facultative changes of physiology and behaviour are mediated through changes in circulating levels of glucocorticoids (corticosterone, cortisol) and their subsequent binding to the high‐affinity mineralocorticoid receptor (MR) or the low‐affinity glucocorticoid receptor (GR). Free‐living male wild Gambels white‐crowned sparrows (Zonotrichia leucophrys gambelii) display annual fluctuations in the stress response with marked attenuation during the transition from the pre‐parental to the parental stage. We investigated whether this rapid reduction in the stress response is mediated through changes in MR and GR mRNA expression in the brain using in situ hybridisation. MR mRNA expression was found to be significantly lower in the hippocampus as the male birds became parental. No changes were observed in GR mRNA expression in the paraventricular nucleus (PVN) or preoptic area (POA) at this time. No significant correlations were found between initial capture levels of corticosterone and GR or MR mRNA expression. No differences were found in basal levels of corticosterone between pre‐parental and parental in birds collected for in situ hybridisation. Stress response data revealed no difference at baseline but reductions in peak levels of corticosterone as birds became parental. These data suggest that changes in MR expression may be important for the regulation of the stress response or behavioural stress sensitivity with respect to promoting parental care and investment.

Decreased expression of the satiety signal receptor CCKAR is responsible for increased growth and body weight during the domestication of chickens

Animal domestication has resulted in changes in growth and size. It has been suggested that this may have involved selection for differences in appetite. Divergent growth between chickens selected for egg laying or meat production is one such example. The neurons expressing AGRP and POMC in the basal hypothalamus are important components of appetite regulation, as are the satiety feedback pathways that carry information from the intestine, including CCK and its receptor CCKAR (CCK1 receptor). Using 16 generations of a cross between a fast and a relatively slow growing strain of chicken has identified a region on chromosome 4 downstream of the CCKAR gene, which is responsible for up to a 19% difference in body weight at 12 wk of age. Animals possessing the high-growth haplotype at the locus have lower expression of mRNA and immunoreactive CCKAR in the brain, intestine, and exocrine organs, which is correlated with increased levels of orexigenic AGRP in the hypothalamus. Animals with the high-growth haplotype are resistant to the anorectic effect of exogenously administered CCK, suggesting that their satiety set point has been altered. Comparison with traditional breeds shows that the high-growth haplotype has been present in the founders of modern meat-type strains and may have been selected early in domestication. This is the first dissection of the physiological consequences of a genetic locus for a quantitative trait that alters appetite and gives us an insight into the domestication of animals. This will allow elucidation of how differences in appetite occur in birds and also mammals.

Dehydration-induced modulation of κ-opioid inhibition of vasopressin neurone activity

Dehydration increases vasopressin (antidiuretic hormone) secretion from the posterior pituitary gland to reduce water loss in the urine. Vasopressin secretion is determined by action potential firing in vasopressin neurones, which can exhibit continuous, phasic (alternating periods of activity and silence), or irregular activity. Autocrine κ‐opioid inhibition contributes to the generation of activity patterning of vasopressin neurones under basal conditions and so we used in vivo extracellular single unit recording to test the hypothesis that changes in autocrine κ‐opioid inhibition drive changes in activity patterning of vasopressin neurones during dehydration. Dehydration increased the firing rate of rat vasopressin neurones displaying continuous activity (from 7.1 ± 0.5 to 9.0 ± 0.6 spikes s−1) and phasic activity (from 4.2 ± 0.7 to 7.8 ± 0.9 spikes s−1), but not those displaying irregular activity. The dehydration‐induced increase in phasic activity was via an increase in intraburst firing rate. The selective κ‐opioid receptor antagonist nor‐binaltorphimine increased the firing rate of phasic neurones in non‐dehydrated rats (from 3.4 ± 0.8 to 5.3 ± 0.6 spikes s−1) and dehydrated rats (from 6.4 ± 0.5 to 9.1 ± 1.2 spikes s−1), indicating that κ‐opioid feedback inhibition of phasic bursts is maintained during dehydration. In a separate series of experiments, prodynorphin mRNA expression was increased in vasopressin neurones of hyperosmotic rats, compared to hypo‐osmotic rats. Hence, it appears that dynorphin expression in vasopressin neurones undergoes dynamic changes in proportion to the required secretion of vasopressin so that, even under stimulated conditions, autocrine feedback inhibition of vasopressin neurones prevents over‐excitation.

Expression of early growth response protein 1 in vasopressin neurones of the rat anterior olfactory nucleus following social odour exposure

The anterior olfactory nucleus (AON), a component of the main olfactory system, is a cortical region that processes olfactory information and acts as a relay between the main olfactory bulbs and higher brain regions such as the piriform cortex. Utilizing a transgenic rat in which an enhanced green fluorescent protein reporter gene is expressed in vasopressin neurones (eGFP‐vasopressin), we have discovered a population of vasopressin neurones in the AON. These vasopressin neurones co‐express vasopressin V1 receptors. They also co‐express GABA and calbinin‐D28k indicating that they are neurochemically different from the newly described vasopressin neurons in the main olfactory bulb. We utilized the immediate early gene product, early growth response protein 1 (Egr‐1), to examine the functional role of these vasopressin neurons in processing social and non‐social odours in the AON. Exposure of adult rats to a conspecific juvenile or a heterospecific predator odour leads to increases in Egr‐1 expression in the AON in a subregion specific manner. However, only exposure to a juvenile increases Egr‐1 expression in AON vasopressin neurons. These data suggest that vasopressin neurones in the AON may be selectively involved in the coding of social odour information.

Regulation of neuronal nitric oxide synthase mRNA expression in the rat magnocellular neurosecretory system.

We examined the activation of nNOS mRNA expression within the supraoptic and paraventricular nuclei (SON and PVN) of the hypothalamus. In salt-loaded rats nNOS mRNA expression was significantly increased in both nuclei. In rats given i.p. injections of 1.5 M NaCl (4 ml/kg), a small but significant increase in nNOS mRNA expression in the SON and PVN was found 6 h after injection; no change was detected 2 or 4 h after injection. In rats in which hyponatraemia had been induced experimentally, nNOS mRNA was downregulated in the SON, and expression levels were not increased within 4 h after intense acute osmotic stimuli. Finally, neurons of the SON were antidromically-activated by neural stalk stimulation for 2 h. No increase of nNOS mRNA expression in the SON was observed 2 h after stimulation. Thus, increased electrical activity is not directly coupled to rapidly increased expression of nNOS mRNA, and hence acute increases in nNOS mRNA expression are unlikely to play a role in short-term adaptation of the magnocellular system to osmotic stimulation.

Reduced levels of dopamine and altered metabolism in brains of HPRT knock-out rats: a new rodent model of Lesch-Nyhan Disease

Lesch-Nyhan disease (LND) is a severe neurological disorder caused by loss-of-function mutations in the gene encoding hypoxanthine phosphoribosyltransferase (HPRT), an enzyme required for efficient recycling of purine nucleotides. Although this biochemical defect reconfigures purine metabolism and leads to elevated levels of the breakdown product urea, it remains unclear exactly how loss of HPRT activity disrupts brain function. As the rat is the preferred rodent experimental model for studying neurobiology and diseases of the brain, we used genetically-modified embryonic stem cells to generate an HPRT knock-out rat. Male HPRT-deficient rats were viable, fertile and displayed normal caged behaviour. However, metabolomic analysis revealed changes in brain biochemistry consistent with disruption of purine recycling and nucleotide metabolism. Broader changes in brain biochemistry were also indicated by increased levels of the core metabolite citrate and reduced levels of lipids and fatty acids. Targeted MS/MS analysis identified reduced levels of dopamine in the brains of HPRT-deficient animals, consistent with deficits noted previously in human LND patients and HPRT knock-out mice. The HPRT-deficient rat therefore provides a new experimental platform for future investigation of how HPRT activity and disruption of purine metabolism affects neural function and behaviour.

Social information changes stress hormone receptor expression in the songbird brain

ABSTRACT Social information is used by many vertebrate taxa to inform decision‐making, including resource‐mediated movements, yet the mechanisms whereby social information is integrated physiologically to affect such decisions remain unknown. Social information is known to influence the physiological response to food reduction in captive songbirds. Red crossbills (Loxia curvirostra) that were food reduced for several days showed significant elevations in circulating corticosterone (a “stress” hormone often responsive to food limitation) only if their neighbors were similarly food restricted. Physiological responses to glucocorticoid hormones are enacted through two receptors that may be expressed differentially in target tissues. Therefore, we investigated the influence of social information on the expression of the mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) mRNA in captive red crossbill brains. Although the role of MR and GR in the response to social information may be highly complex, we specifically predicted social information from food‐restricted individuals would reduce MR and GR expression in two brain regions known to regulate hypothalamic‐pituitary‐adrenal (HPA) activity ‐ given that reduced receptor expression may lessen the efficacy of negative feedback and release inhibitory tone on the HPA. Our results support these predictions ‐ offering one potential mechanism whereby social cues could increase or sustain HPA‐activity during stress. The data further suggest different mechanisms by which metabolic stress versus social information influence HPA activity and behavioral outcomes. HighlightsAnimals use social information to inform decisions.Food‐restricted neighbors induce enhanced corticosterone release during food restriction.Mechanisms for how social information affects changes in behavior are unknown.We propose changes in HPA‐regulation via corticosterone receptors as one mechanism.Reduced receptor expression in key brain regions may sensitize HPA axis and underlie social effect.

Environmental Influences on Neuromorphology in the Non-Native Starling Sturnus vulgaris

Cognitive traits are predicted to be under intense selection in animals moving into new environments and may determine the success, or otherwise, of dispersal and invasions. In particular, spatial information related to resource distribution is an important determinant of neural development. Spatial information is predicted to vary for invasive species encountering novel environments. However, few studies have tested how cognition or neural development varies intraspecifically within an invasive species. In Australia, the non-native common starling Sturnus vulgaris inhabits a range of habitats that vary in seasonal resource availability and distribution. We aimed to identify variations in the brain mass and hippocampus volume of starlings in Australia related to environmental variation across two substantially different habitat types. Specifically, we predicted variation in brain mass and hippocampal volume in relation to environmental conditions, latitude, and climatic variables. To test this, brain mass and volumes of the hippocampus and two control brain regions (telencephalon and tractus septomesencephalicus) were quantified from starling brains gathered from across the species’ range in south eastern Australia. When comparing across an environmental gradient, there was a significant interaction between sex and environment for overall brain mass, with greater sexual dimorphism in brain mass in inland populations compared to those at the coast. There was no significant difference in hippocampal volume in relation to environmental measures (hippocampus volume, n = 17) for either sex. While these data provide no evidence for intraspecific environmental drivers for changes in hippocampus volume in European starlings in Australia, they do suggest that environmental factors contribute to sex differences in brain mass. This study identifies associations between the brain volume of a non-native species and the environment; further work in this area is required to elucidate the mechanisms driving this relationship.