Vasopressin: behavioral roles of an "original" neuropeptide.

Vasopressin (Avp) is mainly synthesized in the magnocellular cells of the hypothalamic supraoptic (SON) and paraventricular nuclei (PVN) whose axons project to the posterior pituitary. Avp is then released into the blood stream upon appropriate stimulation (e.g., hemorrhage or dehydration) to act at the kidneys and blood vessels. The brain also contains several populations of smaller, parvocellular neurons whose projections remain within the brain. These populations are located within the PVN, bed nucleus of the stria terminalis (BNST), medial amygdala (MeA) and suprachiasmatic nucleus (SCN). Since the 1950s, research examining the roles of Avp in the brain and periphery has intensified. The development of specific agonists and antagonists for Avp receptors has allowed for a better elucidation of its contributions to physiology and behavior. Anatomical, pharmacological and transgenic, including "knockout," animal studies have implicated Avp in the regulation of various social behaviors across species. Avp plays a prominent role in the regulation of aggression, generally of facilitating or promoting it. Affiliation and certain aspects of pair-bonding are also influenced by Avp. Memory, one of the first brain functions of Avp that was investigated, has been implicated especially strongly in social recognition. The roles of Avp in stress, anxiety, and depressive states are areas of active exploration. In this review, we concentrate on the scientific progress that has been made in understanding the role of Avp in regulating these and other behaviors across species. We also discuss the implications for human behavior.

A conditional knockout mouse line of the oxytocin receptor.

Oxytocin plays important roles in reproductive physiology and various behaviors, including maternal behavior and social memory. Its receptor (Oxtr) is present in peripheral tissues and brain, so a conditional knockout (KO, -/-) would be useful to allow elimination of the receptor in specific sites at defined times. We created a line of mice in which loxP sites flank Oxtr coding sequence (floxed) enable Cre recombinase-mediated inactivation of the receptor. We expressed Cre recombinase in these mice either in all tissues (Oxtr(-/-)) or the forebrain (Oxtr(FB/FB)) using the Ca(2+)/calmodulin-dependent protein kinase IIalpha promoter. The latter KO has reduced Oxtr binding beginning 21-28 d postnatally, leading to prominent reductions in the lateral septum, hippocampus, and ventral pallidum. The medial amygdala is spared, and there is significant retention of binding within the olfactory bulb and nucleus and neocortex. We did not observe any deficits in the general health, sensorimotor functions, anxiety-like behaviors, or sucrose intake in either Oxtr(-/-) or Oxtr(FB/FB) mice. Females of both KO types deliver pups, but only the Oxtr(FB/FB) mice are able to eject milk. Oxtr(-/-) males show impaired social memory for familiar females, whereas the Oxtr(FB/FB) males appear to recognize their species but not individuals. Our results confirm the importance of oxytocin in social recognition and demonstrate that spatial and temporal inactivation of the Oxtr will enable finer understanding of the physiological, behavioral, and developmental roles of the receptor.

Effects of multiparity on recognition memory, monoaminergic neurotransmitters, and brain-derived neurotrophic factor (BDNF).

Recognition memory and anxiety were examined in nulliparous (NP: 0 litters) and multiparous (MP: 5-6 litters) middle-aged female rats (12 months old) to assess possible enduring effects of multiparity at least 3 months after the last litter was weaned. MP females performed significantly better than NP females on the non-spatial memory task, object recognition, and the spatial memory task, object placement. Anxiety as measured on the elevated plus maze did not differ between groups. Monoaminergic activity and levels were measured in prefrontal cortex, CA1 hippocampus, CA3 hippocampus, and olfactory bulb (OB). NP and MP females differed in monoamine concentrations in the OB only, with MP females having significantly greater concentrations of dopamine and metabolite DOPAC, norepinephrine and metabolite MHPG, and the serotonin metabolite 5-HIAA, as compared to NP females. These results indicate a long-term change in OB neurochemistry as a result of multiparity. Brain-derived neurotrophic factor (BDNF) was also measured in hippocampus (CA1, CA3, dentate gyrus) and septum. MP females had higher BDNF levels in both CA1 and septum; as these regions are implicated in memory performance, elevated BDNF may underlie the observed memory task differences. Thus, MP females (experiencing multiple bouts of pregnancy, birth, and pup rearing during the first year of life) displayed enhanced memory task performance but equal anxiety responses, as compared to NP females. These results are consistent with previous studies showing long-term changes in behavioral function in MP, as compared to NP, rats and suggest that alterations in monoamines and a neurotrophin, BDNF, may contribute to the observed behavioral changes.

Motherhood mitigates aging-related decrements in learning and memory and positively affects brain aging in the rat.

The current work examined spatial learning and memory (i.e., latencies to find a baited food well) in age-matched nulliparous, primiparous and multiparous (NULL, PRIM and MULT, zero, one or two pregnancies and lactations, respectively). We tested at 6, 12, 18 and 24 months of age in a dry land version of the Morris water maze (Main task), and at 12, 18 and 24 months in the same task in which the original location of the baited well was changed (Reversal task). We show that PRIM/MULT rats, compared to the age-matched NULL females, learned the spatial tasks significantly better and exhibited attenuated memory decline, up to 24 months of age. Furthermore, at the conclusion of behavioral testing, we investigated levels of these animals' hippocampal (CA1 and dentate gyrus) immunoreactive amyloid precursor protein (APP), a marker of neurodegeneration and age-related cognitive loss. MULTs had significantly reduced APP in both CA1 and DG, relative to PRIMs and NULLs, and PRIMs had a trend (p<0.06) toward a reduction in APP compared to NULLs in DG. Further, level of APP was negatively correlated with performance in the two tasks (viz., more APP, worse maze performance). Reproduction, therefore, with its attendant natural endocrine and postpartum sensory experiences, may facilitate lifelong learning and memory, and may mitigate markers of neural aging, in the rat. Combining natural hormonal exposure with subsequent substantial experience with stimuli from the offspring may preserve the aged parous female brain relative to that of NULL females.

Changes in anxiety and cognition due to reproductive experience: a review of data from rodent and human mothers.

Rodent research suggests that pregnancy, motherhood and attendant offspring care affect changes in neural function and behaviors that are not directly maternal in nature, but involve cognition, affect, and responses to stress. Thus, female rats having had one pregnancy and bout of rearing (primiparous), or multiple pregnancies and bouts of rearing (multiparous), generally show greater resilience to stress, decreased anxiety, and better memory abilities than female rats that have never experienced motherhood (virgin or nulliparous). Moreover, some studies show that these neural changes remain long after the last pregnancy, persisting even into old age. In the current review, we will begin by discussing these behavioral and neural changes in rodents and provide some information concerning their possible mechanisms. Then we will review data from studies examining anxiety and cognition in postpartum human mothers. While this data is less conclusive than that from non-human animals, it appears that reproductive experience may confer some beneficial changes to human mothers in terms of lowering the anxiety/stress response and enhancing certain aspects of memory.

Normal maternal behavior, but increased pup mortality, in conditional oxytocin receptor knockout females.

Oxytocin (Oxt) and the Oxt receptor (Oxtr) are implicated in the onset of maternal behavior in a variety of species. Recently, we developed two Oxtr knockout lines: a total body knockout (Oxtr-/-) and a conditional Oxtr knockout (OxtrFB/FB) in which the Oxtr is lacking only in regions of the forebrain, allowing knockout females to potentially nurse and care for their biological offspring. In the current study, we assessed maternal behavior of postpartum OxtrFB/FB females toward their own pups and maternal behavior of virgin Oxtr-/- females toward foster pups and compared knockouts of both lines to wildtype (Oxtr+/+) littermates. We found that both Oxtr-/- and OxtrFB/FB females appear to have largely normal maternal behaviors. However, with first litters, approximately 40% of the OxtrFB/FB knockout dams experienced high pup mortality, compared to fewer than 10% of the Oxtr+/+ dams. We then went on to test whether or not this phenotype occurred in subsequent litters or when the dams were exposed to an environmental disturbance. We found that regardless of the degree of external disturbance, OxtrFB/FB females lost more pups on their first and second litters compared to wildtype females. Possible reasons for higher pup mortality in OxtrFB/FB females are discussed.

Housing conditions and stimulus females: a robust social discrimination task for studying male rodent social recognition.

Social recognition (SR) enables rodents to distinguish between familiar and novel conspecifics, largely through individual odor cues. SR tasks utilize the tendency for a male to sniff and interact with a novel individual more than a familiar individual. Many paradigms have been used to study the roles of the neuropeptides oxytocin and vasopressin in SR. However, inconsistencies in results have arisen within similar mouse strains, and across different paradigms and laboratories, making reliable testing of SR difficult. The current protocol details a novel approach that is replicable across investigators and in different strains of mice. We created a protocol that uses gonadally intact, singly housed females presented within corrals to group-housed males. Housing females singly before testing is particularly important for reliable discrimination. This methodology will be useful for studying short-term social memory in rodents, and may also be applicable for longer term studies.

Oxytocin: the great facilitator of life.

Oxytocin (Oxt) is a nonapeptide hormone best known for its role in lactation and parturition. Since 1906 when its uterine-contracting properties were described until 50 years later when its sequence was elucidated, research has focused on its peripheral roles in reproduction. Only over the past several decades have researchers focused on what functions Oxt might have in the brain, the subject of this review. Immunohistochemical studies revealed that magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei are the neurons of origin for the Oxt released from the posterior pituitary. Smaller cells in various parts of the brain, as well as release from magnocellular dendrites, provide the Oxt responsible for modulating various behaviors at its only identified receptor. Although Oxt is implicated in a variety of "non-social" behaviors, such as learning, anxiety, feeding and pain perception, it is Oxt's roles in various social behaviors that have come to the fore recently. Oxt is important for social memory and attachment, sexual and maternal behavior, and aggression. Recent work implicates Oxt in human bonding and trust as well. Human disorders characterized by aberrant social interactions, such as autism and schizophrenia, may also involve Oxt expression. Many, if not most, of Oxt's functions, from social interactions (affiliation, aggression) and sexual behavior to eventual parturition, lactation and maternal behavior, may be viewed as specifically facilitating species propagation.

Behavioural studies using temporal and spatial inactivation of the oxytocin receptor.

Oxytocin (Oxt), synthesized in magnocellular neurons of the paraventricular (PVN) and supraoptic (SON) hypothalamic nuclei for transport to and release from the posterior pituitary, is released during parturition and is essential for lactation. Lesser amounts of Oxt are made by smaller cells of the PVN and a few other forebrain nuclei and released into the central nervous system (CNS) to influence various other behaviours. In both the periphery and CNS, Oxt actions are transduced by the oxytocin receptor (Oxtr). Previously, it has been reported that Oxt(-/-) (knockout, KO) mice show a failure of milk ejection and thus are incapable of rearing their offspring. Unexpectedly, these mice have largely normal reproductive and maternal behaviours, perhaps due to compensatory mechanisms through activation of the Oxtr by vasopressin or through development. To examine the specific roles of the Oxtr during development and in particular brain areas, we created conditional Oxtr(-/-) mice in which we could control the spatial and temporal inactivation of the Oxtr. We flanked the neomycin-resistance selectable marker in an Oxtr intron with FRT sites to enable its removal using FLP recombinase. Coding sequence within exons 2 and 3 was flanked by two loxP sites enabling subsequent inactivation of the gene by targeted expression of Cre recombinase. The first Oxtr KO lines we created have either total or relatively specific forebrain elimination. The latter was achieved by crossing the conditional Oxtr line with a transgenic line in which the Camk2a promoter drives expression of Cre recombinase to significant levels beginning 21-28 days after birth, thus eliminating potential compensation for a deleted Oxtr gene during early development. This Cre-expressing line also significantly spares the main olfactory bulb reducing the potential confound of an olfactory deficit. We have investigated various behaviours, most notably social recognition, in both Oxtr KO strains (Oxtr(-/-) and Oxtr(FB/FB)).