Oxytocin (OT) knock-out mice fail to recognize familiar conspecifics after repeated social exposures, despite normal olfactory and spatial learning abilities. OT treatment fully restores social recognition. Here we demonstrate that OT acts in the medial amygdala during the initial exposure to facilitate social recognition. OT given before, but not after, the initial encounter restores social recognition in OT knock-out mice. Using c-Fos immunoreactivity (Fos-IR) as a marker of neuronal activation in this initial encounter, we found similar neuronal activation in the wild-type (WT) and OT knock-out mouse in olfactory bulbs, piriform cortex, cortical amygdala, and the lateral septum. Wild-type, but not OT knock-out mice exhibited an induction of Fos-IR in the medial amygdala. Projections sites of the medial amygdala also failed to show a Fos-IR induction in the OT knock-out mice. OT knock-out, but not WT, mice showed dramatic increases in Fos-IR in the somatosensory cortex and the hippocampus, suggesting alternative processing of social cues in these animals. With site-specific injections of OT and an OT antagonist, we demonstrate that OT receptor activation in the medial amygdala is both necessary and sufficient for social recognition in the mouse.

This study explores the effects of enhancing vasopressin V1a receptor expression in the septum using viral vector-mediated gene transfer on social discrimination and social interactions. Bilateral infusion of an adeno-associated viral vector containing the prairie vole V1a receptor gene (V1aR-AAV) regulated by a neuron-specific enolase promoter resulted in a stable increase in V1a receptor binding density in the rat septum without affecting oxytocin receptor density. Control animals were infused with a vector expressing the lacZ gene. In a social discrimination paradigm, only V1aR-AAV-treated animals succeeded in discriminating a previously encountered from a novel juvenile after an interexposure interval (IEI) of more than 2 h, demonstrating the functional incorporation of the vole V1a receptor in the rat septal circuits underlying short-term memory processes. Microdialysis administration of synthetic vasopressin during the first juvenile exposure, used to mimic intraseptal release patterns of the neuropeptide, produced similar prolongations in recognition (up to an IEI of 24 h) in both V1aR-AAV and control animals. Septal microdialysis administration of a selective V1a, but not oxytocin, receptor antagonist in both groups prevented discrimination even after an IEI of as short as 0.5 h, confirming the specificity of the vole V1a receptor involvement in social discrimination abilities. In addition, active social interactions were found to be increased among V1aR-AAV rats compared to controls. Viral vector-mediated gene transfer provides a valuable tool for studies on the role of localized gene expression on behavioural parameters.

Genetic background may influence an individual's susceptibility to, and subsequent coping strategy for, an acute stressor. When exposed to social defeat (SD), rats bred for high (HAB) or low (LAB) trait anxiety, which also differ in depression-like behavior, showed highly divergent passive and active coping behaviors, respectively. HABs spent more time freezing and emitted more ultrasound vocalization calls during SD than LABs, which spent more time rearing and grooming. Although the behavioral data confirmed the prediction that heightened trait anxiety would make rats more prone to experience stress, adrenocorticotropin and corticosterone were secreted to a higher extent in LABs than in HABs. In the latter, Fos expression upon SD was enhanced in the amygdala and hypothalamic areas compared with LABs, whereas it was diminished in prefrontal and brainstem areas.

The large brain of modern cetaceans has engendered much hypothesizing about both the intelligence of cetaceans (dolphins, whales, and porpoises) and the factors related to the evolution of such large brains. Despite much interest in cetacean brain evolution, until recently there have been few estimates of brain mass and / or brain-body weight ratios in fossil cetaceans. In the present study, computed tomography (CT) was used to visualize and estimate endocranial volume, as well as to calculate level of encephalization, for two fully aquatic mid-late Eocene archaeocete species, Dorudon atrox and Zygorhiza kochii. The specific objective was to address more accurately and more conclusively the question of whether relative brain size in fully aquatic archaeocetes was greater than that of their hypothesized sister taxon Mesonychia. The findings suggest that there was no increase in encephalization between Mesonychia and these archaeocete species.