Oxytocin in the medial amygdala is essential for social recognition in the mouse.

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.

Social amnesia in mice lacking the oxytocin gene.

The development of social familiarity in rodents depends predominantly on olfactory cues and can critically influence reproductive success. Researchers have operationally defined this memory by a reliable decrease in olfactory investigation in repeated or prolonged encounters with a conspecific. Brain oxytocin (OT) and vasopressin (AVP) seem to modulate a range of social behaviour from parental care to mate guarding. Pharmacological studies indicate that AVP administration may enhance social memory, whereas OT administration may either inhibit or facilitate social memory depending on dose, route or paradigm. We found that male mice mutant for the oxytocin gene (Oxt-/-) failed to develop social memory, whereas wild-type (Oxt+/+) mice showed intact social memory. Measurement of both olfactory foraging and olfactory habituation tasks indicated that olfactory detection of non-social stimuli is intact in Oxt-/- mice. Spatial memory and behavioural inhibition measured in a Morris water-maze, Y-maze, or habituation of an acoustic startle also seemed intact. Treatment with OT but not AVP rescued social memory in Oxt-/- mice, and treatment with an OT antagonist produced a social amnesia-like effect in Oxt+/+ mice. Our data indicate that OT is necessary for the normal development of social memory in mice and support the hypothesis that social memory has a neural basis distinct from other forms of memory.

Effects of prefrontal cortical lesions on neuropeptide and dopamine receptor gene expression in the striatum-accumbens complex.

In the rat, neurochemical, behavioral, and anatomical investigations suggest that medial prefrontal cortical input modulates the activity of the basal ganglia. To understand how prefrontal dysfunction might alter striatal-accumbens function, in situ hybridization histochemistry with S35-labeled oligonucleotide probes was used to assess changes in striatal-accumbens gene expression following bilateral excitotoxic ibotenic acid (IA) lesions of the rat medial prefrontal cortex. Quantitative densitometry was used to measure changes in mRNA levels for preproenkephalin A (ENK), D1 dopamine receptor, protachykinin (SubP), glutamic acid decarboxylase (GAD65), and D2 dopamine receptor. No differences were found between sham and lesion groups for ENK, D1, SubP, or GAD65 mRNA levels in the striatum or nucleus accumbens (NAC). D2 receptor mRNA levels were, however, significantly higher in the dorsomedial striatum and in the core area of the NAC of the lesioned rats. Although the functional significance of increased D2 mRNA is unclear, these findings demonstrate that glutamate mPFC projections modulate gene expression in relatively regionally-localized subcortical neuronal populations.

Effects of chronic naloxone administration on vacuous chewing movements and catalepsy in rats treated with long-term haloperidol decanoate.

Most antipsychotic medications produce motoric side effects, including parkinsonism and tardive dyskinesia (TD). Correlates of these behaviors in rats (catalepsy and vacuous chewing movements, respectively) were used as a model to assess the usefulness of chronic naloxone administration in symptom reduction. Previous studies have suggested that increased neurotransmission in the endogenous opioid system modulates neuroleptic-induced motoric side effects. Rats were treated with haloperidol decanoate or vehicle for 27 weeks, and withdrawn for 30 weeks. Subsequently, naloxone (0.5 to 2.0 mg/kg SC twice daily) was given for 5 weeks. Long-term haloperidol treatment produced a syndrome of vacuous chewing movements (VCMs) that persisted during the drug withdrawal period. Catalepsy developed rapidly and also persisted. Naloxone treatment had little effect on VCMs but increased catalepsy scores in both haloperidol and vehicle treated groups. Naloxone reduced rearing and grooming in haloperidol rats while increasing these measures in vehicle treated rats. The results indicate that neuroleptic-induced motoric side effects are not reversed by naloxone in rats. Furthermore, they suggest that increased opioid neurotransmission may not underlie the expression of VCMs. This does not rule out the possibility that endogenous opioid system may be involved in the development of VCMs. To the extent that this animal model is valid, naloxone may not be effective in treating TD and neuroleptic-induced parkinsonism in humans.