Vasopressin (VP) released within the brain has been shown to be involvement in social recognition. Previously, we reported populations of VP neurons in different areas of the rat olfactory system (OS), and showed that blocking VP actions in the olfactory bulb impairs the social recognition abilities of rats (1). We also reported that VP neurons in the male rat anterior olfactory nucleus (AON) show an increase in the expression of the immediate early gene product, EGR-1, when exposed to juvenile male rat, but not in response to predator odour, suggest that VP in the olfactory system may be selectively involved in the coding of social odour information (2). The aim of this study was to understand how inputs to the OS influence VP neurons in different parts of the OS. Olfactory information is relayed to the AON via inputs from mitral and tufted cells of the olfactory bulb. In these experiments, we stimulated the lateral olfactory tract (LOT), which is formed by the axons of the mitral and tufted cells electrically, and studied the pattern of induction of the immediate early gene c-fos by immunohistochemical localisation of the protein product Fos in wild type Sprague-Dawley (WT) and transgenic rats in which eGFP is expressed under the control of the VP promoter (VP-eGFP) (3). Rats were anaesthetised with Sagatal (60mg/kg, i.p) and a bipolar stainless steel electrode was lowered into the LOT (4mm lateral and 3.2mm anterior to bregma). Two stimulation protocols, 50Hz/10min, and 200Hz/10s (0.1 ms matched biphasic pulses 1mA peak-to-peak), were used. At the end of each experiment brains were processed for Fos and eGFP immunohistochemistry. In the main olfactory bulb of WT rats (control: n=4; 50Hz/10min: n=13), a significant increase in the number of Fos-positive cells was observed after prolonged stimulation, 50Hz/10min (mitral cell layer, control: 20.5±1.7, stimulation: 61.4±4.1 nuclei/section; granule cell layer, 34.5±5.4 vs. 85.7±5.9 nuclei/section; Mean±SEM, P<0.0001; Mann-Whitney Rank Sum Test). Areas of the brain known to receive strong olfactory input, such as the piriform cortex, showed also significant increase in the Fos expression (control: 14.1± 3.6, stimulation: 119.6±18.2 nuclei/section, P<0.0001). In VP-GFP rats (control: n=4, 50Hz/10min: n=5), we hardly detected Fos positive VP expressing cells in the OS in either the control or the stimulated group. Taken together, we established the assay system to antomically determine the effect of LOT activation. It appears that LOT stimulation activates Fos expression in all regions that are directly innervated by mitral cells, but it does not activate Fos expression in VP cells in either the olfactory bulb or the AON, indicating that neither population is activated by direct inputs from mitral or tufted cells, at least in a way coupled to Fos expression.