Morphological changes of synapses induced by urinary stimulation in the hamster accessory olfactory bulb

Urinary pheromones are considered to regulate reproductive functions in various rodent species. The effects of urinary stimuli on synaptic plasticity in the accessory olfactory bulb (AOB), which is the primary nucleus of the vomeronasal system, were studied. Adult male hamsters were divided into four groups and each group was exposed to one of the following four materials: distilled water, female hamster urine, female rat urine, and male hamster urine. After 15 days, the sizes of synapses in the glomerular and the mitral/tufted (MT) cell layers of the AOB were measured. The glomerular synapses, located between the axons of sensory cells and the dendrites of MT cells, were larger in the groups exposed to either the female hamster or the female rat urine compared with those for the distilled water and male hamster urine groups. In the MT cell layer, the synapses are of two types: asymmetrical excitatory synapses and symmetrical inhibitory ones. Exposure of adult male hamsters to female hamster urine induced a reduction in the size of asymmetrical synapses, while on exposure to other kinds of urine there was no synaptic change. The sizes of the symmetrical synapses were not changed by any urinary stimulus. The present study revealed that morphological changes of synapses in the AOB were induced by urinary stimuli. Different urines induced different morphological responses. It is suggested that this synaptic plasticity is responsible for regulation of the output of pheromonal information from the AOB to the higher centers of the vomeronasal system.