Extracellular dopamine increases in the neonatal olfactory bulb during odor preference training.

Young rats learn to approach an odor that has been paired with tactile stimulation. This attraction is accompanied by changes in the metabolism and anatomy within the olfactory bulb glomerular layer. In this study, we examined the changes that occur in the olfactory bulb during early olfactory learning, rather than after such pairings have occurred. Specifically, we determined whether the pairing of an odor with tactile stimulation would produce a modified response by olfactory bulb glomerular-layer neurons. To monitor one large subgroup of these neurons during early learning, we used in vivo microdialysis to assess the activity of dopaminergic neurons in the olfactory bulb of postnatal day (PND) 3 rats during simultaneous presentation of odor and tactile stimulation, tactile stimulation alone, odor alone, or clean air alone. Clean air evokes no change in extracellular dopamine (DA), while both odor alone and stroking alone induce prolonged increases in DA peaking at about 200% of baseline. The combination of odor and tactile stimulation, which allows an olfactory preference to be formed, induces a prolonged increase in DA which peaks at about 400% of baseline. The level of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) increases only in pups receiving both odor and tactile stimulation and peaks at about 200% of baseline. With the exception of the pups exposed to clean air, all groups show an increase in homovanillic acid (HVA) of between 150-200% following stimulation. The large and prolonged increase in DA may be linked to the longer term anatomical and physiological changes in the glomerular layer of the bulb that form as a consequence of early olfactory preference training.