Single-unit analysis of postnatal olfactory learning: modified olfactory bulb output response patterns to learned attractive odors.

Neonatal rats learn to approach odors associated with stimulation normally provided by their mother. The present report describes changes in olfactory bulb single-unit activity following olfactory learning in young rats. Rat pups were exposed from postnatal day 1 to 18 to either (1) peppermint-scented air while receiving tactile stimulation (Pepp-Stroked), (2) peppermint-scented air with no tactile stimulation (Pepp-Only), (3) clean air and tactile stimulation (Stroked-Only), or (4) clean air and no tactile stimulation (Naive). On day 19, single-unit activity was recorded from mitral/tufted cells in urethane-anesthetized, freely breathing pups in response to either peppermint or a novel orange odor. Mitral/tufted cell response patterns to peppermint were significantly altered in Pepp-Stroked animals compared to control pups. Peppermint exposure alone, not associated with tactile stimulation (Pepp-Only), did not affect subsequent single-cell response patterns to that odor. In addition, the modification of response patterns was specific to peppermint and was not associated with a change in respiration rate. Furthermore, Pepp-Stroked pups had a relative behavioral preference for peppermint on day 19 compared to control pups. These results demonstrate that postnatal olfactory learning selectively modifies the subsequent response patterns of olfactory bulb output cells to the attractive odor. Furthermore, these results indicate that the initial coding of an odor's attractive value occurs within the olfactory bulb.