Early odor preference training increases olfactory bulb norepinephrine.

In vivo microdialysis sampling of the olfactory bulbs of awake rats on PND 3 revealed that olfactory stimulation alone does not alter extracellular norepinephrine (NE) levels. Tactile stimulation that is designed to mimic maternal interactions with the young does increase bulb NE and the combined odor and tactile stimulation further increases NE levels. These data are consistent with a critical role for NE in the development of early olfactory preferences in infant rats, induced by odor/tactile stimulation pairings. PND 10 odor/tactile stimulation does not evoke an increase in NE, data consistent with the finding that odor preference training of this kind is ineffective after about the first week of life. Oral infusion of milk on PND 3 also increases bulb NE, a finding consistent with the role of milk as a reinforcing stimulus for the development of early olfactory preferences. Finally, infusion of potassium into the bulb on PND 3 produces a rapid increase in NE, indicating a local neuronal origin of the NE in response to stimulation.