Rats--Physiology

Unlike most neurons in the adult nervous system, olfactory receptor neurons (ORN),
found in the olfactory epithelium (OE), continually turnover in the adult rat. These
neurons project their axons to the olfactory bulb which is their central target. The
present study eliminated target neurons in the bulb using N-methyl-D-aspartate
(NMDA) to examine the effects of target loss on ORN survival and maturation. We
compared the effects of the NMDA lesion to bulbectomy, a permanent surgical
removal of the bulb, which simultaneously causes damage to ORN axons. We found
that unlike bulbectomy, large numbers of dying OE cells were not observed at any
time after the lesion. The number of immature neurons increased relative to the
control side, and the number of mature neurons also slightly increased with time
following NMDA lesion. Survival of ORNs does not seem to be significantly altered
in the absence of its target.

This research was designed to assess the extent to which olfactory
cues, both main olfactory and vomeronasal, are important in the
expression of independent ingestion in 6-day-old rat pups.
Independent ingestion in young rat pups involves the intake of
large volumes of milk (delivered throught an intraoral cannula)
and is accompanied by dramatic behavioral activation. It was found
that behavioral activation in response to milk infusions or milk
odor was eliminated in pups deprived of main olfactory input (by
nasal lavage with ZnS04), or vomeronasal input (by vomeronasal
nerve sections). Similarly, after vomeronasal deafferentation and
ZnS04 lavage pups cease to probe and become active in response to
milk infusions and milk odor accompanied by water infusions. The
results suggest that behavioral activation may be mediated in a
similar fashion by both the main and vomeronasal olfactory system.

Rats with lesions of the ventromedial aspect of the internal
capsule in the vicinty of the entopeduncular nucleus (EP) showed a loss
of forelimb placing (chin, contact and visual) in the contralateral
limb. Spreading depression induced by instillation of KCl (25%) to the
cortex contralateral to the lesion brought back placing in the affected
limb and abolished placing in the normal limb. Within 24 hours the
pre-spreading depression state returned and the impaired limb no longer
placed while the normal limb recovered function. In contrast, KCl on
the ipsilateral cortex did not reinstate placing. These results
suggest that the loss of placing following lesions of the EP are due to
tonic inhibition from the cortex contralateral to the lesion. Sensory
summation was evident during the early recovery period when placing was
accomplished only if two kinds of stimuli were provided
simultaneously. Forelimb placing recovered to its pre-lesion state.