Neural regeneration and functional reconnection following olfactory nerve transection in hamster

The olfactory sensory neurons in the vertebrate nervous system are unique in that they undergo continuous neurogenesis and replacement. Anatomical studies have shown that transection of the olfactory nerves leads to a degeneration of sensory neurons followed by a neurogenesis and replacement with newly formed cells. Replacement neurons grow axonal processes that are capable of reestablishing morphological connections with cells in the olfactory bulb. To determine the functional capacity of these anatomical reconnections, single unit responses to odor stimuli were recorded from cells in the olfactory bulb following recovery from unilateral olfactory nerve transection. A total of 56 cells were studied, taken from hamsters with recovery times of 4,35,60,90,120,180 and 270 days. At day 4, although there was spontaneous activity recorded from cells on the experimental side (n = 10), they did not respond to stimulation of the olfactory epithelium with odors. Control cells (n = 9) from the unoperated side of the same animals showed normal odor responses. By day 35, some of the cells tested on the experimental side responded to odor stimulation, indicating that connections had been reestablished with sensory neurons. With longer recovery times, an increasing percentage of cells responded to odor stimuli. In addition, concentration response functions showed that cells were capable of signaling differences in stimulus intensity. The response of cells to four odors (amyl acetate, 1-butanol, ethyl acetate and ethyl butyrate) showed differences in odor selectivity, suggesting their ability to discriminate among odors.(ABSTRACT TRUNCATED AT 250 WORDS)