Time course of reinnervation of the olfactory bulb after transection of the primary olfactory nerve in the pigeon.

Horseradish peroxidase (HRP) histochemistry was used to study the time course of reinnervation of the pigeon olfactory bulb after simple transection of the primary olfactory nerve. At selected intervals (9, 13, 29, 61 and 93 days) after transection of the right olfactory nerve, a concentrated solution of HRP was instilled in both nasal cavities. Intracarotid perfusion was performed 3 days after nasal instillation of HRP and 40-microns sections of olfactory bulb processed with the tetramethylbenzidine (TMB)-HRP histochemical protocol to visualize olfactory receptor axon terminals reinnervating the glomerular layer of the bulb. The total area of reinnervation of four representative regions in the bulb of the transected side were compared with that on the control bulb. The area of innervation by newly reconstituted olfactory axons was approximately 17% of control values at the 12-day posttransection time interval. A progressive increase in the area of reinnervation was observed over time. Reinnervation of the right bulb was approximately 70% complete at the 32-day posttransection time interval and indistinguishable from the left control bulb at the 64- and 96-day posttransection time intervals. A uniform pattern of reinnervation of different bulb regions was observed at all time intervals. These results indicate that the peripheral olfactory system of the pigeon is capable of complete reconstitution after nerve transection. Our findings should be useful in guiding functional comparisons of normal and newly reconstituted peripheral olfactory systems in the pigeon.