Functional ablation of the olfactory bulb by spreading depression: unit activity changes and transient anosmia

Cortical spreading depression (SD) is widely used to induce functional decortication. Development of a reliable technique for eliciting SD in the olfactory bulb (OB) of rats makes it possible to achieve functional elimination of the first relay of the olfactory pathway. In order to assess the unit activity changes accompanying OBSD, adult male hooded rats (n = 31) were anesthetized with pentobarbital and activity of OB units was recorded with carbon fiber microelectrodes. The predepression activity (12.7 +/- 0.8 Hz) increased up to 35.1 +/- 4.1 Hz during the burst which attained maximum 44 +/- 6 sec after K+ acetate injection and corresponded to the steep depolarization phase of SD slow potential. The burst lasted 20.4 +/- 2.9 sec on the average and was followed by 187 +/- 20 sec of complete silence. Gradual recovery to the predepression level lasted 229 +/- 27 sec. Activity of most units (63%) in the contralateral OB was not changed. Significant reactions of OB neurons to ipsilateral cortical SD found in 57% units were mostly inhibitory (49%). OBSD-induced anosmia was examined in a group of rats (n = 8) with unilateral bulbectomy and a guiding tube implanted into the remaining OB for microinjection of K+ acetate. One week after surgery, the animals were examined in the food-retrieval olfactory test. The microinjection of K+ acetate severely disrupted the food finding behavior in 60% rats during 3-min test. Both electrophysiological and behavioral results indicate that OBSD is a convenient tool for inducing short-lasting anosmia.