The effects of cryogenic blockade of the centrifugal, bulbopetal pathways on the dynamic and static response characteristics of goldfish olfactory bulb mitral cells

Summary

The responses of single goldfish olfactory bulb mitral cells were studied by extracellular recordings before and during cryogenic blockade of the efferent, centrifugal pathways in the ipsilateral olfactory tract. In each experiment the same odour was presented 40 times before and then 40 times during cooling. Each stimulus period (at least 30 s) was preceded by a stimulus-free interval (at least 30 s), during which a steady stream of tap water was applied. These procedures allow the investigation of activity changes of single neurons and of cell ensembles using statistical methods. i) In comparison with the pre-cooling activity, cooling of the efferent pathways did not cause a generalized disinhibition in mitral cell responses. Significant disinhibitory, significant inhibitory and indifferent effects occurred in about the same proportion during repetitive water and odour applications. ii) Abrupt or slow changes of single mitral cell discharge patterns during the 40 water and odour applications were observed before and during blocking of the efferent fibre systems: These pattern changes are therefore not necessarily a consequence of the efferent signals, and may thus have been a result of intrabulbar plasticity. iii) The most notable effect of efferent fibre blockade across all experiments was a significant (Wilcoxon-ranktest, P = 0.01) decrease of the signal to noise ratio i.e., the ratio between the activity during the “spontaneous” (water) and the stimulus (odour) phase, which could be demonstrated for both the phasic (immediately after stimulus onset) and tonic (during long term stimulation) components of the mitral cell responses.