Sensitivity and selectivity of neurons in the medial region of the olfactory bulb to skin extract from conspecifics in crucian carp, Carassius carassius.

To examine the functional subdivision of the teleost olfactory bulb, extracellular recordings were made from the posterior part of the medial region of the olfactory bulb in the crucian carp, Carassius carassius. Bulbar units classified as type I or type II were frequently and simultaneously encountered at a recording site. Type I units displayed a diphasic action potential (AP) with a relatively small amplitude, a short duration (rise time approximately 1 ms) and high spontaneous activity (2.5 per s). Type II units exhibited an AP with a rise time of approximately 1.8 ms and low spontaneous activity (1.5 per s). The AP of this latter unit was nearly always followed by a slow potential, a characteristic diphasic wave with a rise time of approximately 5 ms. Chemical stimulation of the olfactory organ with a graded series of conspecific skin extract induced an increased firing of the type I units. During the period of increased activity of the type I units, the activity of the type II units was suppressed. Stimulation with nucleotides, amino acids and taurolithocholic acid did not induce firing of the type I units of the posterior part of the medial region of the olfactory bulb. These results indicate that the posterior part of the medial region of the olfactory bulb is both sensitive to and selective for skin extract from conspecifics, which has been shown to be a potent stimulus inducing alarm behaviour. The results of the present study indicate that recording single unit activity from a particular region of the olfactory bulb is a suitable method for isolating pheromones or other chemical signals that induce specific activity in the olfactory system. The projection of the neurons categorized as type II was determined by antidromic activation of their axons by electrical stimulation applied to the medial bundle of the medial olfactory tract. The anatomical basis of the type I and type II units in the fish olfactory bulb is discussed.