Effects of cooling on guinea pig olfactory cortex maintained in vitro.

Electric activities from brain slices of guinea pig olfactory cortex were studied during gradual cooling without a temperature gradient. At normal temperature the potential evoked by stimulation of the lateral olfactory tract consists of an initial spike (IS), a negative (N), and a positive (P) potentials. The IS potential has been considered to be presynaptic in origin and the latter two postsynaptic. During cooling from 37 to 15 degrees C, the amplitude of both the IS and the N potentials increased to attain the maximum value (10-36% increase) at about 32 degrees C, then decreased gradually and disappeared at around 15 degrees C. On the other hand, the durations of the IS and the N potentials were prolonged gradually on cooling from 37 to 17 degrees C. Most of the olfactory cortical neurons responded with one or two firings to the tract stimulation. The firing numbers increased on cooling down to about 32 degrees C and declined on further cooling, which corresponds with the behavior of the N potential. All these effects were reversible on rewarming. Augmentation of the N potential might depend primarily on the increase in amplitude of the IS potential and secondarily on the increase of amount of liberated transmitter substance and/or the delayed inactivation process of the transmitter action.