CONTRIBUTION OF SINGLE-UNIT SPIKE WAVE-FORM CHANGES TO TEMPERATURE-INDUCED ALTERATIONS IN HIPPOCAMPAL POPULATION SPIKES

Brain temperature changes accompany exploratory behavior and profoundl y affect field potential amplitudes recorded in hippocampus. The wavef orm alterations in fascia dentata include a reduction in population sp ike area, which might be explained by fewer granule cells firing in re sponse to a given stimulus or by an alteration in the size or shape of the individual action potentials. This study was designed to assess t hese alternate possibilities. In experiment 1, changes in the shape an d firing rates of single cells recorded in the fascia dentata of awake rats were compared with changes in the population spike before and af ter a bout of activity. Single-unit amplitudes were significantly redu ced following exploration, and there was a small (< 3%) change in unit spike-width. These changes, however, were insufficient to account, in a linear fashion, for the entire decline in the population spike. In experiment 2, radiant heat was used to manipulate brain temperature in anesthetized rats. As in the first experiment, the magnitude of chang e in the extracellular units was much smaller than the change in popul ation spike amplitude. The spontaneous firing rates of the cells were also modified by brain temperature changes. In experiment 3, the polys ynaptic, contralateral commissural response (which covaries with chang es in the ipsilateral population spike at a fixed temperature) was mea sured as a function of either exploratory behavior or radiant heat. Th e relationship between the ipsilateral population spike and correspond ing polysynaptic commissural response was altered following exploratio n and passive warming in a manner consistent with a reduction in net g ranule cell output, reduced transmission efficacy through the polysyna ptic circuit, or a combination of these. Taken together these data sug gest that at least two factors contribute to temperature-dependent cha nges in the perforant path-evoked population spikes recorded in the fa scia dentata: changes in the size of individual action potentials and alterations in discharge of action potentials in response to a given s timulus.