Ca. Erickson et al., CONTRIBUTION OF SINGLE-UNIT SPIKE WAVE-FORM CHANGES TO TEMPERATURE-INDUCED ALTERATIONS IN HIPPOCAMPAL POPULATION SPIKES, Experimental Brain Research, 107(3), 1996, pp. 348-360
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.