Journal of neurophysiology
-
In vitro slice experiments were undertaken in adult rats to investigate the physiological origins of a chronic epileptic condition that was initiated in infancy. A unilateral injection of a minute quantity of tetanus toxin into hippocampus on postnatal day 10 produced a severe convulsive syndrome characterized by brief but repeated seizures that lasted for 5-7 days. Hippocampal slices were then taken from these rats in adulthood because at this time previous studies have shown the occurrence of electrographic and behavioral seizures. ⋯ In contrast, slices from control rats produced only brief 100-ms network bursts. These results suggest that a change in excitability within CA3C recurrent excitatory networks likely contributes to seizures in chronically epileptic rats. However, at the same time, this hyperexcitability is controlled to an important degree by functional GABAA-mediated synaptic inhibition.
-
Ionic mechanisms underlying low-threshold (LTO) and high-threshold (HTO) oscillations occurring in a class of spiny neurons within the basolateral amygdaloid complex (see companion paper) were investigated in slice preparations of the guinea pig amygdala in vitro. LTOs were abolished through local application of tetrodotoxin (TTX, 10-20 microM) or a decrease in the extracellular sodium concentration ([Na+]o) from 153 to 26 mM, whereas HTOs were more readily elicited under these conditions. The effects of TTX and low [Na+]o were accompanied by a hyperpolarizing shift of the membrane potential by 3 +/- 1 mV and a decrease in apparent input resistance by 14 +/- 11 MOmega. ⋯ It is concluded that a TTX-sensitive Na+ conductance and the M current contribute to generation of the LTOs, although their exact role in rhythmogenesisremains to be determined. HTOs seem to largely depend on a functional coupling between high-voltage-activated Ca2+ conductances, a Ca2+-activated K+ current presumably carried through BKCa channels, and additional voltage-dependent K+ conductances. In functional terms, the HTOs are important in determining spike frequency adaptation toward a slow-rhythmic firing pattern during maintained depolarizing influence.
-
We evaluated the role of the sodium/glutamate transporter at the synaptic terminals of cone photoreceptors in controlling postsynaptic response kinetics. The strategy was to measure the changes in horizontal cell response rate induced by blocking transporter uptake in cones with dihydrokainate (DHK). DHK was chosen as the uptake blocker because, as we show through autoradiographic uptake measurements, DHK specifically blocked uptake in cones without affecting uptake in Mueller cells. ⋯ This suggests that the transporter was both voltage dependent and robust enough to modulate glutamate concentration. The transporter must be at least as effective as diffusion in removing glutamate from the synapse because there is only a very small light response once the transporter is blocked. The transporter, via its voltage dependence on cone membrane potential, appears to contribute significantly to the control of postsynaptic response kinetics.