TCurrent and Absence Seizures

The acutely dissociated cell, with its advantages for voltage-clamp studies of ionic currents, has been useful for rat subiculum rat CA1

rat subiculum rat CA1

50 ms

FIGURE 1 Examples of acutely dissociated cells and their electrophysiologic discharge patterns. Top row shows images of pyramidal neurons from rat subiculum (left) and hippocampal CA1 (right). Below the cell images are examples of current-clamp responses to small hyperpolarizing and depolarizing current pulses to illustrate differences in the intrinsic firing properties of these cells. Note that the subicular cell (left) discharge consists of an initially complex action potential, which is composed of prominent sodium- and calcium-dependent components (pharmacologic data to dissect out these currents, using tetrodotoxin and cadmium salts, are not shown). The initial response is followed by simpler action potentials that more closely resemble the action potentials generated by the CA1 cell (right). Size, voltage, and time calibrations are given in the figure.

50 ms

FIGURE 1 Examples of acutely dissociated cells and their electrophysiologic discharge patterns. Top row shows images of pyramidal neurons from rat subiculum (left) and hippocampal CA1 (right). Below the cell images are examples of current-clamp responses to small hyperpolarizing and depolarizing current pulses to illustrate differences in the intrinsic firing properties of these cells. Note that the subicular cell (left) discharge consists of an initially complex action potential, which is composed of prominent sodium- and calcium-dependent components (pharmacologic data to dissect out these currents, using tetrodotoxin and cadmium salts, are not shown). The initial response is followed by simpler action potentials that more closely resemble the action potentials generated by the CA1 cell (right). Size, voltage, and time calibrations are given in the figure.

evaluating the role of the transient, low-threshold (T-type), voltage-gated calcium current in absence seizures (Coulter et al., 1989; Huguenard et al., 1991). "Rebound" bursting in thalamic relay neurons was suppressed by ethosuximide (a drug commonly used to treat absence seizures), and the acutely dissociated cell preparation was used to demonstrate that ethosuximide selectively blocks the T-current in these thalamic cells.

A similar study of the T-type calcium current showed that this current is prominent in hippocampal pyramidal cells before postnatal day 30, but it disappears after this age. It is noteworthy that ethosuximide did not appear to affect the T-currents in hippocampal CA1 pyramidal cells (Thompson and Wong, 1991), suggesting that different subtypes of T-channels are present in thalamic versus hippocampal neurons. These studies illustrate how the acutely dissociated cell preparation can be used for the study of specific drug actions.

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