Characteristics of long-duration inhibitory postsynaptic potentials in rat neocortical neurons in vitro

1. The characteristics of long-duration inhibitory postsynaptic
potentials (l-IPSPs) which are evoked in rat frontal neocortical
neurons by local electrical stimulation were investigated with
intracellular recordings from anin vitro slice preparation. 2.
Stimulation with suprathreshold intensities evoked l-IPSPs with
typical durations of 600–900 msec at resting membrane potential.
Conductance increases of 15–60% were measured at the peak amplitude
of l-IPSPs (150–250 msec poststimulus). 3. The duration of the
conductance increases during l-IPSPs displayed a significant
voltage dependence, decreasing as the membrance potential was
depolarized and increasing with hyperpolarization. 4. The reversal
potential of l-IPSPs is significantly altered by reductions in the
extracellular potassium concentration. Therefore it is concluded
that l-IPSPs in rat neocortical neurons are generated by the
activation of a potassium conductance. 5. l-IPSPs exhibit
stimulation fatigue. Stimulation with a frequency of 1 Hz produces
a complete fatigue of the conductance increases during l-IPSPs
after approximately 20 consecutive stimuli. Recovery from this
fatigue requires minutes. 6. l-IPSPs are not blocked by bicuculline
but are blocked by baclofen.