Figure 7. Integration of GABAergic IPSPs by a time-dependent inward rectifying conductance in SPN.

Aa, whole-cell current-clamp recordings from an SPN, held close to threshold for firing (V_h − 40 mV), showing stimulation of the iLF elicited an IPSP that gave rise to rebound action potential firing. Ab, in the same neurone, repetitive high-frequency stimulation of the cLF evoked fIPSPs which summated, followed by a rebound excitation and a burst of action potentials. Ac, recordings from another SPN showing a spontaneous IPSP which gave rise to rebound action potential firing. Ad, high frequency stimulation of the iLF elicited fIPSPs that similarly gave rise to a burst of rebound action potential firing. B, samples of a continuous whole-cell current clamp recording showing repetitive high frequency (10 Hz) stimulation of the iLF, evoked fast IPSPs which summated, followed by a rebound excitation and action potential firing. The IPSP, rebound excitation and associated action potential firing were all insensitive to strychnine, but were completely and reversibly blocked following subsequent application of bicuculline. C, samples of a continuous whole-cell current-clamp recording showing membrane responses to injection of rectangular-wave hyperpolarizing current pulses at holding potentials of −35 mV (top) and −50 mV (bottom). At the resting level of the cell (−50 mV), the SPN was characterized by a transient outward rectification observed as a delayed return to rest of the membrane response to hyperpolarization. At membrane potentials close to threshold (V_hold − 35 mV), a marked decrease in neuronal input resistance was apparent and a time-dependent inward rectification and rebound excitation at the termination of the response revealed.