Figure 10.

A, examination of the effect of acute changes in extracellular glucose levels on the excitability of cholinergic BF neurones. The illustrated recording was made from a cell maintained in culture for 6 days using the perforated-patch recording technique. Downward deflections are membrane voltage responses to current pulses delivered at 0.5 Hz (100 pA, 200 ms) used to monitor changes in input resistance. The threshold current required to evoke spike discharge was determined just prior to changing the extracellular glucose concentration (upward deflections). Insets show firing (200 ms duration pulses) and the threshold current under each condition. Note, prolonged current stimulation (0.5–1 s evoked a sustained slow rate of firing). B, the effect upon excitability of exposure to sodium azide (1 mm). Downward deflections are membrane voltage responses to 50 pA, 200 ms duration current pulses delivered at 1 Hz. On exposure to sodium azide membrane potential typically depolarized transiently before slowly hyperpolarizing. The hyperpolarization was associated with a fall in input resistance (R_in) and inhibited by tolbutamide confirming the involvement of K_ATP channels (data not shown). In the cell shown, R_in fell from 340 to 60 mΩ whilst the current required to evoke an action potential increased from 130 to 850 pA. Asterisk marks the region of the voltage trace where the change in membrane potential was nulled by direct current injection. From this it can be see that the majority of the fall in input resistance was the result of sodium azide-induced channel opening rather than as a secondary consequence of membrane hyperpolarization.