Actions of procaine on specific nociceptive cells in leech central nervous system

The effects of the local anesthetic, procaine, on the electrophysiological properties of the leech neurons responding to nociceptive stimuli (N cells) were examined in isolated segmental ganglia in Macrobdella . In the N cell situated laterally in ganglia 7 to 19, procaine produced a depolarization; whereas in the N cell situated medially, it caused a hyperpolarization. These changes in membrane potential were accompanied by a decrease in input resistance, persisted in solutions containing 20 mM MgCl2, and were reversible after drug washout. The depolarization induced by procaine in the lateral N cell was abolished in Na-free solutions. The hyperpolarization produced by procaine in the medial N cell persisted in low chloride solutions and was enhanced by about 30% when the K concentration was reduced to one-tenth of its control value. Therefore, it seems likely that an increase in the resting K conductance was a contributing factor to this hyperpolarization. Procaine greatly prolonged the action potential of the lateral but not the medial N cell. This effect was not solely a consequence of the membrane potential change produced by the drug. Ganglia 5, 6, 20, and 21 contained two rather than four cells with N-like properties. These neurons were N-like by virtue of the shape of their action potential and their morphological similarities to the N cells found in ganglia 7 to 19, as well as by their selective sensitivities to nociceptive mechanical stimulation of the skin. The cells in ganglia 5 and 6 responded to gut stimulation, as did the medial N cells in ganglia 7 to 19.(ABSTRACT TRUNCATED AT 250 WORDS)