Cyclic nucleotides in spinal cells.

The most striking effects of intracellular injections of adenosine 3'5'-cyclic monophosphate (cAMP) into spinal mononeurons in cats are a speeding-up of the action potential, both its rising and falling phase, and a potentiation of the after-hyperpolarization; the latter porbably indicates a marked enhancement of Ca2+ influx. In this respect, cAMP and guanosine 3'5'-cyclic monophosphate (cGMP) have similar actions, though cAMP appears to be more potent. It is suggested that through this mechanism, cyclic nucleotides may play an important role in synaptic facilitation. Changes in resting membrane potential and resistance are less conspicuous or predictable. By contrast, both agents, when injected into unresponsive cells, presumed to be neuroglia, regularly cause a drop in membrane resistance; this is associated with hyperpolarization and therefore likely to reflect an increase in membrane K+ conductance.