The pharmacology of receptors present on squid giant axon Schwann cells

The adaxonal Schwann cells of the squid giant axon possess a range of receptors for classical neurotransmitters. When activated, these receptors either alter the membrane potential of the Schwann cell or modulate its responsiveness to other inputs. A complex cascade of events is involved in signalling between the giant axon and its surrounding Schwann cells, involving the release of glutamate from the axon, to act on receptors on the Schwann cells, which mediate a release of endogenous acetylcholine. The latter molecule then feeds back to activate nicotinic cholinergic receptors on the Schwann cells, which initiate the production of a long-lasting hyperpolarization of the Schwann cell membrane. The modulation of this cascade of events by a range of biogenic amines and neuropeptides is reviewed. Recent evidence for the presence of both N-methyl-D-aspartate (NMDA) and non-NMDA type glutamate receptors on the Schwann cells is discussed in this chapter, together with evidence for the presence of an active uptake system for L-glutamate in this preparation. The activation of the NMDA receptors associated with the slow depolarization develops slowly during a prolonged exposure to glutamate and initiates a longlasting change in the responsiveness of the Schwann cell to further applications of glutamate. The co-release of acetylcholine and a vasoactive intestinal polypeptide-like peptide from the Schwann cells is also considered. Finally, the physiological roles of the different receptor systems present on the Schwann cells is discussed.