Astrocyte-derived growth factors affect neuronal calcium homeostasis: Roles in neuron survival and plasticity

Abstract

This chapter reviews recent concepts of chemical communication between excitable cells, with particular emphasis on the co-existence of transmitter substances in single nerve terminals. While the classical view is that the autonomic nervous system (ANS) consists largely of antagonistic cholinergic and adrenergic nerves, about fourteen putative neurotransmitters in the ANS have been proposed in the past few years, including various monoamines, polypeptides, purines, and amino acids. Modulatory transmitter mechanisms have also been recognized, including prejunctional inhibition or enhancement of transmitter release, postjunctional modulation of transmitter action, and the secondary involvement of locally synthesized hormones and prostaglandins. The co-transmitters always have synergistic actions on postjunctional effector cells, but two different mechanisms are postulated. (1) If both substances are stored in the same vesicles, release is closely parallel at all impulse frequencies. Upon release, the co-transmitter, in addition to having a direct action on postjunctional cells, may facilitate the action of the other transmitter and act as an inhibitor of its release. (2) If the two substances are stored in separate and different vesicle types, differential release is possible at different impulse frequencies. The peptides released at higher frequencies modulate the role of the classical transmitter, by both prejunctional enhancement of its release and postjunctional facilitation of its action.