Monoaminergic-cholinergic relationships and the chemical communication matrix of the substantia nigra and neostriatum

The historical development of histochemical methods for monoamines and chemicals involved with cholinergic function is reviewed. The use of these methods to elucidate neurochemical interactions in the substantia nigra and caudate-putamen complex is then discussed. Three hypotheses accounting for the localization of acetylcholinesterase within and/or on substantia nigra, pars compacta neurons are presented and evaluated:

1. (a) to catabolize acetylcholine released from afferent cholinergic fibers,

2. (b) to catabolize substance P released from some neostriato-nigral axon terminals, and/or

3. (c) to serve as a communication link with nigral vasculature.

Despite experimental evidence in favor of each of these possibilities, none have met with unqualified acceptance. Possible mechanisms and morphologic substrates accounting for dopaminergic-cholinergic, serotonergic-cholinergic, GABAergic-cholinergic, enkephalinergic-cholinergic, and cholinergic-cholinergic interactions in the caudate-putamen complex are discussed. These include synaptic and non-synaptic relationships, dendroaxonic information flow, and mutual regulatory processes.

Keywords: Acetylcholine; Acetylcholinesterase; Substance P; GABA; Enkephalin; 5-Hydroxytryptamine Substantia nigra; Neostriatum; Interactive mechanisms