Chemical Neuroanatomy of the Mammalian Circadian System

Abstract

The fundamental functional unit of the nervous system is the neuron. Neurons in the central nervous system are organized into networks, or systems, which subserve specialized functions. The communication between neurons within a system, or with other systems, occurs at specialized points of functional contact, synapses. The mechanism of communication is the release of neuroactive substances from the presynaptic element which act on specialized receptor molecules on the postsynaptic element. Thus, neurons may be classified not only with respect to their morphology, location and inclusion in particular functional systems but on the basis of their use of particular neuroactive substances in synaptic transmission. This classification of neurons on the basis of the production of particular neuroactive substances is termed “chemical neuroanatomy.” Over the last 25 years, our knowledge of chemical neuro-anatomy has become very extensive, a fact exemplified by the existence of a Journal of Chemical Neuroanatomy and a long series of volumes which comprise the Handbook of Chemical Neuroanatomy Indeed, at this point we are able to specify at least one neuroactive substance for nearly all of the neuronal components of the mammalian brain. This has been extended recently to the circadian timing system (CTS) as will be described below.