Several criteria have been established for a neurotransmitter, including 1) it is synthesized and released from neurons; 2) it is released from nerve terminals in a chemically or pharmacologically identifiable form; 3) it interacts with postsynaptic receptors and brings about the same effects as are seen with stimulation of the presynaptic neuron; 4) its interaction with the postsynaptic receptor displays a specific pharmacology; and 5) its actions are terminated by active processes (Kandel et al. 2000; Nestler et al. 2001). However, our growing appreciation of the complexity of the central nervous system (CNS) and of the existence of numerous molecules that exert neuromodufatory and neurohormonal effects has blurred the classical definition of neurotransmitters somewhat, and even well-known neurotransmitters do not meet all these criteria under certain situations (Cooper et al. 2001).
Most neuroactive compounds are small polar molecules that are synthesized in the CNS via local machinery or are able to permeate the blood-brain barrier. To date, more than 50 endogenous substances have been found to be present in the brain that appear to be capable of functioning as neurotransmitters. There are many plausible explanations for why the brain would need so many transmitters and receptor subtypes to transmit messages. Perhaps the simplest explanation is that the sheer complexity of the CNS results in many afferent nerve terminals impinging on a single neuron. This requires a neuron to be able to distinguish the multiple information conveying inputs. Although this can be accomplished partially by spatial segregation, it is accomplished in large part by chemical coding of the inputs—that is, different chemicals convey different information. Moreover, as we discuss in detail later, the evolution of multiple receptors for a single neurotransmitter means that the same chemical can convey different messages depending on the receptor subtypes it acts on. Additionally, the firing pattern of neurons is also a means of conveying information; thus, the firing activities of neurons in the brain differ widely, and a single neuron firing at different frequencies can even release different neuroactive compounds depending on the firing rate (e.g., the release of peptides often occurs at higher firing rates than that which is required to release monoamines). These multiple mechanisms to enhance the diversity of responses —chemical coding, spatial coding, frequency coding—are undoubtedly critical in endowing the CNS with its complex repertoire of physiological and behavioral responses (Kandel et al. 2000; Nestler et al. 2001). Finally, the existence of multiple neuroactive compounds also provides built-in safeguards to ensure that vital brain circuits are able to partially compensate for loss of function of particular neurotransmitters.
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