Table 2.2. Criteria which must be fulfilled for a substance to be considered as a transmitter

• It should be present in a nerve terminal and in the vicinity of the area of the brain where it is thought to act.

• It should be released from the nerve terminal, generally by a calcium-dependent process, following stimulation of the nerve.

• The enzymes concerned in its synthesis and metabolism should be present in the nerve ending, or in the proximity of the nerve ending.

• It should produce a physiological response following its release by activating a postsynaptic receptor site. Such changes should be identical to those seen following the local application of the transmitter (e.g. by micro-ionophoresis).

• Its effects should be selectively blocked by a specific antagonist and mimicked by a specific agonist.

These criteria should be regarded as general guidelines, not specific rules.

four different transmitters can occur in the same neuron. In the vertebrate there is also increasing evidence from the seminal studies of Hockfelt and colleagues in Stockholm that some neurons in the central nervous system can also contain more than one transmitter. Such neurons appear to contain a peptide within monoamine-containing terminals. Peptide transmitters (usually referred to as neuropeptides) are contained in specific types of storage vesicles. Thus Dale's Law has to be modified to allow for the presence of neuropeptides in amine-containing nerve terminals, whose function is to act as a neuromodulator of the amine when it acts on the postsynaptic receptor.

There are several criteria which must be fulfilled for a substance to be considered as a transmitter. These are given in Table 2.2.

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