There is now considerable evidence for the presence of dopamine receptors in peripheral tissues, as is documented throughout this volume. There has been, however, a reluctance to accept the possibility of a peripheral dopaminergic neuronal system to innervate these receptors for several reasons. Traditionally, students are taught that the autonomic nervous system is composed of only two types of neurons, cholinergic and noradrenergic. It is difficult to break with tradition. Dopamine is present in autonomic nerves, and it represents about 5–10% of the norepinephrine content. Therefore, it is assumed to be solely a precursor for norepinephrine synthesis and not a neurotransmitter. We should recall that dopamine in the spinal cord represents about 5–10% of the norepinephrine, and, until recently, it was considered to be only a precursor for norepinephrine. There is now a vast literature on the presence of dopaminergic neurons in the cord together with speculation about their possible physiological role (Commissiong et al., 1978; Gentleman et al, 1981; Commissiong and Neff, 1979). What percentage of dopamine should be found in a nerve or tissue to raise suspicion that dopaminergic neurons are present within a structure? Is the percentage of dopamine present meaningful if it is concentrated in a few neurons and nerve endings? Moreover, the quantity of amine stored in a nerve ending may not be as important as its rate of formation and release onto receptors.
KeywordsDopaminergic Neuron Dopamine Receptor Sympathetic Ganglion Superior Cervical Ganglion Ganglionic Transmission
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