The nigrostriatal and tuberoinfundibular-hypophyseal dopaminergic systems subserve different functions, the initiation of motor behaviors and the release of hormones from the pituitary, respectively. These differences are related to specific anatomical characteristics, the most important of which is the postsynaptic element. Although dopamine serves as the transmitter for both systems, the manner in which it reaches the receptive site in each system is fundamentally different. The nigrostriatal neurons form synaptic contacts with other neurons. Thus, the terminal and receptor are in close proximity. Dopamine released from the tuberoinfundibular system, on the other hand, must be transported the length of the infundibulum to reach the receptor, which is on a secretory cell rather than on a neuron. The postsynaptic element of the tuberohypophyseal system has not been as completely defined, although it may consist of neurosecretory cells. Not surprisingly, these systems, which are functionally and anatomically different, are regulated by different mechanisms. A number of afferent inputs can influence nigrostriatal activity at the level of the cell bodies in substantia nigra as well as the terminals in the striatum. In addition to these extrinsic influences, nigrostriatal activity is under tonic homeostatic control. This autoregulation occurs via neuronal feedback loops and/or dopaminergic autoreceptors located at terminals and possibly on neuronal perikarya or dendrites. Nigrostriatal activity is thus maintained within limits despite physiologically or pharmacologically-induced fluctuations in impulse traffic. Because of technical diffculties less known about regulation of tuberoinfundibular-hypophyseal neurons. Certainly, extrinsic neurons must influence their activity, but to date only hormonal influences have been investigated. Prolactin appears to be the major hormonal feedback regulator of this system. Thus, in contrast to the rapid neuronal mechanisms influencing nigrostriatal activity, the tuberoinfundibular-hypophyseal system is regulated, at least in part, by a slow hormonal feedback. © 1979.
