Tuberoinfundibular neurons: dopaminergic and norepinephrinergic sensitivity.

The techniques of microelectrophoresis and antidromic identification have been utilized in the female rat anesthetized with urethane to localize the tuberoinfundibular neurons and to determine their pharmacological sensitivity to dopamine (DA), norepinephrine (NE) and glutamate (GLUT). Extracellular recordings were made of 149 single units in the arcuate (ARC) nucleus; 79 of which were antidromically identified (43 were spontaneously active and 36 were silent) and the remaining 70 cells were determined to be spontaneously active but could not be antidromically identified (unidentified ARC neurons). The mean latency of the antidromically activated potentials was 9,5 msec, which gave a conduction velocity of 0.05 mm/msec. The spontaneous firing rate of antidromically identified ARC neurons varied from less than 1/sec to 8/sec while unidentified cells discharged at rates from less than 3/sec to 10/sec. These data indicate that ARC neurons exist which are capable of conducting impulses and send their axons into the external layer of the ME. Successful drug applications were made on 104 ARC neurons; of the 43 antidromically identified units, 37 were spontaneously active and 6 were silent. There were 61 spontaneously active, unidentified neurons. Two distinct pools of antidromically identified ARC neurons were found based on their sensitivity to NE and DA. Neurons displaying excitation to NE(N = 23) were either inhibited (N = 10) or non-responsive (N = 13) to DA applied electrophoretically; no NE-sensitive ARC neurons were excited by DA. The second pool of neurons (N = 14) was excited by DA. These neurons when tested with NE were either reproducibly inhibited (N = 3) or nonresponsive (N = 11); no DA-sensitive neurons were excited by NE. In the unidentified ARC neurons, iontophoretically applied NE and DA gave reproducible effects on individual neurons, so that some neurons were excited by both NE and DA, some were inhibited by both, and still others were excited by one and inhibited by the other chemical. Glutamate (GLUT) had a powerful excitant action on nearly all the neurons tested, increasing the electrical activity of the spontaneously active neurons as well as initiating activity in quiescent ARC neurons. These data are suggestive of a tuberoinfundibular neuronal system whose activity is modulated by either DA or NE. This evidence is certainly compatible with a functional role for both DA and NE in regulating pituitary function by altering the activity of presumed neurosecretory neurons which release hypophysiotropic hormones.