Comparison of the Effects of Ether and Restraint Stress on the Activity of Tuberoinfundibular Dopaminergic Neurons in Female and Male Rats

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Abstract
The purpose of the present study was to characterize the acute inhibitory effects of restraint stress on the activity of tuberoinfundibular dopamine (DA) neurons as estimated by measuring concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence. The time course of the effects of two types of physical restraint (immobilization in the supine position or confinement in an acrylic cylindrical tube) was determined in unanesthetized and diethylether (ether)-exposed female and male rats. The combination of brief (2 min) exposure to ether followed by 10 and 20 min of supine restraint increased concentrations of prolactin in plasma and decreased DOPAC concentrations in median eminence of both female and male rats. Thirty minutes of supine restraint decreased DOPAC concentrations in the median eminence of female rats that were not exposed to ether, and brief exposure to ether enhanced this effect. By contrast, 30 min of supine restraint failed to alter DOPAC concentrations in the median eminence in either unanesthetized or ether-exposed male rats. Tube restraint in the absence of ether failed to alter DOPAC concentrations in the median eminence of either female or male rats; but in female rats preexposed to ether, 30 min of tube restraint decreased DOPAC concentrations in the median eminence. On the other hand, in the absence of physical restraint, 2 min ether exposure caused a transient increase in prolactin secretion and a concurrent decrease of DOPAC concentrations in median eminence of both female and male rats. These results indicate that physical restraint on the activity of tuberoinfundibular DA neurons in female rats varies as a function of the type of restraint employed, but that when physical restraint is preceded by brief exposure to ether, both supine and tube restraint are equally effective in inhibiting tuberoinfundibular DA neurons. These results also suggest that the mechanism that mediates the sexually differentiated inhibitory effect of physical restraint on the activity of tuberoinfundibular DA neurons in female rats differs from that mediating the inhibitory effects of ether on these neurons in female and male rats.