Single-unit activity of dorsomedial arcuate neurons and diurnal changes oftuberoinfundibular dopaminergic neuron activity in female rats with neonatal monosodium glutamate treatment

Neonatal monosodium glutamate (MSG)-treated rats were used in this study to answer two questions: (1) whether or not the dopamine-responsive dorsomedi al arcuate (dm-ARN) neurons are tuberoinfundibular dopaminergic (TIDA) neur ons, and (2) whether or not the remaining TIDA neurons in MSG-treated rats are functioning normally. MSG (4 mg/g b. wt., subcutaneously [s.c.]) or sal ine was given to neonatal Sprague-Dawley rats on days 1, 3, 5, 7, and 9 aft er birth, The female rats were ovariectomized at 50 days of age and treated with estrogen for 1 week before they were used between 65-90 days of age. The tyrosine hydroxylase-immunoreactive (TH-ir) neurons located in the dm a nd ventrolateral (vl) parts of the ARN were significantly reduced in MSG-tr eated rats, as determined by immunohistochemical method. Some TH-ir cells, however, were visible along the border of the third ventricle. Using single -unit recording in brain slices, we found that dopamine inhibited significa ntly fewer percentage of dm-ARN neurons in MSG-treated (28.2%, n = 39) than in saline-treated rats (73.3%, n = 15). In contrast, bombesin exhibited si milar effects (over 10% excitation) in both groups. Using neurochemical mea ns, neonatal MSG treatment produced significant decreases of both 3,4-dihyd roxyphenylacetic acid and dopamine levels, but not their ratios, in the med ian eminence. Moreover, the diurnal change of TIDA neuronal activity persis ted in the MSG-treated rats; so did the estrogen-induced afternoon prolacti n surge. Alt these results indicate that neonatal MSG-treatment reduced the number and altered the location of TIDA and dopamine-responsive dm-ARN neu rons. The remaining TIDA neurons seemed to be able to maintain their basal activities and diurnal rhythm. (C) 1999 Elsevier Science Inc.