Increased responsiveness of the hypothalamic-pituitary axis after neurotoxin-induced hypothalamic denervation

Neonatal treatment with monosodium glutamate (MSG) induces severe neuronal damage in selected brain areas, which in turn results in a number of neuroendocrine abnormalities during adult life. The present study was designed to determine what effects this partial and selective denervation of the hypothalamic-pituitary axis may have on the sensitivity of two key components of that axis, the median eminence (ME) and the anterior pituitary (AP). In order to test any changes in response that may occur after MSG treatment, the release of several peptide hormones from either the ME or the AP was evaluated in vitro, employing specific or general secretagogues. Male newborn pups of the Holtzman strain were injected with MSG every other day for 5 days, starting on day 2 of life; littermate controls received an injection of 10% NaCl. All animals were used when adult, at about 5-7 months of age. After decapitation, ME and AP tissues from control and MSG-treated rats were dissected out and incubated in vitro. Release of LHRH, SRIF, arginine vasopressin, and oxytocin from the ME was measured by direct RIA, under basal conditions and after stimulation with high K+ medium (28 mM). The results clearly indicate a marked hyperresponse of the release of each of the four neuropeptides by ME fragments from MSG-lesioned animals. The altered release was not attributable to changes in ME peptide content. In the case of the AP, the release of ACTH, LH, PRL, and GH in response to high K+ or, in some cases, to specific releasing factors, was evaluated in a dispersed cell preparation. The release of all four protein hormones was increased by high K+, and again the MSG-lesioned rats showed a pronounced hyperresponse. Corticotropin-releasing factor, at concentrations of 10(-9) and 10(-8) M enhanced ACTH release from control and MSG-lesioned rats, but the latter presented a marked hyperresponse. A similar observation on LH release was seen after stimulation with LHRH (10(-9) M). The results indicate that the neuronal damage induced by neonatal MSG treatment results in a generalized hyperresponsiveness in vitro to either specific or general secretagogues by ME or AP tissues, which may suggest the development of a denervation-type supersensitivity in the hypothalamic-pituitary axis. Since the release of these peptide hormones is sluggish in MSG-lesioned rats under in vivo conditions, it seems plausible to conclude that the major defect after the neurotoxin damage may reside in the loss of neurotransmitter systems normally innervating and regulating the activity of the peptidergic neurons.