Onset of pain threshold changes induced by neonatal monosodium glutamate.

Neonatal administration of monosodium glutamate (MSG) results in a number of anatomical, physiological and behavioral abnormalities, including changes in pain thresholds and analgesic responses. The present study compared the onsets of MSG-induced changes in jump thresholds, hot-plate latencies, and body weight. At 30, 60 and 80 days of age, MSG-treated rats weighed significantly less than rats treated with either saline or hypertonic saline (HSAL), a control for possible osmolarity effects. Jump thresholds of MSG-treated rats were significantly lower than those of saline-treated rats at 80, but not at 45 days of age. In contrast, HSAL treatment decreased jump thresholds at 45, but not at 80 days of age. Hot-plate latencies of MSG-treated rats were significantly longer than saline-treated rats across the time course of testing at 21, 45 and 80 days of age. Thus, not only did MSG treatment induce differential effects upon pain thresholds, but the onsets of such changes varied as a function of the test. The implications of differential onsets of MSG-induced abnormalities are discussed.