Norepinephrine and dopamine activity in microdissected brain areas of the middle-aged and young rat on proestrus

Norepinephrine (NE) and dopamine (DA) neuronal activity were estimated in middle-aged (8-11 months old) and young (3-4 months old) proestrous rats in microdissected brain areas to determine whether alterations in catecholamine concentrations, rate constants and/or turnover rates are detectable at a stage when alterations in proestrous gonadotropin surges are evident. Rate constants and turnover rates were estimated using α-methylparatyrosine (αMPT) at 0900-1100, 1200-1400 and 1500-1700 h in middle-aged and young rats. These parameters were also measured at 1800-2000 h in middle-aged animals. Catecholamines were measured by radioenzymatic assay in rats killed at 0900, 1200, 1500 and 1800 h and in animals which were killed 2 h after receiving αMPT (400 mg/kg i.p.) in the medial preoptic nucleus (MPN), suprachiasmatic nucleus (SCN), median eminence (ME) and arcuate nucleus (AN). In young rats, NE turnover rates increased during the afternoon in all brain areas examined. The rise was significant by 1200-1400 h in the MPN and ME. By 1500-1700 h there was a further increase in the ME, the rise in the MPN was maintained, and AN- and SCN-NE turnover rates were significantly elevated. The pattern of proestrous NE turnover rates was altered in middle-aged rats. Prior to the luteinizing hormone (LH) surge (0900-1100 h), NE turnover rates in the MPN, SCN, and ME were the same as observed in young rats. However, during the afternoon, NE neuronal activity increased only in the ME and AN (1200-1400 h); no change was observed in either the MPN or SCN at any time during the afternoon. In young rats, ME- and AN-DA turnover rates decreased at 1500-1700 h compared to 1200-1400 h. In middle-aged rats, ME-DA decreased at a comparable time, but no change was observed in AN-DA. The results of this study suggest that changes in catecholamine neuronal activity occur in the middle-aged proestrous rat when compared with the young animal. The absence of elevated NE neuronal activity in anterior brain areas (MPN and SCN) may be critically involved in the delayed and attenuated proestrous LH surges which have been reported by several investigators in rats at this age.