120 240 360

120 240 360 480 Minutes

FIG. 4. Plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH), and testosterone (T) levels over the 8 h sampling period of one representative male rat of each age group. *Denotes pulses identified by cluster analysis.

compared to young rats. In the hypothalamus from old rats there was significant reduction in the content of glutamine and y-aminobutyric acid (Bonavera et al 1998). Taken together, these results showed that the NMDA—GnRH—LH axis was altered in old rats, and the decreased hypothalamic content of some of the excitatory amino acids and reduced responsiveness of GnRH neurons to NMDA, both in vivo and in vitro, may play a role in the altered LH pulsatile secretion observed in older rats.

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