Fasting-induced suppression of hypothalamic-pituitary-gonadal axis in the adult rhesus monkey: evidence for involvement of excitatory amino acid neurotransmitters.

The present study was designed to examine whether acute food-restriction in non-human primates, suppresses hypothalamic-pituitary-testicular (HPT) axis via alterations in the excitatory amino acid (EAA) neurotransmitter-utilizing drive to the GnRH neuron. This was achieved indirectly by comparing the plasma testosterone (T) responses to administration of an excitatory amino acid analogue, N-methyl-D,L-aspartic acid (NMA), in acutely fasted and normal fed monkeys. A set of 4 chair-restrained adult male rhesus monkeys, was assigned to the following treatments: a) normal feeding, b) one-day fasting (omission of morning and afternoon meals), c) normal feeding+NMA (15 mg/kg BW) and d) one-day fasting+NMA (15 mg/kg BW). Starting 1 h after the provision or omission of the afternoon meal, frequent blood sampling was initiated at 15-min intervals for a period of 3-h. NMA was administered as an iv bolus 1 h after start of the sampling. Secretion of T was affected (P<0.005) by the treatments. A peak in T was evident during the first h of the sampling in fed but not fasted monkeys. Mean 3-h T concentrations were suppressed (P<0.001) by the fasting. Administration of NMA in fasting conditions resulted into an acute stimulation of T secretion in 2 of the 4 monkeys. However, mean 60-min post-NMA T concentrations were greater (P<0.05) than those prevailing during the same period in fasted animals not given NMA. In contrast, all 4 fed-monkeys showed significant T elevations in plasma immediately following the NMA challenge and mean T levels during the 60-min post-NMA period were higher (P<0.05) than those in fed animals not injected with NMA, at a comparable time. Testosterone area under the curve for the 2-h post-NMA period was greater (P<0.05) in fed- than in fasted-monkeys. These results indicate that although NMA can stimulate GnRH release both in fed and short-term fasting conditions, the response appears to be suppressed in the later situation suggesting that fasting-induced suppression of the HPT axis in the adult male rhesus monkey may involve a reduction in the sensitivity of GnRH neurons to the EAA neurotransmission.