Glutamate supply positively affects serum release of triiodothyronine and insulin across time without increases of glucose during the onset of puberty in female goats

Abstract

The current study evaluated the effect of glutamate supply on the onset of puberty and possible links to changes in serum concentrations of insulin [INS], glucose [GLU] and triiodothyronine [T 3]. The study was conducted from June to November in prepuberal female goats ( n = 18; 3 mo. old, 7/8 Saanen-Alpine, 1/8 Criollo, 26° north) randomly assigned to two experimental groups: (i) excitatory amino acids group (group AA, n = 10; 16.52 ± 1.04 kg LW, 3.4 ± 0.12 body condition score [BCS], receiving an intravenous infusion of 7 mg kg −1 live weight [LW] of l-glutamate, twice a week, and (ii) control group (group CC, n = 8; 16.1 ± 1.04 kg LW, 3.1 ± 0.12 BCS) receiving saline. Blood samples were obtained twice a week, for assessing progesterone [P 4], as well as in a monthly basis to evaluate INS and T 3 by RIA. Mean final LW and BCS were 23.2 ± 0.72 kg, 3.53 ± 0.10 units, without differences between groups. The AA group depicted an earlier onset of puberty (6.9 ± 0.3 compared to 7.5 ± 0.4 mo.; P < 0.05) and an increased ovarian activity (70 ± 0.28% compared to 25 ± 0.26%; P < 0.05). Neither serum INS concentrations nor serum glucose concentrations differed between treatments (1.2 ± 0.06 ng mL −1 and 89.6 ± 1.8 mg 100 mL −1; P > 0.05, respectively). Serum T 3 concentrations, however, were greater in AA goats (1.55 ± 0.03 compared to 1.39 ± 0.04 ng mL −1). In addition, a treatment x time interaction occurred ( P < 0.05) across the experimental period for both T 3 and INS, with increases by the last third of the experimental period, time at which the onset of puberty occurred in both experimental groups. No differences ( P > 0.05) for glucose concentrations across time occurred between treatments. Results indicate that, in prepuberal goats, glutamate acts as a cue for sexual maturation in a glucose-independent pathway, while both T 3 and INS seem to act as metabolic modulators for the establishment of puberty in goats. Actions of INS and T 3 are mediated directly on hypothalamic centers regulating the pulsatile release of GnRH or indirectly by peripheral cues reflecting INS-T 3 actions on somatic development remains to be determined.