EFFECT OF EXOGENOUS GLUTAMATE SUPPLY ON THE ONSET OF PUBERTY IN GOATS. II. SERUM LEVELS OF TRIIODOTHYRONINE

Abstract

Thyroid hormones and their receptors in the ovaries are active regulators of reproductive function; both hyper- and hypo-thyroidism may result in estrous cycle disturbances. In addition, thyroid hormones elicit an extraordinary multiplicity of biochemical, cellular, and physiological responses, both in the simplest and the most complex organisms. On the other hand, glutamate, the main excitatory amino acid of the central nervous system has a marked stimulatory effect on the reproductive axis in mammals. In fact, occurrence of precocious puberty in response to administration of glutamate agonists has been reported in several species. The aim of the present study was to evaluate the effect of administration of glutamate on the onset of puberty in goats, and the association with serum triiodothyronine levels (T3), as a possible metabolic signal for the onset of ovarian activity in juvenile goats. The study was carried out in northern Mexico (26Ëš N) from June to October. Goats (n=18) were offered alfalfa hay (14% PC; 1.14 Mkal Kg-1 ENm), corn silage (8.1% PC, 1.62 ENm Mcal kg-1), and ground corn grain (11.2% PC, 2.38 ENm Mcal kg-1) under natural photoperiod. Location, animals, treatment design, preparation of the glutamate buffer solution, blood sampling scheme and quantification of serum P4 were described in the first part of this study. Serum samples were also evaluated for their content of T3 by RIA. Final averages for live weight (LW) and body condition score (BCS) did not differ (P>0.05) between the Glutamate-supplemented and control groups (23.7±0.72 vs. 22.7±0.72 kg) and (3.69±0.10 vs. 3.38±0.10 units), respectively. The overall average for T3 during the study was 1.47 ng mL-1, with higher levels (P<0.05) in the glutamate-treated-goats (1.55 vs. 1.39±0.04 ng mL-1). Results provide evidence that glutamate administration accelerated onset of puberty and that the onset of ovarian activity was positively related to high levels of serum T3. These data support the hypothesis that goats are able to transduce exogenous glutamate infusions into cues for sexual maturation of juvenile animals, and that T3 could be considered a metabolic modulator of the process leading to puberty in goats.