Cross-talk between excitatory and inhibitory amino acids in the regulationof growth hormone secretion in neonatal rats

Experiments were carried out in 5-day-old male and female rats in order to assess the respective roles of distinct gamma -aminobutyric acid (GABA) rec eptor subtypes in the control of growth hormone (GH) secretion in the newbo rn rat and to identify the potential sexual dimorphism of GABA actions on G H release. The interplay of GABA and excitatory acids (EAAs) to control GH secretion was also analysed. Effects of specific GABA(A), GABA(B) and GABA( C) agonists upon GH secretion as well as of co-ad ministration of GABA and agonists and antagonists of EAA receptors were monitored in 5-day-old male rats. The data confirm that GABA stimulates GH secretion in 5-day-old rats, but indicate that the effect is sexually dimorphic, being greater in femal es. Stimulation of GH secretion by GABA is mainly mediated by GABA(A) recep tors, while GABA(C) appears to be ineffective. Stimulatory effects of GH by GABA and muscimol, a GABA(A) receptor agonist, are blocked by pretreatment with (+)5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-phepten-5-10-imine hyd rogen maleate (MK-801), an antagonist of N-methyl-D-aspartate (NMDA) recept ors, but not by 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]-quinoxaline-7 -sulfonamide (NBOX), an antagonist of (+/-)-alpha -amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA) receptors. Simultaneous administration of GABA and NMDA was additive on GH release while basal GH secretion was not affected by either bicuculline or the respective blockers of GABA(A) and GA BA(B) receptors. In conclusion, our results suggest that cross-talk between inhibitory (GABA) and EAAs is involved in the control of GH secretion in n eonates. Copyright (C) 2001 S. Karger AG, Basel.