Further behavioral analysis of GABA-mediated inhibition in the early chick embryo

Five mumol of gamma-amino-N-butyric acid (GABA) completely inhibited the spontaneous motility of 5-day-old chicken embryos. A molar equivalent dose of baclofen, a specific GABAB receptor agonist, produced a much smaller (but still significant) inhibition of motility. The (-)-isomer of baclofen was significantly more potent than the (+)-isomer. GABAB receptors controlling overt behavior appear in the chick embryo at least as early as day 5. Neither 5 mumol of histamine nor 5 mumol of bradykinin, both potent cerebral vasodilators, inhibited spontaneous motility in 5-day-old chicken embryos. One mumol of serotonin, a potent cerebral vasoconstrictor, did not antagonize the significant (but submaximal) inhibition of motility caused by 1 mumol of GABA, GABA-mediated inhibition of overt behavior in the early chick embryo cannot be the result of GABA-mediated cerebral vasodilation.