Adult-like biphasic neurobehavioral changes induced by a GABA-A agonist in infant and weanling mice.

The neurobehavioral responsivity to peripherally injected muscimol, a gamma-aminobutyric acid-A (GABA-A) agonist, was assessed in infant (14-day-old), weanling (20-day-old) and young adult (53-day-old) outbred male mice. In the first experiment, relatively high doses of muscimol (ranging from 0.05 to 0.40 mg/kg in developing and from 0.50 to 3 mg/kg in adult animals) were found to dose-dependently induced solid catalepsy and ataxia, evaluated 5 times at 20-min intervals. In the second experiment, the GABA agonist was injected in dose ranges which include relatively small concentrations in order to assess its excitatory properties, observable in adults, on rearing and locomotion in developing mice. It appeared that levels of rearing and especially locomotion were enhanced at the low doses (0.025 and 0.050 mg/kg in developing, and 1.3 and 1.9 mg/kg in adult mice) and inhibited at the higher ones (0.150 mg/kg in developing and 1.9 and 2.5 mg/kg in adult mice). This adult-like biphasic action of muscimol in developing mice--excitation at low and depression/sedation at high doses--strongly suggests a full maturation of the GABA-A-related behavioral functions at a period of ontogeny where adult-like locomotion emerges. Given that previous studies have shown that muscimol can biphasically affect behavioral activity in newborn murines as well, it is suggested that GABA-related behavioral functions mature near-monotonically during ontogeny, unlike those related to other major neurotransmitter systems.