Pharmacological studies of the optic system of the chick: Effect of γ-aminobutyric acid and pentobarbital

The optic system of the young chick was selected for the present studies because endogenous γABA levels can be elevated in the optic tectum by parenteral administration of solutions of this compound. The results obtained have shown that intravenously administered γABA can alter the electrical activity of the optic system at both the retinal and tectal levels. The action of γABA upon the electroretinogram (ERG) which has not been previously described in any species, modified the electrical activity of the eye in a manner seemingly different from that reported for other substances tested in various species On the basis of the present results, it is suggested that γABA acts as an inhibitor of neural control of retinal activity. The effect of pentobarbital was also studied. Its effect upon the behavior of the avian optic system is similar to that observed for this drug in the mammalian cortex and on the ERG of a variety of species, including man. The effect of pentobarbital upon the ERG, correlated with anatomical features of the avian optic system, suggests an action mediated through centrifugal nerves, possibly the isthmo-optic tract which arises from a caudal midbrain nucleus—the isthmo-opticus. The data further seem to support the view that this centrifugal system is inhibitory and may be associated with adaptation to the dark. Results of these experiments with pentobarbital and γABA alone or in combination, together with the similarity of action of pentobarbital upon the electrical activity of the avian tectum and mammalian cortex, led to the suggestion that γABA exerts its action in the avian tectum at the presynaptic level. These studies indicate that the optic system of the chick can serve as a useful and subtle tool for neuropharmacological studies in general.