GABA-specificity of behaviour responses to picrotoxin injected into the colliculus superior of cats

The behavioural response to picrotoxin (25-500 ng) injected into the colliculus superior (CS) of freely moving cats was investigated. The maximal response to unilateral injections of picrotoxin (greater than or equal to 200 ng) was characterized by the following sequence of behavioural events. During the first 5 min after the injection the cat executed retroflexions of the contralateral ear. After 1-5 min these contralateral ear movements were followed by short, contralateral head movements. As time progressed the front part of the body, including the forelimbs, became involved in the movements resulting in contralateral torso movements. Finally, as the response was maximal, the whole body became involved in the movements resulting in contralateral body movements. Data are shown indicating that most of these behavioural phenomena were dose-dependent, locus-specific, and GABA-specific. Bilateral injections of picrotoxin resulted in similar characteristic movements, but now directed towards both sides and/or directed 'ventrocaudally'. Finally, it was found that blindfolding the animals did not change the response to unilaterally injected picrotoxin. As the behavioural phenomena described here are dissimilar to the effects observed after disturbing the GABAergic activity in the substantia nigra, pars reticulata (SNR), it is concluded that the CS, being an output station of the SNR, transforms its input signals into new output signals. Finally, it is suggested that picrotoxin resulted in a fixed code at the level of the CS, forcing the animal to execute characteristic motor patterns.