A midbrain–brain stem circuit for conditioned inhibition of the nictitating membrane response in the rabbit (Oryctolagus cuniculus).

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Abstract
A series of experiments was carried out in an effort to delineate the neural system responsible for the development of conditioned inhibition of the rabbit''s nictitating membrane response (NMR). Monopolar electrodes were implanted in a variety of midbrain and brain stem loci. Following acquisition of a conditioned NMR each electrode was tested to determine whether electrical stimulation applied to the electrode inhibited the manifestation of a conditioned response (CR). The most dramatic reduction in the amplitude of the CR was produced by stimulating the nucleus of Darkschewitsch, the interstitial nucleus of Cajal, or the anterior portions of the red nucleus. This disruption of the CR was observed either when the stimulation was applied prior to the onset of the conditioned stimulus or when the stimulation was applied after the initiation of the CR. Radio-frequency lesions placed in the nucleus of Darkschewitsch, interstitial nucleus of Cajal, anterior portions of the red nucleus, or tegmental reticular formation prevented the development of conditioned inhibition. Lesions placed in the rootlets of the oculomotor nerve, posterior red nucleus, or dorsal tegmental nuclei did not prevent the development of conditioned inhibition. A midbrain-brain stem circuit originating in the nucleus of Darkschewitsch and interstitial nucleus of Cajal is responsible for the development of conditioned inhibition of the rabbit''s NMR.