The mesencephalic basis of conditioned inhibition of the rabbit's nictitating membrane response and the use of cyanide as a fiber sparing lesioning technique

Mis (1975; 1977) proposed that a midbrain circuit involving the accessory oculomotor nuclei (nucleus of Darkschewitsch, interstitial nucleus of Cajal) is necessary for inhibition of the conditioned nictitating membrane response (NMR) of the rabbit. The present study attempted to determine: a) whether neurons of the accessory oculomotor nuclei, and not fibers of passage, are necessary for conditioned inhibition (CI) of the NMR and b) the fiber projections from the lesioned areas that disrupt CI. Using neuroanatomical methods, Experiment 1 tested whether an injection of cyanide (NaCN) into the neocortex, hippocampus, or pons resulted in a fiber sparing lesion. Patterns of cell death seen in Experiment 1 suggested that NaCN injections spared fibers of passage. Using electrophysiological methods, Experiment 2 assessed the effects of NaCN injection on the reflexive eyeblink of the rabbit to eye shock. Cyanide injection did not appear to disrupt evoked potentials recorded centrally or peripherally. However, spontaneous neural activity decreased in some animals suggesting destruction of neurons near the recording electrode. In Experiment 3, the transcallosally evoked response of the cerebral cortex was used to further investigate the effects of cyanide on neurally mediated evoked responses: NaCN had no apparent effect on evoked potentials recorded in cell or fiber areas up to 3 hours after injection, iv Experiment 4 assessed the effect of radio-frequency lesions of the midbrain on CI in a retention design. Lesions of the posterior commissure, pretectum, and periaqueductal grey produced significant disruption of CI. Analysis of Fink-Heiraer material suggests that a fiber projection from the pretectum, posterior commissure and periaqueductal grey to the posterior hypothalamus is involved in CI. Since the posterior hypothalamus is part of a brain stem "reward" system, this result is consistent with an opponent-process theory of CI (Dickinson and Pearce, 1977) .