The interrelationship between superior colliculus and substantia nigra pars reticulata in programming movements of...

The present feline study deals with the execution of targeting movements which can be elicited either by injection of picrotoxin into the deeper layers of the superior colliculus (dl-SC) or by application of muscimol into the substantia nigra pars reticulata (SNR), and suppressed either by dl-SC injection of muscimol or by SNR application of picrotoxin: the movements under discussion are the so-called non-externally guided targeting movements, i.e. targeting movements that are elicited but not continuously guided by external (visual, auditory, olfactory and tactile) stimuli. In this study we investigated whether the integrity of the SNR is required for the execution of these targeting movements elicited from the dl-SC. Cats were trained, therefore, to walk from one side of a narrow bar to the other side under stroboscopic illumination (2 flashes/s). The animals received bilateral injections both into the SNR (solvent 0.5 microliter or picrotoxin 500 ng/0.5 microliter) and into the dl-SC (solvent 0.5 microliter or picrotoxin 50-100 ng/0.5 microliter). Injections of picrotoxin into the dl-SC did not evoke non-externally guided targeting movements in case picrotoxin was also injected into the SNR. It is concluded that the integrity of the SNR is required for the execution of non-externally guided targeting movements elicited from the dl-SC. Besides, we investigated whether freezing, i.e., an SNR-specific effect, which can be evoked by injection of picrotoxin into this area, is funnelled through the dl-SC. Therefore, the behaviour of cats which had received bilateral injections both into the SNR (solvent 0.5 microliter or picrotoxin 500 ng/0.5 microliter) and into the dl-SC (solvent 0.5 microliter or picrotoxin 50-100 ng/0.5 microliter) was analysed. Application of picrotoxin into the dl-SC did not suppress the occurrence of freezing, elicited by SNR injection of picrotoxin. It is concluded that the SNR-specific freezing is not channelled through the dl-SC.