Effect of increasing doses of intracaudate haloperidol upon motor expressions that require an intact substantia nigra pars reticulata and/or superior colliculus in cats.

In the present study it was investigated whether intracaudate (CN) injections of increasing doses of haloperidol progressively affect movements that require the integrity of the substantia nigra pars reticulata (SNR) and/or the deeper layers of the superior colliculus (dl-SC) in cats. Therefore, cats were trained to walk from one side of a narrow bar to the other side both under full light and under stroboscopic illumination (2 flashes/s). Previously it has been shown that this set-up is useful for assessing changes in the execution of dl-SC and SNR-related non-externally guided targeting movements. Moreover, cats were trained to step out of a startbox on a rotating cylinder which, according to previous studies, also implies the execution of a dl-SC- and SNR-related non-externally guided targeting movement. CN application of 25 micrograms/5 microliters haloperidol, but not CN injection of 12.5 micrograms/5 microliters haloperidol or apomorphine (0.6 micrograms/5 microliters), reduced the occurrence of these dl-SC- and SNR-related targeting movements. Dl-SC injection of picrotoxin (100 ng/0.5 microliter) did not restore the haloperidol (25 micrograms/5 microliter) induced decrement in the occurrence of these targeting movements. In addition, CN application of 25 micrograms/5 microliter haloperidol, but not 12.5 micrograms/5 microliter haloperidol, reduced the feline ability to execute SNR-specific limb movements while crossing a revolving cylinder. The present results show that only a severe but not a moderate inhibition of dopamine receptors within the CN affects motor expressions characteristic of the SNR and/or dl-SC.