Perioral biting reflex and turning after intranigral injection of a GABA- or metenkephalin-agonist: role of the thalamus and superior colliculus.

Unilateral destruction of the ventromedial thalamus (VMT) with a radiofrequency lesion attenuated turning induced by injection of muscimol (40 ng/0.4 microliter) but not of a metenkephalin-analogue (DAME; 2.5 micrograms/0.4 microliter) into the substantia nigra, pars reticulata (SNR). Unilateral lesions in the deep layers of the superior colliculus (DLSC) attenuated both muscimol- and DAME-induced turning. Lesions in the DLSC but not in the VMT blocked the sensitization of the perioral biting reflex by injection of muscimol or DAME into the SNR on the same side of the lesion. When injected with apomorphine (0.5 mg/kg) all rats with lesions turned ipsiversive to the lesion side and reacted to tactile stimulation of the perioral area on both sides with orienting towards and then biting into the stimulus probe.