The lateral vestibular nucleus is a very important site for depressant actions of benzodiazepines on the crossed extensor reflex

Abstract

We have reported that the crossed extensor reflex (CER) in Wistar male chloralose-anesthetized rats is very susceptible to the depressive action of benzodiazepine derivatives. The present study was designed to elucidate a possible site for this action in the central nervous system. Since the reflex contraction of the femoral quadriceps muscle elicited by an electrical stimulation of contralateral sciatic nerve was little influenced by diazepam (DZ) (0.05-2 mg/kg i.v.) in the chronic spinal rat, it is very probable that DZ may act first on supraspinal structures. DZ (2.5 μg) injected into the 4th ventricle produced much more potent and rapid inhibition of the CER than injection into the lateral ventricle. To find the most sensitive site involved in the area close to the 4th ventricle, microinjections of DZ in doses of 1 and 25 ng into the lateral vestibular nucleus (LVN) ipsilateral to the testing muscle produced a dose-dependent reflex depression which was clearly antagonized by Ro 15-1788, a benzodiazepine antagonist. Injections of 100 ng of DZ into other areas, i.e., the contralateral LVN, other subcerebellar nuclei, the red nucleus and the substantia nigra, produced no effects on the CER. Furthermore, electrolytic lesions of the ipsilateral LVN decreased the CER, while an electrical conditioning stimulation of the same nucleus increased it.