Effects of optic tract stimulation on baroreflex vagal bradycardia in rats

1. Arterial baroreflexes are suppressed in stressful conditions. Intense visual stimuli can cause a threatening sensation and produce defensive reactions. 2. The present study was designed to determine whether and how electrical stimulation of the optic tract (OT) affects arterial baroreflexes, especially the heart rate component, baroreflex vagal bradycardia (BVB), in rats. In chloralose- urethane anaesthetized, beta-adrenoceptor-blocked rats, BVB was evoked by electrical stimulation of the aortic depressor nerve. 3. Electrical stimulation of the OT was found to not only increase blood pressure and heart rate, but also to inhibit BVB. To determine whether these responses were mediated by the lateral genticulate body and/or the superior colliculus, which are major target sites to which the OT projects, each was activated with electrical and chemical stimulation. 4. The lateral genticulate body did not respond to either electrical or chemical stimulation, whereas the superior colliculus increased blood pressure and heart rate while suppressing BVB following electrical stimulation. Essentially similar responses were observed following microinjection of the GABA antagonist bicuculline methiodide. 5. Optic tract-induced inhibition of BVB was abolished by bilateral destruction of the superior colliculus. Furthermore, this inhibition was also largely attenuated by destruction of the midbrain periaqueductal grey (PAG). 6. In conclusion, electrical stimulation of the OT increases blood pressure, heart rate and inhibits BVB. These responses are not mediated by the lateral genticulate body but are mediated by the superior colliculus. The PAG may participate in the subsequent mediation of the responses to electrical stimulation of the OT and the OT-induced inhibition of BVB may contribute to expression of a light-induced defence reaction.