Conflicting data has accumulated regarding the effects of pentobarbital on mono- and poly-synaptic pathways through the vestibular nuclei. On the one hand, investigators believe that pentobarbital interrupts vestibular polysynaptic pathways to the spinal cord while leaving monosynaptic routes intact; conversely, others have shown that even very small doses of pentobarbital virtually silence vestibular neurone discharge to VIIIth nerve stimulation. The purpose of the present investigation was to dispel the uncertainty of the effects of pentobarbital on vestibular mono- and poly-synaptic linkages. When individual branches of the VIIIth nerve are electrically stimulated, mono- and poly-synaptic field potentials are recorded from the ascending MLF. If pentobarbital is administered in varying doses, its consequent influence on MLF response amplitude is noted and a dose-response curve as in Figure 1 is plotted. It is evident from this curve that both the mono- and poly-synaptic discharges of the MLF are depressed by as little as 4 mg/kg of pentobarbital. At anaesthetic doses the depression of both responses was to about 25% of the control amplitude. Single units in the vestibular nuclei which then project in the ascending MLF are also found to discharge mono- and poly-synaptically to stimulation of individual vestibular nerve branches. The monosynaptic and/or polysynaptic response of any of the semicircular canals can be inhibited by conditioning the utricular nerve 4 msec prior to canal nerve stimulation. Figures 2 and 3 show that pentobarbital, in doses of only 3-4 mg/kg, depresses both mono- and poly-synaptic discharges. In fact, the monosynaptic discharge of the test canal is still functional when the polysynaptic responses have already failed. However, the presumed multisynaptic path that mediates the conditioned inhibition is persistent. Thus, from the data presented in this paper it appears that pentobarbital influences both monoand poly-synaptic pathways to a similar extent. It must be assumed then, that at anaesthetic doses of pentobarbital, the responses seen following vestibular nerve stimulation represent only a fraction of the monosynaptic pathways and include a substantial number of polysynaptic pathways as well.
ASJC Scopus subject areas
- Cellular and Molecular Neuroscience