Revised Jn-90933-2008 Network Actions of Pentobarbital in the Rat Mesopontine Tegmentum on Sensory Inflow through the Spinothalamic Tract

The recent discovery of a barbiturate-sensitive “general anesthesia switch” mechanism localized in the rat brainstem mesopontine tegmental anesthesia area (MPTA) has challenged the current view of the non-specific actions of general anesthetic agents in the CNS (Devor and Zalkind 2001). In this study we provide electrophysiological evidence that the antinociception, which accompanies the behavioral state resembling general anesthesia following pentobarbital (PB) microinjections into the MPTA of awake rats, could be accompanied by the attenuation of sensory transmission through the spinothalamic tract (STT). Following bilateral microinjections of PB into the MPTA spontaneous firing rate (SFR), antidromic firing index (FI), and sciatic (Sc) as well as sural (Su) nerve-evoked responses (ER) of identified lumbar STT neurons in the isoflurane-anesthetized rat were quantified using extracellular recording techniques. Microinjections of PB into the MPTA significantly suppressed the SFR (47%), magnitudes of Sc(26%) and Su-ER (36%) as well as FI (41%) of STT neurons. Microinjections of PB-free vehicle control did not alter any of the above electrophysiological parameters. The results from this study suggest that antinociception, which occurs during the anesthesia-like state following PB microinjections into the MPTA (ibid), may be due, in part, to (in)direct inhibition of STT neurons via switching mechanism(s) located in the MPTA. This study provides a provenance for investigating electrophysiologically the actions on STT neurons of other current agents used clinically to maintain the state of general anesthesia.