Photomicrograph of typical microinjection sites. A Dye-spread patches in an otherwise non-stained 50 µm thick brainstem section resulting from two separate microinjection trials, each of 50 nL muscimol mixed with 1% pontamine sky blue dye (mouse # MISO51). The section is overlaid with structural boundaries taken from the mouse brain atlas of (Franklin and Paxinos 2019). The section contains the center of the microinjection on the left; note the tear-shape tracking upward and the relatively dense dye in the center. The microinjection on the right is somewhat more tangential. B, C Images taken at a higher magnification show that the dye is taken up by a fraction of the neurons within the dye-spread patch
The mesopontine tegmental anesthesia area (MPTA) was identified in rats as a singular brainstem locus at which microinjection of minute quantities of GABAergic agents rapidly and reversibly induces loss-of-consciousness and a state of general anesthesia, while lesioning renders animals insensitive to anesthetics at normal systemic doses. Obtaining...
Although general anesthesia is normally induced by systemic dosing, an anesthetic state can be induced in rodents by microinjecting minute quantities of GABAergic agents into the brainstem mesopontine tegmental anesthesia area (MPTA). Correspondingly, lesions to the MPTA render rats relatively insensitive to standard anesthetic doses delivered systemically. Using a chemogenetic approach we have identified and characterized a small subpopulation of neurons restricted to the MPTA which, when excited, render the animal anesthetic by sensorimotor (immobility) and electroencephalographic (EEG) criteria. These “effector-neurons” do not express GABAAδ-Rs, the likely target of GABAergic anesthetics. Rather, we report a distinct sub-population of nearby MPTA neurons which do. During anesthetic induction these likely excite the effector-neurons by disinhibition. Within the effector population ~ 70% appear to be glutamatergic, ~30% GABAergic and ~ 40% glycinergic. Most are projection neurons that send ascending or descending axons to distant targets associated with the individual functional components of general anesthesia: atonia, analgesia, amnesia, and loss-of-consciousness.