Bulbospinal neurons implicated in mesopontine-induced anesthesia are substantially collateralized

Microinjection of pentobarbital or other γ-aminobutyric acid type A receptor (GABA_A-R) active anesthetics into a brainstem region in the rat that we have called the mesopontine tegmental anesthesia area (MPTA) induces a general anesthesia-like state that includes suppression of locomotor activity, loss of the righting reflex, atonia, antinociception, and apparent loss of consciousness. The suppression of muscle tone and of nocifensive spinal reflexes suggests a direct or indirect effect at the level of the spinal cord itself, an inference supported by anterograde tracing from the MPTA area. We have now used single and double retrograde tracing to characterize this bulbospinal pathway further. The MPTA contains the majority of all bulbospinal neurons present at mesopontine levels (65.8%). Many of these neurons, although not all, appear to have a highly collateralized projection pattern within the spinal cord. About 40% of the MPTA neurons that project to the lumbar spinal cord also have collaterals at cervical levels, and about 60% of those with projections to the ventral horn also have projections to the dorsal horn (at cervical levels). However, the large majority projects either ipsilaterally or contralaterally. Relatively few (∼13%) send collaterals to both sides of the spinal cord. The pattern of connectivity revealed appears to be consistent with a system designed primarily to modulate motor and sensory functions globally, over the entire neuraxis, rather than regionally or segmentally.