Immunostaining for the alpha3 isoform of the Na+/K+-ATPase is selective for functionally identified muscle spindle afferents in vivo.

摘要

Muscle spindle afferent (MSA) neurons can show rapid and sustained firing. Immunostaining for the alpha3 isoform of the Na(+)/K(+)-ATPase (alpha3) in some large dorsal root ganglion (DRG) neurons and large intrafusal fibres suggested alpha3 expression in MSAs (Dobretsov et al. 2003), but not whether alpha3-immunoreactive DRG neuronal somata were exclusively MSAs. We found that neuronal somata with high alpha3 immunointensity were neurofilament-rich, suggesting they have A-fibres; we therefore focussed on A-fibre neurons to determine the sensory properties of alpha3-immunoreactive neurons. We examined alpha3 immunointensity in 78 dye-injected DRG neurons whose conduction velocities and hindlimb sensory receptive fields were determined in vivo. A dense perimeter or ring of staining in a subpopulation of neurons was clearly overlying the soma membrane and not within satellite cells. Neurons with clear alpha3 rings (n = 23) were all MSAs (types I and II); all MSAs had darkly stained alpha3 rings, that tended to be darker in MSA1 than MSA2 units. Of 52 non-MSA A-fibre neurons including nociceptive and cutaneous low-threshold mechanoreceptive (LTM) neurons, 50 had no discernable ring, while 2 (Aalpha/beta cutaneous LTMs) had weakly stained rings. Three of three C-nociceptors had no rings. MSAs with strong ring immunostaining also showed the strongest cytoplasmic staining. These findings suggest that alpha3 ring staining is a selective marker for MSAs. The alpha3 isoform of the Na(+)/K(+)-ATPase has previously been shown to be activated by higher Na(+) levels and to have greater affinity for ATP than the alpha1 isoform (in all DRG neurons). The high alpha3 levels in MSAs may enable the greater dynamic firing range in MSAs.