Immunostaining for the α3 isoform of the Na⁺/K⁺-ATPase is selective for functionally identified muscle spindle afferents in vivo

Muscle spindle afferent (MSA) neurons can show rapid and sustained firing. Immunostaining for the α3 isoform of the Na⁺/K⁺-ATPase (α3) in some large dorsal root ganglion (DRG) neurons and large intrafusal fibres suggested α3 expression in MSAs (Dobretsov 2003), but not whether α3-immunoreactive DRG neuronal somata were exclusively MSAs. We found that neuronal somata with high α3 immunointensity were neurofilament-rich, suggesting they have A-fibres; we therefore focussed on A-fibre neurons to determine the sensory properties of α3-immunoreactive neurons. We examined α3 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 α3 rings (n= 23) were all MSAs (types I and II); all MSAs had darkly stained α3 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 (Aα/β 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 α3 ring staining is a selective marker for MSAs. The α3 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 α1 isoform (in all DRG neurons). The high α3 levels in MSAs may enable the greater dynamic firing range in MSAs.Sensory neurons that carry different types of sensory information to the central nervous system (CNS) express different molecules that contribute to their different properties. Understanding of their chemical/molecular properties would be enhanced by markers selective for functionally defined sensory subgroups. There are few of these. In many species, immunoreactivity for the α3 sodium pump isoform in some large sensory neurons and in fibres projecting to muscle spindles suggested a possible functional selectivity. We confirm this in functionally identified sensory neurons in rat and guinea pig, and show that immunocytochemistry for α3 clearly labels muscle spindle afferent neurons (MSAs), labels all MSAs and does not label other sensory neuron types examined. This reliable marker for MSAs should enable further studies of their properties without the necessity and complexity of identifying their sensory functions. Functionally, α3 itself may enable the sustained firing that is so important in MSAs.