Protein expression and mRNA cellular distribution of the NKCC1 cotransporter in the dorsal root and trigeminal ganglia of the rat

Primary afferent neurons maintain depolarizing responses to GABA into adulthood. The molecular basis for this GABAergic response appears to be the Na⁺K⁺2Cl^- cotransporter NKCC1 that contributes to the maintenance of a high intracellular chloride concentration. Recently, a role for NKCC1 has been proposed in nociceptive processing which makes it timely to gain a better understanding of the distribution of NKCC1 in sensory ganglia. Here, we describe that, in the rat, NKCC1 mRNA is predominately expressed by small and medium diameter dorsal root (DRG) and trigeminal (TG) ganglion neurons. The colocalization of NKCC1 mRNA with sensory neuron population markers was assessed. In the DRG, many NKCC1 mRNA-expressing neurons colocalized peripherin (57.0 ± 2.5%), calcitonin-gene-related peptide (CGRP, 39.2 ± 4.4%) or TRPV1 immunoreactivity (50.0 ± 1.9%) whereas only 8.7 ± 1.2% were co-labeled with a marker for large diameter afferents (N52). Similarly, in the TG, NKCC1 mRNA-expressing neurons frequently colocalized peripherin (50.0 ± 3.0%), CGRP (35.4 ± 2.6%) or TRPV1 immunoreactivity (44.7 ± 1.2%) while 14.8 ± 1.3% were co-labeled with the N52 antibody. NKCC1 mRNA was also detected in satellite glial (SGCs) in both the DRG and TG. Colocalization of NKCC1 protein with the SGC marker NG2 confirmed the phenotype of these NKCC1-expressing glial cells. In contrast to in situ hybridization experiments, we did not observe NKCC1 immunoreactivity in primary afferent somata. These findings suggest that NKCC1 is expressed in anatomically appropriate cells in order to modulate GABAergic responses in nociceptive neurons. Moreover, these results suggest the possibility of a functional role of NKCC1 in the glial cells closely apposed to primary sensory afferents.