Different basal levels of CaMKII phosphorylated at Thr286/287 at nociceptive and low-threshold primary afferent synapses

Postsynaptic autophosphorylation of Ca2+/calmodulin‐dependent protein kinase II (CaMKII) at Thr286/287 is crucial for the induction of long‐term potentiation at many glutamatergic synapses, and has also been implicated in the persistence of synaptic potentiation. However, the availability of CaMKII phosphorylated at Thr286/287 at individual glutamatergic synapses in vivo is unclear. We used post‐embedding immunogold labelling to quantitatively analyse the ultrastructural localization of CaMKII phosphorylated at Thr286/287 (pCaMKII) at synapses formed by presumed nociceptive and low‐threshold mechanosensitive primary afferent nerve endings in laminae I–IV of rat spinal cord. Immunogold labelling was enriched in the postsynaptic densities of such synapses, consistent with observations in pre‐embedding immunoperoxidase‐stained dorsal horn. Presynaptic axoplasm also exhibited sparse immunogold labelling, in peptidergic terminals partly associated with dense core vesicles. Analysis of single or serial pCaMKII‐immunolabelled sections indicated that the large majority of synapses formed either by presumed peptidergic or non‐peptidergic nociceptive primary afferent terminals in laminae I–II of the spinal cord, or by presumed low‐threshold mechanosensitive primary afferent terminals in laminae IIi–IV, contained pCaMKII in their postsynaptic density. However, the postsynaptic levels of pCaMKII immunolabelling at low‐threshold primary afferent synapses were only ∼ 50% of those at nociceptive synapses. These results suggest that constitutively autophosphorylated CaMKII in the postsynaptic density is a common characteristic of glutamatergic synapses, thus potentially contributing to maintenance of synaptic efficacy. Furthermore, pCaMKII appears to be differentially regulated between high‐ and low‐threshold primary afferent synapses, possibly reflecting different susceptibility to synaptic plasticity between these afferent pathways.