Novel coupling between TRPC-like and KNa channels modulates low threshold spike-induced afterpotentials in rat thalamic midline neurons

- A TTX-insensitive LTS-induced sAHP in midline thalamic neurons depends on [Na+]o.
- KNa-like channels are sole contributors to this LTS-induced sAHP.
- T-type Ca2+ channel opening appears to trigger Ca2+-dependent TRPC-like channel opening.
- Ensuing Na+ entry via TRPC-like channels opens the KNa-like channels.
- An isolated LTS-induced sADP can be modulated by neurotransmitters.

Neurons in thalamic midline and paraventricular nuclei (PVT) display a unique slow afterhyperpolarizing potential (sAHP) following the low threshold spike (LTS) generated by activation of their low voltage Ca2+ channels. We evaluated the conductances underlying this sAHP using whole-cell patch-clamp recordings in rat brain slice preparations. Initial observations recorded in the presence of TTX revealed a marked dependency of the LTS-induced sAHP on extracellular Na+: replacing Na+ with TRIS+ in the external medium eliminated the LTS-induced sAHP; substitution of Na+ with either Li+ or choline+ in the external medium resulted in a gradual loss of the sAHP and its replacement with a prolonged slow afterdepolarizing potential (sADP). The LTS-induced sAHP was reduced by quinidine and potentiated by loxapine, suggesting involvement of KNa-like channels. Canonical transient receptor potential (TRPC) channels were considered the source for Na+ based on observations that the sAHP was suppressed by nonselective TRPC channel blockers (2-APB, flufenamic acid and ML204) but unchanged in the presence of TRPV1 channel blocker (SB-366791). In addition, after replacement of Na+ with Li+, the isolated LTS-induced sADP was significantly suppressed in the presence of 2-APB or ML204, after replacement of extracellular Ca2+ with Sr2+, and by intracellular Ca2+ chelation with EGTA, data that collectively suggest involvement of Ca2+-activated TRPC-like conductances containing TRPC4/5 subunits. The isolated LTS-induced sADP also exhibited a strong voltage dependency, decreasing at hyperpolarizing potentials, further support for involvement of TRPC4/5 subunits. This sADP exhibited neurotransmitter receptor sensitivity, with suppression by 5-CT, a 5-HT7 receptor agonist, and enhancement by the neuropeptide orexin A. These data suggest that LTS-induced slow afterpotentials reflect a simultaneous interplay between KNa and TRPC-like conductances, novel for midline thalamic neurons.