Temperature modulates PVN pre-sympathetic neurones via transient receptor potential ion channels.

Abstract

Background and purposeThe paraventricular nucleus (PVN) of the hypothalamus plays a vital role in maintaining homeostasis and controls cardiovascular function via autonomic pre-sympathetic neurones. We have previously shown that coupling between transient receptor potential cation channel subfamily V member 4 (Trpv4) and small-conductance calcium-activated potassium channels (SK) in the PVN facilitate osmosensing. TRP channels are also thermosensitive and therefore, in this report we investigated the temperature sensitivity of PVN neurones. Experimental approachWe identified TRP channel mRNA in mouse PVN using quantitative reverse transcription-PCR (RT-PCR). Using cell-attached patch-clamp electrophysiology, we characterised the thermosensitivity of Trpv4-like ion channels on mouse PVN neurones. Following recovery of temperature sensitive single channel kinetic schema, we constructed a novel and predictive stochastic mathematical model of these neurones. We then validated this model with electrophysiological recordings of action current frequency from mouse PVN neurones. ResultsWe identified 7 TRP channel genes in the PVN with known thermosensitive capabilities. Trpv4 was the most abundant of these and was easily identified at the single channel level using cell-attached patch-clamp electrophysiology on PVN neurones. We investigated the thermosensitivity of these Trpv4-like channels; open probability (Po) markedly decreased when temperature was decreased, mediated by a decrease in mean open dwell times. Our neuronal model predicted that PVN spontaneous action current frequency (ACf) would increase as temperature is decreased and in our electrophysiological experiments, we found that ACf from PVN neurones was significantly higher at lower temperatures. The broad-spectrum channel blocker, gadolinium (100 M), was used to block the warm-activated Ca2+-permeable Trpv4 and Trpv3 channels. In the presence of gadolinium (100 M), the temperature effect was largely retained. Using econazole (10 M), a blocker of Trpm2, we found there were significant increases in overall ACf and the temperature effect was inhibited. ConclusionOur work identified Trpv4 mRNA as an abundantly expressed thermosensitive TRP channel gene in the PVN and this ion channel contributes to the intrinsic thermosensitive properties of PVN neurones. At physiological temperatures (37{degrees}C), we observed relatively low ACf primarily due to the activity of Trpm2 channels, whereas at room temperature, where most of the previous characterisation of PVN neuronal activity has been performed, ACf is much higher, and appears to be predominately due to reduced Trpv4 activity. This work gives insight into the fundamental mechanisms by which the body decodes temperature signals and maintains homeostasis.