Serotonin 2C receptor activates a distinct population of arcuate pro-opiomelanocortin neurons via TRPC channels

Abstract

Serotonin 2C receptors (5-HT(2C)Rs) expressed by pro-opiomelanocortin (POMC) neurons of hypothalamic arcuate nucleus regulate food intake, energy homeostasis and glucose metabolism. However, the cellular mechanisms underlying the effects of 5-HT to regulate POMC neuronal activity via 5-HT(2C)Rs have not yet been identified. In the present study, we found the putative transient receptor potential C (TRPC) channels mediate the activation of a subpopulation of POMC neurons by mCPP (a 5-HT(2C)R agonist). Interestingly, mCPP-activated POMC neurons were found to be a distinct population from those activated by leptin. Together, our data suggest that 5-HT(2C)R and leptin receptors are expressed by distinct subpopulations of arcuate POMC neurons and that both 5-HT and leptin exert their actions in POMC neurons via TRPC channels.

Figure 1. mCPP depolarizes POMC-hrGFP neurons

(A) Brightfield illumination of POMC-hrGFP neuron during acquisition of a whole-cell recording (arrow indicates the targeted cell). Scale bar = 10 μm. (B) The same neuron under fluorescent (FITC) illumination to identify POMC-hrGFP signal. (C) Image shows the complete dialysis of Alexa Fluor 594 from the intracellular pipette at the end of the recording. (D) Image illustrates colocalization of Alexa Fluor 594 with the GFP expression in the same POMC-hrGFP neuron. (E) Post hoc identification of recorded neuron within the arcuate nucleus. Scale bar = 100 μm. (F) Current-clamp recording demonstrates that mCPP (4 μM) depolarizes some POMC-hrGFP neurons. Continuous recordings were interrupted to apply current step pulses as indicated (arrows). Dashed line indicates the resting membrane potential. (G) Current-clamp recording demonstrates that mCPP still depolarizes POMC-hrGFP neurons in the presence of TTX (1 μM). (H) Bar graph summarizing the averaged changes in membrane potential of POMC-hrGFP neurons. Numbers in parentheses indicate number of cells tested. Changes in membrane potential were not affected by TTX, but were significantly attenuated by TRPC channel blockers (SKF96365 & 2-APB), by replacing external Na⁺ and Ca²⁺ (in Ca²⁺ free/Choline), and by a PLC blocker (U73122). Data are presented in mean ± SEM and * indicates P<0.05.

Figure 2. mCPP-activated POMC neurons are located in distinct areas of arcuate nucleus

(A) Representative drawings at four rostrocaudal levels of the mouse hypothalamus illustrate the location of mCPP-activated POMC-hrGFP neurons (red dots). (B) Drawings at three rostrocaudal levels of the mouse hypothalamus demonstrate the location mCPP-activated POMC-hrGFP neurons (red dots) in the presence of TTX. 3V: 3rd ventricle, arc: arcuate nucleus, d3V: dorsal 3rd ventricle, DMH: dorsomedial hypothalamic nucleus, F: fornix, LV: lateral ventricle, ME: median eminence, opt: optic tract, PMD: premammillary nucleus, dorsal part, PMV: premammillary nucleus, ventral part, RCA: retrochiasmatic area, sox: supraoptic decussation, VMH: ventromedial hypothalamic nucleus.

Figure 3. POMC-hrGFP neuron depolarization by mCPP is not mediated by the inhibition of GIRK channels

(A) Voltage clamp recordings of membrane currents at a holding potential of −40 mV. TTX (0.5 μM) was present in the bath solution throughout the recording. Successive application of baclofen (50 μM) elicited outward GIRK currents with similar amplitudes. (B) Pretreatment with mCPP prior to the second application of baclofen significantly reduced the second GIRK amplitudes. (C) Bar graph summarizing the averaged ratio of first and second GIRK current amplitudes (I₂/I₁). Data are presented in mean ± SEM and ** indicates P<0.01. Note that GABA_B-activated GIRK currents are almost completely blocked by CGP54626 (2 μM). (D) Bath application of CGP54626 (2 μM) failed to affect the membrane potential of 9 POMC-hrGFP neurons tested. (E) mCPP depolarized some POMC-hrGFP neurons from GIRK1 knockout mice. Continuous recordings were interrupted to apply current step pulses as indicated (arrows) (a). mCPP decreased voltage deflections in response to hyperpolarizing current steps as shown at the bottom (b). Current versus voltage (I–V) plot from same neuron illustrating a characteristic decrease in input resistance subsequent to mCPP application. Shown are responses before (control) and during mCPP application. Note that the input resistance was decreased by mCPP (c).

Figure 4. mCPP decreases input resistance of POMC-hrGFP neurons by activating TRPC channels

(A) Current clamp recording from a POMC-hrGFP neuron shows decreased voltage deflection and increased action potential frequency after mCPP application (upper and middle traces), and its recovery (lower trace). Hyperpolarizing current steps were applied as shown at the bottom (500 ms; −10 to −50 pA). (B) Current versus voltage (I–V) plot from same neuron illustrating a characteristic decrease in input resistance subsequent to mCPP application. Shown are responses before (control) and during mCPP application. Note that the input resistance was decreased by mCPP. (C) Decreased whole-cell input resistance from the neuron groups examined. Decreases in input resistance were not affected by TTX, but were significantly attenuated TRPC channel blockers (SKF96365 & 2-APB), by replacing external Na⁺ and Ca²⁺ (in Ca²⁺ free/Choline), and by a PLC blocker (U73122). Data are presented in mean ± SEM and ** indicates P<0.01. (D) mCPP failed to depolarize POMC-hrGFP neurons in the presence of SKF96365. (E) mCPP failed to depolarize when external Na⁺ and Ca²⁺ were replaced. 1 μM TTX was added to the external solution.

Figure 5. POMC-hrGFP neurons are activated by either mCPP or leptin, but not by both

(A) In 4 POMC-hrGFP neurons depolarized by mCPP, subsequent application of leptin did not change the membrane potential. (B) Leptin depolarized the membrane potential in 7 out of 28 POMC-hrGFP neurons that were not depolarized by mCPP. (C) Drawings at four rostrocaudal levels of the mouse hypothalamus demonstrate the location mCPP-activated (red dots) and leptin-activated (blue dots) POMC-hrGFP neurons. See Figure 2 for abbreviations.

Figure 6. mCPP and leptin activates distinct population of POMC-hrGFP neurons

(A-1) LepR-positive POMC-hrGFP neurons with both red and green fluorescence were targeted for electrophysiological characterization with recording pipettes containing Alexa Fluor 350. (A-2) Leptin depolarized 11 out of 16 LepR-positive POMC-hrGFP neurons. (B-1) LepR-negative POMC-hrGFP neurons with green fluorescence only were targeted for electrophysiological characterization. (B-2) mCPP depolarized 5 out of 11 LepR-negative POMC-hrGFP neurons. Scale bar = 10 μm. (C) Drawings at three rostrocaudal levels of the mouse hypothalamus demonstrate the location leptin-activated LepR-positive POMC-hrGFP neurons (red dots). Note that mCPP did not activate any of these neurons. (D) Drawings at three rostrocaudal levels of the mouse hypothalamus demonstrate the location mCPP-activated LepR-negative POMC-hrGFP neurons (red dots). Note that leptin did not activate any of these neurons. See Figure 2 for abbreviations.