Infrared heat treatment reduces food intake and modifies expressions of TRPV3-POMC in the dorsal medulla of obesity prone rats

Abstract

Purpose: Infrared heat, a transient receptor potential vanilloid type-3 (TRPV3) sensitive stimulus, may have potential physiological effects beneficial to treating metabolic syndrome.

Materials and methods: Obesity prone (OP) and obesity resistant (OR) rats were fed for seven days on a high-fat diet. Heat treated OP rats were exposed twice daily to infrared light for 20 min each, separated by 80 min of rest. Food intake, blood pressure, blood glucose, and body weight measurements were taken daily and compared between treated OP rats, untreated OP rats, and OR controls. The animals were perfused with 4% paraformaldehyde, and immunohistochemistry was performed on the coronal brainstem sections with polyclonal antibodies against TRPV3 and pro-opiomelanocortin (POMC). The positive-staining cells in the medulla nuclei were quantified using a microscope with reticule grid.

Results: Food intake, body weight, and mean arterial blood pressure (MAP) were higher in OP rats, a diet-induced metabolic syndrome model, accompanied by a reduced expression of POMC, an anorectic agent, in the hypoglossal nucleus (HN) and medial nucleus tractus solitarius (mNTS). Food intake in heat-treated OP rats was significantly decreased. POMC positive neuron count was increased in the HN and mNTS of OP rats following treatment. TRPV3 positive staining neurons were increased in the HN and mNTS of OP control rats and decreased following the heat treatments.

Conclusion: Lowered POMC and heightened TRPV3 expressions in the HN and mNTS are involved in development of hyperphagia and obesity in OP rats. Exposure to infrared heat modifies TRPV3 and POMC expression in the brainstem, reducing food intake.

Figure 1

Photograph illustrating the experimental set-up used in infrared heat application. The infrared heating device, TDP CQ-27 (Lhasa OMS, Weymouth, MA), was utilised as a heat source. The infrared heat lamp is placed above an open-filtered mouse cage, and temperature is maintained by adjusting the proximity of the lamp to the chamber. Temperature is monitored and kept between 36–40°C by a digital temperature controller (Harvard Bioscience, Holliston, MA), with the sensor placed inside the heating chamber, as labelled.

Figure 2

Bar graph representing food intake of all rats across 8 days of experimentation. Infrared heat treatment started on day 3. OR rats showed significantly lowered food intake compared to OP controls throughout the experiment. OP heated rats showed significantly reduced food intake after 4 days of daily heat treatment. Each bar represents the mean values and vertical lines represent SEM (n=5–7). *P<0.05 (analysis of variance) compared with OR control; #P<0.05 compared with OP control.

Figure 3

These low (left) and high (right) magnification micrographs from medulla sections show the structures and distributions of TRPV3 staining in response to infrared heat treatment in an OR control rat (top), OP control (middle), and OP heated (bottom) rat. Full body infrared heat treatment resulted in a decrease of TRPV3 staining neurons (indicated by arrows) in the hypoglossal nucleus of the medulla (HgN) in an OP heated rat (bottom) compared to an OP control animal (middle).

Figure 4

Quantitation of TRPV3 immunoreactivity in the hypoglossal nucleus and medial nucleus tractus solitarius (mNTS) in infrared heat-treated OP rats compared with OP control and OR control rats. TRPV3 immunostaining cells in the hypoglossal nucleus (left side) are significantly reduced in heat-treated OP rats and OR control rats, in comparison to OP control rats. In the mNTS (right side), OP heated and OR rats also displayed significantly lowered positive staining neuron counts, but the absolute difference was less. Each bar represents the mean values and the vertical lines represent SEM (n=5–7). *P<0.05 (analysis of variance), compared with OR control; #P<0.05 compared with OP control.

Figure 5

Micrographs, low (left) and high (right) magnification, showing number of POMC positive staining neurons in the medial NTS (mNTS) of an OR control (top), OP control (middle), and OP heated rat (bottom). POMC staining cells (indicated by arrows) were increased in the mNTS in the heat-treated OP rat (bottom) and OR control rat (top) when compared to the OP control (middle).

Figure 6

Quantitation of POMC immunoreactivity in the hypoglossal nucleus and medial nucleus tractus solitarius (mNTS) in infrared heat-treated OP rats compared with OP control and OR control rats. POMC immunostaining cells in the hypoglossal nucleus (left side) are increased in heat-treated OP rats and OR control rats, in comparison to OP control rats, but this change was short of statistical significance. In the mNTS (right side), OP heated and OR rats displayed significantly increased POMC positive staining neuron counts. Each bar represents the mean values and the vertical lines represent SEM (n=5–7). *P<0.05 (analysis of variance), compared with OR control; #P<0.05 compared with OP control.