Blood pressure regulation and cardiac autonomic control in mice overexpressing alpha- and gamma-melanocyte stimulating hormone.

Melanocyte stimulating hormones (MSH) derived from pro-opiomelanocortin have been demonstrated to participate in the central regulation of cardiovascular functions. The aim of the present study was to elucidate the chronic effects of increased melanocortin activation on blood pressure regulation and autonomic nervous system function. We adapted telemetry to transgenic mice overexpressing alpha- and gamma-MSH and measured blood pressure, heart rate and locomotor activity, and analyzed heart rate variability (HRV) in the frequency-domain as well as baroreflex function by the sequence technique. Transgenic (MSH-OE) mice had increased systolic blood pressure but their heart rate was similar to wild-type (WT) controls. The 24-h mean of systolic blood pressure was 132+/-7mmHg in MSH-OE and 113+/-4mmHg in WT mice. Locomotor activity was decreased in the MSH-OE mice. Furthermore, MSH-OE mice showed slower adaptation to mild environmental stress in terms of blood pressure changes. The low frequency (LF) power of HRV tended to be higher in MSH-OE mice compared to WT mice, without a difference in overall variability. The assessment of baroreflex function indicated enhanced baroreflex effectiveness and more frequent baroreflex operations in MSH-OE mice. Baseline heart rate, increased LF power of HRV and increased baroreflex activity may all reflect maintenance of baroreflex integrity and an increase in cardiac vagal activity to counteract the increased blood pressure. These results provide new evidence that long-term activation of the melanocortin system elevates blood pressure without increasing heart rate.

Genetic and pharmacological mouse models of chronic melanocortin activation show enhanced baroreflex control of heart rate.

The central melanocortin system is an important regulator of energy homeostasis and cardiovascular functions. Although the acute effects of melanocortins on central blood pressure regulation are well-established, their long-term effects on autonomic balance and baroreflex function remain largely unexplored. Here we investigated the impact of chronic melanocortin activation on cardiovascular and autonomic nervous system functions by studying α- and γ-MSH overexpressing (MSH-OE) mice and, as pharmacological model, mice treated with the stable α-MSH analogue melonotan-II (MT-II, 0.3 mg/kg/day for 7 days, i.p.). Mean arterial pressure (MAP) and heart rate (HR) were measured in conscious mice by radiotelemetry. MSH-OE mice did not differ from their wild-type littermates in terms of MAP, but displayed reduced HR under physiological baseline conditions. To evaluate the relative activities of sympathetic and parasympathetic nervous systems, we applied autonomic receptor blockers and found an enhanced HR response to atropine in MSH-OE mice, indicating increased cardiac vagal activity. The compensatory increase in HR after drug-evoked vasodilatation was also augmented in MSH-OE mice. Exposure to a high-sodium diet (8% NaCl) markedly reduced HR in MSH-OE mice without concomitant changes in blood pressure, suggesting improved reflex regulation of HR. Chronic treatment with MT-II did not change 24-h MAP or HR regardless of the acute pressor and tachycardic actions of MT-II. Consistent with the finding in MSH-OE mice, MT-II-treated mice showed an enhanced HR response to vasodilatation. These observations suggest that chronic melanocortin activation may provide cardioprotective regulation by enhancing vagal nerve activity and baroreflex control of heart rate.