Optogenetic Activation of Leptin‐Receptor Expressing Neurons in the Region of the Dorsomedial Hypothalamus Increases Respiratory Motor Output in Mice

Abstract

Administration of the anorexigenic hormone leptin to rodents increases energy expenditure with a parallel increase in respiratory motor output in order to maintain arterial blood gas and acid‐base balance. This occurs at least in part through activation of CNS neurons that express the long form of the leptin receptor (LepRb). LepRb expressing neurons are concentrated in several clusters, most notably in the hypothalamus and medulla. Previous studies have linked respiratory motor stimulation to activation of LepRb neurons in the medulla. In this study we sought to determine whether hypothalamic neurons also contribute to the respiratory stimulation. Specifically, LepRb neurons are concentrated in the dorsomedial nucleus of the hypothalamus (DMH) and activation of DMH neurons is known to stimulate breathing. However, whether LepRb expressing neurons contribute to the stimulation is not known. We have initiated studies to determine whether DMH LepRb neurons contribute to the respiratory stimulation using mice that express channelrhodopsin‐2 (ChR2) in LepRb neurons. Mice were anesthetized and an optical fiber (200 um diameter) conducting 473 nm wavelength light was used to activate the ChR2‐LepRb neurons. Light intensity was 4 mW, below the threshold for nonspecific activation or damage to tissue in the vicinity of the fiber. For stimulation, 3 min (or shorter) trains of light pulses at 5, 10, 20 and 50 Hz (≤10 ms pulse width) were used. In parallel control experiments, areas dorsal or lateral to the DMH were stimulated. Activating the DMH elicited a progressive increase in respiratory motor output, consisting primarily of an increase in the peak amplitude of integrated phrenic nerve activity with little change in the number of phrenic bursts per minute. The phrenic amplitude continued to increase after the end of the stimulation, averaging 189 +/− 23% of control (n = 3) at 30 min and 212 +/−17% (n = 2) at 60 min after the end of stimulation. Our preliminary conclusion is that the DMH contributes significantly to the leptin‐mediated stimulation of breathing.Support or Funding InformationWe are grateful for the support from NIH grant HL122921.