Physiological changes in glucose differentially modulate the excitability of hypothalamic melanin-concentrating hormone and orexin neurons in situ

Abstract

The physiological signaling mechanisms that link normal variations in body energy status to the activity of arousal- and metabolism-regulating brain centers are not well understood. The melanin-concentrating hormone (MCH) and orexin/hypocretin types of neurons of the lateral hypothalamus (LH) exert opposing effects on arousal and metabolism. We examined whether shifts in brain extracellular glucose that correspond to physiological changes in blood glucose can alter the electrical output of neurochemically and biophysically defined LH cells in mouse brain slices. Here, we show that physiologically relevant concentrations of glucose dose-dependently enhance the electrical excitability of MCH neurons by inducing depolarization and increasing membrane resistance. We also demonstrate that the same physiological shifts in glucose have the opposite effects on the electrical activity of orexin neurons. We propose that these direct actions of glucose on the arousal- and metabolism-regulating LH neurons play a key role in the translation of normal variations in body energy resources into appropriate changes in arousal and metabolism.

Figure 1.

Electrical responses of LH MCH neurons to physiological changes in glucose. A, Defining electrical signature of MCH neurons: absence of spontaneous firing (i), no H-current-mediated sag (ii), and spike-rate adaptation (iii). Current-clamp protocols used to elicit these responses are shown schematically below the corresponding traces. B, Immunofluorescence imaging of the cell shown in A, identified by Neurobiotin staining (green); the cell contains MCH (red) but not orexin-A (yellow). Scale bar, 20 μm. C, Glucose induced reversible depolarization and spiking in an MCH neuron. D, Glucose enhanced spiking evoked by depolarizing current injection (20 pA for 3 s; protocol shown schematically below the traces); this effect was reversible after glucose washout. E, Dose-response relationship of glucose-induced depolarization of MCH cells (EC₅₀ = 0.8 mm; h = 3.2; V_max = -43.5 mV; V₀ = -54.8 mV; the general equation of the fit is given in Materials and Methods). Numbers of cells are indicated above corresponding points. F, In the presence of tetrodotoxin (TTX) (300 nm), glucose induced depolarization and increased membrane resistance (the latter effect is manifested as increased amplitude of membrane potential responses to hyperpolarizing current pulses; 40 pA for 500 ms at 30 s intervals).

Figure 2.

Electrical responses of LH orexin neurons to physiological changes in glucose. A, Defining electrical signature of orexin neurons: tonic spontaneous firing (i), H-current-mediated sag (ii), low-threshold spike (iii), and little spike-rate adaptation (iv). Current-clamp protocols used to elicit these responses are shown schematically below corresponding traces. B, Immunofluorescence imaging of the cell shown in A, identified by Neurobiotin staining (green); the cell contains orexin-A (yellow) but not MCH (red). Scale bar, 20 μm. C, Glucose induced reversible hyperpolarization and inhibited spiking in an orexin neuron. D, Glucose suppressed the spiking response to depolarizing current injection (40 pA for 3 s; protocol shown schematically below the traces); this effect was reversible after glucose washout. E, Dose-response relationship of glucose-induced hyperpolarization of orexin cells (IC₅₀ = 3.5 mm; h = 1.8; V_max = -78.8 mV; V₀ = -42.9 mV; the general equation of the fit is given in Materials and Methods). Numbers of cells are indicated above corresponding points. F, In the presence of tetrodotoxin (TTX) (300 nm), glucose induced hyperpolarization and decreased membrane resistance (resistance was monitored as described in Fig. 1F).