Neurones containing orexin A are clustered in a region classically associated with feeding regulation. Many of the neurones in this region are thought to contribute to feeding regulation by monitoring plasma glucose levels (Song et al., 2001; Eggerman et al., 2003). Here we have investigated the glucose sensitivity of orexin containing neurones.

Wistar rats, either sex, 15-30 days old were deeply anaesthetised (ketamine HCl, 60mg/kg), decapitated and hypothalamic slices prepared. Intracellular recordings were made from 62 neurones in the perifornical region and the responses of most of these neurones to 0.1, 2.5 and 5mM glucose and an application of orexin A (1µM) were tested. The neurones were then filled with neurobiotin for subsequent double immunofluorescent staining to identify those neurones that expressed orexin.

Seven neurones were found to contain orexin A. Their resting membrane potential ranged from -35 to -55 mV (Mean±SD = -44.29±9.72) (Fig.1). All but one showed a rebound spike following hyperpolarising pulses (Fig.2). Four of the five neurones tested were hyperpolarised (-3.40±4.4mV) by increased glucose, or depolarised by lowering glucose (5.03±2.48). Bath application of orexin A peptide (1µM) produced hyperpolarization and reduction of spontaneous firing in two of four neurones tested.

Fig.1. Spontaneous activity of orexin cell.

Fig.2. After-hyperpolarisation rebound spike

Orexin A containing neurones were multipolar with long thin beaded dendrites with few branches. Axons of four neurones projected ventrally towards the arcuate nucleus, two projected ventrolaterally towards the optic tract and one projected dorsally reaching zona incerta area, suggesting that different groups of orexin neurones project to different targets. Some received synaptic contacts from orexin A containing boutons, suggesting that they express autoreceptors that could account for their hyperpolarisation by orexin A.

Eggerman et al. (2003). J Neurosci. 23, 1557-1562.
Song et al. (2001). Diabetes. 50, 2673-2674.

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