Glutamatergic input to the lateral hypothalamus stimulates ethanol intake: role of orexin and melanin-concentrating hormone.

The neurotransmitter glutamate (GLUT), which exerts its fast actions by binding to the ionotropic receptors N-methyl-D-aspartate (NMDA), alpha-amino-5-methyl-3-hydroxy-4-isoxazole propionic acid (AMPA), and kainate (KA) (Monaghan et al., 1989), has been widely studied in mesolimbic brain regions, where GLUT mediates consummatory and reward-related behaviors. These regions include the nucleus accumbens (NAc), where stimulation of the glutamatergic pathway increases cue-evoked sucrose intake (Stuber et al., 2011). They also include the ventral tegmental area (VTA), where blockade of GLUT receptors decreases drug self-administration (Kenny et al., 2009; You et al., 2007). Together with evidence showing relapse to drug use to be attenuated by intra-VTA infusions of a GLUT antagonist (Sun et al., 2005), the ionotropic GLUT receptors in this region appear to be crucial not only in reinforcement but also in the reinstatement of drug intake. A third area implicated in GLUT regulation is the lateral hypothalamus (LH), together with the perifornical area (PF) and the zona incerta (ZI) dorsal to the LH. The LH/PF area, which interacts closely with the NAc and VTA, is important in mediating reward (Aston-Jones et al., 2010; Millan et al., 2010). This may be achieved through GLUT transmission, which has extensive afferents throughout the LH/PF and ZI where it also exists in local interneurons (Gao & van den Pol, 2001; van den Pol & Trombley, 1993). The possibility that GLUT in the LH/PF mediates consummatory behavior is supported by feeding studies, showing local injection of GLUT agonists NMDA and AMPA to elicit feeding (Stanley et al., 2011). Whereas there is indirect evidence for the involvement of LH/PF GLUT in drug addiction (Hamlin et al., 2008; Levy et al., 2007), it remains unclear whether GLUT inputs to the area control the consumption of drugs such as alcohol. This is possible since neural mechanisms underlying alcohol intake are similar to those controlling food intake (Leibowitz, 2007). If GLUT in the LH is involved in controlling alcohol consumption, a neurochemical mechanism through which it acts may include the peptide, hypocretin/orexin (OX), which is expressed in the LH and nearby PF and controls feeding and arousal (de Lecea et al., 1998; Sakurai et al., 1998). This peptide also mediates motivated behaviors such as intake of drugs of abuse (Aston-Jones et al., 2010), including alcohol, as both ethanol exposure and an ethanol-paired cue stimulate LH OX expression (Dayas et al., 2008; Morganstern et al., 2010a) and local injection of OX enhances ethanol intake (Schneider et al., 2007). While evidence has shown OX neurons to express both NMDA and AMPA receptors (Li et al., 2002), it remains to be tested whether GLUT in the LH has effects on OX that, in turn, may influence ethanol intake. In addition to OX, GLUT in the LH may affect neurons expressing the melanin-concentrating hormone (MCH), which also contain GLUT receptors and are expressed in the LH as well as the ZI (Skofitsch et al., 1985; van den Pol et al., 2004). Being an inhibitory peptide, MCH acts differently from OX in many respects. For example, MCH plays an opposite role from OX in regulating arousal state and sleep cycle (Hassani et al., 2009; Willie et al., 2008), and the administration of MCH into the LH suppresses ethanol drinking (Morganstern et al., 2010b). Thus, if GLUT in the LH influences ethanol intake, it may have differential effects on MCH as compared to OX. The current study using brain-cannulated rats investigated the GLUT system in the LH/PF in terms of its impact on the consumption of ethanol and also on local peptide-expressing neurons. In Experiments 1 and 2, Sprague-Dawley rats were trained to voluntarily drink 9% ethanol, and the effects of NMDA and AMPA agonists or antagonists in the LH on ethanol, food, and water consumption were examined. To investigate the mechanism underlying the GLUT-induced changes in ethanol drinking, Experiment 3 tested whether LH injections of GLUT agonists could influence local expression of OX and MCH as measured by in situ hybridization.