RIPping off GABA Release in Hypothalamic Circuits Causes Obesity

Abstract

Hypothalamic RIP-expressing neurons regulate energy balance, but the precise neural pathways and neurotransmitters mediating this effect remain undetermined. Kong et al. (2012) now demonstrate that RIP neurons regulate energy expenditure and BAT thermogenesis predominantly via a GABAergic arcuateparaventricular-hindbrain pathway.

The hypothalamus is a critical regulator of energy balance and glucose homeostasis. Over the past 10–15 years a great deal of attention has been devoted to hypothalamic neurons that produce agouti-related peptide (AgRP) and neurons that express proopiomelanocortin (POMC) in part because of the importance of these neuronal populations in the integration of peripheral blood-born signals, including endocrine signals and nutrients (Williams and Elmquist, 2012; Sánchez-Lasheras et al., 2010; Gao and Horvath, 2007). However, the hypothalamus contains a variety of other neuronal systems, and their relative contributions to the regulation of energy balance remains incompletely understood. In addition, a better understanding of the complex neuronal network that regulates appetite and body weight is needed. In a recent issue of Cell, Kong et al. (2012) mapped the synaptic interactions of a subpopulation of GABAergic neurons from the arcuate nucleus (ARH) with other hypothalamic and hindbrain pathways and demonstrated the importance of these neural circuits in the regulation of energy expenditure and brown adipose tissue (BAT) activity (Figure 1).

Figure 1. RIP Neurons Selectively Regulate Energy Expenditure and BAT Activity via a GABAergic ARH-PVH-Hindbrain Pathway.

.A distinct population of neurons in the arcuate nucleus (ARH) expresses RIP and represents a major route for regulation of energy expenditure by leptin, as demonstrated in the study by Kong et al. (2012). These neurons send direct projections to discrete populations of neurons located in the medial parvicellular part of the paraventicular nucleus (PVHmpv), where they release the inhibitory neurotransmitter GABA. The RIP target PVHmpv neurons then send functional efferent connections to the nucleus of the tractus solitarius (NTS), which in return sends GABAergic projections to the raphe pallidus (RPa), another hindbrain nucleus known to regulate sympathetic outflow to brown adipose tissue (BAT).

Empirical experiments using physical lesions of specific hypothalamic nuclei and, more recently, genetic and optogenetic studies have shown the importance of neurons within the ARH, the ventromedial nucleus (VMH), the dorsomedial nucleus (DMH), the paraventricular nucleus (PVH), and the lateral hypothalamic area (LHA) in the regulation of feeding and body weight. Lowell and colleagues used the cre-loxP approach to specifically delete GABAergic neurotransmission in rat insulin promoter (RIP)-expressing neurons; this neuronal population is distinct from the POMC and AgRP neurons and is present in the ARH, VMH, and DMH (Kong et al., 2012). Mice lacking synaptic release of GABA from RIP-containing neurons (Rip-Cre; Vgat^flox/flox mice) display higher body weights and a marked increase in fat mass. The mutant mice also show a greater sensitivity to weight gain on a high-fat diet (HFD). However, the elevated body weight in chow and HFD-fed mice could not be explained by changes in food intake or locomotor activity, but Kong et al. found a marked decrease in oxygen consumption and altered BAT thermogenesis, revealed by reduced interscapular temperature and UCP1 expression, which may be partially responsible for the reduced energy expenditure observed in mice lacking GABA neurotransmission in RIP neurons. In contrast, blockade of glutamatergic release from RIP-expressing neurons did not affect energy balance, indicating that GABA is the major neurotransmitter involved in this process.

One of the most important functions of RIP neurons is to mediate leptin’s effect on energy balance (Covey et al., 2006). Kong et al. (2012) show that the ability of leptin to reduce body weight requires GABA release from RIP-containing neurons. In the absence of GABA neurotransmission, leptin does not reduce body weight, likely due to an inability to increase energy expenditure and BAT activity. RIP neurons are primarily present in the ARH, VMH, and DMH, and each of these nuclei is a direct target for leptin (Patterson et al., 2011). To define the RIP neurons responsible for the bulk of leptin action, Kong et al. (2012) examined leptin-induced pSTAT3, a marker of leptin receptor activation, in Rip-Cre; lox-GFP mice and found cells coexpressing pSTAT3 and Cre mainly in the ARH and VMH, but since VMH neurons are glutamatergic and not GABAergic, the authors conclude that all leptin-responsive Rip-Cre neurons are located in the ARH.

To further demonstrate the relative contribution of ARH RIP neurons to energy balance regulation, Lowell and colleagues used DREADD (designer receptors exclusively activated by designer drugs) technology to specifically control the activation of this neuronal population. They found that chronic stimulation of ARH neurons increases oxygen consumption and BAT activity and that this effect is blunted in the absence of GABA release from ARH RIP neurons. Arcuate neurons project extensively to various parts of the brain (Bouret et al., 2004). The researchers next pursued creative approaches combining electrophysiological, tract-tracing, and optogenetic circuit mapping methods to delineate neural circuits involved in BAT thermogenesis. They found that the vast majority of GABAergic leptin-responsive RIP neurons in the ARH provide direct functional projections to the PVH. This is of particular interest because the PVH is the most thoroughly characterized hypothalamic interface between the endocrine and autonomic systems that influence energy balance (Sawchenko, 1998). Of fundamental importance to the function of the PVH are its parvicellular neurons that regulate sympathetic outflow and BAT function. Extending these findings, Kong et al. (2012) demonstrate that neurons located in the medial parvicellular part of the PVH (PVHmpv) receive direct projections from GABAergic ARH RIP neurons and in return send efferent connections to the nucleus of the tractus solitarius (NTS). They further show that the neural circuits linking the NTS with BAT involve GABAergic projections to the raphe pallidus (RPa), a hindbrain nucleus that contains BAT sympathetic preautonomic neurons.

Like most new and exciting observations, the current study by Kong et al. (2012) prompts many questions for future investigation. Perhaps the most intriguing finding of this paper is that GABAergic ARH RIP neural circuits selectively drive energy expenditure but not other components of energy balance such as food intake. However, it is still not clear which specific GABAergic ARH neuronal population influences energy expenditure. The ARH contains a plethora of intermingled and molecularly defined cell types, each characterized by the expression of specific neuropeptides. The present study rules out the involvement of AgRP/NPY neurons, a well-characterized population of GABAergic neurons in the ARH. However, neuropeptides such as galanin, dynorphin, enkephalin, growth-hormone releasing hormone, neurotensin, neuromedin U, and somatostatin (Lantos et al., 1995) are also expressed in subpopulations of arcuate neurons, and their specific contribution to energy expenditure remains to be established.

The study by Lowell and colleagues reinforces the concept that a sophisticated network of neural circuits is responsible for the regulation of energy expenditure. However, these circuits are probably even more complex than the ones described in this study. For example, the DMH has long been considered an important component of thermogenic BAT circuits. Although Kong et al. (2012) report that the DMH only receives a few GABAergic projections from ARH RIP neurons, it may still mediate some of the thermogenic effects of leptin. Consistent with this idea, a recent study showed that DMH neurons expressing leptin receptor project transsynaptically to the BAT via projections to the RPa (Zhang et al., 2011).

In the face of the growing obesity epidemic and with the realization that obesity is an induced disease of the brain, it is critical that we acquire a better understanding of the neural circuits and networks involved in energy balance regulation, which, in turn, will make it possible to define targets for the development of more specific medicines. The study by Kong et al. (2012) reveals the neural substrates that are selectively involved in energy expenditure and BAT thermogenesis, uncovers a significant piece of the obesity puzzle, and will undoubtedly have important implications for metabolic disease.