Ghrelin Increases GABAergic Transmission and Interacts with Ethanol Actions in the Rat Central Nucleus of the Amygdala

Abstract

The neural circuitry that processes natural rewards converges with that engaged by addictive drugs. Because of this common neurocircuitry, drugs of abuse have been able to engage the hedonic mechanisms normally associated with the processing of natural rewards. Ghrelin is an orexigenic peptide that stimulates food intake by activating GHS-R1A receptors in the hypothalamus. However, ghrelin also activates GHS-R1A receptors on extrahypothalamic targets that mediate alcohol reward. The central nucleus of the amygdala (CeA) has a critical role in regulating ethanol consumption and the response to ethanol withdrawal. We previously demonstrated that rat CeA GABAergic transmission is enhanced by acute and chronic ethanol treatment. Here, we used quantitative RT-PCR (qRT-PCR) to detect Ghsr mRNA in the CeA and performed electrophysiological recordings to measure ghrelin effects on GABA transmission in this brain region. Furthermore, we examined whether acute or chronic ethanol treatment would alter these electrophysiological effects. Our qRT-PCR studies show the presence of Ghsr mRNA in the CeA. In naive animals, superfusion of ghrelin increased the amplitude of evoked inhibitory postsynaptic potentials (IPSPs) and the frequency of miniature inhibitory postsynaptic currents (mIPSCs). Coapplication of ethanol further increased the ghrelin-induced enhancement of IPSP amplitude, but to a lesser extent than ethanol alone. When applied alone, ethanol significantly increased IPSP amplitude, but this effect was attenuated by the application of ghrelin. In neurons from chronic ethanol-treated (CET) animals, the magnitude of ghrelin-induced increases in IPSP amplitude was not significantly different from that in naive animals, but the ethanol-induced increase in amplitude was abolished. Superfusion of the GHS-R1A antagonists D-Lys3-GHRP-6 and JMV 3002 decreased evoked IPSP and mIPSC frequency, revealing tonic ghrelin activity in the CeA. D-Lys3-GHRP-6 and JMV 3002 also blocked ghrelin-induced increases in GABAergic responses. Furthermore, D-Lys3-GHRP-6 did not affect ethanol-induced increases in IPSP amplitude. These studies implicate a potential role for the ghrelin system in regulating GABAergic transmission and a complex interaction with ethanol at CeA GABAergic synapses.

Ghrelin in psychiatric disorders – A review

“…Furthermore, GABAergic transmission, repeatedly shown to be involved in alcohol intake (Hyytiä and Koob, 1995;McBride and Li, 1998;Cruz et al, 2013), was increased after local ghrelin application in rats (Cruz et al, 2013). Importantly, ghrelin's effects on dopamine release were shown to be mediated by GHSR-1a.…”

“…Since then, a large body of evidence has accumulated which not only confirms this observation but also strongly suggests a role of ghrelin in neurogenesis, neural plasticity, learning, memory and neurodegenerative diseases, most notably Alzheimer's disease (AD) (Carlini et al, 2002(Carlini et al, , 2004Diano et al, 2006;Chung et al, 2008;Atcha et al, 2009;Moon et al, 2011;Bayliss and Andrews, 2013;Martins et al, 2013;Walker et al, 2014). Ghrelin receptors are widely expressed throughout the CNS, especially in the hypothalamus, pituitary gland, hippocampus, ventral tegmental area (VTA) and the amygdala (Diano et al, 2006;Zigman et al, 2006;Andrews, 2011;Landgren et al, 2011;Cruz et al, 2013;Gahete et al, 2014). The findings of the majority of studies are unanimous, suggesting a memory-enhancing and neurogenetic effect as specified below.…”

Central Amygdala Neuroplasticity in Alcohol Dependence

“…In addition, D-Lys3-GHRP-6 does not affect alcoholinduced increases in IPSP. These studies show that ghrelin modulates basal and alcohol-induced changes in GABAergic transmission in the CeA (Cruz et al, 2013).…”

“…When applied alone, alcohol significantly increases IPSP amplitude, but this effect is attenuated by the application of ghrelin. In neurons from alcoholdependent rats, the magnitude of ghrelin-induced increases in IPSP amplitude is not significantly different from that in naïve animals, but the alcohol-induced increase in amplitude is abolished (Cruz et al, 2013). Superfusion of the GHS-R1A antagonists D-Lys3-GHRP-6 and JMV 3002 decrease evoked IPSPs and mIPSC frequency, revealing tonic ghrelin activity in the CeA of naïve rats.…”

“…Using quantitative RT-PCR (qRT-PCR), we detected Ghsr mRNA in the CeA and showed that in naïve animals superfusion of ghrelin increases the amplitude of evoked IPSPs and the frequency of mIPSCs (Cruz, Herman, Cote, Ryabinin, & Roberto, 2013). Coapplication of alcohol further increases the ghrelin-induced enhancement of IPSP amplitude but to a lesser extent than alcohol alone.…”