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Starski PA, De Oliveira Sergio T, Hopf FW. Using lickometry to infer differential contributions of salience network regions during compulsion-like alcohol drinking. ADDICTION NEUROSCIENCE 2023; 7:100102. [PMID: 38736902 PMCID: PMC11086682 DOI: 10.1016/j.addicn.2023.100102] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Subscribe] [Scholar Register] [Indexed: 05/14/2024]
Abstract
Alcohol use disorder extracts substantial personal, social and clinical costs, and continued intake despite negative consequences (compulsion-like consumption) can contribute strongly. Here we discuss lickometry, a simple method where lick times are determined across a session, while analysis across many aspects of licking can offer important insights into underlying psychological and action strategies, including their brain mechanisms. We first describe studies implicating anterior insula (AIC) and dorsal medial prefrontal cortex (dMPF) in compulsion-like responding for alcohol, then review work suggesting that AIC/ventral frontal cortex versus dMPF regulate different aspects of behavior (oral control and overall response strategy, versus moment-to-moment action organization). We then detail our lickometer work comparing alcohol-only drinking (AOD) and compulsion-like drinking under moderate- or higher-challenge (ModChD or HiChD, using quinine-alcohol). Many studies have suggested utilization of one of two main strategies, with higher motivation indicated by more bouts, and greater palatability suggested by longer, faster bouts. Instead, ModChD shows decreased variability in many lick measures, which is unexpected but consistent with the suggested importance of automaticity for addiction. Also surprising is that HiChD retains several behavior changes seen with ModChD, reduced tongue variability and earlier bout start, even though intake is otherwise disrupted. Since AIC-related measures are retained under both moderate- and higher-challenge, we propose a novel hypothesis that AIC sustains overall commitment regardless of challenge level, while disordered licking during HiChD mirrors the effects of dMPF inhibition. Thus, while AIC provides overall drive despite challenge, the ability to act is ultimately determined within the dMPF.
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Affiliation(s)
- Phillip A. Starski
- Indiana University School of Medicine, Department of Psychiatry, Indianapolis IN, USA
| | | | - Frederic W. Hopf
- Indiana University School of Medicine, Department of Psychiatry, Indianapolis IN, USA
- Stark Neurosciences Research Institute, Indianapolis IN, USA
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2
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White SR, Laubach M. The rostral medial frontal cortex is crucial for engagement in consummatory behavior. Behav Neurosci 2022; 136:551-560. [PMID: 35771511 PMCID: PMC9671839 DOI: 10.1037/bne0000523] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The medial frontal cortex (MFC) in rodents emits rhythmic activity that is entrained to the animal's licking cycle during consumption and encodes the value of consumed fluids. These signals are especially prominent in the rostral half of the MFC. This region is located above an orbitofrontal region where mu-opioid receptors regulate intake and reversible inactivation reduces behavioral measures associated with the incentive value and palatability of liquid sucrose. Here, we examined the effects of reversible inactivation and stimulation of mu-opioid receptors in rostral MFC on behavior in an incentive contrast licking task. Adult male rats licked to receive access to liquid sucrose, which alternated between high (16%) and low (4%) values over 30 s periods. Bilateral infusion of muscimol reduced the total number of licks over the 30 min test sessions, the time spent actively consuming sucrose, and the ratio of licks for the higher and lower value fluids. Inactivation did not alter licking frequency or variability or microstructural measures such as the duration of licking bouts that are classically associated with the palatability of a liquid reward. Infusions of [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO; 1 μg/μL) at the same sites had inconsistent behavioral effects across different subjects. Our findings suggest that the rostral MFC has a distinct role in the control of consummatory behavior and contributes to persistent consumption and not to the expression of palatability. (PsycInfo Database Record (c) 2022 APA, all rights reserved).
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Affiliation(s)
- Samantha R. White
- Department of Neuroscience, American University, Washington, DC, USA, 20016
| | - Mark Laubach
- Department of Neuroscience, American University, Washington, DC, USA, 20016
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3
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Amarante LM, Laubach M. Coherent theta activity in the medial and orbital frontal cortices encodes reward value. eLife 2021; 10:e63372. [PMID: 34505830 PMCID: PMC8457826 DOI: 10.7554/elife.63372] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2020] [Accepted: 09/09/2021] [Indexed: 01/03/2023] Open
Abstract
This study examined how the medial frontal (MFC) and orbital frontal (OFC) cortices process reward information. We simultaneously recorded local field potentials in the two areas as rats consumed liquid sucrose rewards. Both areas exhibited a 4-8 Hz 'theta' rhythm that was phase-locked to the lick cycle. The rhythm tracked shifts in sucrose concentrations and fluid volumes, demonstrating that it is sensitive to differences in reward magnitude. The coupling between the rhythm and licking was stronger in MFC than OFC and varied with response vigor and absolute reward value in the MFC. Spectral analysis revealed zero-lag coherence between the cortical areas, and found evidence for a directionality of the rhythm, with MFC leading OFC. Our findings suggest that consummatory behavior generates simultaneous theta range activity in the MFC and OFC that encodes the value of consumed fluids, with the MFC having a top-down role in the control of consumption.
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Affiliation(s)
- Linda M Amarante
- Department of Neuroscience, American UniversityWashington DCUnited States
| | - Mark Laubach
- Department of Neuroscience, American UniversityWashington DCUnited States
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Laubach M, Amarante LM, Caetano MS, Horst NK. Reward signaling by the rodent medial frontal cortex. INTERNATIONAL REVIEW OF NEUROBIOLOGY 2020; 158:115-133. [PMID: 33785143 DOI: 10.1016/bs.irn.2020.11.012] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
Abstract
The anatomical relevance and functional significance of medial parts of the rodent frontal cortex have been intensely debated over the modern history of neuroscience. Early studies emphasized common functions among medial frontal regions in rodents and the dorsolateral prefrontal cortex of primates. Behavioral tasks emphasized memory-guided performance and persistent neural activity as a marker of working memory. Over time, it became clear that long-standing concerns about cross-species homology were justified and the view emerged that rodents are useful for understanding medial parts of the frontal cortex in primates, and not the dorsolateral prefrontal cortex. Here, we summarize a series of studies on the rodent medial frontal cortex that began with an interest in studying working memory in the perigenual prelimbic area and ended up studying reward processing in the medial orbital region. Our experiments revealed a role for a 4-8Hz "theta" rhythm in tracking engagement in the consumption of rewarding fluids and denoting the value of a given reward. Evidence for a functional differentiation between the rostral and caudal medial frontal cortex and its relationship to other frontal cortical areas is also discussed with the hope of motivating future work on this part of the cerebral cortex.
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Affiliation(s)
- Mark Laubach
- Department of Neuroscience, American University, Washington, DC, United States.
| | - Linda M Amarante
- Department of Neuroscience, Johns Hopkins University, Baltimore, MD, United States
| | - Marcelo S Caetano
- Center for Mathematics, Computing, and Cognition, Universidade Federal do ABC (UFABC), Santo André, Brazil
| | - Nicole K Horst
- Centre for Teaching and Learning and Postdoc Academy, University of Cambridge, Cambridge, England
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Darevsky D, Hopf FW. Behavioral indicators of succeeding and failing under higher-challenge compulsion-like alcohol drinking in rat. Behav Brain Res 2020; 393:112768. [PMID: 32544510 PMCID: PMC7444822 DOI: 10.1016/j.bbr.2020.112768] [Citation(s) in RCA: 22] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2020] [Revised: 06/05/2020] [Accepted: 06/07/2020] [Indexed: 01/04/2023]
Abstract
Intake despite negative consequences (compulsivity) contributes strongly to the harm of alcohol use disorder, making the underlying psychological and circuit mechanisms of great importance. To gain insight into possible underlying action strategies, we compared rat licking microstructure across compulsion-like and non-compulsive conditions. We previously showed that drinking under a moderate-challenge, quinine-alcohol model (Alc-ModQ) shows less variable responding in many measures, suggesting a more automatic strategy to overcome challenge. Here, we reanalyzed our original data, newly focusing on the behavioral profile of higher-challenge intake (100 mg/L quinine in alcohol, Alc-HighQ). Alc-HighQ greatly dropped consumption, yet retained aspects of greater automaticity and drive seen with Alc-ModQ, including earlier bout initiation and measures suggesting more stereotyped tongue control. In contrast, Alc-HighQ disordered bout generation and timing. Importantly, only fast-starting bouts persisted under Alc-HighQ, and while there were many fewer longer Alc-HighQ bouts, they still contributed >50 % of consumption. Also, longer bouts under Alc-HighQ had an early, several-second period with greater chance of stopping, but afterwards showed similar persistence and recovery from slow licking as other drinking conditions. Together, our findings elucidate novel behavioral indicators of successful and unsuccessful epochs of Alc-HighQ, compulsion-like intake. We also relate findings to congruent human and animal work implicating anterior insula and medial prefrontal cortices as critical for compulsion-like alcohol responding, and where ventral frontal cortex has been more associated with overall action plan and tongue control (retained under Alc-HighQ), with medial cortex more related to proximal action timing (disrupted under Alc-HighQ except after faster bout initiation).
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Affiliation(s)
- David Darevsky
- University of California at Berkeley, University of California at San Francisco (UCSF), Graduate Program in Bioengineering, United States; UCSF Medical Scientist Training Program, San Francisco, CA, United States; Department of Neurology, UCSF, United States
| | - Frederic W Hopf
- Department of Neurology, UCSF, United States; Department of Psychiatry, Indiana University School of Medicine, Indianapolis, IN, United States.
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Yang AK, Mendoza JA, Lafferty CK, Lacroix F, Britt JP. Hippocampal Input to the Nucleus Accumbens Shell Enhances Food Palatability. Biol Psychiatry 2020; 87:597-608. [PMID: 31699294 DOI: 10.1016/j.biopsych.2019.09.007] [Citation(s) in RCA: 22] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/10/2019] [Revised: 08/28/2019] [Accepted: 09/12/2019] [Indexed: 01/12/2023]
Abstract
BACKGROUND Insight into the neural basis of hedonic processing has come from studies of food palatability in rodents. Pharmacological manipulations of the nucleus accumbens shell (NAcSh) have repeatedly been demonstrated to increase hedonic taste reactivity, yet the contribution of specific NAcSh circuit components is unknown. METHODS Bidirectional optogenetic manipulations were targeted to the principal NAcSh projection neurons and afferent pathways in mice during free feeding assays. Number of licks per bout of consumption was used as a measure of food palatability as it was confirmed to track sucrose concentration and subjective flavor preferences. RESULTS Photoinhibition of NAcSh neurons, whether general or cell-type specific, was found to alter consumption without affecting its hedonic impact. Among the principal excitatory afferent pathways, we showed that ventral hippocampal (vHipp) input alone enhances palatability upon low-frequency photostimulation time-locked to consumption. This enhancement in palatability was independent of opioid signaling and not recapitulated by NAcSh or dopamine neuron photostimulation. We further demonstrated that vHipp input photostimulation is sufficient to condition a flavor preference, while its inhibition impedes sucrose-driven flavor preference conditioning. CONCLUSIONS These results demonstrate a novel contribution of vHipp-NAcSh pathway activity to palatability that may relate to its innervation of a particular region or neuronal ensemble in the NAcSh. These findings are consistent with the evidence that vHipp-NAcSh activity is relevant to the pathophysiology of anhedonia and depression as well as the increasing appreciation of hippocampal involvement in people's food pleasantness ratings, hunger, and weight.
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Affiliation(s)
- Angela K Yang
- Integrated Program in Neuroscience, McGill University, Montreal, Quebec, Canada; Center for Studies in Behavioral Neurobiology, Concordia University, Montreal, Quebec, Canada
| | - Jesse A Mendoza
- Department of Psychology, McGill University, Montreal, Quebec, Canada; Center for Studies in Behavioral Neurobiology, Concordia University, Montreal, Quebec, Canada
| | - Christopher K Lafferty
- Department of Psychology, McGill University, Montreal, Quebec, Canada; Center for Studies in Behavioral Neurobiology, Concordia University, Montreal, Quebec, Canada
| | - Franca Lacroix
- Department of Psychology, McGill University, Montreal, Quebec, Canada
| | - Jonathan P Britt
- Integrated Program in Neuroscience, McGill University, Montreal, Quebec, Canada; Department of Psychology, McGill University, Montreal, Quebec, Canada; Center for Studies in Behavioral Neurobiology, Concordia University, Montreal, Quebec, Canada.
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Arnsten AF, Jin LE, Gamo NJ, Ramos B, Paspalas CD, Morozov YM, Kata A, Bamford NS, Yeckel MF, Kaczmarek LK, El-Hassar L. Role of KCNQ potassium channels in stress-induced deficit of working memory. Neurobiol Stress 2019; 11:100187. [PMID: 31832507 PMCID: PMC6889760 DOI: 10.1016/j.ynstr.2019.100187] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2018] [Revised: 06/21/2019] [Accepted: 07/23/2019] [Indexed: 12/25/2022] Open
Abstract
The prefrontal cortex (PFC) mediates higher cognition but is impaired by stress exposure when high levels of catecholamines activate calcium-cAMP-protein kinase A (PKA) signaling. The current study examined whether stress and increased cAMP-PKA signaling in rat medial PFC (mPFC) reduce pyramidal cell firing and impair working memory by activating KCNQ potassium channels. KCNQ2 channels were found in mPFC layers II/III and V pyramidal cells, and patch-clamp recordings demonstrated KCNQ currents that were increased by forskolin or by chronic stress exposure, and which were associated with reduced neuronal firing. Low dose of KCNQ blockers infused into rat mPFC improved cognitive performance and prevented acute pharmacological stress-induced deficits. Systemic administration of low doses of KCNQ blocker also improved performance in young and aged rats, but higher doses impaired performance and occasionally induced seizures. Taken together, these data demonstrate that KCNQ channels have powerful influences on mPFC neuronal firing and cognitive function, contributing to stress-induced PFC dysfunction.
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Affiliation(s)
- Amy F.T. Arnsten
- Departments of Neuroscience, Yale University School of Medicine, USA
| | - Lu E. Jin
- Departments of Neuroscience, Yale University School of Medicine, USA
| | - Nao J. Gamo
- Departments of Neuroscience, Yale University School of Medicine, USA
| | - Brian Ramos
- Departments of Neuroscience, Yale University School of Medicine, USA
| | | | - Yury M. Morozov
- Departments of Neuroscience, Yale University School of Medicine, USA
| | - Anna Kata
- Departments of Neuroscience, Yale University School of Medicine, USA
| | - Nigel S. Bamford
- Pediatric Neurology, Cellular and Molecular Physiology, Yale University School of Medicine, USA
| | - Mark F. Yeckel
- Departments of Neuroscience, Yale University School of Medicine, USA
| | | | - Lynda El-Hassar
- Departments of Neuroscience, Yale University School of Medicine, USA
- Pharmacology, Yale University School of Medicine, USA
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Al Massadi O, Nogueiras R, Dieguez C, Girault JA. Ghrelin and food reward. Neuropharmacology 2019; 148:131-138. [PMID: 30615902 DOI: 10.1016/j.neuropharm.2019.01.001] [Citation(s) in RCA: 45] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/03/2018] [Revised: 01/02/2019] [Accepted: 01/02/2019] [Indexed: 12/14/2022]
Abstract
Food intake is tightly regulated by homeostatic and reward mechanisms and the adequate function of both is necessary for the proper maintenance of energy balance. Ghrelin impacts on these two levels to induce feeding. In this review, we present the actions of ghrelin in food reward, including their dependence on other relevant modulators implicated in the motivational aspects of feeding, including dopamine, opioid peptides, and endocannabinoids. We also describe the interaction between brain areas involved in homeostatic regulation of feeding and the reward system, with a special emphasis on the role of arcuate nucleus melanocortins and lateral hypothalamus orexins in ghrelin function. Finally, we briefly discuss the actions of ghrelin in food reward in obesity. We propose that new insights into the mechanism of action of ghrelin in the rewarding and motivational control of food intake will help to understand food-related disorders including obesity and anorexia.
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Affiliation(s)
- Omar Al Massadi
- Inserm UMR-S 839, 75005, Paris, France; Sorbonne Université, Sciences and Engineering Faculty, 75005 Paris, France; Institut du Fer a Moulin, 75005, Paris, France.
| | - Ruben Nogueiras
- Department of Physiology, CIMUS, University of Santiago de Compostela-Instituto de Investigación Sanitaria, Santiago de Compostela, 15782, Spain; CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn), 15706, Spain
| | - Carlos Dieguez
- Department of Physiology, CIMUS, University of Santiago de Compostela-Instituto de Investigación Sanitaria, Santiago de Compostela, 15782, Spain; CIBER Fisiopatología de la Obesidad y Nutrición (CIBERobn), 15706, Spain
| | - Jean-Antoine Girault
- Inserm UMR-S 839, 75005, Paris, France; Sorbonne Université, Sciences and Engineering Faculty, 75005 Paris, France; Institut du Fer a Moulin, 75005, Paris, France
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Amarante LM, Caetano MS, Laubach M. Medial Frontal Theta Is Entrained to Rewarded Actions. J Neurosci 2017; 37:10757-10769. [PMID: 28978665 PMCID: PMC5666591 DOI: 10.1523/jneurosci.1965-17.2017] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/11/2017] [Revised: 09/21/2017] [Accepted: 09/25/2017] [Indexed: 01/13/2023] Open
Abstract
Rodents lick to consume fluids. The reward value of ingested fluids is likely to be encoded by neuronal activity entrained to the lick cycle. Here, we investigated relationships between licking and reward signaling by the medial frontal cortex (MFC), a key cortical region for reward-guided learning and decision-making. Multielectrode recordings of spike activity and field potentials were made in male rats as they performed an incentive contrast licking task. Rats received access to higher- and lower-value sucrose rewards over alternating 30 s periods. They learned to lick persistently when higher-value rewards were available and to suppress licking when lower-value rewards were available. Spectral analysis of spikes and fields revealed evidence for reward value being encoded by the strength of phase-locking of a 6-12 Hz theta rhythm to the rats' lick cycle. Recordings during the initial acquisition of the task found that the strength of phase-locking to the lick cycle was strengthened with experience. A modification of the task, with a temporal gap of 2 s added between reward deliveries, found that the rhythmic signals persisted during periods of dry licking, a finding that suggests the MFC encodes either the value of the currently available reward or the vigor with which rats act to consume it. Finally, we found that reversible inactivations of the MFC in the opposite hemisphere eliminated the encoding of reward information. Together, our findings establish that a 6-12 Hz theta rhythm, generated by the rodent MFC, is synchronized to rewarded actions.SIGNIFICANCE STATEMENT The cellular and behavioral mechanisms of reward signaling by the medial frontal cortex (MFC) have not been resolved. We report evidence for a 6-12 Hz theta rhythm that is generated by the MFC and synchronized with ongoing consummatory actions. Previous studies of MFC reward signaling have inferred value coding upon temporally sustained activity during the period of reward consumption. Our findings suggest that MFC activity is temporally sustained due to the consumption of the rewarding fluids, and not necessarily the abstract properties of the rewarding fluid. Two other major findings were that the MFC reward signals persist beyond the period of fluid delivery and are generated by neurons within the MFC.
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Affiliation(s)
- Linda M Amarante
- Department of Biology and Center for Behavioral Neuroscience, American University, Washington, DC 20016, and
| | - Marcelo S Caetano
- Center for Mathematics, Computation and Cognition, Universidade Federal do ABC (UFABC), Rua Santa Adélia, 166. Santo André-SP-09210-170, Brazil
| | - Mark Laubach
- Department of Biology and Center for Behavioral Neuroscience, American University, Washington, DC 20016, and
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Pharmacological modulation of the voltage-gated neuronal Kv7/KCNQ/M-channel alters the intrinsic excitability and synaptic responses of pyramidal neurons in rat prefrontal cortex slices. Acta Pharmacol Sin 2017; 38:1248-1256. [PMID: 28603289 DOI: 10.1038/aps.2017.72] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2016] [Accepted: 02/09/2017] [Indexed: 12/20/2022] Open
Abstract
The prefrontal cortex (PFC) critical for higher cognition is implicated in neuropsychiatric diseases, such as Alzheimer's disease, depression and schizophrenia. The voltage-activated Kv7/KCNQ/M-channel or M-current modulates the neuronal excitability that defines the fundamental mechanism of brain function. However, whether M-current functions to regulate the excitability of PFC neurons remains elusive. In this study, we recorded the native M-current from PFC layer V pyramidal neurons in rat brain slices and showed that it modulated the intrinsic excitability and synaptic responses of PFC pyramidal neurons. Application of a specific M-channel blocker XE991 (40 μmol/L) or opener retigabine (10 μmol/L) resulted in inhibition or activation of M-current, respectively. In the current-clamp recordings, inhibition of M-current was evidenced by the increased average spike frequency and the reduced first inter-spike interval (ISI), spike onset latency and fast afterhyperpolarization (fAHP), whereas activation of M-current caused opposite responses. Furthermore, inhibition of M-current significantly increased the amplitude of excitatory postsynaptic potentials (EPSPs) and depolarized the resting membrane potential (RMP) without affecting the miniature EPSC (mEPSC) frequency. These data demonstrate that voltage-gated neuronal Kv7/KCNQ/M-current modulates the excitability and synaptic transmission of PFC neurons, suggesting that pharmacological modulation of M-current in the PFC may exert beneficial effects on cognitive deficits implicated in the pathophysiology of neuropsychiatric disorders.
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From Belly to Brain: Targeting the Ghrelin Receptor in Appetite and Food Intake Regulation. Int J Mol Sci 2017; 18:ijms18020273. [PMID: 28134808 PMCID: PMC5343809 DOI: 10.3390/ijms18020273] [Citation(s) in RCA: 92] [Impact Index Per Article: 13.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2016] [Accepted: 01/19/2017] [Indexed: 12/20/2022] Open
Abstract
Ghrelin is the only known peripherally-derived orexigenic hormone, increasing appetite and subsequent food intake. The ghrelinergic system has therefore received considerable attention as a therapeutic target to reduce appetite in obesity as well as to stimulate food intake in conditions of anorexia, malnutrition and cachexia. As the therapeutic potential of targeting this hormone becomes clearer, it is apparent that its pleiotropic actions span both the central nervous system and peripheral organs. Despite a wealth of research, a therapeutic compound specifically targeting the ghrelin system for appetite modulation remains elusive although some promising effects on metabolic function are emerging. This is due to many factors, ranging from the complexity of the ghrelin receptor (Growth Hormone Secretagogue Receptor, GHSR-1a) internalisation and heterodimerization, to biased ligand interactions and compensatory neuroendocrine outputs. Not least is the ubiquitous expression of the GHSR-1a, which makes it impossible to modulate centrally-mediated appetite regulation without encroaching on the various peripheral functions attributable to ghrelin. It is becoming clear that ghrelin’s central signalling is critical for its effects on appetite, body weight regulation and incentive salience of food. Improving the ability of ghrelin ligands to penetrate the blood brain barrier would enhance central delivery to GHSR-1a expressing brain regions, particularly within the mesolimbic reward circuitry.
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