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Abstract
Attention deficit hyperactivity disorder (ADHD) presents special challenges for drug development. Current treatment with psychostimulants and nonstimulants is effective, but their mechanism of action beyond the cellular level is incompletely understood. We review evidence suggesting that altered reinforcement mechanisms are a fundamental characteristic of ADHD. We show that a deficit in the transfer of dopamine signals from established positive reinforcers to cues that predict such reinforcers may underlie these altered reinforcement mechanisms, and in turn explain key symptoms of ADHD. We argue that the neural substrates controlling the excitation and inhibition of dopamine neurons during the transfer process are a promising target for future drug development. There is a need to develop animal models and behavioral paradigms that can be used to experimentally investigate these mechanisms and their effects on sensitivity to reinforcement. More specific and selective targeting of drug development may be possible through this approach.
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Affiliation(s)
- Gail Tripp
- Human Developmental Neurobiology Unit, Okinawa Institute of Science and Technology Graduate University, Kunigami, Okinawa 904-0412, Japan.
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Wickens JR, Hyland BI, Tripp G. Animal models to guide clinical drug development in ADHD: lost in translation? Br J Pharmacol 2012; 164:1107-28. [PMID: 21480864 DOI: 10.1111/j.1476-5381.2011.01412.x] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
We review strategies for developing animal models for examining and selecting compounds with potential therapeutic benefit in attention-deficit hyperactivity disorder (ADHD). ADHD is a behavioural disorder of unknown aetiology and pathophysiology. Current understanding suggests that genetic factors play an important role in the aetiology of ADHD. The involvement of dopaminergic and noradrenergic systems in the pathophysiology of ADHD is probable. We review the clinical features of ADHD including inattention, hyperactivity and impulsivity and how these are operationalized for laboratory study. Measures of temporal discounting (but not premature responding) appear to predict known drug effects well (treatment validity). Open-field measures of overactivity commonly used do not have treatment validity in human populations. A number of animal models have been proposed that simulate the symptoms of ADHD. The most commonly used are the spontaneously hypertensive rat (SHR) and the 6-hydroxydopamine-lesioned (6-OHDA) animals. To date, however, the SHR lacks treatment validity, and the effects of drugs on symptoms of impulsivity and inattention have not been studied extensively in 6-OHDA-lesioned animals. At the present stage of development, there are no in vivo models of proven effectiveness for examining and selecting compounds with potential therapeutic benefit in ADHD. However, temporal discounting is an emerging theme in theories of ADHD, and there is good evidence of increased value of delayed reward following treatment with stimulant drugs. Therefore, operant behaviour paradigms that measure the effects of drugs in situations of delayed reinforcement, whether in normal rats or selected models, show promise for the future.
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Yu H, Pandit B, Klett E, Lee MH, Lu K, Helou K, Ikeda I, Egashira N, Sato M, Klein R, Batta A, Salen G, Patel SB. The rat STSL locus: characterization, chromosomal assignment, and genetic variations in sitosterolemic hypertensive rats. BMC Cardiovasc Disord 2003; 3:4. [PMID: 12783625 PMCID: PMC165443 DOI: 10.1186/1471-2261-3-4] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2003] [Accepted: 06/03/2003] [Indexed: 11/21/2022] Open
Abstract
BACKGROUND Elevated plant sterol accumulation has been reported in the spontaneously hypertensive rat (SHR), the stroke-prone spontaneously hypertensive rat (SHRSP) and the Wistar-Kyoto (WKY) rat. Additionally, a blood pressure quantitative trait locus (QTL) has been mapped to rat chromosome 6 in a New Zealand genetically hypertensive rat strain (GH rat). ABCG5 and ABCG8 (encoding sterolin-1 and sterolin-2 respectively) have been shown to be responsible for causing sitosterolemia in humans. These genes are organized in a head-to-head configuration at the STSL locus on human chromosome 2p21. METHODS To investigate whether mutations in Abcg5 or Abcg8 exist in SHR, SHRSP, WKY and GH rats, we initiated a systematic search for the genetic variation in coding and non-coding region of Abcg5 and Abcg8 genes in these strains. We isolated the rat cDNAs for these genes and characterized the genomic structure and tissue expression patterns, using standard molecular biology techniques and FISH for chromosomal assignments. RESULTS Both rat Abcg5 and Abcg8 genes map to chromosome band 6q12. These genes span ~40 kb and contain 13 exons and 12 introns each, in a pattern identical to that of the STSL loci in mouse and man. Both Abcg5 and Abcg8 were expressed only in liver and intestine. Analyses of DNA from SHR, SHRSP, GH, WKY, Wistar, Wistar King A (WKA) and Brown Norway (BN) rat strains revealed a homozygous G to T substitution at nucleotide 1754, resulting in the coding change Gly583Cys in sterolin-1 only in rats that are both sitosterolemic and hypertensive (SHR, SHRSP and WKY). CONCLUSIONS The rat STSL locus maps to chromosome 6q12. A non-synonymous mutation in Abcg5, Gly583Cys, results in sitosterolemia in rat strains that are also hypertensive (WKY, SHR and SHRSP). Those rat strains that are hypertensive, but not sitosterolemic (e.g. GH rat) do not have mutations in Abcg5 or Abcg8. This mutation allows for expression and apparent apical targeting of Abcg5 protein in the intestine. These rat strains may therefore allow us to study the pathophysiological mechanisms involved in the human disease of sitosterolemia.
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Affiliation(s)
- Hongwei Yu
- Division of Endocrinology, Diabetes and Medical Genetics, Medical University of South Carolina, STR 541, 114 Doughty Street, Charleston, SC 29403, USA
| | - Bhaswati Pandit
- Division of Endocrinology, Diabetes and Medical Genetics, Medical University of South Carolina, STR 541, 114 Doughty Street, Charleston, SC 29403, USA
| | - Eric Klett
- Division of Endocrinology, Diabetes and Medical Genetics, Medical University of South Carolina, STR 541, 114 Doughty Street, Charleston, SC 29403, USA
| | - Mi-Hye Lee
- Division of Endocrinology, Diabetes and Medical Genetics, Medical University of South Carolina, STR 541, 114 Doughty Street, Charleston, SC 29403, USA
| | - Kangmo Lu
- Division of Endocrinology, Diabetes and Medical Genetics, Medical University of South Carolina, STR 541, 114 Doughty Street, Charleston, SC 29403, USA
| | - Khalil Helou
- Genetics Branch, Center for Cancer Research, National Cancer, Institute/NIH, Bethesda, Maryland 20892, USA
- Department of Oncology, Institute of Selected Clinical Sciences, Goteborg University, SE 413 45, Gothenburg, Sweden
| | - Ikuo Ikeda
- Laboratory of Nutrition Chemistry, Department of Bioscience and Biotechnology, Faculty of Agriculture, Graduate School Kyushu University, Fukuoka, 812-8581, Japan
| | - Nami Egashira
- Laboratory of Nutrition Chemistry, Department of Bioscience and Biotechnology, Faculty of Agriculture, Graduate School Kyushu University, Fukuoka, 812-8581, Japan
| | - Masao Sato
- Laboratory of Nutrition Chemistry, Department of Bioscience and Biotechnology, Faculty of Agriculture, Graduate School Kyushu University, Fukuoka, 812-8581, Japan
| | - Richard Klein
- Division of Endocrinology, Diabetes and Medical Genetics, Medical University of South Carolina, STR 541, 114 Doughty Street, Charleston, SC 29403, USA
| | - Ashok Batta
- Research Service and Medical Service, Department of Veterans Affairs Medical Center, East Orange, NJ, USA
| | - Gerald Salen
- Research Service and Medical Service, Department of Veterans Affairs Medical Center, East Orange, NJ, USA
| | - Shailendra B Patel
- Division of Endocrinology, Diabetes and Medical Genetics, Medical University of South Carolina, STR 541, 114 Doughty Street, Charleston, SC 29403, USA
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