Transgenic mice in drug dependence research

Contributions of A2A and A2B adenosine receptors in coronary flow responses in relation to the KATP channel using A2B and A2A/2B double-knockout mice

Adenosine plays a role in physiological and pathological conditions, and A(2) adenosine receptor (AR) expression is modified in many cardiovascular disorders. In this study, we elucidated the role of the A(2B)AR and its relationship to the A(2A)AR in coronary flow (CF) changes using A(2B) single-knockout (KO) and A(2A/2B) double-KO (DKO) mice in a Langendorff setup. We used two approaches: 1) selective and nonselective AR agonists and antagonists and 2) A(2A)KO and A(2B)KO and A(2A/2B)DKO mice. BAY 60-6583 (a selective A(2B) agonist) had no effect on CF in A(2B)KO mice, whereas it significantly increased CF in wild-type (WT) mice (maximum of 23.3 ± 9 ml·min(-1)·g(-1)). 5'-N-ethylcarboxamido adenosine (NECA; a nonselective AR agonist) increased CF in A(2B)KO mice (maximum of 34.6 ± 4.7 ml·min(-1)·g(-1)) to a significantly higher degree compared with WT mice (maximum of 23.1 ± 2.1 ml·min(-1)·g(-1)). Also, CGS-21680 (a selective A(2A) agonist) increased CF in A(2B)KO mice (maximum of 29 ± 1.9 ml·min(-1)·g(-1)) to a significantly higher degree compared with WT mice (maximum of 25.1 ± 2.3 ml·min(-1)·g(-1)). SCH-58261 (an A(2A)-selective antagonist) inhibited the NECA-induced increase in CF to a significantly higher degree in A(2B)KO mice (19.3 ± 1.6 vs. 0.5 ± 0.4 ml·min(-1)·g(-1)) compared with WT mice (19 ± 3.5 vs. 3.6 ± 0.5 ml·min(-1)·g(-1)). NECA did not induce any increase in CF in A(2A/2B)DKO mice, whereas a significant increase was observed in WT mice (maximum of 23.1 ± 2.1 ml·min(-1)·g(-1)). Furthermore, the mitochondrial ATP-sensitive K(+) (K(ATP)) channel blocker 5-hydroxydecanoate had no effect on the NECA-induced increase in CF in WT mice, whereas the NECA-induced increase in CF in WT (17.6 ± 2 ml·min(-1)·g(-1)), A(2A)KO (12.5 ± 2.3 ml·min(-1)·g(-1)), and A(2B)KO (16.2 ± 0.8 ml·min(-1)·g(-1)) mice was significantly blunted by the K(ATP) channel blocker glibenclamide (to 0.7 ± 0.7, 2.3 ± 1.1, and 0.9 ± 0.4 ml·min(-1)·g(-1), respectively). Also, the CGS-21680-induced (22 ± 2.3 ml·min(-1)·g(-1)) and BAY 60-6583-induced (16.4 ± 1.60 ml·min(-1)·g(-1)) increase in CF in WT mice was significantly blunted by glibenclamide (to 1.2 ± 0.4 and 1.8 ± 1.2 ml·min(-1)·g(-1), respectively). In conclusion, this is the first evidence supporting the compensatory upregulation of A(2A)ARs in A(2B)KO mice and demonstrates that both A(2A)ARs and A(2B)ARs induce CF changes through K(ATP) channels. These results identify AR-mediated CF responses that may lead to better therapeutic approaches for the treatment of cardiovascular disorders.

The endogenous cannabinoid anandamide shares discriminative stimulus effects with ∆(9)-tetrahydrocannabinol in fatty acid amide hydrolase knockout mice

The endogenous cannabinoid system has been noted for its therapeutic potential, as well as the psychoactivity of cannabinoids such as Δ9-tetrahydrocannabinol (THC). However, less is known about the psychoactivity of anandamide (AEA), an endocannabinoid ligand. Thus, the goals of this study were to establish AEA as a discriminative stimulus in transgenic mice lacking fatty acid amide hydrolase (i.e., FAAH -/- mice unable to rapidly metabolize AEA), evaluate whether THC or oleamide, a fatty acid amide, produced AEA-like responding, and assess for CB(1) mediation of AEA's discriminative stimulus. Mice readily discriminated between 6mg/kg AEA and vehicle in a two-lever drug discrimination task. AEA dose-dependently generalized to itself. THC elicited full AEA-like responding, whereas oleamide failed to substitute. The CB(1) antagonist rimonabant attenuated AEA- and THC-induced AEA-appropriate responding, demonstrating CB(1) mediation of AEA's discriminative stimulus. These findings suggest that, in the absence of FAAH, AEA produces intoxication comparable to THC, and consequently to marijuana.

Pitx3-deficient aphakia mice display unique behavioral responses to psychostimulant and antipsychotic drugs

The dorsal (A9) and ventral striatum (A10) of the midbrain mediate many of the effects of psychoactive drugs that alter emotion, cognition, and motor activity within the contexts of therapy or abuse. Although transgenic and knockout technologies have enabled development of genetic models to dissect contributions of specific dopamine (DA) receptor subtypes to psychoactive drug effects, few models exist that can distinguish contributions of A9 versus A10 circuits. Pitx3 is a transcription factor enriched in DA neurons. Aphakia (ak) mice deficient in Pitx3 show selective loss of nigrostriatal DA, while other DA pathways are relatively spared, and therefore could be a useful tool for investigating the role of this subclass of DA projections. We investigated the effects of stimulants amphetamine, apomorphine, and MK-801 and the antipsychotic drug haloperidol on behavior in ak mice. Whereas wild-type mice showed the characteristic locomotor hyperactivity in response to amphetamine (5 mg/kg) and apomorphine (4 mg/kg), these drugs caused a paradoxical suppression of locomotor hyperactivity in ak mice. MK-801 (0.2 mg/kg) induced hyperactivity was maintained in both wt and ak mice. Additionally, mutant but not wild-type mice were insensitive to the cataleptic effects of haloperidol (1 mg/kg). These studies indicate that the nigrostriatal DA circuit plays a critical role in maintaining normal responsiveness to psychotropic drugs that either stimulate or block DA neurotransmission. We propose that ak mice may represent a valuable genetic model not only to study Parkinson's disease, but also to dissect the pathophysiologic and pharmacotherapuetic mechanisms of other DA-mediated disorders such as attention-deficit hyperactivity disorder, drug abuse and schizophrenia.

The contribution of MOR-1 exons 1-4 to morphine and heroin analgesia and dependence

Although morphine and heroin analgesia is mediated by mu-opioid receptors encoded by the MOR-1 gene, distinct isoforms are involved. Both opioids also induce dependence by acting at mu-opioid receptors, but which variants are utilized is not known. Here, we assayed morphine and heroin analgesia and dependence in mice treated with antisense oligodeoxynucleotides (AO) targeting MOR-1 exons 1-4. Whereas AOs targeting exons 1 and 4 blocked morphine analgesia, those targeting exons 2 and 3 blocked heroin analgesia. Neither morphine nor heroin analgesia was compromised 5 days after the last AO injection. In morphine and heroin dependent mice, only exon 1 AO significantly reduced jumping incidence during naloxone (50mg/kg) precipitated withdrawal. Neither analgesia nor withdrawal jumping was attenuated in controls pretreated with saline or a mismatch oligodeoxynucleotide control sequence. While these data confirm previous reports that morphine and heroin analgesia are not mediated by a single mu-opioid receptor, both opiates nonetheless apparently induce dependence via a mu-opioid receptor isoform containing exon 1. For heroin, the possibility that analgesia and dependence are mediated by distinct mu-opioid receptor isoforms offers the prospect of developing potent opiate analgesics possessing reduced dependence liability.

Novel therapeutic modalities to address nondrugable protein interaction targets

Small molecule drugs are relatively effective in working on 'drugable' targets such as GPCRs, ion channels, kinases, proteases, etc but ineffective at blocking protein-protein interactions that represent an emerging class of 'nondrugable' central nervous system (CNS) targets. This article provides an overview of novel therapeutic modalities such as biologics (in particular antibodies) and emerging oligonucleotide therapeutics such as antisense, small-interfering RNA, and aptamers. Their key properties, overall strengths and limitations, and their utility as tools for target validation are presented. In addition, issues with regard to CNS targets as it relates to the blood-brain barrier penetration are discussed. Finally, examples of their application as therapeutics for the treatment of pain and some neurological disorders such as Alzheimer's disease, multiple sclerosis, Huntington's disease, and Parkinson's disease are provided.

Unaltered D1, D2, D4, and D5 dopamine receptor mRNA expression and distribution in the spinal cord of the D3 receptor knockout mouse

Dopamine (DA) acts through five receptor subtypes (D1-D5). We compared expression levels and distribution patterns of all DA mRNA receptors in the spinal cord of wild-type (WT) and loss of function D3 receptor knockout (D3KO) animals. D3 mRNA expression was increased in D3KO, but no D3 receptor protein was associated with cell membranes, supporting the previously reported lack of function. In contrast, mRNA expression levels and distribution patterns of D1, D2, D4, and D5 receptors were similar between WT and D3KO animals. We conclude that D3KO spinal neurons do not compensate for the loss of function of the D3 receptor with changes in the other DA receptor subtypes. This supports use of D3KO animals as a model to provide insight into D3 receptor dysfunction in the spinal cord.

Up-regulation of A 2B adenosine receptor in A 2A adenosine receptor knockout mouse coronary artery

In this study, we looked into possible compensatory changes of other adenosine receptors (ARs) in A(2A) genetic knockout mice (A2AKO) as well as the functional role of nitric oxide (NO) in A(2A) AR-mediated vasodilation. Gene expression of ARs from coronary arteries of A(2A) AR wild type mice (A2AWT) and A2AKO was studied using real-time PCR. Functional studies were carried out in isolated heart and isolated coronary artery preparations. A(2B) AR was found to be 4.5 fold higher in A2AKO than in A2AWT, while A(2A) AR expression was absent in A2AKO. There was no difference in A(1) and A(3) ARs between WT and KO animals. The concentration-relaxation curve for adenosine-5'-N-ethylcarboxamide (NECA, non-selective AR agonist) in isolated coronary arterial rings in A2AKO was shifted to the left when compared to A2AWT. The concentration-response curve for A(2B) selective agonist (BAY 60-6583) was also shifted to the left in A2AKO hearts. L-NAME, a non-specific NO synthase inhibitor, did not affect baseline coronary flow (CF) until the concentration reached 10 microM in A2AWT (76.32+/-11.35% from baseline, n=5). In A2AKO, the CF decreased significantly by L-NAME only at a higher concentration (100 microM, 93.32+/-5.8% from baseline, n=5). L-NMA (1 microM, n=4), another non-specific NO synthase inhibitor, also demonstrated similar results in decreasing CF (59.66+/-3.23% from baseline in A2AWT, while 81.76+/-8.91% in A2AKO). It was further demonstrated that the increase in CF by 100 microM NECA was significantly blunted with 10 microM L-NAME (377.08+/-25.23% to 305.41+/-30.73%, n=9) in A2AWT but not in A2AKO (153.66+/-22.7% to 143.88+/-36.65%, n=5). Similar results were also found using 50 nM of CGS-21680 instead of NECA in A2AWT (346+/-22.85 to 277+/-31.39, n=6). No change in CF to CGS-21680 was noted in A(2A)AKO. Our data demonstrate, for the first time, that coronary A(2B) AR was up-regulated in mice deficient in A(2A) AR. We also provide direct evidence supporting a role for NO in A(2A) AR-mediated coronary vasodilation. The data further support the role for A(2A) AR in the regulation of basal coronary tone through the release of NO.

Anxiogenic effects of cocaine withdrawal in zebrafish

Continued usage of cocaine is determined by genetic, conditioned and homeostatic factors, while it is reinforced by drug-induced reward and the emotionally negative state of drug withdrawal, which includes anxiety. The molecular mechanisms of these long-term behavioral and physiological alterations have yet to be fully elucidated. Here we demonstrate that in zebrafish, a wide range of non-anesthetic cocaine doses, 0.015-15 muM, does not result in acute alterations in locomotor activity, in spite of the high brain cocaine levels induced (7-120 pg/microg protein). Conversely, cocaine withdrawal causes hyperactivity associated with stereotypy. The behavioral hyperactivity is progressively increased during the initial period of withdrawal (24-72 h) and is maintained for at least 5 days. Such effect of cocaine withdrawal is aggravated by environmental stimulation and attenuated in the home environment. Administration of cocaine (1.5 microM) or a non-sedative dose of diazepam (5 microM, immersion) acutely counteracts withdrawal-associated hyperactivity and stereotypy in zebrafish, with the magnitude of these effects positively correlating with the degree of prior increase in basal activity. Administration of an anxiogenic benzodiazepine inverse agonist, FG-7142, results in zebrafish behavior similar to that observed during cocaine withdrawal. Together, the results suggest that cocaine withdrawal produces long-lasting behavioral effects in zebrafish which are consistent with an anxiety-like state. Thus, zebrafish, a powerful model for the study of vertebrate genetics, could provide insights into the molecular mechanisms of drug withdrawal.

Analysis of the endocannabinoid system by using CB1 cannabinoid receptor knockout mice

The endocannabinoid system has been involved in the control of several neurophysiological and behavioural responses. To date, three lines of CB1 knockout mice have been established independently in different laboratories. This chapter reviews the main results obtained with these lines of CB1 knockout mice in several physiological responses that have been previously related to the activity of the endocannabinoid system. Studies using CB1 knockout mice have demonstrated that this receptor participates in the control of several behavioural responses including locomotion, anxiety- and depressive-like states, cognitive functions such as memory and learning processes, cardiovascular responses and feeding. Furthermore, the CB1 cannabinoid receptor is involved in the control of pain by acting at peripheral, spinal and supraspinal levels. The involvement of the CB1 cannabinoid receptor in the behavioural and biochemical processes underlying drug addiction has also been investigated. These CB1 knockouts have provided new findings to clarify the interactions between cannabinoids and the other drugs of abuse such as opioids, psychostimulants, nicotine and ethanol. Recent studies have demonstrated that endocannabinoids can function as retrograde messengers, modulating the release of different neurotransmitters, including opioids, gamma-aminobutyric acid (GABA), and cholecystokinin (CCK), which could explain some of the responses observed after the stimulation of the CB1 cannabinoid receptor. This review provides an update of the apparently controversial data reported in the literature using the three different lines of CB1 knockout mice, which seem to be mainly due to the use of different experimental procedures rather than any constitutive alteration in these lines of knockouts.

Genetically engineered animals in drug discovery and development: a maturing resource for toxicologic research

Genetically engineered mice that either over-express a foreign gene (transgenic) or in which the activity of a specific gene has been removed ("knock-out") or replaced ("knock-in") will be used increasingly to investigate molecular mechanisms of disease, to evaluate innovative therapeutic targets, and to screen novel agents for efficacy and/or toxicity. Recent innovations of relevance to toxicologic researchers include the construction of genetically engineered mice with (1) multiple engineered genes, (2) mutations that can be induced at specific sites and times throughout life, and (3) the substitution of human genes for their mouse counterparts ("humanized" mice) to allow in vivo investigation of xenobiotic toxicity. Contemporary applications of genetically engineered mice in toxicology include basic mechanistic research exploiting newly engineered mouse lines as well as applied screening for genotoxicity and carcinogenicity using commercially available animals. Many caveats must be considered when interpreting genetically engineered mice-derived toxicity data, the chief of which will be the extent to which the model's phenotype has been fully characterized, the type and incidence of background lesions for the given mouse strain and engineered gene, and the possibility of misinterpreting the presence or absence of a phenotype due to compensatory physiologic processes that mask the outcome produced by the engineering event. Toxicity data acquired using genetically engineered mice currently supplements and in time likely will supplant those gathered using the present "gold standard" bioassays, as genetically engineered mice typically develop more lesions after a shorter latency period than do age- and strain-matched, wild-type mice.

Toxicology of antisense therapeutics

Targeting unique mRNA molecules using antisense approaches, based on sequence specificity of double-stranded nucleic acid interactions should, in theory, allow for design of drugs with high specificity for intended targets. Antisense-induced degradation or inhibition of translation of a target mRNA is potentially capable of inhibiting the expression of any target protein. In fact, a large number of proteins of widely varied character have been successfully downregulated using an assortment of antisense-based approaches. The most prevalent approach has been to use antisense oligonucleotides (ASOs), which have progressed through the preclinical development stages including pharmacokinetics and toxicological studies. A small number of ASOs are currently in human clinical trials. These trials have highlighted several toxicities that are attributable to the chemical structure of the ASOs, and not to the particular ASO or target mRNA sequence. These include mild thrombocytopenia and hyperglycemia, activation of the complement and coagulation cascades, and hypotension. Dose-limiting toxicities have been related to hepatocellular degeneration leading to decreased levels of albumin and cholesterol. Despite these toxicities, which are generally mild and readily treatable with available standard medications, the clinical trials have clearly shown that ASOs can be safely administered to patients. Alternative chemistries of ASOs are also being pursued by many investigators to improve specificity and antisense efficacy and to reduce toxicity. In the design of ASOs for anticancer therapeutics in particular, the goal is often to enhance the cytotoxicity of traditional drugs toward cancer cells or to reduce the toxicity to normal cells to improve the therapeutic index of existing clinically relevant cancer chemotherapy drugs. We predict that use of antisense ASOs in combination with small molecule therapeutics against the target protein encoded by the antisense-targeted mRNA, or an alternate target in the same or a connected biological pathway, will likely be the most beneficial application of this emerging class of therapeutic agent.

Novel fluorescence-based approaches for the study of biogenic amine transporter localization, activity, and regulation

Pre-synaptic norepinephrine (NE) and dopamine (DA) transporters (NET and DAT) terminate catecholamine synaptic transmission through reuptake of released neurotransmitter. Recent studies reveal that NET and DAT are tightly regulated by receptor and second messenger-linked signaling pathways. Common approaches for studying these transporters involve use of radiolabeled substrates or antagonists, methods possessing limited spatial resolution and that bear limited opportunities for repeated monitoring of living preparations. To circumvent these issues, we have explored two novel assay platforms that permit temporally resolved quantitation of transport activity and transporter protein localization. To monitor the binding and transport function of NET and DAT in real-time, we have investigated the uptake of the fluorescent organic compound 4-(4-diethylaminostyryl)-N-methylpyridinium iodide (ASP+). We have extended our previous single cell level application of this substrate to monitor transport activity via high-throughput assay platforms. Compared to radiotracer uptake methods, acquisition of ASP+ fluorescence is non-isotopic and allows for continuous, repeated transport measurements on both transfected and native preparations. Secondly, we have extended our application of small-molecule-conjugated fluorescent CdSe/ZnS nanocrystals, or quantum dots (Qdots), to utilize antibody and peptide ligands that can identify surface expressed transporters, receptors and other membrane proteins in living cell systems. Unlike typical organic fluorophores, Qdots are highly resistant to bleaching and can be conjugated to multiple ligands. They can also be illuminated by conventional light sources, yet produce narrow, gaussian emission spectra compatible with multiple target visualization (multiplexing). Together, these approaches offer novel opportunities to investigate changes in transporter function and distribution in real-time with superior spatial and temporal resolution.

Ibogaine signals addiction genes and methamphetamine alteration of long-term potentiation

The mapping of the human genetic code will enable us to identify potential gene products involved in human addictions and diseases that have hereditary components. Thus, large-scale, parallel gene-expression studies, made possible by advances in microarray technologies, have shown insights into the connection between specific genes, or sets of genes, and human diseases. The compulsive use of addictive substances despite adverse consequences continues to affect society, and the science underlying these addictions in general is intensively studied. Pharmacological treatment of drug and alcohol addiction has largely been disappointing, and new therapeutic targets and hypotheses are needed. As the usefulness of the pharmacotherapy of addiction has been limited, an emerging potential, yet controversial, therapeutic agent is the natural alkaloid ibogaine. We have continued to investigate programs of gene expression and the putative signaling molecules used by psychostimulants such as amphetamine in in vivo and in vitro models. Our work and that of others reveal that complex but defined signal transduction pathways are associated with psychostimulant administration and that there is broad-spectrum regulation of these signals by ibogaine. We report that the actions of methamphetamine were similar to those of cocaine, including the propensity to alter long-term potentiation (LTP) in the hippocampus of the rat brain. This action suggests that there may be a "threshold" beyond which the excessive brain stimulation that probably occurs with compulsive psychostimulant use results in the occlusion of LTP. The influence of ibogaine on immediate early genes (IEGs) and other candidate genes possibly regulated by psychostimulants and other abused substances requires further evaluation in compulsive use, reward, relapse, tolerance, craving and withdrawal reactions. It is therefore tempting to suggest that ibogaine signals addiction gene products.

Use of genetically engineered mice in drug discovery and development: wielding Occam's razor to prune the product portfolio

Genetically engineered mice (GEMs) that either overexpress (transgenic) or lack (gene-targeted, or "knock-out") genes are used increasingly in industry to investigate molecular mechanisms of disease, to evaluate innovative therapeutic targets, and to screen agents for efficacy and/or toxicity. High throughput GEM construction in drug discovery and development (DDD) serves two main purposes: to test whether a given gene participates in a disease condition, or to determine the function(s) of a protein that is encoded by an expressed sequence tag (EST, an mRNA fragment for a previously uncharacterized protein). In some instances, phenotypes induced by such novel GEMs also may yield clues regarding potential target organs and toxic effects of potential therapeutic molecules. The battery of tests used in phenotypic analysis of GEMs varies between companies, but the goal is to define one or more easily measured endpoints that can be used to monitor the disease course--especially during in vivo treatment with novel drug candidates. In many DDD projects, overt phenotypes are subtle or absent even in GEMs in which high-level expression or total ablation of an engineered gene can be confirmed. This outcome presents a major quandary for biotechnology and pharmaceutical firms: given the significant expense and labor required to generate GEMs, what should be done with "negative" constructs? The 14th century philosophical principle known as Occam's razor-that the simplest explanation for a phenomenon is likely the truth-provides a reasonable basis for pruning potential therapeutic molecules and targets. In the context of DDD, Occam's razor may be construed to mean that correctly engineered GEMs lacking obvious functional or structural phenotypes have none because the affected gene is not uniquely essential to normal homeostasis or disease progression. Thus, a "negative" GEM construct suggests that the gene under investigation encodes a ligand or target molecule without significant therapeutic potential. This interpretation indicates that, at least in a market-driven industrial setting, such "negative" projects should be pruned aggressively so that resources may be redirected to more promising DDD ventures.

Availability and characterization of transgenic and knockout mice with behavioral manifestations: where to look and what to search for

Mice altered by transgenesis or gene targeting ("knockouts") have increasingly been employed as alternative effective tools in elucidating the genetic basis of neurophysiology and behavior. Standardization of specific behavioral paradigms and phenotyping strategies will ensure that these behavioral mouse mutants offer robust models for evaluating the efficacy of novel therapeutics in the treatment of hereditary neurological disorders. The Induced Mutant Resource (IMR) at The Jackson Laboratory (Bar Harbor, Maine, USA) imports, cryopreserves, develops, maintains, and distributes to the research community biomedically valuable stocks of transgenic and targeted mutant mice. Information on behavioral and neurological strains-including a phenotypic synopsis, husbandry requirements, strain availability, and genetic typing protocols-is available through the IMR database (http://www.jax.org/resources/documents/imr/). A current catalog of available strains is readily accessible via the JAX Mice Web site at http://jaxmice.jax.org/index.shtml. In addition, The Jackson Laboratory is now home to TBASE (http://tbase.jax.org/), a comprehensive, community database whose primary focus is on mouse knockouts. TBASE accommodates an exhaustive bibliographical resource for transgenic and knockout mice and provides a detailed phenotypic characterization of numerous behavioral knockouts that is primarily extracted from the literature. Concerted efforts to merge the two resources into a new, schematically reformed database are underway.

Transgenic and knockout databases: behavioral profiles of mouse mutants

Genetically engineered strains of mice, modified by transgenesis or gene targeting ("knockouts") are being generated at an impressive rate and used, among other areas, as premiere research tools in deciphering the genetic basis of behavior. As behavioral phenotyping strategies continue to evolve, characterization of these "designer" mice will provide models to evaluate the efficacy of new pharmacological and gene therapy treatments in human hereditary diseases. Reported behavioral profiles include aberrant social, reproductive, and parental behaviors, learning and memory deficits, feeding disorders, aggression, anxiety-related behaviors, pain/analgesia, and altered responses to antidepressants, antipsychotics, ethanol, and psychostimulant drugs of abuse. The Induced Mutant Resource (IMR) at The Jackson Laboratory (TJL, Bar Harbor, ME, USA) imports, cryopreserves, develops, maintains, and distributes biomedically important stocks of transgenic and targeted mutant mice to the research community. Information on neurological/behavioral strains--including behavioral performance, husbandry requirements, strain availability, and genetic typing protocols--is provided through the IMR database (http://www.jax.org/resources/documents/imr/). A catalog of available strains is readily accessible via the JAX Mice website at http://jaxmice.jax.org/index.shtml. In addition, TJL is now host to TBASE (http://tbase.jax.org/), a comprehensive, public-domain database with primary emphasis on mouse knockouts. TBASE contains an exhaustive list of knockout-related citations and provides an extensive phenotypic characterization of numerous behavioral mutants that is extracted directly from the literature. Present efforts to merge the two resources into a novel, schematically enhanced database, provisionally named Transgenic and Targeted Mutation Database (TTMD), will be briefly discussed.

A single vector containing modified cre recombinase and LOX recombination sequences for inducible tissue-specific amplification of gene expression

The selective alteration of the genome using Cre recombinase to target the rearrangement of genes flanked by LOX recognition sequences has required the use of two separate genetic constructs in trans, one containing cre and the other containing the gene of interest flanked by LOX sites. We have developed a strategy in which both the cre recombinase gene and LOX recombination sites may be cloned within a single vector in cis. This method uses a modified form of Cre (CREM) that contains alterations to the 5' region including the introduction of a Kozak consensus sequence and insertion of a functional intron. This system allows for the inducible, tissue-specific activation or inactivation of gene expression in a single vector and can be utilized for the 300-fold amplification of gene expression from a weak promoter. This approach can be applied to targeting strategies for generating genetically altered mice and gene therapy.

Facilitated glutamatergic transmission in the striatum of D2 dopamine receptor-deficient mice

Dopamine (DA) receptors play an important role in the modulation of excitability and the responsiveness of neurons to activation of excitatory amino acid receptors in the striatum. In the present study, we utilized mice with genetic deletion of D2 or D4 DA receptors and their wild-type (WT) controls to examine if the absence of either receptor subtype affects striatal excitatory synaptic activity. Immunocytochemical analysis verified the absence of D2 or D4 protein expression in the striatum of receptor-deficient mutant animals. Sharp electrode current- and whole cell patch voltage-clamp recordings were obtained from slices of receptor-deficient and WT mice. Basic membrane properties were similar in D2 and D4 receptor-deficient mutants and their respective WT controls. In current-clamp recordings in WT animals, very little low-amplitude spontaneous synaptic activity was observed. The frequency of these spontaneous events was increased slightly in D2 receptor-deficient mice. In addition, large-amplitude depolarizations were observed in a subset of neurons from only the D2 receptor-deficient mutants. Bath application of the K+ channel blocker 4-aminopyridine (100 microM) and bicuculline methiodide (10 microM, to block synaptic activity due to activation of GABA(A) receptors) markedly increased spontaneous synaptic activity in receptor-deficient mutants and WTs. Under these conditions, D2 receptor-deficient mice displayed significantly more excitatory synaptic activity than their WT controls, while there was no difference between D4 receptor-deficient mice and their controls. In voltage-clamp recordings, there was an increase in frequency of spontaneous glutamate receptor-mediated inward currents without a change in mean amplitude in D2 receptor-deficient mutants. In WT mice, activation of D2 family receptors with quinpirole decreased spontaneous excitatory events and conversely sulpiride, a D2 receptor antagonist, increased activity. In D2 receptor-deficient mice, sulpiride had very little net effect. Morphologically, a subpopulation of medium-sized spiny neurons from D2 receptor-deficient mice displayed decreased dendritic spines compared with cells from WT mice. These results provide evidence that D2 receptors play an important role in the regulation of glutamate receptor-mediated activity in the corticostriatal or thalamostriatal pathway. These receptors may function as gatekeepers of glutamate release or of its subsequent effects and thus may protect striatal neurons from excessive excitation.

Nerve growth factor (NGF) influences differentiation and proliferation of myogenic cells in vitro via TrKA

Classic studies have established that muscle cells exert trophic actions on neurons of the developing peripheral nervous system through the production of neurotrophins. For this reason neurotrophins are also known as 'target-derived factors'. During differentiation, muscle cells also express some neurotrophin receptors, such as the low-affinity p75 neurotrophin receptor, which binds all neurotrophins, and the high affinity tyrosine kinase receptor TrKA, nerve growth factor (NGF) transducing receptor. The functional roles of these receptors in muscle cells are still unclear and only fragmentary and controversial data are available regarding the responsiveness of muscle cells to NGF. The aim of the present study is to investigate the effects of NGF on cells of myogenic lineage. The rat myogenic cell line L6, primary cultures of adult human myoblasts, and the human rhabdomyosarcoma cell line TE-671 were used in this study. As expected, all the three cell types expressed NGF, p75 and TrKA. NGF was expressed by L6 and primary myoblasts following differentiation, but it was constitutively expressed at high levels in the TE-671 rhabdomyosarcoma cells. In L6 myoblasts, p75 receptor was expressed in myoblasts but not in myotubes early after plating; while some primary human myoblasts expressed it at all the time-points tested. Some fusiform cells of the TE-671 rhabdomyosarcoma cell line also expressed p75. TrKA was constitutively immunodetected in all the three cell lines, suggesting that these cells may respond to NGF. Addition of exogenous NGF increased the fusion rate of both primary and L6 myoblasts, as well as the proliferation of the slowly dividing primary myoblasts. Consistently, blocking the action of endogenously produced NGF with a specific neutralizing antibody decreased the percentage of fusion in both primary and L6 myoblasts. On the contrary, blocking the binding of NGF to p75 did not affect the percentage of fusion. Furthermore, neither exogenous NGF nor NGF- or p75-neutralizing antibodies appeared to affect the rhabdomyosarcoma cells, which have a high proliferation rate and do not fuse. Pharmacological inhibition of TrKA signal transduction with K252a (in the nM range) and tyrphostin AG879 (in the low microM range) resulted in a dramatic dose-dependent decrease in proliferation of all of the myogenic cell lines tested. Interestingly, this was especially evident in the rapidly dividing rhabdomyosarcoma cell line. The TrKA inhibitors also blocked fusion of L6 and primary myoblasts and induced morphological changes characterized by the flattening of the cells and a 'spider-like' rearrangement of the intermediate filaments in all three cell lines with some minor differences. A transfection study showed that p75-overexpressing L6 cells do not fuse and present changes in their morphology similar to the TrKA-inhibitors treated L6 cells. These data support the notion that NGF expression in skeletal muscle is not only associated with a classical target-derived neurotrophic function for peripheral nervous system neurons, but also with an autocrine action which affects the proliferation, fusion into myotubes, and cell morphology of developing myoblasts. The present data also suggest that these effects of NGF are mediated by TrKA receptors and that a sustained presence of NGF is needed for increase fusion into myotubes. Lastly, the dramatic anti-proliferative effect of TrKA inhibitors on myogenic cells, and especially on the TE-671 rhabdomyosarcoma cell line, suggests that pharmacological interference with NGF signal transduction could be effective in the control of these malignancies.

Genetic dissection of nicotine-related behaviour: a review of animal studies

Nicotine has a broad spectrum of behavioural effects. A considerable body of data has emerged indicating genetic factors regulate the behavioural effects of nicotine. Experimental genetic techniques have been invaluable in generating knowledge on the interrelationship of genetic factors and behavioural responsiveness to nicotine. Three different approaches have been invoked to explore the relationship of genetic factors to response to nicotine. Firstly, the classical genetic tool of inbred lines has been exploited to delineate genetic influences in the effects of nicotine. Secondly, the use of selectively bred lines has been profitably employed to reveal genetic differences in behavioural responses, such as cognition and exploration, to nicotine. These approaches have also provided useful information on the contribution of genetic factors influencing nicotinic receptors function. Finally the molecular genetic technique of gene targetting to create mice with null mutations of specific genes in the central nervous system, which is having a tremendous impact in drug addiction research, has also been employed to gain insight into the molecular and cellular basis of nicotine action. These techniques are proving to be invaluable in dissecting the role of different subunits of the nicotinic acetylcholine receptors on behaviour. This paper provides a survey of the animal studies that have used the above mentioned techniques to gain insight into genetic basis of the behavioural effects of nicotine.

Could a common biochemical mechanism underlie addictions?

The subject of 'drug addiction' is multifaceted and many aspects of it (even some of the definitions) are controversial. Collateral medical problems include the spread of HIV and hepatitis C virus secondary to i.v. drug abuse and effects on prenatal brain development (1). Progress in the understanding of the causes of addictions and its treatment has been impeded by the lack of a unifying biochemical theory. However, recent evidence suggests that some common mechanism might underlie addictions to otherwise apparently unrelated drugs. A major hypothesis has emerged suggesting that the neurotransmitter dopamine (DA) might play a central role in the molecular mechanisms of at least some addictions. If so, it would represent an important target for discovery of effective pharmacotherapy and revolutionize the pharmacist's role in treating addictions. This short overview outlines the status of the theory of a common biochemical mechanism of drug addiction.