A Genetic Animal Model of Alcoholism for Screening Medications to Treat Addiction.

The purpose of this review is to present up-to-date pharmacological, genetic, and behavioral findings from the alcohol-preferring P rat and summarize similar past work. Behaviorally, the focus will be on how the P rat meets criteria put forth for a valid animal model of alcoholism with a highlight on its use as an animal model of polysubstance abuse, including alcohol, nicotine, and psychostimulants. Pharmacologically and genetically, the focus will be on the neurotransmitter and neuropeptide systems that have received the most attention: cholinergic, dopaminergic, GABAergic, glutamatergic, serotonergic, noradrenergic, corticotrophin releasing hormone, opioid, and neuropeptide Y. Herein, we sought to place the P rat's behavioral and neurochemical phenotypes, and to some extent its genotype, in the context of the clinical literature. After reviewing the findings thus far, this chapter discusses future directions for expanding the use of this genetic animal model of alcoholism to identify molecular targets for treating drug addiction in general.

In vivo wireless ethanol vapor detection in the Wistar rat.

Traditional alcohol studies measure blood alcohol concentration to elucidate the biomedical factors that contribute to alcohol abuse and alcoholism. These measurements require large and expensive equipment, are labor intensive, and are disruptive to the subject. To alleviate these problems, we have developed an implantable, wireless biosensor that is capable of measuring alcohol levels for up to six weeks. Ethanol levels were measured in vivo in the interstitial fluid of a Wistar rat after administering 1 g/kg and 2 g/kg ethanol by intraperitoneal (IP) injection. The data were transmitted wirelessly using a biosensor selective for alcohol detection. A low-power piezoresistive microcantilever sensor array was used with a polymer coating suitable for measuring ethanol concentrations at 100% humidity over several hours. A hydrophobic, vapor permeable nanopore membrane was used to screen liquid and ions while allowing vapor to pass to the sensor from the subcutaneous interstitial fluid.

The role of 5-HT3 receptors in drug abuse and as a target for pharmacotherapy.

Alcohol and drug abuse continue to be a major public health problem in the United States and other industrialized nations. Extensive preclinical research indicates the mesolimbic dopamine (DA) pathway and associated regions mediate the rewarding and reinforcing effects of drugs of abuse and natural rewards, such as food and sex. The serotonergic (5-HT) system, in concert with others neurotransmitter systems, plays a key role in modulating neuronal systems within the mesolimbic pathway. A substantial portion of this modulation is mediated by activity at the 5-HT3 receptor. The 5-HT3 receptor is unique among the 5-HT receptors in that it directly gates an ion channel inducing rapid depolarization that, in turn, causes the release of neurotransmitters and/or peptides. Preclinical findings indicate that antagonism of the 5-HT3 receptor in the ventral tegmental area, nucleus accumbens or amygdala reduces alcohol self-administration and/or alcohol-associated effects. Less is known about the effects of 5-HT3 receptor activity on the self-administration of other drugs of abuse or their associated effects. Clinical findings parallel the preclinical findings such that antagonism of the 5-HT3 receptor reduces alcohol consumption and some of its subjective effects. This review provides an overview of the structure, function, and pharmacology of 5-HT3 receptors, the role of these receptors in regulating DA neurotransmission in mesolimbic brain areas, and discusses data from animal and human studies implicating 5-HT3 receptors as targets for the development of new pharmacological agents to treat addictions.

Neuroadaptive changes in the mesoaccumbens dopamine system after chronic nicotine self-administration: a microdialysis study.

There is little evidence to date to indicate if mesoaccumbens dopamine function at the neurochemical level is altered during early abstinence from chronic i.v. nicotine self-administration. Thus, a quantitative microdialysis (no-net-flux) approach was used to measure basal extracellular concentrations and extraction fractions of dopamine in the nucleus accumbens (ACB) of rats that self-administered nicotine i.v. for 25 days, as well as in rats serving as yoked comparison groups (yoked nicotine and yoked saline). After 24-48 h of the final self-administration session, there was a significant reduction in basal extracellular dopamine levels in the ACB of the self-administration group compared with the yoked saline group (1.35+/-0.15 nM versus 3.70+/-0.28 nM). The basal extracellular dopamine levels in the yoked nicotine group (1.46+/-0.20 nM) were not significantly different compared with the nicotine self-administration group. The in vivo extraction fraction of dopamine, an indirect measure of dopamine uptake, was significantly increased in the nicotine self-administration (86%) and yoked nicotine (91%) groups compared with the yoked saline group (77%). In addition, a marked reduction in the elevation of extracellular dopamine levels in the ACB occurred after a nicotine challenge as measured by conventional microdialysis in the self-administration (112% of basal) and yoked nicotine (121% of basal) groups as compared with a yoked saline (154% of basal) group. The reduced basal ACB dopamine levels in the nicotine groups during early abstinence appears to be due to increased clearance, suggesting increased dopamine uptake is occurring as a result of the chronic nicotine treatment. The reduced elevation of extracellular dopamine levels in the ACB upon nicotine challenge suggests a functional desensitization or downregulation phenomenon involving acetylcholine receptors (nicotinic nAChRs). Overall, these results provide clear evidence for a neuroadaptive change that alters dopamine transmission in the ACB during abstinence from chronic i.v. nicotine exposure.

Ethanol drinking and deprivation alter dopaminergic and serotonergic function in the nucleus accumbens of alcohol-preferring rats.

The alcohol deprivation effect is a temporary increase in the intake of, or preference for, ethanol after a period of deprivation that may result from persistent changes in key limbic regions thought to regulate alcohol drinking, such as the nucleus accumbens. The present study tested the hypothesis that chronic alcohol drinking under continuous 24-h free-choice conditions alters dopamine and serotonin neurotransmission in the nucleus accumbens and that these alterations persist in the absence of alcohol. Using the no-net-flux microdialysis method, the steady-state extracellular concentration (point of no-net-flux) for dopamine was approximately 25% higher in the adult female alcohol-preferring P rats given prior access to 10% ethanol, even after 2 weeks of ethanol abstinence, compared with the P rats gives access only to water. However, the extracellular concentration of serotonin was approximately 35% lower in animals given 8 weeks of continuous access to ethanol compared with water controls and animals deprived of ethanol for 2 weeks. The effect of local perfusion with 100 microM sulpiride (D(2) receptor antagonist) and 35 microM 1-(m-chlorophenyl)-biguanide (5-hydroxytryptamine(3) receptor agonist) on dopamine overflow were reduced approximately 33% in both groups of ethanol-exposed P rats compared with water controls. Free-choice alcohol drinking by P rats alters dopamine and serotonin neurotransmission in the nucleus accumbens, and many of these effects persist for at least 2 weeks in the absence of ethanol, suggesting that these underlying persistent changes may be in part responsible for increased ethanol drinking observed in the alcohol-deprivation effect.

Reverse microdialysis of a dopamine uptake inhibitor in the nucleus accumbens of alcohol-preferring rats: effects on dialysate dopamine levels and ethanol intake.

BACKGROUND: The mesolimbic dopamine (DA) system has been implicated in mediating the reinforcing actions of ethanol (EtOH). This study examines the effects of local perfusion of the DA uptake inhibitor GBR12909 (GBR) on (1) DA levels in the nucleus accumbens (NAc) and (2) EtOH drinking in alcohol-preferring rats. METHODS: Stable drinking of a 15% (v/v) EtOH solution (minimum of 0.75 g/kg body weight) was established in daily 1 hr limited access sessions. Rats were then implanted with bilateral guide cannulae aimed 4 mm above the NAc. After recovery from surgery, concentric microdialysis probes (2 mm dialysis membrane surface) were inserted into the NAc. Most placements were in the shell or overlapping both shell and core. Two days later, the probes were perfused at 1.0 microl/min with artificial cerebral spinal fluid (aCSF) for at least a 90 min washout period followed by collection of five basal samples over 150 min. Rats were then perfused with either aCSF alone or 10, 25, 100, or 200 microM of GBR for 240 min on the first day of microdialysis. During the last 60 min of the drug treatment phase, rats were given their scheduled access to 15% EtOH. All rats were then perfused with aCSF for the last 90 min of the experiment. The following day, the procedure was repeated, but animals that received aCSF on the first day were given a dose of GBR and rats given GBR on the first day received only aCSF. RESULTS: GBR perfusion increased extracellular NAc DA levels dose dependently to more than 800% of basal levels at 100 to 200 microM but failed to alter EtOH intake (p > 0.05, paired t test) at any concentration tested. Moreover, after 100 microM of GBR perfusion had terminated, the extracellular levels of DA in the NAc remained elevated for approximately 24 hr (790% of day 1 basal; p < 0.05). The increase in dialysate DA levels observed during GBR perfusion with 100 microM was significantly greater for EtOH-experienced rats than for EtOH-naïve rats [F(7,59) = 14.85, p < 0.0001, analysis of variance, Student-Newman-Keuls post hoc test]. CONCLUSIONS: The results suggest (1) that EtOH drinking experience induces neuroadaptations that increase DA release in the NAc, and (2) that additional elevation in synaptic levels of DA in the NAc does not influence the maintenance of ongoing alcohol drinking under scheduled access conditions in alcohol-preferring animals.

Enhancement of fluoxetine-dependent increase of extracellular serotonin (5-HT) levels by (-)-pindolol, an antagonist at 5-HT1A receptors.

The somatodendritic 5-HT1A autoreceptor is known to regulate activity of 5-HT neurons and consequently 5-HT release. Administration of a selective 5-HT uptake inhibitor, fluoxetine (10 mg/kg, i.p.) increased extracellular 5-HT levels in rat hypothalamus up to 260 percent of basal levels. (-)-Pindolol, and antagonist at the somatodendritic 5-HT1A autoreceptor, dose-dependently (1, 3 and 5 mg/kg, s.c.) potentiated the fluoxetine dependent increase up to 458 percent of basal 5-HT levels for approximately 1.5 hours. Continuous infusion of ( +/- )-pindolol at 30 mg/kg/h s.c. enhanced the fluoxetine dependent elevation of extracellular 5-HT concentrations in hypothalamus up to 464 percent of basal levels and lasted for 3 hours. Thus, the combination of 5-HT uptake inhibition with antagonism at the somatodendritic 5-HT1A autoreceptor can enhance 5-HT release to levels beyond those achieved with uptake inhibition alone. The present findings are consistent with the hypothesis that blockade of somatodendritic 5-HT1A autoreceptors removes the inhibitory effect exerted by the elevated 5-HT levels resulting from uptake inhibition.

Antagonism of serotonin 5-HT1A receptors potentiates the increases in extracellular monoamines induced by duloxetine in rat hypothalamus.

In the current study we examined the effects of coadministration of a serotonin 5-HT1A antagonist, (+-)-1-(1H-indol-4-yloxy)-3-(cyclohexylamino)-2-propanol maleate (LY 206130), and a dual 5-HT and norepinephrine (NE) uptake inhibitor, duloxetine, on extracellular levels of NE, 5-HT, dopamine (DA), 5-hydroxyindoleacetic acid, and 3,4-dihydroxyphenylacetic acid in rat hypothalamus microdialysates. LY 206130 (3.0 mg/kg, s.c.) alone significantly increased NE and DA levels by 60 and 34%, respectively, without affecting 5-HT levels. Duloxetine administration at 4.0 mg/kg, i.p. alone produced no significant changes in levels of 5-HT, NE, or DA. In contrast, when LY 206130 and duloxetine were coadministered at 3.0 mg/kg, s.c. and 4.0 mg/kg, i.p., respectively, 5-HT, NE, and DA levels increased to 5.7-, 4.8-, and threefold over their respective basal levels. These data demonstrate that antagonism of somatodendritic 5-HT1A autoreceptors and concomitant inhibition of 5-HT and NE uptake with duloxetine may promote synergistic increases in levels of extracellular 5-HT, NE, and DA in hypothalamus of conscious, freely moving rats.

Regulation of extracellular concentrations of norepinephrine in hypothalamus of the conscious rat: effect of amitriptyline.

The current study examines the effects of amitriptyline administration on extracellular levels of monoamines and their metabolites in hypothalamus of conscious rat using in vivo microdialysis. Systemic amitriptyline (30 mg/kg, i.p.) maximally increased extracellular norepinephrine to nearly 19 times, dopamine 12 times, and serotonin 2.5 times that of their respective basal values. Local administration of amitriptyline (10 microM) in the perfused artificial cerebrospinal fluid increased extracellular serotonin levels to 2.7 times basal levels - a similar increase to that observed with systemic administration. In contrast, local administration of amitriptyline (10 microM) increased extracellular norepinephrine and dopamine to only 5.3 and 3.8 times basal levels. Systemic administration of amitriptyline (30 mg/kg, i.p.) during local amitriptyline perfusion increased norepinephrine and dopamine levels to 28 and 12.9 times their respective basal levels without affecting serotonin levels. Systemic amitriptyline administration preferentially increases extracellular norepinephrine and dopamine levels over serotonin levels in the rat hypothalamus and the effects on the catecholamine levels may be partially due to mechanisms other than local uptake inhibition.

Operant response suppression induced with systemic administration of 5-hydroxytryptophan is centrally mediated.

Intracerebroventricular (ICV) administration of selective serotonergic agents was used to examine the extent of central mediation of 5-HTP-induced operant response suppression in rats. ICV administration of LY53857 (1.0, 3.75, or 7.5 micrograms/5 microliters/5 min) dose dependently blocked response suppression induced with systemically administered 5-HTP (25 mg/kg, IP), whereas ICV 0.9% saline (5 microliters over 5 min) had no significant effect on 5-HTP-induced response suppression. ICV ketanserin (7.5 micrograms/5 microliters/5 min) also blocked response suppression induced with systemically administered 5-HTP. ICV administration of the 5-HT2A/2C receptor agonist DOI (80 micrograms/5 microliters/5 min) induced significant periods of response suppression in this model, which was blocked with LY53857 (1.0 mg/kg, IP) pretreatment. These data demonstrate that central administration of 5-HT2A/2C antagonists potently attenuate operant response suppression induced with systemically administered 5-HTP or DOI and are in agreement with previous findings suggesting central mediation of 5-HTP-induced operant response suppression.

Simultaneous increases of extracellular monoamines in microdialysates from hypothalamus of conscious rats by duloxetine, a dual serotonin and norepinephrine uptake inhibitor.

Duloxetine (LY248686, [+]-N-methyl-3-(1-napthalenyloxy)-2-thiophene-propanamine) is a potent dual inhibitor of serotonin (5-hydroxytryptamine, 5-HT) and norepinephrine (NE) uptake in hypothalamus and cerebral cortex of rat brain (Wong et al. 1993; Fuller et al. 1994). Consistent with the dual mechanisms of inhibiting 5-HT and NE uptake, duloxetine at 15 mg/kg IP produced large increases in extracellular levels of 5-HT (250%) and NE (1,100%) 30 minutes after systemic administration. Levels of 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG) and 5-hydroxyindoleacetic acid (5-HIAA), metabolites of NE and 5-HT, respectively, were reduced, whereas those of dopamine (DA) and its metabolite 3, 4-dihydroxyphenylacetic acid (DOPAC) were not significantly altered. Duloxetine at 7 mg/kg produced less pronounced increases while no consistent effects were observed at 4 mg/kg. In this dose range, duloxetine inhibited 5-HT uptake in platelets ex vivo without inhibiting striatal dopamine (DA) uptake. In the present study we also found that the primary amine (a racemate) of duloxetine is about one-fourth as active as duloxetine to inhibit 5-HT and NE uptake. The potential primary amine metabolite of duloxetine might contribute, in part, to the inhibition of 5-HT and NE uptake in vivo. Thus the ability to produce robust increases of extracellular 5-HT and NE levels suggests that duloxetine may potentially be a highly effective antidepressant agent.

Prozac (fluoxetine, Lilly 110140), the first selective serotonin uptake inhibitor and an antidepressant drug: twenty years since its first publication.

In this review, we describe the evolutionary process involved in the discovery of the selective 5-HT uptake inhibitor, fluoxetine, and summarize some of the large body of scientific research performed on fluoxetine in the 20 years since the first publication. The historical background of the proposed involvement of 5-HT in psychiatric disorders and the activity of tricyclic antidepressants in depression is reviewed. The effects of fluoxetine in various in vitro assays and in animal studies including receptor down-regulation, neurochemical and behavioral models are summarized. In addition, the clinical effectiveness of fluoxetine in depression and obsessive compulsive disorders and its potential use in other disorders are examined.

Response suppression induced with selective 5-HT agonists can be differentially blocked with LY53857 in an animal model of depression.

Studies from this laboratory have demonstrated that administration of the selective 5-HT2/1C antagonist LY53857 can block 5-HTP-induced response suppression. To further investigate the serotonergic mechanisms involved in this effect, we decided to test the capacity of LY53857 to block response suppression induced with two selective 5-HT agonists. After a 15 minute baseline period, rats trained to press a lever for milk reinforcement on a VI 1' schedule were given IP injections of 1.0 mg/kg DOI, or 1.0 mg/kg 8-OH-DPAT to induce response suppression. Subsequently, rats were injected with 1.0 mg/kg LY53857 1 hour prior to DOI- or 8-OH-DPAT-induced response suppression. Preinjections with LY53857 resulted in a 100% blockade of DOI-induced response suppression whereas the same dose did not block response suppression induced with 8-OH-DPAT. These results indicate that the 5-HTP-induced response suppression shows some pharmacological similarity to DOI-induced response suppression and may be mediated through 5-HT2 and/or 5-HT1C receptors.