A method for recognizing specific effects on ethanol intake by experimental animals

Genetics of dopamine and its contribution to cocaine addiction

Cocaine addiction is a major health and social problem for which there are presently no effective pharmacotherapies. Many of the most promising medications target dopamine based on the large literature that supports its role in addiction. Recent studies show that genetic factors are also important. Rodent models and gene knock-out technology have helped elucidate the involvement of specific genes in the function of the dopamine reward system and intracellular cascades that lead to neuronal changes in this system. Human epidemiological, linkage, and association studies have identified allelic variants (polymorphisms) that give rise to altered metabolism of dopamine and its functional consequences. Individuals with these polymorphisms respond differently to psychostimulants and possibly to pharmacotherapies. Here we review the literature on genetic variations that affect dopamine neurotransmission, responses to psychostimulants and potential treatments for cocaine addiction. Behavioral responses to psychostimulants in animals with different or modified genetics in dopamine signaling are discussed. We also review polymorphisms in humans that affect dopaminergic neurotransmission and alter the subjective effects of psychostimulants. Pharmacotherapies may have increased efficacy when targeted to individuals possessing specific genetic polymophisms in dopamine's metabolic and intracellular messenger systems.

Disulfiram facilitates the development and expression of locomotor sensitization to cocaine in rats

BACKGROUND

Disulfiram (DS; Antabuse) inhibits dopamine-beta-hydroxylase leading to increased brain dopamine levels and shows treatment efficacy for cocaine addiction. Yet few preclinical studies have been performed. This study establishes the effects of DS on locomotor sensitization to cocaine in rats.

METHODS

Rats were administered vehicle, cocaine (10 mg/kg; intraperitoneally [IP]), or DS (50 or 100 mg/kg; IP) alone or in combination for 5 days (development phase). Locomotor activity was measured for 60-min each day. After a 10-day drug washout, rats were administered cocaine, and locomotor activity was measured (expression phase). Plasma cocaine levels were assessed in separate groups of rats administered one of two cocaine doses (0 or 10 mg/kg) and one of two DS doses (0 or 100 mg/kg) for 5 days. Ten days later, blood was collected 60-min postcocaine injection.

RESULTS

The development of cocaine locomotor sensitization was facilitated by DS even though DS alone had minimal effect on activity levels. Furthermore, expression of sensitization was greatest in rats previously administered DS, an effect that cannot be attributed to altered plasma cocaine levels.

CONCLUSIONS

Because DS shows treatment efficacy for cocaine addiction, results from this study suggest potential treatment agents should enhance, not attenuate, locomotor sensitization in rats.

Effect of low doses of ethanol on spontaneous locomotor activity in UChB and UChA rats

The effects of low to moderate doses of ethanol on spontaneous locomotor activity were studied in the selectively bred high-ethanol drinking (UChB) and the low-ethanol drinking (UChA) strain of rats. Alcohol-naive rats had food and water available ad libitum, although food was removed 24 hours before and during activity testing. After an injection of c.15 M NaCl or ethanol (0.25-1.0 g/kg), spontaneous locomotor activity was monitored every 5 minutes for 20 minutes in an open field apparatus. The UChB rats exhibited increased locomotor activity after doses of 0.25 and 0.50 g/kg of ethanol, while UChA rats failed to show increased locomotor activity at any ethanol dose. Moreover, the UChA rats appeared to be more sensitive to the sedating effects of 1.0 g/kg of ethanol than the UChB rats. These differences were not the result of different brain-blood alcohol levels. Ethanol intakes by the UChB and UChA rats determined at the conclusion of activity testing averaged 5.0 ± 0.5 and 1.9 ± 0.4 g/kg/day, respectively. The data suggest that ethanol-induced locomotor stimulation may be associated with ethanol preference and that hyperactivity may be an expression of the positive reinforcing effect of ethanol in UChB rats.

Effect of nicotinamide administration on ethanol consumption and on liver and brain acetaldehyde oxidation rate, by UChB rats

The activities of liver and brain aldehyde dehydrogenase, an NAD(+) dependent enzyme, which controls acetaldehyde oxidation have been reported to play a role in voluntary ethanol consumption. It has been reported that nicotinamide administration to rats increases NAD(+) levels, that may increase acetaldehyde oxidation rates if basal NAD(+) levels are not saturating for the enzyme. In the present paper the effect of nicotinamide administration on voluntary ethanol consumption by genetically high ethanol consumer UChB rats and brain and liver mitochondrial acetaldehyde oxidation were studied. Administration of nicotinamide 250 or 500 mg/kg i.p. to UChB rats, produced a significant reduction in their voluntary ethanol consumption and increased brain acetaldehyde oxidation in brain but not liver homogenates.These results suggest that basal NAD(+) levels are not saturating for brain aldehyde dehydrogenase and that the reduction of ethanol consumption by UChB rats may be the consequence of a change in the brain redox state, rather than the local level of acetaldehyde.

Effect of captopril on voluntary consumption of ethanol, water and solid food by UChA and UChB rats

Angiotensin converting enzyme inhibitors (ACEI) have been reported to reduce ethanol consumption in rats, but it is unclear whether this effect is specific for ethanol or secondary to effects on appetite or satiation for calories or water. In the present study we assessed the effect of captopril, an ACEI, on the voluntary consumption of 10% ethanol solution, water and solid food in our strain of rats genetically selected for their voluntary consumption of ethanol, namely UChA (low consumer) and UChB (high consumer). Captopril (30 mg/kg) was injected intraperitoneally for 3 consecutive days to UChA and UChB rats and ethanol, water and food intake were measured before, during and after captopril treatment; these results were compared with those produced by a control saline solution. Results showed that captopril produced a significant reduction of alcohol voluntary consumption in UChB but not in UChA rats. However, this effect was not specific for ethanol since captopril also induced a significant decrease in food intake leading to a loss of weight in both rat strains, suggesting that it seems to be secondary to changes in appetite for calories.

Effects of bromocriptine on the voluntary consumption of ethanol, water, and solid food by UChA and UChB rats

The effect of bromocriptine (stimulant of dopaminergic D2 receptors) on the daily consumption of 10% v/v ethanol solution, distilled water, and solid food, under free-choice conditions, was measured in nine genetically low (UChA) and six high ethanol consumer (UChB) adult female rats. Animals were housed in individual cages and maintained at room temperature of 23 +/- 1 degree C and with 12/12 h dark/light rhythm. The consumption of ethanol solution, water, and solid food was measured in pretreatment, treatment, and posttreatment periods of 3 days (Tuesday through Thursday) of 3 consecutive weeks. During the treatment period rats received daily a single oral dose of 8 mg of bromocriptine mesylate (Sandoz) suspended in 1 ml of water per kg of body weight. Data analysis was performed with a method previously reported, which allows to recognize specific effect of ethanol intake, depurated from the effects on calories and/or water consumption. Results showed that all UChB rats decreased significantly and specifically the consumption of ethanol solution during the treatment period compared to the pretreatment period (mean: -58 +/- 15%) and recovered the pretreatment consumption in the posttreatment period, without significant changes in the consumption of food and/or total water. The only significant change observed in UChA rats was a decrease of the consumption of solid food (mean: -15 +/- 5%). Results are consistent with the idea that a dopaminergic D2 synapsis participates in the neural network responsible for satiation with ethanol.

Effects of aminotriazole on ethanol, water, and food intake and on brain catalase in UChA and UChB rats

Aminotriazole (AT), a catalase inhibitor, was administered to UChA (low ethanol consumer) and UChB (high ethanol consumer) rats. Ethanol, water, and solid food intake were measured during basic, treatment, and posttreatment periods. The effects of AT on brain catalase activity and acetaldehyde recovered during incubation of brain homogenates with ethanol were also studied in rats of both strains. Results showed that AT decreased voluntary ethanol intake in UChB rats, and also diminished the consumption of food by rats of both strains. No strain difference in brain catalase activity and acetaldehyde recovered during ethanol incubation was observed. The results suggest that AT effect on ethanol consumption is secondary to a reduction in the appetite for calories and not related to its catalase blocking effect.

Effects of p-chlorophenylalanine on the voluntary consumption of ethanol, water and solid food by UChA and UChB rats

The immediate and the long-lasting effects of PCPA (126 mg/kg IP for 3 days) on the voluntary consumption of 10% v/v ethanol solution, water and solid food were studied in genetically low (UChA) and high (UChB) ethanol consumer rats. Data were analysed according to the method proposed by the authors for recognizing specific effects on ethanol consumption. Results confirmed immediate specific decrease of ethanol consumption in UChB rats, while a nonspecific decrease of it was observed in UChA rats. In UChB rats ethanol consumption recovered the basic level about nine days after the first dose. By contradistincion, in UChA rats a significant specific increase of ethanol consumption, starting at the first week after the treatment, was observed. During this period 12 out of 21 UChA rats reached an ethanol consumption level commonly observed in UChB rats, and 10 of them recovered the pretreatment level in 15 to 30 weeks. The other 2 maintained the high consumption until 38 weeks of observation. At that time the serotonin content of cortex and hippocampus of these 2 rats was normal.

Effects of serotonin uptake blockers and of 5-hydroxytryptophan on the voluntary consumption of ethanol, water and solid food by UChA and UChB rats

The effects of zimelidine, fluvoxamine, and citalopram (serotonin uptake blockers), as well as those of 5-hydroxytryptophan (serotonin precursor), on the voluntary consumption of 10% ethanol solution, distilled water and solid food were tested in UChA (genetically low ethanol consumer) and UChB (genetically high ethanol consumer) rats. Since it is well known that drugs which stimulate central serotonergic synapses decrease food and water intake, the data concerning the difference of the respective consumption during the treatment period and the pretreatment one were analysed with a method previously proposed (Alcohol 5:15-19; 1988) to recognize specific effects on ethanol intake. The results showed that while the decrease of ethanol consumption induced by the three serotonin uptake blockers appeared not to be specific of ethanol, the effects of 5-hydroxytryptophan in UChB rats satisfy the criteria for being considered as an expression of a decrease of the specific appetite--or increase satiety--for ethanol. Experimental results cannot help in the explanation of this difference.

Vasoactive intestinal peptide: behavioral effects in the rat and hamster

The behavioral effects of intracerebroventricular (ICV) injection of the brain-gut peptide vasoactive intestinal peptide (VIP) were quantified with a behavioral sampling technique in home-caged, nondeprived, male and female albino rats and golden hamsters. ICV VIP sex-dependently decreased observed resting behavior during 1 hr after injections in both rats and hamsters at 0.1-10.0 micrograms. Grooming behavior was increased in hamsters, and rearing and standing behaviors were increased in rats, sex-dependently at VIP doses that decreased resting. Drinking behavior was suppressed in rats by VIP at 10.0 micrograms. Intraperitoneal (IP) VIP (100.0 micrograms/kg) increased 5% ethanol intake and decreased eating behavior in fluid-deprived male rats. The increase in ethanol intake produced by IP VIP was prevented by IP cholecystokinin octapeptide (CCK, 4.0 micrograms/kg). VIP potently controls resting and ingestive behaviors, suggesting a role for this neuropeptide, along with CCK, in the feedback regulation of rodent behavior.

Effects of pyrazole on the voluntary consumption of ethanol, water and solid food in UChA and UChB rats

The effect of pyrazole (35 or 70 mg/kg IP) on the voluntary consumption of ethanol, water and solid food was studied in UChA (genetically low ethanol consumer) and UChB (genetically high ethanol consumer) rats of both sexes, under a free choice of 10% v/v ethanol solution, distilled water and solid food. the data were analyzed according to the method previously proposed for recognizing the specific effect on appetite-satiety of ethanol. The effect of pyrazole in UChB rats correlated with the pattern of specific effects on appetite-satiety for ethanol. The only effect observed in UChA rats was a decrease in the intake of solid food and total water. No significant sex differences were observed.