Drinking patterns in genetic low-alcohol-drinking (LAD) rats after systemic cyanamide and cerebral injections of THP or 6-OHDA.

A key question related to the role of acetaldehyde and aldehyde adducts in alcoholism concerns their relationship to the genetic mechanisms underlying drinking. Experimentally, the low-alcohol-drinking (LAD) rat represents a standard rodent model having a strong aversion to alcohol. In these experiments, preferences for water vs. alcohol, offered in concentrations from 3% to 30%, were determined over 10 days in adult LAD rats (N = 6 per group). Then a saline vehicle or either 10 or 20 mg/kg of the aldehyde dehydrogenase (AIDH) inhibitor, cyanamide, was injected s.c. twice daily for 3 days. Secondly, either 0.5 or 1.0 microg of tetrahydropapaveroline (THP) was infused i.c.v. twice daily for 3 days in LAD rats (N = 8) and, as a genetic control, THP also was infused identically in Sprague-Dawley (SD) rats (N = 8). The results showed that the lower and higher doses of cyanamide augmented alcohol intakes in 33% and 50% of the LAD rats, respectively, with the patterns of drinking resembling that of genetic high-alcohol-drinking HAD or P rats. Although i.c.v. infusions of THP had little effect on alcohol preference of LAD rats, alcohol drinking was enhanced significantly in the SD rats. In a supplementary study, 200 microg of 6-hydroxydopamine (6-OHDA) also was infused i.c.v. in LAD rats (N = 7) on two consecutive days; no change occurred in the characteristic aversion to alcohol. These findings suggest that in certain individuals, a perturbation in the synthesis of AIDH can modify the genetically based aversion to alcohol, thus precipitating the liability for alcoholism. In that neither THP nor 6-OHDA lesioning exerted any effect on the genetic nondrinking LAD animal suggests that an unknown endogenous factor in the brain must underlie the cyanamide-induced shift to alcohol preference. We conclude that the genetic elements that normally prevent the progression to addictive drinking in most individuals appear to be invariant and irreversible.

Study of the antinociceptive effect of tetrahydro-papaveroline derivatives. Interaction with opioids.

The antinociceptive effect of racemic tetrahydropapaveroline (THP), of its two R(+)- and S(-) enantiomers, of 1-2-dehydro-THP and of 1-carboxy-THP was assessed using different pain tests in mice. None of these drugs possessed a significant activity in the hot-plate and tail-flick tests. However, after i.p. injection, they reduced the number of abdominal writhes induced by phenylbenzoquinone, with ED50 values of 51 +/- 7, 73 +/- 9 and 79 +/- 7 mg/kg for the most potent compounds: 1,2-dehydro-THP, +/- THP and -THP, respectively. This activity was not antagonized by naloxone (1 mg/kg, s.c.). However combination of inactive doses of these three compounds (32 mg/kg, i.p.) and of morphine (0.5 mg/kg, s.c.) led to a significant antinociceptive effect (83 to 85% of reduction of the number of writhes). This synergistic potentiation confirmed with the combination of +/- THP (16 mg/kg, i.p.) and morphine (0.5 mg/kg, s.c.) was totally inhibited by naloxone (1 mg/kg, s.c.). These results, although excluding a direct agonistic effect of THP derivatives on opiate receptors, suggest an indirect interaction of these drugs with the endogenous opioid system.

Tetrahydropapaveroline and the blood-brain barrier in rats.

The blood-brain barrier penetration of tetrahydropapaveroline (THP) was studied in male rats of the Sprague-Dawley strain. THP was not found in brains of untreated animals. However, THP was observed in the brains of animals that received THP.HBr by intraperitoneal (IP) injection 30 minutes before decapitation. Animals that received IP injections of 0.10, 1.0, 5.0, or 10 mg THP.HBr/kg exhibited brain levels of 3.1, 25, 95 or 126 pmoles THP per gram brain, respectively. Another group of rats received THP.HBr (5.0 mg/kg) IP 30 minutes before decapitation. The brain of each animal was dissected into nine regions and each region assayed for THP. All brain regions assayed had measurable levels of THP. The highest concentration of THP (132 pmoles/g) was observed in the olfactory lobes-frontal cortex while the lowest concentration of THP (27.3 pmol/g) was in the striatum. These results demonstrate that THP penetrates the blood-brain barrier in rats. They also suggest that if THP is formed in the periphery, it may penetrate the blood-brain barrier, and be localized in discrete brain regions.

Effect of acute ethanol administration on brain levels of tetrahydropapaveroline in L-dopa-treated rats.

The effect of ethanol on the concentration of the aberrant dopamine metabolite, tetrahydropapaveroline (THP), in brains of L-dopa-treated rats has been evaluated. THP was isolated from rat brain extract by a newly developed multiple stage separation technique that is highly specific for the alkaloid. THP, dopa, and dopamine were assayed by high-performance liquid chromatography with electrochemical detection. THP was not found in brains of untreated animals. However, levels of 0.42 pmol THP per g brain were observed in animals that received L-dopa (200 mg/kg) by intraperitoneal injection (IP) 90 min before decapitation. Administration of ethanol (3g/kg) IP to L-dopa-treated animals at time intervals ranging from 60 to 240 min before decapitation resulted in significant increases in brain levels of THP as compared to L-dopa-treated animals. Maximum levels of THP (4.02 to 4.82 pmol/g brain) were observed when ethanol was given at time intervals ranging from 80 to 180 min before the animals were killed. Administration of ethanol and L-dopa, as compared to the administration of L-dopa only, markedly increased brain levels of dopa and dopamine. Maximum brain levels of THP, dopa, and dopamine in animals administered ethanol plus L-dopa as compared with L-dopa-treated animals represented a 1048%, 325%, and 84% increase, respectively. These results strongly support the concept that the concentration of THP in the brain of intact animals can be enhanced by ethanol administration.

MAO activity in rat brain stem and cerebral cortex: effect of acute and chronic treatment with ethanol and tetrahydropapaveroline.

Brain mitochondria were prepared from rat brain stem and cerebral cortex. Four substrates were used for enzyme characterization: dopamine, serotonin, noradrenaline and tryptamine. Monoamine oxidase (MAO) activities were tested on two different groups of rats treated with ethanol and tetrahydropapaveroline (THP); another group was used as control group. Data obtained show that there is almost always activation of MAO activities after acute treatment with either ethanol or THP, while after chronic treatment, the two drugs show opposite effects. In particular, the same type of MAO activity continues to be activated by ethanol, whereas it is inhibited by THP.