Effects of modulation of NMDA neurotransmission on response rate and duration in a conflict procedure in rats

N-Methyl-D-aspartate (NMDA) antagonists and gamma-aminobutyric acid agonists share a number of common pharmacological properties, including motor and anticonvulsant effects. In the present study, site-selective NMDA antagonists were evaluated for potential anxiolytic efficacy and motor impairment in a modified Geller-Seifter conflict procedure, an animal model widely used to screen drugs for anxiolytic effects. Male Sprague-Dawley rats were trained to respond for food reward under a multiple FI 30 s (food only), FR 10 (food + shock) operant schedule. Consistent with the results of previous studies, the benzodiazepines chlordiazepoxide and diazepam selectively increased punished responding and increased response durations at higher doses. The competitive NMDA antagonist CGP 37,849 increased punished responding at some doses, though not selectively, and also increased response duration in both schedule components. The glycine-site modulators milacemide, ACEA 1011 and ACEA 1021, the NR2B-selective polyamine site antagonist eliprodil and NMDA did not produce anticonflict effects at any dose and had inconsistent effects on response durations. These results suggest that the anticonflict effects of NMDA antagonists are not as reliable as those of the benzodiazepines. Further research is needed to clarify the experimental conditions under which the anxiolytic potential of NMDA antagonists is most evident.

The effects of inhaled isoparaffins on locomotor activity and operant performance in mice

Very little is known qualitatively or quantitatively about the acute central nervous system effects of isoparaffin solvents that are widely used in household and commercial applications. Four isoparaffinic hydrocarbon solvent products differing in predominant carbon number and volatility (ISOPAR-C, -E -G, -H) were tested for their acute effects on locomotor activity and operant performance after inhalation exposure in mice. For both measures, concentration-effect curves were obtained for 30-min exposures using a within-subject design. The more volatile products, ISOPAR-C and -E, were as easily vaporized under our conditions as vapors such as toluene and TCE, which have acute effects on human behavior and are abused. ISOPAR-G was slowly volatilized and ISOPAR-H could not be completely volatilized within our 30-min exposures, suggesting that acute human exposures may be less likely and that it may be more difficult to abuse them. ISOPAR-C, -E, and -G produced reversible increases in locomotor activity of mice at 4000 and 6000 ppm while ISOPAR-C and -E produced reversible concentration-dependent decreases in rates of schedule-controlled operant behavior in approximately the same concentration range as they affected locomotor activity. The fact that only locomotor activity increases were observed with these isoparaffins provides evidence that they produce a different pattern of effects than those reported for abused solvents such as toluene and TCE. Further research will be needed to determine if this different pattern of effects on animal behavior between isoparaffins and abused solvents is correlated with a different pattern of acute intoxication and abuse potential in humans.

Phencyclidine-like discriminative stimulus effects of polyamine modulators of N-methyl-D-aspartate receptor activity in rats

Excessive N-methyl-D-aspartate (NMDA) receptor activation has been implicated in many acute and chronic neuropathologies. NMDA antagonists might prove to be useful treatments, unfortunately, some can produce phencyclidine (PCP)-like side effects. The polyamine-site modulators, spermine (SPM) and spermidine (SPD), produce dose related biphasic modulation of NMDA channel currents while another polyamine, arcaine (ARC), produces only negative modulatory effects. The PCP-like effects of these compounds were tested in rats trained to discriminate PCP from saline in a standard two-lever drug discrimination paradigm under a fixed ratio schedule of food reinforcement. SPM, SPD and ARC occasioned little, if any, responding on the PCP-associated lever, even at response rate suppressing doses. The results provide further evidence that differences exist between the discriminative stimulus effects produced by drugs active at different sites on the NMDA receptor and suggest that the polyamine modulatory site should be a good target for development of NMDA antagonist medications with a reduced propensity for PCP-like acute behavioral effects.

A direct comparison of inhalant effects on locomotor activity and schedule-controlled behavior in mice

Increases in rates of rodent behavior have been commonly seen with exposure to abused vapors. In the 1st study, 30-min exposures to vapors of toluene, trichloroethane (TCE), or methoxyflurane produced increases in locomotor activity of mice at lower concentrations and decreases at higher concentrations. In the 2nd study, the effects of these vapors on schedule-controlled behavior were determined in mice lever pressing under a multiple fixed-ratio, 20-fixed interval (FI), 3-min schedule. Only concentration-related decreases in response rates were obtained in both components. In the 3rd study, toluene and TCE again produced only decreases in rates of responding under a simple FI 3-min schedule; biphasic effects were produced by methoxyflurane and amyl nitrite. The increases in rates of behavior often seen with abused vapors depend on the testing conditions.

Effects of the abused solvent toluene on recombinant N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors expressed in Xenopus oocytes

Previous studies have shown that toluene, which is commonly abused, depresses neuronal activity and causes behavioral effects in both animals and man similar to those observed for ethanol. In this study, the oocyte expression system was used to test the hypothesis that toluene, like ethanol, inhibits the function of ionotropic glutamate receptors. Oocytes were injected with mRNA for specific N-methyl-D-aspartate (NMDA) or non-NMDA subunits and currents were recorded using conventional two-electrode voltage clamp. To enhance the low water solubility of toluene, drug solutions were prepared by mixing toluene with alkamuls (ethoxylated castor oil) at a 1:1 ratio (v:v) and diluting this mixture to the appropriate concentration with barium-containing normal frog Ringer solution. Alkamuls, up to 0.1%, had no significant effects on membrane leak currents or on NMDA-induced currents. Toluene, up to approximately 9 mM, had only minor effects on membrane leak currents but dose-dependently inhibited NMDA-mediated currents in oocytes. The inhibition of NMDA receptor currents by toluene was rapid, reversible and the potency for toluene's effects was subunit dependent. The NR1/2B subunit combination was the most sensitive with an IC50 value for toluene-induced inhibition of 0.17 mM. The NR1/2A and NR1/2C receptors were 6- and 12-fold less sensitive with IC50 values of 1.4 and 2.1 mM, respectively. In contrast, toluene up to approximately 9 mM did not inhibit kainate-induced currents in oocytes expressing GluR1, GluR1(+)R2 or GluR6 subunits. These results suggest that some of the effects of toluene on neuronal activity and behavior may be mediated by inhibition of NMDA receptors.

Interactions between N-methyl-D-aspartate receptor antagonists and the discriminative stimulus effects of morphine in rats

NMDA receptor antagonists have previously been reported to alter some pharmacological and behavioral effects of acute and chronic opioid administration. The present study assessed the interactions of NMDA antagonists with the discriminative stimulus properties of morphine. Adult male Long Evans rats were trained to discriminate 3.2 mg/kg of s.c. morphine from water under a two-lever fixed-ratio 10 schedule of food reinforcement. During test sessions. I.p. injections of the noncompetitive NMDA receptor antagonist dizocilpine (0.03-0.2 mg/kg), the competitive antagonists NPC 17742 (1-16 mg/kg), and SDZ 220-581 (0.1-3 mg/kg), the polyamine site antagonist eliprodil (3-17.3 mg/kg), the glycine-site partial agonist (+)-HA-966 (3-56 mg/kg), and the nonselective glutamate antagonist kynurenic acid (30-150 mg/kg) were coadministered with s.c. morphine (1-3.2 mg/kg; interaction tests) or water (generalization tests). In generalization tests, none of the compounds completely substituted for morphine. Concurrent administration of morphine and NMDA antagonists did not greatly alter the discriminative stimulus properties of morphine. Various doses of NPC 17742, SDZ 220-581, or (+)-HA-966 somewhat increased levels of morphine-appropriate lever selection, whereas some attenuation of morphine-lever selection was obtained when morphine was coadministered with eliprodil. These results show that NMDA antagonists have minimal interactions with the discriminative stimulus effects of morphine.

Neural basis of inhalant abuse

It should be apparent from this review that far less is known about the neural basis for inhalant abuse than for other forms of drug abuse. This reflects a lack of research interest in this area (Balster, 1997). Indeed, conclusions are difficult to draw. In the case of the volatile alkyl nitrites, the most reasonable hypothesis at this time is that the cellular basis for their abuse resides in their actions on smooth muscles to produce vasodilation and relaxation, however, direct effects on the brain cannot be ruled out. Although there is some evidence that analgesic effects of nitrous oxide may involve opiate systems, even this conclusion is controversial. There is no evidence that opiate systems play a role in nitrous oxide intoxication or reinforcement. The mechanisms for these effects are unknown. They may reflect the same actions on lipid membranes or on hydrophobic sites on unspecified proteins that have been proposed as mechanisms for nitrous oxide anesthesia. In the case of the volatile solvents, fuels and anesthetics we are faced with a wide variety of specific chemicals which may produce different profiles of pharmacological effects. There is evidence that the prototypic abused solvents toluene and trichloroethane produce acute effects similar to subanesthetic concentrations of general anesthetics, as well as to the effects of classical CNS depressant drugs, such as alcohol and the barbiturates. For the anesthetics, evidence suggests that enhancement of GABAergic inhibition may be an important cellular target for their acute effects, just as it is for alcohol and other depressant drugs. For toluene, as with alcohol, recent evidence suggests a possible role for inhibition of glutamatergic neurotransmission involving NMDA receptors. Toluene has also been shown to have some dopaminergic effects which may be important to its abuse. As for the large number of other abused vapors, practically no information can be found on their cellular actions, and certainly not on actions that may be relevant to their abuse. This entire area would seem an important direction for future research.

Evaluation of the reinforcing and discriminative stimulus properties of the low-affinity N-methyl-D-aspartate channel blocker memantine

Memantine (MEM) is currently in clinical use in Europe for the treatment of various neurological disorders. It is a low-affinity channel blocker of N-methyl-D-aspartate (NMDA) receptors whose voltage-dependent, rapid binding kinetics are believed to limit its phencyclidine (PCP)-like side effects. MEM, and its analog amantadine (AMA), which has also been demonstrated to have some NMDA antagonist activity, were evaluated for PCP-like behavioral effects. The discriminative stimulus properties of MEM and AMA were tested in monkeys and rats trained to discriminate PCP from saline using a standard two-lever drug discrimination paradigm under a fixed ratio (FR) schedule of food reinforcement. In rats, MEM resulted in a dose-dependent substitution for PCP; however, full substitution occurred only at response rate suppressing doses. AMA failed to substitute for PCP at any dose tested. For MEM, all four monkeys showed complete substitution for PCP at doses which did not greatly decrease rates of responding. Conversely, AMA occasioned little or no responding on the PCP-associated level in any of the subjects. Intravenous self-administration of MEM and AMA was tested under a FR schedule of reinforcement in four monkeys trained to lever press for infusions of PCP. MEM served as a reinforcer in all subjects at one or more doses tested. For two of the subjects, at least one dose of AMA maintained rates of self-administration above those for saline. For both MEM and AMA, maximal response rates were considerably lower than with PCP self-administration and both drugs were much less potent in monkeys than would be predicted from rodent studies. The data show that MEM shares discriminative stimulus effects with PCP under these testing conditions, whereas the chemically similar compound AMA does not. MEM also serves as a positive reinforcer in rhesus monkeys, whereas AMA can serve as a weak reinforcer in only some subjects. Both AMA and MEM are reported to function as NMDA antagonists, yet clear differences exist in their behavioral effects with MEM acting more like a PCP-like antagonist. In addition, despite the rapid channel kinetics of MEM's NMDA receptor blockade, it may have some PCP-like abuse potential in humans at doses above the normal therapeutic levels.

Inhalant abuse in pregnancy

Information from a variety of sources suggests the possibility of adverse effects of maternal inhalant abuse, although a well-controlled, prospective study in this area has not been conducted. One source of this concern is the data from occupational exposure to some of the abused solvents, specifically toluene and TCE, with numerous reports suggesting increased spontaneous abortion and fetal malformations. There are also data suggesting decreased fertility and an increased risk for spontaneous abortion in health care workers exposed to nitrous oxide. The relevance of these studies to problems of inhalant abuse is not clear. Although the chemicals involved are the same, there are many differences in the exposure parameters, the populations exposed, and the types of associated risk factors. Nonetheless, there are more than 100 cases reported in the literature of children born to solvent-abusing mothers. Many of these children were small at birth, and some have craniofacial abnormalities not unlike that seen in children with FAS. In the few studies reporting the findings of follow-up in these children, some evidence has been obtained for retardation in growth and development and for residual deficits in cognitive, speech, and motor skills. Clearly, more research is needed to rule out the concomitant risk factors and to identify specific chemicals and patterns of use associated with adverse effects. Animal studies provide more direct evidence that prenatal exposure to toluene or TCE can produce reduced birth weights, occasional skeletal abnormalities, and delayed neurobehavioral development, even under conditions designed to mimic inhalant abuse patterns. Additional research is needed to identify other chemicals with adverse effects, critical periods of exposure, effects of combinations of inhalants, or interactions with drugs of abuse. The research literature seems sufficient to alert clinicians to possible problems in patients who abuse inhalants while pregnant. Diagnosis and good prenatal care for these women are important. The evidence for neonatal withdrawal is limited at this time; however, infants born to women who have recently used inhalants should be observed carefully for an alcohol-like withdrawal syndrome. Although it is not possible to link a specific birth defect or developmental problem in the child of an inhalant abuser to prenatal exposure to a specific chemical, it is clear that inhalant abuse and its associated lifestyle place children at increased risk. A wider appreciation of this is needed among health care professionals and the general public.

A method for adjusting exposure levels of volatile solvents based on effects on schedule-controlled behavior

A novel adjusting procedure was employed to assess the acute behavioral effects of inhaled 1,1,1-trichloroethane (TCE) and m-xylene. Mice were trained to lever press under a fixed ratio 20 schedule of milk reinforcement. During 30-min test sessions, TCE concentrations were altered every 5 min dependent upon rates of responding in the preceding 5-min segment of the session. When response rates during a 5-min interval were not decreased by greater than 30% from control rates, the TCE concentration for the next interval was increased. Reduction in response rates of more than 30% from the previous interval resulted in a lowering of the TCE concentration in the subsequent 5-min interval. TCE produced concentration-dependent decreases in response rates similar to what has been shown previously and many mice adjusted their exposure levels such that there were alternating intervals of increasing and decreasing concentration exposures. This provided a means of determining, in a single test session, no effect (subthreshold) and minimal effect concentrations of TCE for effects on schedule controlled behavior. Alteration of the starting TCE concentration from 1000 to 6000 ppm increased the TCE threshold for effects. When the response rate contingency was removed and TCE concentrations were "played back" from an earlier adjustment session, concentration-effect curves were similar to what were obtained under adjusting conditions. Using the adjusting procedure, we were also able to rapidly obtain a concentration-effect curve and threshold concentrations for m-xylene that are consistent with published results. This procedure for response rate adjusting exposure concentrations should be useful for rapid assessment of inhalant effects on schedule-controlled behavior and for focusing attention on near threshold levels of exposure.

Muscimol-like discriminative stimulus effects of GABA agonists in rats

The discriminative stimulus effects of GABAergic drugs were evaluated in rats trained to discriminate the direct GABA(A) agonist, muscimol (1.0 mg/kg I.P.), from saline under a two-lever fixed ratio (FR) 32 schedule of food reinforcement. Another direct GABA(A) agonist, THIP, produced full substitution for muscimol, however, at doses producing response rate decreasing effects. Diazepam, an allosteric modulator of GABA-mediated postsynaptic inhibition, yielded a maximum of 50% muscimol-lever responding at a dose that also decreased rates of responding. Partial substitution for muscimol (maximal levels of 71% muscimol-lever responding) was also produced by the GABA agonist progabide. Propofol, an anesthetic that potentiates GABA(A) receptor function, and the GABA uptake inhibitor, tiagabine, produced no greater than 53 and 48% muscimol-lever responding, respectively. Valproic acid, a reversible GABA transaminase inhibitor, failed to substitute for muscimol, and vigabatrin, an irreversible GABA transaminase inhibitor, yielded a maximal 46% muscimol-lever responding. These results demonstrate the pharmacological specificity of muscimol discrimination by showing that only direct agonists for the GABA site on the GABA(A) receptor complex produce full substitution. GABA agonists acting by other mechanisms can be distinguished from muscimol and THIP in this procedure.

Failure of ibogaine to produce phencyclidine-like discriminative stimulus effects in rats and monkeys

The discriminative stimulus properties of ibogaine were investigated in rats trained to discriminate phencyclidine (PCP; 2.0 mg/kg, I.P.) from saline under a two-lever fixed-ratio (FR) 32 schedule of food reinforcement. Ibogaine (5.6-17.6 mg/kg, I.P.) showed a complete lack of substitution. Ibogaine (0.5-4.0 mg/kg, I.M.) also failed to generalize in rhesus monkeys trained to discriminate PCP (0.1 mg/kg, I.M.) from sham injection. Lysergic acid diethylamide (LSD), tested as a reference compound, produced partial substitution for PCP in rats and occasioned little responding on the PCP-associated lever in monkeys. These results demonstrate important differences between the behavioral effects of PCP and other types of hallucinogenic drugs such as LSD and ibogaine and do not support the hypothesis that the affinity of ibogaine for the PCP site on N-methyl-D-aspartate (NMDA) receptors plays a major role in its acute behavioral effects.

Evaluation of cannabimimetic discriminative stimulus effects of anandamide and methylated fluoroanandamide in rhesus monkeys

In previous research arachidonylethanolamide (anandamide) has been shown to produce behavioral effects in mice characteristic of psychoactive cannabinoids, including antinociception, catalepsy, hypothermia, and hypomotility. However, differences have also been found between anandamide and delta9-tetrahydrocannabinol (delta9-THC), with anandamide having lower potency, a more rapid onset, and shorter duration of action than delta9-THC. Although it can produce delta9-THC like discriminative stimulus effects in rats, anandamide also produces concomitant response rate decreasing effects, whereas with delta9-THC there is a better separation of these two behavioral effects. The present study was designed to examine the discriminative stimulus effects of anandamide in rhesus monkeys trained to discriminate delta9-THC from vehicle. While anandamide failed to produce reliable substitution for delta9-THC and did not reduce response rates at doses up to 10 mg/kg, 2-methylarachidonyl-2'-fluoroethylamide (methylated fluoroanandamide), a putative stable analog of anandamide, produced full dose-dependent substitution for delta9-THC at doses that caused no significant changes in response rates. These results suggest that systemically administered anandamide may be metabolized in monkeys before behaviorally active concentrations could reach the brain and further suggest that the metabolically more stable analog of anandamide, methylated fluoroanandamide, may aid in the discovery of functional properties of the endogenous cannabinoid system.

Evaluation of the reinforcing and discriminative stimulus effects of the N-methyl-D-aspartate competitive antagonist NPC 17742 in rhesus monkeys

The phencyclidine (PCP)-like effects of the N-methyl-D-aspartate (NMDA) competitive antagonist 2R,4R,5S-2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid (NPC 17742) were evaluated in three behavioral tests in rhesus monkeys. The discriminative stimulus properties of NPC 17742 (2-24 mg/kg, i.m.) were tested in four rhesus monkeys trained to discriminate PCP from saline under a fixed-ratio (FR) 50 schedule of food reinforcement. In three of the monkeys, NPC 17742 showed complete substitution for PCP at doses which did not decrease rates of responding. Intravenous self-administration of NPC 17742 (50-800 micrograms/kg/infusion) was tested under a FR schedule of reinforcement in four monkeys trained to lever press for infusions of PCP. At least one dose of NPC 17742 functioned as a reinforcer in two of the monkeys. A second self-administration study, employing a 10 min fixed interval schedule of reinforcement, was performed in three monkeys trained to self-administer PCP during three daily sessions. Compared with PCP, NPC 17742 (0.4-1.6 mg/kg/infusion) maintained very low rates of responding; NPC 17742 could not be clearly established as a reinforcer in this procedure. The data show that NPC 17742 has some PCP-like behavioral effects, and may function as a weak reinforcer in some subjects under specific conditions. The results provide further evidence that both similarities and differences exist between the behavioral effects of PCP and competitive NMDA antagonists.

Discriminative stimulus effects of site-selective N-methyl-D-aspartate antagonists in NPC 17742-trained rats and squirrel monkeys

Drug discrimination studies in rats and monkeys with competitive N-methyl-D-aspartate. (NMDA) antagonists as training drugs have shown that these drugs typically cross-substitute for each other, whereas phencyclidine (PCP)-like NMDA channel blockers produce partial, if any, substitution. In the present study, rats and squirrel monkeys were trained to discriminate the competitive NMDA antagonist, NPC 17742, from vehicle in a two-lever drug discrimination procedure for food reinforcement. The competitive NMDA antagonists, NPC 12626, SDZ EAA 494 (D-CPPene), and MDL 100,453 fully substituted for NPC 17742 in monkeys or in rats. The relative potencies of these compounds were similar across species. Open channel blockers, PCP and dizocilpine, and the tricyclic antidepressant and low affinity PCP-site ligand, desipramine, produced minimal responding on the NPC 17742-associated lever in rats or monkeys. The glycine-site modulators, (+)-HA-966, ACEA 1021 and milacemide, and the polyamine/NR2B-selective antagonist, eliprodil, also failed to substitute fully for NPC 17742 in rats and monkeys. These data complement and extend results of previous studies which have shown a lack of PCP-like discriminative stimulus effects of these non-competitive NMDA antagonists by further showing that they also do not share discriminative stimulus effects with those produced by many competitive NMDA antagonists. These observations would support a prediction that differences in side-effect profiles should emerge among types of NMDA antagonists.

Evaluation of the acute behavioral effects and abuse potential of a C8-C9 isoparaffin solvent

We hypothesized that the abuse potential of certain types of inhalants could be evaluated in animals by determining the overlap in their profile of behavioral effects with that of CNS depressant drugs and other depressant-like abused inhalants. For our first attempt in evaluating a solvent with an unknown abuse potential we tested ISOPAR-E. ISOPAR-E is a mixture of predominantly C8-C9 isoparaffinic hydrocarbons that is being used more and more frequently as a solvent in industrial and consumer products, including, but not limited to, typewriter correction fluids. Presently, nothing is known about the potential for abuse of products containing this solvent. In the present studies, we compared the volatility of ISOPAR-E and the abused solvent 1,1,1-trichloroethane (TCE) in our exposure systems. Additionally, five behavioral procedures were conducted in mice to compare the effects of the two compounds. The results demonstrate that: (1) ISOPAR-E was less volatile than TCE; (2) ISOPAR-E produced a somewhat different profile of effects than did TCE as assessed with a functional observational battery; (3) unlike TCE, ISOPAR-E did not affect performance on tests of motor coordination; (4) TCE and ISOPAR-E produced concentration-related decreases in schedule-controlled operant performance with recovery from TCE being somewhat more rapid; (5) ISOPAR-E produced cross dependence in TCE-dependent mice; and (6) both TCE and ISOPAR-E produced substantial levels of ethanol-lever responding in a drug discrimination procedure, although the ethanol-like effects of ISOPAR-E only occurred at response rate decreasing concentrations. Overall, there was a poorer separation of behavioral and lethal concentrations for ISOPAR-E than for TCE. Although a somewhat different profile of behavioral effects was obtained with ISOPAR-E and TCE, we cannot say with certainty if enough similarities exist with abused inhalants to predict that ISOPAR-E would be subject to depressant-like abuse. Nonetheless, the feasibility of preclinical assessment of abuse potential of inhalants was demonstrated.

Neurobehavioral consequences of intermittent prenatal exposure to high concentrations of toluene

The effects of several concentrations of toluene on physical and behavioral development were examined in CD-1 mice prenatally exposed during the last week of gestation. Pregnant mice were exposed to either 200, 400, or 2000 ppm toluene (TOL) for 60 min three times a day during gestational days 12-17. A sham group was exposed concurrently to filtered air. No group differences were observed in maternal weight gain or food consumption, common measures of maternal toxicity. Initial litter characteristics including gestation length, number of litters delivered, and litter size were also similar. At birth, mean initial individual pup weight from representative male and female 2000 TOL-exposed pups was less than sham-exposed pups; however, entire litter weight did not differ. Pups were evaluated on postnatal days 1-20. Pups exposed to 2000 TOL gained less weight and performed more poorly on the behavioral tests of the righting reflex, grip strength, and inverted screen. In contrast, pups exposed to either 200 or 400 TOL did not differ from sham-exposed pups on any of the measures of development or behavior. These data provide evidence for the neurobehavioral teratogenicity of prenatal exposure to high levels of toluene late in gestation. Because this exposure regimen of intermittent high-concentration exposure was designed to simulate human exposures that might occur with toluene abuse, these results are consistent with case reports of adverse consequences of inhalant abuse by pregnant women.

Desflurane, enflurane, isoflurane and ether produce ethanol-like discriminative stimulus effects in mice

In the present studies, drug discrimination procedures were used to compare the discriminative stimulus effects of ethanol (ETOH) and several volatile anesthetics. Male albino mice were trained to discriminate between IP injections of ETOH (1.25 g/kg) and saline in a two-lever operant task in which responding was under the control of a fixed-ratio 20 (FR20) schedule of food presentation. Stimulus generalization was examined after 20-min inhalation exposures to desflurane (4,000-32,000 ppm), enflurane (3,000-12,000 ppm), isoflurane (1,000-8,000 ppm) and ether (4,000-32,000 ppm). Concentration-related increases in ETOH-lever responding were observed for all four volatile anesthetics. For enflurane and ether, maximal levels of > 85% ETOH-lever responding were obtained at one or more concentrations. For desflurane and isoflurane, the maximal mean percentages of ETOH-lever responding were somewhat lower, but 6 out of 7 mice showed full substitution with desflurane and 5 out of 7 for isoflurane. The shared discriminative properties of these compounds with ETOH suggest that these anesthetics may share some of ETOH's pharmacological properties. These results are similar to previous research results showing ETOH-like discriminative stimulus effects in mice with other anesthetics and abused volatile inhalants (i.e. halothane, toluene and 1.1,1-trichloroethane) and may reflect the CNS-depressant drug-like effects of inhaled anesthetics and abused solvents.

Effects of gamma-aminobutyric acid agonists and N-methyl-D-aspartate antagonists on a multiple schedule of ethanol and saccharin self-administration in rats

Recently, it has been shown at both the cellular and behavioral levels that ethanol has effects on the N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid (GABA)a receptor systems, leading to the possibility that the reinforcing effects of ethanol may be, at least partially, mediated via these receptor ionophores. In this study, a multiple schedule of ethanol and saccharin self-administration was used to study that possibility. Adult male Long-Evans rats were trained during 1-hr sessions to press on two different levers for 10% (w/v) ethanol and 0.1% (w/v) saccharin solutions, under an alternating 5-min, fixed-ratio-4 schedule of liquid availability. After training, tests were conducted with ethanol, NMDA antagonists and GABA agonists given before six consecutive sessions. Pretreatment with ethanol selectively decreased ethanol self-administration without altering saccharin self-administration. The competitive NMDA antagonist CPPene (D-3-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid [SDZ EAA 494]) and the noncompetitive NMDA antagonist phencyclidine decreased both ethanol and saccharin self-administration. The GABA agonists pentobarbital and diazepam also failed to reduce ethanol self-administration, relative to saccharin. Although these results do not support the hypothesis that antagonism of the NMDA receptor system or activation of the GABA receptor system can selectively modify ethanol-reinforced responding, they identify important issues for designing the best strategies to be used to assess selective drug effects on ethanol self-administration.

A comparison of the acute behavioral effects of inhaled amyl, ethyl, and butyl acetate in mice

The acute neurobehavioral effects of three acetates (amyl, ethyl, and n-butyl acetate) were investigated after 20-min inhalation exposures in mice using locomotor activity and a functional observational battery (FOB). Ethyl and n-butyl acetate produced significant decreases in locomotor activity at the highest concentrations examined, while amyl acetate was without effect. Minimally effective concentrations for activity-decreasing effects were 2000 ppm for ethyl acetate and 8000 ppm for n-butyl acetate. The potency order was similar in the FOB where ethyl acetate was more potent in disrupting the neurobehavioral measures. The FOB profile of effects for all three acetates included changes in posture, decreased arousal, increased tonic/clonic movements, disturbances in gait, delayed righting reflexes, and increased sensorimotor reactivity. Furthermore, handling-induced convulsions were produced in some mice acutely exposed to each of these acetates. Recovery from the acute effects of these acetates was rapid and began within minutes of removal from the exposure chamber. The acetates produced a profile of neurobehavioral effects that were different from those reported for depressant solvents (i.e., toluene, 1,1,1-trichloroethane) that are subject to abuse. Evidence is emerging for qualitative differences in the acute neurobehavioral effects of various volatile chemicals.

The discriminative stimulus effects of muscimol in rats

To evaluate the discriminative stimulus effects of a direct-acting GABAA agonist, seven rats were trained to discriminate 1 mg/kg IP muscimol from saline under a two-lever fixed ratio (FR) 20 schedule of food reinforcement. The direct GABAA agonist THIP (4,5,6,7-tetrahydro-isoxazolo [5, 4,c]-pyridin-3-ol) produced increases in muscimol lever responding and substituted for muscimol in all subjects. Unlike results with muscimol, the highest levels of muscimol lever responding following THIP administration were often produced at doses which also decreased rates of responding. The GABAB agonist baclofen and the indirect-acting GABAA agonists pentobarbital and midazolam produced substitution for muscimol in some subjects, but not in others. The non-competitive NMDA antagonist phencyclidine (PCP) produced mixed results in these rats, from partial to full substitution (both dose-dependently and exhibiting in lack of dose-dependence) in some animals and a complete failure to substitute in another. The selective GABAA antagonist bicuculline dose-dependently blocked the muscimol discriminative stimulus in a majority of subjects. This study is the first report of successful training of a drug discrimination in rats using muscimol. Evidence is provided from substitution and antagonism testing with THIP and bicuculline, respectively, that the muscimol discrimination was mediated by actions at the GABA binding site on the GABAA receptor-ionophore complex. Results, also suggest that drug stimulus control by muscimol is weak compared to that of other types of GABA agonists previously studied using drug discrimination procedures in rodents.

Developmental consequences of intermittent and continuous prenatal exposure to 1,1,1-trichloroethane in mice

The effects of 1,1,1-trichloroethane (TCE) on physical and behavioral development were examined in CD-1 mice prenatally exposed under two regimens. In the first study, pregnant mice were exposed to either 2,000 ppm TCE or filtered air for 17 hrs. during gestational days (GD) 12-17. A third group remained untreated. The results revealed no differences on pregnancy outcome. TCE-exposed pups gained less weight, exhibited delays in developmental landmarks and acquisition of the righting reflex, had poorer performance on tests of motor coordination and exhibited delays in negative geotaxis relative to sham or untreated pups. A second experiment was designed to more closely parallel the intermittent, acute, high-concentration pattern of solvent abuse. Pregnant mice were exposed for 60 min. to 8,000 ppm TCE or sham placement in exposure chambers three times/day during GD's 12-17. The results were very similar to what were obtained in the more continuous exposure study. TCE-exposed pups gained less weight, had delays in developmental landmarks and acquisition of the righting reflex and exhibited weaker grip strength, poorer negative geotaxis and less rooting intensity in comparison to sham pups. These data provide evidence for the behavioral and developmental teratogenicity of prenatal TCE exposure late in gestation.

Evaluation of the discriminative stimulus and reinforcing effects of gammahydroxybutyrate (GHB)

Gammahydroxybutyrate (GHB) satisfies many of the criteria for consideration as a neuro-transmitter including having specific receptor sites, endogenous synthesis, and heterogeneous CNS distribution. GHB has been reported to be illicitly used, to induce physical dependence, and to relieve effects from alcohol and heroin withdrawal. GHB has also been shown to have antidopaminergic activity to displace 3H[MK-801] binding in brain membranes, and to have some in vivo effects similar to the typical antipsychotics. To characterize the behavioral pharmacology of GHB further, we evaluated it for its reinforcing effects upon IV administration in rhesus monkeys with PCP self-administration histories, its ability to produce heroin- and PCP-like discriminative stimulus effects, and for its ability to antagonize cocaine discrimination in rats. The results indicated that GHB (300-7500 micrograms/kg per infusion) was not self-administered above vehicle control rates, although self-infusions occurred at levels sufficient to produce signs indicative of sedation. Also, neither heroin nor PCP discriminative stimulus effects generalized to injections of GHB up to 300 mg/kg IP, and GHB did not effectively antagonize the cocaine discriminative stimulus when tested up to 300 mg/kg IP. These data indicate that GHB is unlike PCP as a reinforcer and that neither PCP nor heroin generalize to injections of GHB, nor can injections of GHB attenuate the discriminative stimulus effects of cocaine.

The effects of abused inhalants on mouse behavior in an elevated plus-maze

Previous research has shown that abused inhalants (i.e., the volatile solvents) share some of the pharmacological properties of drugs used in the treatment of anxiety. In an attempt to further examine commonalities in the effects of inhalants and central nervous system depressant drugs, the behavioral effects of inhaled 1,1,1-trichloroethane, toluene, methoxyflurane and the convulsant vapor flurothyl were examined and compared to those of diazepam in the elevated plus-maze, a test used to predict antianxiety effects. After inhalant exposure or diazepam injection, mice were placed in the center of an elevated plus-maze and the number of entries and time spent in each type of arm (open versus closed) were measured during 5-min tests. Exposure to increasing concentrations of toluene produced concentration-related increases in the total number of open arm entries and the total time spent on the open arms, a pattern of behavioral effects similar to that produced by diazepam. A similar pattern was observed for increasing concentrations of 1,1,1-trichloroethane and methoxyflurane but changes in open arm activity were only observed at concentrations that increased locomotor activity. Conversely, only decreases in open arm time and number of entries were observed for flurothyl. The increasing evidence for commonalities in the behavioral effects of volatile solvents and depressant drugs may provide a foundation for understanding the neurobehavioral basis of inhalant abuse.