Laurel, MD), which could be operated by a downward force of 20 g or more. .... nicotine trial of the session averaged 97.7% (with a stan- dard error of _+ 3.1%) if ...
Psychopharmacology
Psychopharmacology (1989) 99: 208-212
9 Springer-Verlag 1989
Discriminative stimulus effects of intravenous/-nicotine and nicotine analogs or metabolites in squirrel monkeys K. Takada*, M.D.B. Swedberg**, S.R. Goldberg, and J.L. Katz Preclinical Pharmacology Branch, NIDA Addiction Research Center, P.O. Box 1580 Baltimore, MD 21224, USA
Abstract. Squirrel monkeys were trained to emit one response after IV administration of /-nicotine (0.4 or 0.2 gmol/kg) and a different response after IV administration of saline. After stable discriminative performances were established, subjects were tested with cumulative doses of /-nicotine (0.02-2.2 ~tmol/kg), d-nicotine (0.02-19.7 gmol/ kg), l-nornicotine (0.~12.0 gmol/kg), l-cotinine (56.8-567.5 gmol/kg), and dl-anabasine (0.6--19.7 gmol/kg). All of the drugs produced dose-related increases in the percentage of drug-appropriate responses emitted, from predominately saline-appropriate responses after low doses, to predominately drug-appropriate responses at the highest doses studied. Relative potency comparisons indicated that /-nicotine was 28 times more potent than d-nicotine, 29 times more potent than l-nornicotine, and approximately 2000 times more potent than l-cotinine. Each of the drugs also produced decreases in rates of responding, with potency order similar to that obtained for the discriminative effects. The effects of l-cotinine may be attributed to trace amounts of/-nicotine, which existed within the/-cofinine. The effects of dl-anabasine were lethal in one subject and were consequently not studied in the other subjects. Key words: Nicotine - d-Nicotine - l-Nornicotine - l-Cotinine - Drug discrimination
Nicotine, like other psychomotor stimulants such as cocaine and amphetamine, functions as a reinforcer (Spealman and Goldberg 1982), and can increase or decrease rates of schedule-controlled behavior, depending on dose and the control rate of responding (Spealman et al. 1981; Risner et al. 1985). Several, but not all, of these psychomotor stimulant effects are shared by the metabolites of nicotine. For example, the N-demethylated metabolite, nornicotine, only decreased response rates under both fixed-interval and fixedratio schedules in dogs and in squirrel monkeys. In contrast, /-cotinine the principal metabolite, had effects in squirrel monkeys similar to those of/-nicotine; rates of responding under the fixed-interval schedule were increased whereas rates under the fixed-ratio schedule were decreased. AddiPresent addresses: * K. Takada, CIEA Preclinical Research Laboratories, 1433 Nogawa, Miyamae, Kawasaki, Kanagawa, Japan 213 9* M.D.B. Swedberg, Behavioral Pharmacology Laboratory, Research Division, Ferrosan Group, Sydmarken 5, DK-2860 Soeborg, Denmark Offprint requests to: J.L. Katz
tionally, the lowest doses of l-cotinine that increased rates of responding were similar to the doses of/-nicotine that increased response rates. In beagle dogs,/-nicotine also increased response rates under the fixed-interval schedule, however, l-cotinine only decreased response rates (Risner et al. 1985). The discriminative-stimulus effects of nicotine have been studied extensively in rats and have been shown to be pharmacologically more specific than some of the effects outlined above (Rosecrans 1979). Despite the similarities of effects of nicotine and other psychomotor stimulants like cocaine and d-amphetamine, neither of these drugs has been reported to fully substitute for the discriminative effects of nicotine in rats (Rosecrans and Chance 1977) or primates (Takada et al. 1988). Drugs that completely substitute for nicotine are typically restricted to centrally-acting nicotiniccholinergic agents, e.g., the optical isomer d-nicotine (Meltzer et al. 1980; Romano et al. 1981) and the nicotine analogs 3-pyridylmethyl-pyrollidine (Chance et al. 1978), and anabasine (Romano et al. 1981). The effects of nicotine metabolites have been studied in rats trained under nicotine discrimination procedures. A preliminary report of complete substitution for nicotine by nornicotine (Rosecrans et al. 1978) was confirmed with the stereoisomers of nornicotine (Goldberg et al. 1989). Results with cotinine, however, have been inconsistent. For example Rosecrans and Chance (1977) found cotinine to be inactive when administered systemically, whereas one dose of cotinine completely substituted for nicotine when administered intraventricularly. Goldberg et al. (1989) reported full substitution of l-cotinine for/-nicotine. The present study examined the effects of metabolites of/-nicotine, and in addition the nicotine analogs, d-nicotine and dl-anabasine, in primates. Specific interest was directed at the effects of l-cotinine in order to determine if, as under the fixed-interval schedule (Risner et al. 1985), l-cotinine would have effects similar to those of/-nicotine, and whether the two drugs would be similar in potency. The results of the study by Risner et al. (1985) suggested that the effects of at least l-cotinine, and possibly other nicotinic drugs may differ substantially in squirrel monkeys.
Methods Subjects. Four adult male squirrel monkeys (Saimiri sciureus) were surgically prepared with intravenous catheters according to procedures described by Herd et al. (1969), and were maintained at 80% of their unrestricted-feeding
209 weights (863-878 g). Three of the subjects (S-965, S-955 and S-974) had been trained previously lander a similar procedure (Takada et al. 1988). The remaining subject (S-866) was experimentally naive at the start of these studies. Each subject was provided sufficient Teklad Monkey Diet (Teklad, Inc., Monmouth, IL) and Purin~ Monkey Chow (Ralston-Purina Co., St. Louis, MO) to maintain its weight after experimental sessions. Water was available at all times in home cages.
Apparatus. Experimental sessions were conducted with subjects seated in restraint chairs (Hake and Azrin 1963), and enclosed within ventilated, sound-zLttenuating chambers (Model AC-2, Industrial Acoustics Co., Bronx, NY). Mounted on the front panels of the chairs in front of the subjects were two response keys (Model 121-05, BRS/LVE, Laurel, MD), which could be operated by a downward force of 20 g or more. Each response produced an audible click of a relay mounted within the chamber. Three pairs of red, white or green (left to right) stimulus lights were also mounted on the front panel of the chair at about eye level. A pellet dispenser (Model D-I, Ralph Gerbrands, Co., Arlington, MA) could deliver 190 mg food pellets (banana flavored, Bioserv Inc., Frenchtown, N J) to a food receptacle located between the two levers. White noise was used to mask extraneous sounds during experimental sessions. Injection pumps (Model 355, Sage Instruments, Cambridge, MA) located on top of the chamber were used to deliver intravenous injections, via the catheter, without disturbing the subject. The control of experimental contingencies and data collection were accomplished by a PDP-SE computer located in an adjoining room. Key-press responses were also monitored by cumulative recorders (Model C-3, Ralph Gerbrands Co., Arlington, MA). Training procedures. Discriminative control of responding was established with the training dose of 0.4 (S-866, S-955 and S-974) or 0.2 (S-965) gmol/kg l-tricotine. The training doses were chosen to be doses producing consistent drugappropriate responding. Baseline training sessions generally consisted of two trials, each preceded by an IV infusion and separated by a timeout. Either saline or/-nicotine solutions (approximately 0.5 ml) were infused manually into the tubing connecting the pump and the catheter. Thirty seconds before the start of each trial, the center white lamps were illuminated, the pump was activated, and 2 ml saline (0.9% NaC1) was infused through the connecting tubing at a rate of 0.36 ml/s, flushing the contents of the catheter into the subject. At the end of the 30 s, during which responses of the subject had no scheduled consequences, the white lights were turned off, and the trial was started with both red and green lights illuminated. In order to hold the interval between infusions constant at 30 rain, the duration of the timeout between trials was varied according to the time required by the subject to complete the preceding trial. During trials, 20 consecutive responses on the appropriate lever produced food (FR 20 schedule). Responses on the inappropriate lever reset the FR requirement on the appropriate lever. Each food presentation was followed by a 5-s timeout. Each trial ended after l0 min had elapsed or 20 pellets had been obtained. The appropriate lever for /-nicotine or saline was arbitrarily selected for each subject;
the/-nicotine lever was on the left for three subjects (S-866, S-955 and S-965) and was on the right for the remaining subject (S-974). The sequence of trials in a training session was generally either saline-saline, saline-nicotine, or nicotine-nicotine; a saline trial never followed a nicotine trial. Additional trials were occasionally conducted when the performance of the subject indicated that further training would be beneficial. Sessions were conducted 5 days per week. Training continued until each subject met the following discrimination criteria in three consecutive saline and nicotine trials: (1) less than 40 responses were emitted before the first pellet delivery, and (2) at least 80% of the total responses were emitted on the appropriate lever.
Drug testing procedures. After the subjects met the above criteria, the dose-effects of/-nicotine were determined. Subsequently, effects of d-nicotine,/-nornicotine, and l-cotinine were tested in sequence. Test sessions started with a saline trial, after which effects of successive doses were determined. Doses were administered in a cumulative manner before sequential trials (Bertalmio et al. 1982). Trials lasted until ten pellets were presented or 5 rain had elapsed. Each trial started ] 5 min after the previous injection. Trials were conducted until response rates were markedly decreased or until more than 80% of responses were emitted on the nicotine-appropriate lever. During test trials, 20 consecutive responses on either lever produced a food pellet; switching to the alternate lever reset the FR requirement. Test sessions were generally separated by two training sessions, including at least one nicotine trial, and were conducted only after the discrimination criteria were met. Generally, each drug was studied once in each subject; however cotinine was studied on several occasions in some subjects in order to find the appropriate dose range. Drugs. Drugs and dose ranges tested (expressed in terms of gmol per kg of the body weight of the subject) were as follows: /-nicotine tartrate (0.02-2.2); d-nicotine d-tartrate (0.02-19.7); /-nornieotine camsylate (0.2-12.0); and /-cotinine base (56.8-567.5) The effects of dl-anabasine (0.6-19.7) were studied with only one subject (S-965) immediately after determination of the /-nicotine dose-effect curve. After the highest dose tested this subject was found dead; consequently, this drug was not studied in additional subjects. Dose-effect curves for the other drugs were determined at least once in each subject. The/-nicotine tartrate was purchased from Pfaltz and Bauer Inc. (Stamford, CT); d-nicotine and/-nornicotine were kindly supplied by Payton Jacob (synthesis to be found in Jacob I982); l-cotinine and dl-anabasine were purchased from Sigma Chemical Company (St. Louis, MO).
Analysis of results. The effects of drugs were evaluated in terms of the per cent of nicotine-appropriate responses emitted (responses on the nicotine-appropriate lever divided by the total number of responses emitted x 100) and in terms of the rate of responding (number of responses divided by elapsed time in the trial excluding the brief timeout periods that followed each presentation of food). Response rates in each trial were expressed as a percentage of the rate during the first saline trial of the test session. For comparisons of dose-effect curves, analysis of variance and linear regression techniques (Snedecor and Cochran 1967)
210 were used to determine EDso values (the dose producing 50% responding on the nicotine key, or a decrease in response rates to 50% of the control rate) and 95% confidence limits. Data from the linear portions of the doseeffect curves were analyzed by standard parallel line bioassay techniques (Finney 1964) to obtain potency estimates. The results of studies in subject S-965 were not included in any of the data presented in graphs or tables. Results
At criterion, responding occurred primarily on the appropriate keys following/-nicotine or saline injections. Further, the per cent drug-appropriate responding did not depend on the number of saline trials preceding a/-nicotine trial. For example, drug-appropriate responding on the first /nicotine trial of the session averaged 97.7% (with a standard error of _+3.1%) if l-nicotine was administered before the first trial of the session, and averaged 99.7% (+_0.3% if/-nicotine was administered before the second trial of the experimental session. Further, the average per cent drug-appropriate responding after the first and second saline trials of the session was 1.3 (___1.7) and 4.8 (+_5.9), respectively. Response rates during /-nicotine trials averaged 46% of those during saline trials in the three subjects studied at 0.4 gmol/kg. /-Nicotine produced dose-related increases in drug-appropriate responding from the lowest dose (0.02 gmol/kg), producing primariliy saline-appropriate responding, to a dose of 0.2 gmol/kg, producing almost exclusive responding on the nicotine-appropriate key (Fig. 1). The EDso value for/-nicotine was 0.09 gmol/kg (Table 1). Each of the other drugs studied also produced doserelated increases in the percentage of/-nicotine-appropriate responses, d-Nicotine and /-nornicotine were equipotent, producing /-nicotine-appropriate responding with ED50 values of 2.69 and 2.83 gmol/kg, respectively (Table 1), and each less potent than/-nicotine (Fig. 1). l-Cotinine was the least potent of the drugs studied (Fig. 1). The large SE of/-cotinine at 170.3 tamol/kg was due to differences between subjects: one responded ahnost exclusively on the /-nicotine-appropriate key, another divided responses about equally between the two keys, and the remaining subject responded exclusively on the salineappropriate key. The EDso value for l-cotinine was 177.44, yielding a relative potency of 0.0005 (Table 1). Information supplied with the compound from the manufactures indicated that the l-cotinine sample contained 0.1% nicotine. Thus, the relative potency of l-cotinine approximated that which would be predicted from the impurity in the sample of drug studied. dl-Anabasine produced dose-related increases in/-nicotine-appropriate responding, with a dose of 6.2 ~tmol/kg producing approximately 50% drug-appropriate responding. That dose also decreased response rates to about 50% of control level (data not shown). At the highest dose (19.7 gmol/kg) there was very little responding and the subject was found dead following the triM. Cause of death was not determined. Each of the drugs produced dose-related decreases in rates of responding (Fig. 2), with dose-effect curves not different from parallel. As with the discriminative effects, lnicotine was most potent followed by d-nicotine and/-nornicotine which were approximately equipotent. Finally, l-
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Fig. 1. Dose-effect relationships of intravenous /-nicotine, d-nicotine, /-nornicotine, and 1-cotinine in squirrel monkeys trained to discriminate IV nicotine at 0.4 gmoI/kg from IV saline, Abscissae: dose in gmol/kg, log scale. Ordinates: per cent of the total number of responses on the nicotine-appropriate lever. Each symbol represents the mean value from three individual subjects with vertical bars representing standard errors of the means. 9 /-Nicotine; [] d-nicotine; ~ l-nornicotine; o l-cotinine Table 1. EDso values and relative potency estimates for effects of /-nicotine and related drugs, EDso values are expressed as/amol/kg and are defined under Methods. Values in parentheses are 95% confidence limits. Standard bioassay procedures, using /-nicotine as the standard drug for all comparisons, were used to calculate the relative potency estimates. The values for relative potency represent the dose of the standard drug, in gmol/kg, of equal effectiveness with 1.0 gmol/kg of the test drug Drug
ED5o value
Relative potency
Discriminative effects
/-nicotine d-nicotine Lnornicotine /-cotinine
0.09 (0.08-0.11) 2.69 (1.27-5.70 2.83 (1.89-4.23) 177.44 (96.44-326.48)
1.0 0.036 (0.018-0.075 0.034 (0.024-0.046 0.0005 (0.0003-0.0009)
Response rates
/-nicotine 0.17 (0.13-0.23) d-nicotine 2.49 (1.06--5.88) /-nornicotine 0.99 (0.55-1.75) /-cotinine 1095.6 (231.6-6809.6)
1.0 0.063 (0.027-0.130) 0.169 (0.094-0.313) ND*
* Not determined due to a significant preparations effect cotinine was the least potent of the drugs studied. The EDso values and relative potency calculations for the effects on response rates are also shown in Table 1. These relative potencies were in close agreement with those determined for the discriminative effects. Since doses of l-cotinine that produced /-nicotine-appropriate responding did not have substantial effects on response rates, the EDso value is an estimate and confidence limits cannot be accurately determined. Discussion
The present discriminative effects of IV/-nicotine were in most respects similar to those obtained in a previous study in which subjects were trained with a lower dose (0.2 gmol/ kg) of/-nicotine (Takada et al. 1988). Specifically, respond-
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100
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Fig. 2. Dose-effect relationships of intravenous /-nicotine, d-nicotine, l-nornicotine, and l-cotinine in squirrel monkeys trained to discriminate IV nicotine at 0.4 gmol/kg from IV saline. Abscissae: dose in gmol/kg, log scale. Ordinates: response rate as a per cent of that during the first saline trial in a session. Each symbol represents the mean values from three individual subjects. For other details, see legend for Fig. 1. []/-Nicotine; ~ d-nicotine; zxl-nornicotine; o l-cotinine
ing occurred almost exclusively on the saline-appropriate key at 0.02 pmol/kg and on the/-nicotine-appropriate key at doses of 0.2 btmol/kg or greater. The effects on response rates, however, differed according to the training dose used: rates after training doses of 0.4 gmol/kg /-nicotine in the present study were often lower than those after saline, whereas rates after training doses of 0.2 pmol/kg in the previous study were simular to those after saline. In the present study,/-nicotine was 28 or 16 times more potent than d-nicotine in producing discriminative effects or decreases in response rates, respectively. This potency ratio is slightly greater than that previously published for discriminative effects (e.g., Meltzer etal. 1980; Romano et al. 1981, Goldberg et al. 1989) and effects on response rates (e.g., Risner et al. 1988). The present relative potency is in agreement with studies of other effects of nicotine. For example, /-nicotine was approximately 20 times more potent than d-nicotine in producing convulsions and lethality in mice; and approximately 8-10 times more potent than d-nicotine in increasing heart rate and blood pressure in anesthetized rats, and inducing contractions of the isolated rat ileum. In contrast, the two isomers were relatively impotent, but of equal potency in blocking the contraction of isolated rat phrenic nerve-diaphragm preperation (Shimada et al. 1981). Thus, as noted by Shimada et al., generally the effects of the isomers of nicotine are qualitatively similar, with the d-isomer from 1/8th to 1/20th as potent as the/-isomer. The present results extended those findings to primates, at least with regard to the behavioral effects of these drugs. Studies with /-nornicotine, which is also present in tobacco plants (Jacob 1982), have shown that it is an active metabolite of nicotine and partially (Rosecrans et al. 1978) or fully (Goldberg et al. 1989) substitutes for /-nicotine, with about 1/14th to 1/20th the potency. Similarly, in the present study full substitution of the/-isomer of nornicotine for/-nicotine was observed with a potency of 1/29th that of/-nicotine. The EDso value for the effects on response rates was lower than that for the discriminative effects, with
a relative potency of 1/17th that of/-nicotine. Thus,/-nornicotine was somewhat more potent in decreasing response rates than in producing/-nicotine-like discriminative effects in squirrel monkeys. Previous results with/-cotinine, the principal metabolite of nicotine, have been somewhat inconsistent. This metabolite was reported to substitute for/-nicotine only when given centrally, and only at one training dose of/-nicotine (Rosecrans and Chance 1977). Goldberg et aI. (1989) studied higher doses and found full substitution of/-cotinine for/-nicotine. That the potency ratio of/-nicotine and/-cotinine was in accord with the degree of/-nicotine impurity in the lcotinine sample in the present study and the study by Goldberg et al. (1989) suggested that the discriminative effect obtained was probably due exclusively to the extant nicotine contamination. Consequently, it remains to be determined if pure/-cotinine would be active in producing nicotine-like discriminative effects. A previous study (Risner et al. 1985) has shown that /-cotinine, like /-nicotine, increased response rates maintained under fixed-interval schedules in squirrel monkeys but not beagle dogs. In that study/-cotinine and/-nicotine were approximately equally potent and efficacious. Thus, it is unlikely that the increases produced by/-cotinine were due to a small degree of nicotine impurity in the/-cotinine sample. It is equally unlikely that the increase in response rates produced by/-cotinine in squirrel monkeys was due to an action at nicotinic receptors, since the/-cotinine and /-nicotine did not produce nicotine-like discriminative effects at comparable doses. Studies of the antagonism by mecamylamine of the effects of cotinine on response rates are necessary to confirm this conclusion. All of the nicotine metabolites and analogs examined in the present study produced discriminative effects similar to those of/-nicotine, and these results are for the most part comparable to results of previous studies with rodents. Studies with /-nornicotine have shown that it substitutes for/-nicotine partially (Rosecrans et al. 1978) or, as in the present study, fully (Goldberg et al. 1989). Others have shown that d-nicotine (Meltzer et al. 1980; Romano et al. 1981; Goldberg et al. 1989),/-anabasine (Pratt et al. 1983; Stolerman etal. 1984) and d/-anabasine (Romano et al. 1981) have discriminative effects similar to those of nicotine. The d/-anabasine in the present study proved fatal in contrast to the earlier finding with rodents (Romano et al. 1981). Since the sublethal dose produced drug-appropriate responding, there may be different potency relations for the lethal and discriminative effects of this compound. Despite indications that /-cotinine may have a different spectrum of effects in the squirrel monkey, the discriminative effects of/-cotinine were similar to those recently reported (Goldberg et al. 1989). The relative low potency of this drug is consistent with previous reports with rodents (Rosecrans and Change 1977). Taken as a whole, the results of this study suggest that the discriminative effects of IV nicotine analogs and metabolites in the squirrel monkey are similar to those obtained by other routes of administration in other species. References
Bertalmio A, Herling S, Hampton RY, Winger G, Woods JH (1982) A procedure for rapid evaluation of the discriminative stimulus effects of drugs. J Pharmacol Methods 7:289-299 Chance WT, Kallman MD, Rosecrans JA, Spencer RM 0978)
212 A comparison of nicotine and structurally related compounds as discriminative stimuli. Br J Pharmacol 63:609-616 Finney DJ (1964) Statistical methods in biological assay, 2rid edn. Hafner, New York Goldberg SR, Risner ME, Stolerman IP, Reavill C, Garcha HS (1989) Nicotine and some related compounds: effects on schedule-controlled behaviour and discriminative properties in rats. Psychopharmacology 97 : 295-302 Hake DF, Azrin NH (1963) An apparatus for delivering pain shock to monkeys. J Exp Anal Behav 6:297-298 Herd JA, Morse WH, Kelleher RT, Jones LG (1969) Arterial hypertension in the squirrel monkey during behavioral experiments. Am J Physiol 217:24--29 Jacob P, III (1982) Resolution of (+)-5-bromonornicotine. Synthesis of (R)- and (S)-nornicotine of high enantomeric purity. J Org Chem 47:4165-4167 Meltzer LT, Rosecrans JA, Aceto MD, Harris LS (1980) Discriminative stimulus properties of the optical isomers of nicotine. Psychopharmacology 68: 283-286 Pratt JA, Stolerman IP, Garcha HS, Giardini V, Feyerabend C (1983) Discriminative stimulus properties of nicotine: further evidence for mediation at a cholinergic receptor. Psychopharmacology 81:54-60 Risner ME, Goldberg SR, Prada JA, Cone EJ (1985) Effects of nicotine, cocaine and some of their metabolites on schedulecontrolled responding by beagle dogs and squirrel monkeys. J Pharmacol Exp Ther 234:113-119 Risner ME, Cone EJ, Benowitz NL, Jacob P (1988) Effects of the stereoisomers of nicotine and nornicotine on schedule-controlled responding and physiological parameters of dogs. J Pharmacol Exp Ther 244: 807-813 Romano C, Goldstein A, Jewel NP (1981) Characterization of the
receptor mediating the nicotine discriminative stimulus. Psychopharmacology 74:310-315 Rosecrans JA (1979) Nicotine as a discriminative stimulus to behavior : its characterization and relevance to smoking behavior. In: Kransnegor NA (ed) NIDA Res Monogr Ser 23, pp 58-69 Rosecrans JA, Chance WT (1977) Cholinergic and non-cholinergic aspects of the discriminative stimulus properties of nicotine. In: Lal H (ed) Discriminative stimulus properties of drugs. Plenum Press, New York, pp 155-186 Rosecrans JA, Kallman MJ, Glennon R (1978) The nicotine cue: an overview. In: Colpaert FC, Rosecrans JA (eds) Stimulus properties of drugs: ten years of progress. Elsevier/North-Holland Biomedical Press, Amsterdam, pp 6%81 Shimada A, Iizuka H, Kawaguchi T, Yanagita T (1981) Pharmacodynamic effects of d-nicotine - comparison with/-nicotine. Folia Pharmacol Japon 84:1-10 (article in Japanese) Snedecor GW, Cochran WG (1967) Statistical methods, 6th edn. Iowa State University Press, Ames, Iowa, pp 135-171 Spealman RD, Goldberg SR (1982) Maintenance of schedule-controlled behavior by intravenous injections of nicotine in squirrel monkeys. J Pharmacol Exp Ther 223 : 402-408 Spealman RD, Goldberg SR, Gardner ML (1981) Behavioral effects of nicotine: schedule-controlled responding by squirrel monkeys. J Pharmacol Exp Ther 216:484491 Stolerman IP, Garcha HS, Pratt JA, Kumar R (1984) Role of training dose in discrimination of nicotine and related compounds by rats. Psychopharmacology 84: 413-419 Takada K, Hagen TJ, Cook JM, Goldberg SR, Katz JL (]988) Discriminative stimulus effects of intravenous nicotine in squirrel monkeys. Pharmacol Bioehem Behav 30:243-247 Received August 22, 1988 / Final version March 14, 1989
