Introduction

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Sigma binding sites have been differentiated from PCP receptors in radioligand binding (Su, 1982; Tam, 1983; Tam and Cook, 1984) and autoradiographic (Gundlach et al., 1985, 1986; Largent et al., 1984) studies. Because many antipsychotics, including haloperidol, exhibit high affinity for sigma sites, this site is suspected to have relevance in the etiology of schizophrenia (Itzhak, 1988; Tam and Cook, 1984). Moreover, some psychotomimetic compounds, such as (+)-SKF 10,047, bind to sigma sites in addition to PCP receptors (Largent et al., 1988; Su, 1982; Tam et al., 1988). The potential clinical relevance of the sigma site has heightened interest in the development of functional assays that can be used to evaluate the role of this site in behavioral processes.

Drug discrimination studies can provide relatively sensitive, pharmacologically selective characterizations of the stimulus effects produced by psychoactive drugs (e.g., Appel et al., 1978). To date, several compounds that have relatively high affinity for sigma binding sites, such as DTG, (+)-SKF 10,047 and (+)-pentazocine, have been trained as discriminative stimuli. These drug discrimination studies have not identified a discriminative stimulus effect distinct from PCP involvement that can be unequivocally attributed to the sigma site. For example, PCP substitutes for the training drug in animals trained to discriminate (+)-SKF 10,047 (Balster, 1989; Holtzman, 1993; Singh et al., 1990; Steinfels et al., 1987; Tam et al., 1988) or DTG (Holtzman, 1989), and it partially generalizes to (+)-pentazocine (Steinfels et al., 1988). Conversely, (+)-SKF 10,047 and DTG substitute in PCP-trained animals (Holtzman, 1980, 1989). Such studies also demonstrate that cross-generalization between sigma ligands is inconsistent. Although (+)-SKF 10,047 substitutes in rats trained to discriminate (+)-pentazocine (Steinfels et al., 1988), haloperidol and other sigma ligands, including BMY 14802, do not substitute in (+)-SKF 10,047-discriminating rats or squirrel monkeys (Balster, 1989; Holtzman, 1993; Singh et al., 1990; Tam et al., 1988).

BMY 14802 is a potent displacer of sigma binding (Taylor and Dekleva, 1987a,b). Because BMY 14802 blocks conditioned avoidance responding and inhibits apomorphine-induced stereotypy in rats, it is suggested to have potential as an antipsychotic (Taylor et al., 1985). However, unlike classic antipsychotics, BMY 14802 does not induce catalepsy in rats (Matthews et al., 1986; Taylor et al., 1985), shows relatively low affinity for D₂ dopamine receptors (Matthews et al., 1986; Taylor et al., 1985) and appears to influence dopaminergic neurotransmission through a nondopaminergic mechanism (Matthews et al., 1986; Wachtel and White, 1987). BMY 14802 also demonstrates low affinity for a variety of other neurotransmitter receptors, including, D₁ dopamine; alpha-1-, alpha-2- and beta-adrenergic; benzodiazepine; glycine (strychnine-sensitive and insensitive subtypes); glutamate (NMDA, kainate (and quisqualate subtypes); mu and kappa opioid; imipramine; nitrendipine; and 5-HT₂ receptors (Taylor et al., 1985).

Preliminary experiments in this laboratory with pigeons trained to discriminate PCP from saline suggested that BMY 14802 does not share stimulus effects with PCP.⁵ The failure of BMY 14802 to substitute for PCP and its apparent selectivity for sigma sites identified it as a strong candidate for demonstrating a selective sigma-mediated discriminative stimulus effect. Thus, this study pharmacologically characterized the discriminative stimulus effects of BMY 14802. For this purpose, pigeons were trained to discriminate 5.6 mg/kg BMY 14802 from saline for food reinforcement using a two-key operant procedure. Experiments were performed to identify compounds that either evoked or attenuated the discriminative stimulus effects of BMY 14802 to determine the pharmacological mechanism responsible for mediating these effects.

	Methods

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Animals. Six experimentally naive White Carneaux pigeons (Palmetto, Sumter, SC) were used in the study. They were maintained at ~80% (380-450 g) of their free feeding weights by restricted provisions of Purina Pigeon Checkers (Purina Mills, St. Louis, MO). Each subject was individually housed with water and grit freely available in the home cage.

Apparatus. Experimental sessions were conducted in operant chambers (36 cm high × 28 cm wide × 33 cm long) in sound-attenuating isolation boxes. Three translucent response keys (2-cm diameter), which could be transilluminated by 7-W red lightbulbs, were located inside the front panel of each chamber ~25 cm above a grid floor. A square orifice (6 × 6 cm) was located directly below the center response key, ~10 cm above the floor. Access to mixed grain could be made available through this opening from a hopper that could be raised via solenoid activation. Each food presentation was signaled with illumination of the hopper area by a 7-W white lightbulb. During food presentation, house and key lights were extinguished, and responses had no programmed consequences. An exhaust fan ventilated each chamber, and white noise was provided to mask extraneous sounds. Control of experimental contingencies and data collection were accomplished with a Texas Instruments (Dallas, TX) 960A computer.

Discrimination training. Experimental sessions were conducted once a day on Monday through Friday. Initially, pigeons were placed into chambers for a 30-min session and autoshaped to make a key-peck response to an illuminated response key for 4-sec access to food. During subsequent sessions, the FR requirement on this key was gradually increased until an FR 20 was in effect. During the next two sessions, a second response key was illuminated, and 20 consecutive responses to either of the two keys was reinforced.

Discrimination testing. To provide a standard of comparison for a cumulative-dosing, multiple-trial testing method, the stimulus effects of single doses of BMY 14802 were evaluated using single-trial test sessions. During single-trial test sessions, 20 consecutive responses to either key resulted in food reinforcement. Each first response to a key reset the ratio requirement on that key. All other parameters of single-trial test sessions were identical to those of training sessions.

Antagonism testing. During antagonism experiments, potential antagonists were administered before the first cumulative dose of another compound. The doses chosen did not engender BMY 14802-appropriate responding or markedly suppress responding during substitution testing (DTG, metergoline, MR2035, NAN 190, ondansetron and tiospirone). Alternatively, doses of pirenpirone were chosen on the basis that they had been shown to be effective as antagonists in other pigeon discrimination studies conducted within this laboratory (Yamamoto et al., 1991).

Data analysis. The discrimination data are expressed as the percentage of responses made on the BMY 14802-appropriate key. Responses across all keys were used to calculate rates of responding and are expressed as responses per second. Group averages are presented unless otherwise indicated. Dose-effect functions were constructed by pooling measures of discriminative performance or response rate for all subjects and plotting the resulting mean ± 1 S.E.M. as a function of dose. Doses required to evoke 50% drug-appropriate responding (ED₅₀) and 95% CL were determined by regression analysis and analysis of variance (Snedecor and Cochran, 1967). Compounds were tested in each subject once unless otherwise stated. Drugs were considered to share stimulus effects with BMY 14802 if the majority of subjects emitted 90% BMY 14802-appropriate responding during any trial of a test session. Discrimination data for an individual subject were not included in the group average for a trial if response rates were suppressed <0.07 responses/sec during that trial. Compounds were tested in the same order in all pigeons.

Drugs. All drugs were administered intramuscularly, with the injections alternating between the left and right pectoral muscles. Unless otherwise indicated, all drugs were dissolved in sterile water with the aid of a few drops of lactic acid if required. The following drugs were used: BMY 14802, buspirone and tiospirone (Bristol-Myers Squibb, Wallingford, CT); DTG (Eckard Weber, University of Oregon Medical School, Portland, OR); NAN 190 (Richard A. Glennon, Department of Medicinal Chemistry, Medical College of Virginia, Richmond, VA); haloperidol, pirenperone and spiroxatrine (Janssen Pharmaceutica, Beerse, Belgium); eticlopride, 8-OH-DPAT, m-CPP and (+)-3-PPP (Research Biochemical, Natick, MA); rimcazole (Burroughs Wellcome Co., Research Triangle Park, NC); (+)-SKF 10,047 and PCP (National Institute on Drug Abuse, Rockville, MD); MR 2035 (H. Mertz, Boehringer Ingelheim, Ingelheim am Rhein, Germany); quipazine maleate (Miles Scientific, Naperville, IL); DMI and ondansetron (M. B. Tyers, Glaxo, Ware, England); RU24969 (Roessel-UCLAF, Romanville, France); l-5-HTP (United States Biochemical, Cleveland, OH); metergoline (Farmatalia, Milan, Italy); clozapine (Sandoz Pharmaceuticals, East Hanover, NJ); and cinuperone (Hoechst Aktiengesellschaft, Frankfurt am Main, Germany). Cinuperone was dissolved in a vehicle consisting of 40% propylene glycol, 10% ethanol (95%) and 50% sterile water by volume. Doses of drugs are expressed as the forms listed above.

	Results

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The pigeons met testing criteria in an average of 31 ± 9 sessions. Figure 1 shows the dose-effect profiles obtained for BMY 14802 using single-dosing, single-trial and cumulative-dosing, multiple-trial test sessions. The results of both methods show that subjects directed all responses to the saline-appropriate key at the lowest doses tested, whereas the majority of responses were emitted on the BMY 14802-appropriate key at doses of 3.2 mg/kg. The ED₅₀ values were 1.5 mg/kg (95% CL, 1.0-2.1 mg/kg) during single-trial and 2.5 mg/kg (95% CL, 2.0-3.2 mg/kg) during multiple-trial test sessions. Response rates were completely or greatly suppressed in all subjects by a dose of 32 mg/kg during single- or multiple-trial test sessions. Redeterminations of the BMY 14802 cumulative dose-effect gradient indicated that it remained stable throughout the course of the study (data not shown).

View larger version (16K): [in this window] [in a new window]   Fig. 1.   Dose-response functions for stimulus (top) and rate-altering (bottom) effects of BMY 14802 obtained during single- () and multiple- () trial tests. Each point represents the mean value obtained for six subjects unless the number of subjects (n) represented at a particular point is indicated to be fewer than six. Error bars represent ± 1 S.E.M. Abscissae, dose of BMY 14802 in mg/kg. Ordinates, percentage of responses directed to the BMY 14802-appropriate key (top) or response rate in responses per second (bottom). a, n = 5, multiple trial. b, n = 1.

The ()-isomer (ED₅₀, 0.44 mg/kg; 95% CL, 0.28-0.70 mg/kg) of BMY 14802 was slightly more potent than the (+)-isomer (ED₅₀, 2.1 mg/kg; 95% CL, 1.4-3.4 mg/kg) in occasioning BMY 14802-appropriate responding (fig. 2, top). A dose of 3.2 mg/kg ()-BMY 14802 or 10 mg/kg (+)-BMY 14802 was required to evoke 90% generalization in all six subjects. Thus, the ()-isomer of BMY 14802 was more potent in producing BMY 14802-like stimulus effects than the (+)-isomer, which was essentially equipotent to the racemate training stimulus. Rates of responding were similarly affected by the stereoisomers (fig. 2, bottom).

View larger version (17K): [in this window] [in a new window]   Fig. 2.   Stimulus (top right) and rate-altering (bottom right) effects of the ()-isomer () and (+)-isomer () of BMY 14802 in multiple-trial tests. Data are presented as in figure 1.

Substitution tests with compounds having affinity for sigma binding sites. Several compounds that show affinity for the sigma site in binding assays failed to substitute for the training stimulus (table 1). Cinuperone (1.8-100 mg/kg), DTG (1.0-18 mg/kg), haloperidol (0.032-3.2 mg/kg), MR 2035 (5.6-56 mg/kg), rimcazole (1.8-100 mg/kg), (+)-SKF 10,047 (0.1-32 mg/kg) and tiospirone (0.32-32 mg/kg) failed to occasion substantial generalization in any subject. (+)-3-PPP produced little or no BMY 14802-appropriate responding in five pigeons over a dose range of 1.0 to 32 mg/kg, but it did substitute for BMY 14802 in one pigeon at a dose of 3.2 mg/kg. On retesting several months later, (+)-3-PPP again failed to occasion substantial generalization in five of six subjects, and it evoked 70% generalization in the subject in which it had previously substituted. PCP (0.10-10 mg/kg) did not substitute for the BMY 14802 training stimulus in any of the pigeons. A second study of PCP yielded similar results, with the exception that it substituted in one subject at a dose of 0.32 mg/kg, whereas it had previously produced no generalization in this subject.

Substitution tests of other compounds. Eticlopride (0.32-10.0 mg/kg) and DMI (0.1-18.0 mg/kg) failed to substitute for the BMY 14802 training stimulus in any of the pigeons (table 1). Although it did not substitute for the training stimulus, eticlopride evoked 70% to 88% generalization in three subjects. DMI evoked saline-appropriate responding from all subjects over the dose range tested.

Sigma ligands as potential antagonists of the BMY 14802 stimulus. The discriminative stimulus potency of BMY 14802 was not changed after pretreatments with selected doses of MR 2035, tiospirone and DTG (data not shown). Pretreatments of either 10 mg/kg MR 2035 or 0.32 mg/kg tiospirone did not change the discriminative stimulus or rate-reducing potency of BMY 14802. Similarly, pretreatment with 5.6 mg/kg DTG did not affect the discriminative stimulus profile of BMY 14802; however, this treatment did enhance the rate-reducing effects of BMY 14802.

5-HT_1A agonist substitution results. Buspirone (ED₅₀, 0.67 mg/kg; 95% CL, 0.40-1.1 mg/kg) and 8-OH-DPAT (ED₅₀, 0.66 mg/kg; 95% CL, 0.24-1.9 mg/kg) dose-dependently substituted for BMY 14802 in all pigeons (fig. 3). Similarly, spiroxatrine (ED₅₀, 1.2 mg/kg; 95% CL, 0.78-1.8 mg/kg) substituted in five of six subjects (fig. 3). Buspirone substituted for BMY 14802 in all pigeons by a dose of 10 mg/kg. Responding was completely eliminated in two pigeons with a dose of 10 mg/kg buspirone and a dose of 18 mg/kg buspirone in four other pigeons. Substitution of 8-OH-DPAT for BMY 14802 occurred in three pigeons at a dose of 0.32 mg/kg and in the remaining subjects at 3.2 mg/kg. Responding was completely suppressed at a dose of 3.2 mg/kg in the subjects for which 8-OH-DPAT substituted at the lower dose, whereas the other three subjects stopped responding at a dose of 32 mg/kg. Spiroxatrine substituted for BMY 14802 in five pigeons at a dose of 3.2 mg/kg and suppressed responding in these subjects at 10 mg/kg. In another subject, spiroxatrine engendered <40% BMY 14802-appropriate responding at any dose tested and suppressed responding at a dose of 18 mg/kg. The relative potencies of BMY 14802, 8-OH-DPAT, buspirone and spiroxatrine to produce BMY 14802-like discriminative effects corresponded to the minimal doses of each of these drugs to produce BMY 14802-like effects and their affinity for 5-HT_1A receptors (table 2).

View larger version (20K): [in this window] [in a new window]   Fig. 3.   Dose-response functions obtained during substitution tests with buspirone (), spiroxatrine () and 8-OH-DPAT () in BMY 14802-trained pigeons. Abscissa, dose of drug in mg/kg. Remaining data are presented as in figure 1. a, n = 3. b, n = 4. c, n = 1.

Substitution experiments with other serotonergic compounds. Serotonergic compounds that are relatively nonselective or selective for receptors other than those of the 1A subtype failed to substitute in a majority of the pigeons (table 3). RU24969 (0.32-18 mg/kg) substituted in three animals by a dose of 10 mg/kg. Clozapine (0.32-56 mg/kg) substituted in two subjects and produced ~70% BMY 14802-appropriate responding in another subject. Quipazine substituted in two subjects at 3.2 mg/kg, as did m-CPP over a dose range of 0.1 to 10 mg/kg. l-5-HTP substituted in two subjects at a dose of 18 mg/kg. Metergoline (0.32-100 mg/kg) substituted in one subject at 10 mg/kg and another at 56 mg/kg. NAN 190 (0.032-3.2 mg/kg) substituted in one pigeon at 1.0 mg/kg. Ondansetron (0.0032-1.0 mg/kg) did not substitute in any pigeon.

Serotonergic compounds as potential antagonists of the BMY 14802 stimulus. NAN 190 appeared to competitively antagonize the discriminative effects of BMY 14802 (fig. 4, top). A subset of four pigeons were tested with cumulative BMY 14802 (0.10-32 mg/kg) after pretreatments of 0.32 and 3.2 mg/kg NAN 190. The ED₅₀ for the discriminative effects of BMY 14802 was increased 4.9-fold after pretreatment with 0.32 mg/kg NAN 190 (table 4). Pretreatment with the higher dose of NAN 190 shifted the BMY 14802 dose-effect gradient downward such that no dose of BMY 14802 evoked 50% drug-appropriate responding. The doses of BMY 14802 that abolished responding were unaffected by either pretreatment (fig. 4, bottom).

View larger version (18K): [in this window] [in a new window]   Fig. 4.   Dose-response functions for BMY 14802 alone () and for 0.32 () and 3.2 () mg/kg NAN 190 followed by BMY 14802. Data are presented as in figure 1 except that each point represents the mean value from four subjects unless otherwise indicated. a, n = 3. b, n = 2.

View larger version (15K): [in this window] [in a new window]   Fig. 5.   Dose-response gradients for 8-OH-DPAT alone () and 0.32 () and 3.2 () NAN 190 followed by 8-OH-DPAT. Abscissae, dose of 8-OH-DPAT in mg/kg. Data are presented as in figure 1 except that each point represents the mean value from four subjects.

	Discussion

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The results of this study show that BMY 14802 can be rapidly established as a discriminative stimulus in pigeons. Because BMY 14802 has relatively high affinity for sigma sites and it binds in a stereospecific and competitive manner (Taylor and Dekleva, 1987a,b, 1988), investigators have speculated that these sites may be responsible for its unique profile of activity. Although neurochemical findings suggest that BMY 14802 can produce functional effects through sigma sites (Taylor et al., 1991), results of this study do not suggest that the discriminative stimulus effects of BMY 14802 are related to sigma sites. Instead, the results indicate that these effects are serotonergically mediated primarily through 5-HT_1A receptors.

Several lines of evidence from this study suggest that the discriminative stimulus effects of BMY 14802 are unrelated to sigma sites. First, ()-BMY 14802 is more potent than (+)-BMY 14802 in producing BMY 14802-like discriminative effects (fig. 1, top right). This is in contrast to the stereoselectivity favoring the dextrorotatory form of sigma ligands, including BMY 14802, demonstrated in sigma radioligand binding assays (Taylor and Dekleva, 1987, 1988). Moreover, a variety of compounds that have affinity for sigma sites fail to substitute for the BMY 14802 stimulus (table 1). DTG, (+)-3-PPP, MR 2035 (Weber et al., 1986),⁶ as well as (+)-SKF 10,047, haloperidol (Largent et al., 1988; Tam and Cook, 1984) and the putative antipsychotics tiospirone, cinuperone and rimcazole (Ferris et al., 1986; Su, 1986; Taylor and Dekleva, 1988) bind sigma sites with moderate to high affinity. Although the efficacy of these drugs must be considered with respect to their potency, none of these compounds substituted for the BMY 14802 stimulus. Also, DTG, MR 2035 and tiospirone failed to antagonize the discriminative stimulus effects of BMY 14802. Thus, BMY 14802 and these other drugs did not appear to produce discriminative stimulus effects mediated by a common population of receptors.

In addition to providing evidence that sigma sites do not mediate the discriminative stimulus effects of BMY 14802, experiments with the sigma compounds provide evidence to exclude the possible involvement of certain other neurotransmitter systems. PCP receptors do not appear to be involved because PCP and (+)-SKF 10,047, which also bind with high affinity to PCP receptors (Hampton et al., 1982; Quirion et al., 1987), do not substitute for the BMY 14802 stimulus. Dopamine receptors are also unlikely to play a role in mediating the stimulus effects of BMY because haloperidol, tiospirone and cinuperone, sigma compounds that also bind to D₂ receptors and have action as D₂ antagonists in some assays (Riblet et al., 1982; Seeman, 1981; Su, 1986), do not produce BMY 14802-like stimulus effects. In addition, sigma ligand (+)-3-PPP, which functions as a dopamine autoreceptor agonist (Hjorth et al., 1983), did not substitute.

Although results from this study do not provide evidence to support the hypothesis that sigma sites are involved in mediating the discriminative stimulus effects of BMY 14802, evidence was found to suggest that these effects are serotonergically mediated. The clinically efficacious anxiolytic buspirone (Goldberg and Finnerty, 1979) produces actions in a variety of assays that are attributed to 5-HT_1A (Eison et al., 1982, 1986; Eison and Temple, 1986; Peroutka, 1985) and D₂ dopamine (Riblet et al., 1982; Taylor et al., 1982) receptors. This drug, which is a structural analog of BMY 14802 that also has affinity for sigma sites (Taylor and Dekleva, 1988), substitutes for BMY 14802. Similarly, spiroxatrine, a compound characterized as a 5-HT_1A agonist with dopaminergic and opioid components on the basis of neurochemical and behavioral data (Barrett et al., 1989; Niemegeers et al., 1964), substituted. Although the selective D₂ antagonist eticlopride failed to produce BMY 14802-like discriminative effects, the serotonergic ligand 8-OH-DPAT, which has relatively high affinity and selectivity for 5-HT_1A receptors (Middlemiss and Fozard, 1983), did substitute for BMY 14802. These results strongly imply that the discriminative stimulus effects of BMY 14802 are serotonergically mediated through 5-HT_1A receptors.

In agreement with findings from this study, other drug discrimination studies demonstrate that BMY 14802, buspirone, spiroxatrine and 8-OH-DPAT share discriminative stimulus effects. For example, BMY 14802 (Bristow et al., 1991; Sanger and Schoemaker, 1992) and buspirone (Cunningham et al., 1987; Sanger and Schoemaker, 1992) substitute for an 8-OH-DPAT stimulus in rats. Spiroxatrine and 8-OH-DPAT substitute for a buspirone stimulus in pigeons (Mansbach and Barrett, 1986; Nader et al., 1989). Behavioral studies also show that buspirone, 8-OH-DPAT and spiroxatrine each increases punished responding in the pigeon (Barrett et al., 1989; Witkin and Barrett, 1986) as does BMY 14802 in the rat (Bristow et al., 1991). The mechanism responsible for these shared actions may also explain other common effects. For instance, both BMY 14802 and buspirone increase nigral dopamine neuronal impulse flow in the presence of haloperidol (McMillen et al., 1983), reverse neuroleptic-induced catalepsy and do not induce catalepsy themselves (Eison et al., 1982; Matthews et al., 1986; McMillen and Mattiace, 1983; McMillen and Williams, 1985). Future investigations may reveal the extent of serotonergic contribution in producing such actions.

Receptor binding studies indicate that compounds that produce BMY 14802-like discriminative effects share affinity for 5-HT_1A receptors. Buspirone and 8-OH-DPAT both potently inhibit (³H)8-OH-DPAT binding in rat hippocampus, as does BMY 14802 with lower affinity.⁷ Similar findings have been reported for BMY 14802 and 8-OH-DPAT in rat frontal cortex (Bristow et al., 1991) and for spiroxatrine as well as buspirone in pigeon cerebral membranes (Barrett et al., 1989). Because BMY 14802, buspirone, spiroxatrine and 8-OH-DPAT share stimulus effects and have in common affinity for 5-HT_1A receptors, the discriminative stimulus effects of BMY 14802 may be serotonergically mediated through this receptor population.

The contention that 5HT_1A receptors mediate the discriminative stimulus effects of BMY 14802 is strengthened by findings from this study with other serotonergic compounds. First, the inability of quipazine, metergoline and clozapine to substitute for BMY 14802 argues against the involvement of 5-HT_1C/2 receptors. Quipazine produces discriminative stimulus effects through activation of 5-HT₂ receptors in pigeons (Walker et al., 1991; Yamamoto et al., 1991) and rats (Friedman et al., 1984). Metergoline has been suggested to act through these receptors in drug discrimination studies due to its ability to antagonize a quipazine training stimulus in pigeons (Yamamoto et al., 1991) and in rats (Friedman et al., 1984). Experiments in this laboratory have shown that the discriminative stimulus effects of the atypical antipsychotic clozapine are mediated through 5-HT_1C/2 receptors because compounds such as metergoline that share antagonist action through these receptors substitute for a clozapine stimulus in pigeons (Hoenicke et al., 1992). BMY 14802 does not substitute for a clozapine stimulus (Hoenicke et al., 1992), which provides further evidence that the BMY 14802 stimulus is not mediated through 5-HT_1C/2 receptors. The failure of the selective 5-HT₃ antagonist ondansetron (Butler et al., 1988) to substitute for the BMY 14802 stimulus suggests that 5-HT₃ receptors are also unlikely to be involved in mediating the discriminative stimulus effects of BMY 14802. Finally, interaction with 5-HT_1B/1C receptors is not evident because m-CPP, a compound that produces behavioral effects through 5-HT_1B/1C receptors (Lucki et al., 1989), did not substitute.

RU24969, a compound that has been suggested to act through several serotonin receptor subtypes (e.g., Lucki, 1992), substituted in 50% of the BMY 14802-discriminating pigeons. Interestingly, this compound substitutes for a buspirone stimulus in pigeons (Nader et al., 1989), which suggests that it has activity as a 5-HT_1A agonist. However, results from other drug discrimination studies using this compound are not easily reconciled with the findings from this study. RU24969 and 8-OH-DPAT substitute for a l-5-HTP stimulus in pigeons (Walker et al., 1991); however, l-5-HTP did not substitute for BMY 14802. Further research, possibly involving RU24969-discriminating subjects, will be necessary to fully elucidate the mechanism responsible for the discriminative effects produced by RU24969.

Evidence of a 5-HT_1A mechanism of action is also provided from experiments that assessed whether different serotonergic compounds could attenuate the discriminative stimulus effects of BMY 14802. The mixed 5-HT_1A agonist/antagonist NAN 190 (Hjorth and Sharp, 1990) can antagonize the stimulus effects of 8-OH-DPAT in rats (Glennon et al., 1988, 1989). In this study, NAN 190 failed to substitute for BMY 14802; however, it antagonized the stimulus effects of BMY 14802 and the BMY 14802-like stimulus effects of 8-OH-DPAT (fig. 3, top). Although involvement of 5-HT_1A receptors was further substantiated, additional antagonism experiments provided more evidence against involvement of 5-HT₂ and 5-HT₃ receptors. Pirenpirone, a highly selective 5-HT₂ drug (Leysen and Tollenaere, 1982) that antagonizes 5-HT₂-mediated discriminative stimuli in pigeons (Yamamoto et al., 1991) and rats (Glennon et al., 1983), did not antagonize the discriminative stimulus effects of BMY 14802. Ondansetron also failed to act as an antagonist. Although metergoline did not antagonize the discriminative stimulus effects of BMY 14802, it is interesting to note that a relatively large dose of this drug given 3 hr before cumulative BMY 14802 appeared to potentiate the discriminative effects of BMY 14802. Because metergoline is a relatively nonselective drug and its onset of action varies with respect to its disparate actions, future studies of metergoline alone and in combination with selective 5-HT_1A and 5-HT₂ compounds may clarify this finding.

In summary, results from this study suggest that the discriminative stimulus effects of BMY 14802 are serotonergically mediated primarily through 5-HT_1A receptors and unrelated to the sigma site. This conclusion is based on the finding that several compounds identified in radioligand binding studies as potent sigma ligands did not engender or antagonize BMY 14802-like discriminative effects. In contrast, compounds that either evoked or attenuated BMY 14802-like discriminative effects share affinity for 5-HT_1A serotonin receptors and actions that are attributed to these receptors. Results from experiments using serotonergic compounds that produce actions through receptors other than those of the 1A subtype suggest that the discriminative stimulus effects of BMY 14802 are primarily mediated through 5-HT_1A receptors. These findings suggest that the unique profile of BMY 14802 obtained in antipsychotic screening assays may be due to serotonergically influenced activity. The functional relevance of the sigma site remains to be determined.