Introduction

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An understanding of the neuropharmacological actions of cocaine is necessary to guide the development of novel pharmacotherapies for cocaine dependence, which remains a widespread problem in the Unites States. The observations that inhibition of dopamine (DA) reuptake (Koe, 1976) and stimulation of D₁- and D₂-like receptors underlie many of the behavioral effects of cocaine in animals (Spealman et al., 1992) have provided a key rationale for the use of dopaminergic compounds in the treatment of cocaine addiction. However, the utility of DA receptor ligands in treatment settings has been limited by adverse side effects (for review, see Klein, 1998). Cocaine also inhibits serotonin (5-hydroxytryptamine; 5-HT) reuptake (Koe, 1976) and stimulation and blockade of 5-HT receptors have been shown to modulate the behavioral effects of cocaine in preclinical studies (for review, see Walsh and Cunningham, 1997; McMahon and Cunningham, 1999). A more thorough appreciation of this modulatory role for 5-HT in the behavioral effects of cocaine may yield insight into how manipulations of 5-HT systems may prove useful in facilitating treatment goals to maintain abstinence in a cocaine abuser.

Of the 14 5-HT receptor subtypes characterized to date, the 5-HT₂ receptors (5-HT₂Rs), including 5-HT_2AR, 5-HT_2BR, and 5-HT_2CR, are products of different genes and exhibit differential anatomical localization (Hoyer et al., 1994). Stimulation of 5-HT₂Rs, which are linked to phospholipase C, results in increased phosphatidyl inositol turnover, increased intracellular levels of Ca²⁺, and neuronal depolarization (Hoyer et al., 1994). The 5-HT_2AR is synonymous with the classical 5-HT₂R and has been implicated in hallucinosis, psychosis, and affective disorders (Baxter et al., 1995). The distributions of 5-HT_2AR mRNA and protein in rat brain closely overlap, with moderate-to-high levels observed in DA cell body and terminal fields of the mesocorticolimbic and nigrostriatal systems, including lamina V of cortex, dorsal striatum, and nucleus accumbens (Lopez-Gimenez et al., 1997; Cornea-Hebert et al., 1999). In fact, 5-HT_2AR protein has been recently localized to DA neurons in the ventral tegmental area of rats (Doherty and Pickel, 2000) and humans (Ikemoto et al., 2000). Dopamine neurotransmission appears to be under afferent regulation by 5-HT_2AR (Schmidt et al., 1992, 1995; Sorensen et al., 1992, 1993; De Deurwaerdere and Spampinato, 1999; Lucas and Spampinato, 2000), and therefore 5-HT_2AR might be a target through which to treat disorders associated with dysfunctional DA systems, including drug dependence and psychosis.

Attempts to elucidate a role for 5-HT_2AR in the behavioral effects of cocaine have used ligands that do not effectively differentiate between 5-HT₂R subtypes, and frequently possess affinity for other monoamine receptors such as -noradrenergic receptors (e.g., ketanserin; Dudley et al., 1988). Cocaine-evoked hyperactivity has been reported to be both enhanced and attenuated by the nonselective 5-HT₂R antagonist ketanserin (Herges and Taylor, 1998; O'Neill et al., 1999) and the 5-HT_2B/2CR antagonist SB 206553 (McCreary and Cunningham, 1999). In drug discrimination studies, ketanserin and ritanserin were reported to attenuate the stimulus effects of cocaine in squirrel monkeys (Schama et al., 1997). However, the stimulus effects of cocaine in rats were reportedly unaltered by nonselective 5-HT₂R antagonists (Meert and Janssen, 1992; Peltier et al., 1994; Callahan and Cunningham, 1995). Ketanserin was also reported to decrease response rates maintained by a second order schedule of cocaine self-administration in squirrel monkeys, at doses that also decreased responding for food (Nader and Barrett, 1990). Failing to alter breakpoints on a progressive ratio schedule of cocaine delivery, ketanserin did modestly reduce cocaine intake in rats (Lacosta and Roberts, 1993). In contrast, ritanserin increased response rates for self-administration of low doses of cocaine (Howell and Byrd, 1995). Thus, a potential role for 5-HT₂R in the hypermotive, stimulus and reinforcing effects of cocaine is suggested; however, the identification of roles for specific 5-HT₂R subtypes has been hampered by the use of compounds with poor selectivity.

Although only a 2- to 10-fold selectivity of ketanserin for 5-HT_2A (K_i = 3.2 nM) over either 5-HT_2C (K_i = 28 nM) or ₁-adrenergic receptors (K_i = 6.7 nM; Dudley et al., 1988; Yagaloff and Hartig, 1985) is evident, the more recently developed compound M100907 is an antagonist that possesses nanomolar affinity for 5-HT_2AR (K_i = 0.85 nM), considerably less affinity for 5-HT_2CR (K_i = 88 nM), and ₁-adrenergic receptors (K_i = 128 nM), and negligible affinity for most other receptors, including DA D₁- and D₂-like receptors (>500 nM; Kehne et al., 1996a). Thus, M100907 presents a profile of >100-fold selectivity for the 5-HT_2AR versus the 5-HT_2CR or ₁-adrenergic receptor. The aim of the present study was to more thoroughly analyze the possible role of 5-HT_2AR in the in vivo effects of cocaine by directly comparing the abilities of the nonselective 5-HT₂R antagonist ketanserin and the selective 5-HT_2AR antagonist M100907 to block the hypermotive and discriminative stimulus effects of cocaine.

	Materials and Methods

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Animals

Male Sprague-Dawley rats (n = 50; Harlan, Houston, TX) weighing between 300 and 350 g at the beginning of the study were used. The rats were housed in pairs in a colony room that was maintained at a constant temperature (21-23°C) and humidity (40-50%); lighting was maintained on a 12-h light/dark cycle (7:00 AM-7:00 PM). For locomotor testing, rats were provided with continuous access to tap water and rodent chow throughout the experiment except during experimental sessions. For drug discrimination testing, food was always available in the home cage but not during experimental sessions. The amount of water each animal received during drug discrimination studies was restricted to that given during operant training sessions, after test sessions (10-15 min), and on weekends (36 h). All experiments were conducted during the light phase (9:00 AM-3:00 PM).

Drugs

Doses of all drugs refer to the weight of the salt. Cocaine hydrochloride (National Institute on Drug Abuse, Research Triangle Park, NC) was prepared in 0.9% NaCl. Ketanserin tartrate (Sigma/Research Biochemicals International, Natick, MA) was prepared in a solution of 45% 2-hydroxypropyl-cyclodextrin (Sigma/Research Biochemicals International) in sterile distilled water. M100907 [R-(+)-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol] (Hoechst Marion Roussel, Cincinnati, OH) was prepared in a solution of 1% Tween 80 (Sigma, St. Louis, MO) in sterile distilled water. All drugs were injected in a volume of 1 ml/kg.

Measurement of Locomotor Activity

Apparatus. Locomotor activity was monitored and quantified using an open field activity system (San Diego Instruments, San Diego, CA). Each clear Plexiglas chamber (40 × 40 × 40 cm) was housed within sound-attenuating enclosures and was surrounded with a 4 × 4 photobeam matrix located 4 cm from the floor surface. Interruptions of the photobeams resulted in counts of activity in the peripheral and central fields of the chamber. Activity recorded in the inner 16 × 16 cm of the open field was counted as central activity, whereas the field bounded by the outer 16-cm band registered peripheral activity. Another horizontal row of 16 photobeams, located 16 cm from the floor surface, provided each chamber with a measurement of vertical activity. Separate counts of peripheral, central, and vertical activity were made by the control software (Photobeam Activity Software; San Diego Instruments) and stored for subsequent statistical evaluation. Peripheral and central activity counts were summed to provide a single measure of total horizontal activity. Video cameras positioned above the chambers permitted continuous observation of behavior without disruption.

Behavioral Protocols. All rats were maintained in the colony room for a minimum of 1 week before behavioral testing for acclimation to daily handling procedures. All rats were habituated to the test environment for 3 h/day on each of the 2 days before the start of the experiment. On each of the test days, rats were habituated to the activity monitors for 1 h before administration of drugs. Using a repeated measures design, one group of rats (n = 8) was pretreated with an injection of vehicle (1 ml/kg i.p.) or M100907 (0.02, 0.2, or 2 mg/kg i.p.), followed 45 min later by an injection of saline (1 ml/kg i.p.) or cocaine (10 mg/kg i.p.). Using the same design, another group of rats (n = 6) was pretreated with an injection of vehicle (1 ml/kg i.p.) or ketanserin (0.04, 0.4, or 4 mg/kg i.p.), followed 45 min later by an injection of saline (1 ml/kg i.p.) or cocaine (10 mg/kg i.p.). Measurement of locomotor activity counts began immediately following injection of saline or cocaine and was divided into 5-min bins for a total of 1 h. Test sessions were conducted every other day, and the order of drug tests was counterbalanced with the caveat that cocaine was administered no more than once per week.

Data Analysis. Locomotor activity was totaled across the 60-min session and analyzed as horizontal (peripheral + central activity counts) and vertical activity counts. Horizontal and vertical activity was analyzed using a two-way ANOVA for repeated measures for the factors of pretreatment (0, 0.02, 0.2, or 2 mg/kg M100907 or 0, 0.04, 0.4, or 4 mg/kg ketanserin), treatment (0 or 10 mg/kg cocaine), and the pretreatment × treatment interaction (SAS for Windows, version 6.12). Horizontal and vertical activity was also divided into four separate 15-min time bins and analyzed with a three-way ANOVA with time as a repeated measures factor to examine the pretreatment × treatment × time interaction. The Student-Newman-Keuls procedure was used to analyze preplanned, pairwise comparisons; all comparisons were conducted with an experimentwise  (_EW) equal to 0.05.

Drug Discrimination Assays

Apparatus. The procedures were conducted in commercially available, two-lever operant chambers (model 80001; Lafayette Instrument, Lafayette, IN). Each chamber was equipped with a water-filled dispenser mounted equidistant between two response levers on one wall, and was housed in a light- and sound-attenuating shell (model 80015; Lafayette Instrument). Illumination was provided by a 28-V house light; ventilation and masking noise were supplied by a blower. An interface (MedAssociates, St. Albans, VT) connected the chambers to a computer, which controlled and recorded all experimental events.

Procedures. Before the initiation of the errorless training phase, the rats were trained to discriminate an injection of cocaine (10 mg/kg i.p.) from saline (1 ml/kg i.p.) given 15 min before daily (Monday through Friday) 30-min sessions. Training began under a schedule of continuous water reinforcement (FR 1) with only the stimulus-appropriate (cocaine or saline) lever present ("errorless" training); the schedule of reinforcement was increased until each rat was responding reliably under an FR 20 schedule for each experimental condition. For half of the rats, left-lever responses were reinforced after cocaine administration, whereas right-lever responses were reinforced after saline administration; conditions were reversed for the remaining rats. During this phase of training, cocaine and saline were administered irregularly with the restriction that neither condition prevailed for more than three consecutive sessions.

Data Analysis. Accuracy was defined as the percentage of correct responses to total responses before the delivery of the first reinforcer. During test sessions, performance was expressed as the percentage of drug-lever responses to total responses upon completion of an FR 20 on either lever. The response rate (responses per min) was calculated as the total number of responses emitted before completion of the first FR 20 divided by the number of minutes taken to complete the first ratio. For cocaine dose-response tests, Student's t test for repeated measures was used to compare the percentage of cocaine-appropriate responding and response rates during test sessions with the previous cocaine session. For combination tests, Student's t test for repeated measures was used to compare performance following a fixed dose of cocaine (5 mg/kg) and that following the comparable dose of cocaine plus various doses of M100907 or ketanserin. Only data from animals that completed the FR 20 during tests were included for analysis. Log-probit analyses were used to estimate the dose of cocaine predicted to elicit 50% cocaine-appropriate responding (ED₅₀), the doses of M100907 or ketanserin to decrease cocaine-appropriate responding by 50% (AD₅₀), and the 95% confidence limits for each (Tallarida and Murray, 1987). All statistical analyses were conducted with _EW = 0.05.

	Results

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Effects of M100907 on Cocaine-Induced Hyperactivity. A significant main effect of pretreatment (F_3,21 = 8.61, p < 0.001), treatment (F_1,7 = 289.58, p < 0.001), and a pretreatment × treatment interaction (F_3,21 = 4.06, p < 0.05) were observed for total horizontal activity summed across the 60-min session. Pretreatment with M100907 at each dose significantly attenuated cocaine-induced increases in horizontal activity (p < 0.05), with cocaine-induced horizontal activity after 0.2 mg/kg M100907 not differing significantly from activity observed in vehicle-saline controls (p < 0.05; Fig. 1A). A significant pretreatment × treatment × time interaction was observed for horizontal activity divided into four separate 15-min time bins (F_9,63 = 2.06, p < 0.05). Cocaine-induced increases in horizontal activity were significantly attenuated by each dose of M100907 during the first and second 15-min intervals, whereas only 0.2 or 2 mg/kg M100907 significantly attenuated cocaine-induced horizontal activity during the third 15-min interval (p < 0.05; Fig. 1B). None of the doses of M100907 significantly altered cocaine-induced horizontal activity during the fourth 15-min interval (p > 0.05). M100907 (0.02-2 mg/kg) did not significantly alter spontaneous activity after saline injection at any time point (p > 0.05).

View larger version (39K): [in this window] [in a new window]   Fig. 1.   Horizontal activity following pretreatment with the 5-HT2AR antagonist M100907. A, mean total horizontal activity (counts/60 min) (±S.E.M.) is depicted following i.p. injection of vehicle (Veh) or M100907 (M100; 0.02, 0.2, or 2 mg/kg) followed by saline or cocaine injection (Coc; 10 mg/kg). *Activity levels that were significantly different from vehicle-saline controls; activity levels that were significantly different from vehicle-cocaine controls based on a preplanned comparison using Student-Newman-Keuls procedure (p < 0.05). B, time course of horizontal activity plotted in 15-min bins across the 60-min test session is depicted following i.p. injection of vehicle or M100907 (0.02, 0.2, or 2 mg/kg) followed by saline (open symbols) or cocaine injection (10 mg/kg; closed symbols). **Activity levels that were significantly different from vehicle-cocaine controls at the corresponding time point based upon a Student-Newman-Keuls procedure (p < 0.05).

View larger version (22K): [in this window] [in a new window]   Fig. 2.   Vertical activity following pretreatment with the 5-HT2AR antagonist M100907. Mean total vertical activity (counts/60 min) (±S.E.M.) is depicted following i.p. injection of vehicle (Veh) or M100907 (M100; 0.02, 0.2, or 2 mg/kg) followed by saline or cocaine injection (Coc; 10 mg/kg). *Activity levels that were significantly different from vehicle-saline controls; activity levels that were significantly different from vehicle-cocaine controls based on a preplanned comparison using Student-Newman-Keuls procedure (p < 0.05).

Effects of Ketanserin on Cocaine-Induced Hyperactivity. A significant main effect of pretreatment (F_3,15 = 8.28, p < 0.001), treatment (F_1,5 = 21.05, p < 0.01), and a pretreatment × treatment interaction (F_3,15 = 6.00, p < 0.01) were observed for total horizontal activity summed across the 60-min session. Pretreatment with 0.4 or 4 mg/kg ketanserin significantly attenuated cocaine-induced increases in horizontal activity (p < 0.05), with cocaine-induced horizontal activity after 4 mg/kg ketanserin not differing significantly from activity observed in vehicle-saline controls (p < 0.05; Fig. 3A). The pretreatment × treatment × time interaction was not significant for horizontal activity divided into four separate 15-min time bins (F_9,45 = 1.59, p > 0.05; data not shown).

View larger version (31K): [in this window] [in a new window]   Fig. 3.   Horizontal and vertical activity following pretreatment with the 5-HT2R antagonist ketanserin. See legends to Figs. 1A and 2 for explanation of the figures.

Effects of M100907 or Ketanserin on the Stimulus Effects of Cocaine. The mean number of sessions required to meet the cocaine versus saline discrimination criterion was 13 (range 10-22). During dose-response tests, cocaine (0.625-10 mg/kg) produced a dose-related increase in cocaine-appropriate responding, whereas saline administration resulted in <10% cocaine-appropriate responding (data not shown). Drug-appropriate responding after cocaine doses of 0.625, 1.25, and 2.5 mg/kg was significantly different from the previous cocaine session (p < 0.05; data not shown). The dose of cocaine predicted to elicit 50% cocaine-lever responding (ED₅₀) was 2.2 mg/kg (95% CI, 0.8-6.1 mg/kg). Response rates after cocaine (0.625-10 mg/kg) did not differ significantly from the previous cocaine maintenance session (p > 0.05; data not shown).

View larger version (18K): [in this window] [in a new window]   Fig. 4.   Results of combination tests with the 5-HT2AR antagonist M100907 or the 5-HT2R antagonist ketanserin plus 5 mg/kg cocaine. Closed symbols denote the mean percentage of cocaine-appropriate responses (±S.E.M.; left ordinate); open symbols denote the mean response rate per minute (±S.E.M.; right ordinate). M100907 data points represent the mean values from 13/13 or 14/14 rats; ketanserin data points represent the mean values from 8/8 rats [n/N, number of subjects (n) completing at least 20 responses out of the number of subjects tested (N)]. *Performances during test sessions that were significantly different from cocaine (5 mg/kg) administered alone (p < 0.05). For comparison, mean percentage of cocaine-appropriate responses (±S.E.M.; ) and mean response rate per minute (±S.E.M.; ) are depicted after cocaine (5 mg/kg) alone.

	Discussion

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The 5-HT_2AR antagonist M100907 and nonselective 5-HT₂R antagonist ketanserin significantly attenuated hyperactivity induced by cocaine, in keeping with previous observations (Herges and Taylor, 1998; O'Neill et al., 1999). Because ketanserin discriminates poorly among 5-HT_2A, 5-HT_2C, and ₁-adrenergic receptors (Yagaloff and Hartig, 1985; Dudley et al., 1988), the reduction of cocaine-induced hyperactivity by ketanserin may reflect actions at any of these receptor subtypes. Suppression of cocaine-induced hyperactivity has been observed following pretreatment with the 5-HT_2B/2CR antagonist SB 206553 (McCreary and Cunningham, 1999) and the ₁-adrenergic receptor antagonist prazosin (Snoddy and Tessel, 1985); however, the efficacious blockade afforded by low doses of M100907, which has a 100-fold higher affinity for 5-HT_2A over 5-HT_2C and ₁-adrenergic receptors, supports the hypothesis that antagonism of 5-HT_2AR may account in whole (M100907) or part (ketanserin) for the attenuation of the motor stimulant properties of cocaine. This argument is further reinforced by the finding that M100907, at doses up to 30 times higher than those used here, did not antagonize the behavioral effects of 5-HT_2CR, D₂-like, and ₁-adrenergic agonists (Kehne et al., 1996a; Dekeyne et al., 1999).

This effective antagonism of cocaine-stimulated hyperactivity afforded by M100907 and ketanserin occurred in the absence of altered basal activity seen after the same doses of M100907 or ketanserin administered alone. Because activity measurements were obtained from rats well habituated to the test environment, the argument can be made that the resultant low levels of spontaneous activity may render this baseline insensitive to potential behavioral suppression due to administration of M100907 or ketanserin alone, and thus, raise suspicion concerning the specificity of the antagonism. However, behaviorally suppressive effects of M100907 (1 mg/kg) have not been noted in unhabituated rats tested in our activity monitors (P. S. Frankel and K. A. Cunningham, unpublished data); this finding is consistent with observations that appreciable sedation is seen only at much higher doses of M100907 (>8 mg/kg; Sorensen et al., 1993; Kehne et al., 1996a,b; but see Gleason and Shannon, 1997). Similarly, we noted no statistically significant effect of ketanserin on basal activity, although a disruptive effect of this compound at 3 mg/kg has been observed in mice with little prior exposure to the activity monitor (Gleason and Shannon, 1997), but not in acclimatized rats (Wing et al., 1990). Thus, although modest behaviorally suppressant effects of M100907 and ketanserin may be associated with systemic injection, 5-HT_2AR antagonism appears to evoke significantly less sedation than that associated with DA or ₁-adrenergic receptor antagonism (Jackson et al., 1994; Mathe et al., 1996).

The present study revealed that M100907 (ED₅₀ = 0.25 mg/kg) and ketanserin (ED₅₀ = 1.48 mg/kg) effectively antagonized the stimulus properties of cocaine. This finding is in keeping with the observation that ketanserin (and a similar nonselective 5-HT₂R antagonist, ritanserin) attenuated the cocaine cue in squirrel monkeys (Schama et al., 1997), although ritanserin did not alter the stimulus properties of cocaine in rats (Meert and Janssen, 1992; Peltier et al., 1994). The inconsistencies in the ability of 5-HT₂ antagonists to block the stimulus effects of cocaine are likely to be related to the varying affinity profiles of these ligands for monoamine receptors, particularly given the difficulty in predicting the exact profile of receptor occupancy after systemic administration. However, the efficacy of M100907 to block the stimulus effects of cocaine in the face of its selective receptor affinity profile and the failure of this ligand to antagonize the behavioral effects of 5-HT_2C, D₂-like, and ₁-adrenergic receptor agonists (Kehne et al., 1996a; Dekeyne et al., 1999) provide the most compelling evidence to date in support for a modulatory role of 5-HT_2AR in the discriminative stimulus properties of cocaine. Furthermore, in contrast to the overt behavioral suppression evident at doses of the classical DA antagonists that block the stimulus effects of cocaine (Callahan et al., 1991), M100907 effectively attenuated the cocaine cue in the absence of robust changes in response rates. Thus, selective antagonism of 5-HT_2AR with M100907 appears to be an effective strategy for attenuating the behavioral effects of cocaine in the absence of prominent sedative effects.

The lack of overt changes in basal behavior upon administration of M100907 may reflect the relative absence of a tonic regulatory influence of 5-HT_2AR on DA function, a hypothesis supported by other neuropharmacological investigations. For example, systemic administration of 5-HT_2AR antagonists did not alter basal levels of DA cell firing (Sorensen et al., 1993) or striatal (Schmidt et al., 1992) and accumbal DA efflux (De Deurwaerdere and Spampinato, 1999). Furthermore, striatal DA efflux was not evoked by coperfusion of the 5-HT_2A/2B/2CR agonist 1-(2,5-dimethoxy-4-iodo)-2-aminopropane (DOI) and the 5-HT_2B/2CR antagonist SB 206553, supporting the concept that sole stimulation of 5-HT_2AR is not sufficient to enhance DA efflux (Lucas and Spampinato, 2000). However, under conditions of DA stimulation, the 5-HT_2AR does appear to positively modulate DA outflow. For example, antagonism of 5-HT_2AR has been shown to attenuate striatal DA efflux stimulated by systemic administration of (±)-3,4-methylenedioxymethamphetamine (MDMA; Schmidt et al., 1992), blockade of D₂-like autoreceptors with haloperidol (Lucas and Spampinato, 2000), and electrical stimulation of the dorsal raphe nucleus (De Deurwaerdere and Spampinato, 1999). Despite the evidence to suggest that the 5-HT_2AR exercises little tonic control over DA function under normal conditions (see above), basal 5-HT concentrations may provide sufficient tone on 5-HT_2AR such that, under conditions of stimulated DA function, antagonism of 5-HT_2AR triggers functional mechanisms to compensate for the overactivation of DA neurons. Such a mechanism might help to explain why M100907 blocks the in vivo consequences of drugs thought to predominantly enhance DA efflux, such as amphetamine (Sorensen et al., 1993; Moser et al., 1996) and the DA reuptake inhibitor GBR 12909 (Carlsson, 1995). On the other hand, further increases in interstitial levels of 5-HT, such as the case following cocaine (present results) or the 5-HT- and DA-releaser MDMA (Schmidt et al., 1992; Kehne et al., 1996b), may be required to uncover a 5-HT_2AR control of stimulated DA neurotransmission. In this case, one could postulate that amphetamine and GBR 12909 might alter interstitial levels of 5-HT, possibly below the limits of detectability in some microdialysis experiments, that might contribute to the ability of 5-HT_2AR to control DA function. To complicate matters further, in addition to 5-HT-evoked increases in DA efflux (Benloucif and Galloway, 1991; Steward et al., 1996; De Deurwaerdere and Spampinato, 1999; Lucas and Spampinato, 2000), DA has also been shown to increase 5-HT release (Matsumoto et al., 1996) and these effects appear to be mediated, at least in part, by stimulation of specific 5-HT and DA receptors, respectively.

The mechanism of action for M100907 to control DA function has been suggested to involve blockade of 5-HT_2AR, which putatively controls DA synthesis under conditions of stimulated DA neurotransmission (Schmidt et al., 1992, 1995; Lucas and Spampinato, 2000). This hypothesis is based upon the fact that the DA precursor l-dihydroxyphenylalanine reversed the inhibitory actions of M100907 on amphetamine-evoked suppression of DA cell firing (Sorensen et al., 1992) and MDMA-induced DA synthesis (Schmidt et al., 1995) and is further strengthened by the reported ineffectiveness of M100907 to reduce hyperactivity induced by direct stimulation of postsynaptic DA receptors (Carlsson, 1995; O'Neill et al., 1999). However, this hypothesis is most likely too simplistic to encompass the ability of M100907 to block cocaine-evoked behavioral effects because cocaine inhibits DA reuptake (Koe, 1976) and decreases DA synthesis for a duration encompassing the behavioral effects of cocaine reported here, presumably via increased DA acting upon D₂-like synthesis-modulating autoreceptors (Galloway, 1990). This also appears to be the case for GBR 12909 (Nissbrandt et al., 1991). Thus, blockade of a facilitatory role of 5-HT_2AR on DA synthesis is unlikely to account for the suppressive effect of M100907 on cocaine- (and GBR 12909)-induced behavior. Despite this incongruity, intracranial microinjection studies from our laboratory show that the ventral tegmental area, but not the nucleus accumbens shell, is a critical locus of action for M100907 to block the systemic effects of cocaine (L. R. McMahon, M. Filip, and K. A. Cunningham, submitted). Thus, we propose that other mechanisms of action, in addition to the control of DA synthesis ascribed to 5-HT_2AR, contribute to 5-HT_2AR-mediated control of DA neurotransmission and that actions at the level of the DA cell body are particularly critical to the interaction between 5-HT_2AR and DA systems.

In summary, the present study is the first to demonstrate an effective antagonism of the hypermotive and discriminative stimulus effects of cocaine by the 5-HT_2AR antagonist M100907; similar results were obtained with the 5-HT₂R antagonist ketanserin. M100907 appeared to selectively attenuate cocaine-induced behavior at doses that did not elicit overt changes in basal behavior. These results suggest that the 5-HT_2AR plays an important role in the behavioral effects of cocaine and that 5-HT_2AR should be considered a viable target for analysis in the search for pharmacotherapies for the treatment of cocaine dependence, particularly in light of a potentially more acceptable side effect profile presented by M100907 than DA receptor antagonists.