Effects of the 5-HT2C/2B Antagonist SB 206553 on Hyperactivity Induced by Cocaine Andrew C. McCreary, Ph.D., and Kathryn A. Cunningham, Ph.D. Serotonin (5-HT) appears to play a modulatory role in the However, the 4-mg/kg dose of SB 206553 significantly behavioral effects of cocaine, although the impact of 5-HT2C enhanced the effects of cocaine on peripheral activity. Based receptors in this control has not been fully established. The upon the present observations and an interpretation of aim of the present study was to establish whether acute previous research to implicate 5-HT2C receptor control of pretreatment with the selective 5-HT2C/2B antagonist SB the dopamine (DA) mesoaccumbens pathways in behavior, 206553 (1, 2, and 4 mg/kg IP) altered hyperactivity induced a thorough and systematic analysis of the role of 5-HT2C by cocaine (15 mg/kg, IP) using an open field activity (and 5-HT2B) receptors in psychostimulant-induced system which recorded central, peripheral, and rearing behaviors is warranted. [Neuropsychopharmacology activity. Pretreatment with 1 and 2 mg/kg of SB 206553 20:556–564, 1999] © 1999 American College of attenuated cocaine-induced central and peripheral activity, Neuropsychopharmacology. Published by Elsevier respectively; rearing was also attenuated by the latter dose. Science Inc. KEY WORDS: Cocaine; Hyperactivity; SB 206553; Serotonin; vated DA neurotransmission, particularly within the 5-HT2C; 5-HT2B mesoaccumbens pathway originating from DA soma in the ventral tegmental area (VTA) and terminating in the The behavioral effects of cocaine in rodents include the nucleus accumbens (NAc), is especially important in stimulation of locomotor activity and stereotypy as well mediating the locomotor, discriminative stimulus and as convulsions at higher doses (Scheel-Kruger et al. reinforcing effects of cocaine (for review, Amalric and 1977; McCreary and Marsden 1993). Cocaine also serves Koob 1993). as a positive reinforcer for operant behavior (de Wit Despite the pronounced involvement of DA in its in and Wise 1977) and elicits interoceptive stimulus effects vivo effects, cocaine is not a selective DA reuptake in- easily discriminable from saline (Callahan et al. 1994). hibitor as it binds to 5-hydroxytryptamine (5-HT) and The locomotor stimulant effects have been linked to the norepinephrine (NE) transporters and inhibits the re- inhibition of dopamine (DA) reuptake (Koe 1976) and a uptake of 5-HT and NE with at least equal potency and subsequent enhancement of DA activation via both DA efficacy as that of DA reuptake (Koe 1976). Recent stud- D and D receptors (Scheel-Kruger et al. 1977; Mc- 1 2 ies of the locomotor stimulant, reinforcing and stimulus Creary and Marsden 1993; White et al. 1998). The ele- effects of cocaine suggest that enhanced 5-HT transmis- sion and subsequent stimulation of 5-HT receptors play a modulatory role in the in vivo effects of cocaine (for re- views, see Cunningham and Callahan 1994; Walsh and From the Department of Pharmacology and Toxicology, Univer- sity of Texas Medical Branch, Galveston, Texas, USA Cunningham 1997). Address correspondence to: Dr. K. A. Cunningham, Department The exact contribution of specific 5-HT receptors to of Pharmacology and Toxicology, University of Texas Medical the behavioral profile of cocaine has not yet been fully Branch, Galveston, TX 77555-1031, USA. Received May 13, 1998; revised August 20, 1998; accepted Sep- elucidated, largely due to the multiplicity of 5-HT re- tember 3, 1998. ceptors in mammalian systems and the lack of selective NEUROPSYCHOPHARMACOLOGY 1999–VOL. 20, NO. 6 © 1999 American College of Neuropsychopharmacology Published by Elsevier Science Inc. 0893-133X/99/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0893-133X(98)00087-6 NEUROPSYCHOPHARMACOLOGY 1999–VOL. 20, NO. 6 5-HT2C Receptors and Cocaine Hyperactivity 557 ligands for each of the respective receptor subtypes. At antagonists have been shown to enhance cocaine-induced least 14 5-HT receptor subtypes have been shown to ex- hyperactivity (Scheel-Kruger et al. 1977; Berman et al. ist, although the operational characteristics are un- 1982; Herges and Taylor 1998; but see Reith 1990 and known for some receptors (Hoyer and Martin 1997). Peltier et al. 1994). Since 5-HT2C antagonists appear to The 5-HT2C receptor subtype (formerly 5-HT1C) has re- increase the activity of a subpopulation of VTA DA ceived attention as a possible therapeutic target in psy- neurons (Prisco et al. 1994; Prisco and Esposito 1995; chiatric conditions such as psychoses and anxiety (Ken- Ashby and Minabe 1996; Lejeune et al. 1997) and 5-HT2C nett 1993) and evidence suggests that this receptor is agonists induce hypomotility (Lucki et al. 1989; Kennett important in the serotonergic regulation of the me- 1993; Kennett et al. 1996), a 5-HT2C antagonist would be solimbic DA system. The transcript for the 5-HT2C re- predicted to increase the locomotor activation observed ceptor is richly expressed in DA somatic and terminal after cocaine administration. To test this hypothesis, we field regions, including the VTA and NAc (Hoffman assessed the ability of the 5-HT2C/2B antagonist SB 206553 and Mezey 1989; Eberle-Wang et al. 1997). Although (Forbes et al. 1995; Kennett et al. 1996) to modulate not yet established for the VTA, 5-HT2C mRNA was lo- spontaneous and acute cocaine-induced locomotor acti- calized in g-aminobutyric acid (GABA), but not DA, vation. The doses of SB 206553 (1–4 mg/kg) chosen for neurons in the substantia nigra suggesting that the in- this study encompass those doses that effectively in- 5 fluence of 5-HT2C receptors on DA cell bodies may be hibit mCPP-induced hypermotility (ID50 1.1 mg/kg; indirect (Eberle-Wang et al. 1997). In keeping with the G. Kennett, personal communication), increase cell fir- distribution of mRNA, ligand binding studies have ing of DA VTA neurons (Lejeune et al. 1997) and in- demonstrated labeling of 5-HT2C receptors in the VTA crease DA release in the NAc (Lejeune et al 1997; Spam- and NAc (Pazos and Palacios 1985; Mengod et al. 1990). pinato et al. 1997). Mesolimbic DA circuits appear to be under a modu- latory influence of 5-HT2C receptors at the level of both the VTA and NAc. For example, systemic and micro- iontophoretic administration of the 5-HT2C/1B agonist MATERIALS AND METHODS meta-chlorophenylpiperazine (mCPP) decreased the firing rate of VTA DA neurons; the systemic actions of Animals mCPP were antagonized by the 5-HT2/1A antagonist Male Sprague–Dawley rats (Harlan Sprague–Dawley, mesulergine, which alone increased firing rates of a Indianapolis, IN, USA) weighing 225–250 g were housed subpopulation of these cells (Prisco et al. 1994; Prisco (4 rats/cage) for 1 week prior to experimentation in a and Esposito 1995). Although 5-HT2C/2B/1B ligands temperature (21 6 28C) and humidity (40–50%) con- failed to alter the firing characteristics of DA neurons in trolled environment with food and tap water available the substantia nigra (Kelland et al. 1990; but see Stan- ad libitum, except during experimental sessions. Light- ford and Lacey 1996), the activity of VTA DA neurons ing was maintained under a 12-hr light-dark cycle (lights is reported to increase following administration of the on 07:00–19:00 hrs). All experimental procedures were 5-HT2C/2B antagonists SB 200646A (Ashby and Minabe performed between 08:00 and 14:00 hrs. 1996) or SB 206553 (Lejeune et al. 1997). These results suggest that 5-HT2C/2B receptors may tonically inhibit cell firing of DA neurons in the VTA. In keeping with this possibility, preliminary microdialysis studies have Apparatus shown that systemic injection of the 5-HT2C/2B antago- nist SB 206553 (Lejeune et al. 1997; Spampinato et al. Behavioral activity was monitored and quantified using 1997) or the 5-HT2/1A receptor antagonist mesulergine an open field activity system (San Diego Instruments, (Spampinato et al. 1997) enhanced basal DA overflow San Diego, CA, USA). Housed within sound attenuat- in the NAc. ing enclosures, each chamber was a 40 cm 3 40 cm 3 40 3 Consonant with the observation that 5-HT2C recep- cm clear Plexiglas enclosure containing a 4 4 photo- tor activation may dampen DA activity in mesolimbic beam matrix located 4 cm from the floor surface; inter- circuits, pretreatment with the 5-HT2C/2B/1B agonists ruptions of these photobeams resulted in counts of mCPP and MK 212 effectively attenuated the stimulus activity in the peripheral and central fields of the cham- effects of cocaine (Callahan and Cunningham 1995). ber. Another horizontal row of 16 photobeams located Both mCPP (Nader and Barrett 1990) and the 5-HT2 an- 16 cm from the floor provided a measure of rearing tagonist ritanserin (Meert and Janssen 1992; Howell and activity. Video cameras, located above the chambers, Byrd 1995) altered rates of responding for cocaine in provided the ability to observe activity of animals self-administration tasks. The ability of selective 5-HT2C continuously without disrupting behavior. Separate antagonists to affect hyperactivity induced by cocaine counts of peripheral, central, and rearing activity were has not been analyzed, however, non-selective 5-HT1/2 made by the control software (Photobeam Activity 558 A.C. McCreary and K.A. Cunningham NEUROPSYCHOPHARMACOLOGY 1999–VOL. 20, NO. 6 Software, San Diego Instruments, San Diego, CA, USA) RESULTS and stored for subsequent statistical evaluation. Effects of SB 206553 (1 mg/kg) on Basal and Cocaine-induced Locomotor Activity Behavioral Procedures A significant main effect of pretreatment was observed 5 , 5 Three separate groups of rats (n 5 8 rats/group) were for central (F(3,28) 19.60, p .0001), peripheral (F(3,28) , 5 , habituated to the activity monitors for 2 hr per day on 21.93, p .0001), and rearing activity (F(3,28) 4.01, p the 2 days prior to the start of the experiment.