Effects of Antipsychotic Drugs on Extracellular Dopamine Levels in Rat Medial Prefrontal Cortex and Nucleus Accumbens1

The present study was designed to compare the effects of typical and atypical antipsychotic drugs on extracellular dopamine (DA) levels in the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAC), using in vivo microdialysis with dual probe implantation in awake, freely moving rats. Amperozide (2 and 10 mg/kg), clozapine (5 and 20 mg/kg), and olanzapine (10 mg/kg), all of which are atypical antipsychotics, produced greater increases in extracellular DA levels in the mPFC than in the NAC. Olanzapine (1 mg/kg), risperidone (0.1 and 1 mg/kg), also an atypical antipsychotic, and S-(2)-sulpiride (25 mg/kg), a typical antipsychotic, produced comparable increases in extracellular DA levels in the mPFC and the NAC. S-(2)-sulpiride (10 mg/kg) and haloperidol (0.1 and 1 mg/kg), another typical antipsychotic, significantly increased extracellular DA levels in the NAC but not in the mPFC. The effects of the six antipsychotic drugs to increase extracellular DA levels in the mPFC relative to those in the NAC was positively correlated with the difference between their pKi values for serotonin (5-hydroxytryptamine, 5-HT2A) and DA-D2 receptors and was inversely correlated to their pKi values for D2 or D3 receptors, but was not for 5-HT2A receptors alone. These results are consistent with the hypothesis that the ability of antipsychotic drugs to produce a greater increase in prefrontal compared with NAC extracellular DA levels may be related, in part, to weak D2 and D3 receptor affinity relative to 5-HT2A receptor antagonism. Antipsychotic drugs such as haloperidol (HAL) and S-(2)sulpiride (SUL) increase dopamine (DA) release in the striatum (STR) and the nucleus accumbens (NAC), most likely due to the blockade of presynaptic DA-D2-like autoreceptors (Westerink and DeVries, 1989). The atypical antipsychotics clozapine (CLOZ) and amperozide (APZ), both of which have greater affinities for serotonin (5-hydroxytryptamine, 5-HT)2A receptors relative to D2 receptors (Meltzer et al., 1989; Leysen et al., 1993), have been reported to produce greater increases in extracellular DA levels in the medial prefrontal cortex (mPFC) compared with the STR and the NAC (Moghaddam and Bunney, 1990; Nomikos et al., 1994; Pehek and Yamamoto, 1994; Volonté et al., 1997). By contrast, HAL has a limited effect on extracellular DA levels in the mPFC. Compounds like CLOZ and APZ that are relatively more potent as 5-HT2A than as D2 receptor antagonists, e.g., melperone, olanzapine (OLAN), risperidone (RISP), quetiapine, sertindole, and ziprasidone (Meltzer et al., 1989; Leysen et al., 1993), have been commonly referred to 5-HT2A/D2 receptor antagonists (Meltzer, 1995). Therefore, it is of interest to test the hypothesis that all antipsychotic drugs of the 5-HT2A/D2 receptor antagonist type produce a greater increase in extracellular DA levels in the mPFC compared with the NAC. Dopaminergic hypofunction in the prefrontal cortex has been suggested to be related to the etiology of negative symptoms (Davis et al., 1991; Weinberger and Lipska, 1995) and cognitive dysfunction of schizophrenia (Sawaguchi and Goldman-Rakic, 1991). A number of the 5-HT2A/D2 receptor antagonists, e.g., CLOZ, RISP, OLAN, and sertindole, have been reported to have a greater ability to improve negative symptoms than do neuroleptics such as HAL or chlorpromazine (Kane et al., 1988; Chouinard et al., 1993; Meltzer, 1995; Tollefson and Sanger, 1997). CLOZ and RISP also improve some aspects of cognitive function (Hagger et al., 1993; Green Received for publication April 10, 1998. 1 This work was supported in part by U.S. Public Health Services Grant MH 41684, Department of Veterans Affairs grant GCRC MO1RR00080, and the National Alliance for Research on Schizophrenia and Depression (NARSAD) Young Investigator Award grant, as well as grants from Elisabeth Severance Prentiss Foundation and Mr. Larry Freedman. Preliminary data from this study have been reported in abstract forms of the annual meetings of the Society for Neuroscience, 1995, and the Society of Biological Psychiatry, 1996. ABBREVIATIONS: HAL, haloperidol; SUL, S-(2)-sulpiride; DA, dopamine; STR, striatum; NAC, nucleus accumbens; CLOZ, clozapine; APZ, amperozide; mPFC, medial prefrontal cortex; OLAN, olanzapine; RISP, risperidone; 5-HT, 5-hydroxytryptamine; AUC, area under the curve; VEH, vehicle control. 0022-3565/99/2882-0774$03.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 288, No. 2 Copyright © 1999 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. JPET 288:774–781, 1999 774 at A PE T Jornals on M ay 0, 2017 jpet.asjournals.org D ow nladed from et al., 1997) which may be related to frontal lobe function. Thus, the greater ability of 5-HT2A/D2 receptor antagonists to increase extracellular DA levels in the cortical regions compared with subcortical regions may contribute to their ability to improve negative symptoms (Deutch et al., 1991). This hypothesis is consistent with preferential in vivo and in vitro binding of 5-HT2A/D2 receptor antagonists to cortical 5-HT2A compared with striatal D2 receptors (Meltzer et al., 1989; Leysen et al., 1993; Stockmeier et al., 1993). Recently, evidence for the modulation of the activity of mesolimbocortical DA neurons by 5-HT2A receptors has been reported. Systemic administration of the 5-HT2A/2C receptor antagonist ritanserin has been demonstrated to enhance the activity of midbrain DA neurons (Ugedo et al., 1989). Ritanserin has also been reported to potentiate the ability of the selective D2/3 receptor antagonist raclopride to increase DA release in the mPFC but not in the STR, whereas ritanserin alone had no effect on DA release in either region (Andersson et al., 1995). These data suggest that the 5-HT2A receptor antagonism, together with blockade of D2-like receptors (D2, D3, and perhaps D4 subtype), may contribute to the preferential increase in extracellular DA levels in the mPFC compared with the NAC and the STR produced by APZ and CLOZ. However, this hypothesis is not consistent with the reports that RISP and OLAN, which are more potent as 5-HT2A than as D2 receptor antagonists (Bymaster et al., 1996a; Schotte et al., 1996), produced comparable increases in extracellular DA levels in the mPFC and the NAC or the STR (Hertel et al., 1996; Volonté et al., 1997). The present study was designed to characterize antipsychotic drugs of the 5-HT2A/D2 receptor antagonist type in terms of their relative ability to produce a greater increase in extracellular DA levels in the mPFC compared with the NAC. Importantly, antipsychotic drugs differ in their affinities for D1, D3, D4, 5-HT2C, 5-HT6, and 5-HT7 receptors (Meltzer et al., 1989; Roth et al., 1992; Schotte et al., 1996). These affinities may also influence their effects on extracellular DA levels in the mPFC, the NAC, and the STR. Therefore, this study also investigated the relationships between relative potency to increase extracellular DA levels and in vivo affinities for multiple 5-HT and DA receptors as indicated by pKi values obtained from published sources (Table 1). Six antipsychotic drugs were chosen on the basis of the difference in in vitro and in vivo affinity between rat cortical 5-HT2A and striatal D2 receptors in the following rank order: APZ . CLOZ . RISP . OLAN . HAL . SUL. Materials and Methods Animals. Male Sprague-Dawley albino rats (Zivic-Miller, Pittsburgh, PA) weighing 250 to 300 g were used throughout the study. Rats were housed two or three per cage and were maintained on a 12-h light/dark cycle and under constant temperature at 22°C, with ad libitum access to food and water. Surgery and Microdialysis. Rats were anesthetized with a combination of xylazine (Rompun, 6 mg/kg, i.p.; Miles, Kansas City, KS) and ketamine hydrochloride (Ketaset, 70 mg/kg, i.p.; Fort Dodge Lab., Fort Dodge, IA) and mounted in a stereotaxic frame (David Kopf Instruments, Tujunga, CA). Two stainless guide cannula (21 gauge) with a dummy probe were placed and fixed by cranioplastic cement onto the cortex dorsal both to the right mPFC and the left NAC (dual probe implantation). Stereotaxic coordinates of each probe when implanted were A 13.2, L 10.8 (10° inclination), V 25.5 mm for the mPFC and A 12.0, L 11.7, V 27.5 mm for the NAC, respectively, relative to bregma; incision bar level: 23.0 mm, according to the atlas of Paxinos and Watson (1986). Microdialysis probes were constructed in our laboratory according to the method of Robinson and Whishaw (1988). Briefly, the probe was a concentric design consisting of 25-gauge stainless steel tubing (460 mm o.d.) connected to PE20 tubing as an inlet. The outlet consisted of a fused silica capillary (150 mm o.d.; Polymicro Technologies, Phoenix, AZ) passed through the wall of the PE20, threaded down through the stainless steel tubing and into a 2-mm length of sealed dialysis tubing (polyacrylonitrile/sodium methalylsulfonate polymer, 310 mm o.d., 220 mm i.d., 40,000 Da cutoff, AN69HF, Hospal; CGH Medical, Lakewood, CO). PE10 tubing was attached to the silica capillary where it exits the probe. All junctions were glued with 2-ton Epoxy (Devon Bearings, Brooklyn Heights, OH). Because the variability in percentage of recovery of DA between probes varied by less than 10%, basal extracellular DA levels in each region were expressed as absolute values and were not corrected by percentage of