Generalization of serotonin (5-HT)_1A agonists and the antipsychotics, clozapine, ziprasidone and S16924, but not haloperidol, to the discriminative stimuli elicited by PD128,907 and 7-OH-DPAT

Abstract

Rats were trained to recognize a discriminative stimulus (DS) elicited by the dopamine D₂/D₃ receptor agonist, PD128,907 (0.16 mg/kg, i.p.), which suppressed frontocortical release of dopamine (DA) but not 5-HT. The selective 5-HT_1A receptor agonists, 8-OH-DPAT and flesinoxan, dose-dependently generalized to PD128,907 with effective dose₅₀s (ED₅₀s) of 0.08 and 1.5 mg/kg, s.c., respectively, and inhibited the release and synthesis of 5-HT but not of DA. The ‘atypical’ antipsychotic, clozapine, which displays weak partial agonist properties at 5-HT_1A receptors, dose-dependently, though partially, generalized to PD128,907 (50%, 2.5 mg/kg, s.c.). Further, S16924 and ziprasidone, which in a like manner, display partial agonist activity at 5-HT_1A receptors, generalized with ED₅₀s of 0.6 and 2.3 mg/kg, s.c., respectively. In contrast, haloperidol, which is devoid of affinity at 5-HT_1A sites, was inactive. At doses equivalent to those generalizing to PD128,907, clozapine, S16924 and ziprasidone reduced serotonergic (but not dopaminergic) transmission, whereas haloperidol was inactive. In rats trained to recognize a further D₂/D₃ agonist, 7-OH-DPAT (0.16 mg/kg, i.p.), generalization was obtained similarly with 8-OH-DPAT (ED₅₀=0.07 mg/kg, s.c.), flesinoxan (3.4) and clozapine (0.6), but not with haloperidol. In conclusion, although PD128,907 and 7-OH-DPAT do not directly interact with 5-HT_1A receptors or influence serotonergic transmission, their DS properties are mimicked by 5-HT_1A receptor agonists at doses activating 5-HT_1A but not D₂/D₃ (auto)receptors. These observations likely account for generalization of clozapine, S16924 and ziprasidone to PD128,907 and 7-OH-DPAT inasmuch as they behave as antagonists at D₂/D₃ receptors, yet agonists at 5-HT_1A (auto)receptors.

Introduction

Drug discrimination (DD) procedures provide an important tool for the characterization of mechanisms underlying interoceptive properties of psychoactive drugs (Goudie and Leathley, 1993, Goudie and Taylor, 1998, Millan et al., 1999). DD procedures have also been extensively employed in the characterization of drugs interacting with specific receptor types, and dopaminergic agonists elicit robust DS, probably via actions at autoreceptors on dopaminergic neurones (Kleven and Koek, 1997, Bristow et al., 1998, Millan et al., 2000a). However, the discovery that dopaminergic agonists interact with D₃ as well as D₂ receptors (Levant, 1997) questions their respective DS properties.

In this light, several investigations have evaluated the contribution of D₃ versus D₂ receptors to DS elicited by 7-OH-DPAT and PD128,907, which interact preferentially with D₃ versus D₂ sites (Newman-Tancredi et al., 1999b, Levant, 1997). Sanger et al. (1997) and Varty and Higgins (1997) suggested, on the basis of correlation analyses, that D₃ receptors mediate the DS properties of 7-OH-DPAT. However, subsequent studies employing both 7-OH-DPAT and PD128,907 observed that selective antagonists at D₃ receptors do not block their DS properties, indicating that D₂ sites are probably involved (Baker et al., 1999, Bristow et al., 1998, Depoortere et al., 2000, Millan et al., 2000a). A predominant role of D₂ sites is in line with the studies suggesting that these agonists are less selective for D₃ versus D₂ sites than originally claimed, and that D₂ receptors play a major role in their actions in vivo (see Levant, 1997).

Pertaining to the selectivity of 7-OH-DPAT, it possesses partial agonist properties at cloned, human (h)5-HT_1A receptors while, via activation of 5-HT_1A autoreceptors, it inhibits dorsal raphe nucleus (DRN)-localized serotonergic neurones (Lejeune et al., 1997). These actions of 7-OH-DPAT are seen only at doses much higher than those engaging D₂/D₃ receptors, and PD128,907 has not been reported to significantly interact with 5-HT_1A receptors. Nevertheless, other observations suggest an indirect interrelationship between the DS properties of PD128,907 and 7-OH-DPAT and 5-HT_1A receptors. There is, thus, an extensive pattern of reciprocal, functional interactions amongst corticolimbic dopaminergic and serotonergic pathways, in which both D₂/D₃ and 5-HT_1A autoreceptors are implicated (Millan et al., 2000a, Millan et al., 2000b). For example, dopaminergic pathways may, via D₂ receptors, modulate the activity of serotonergic neurones (Adell and Artigas, 1999, Mendlin et al., 1999, Nakazato et al., 1998, Thorré et al., 1998). Further, serotonergic neurones exert a tonic, inhibitory influence on mesocortical dopaminergic pathways, relief of which by the activation of 5-HT_1A autoreceptors reinforces DA release in frontal cortex (FCX) (Ichikawa and Meltzer, 1999, Millan et al., 1998, Millan et al., 2000b). Correspondingly, certain functional actions of 5-HT_1A agonists are mediated by dopaminergic mechanisms (Ichikawa and Meltzer, 1999, Lejeune et al., 1997). Further, 5-HT_1A receptor agonists modify the influence of selective D₂ receptor antagonists on mood and motor behaviour (Prinssen et al., 1999, Walsh and Cunningham, 1997). Notably, they modulate the interoceptive properties of both cocaine and amphetamine (Munzar et al., 1999, Nader and Woolverton, 1994, Walsh and Cunningham, 1997). Notwithstanding these observations there has been, to date, no systematic examination of potential actions of 5-HT_1A receptor ligands in rats discriminating PD128,907 or 7-OH-DPAT. This question was examined herein. The approach adopted was as follows.

Firstly, we determined the ability of the selective 5-HT_1A agonists, 8-OH-DPAT and flesinoxan (Glennon, 1986, Schreiber et al., 1995, Tricklebank, 1987, Ybema et al., 1994), to generalize to 7-OH-DPAT and PD128,907. In addition, the influence of the selective 5-HT_1A antagonist, WAY100,635, on DS properties of PD128,907 and 7-OH-DPAT was examined. Secondly, potential generalization was also evaluated with a further high-efficacy 5-HT_1A receptor agonist, roxindole, which shows partial agonist properties at D₂ and D₃ receptors (Newman-Tancredi et al., 1999a). Interestingly, roxindole is a potential antipsychotic agent (Seyfried et al., 1989) and the atypical antipsychotic, clozapine, exerts partial agonist properties at 5-HT_1A receptors at doses similar to those blocking the D₂ receptors (Millan et al., 1998, Newman-Tancredi et al., 1996). Further, ziprasidone and the putative antipsychotic agent, S16924, likewise behave as partial agonists at 5-HT_1A receptors (Millan et al., 1999, Seeger et al., 1995, Sprouse et al., 1999). Thirdly, we examined their ability to generalize to PD128,907 and 7-OH-DPAT. Their actions were compared to those of the neuroleptic, haloperidol, which is devoid of affinity for 5-HT_1A sites (Millan et al., 1998). Fourthly, in independent studies, we monitored the influence of PD128,907 and 7-OH-DPAT on release of DA versus 5-HT, which is subjected to inhibition by the activation of D₂/D₃ and 5-HT_1A autoreceptors, respectively (Millan et al., 2000b). Thus, extracellular levels of DA and 5-HT were quantified in dialysates of the FCX of freely moving rats. Although precise neuronal substrates underlying DS properties of PD128,907 and 7-OH-DPAT are not currently known, the FCX was selected for these studies as the influence of D₂/D₃ and 5-HT_1A agonists (as well as antipsychotic drugs) on DA and 5-HT release in this structure are well-characterized (Millan et al., 2000b), and because the FCX plays an important role in the actions of antipsychotic agents. Moreover, if D₂ autoreceptors are involved in DS properties of dopaminergic agonists (Bristow et al., 1998, Millan et al., 2000b), the FCX is an appropriate structure to monitor changes in DA release caused by their engagement. The influence of 5-HT_1A agonists and antipsychotic agents on DA and 5-HT synthesis in striatum (DA and 5-HT) and hippocampus (5-HT) was also investigated. Finally, the actions of the above-mentioned drugs were interpreted in the light of their respective affinities and efficacies at native, rat and cloned, human 5-HT_1A, D₂ and D₃ receptors (Levant, 1997, Millan et al., 2000b) as established in our previous studies (Table 1).