Effect of the selective 5-HT7 receptor antagonist SB 269970 in animal models of anxiety and depression
Introduction
It has been well documented that the serotonergic (5-HT) system of the brain is involved in mental illnesses such as anxiety and depression (Blier and de Montigny, 1999, Mann, 1999). The neurotransmitter 5-HT mediates its effects by interacting with seven distinct families of the receptors located pre- and/or postsynaptically (Barnes and Sharp, 1999, Hoyer et al., 2002). However, the role of different 5-HT receptor subtypes in the pathophysiology and treatment of mood diseases has still to be clarified. Recently it was suggested that 5-HT7 receptors (whose biological functions are poorly understood) may play some role in the control of circadian rhythms, sleep, cognitive processes, pain and migraine, as well as in the pathophysiology of many psychiatric disorders including depression and anxiety (Hedlund and Sutcliffe, 2004, Thomas and Hagan, 2004).
The 5-HT7 receptor has been cloned from human, rat, mouse and guinea-pig cDNA libraries and it shows a distinct pharmacological profile, different from that of other 5-HT receptors; successively, four isoforms of the 5-HT7 receptor, all of them being positively coupled to adenylyl cyclase (presumably via Gs) and having analogous pharmacology, have been identified (Barnes and Sharp, 1999). The distribution of 5-HT7 receptor mRNA, the immunolabelling and radioligand binding studies are consistent across species, the highest 5-HT7 receptor densities being found in the thalamus, hypothalamus, hippocampus, frontal cortex and amygdala (Hedlund and Sutcliffe, 2004, Thomas and Hagan, 2004). Neurochemical studies indicate that at least in the hypothalamus the 5-HT7 receptor is located postsynaptically to 5-HT nerve terminals (Clemett et al., 1999, Belenky and Pickard, 2001). Electrophysiological studies show that activation of 5-HT7 receptors increases neuronal excitability in a number of brain regions (Tokarski et al., 2003, Thomas and Hagan, 2004).
Although 5-HT7 receptor selective agonists have not been reported to date, recent development of selective 5-HT7 receptor antagonists and the possibility of using 5-HT7 antisense oligonucleotides, as well as the availability of a 5-HT7 receptor knockout mouse strain make it possible to better test the effects produced by 5-HT7 receptor manipulation in different experimental models. However, conclusive evidence for the 5-HT7 receptor function in vivo is still insufficient. It was demonstrated that the 5-HT7 receptor was involved in the hypothermia induced by the non-selective 5-HT7 agonist 5-carboxytryptamine (5-CT) in guinea-pigs (Hagan et al., 2000) and mice (Guscott et al., 2003); furthermore, it was shown that 5-HT and 5-CT failed to induce hypothermia in 5-HT7 receptor knockout mice (Guscott et al., 2003, Hedlund et al., 2003). Additionally, 5-HT7 knockout mice showed antidepressant-like activity in the Porsolt forced swimming test (Guscott et al., 2005, Hedlund et al., 2005) and in the tail suspension test (Hedlund et al., 2005) as their immobility was reduced compared to wild-type controls. Administration of the 5-HT7 receptor antagonist (R)-3,N-dimethyl-N-[1-methyl-3-(4-methyl-piperidine-1-yl)propyl]benzene-sulfonamide (SB 258719) to wild-type mice caused only a small, non-significant decrease in immobility time in the forced swimming test, measured in the light phase; in the dark phase, however, it induced an effect similar to that observed in knockout mice (Guscott et al., 2005). Recently Hedlund et al. (2005) demonstrated that the 5-HT7 receptor antagonist (2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]-pyrrolidine (SB 269970) exhibited antidepressant-like activity (in the light phase) in both the mouse forced swimming and the tail suspension tests. It is noteworthy that only one dose of either 5-HT7 antagonist was examined (Guscott et al., 2005, Hedlund et al., 2005). In view of the fact that acute restraint stress up-regulates 5-HT7 receptor mRNA in the hippocampus (Yau et al., 2001), and that chronic treatment with antidepressants down-regulates 5-HT7 receptor binding in rat hypothalamus (Sleight et al., 1995, Mullins et al., 1999), the above-quoted results suggest some role of 5-HT7 receptors in depression.
A few papers are available so far on the effect of 5-HT7 receptor manipulation in animal models of anxiety. Treatment with 5-HT7 antisense oligonucleotides had no significant effect on the percentage of either open entries or the time spent in the open arms in the elevated plus-maze in rats (Clemett et al., 1998). In the same model, both wild-type and 5-HT7 receptor knockout mice showed no difference in the time spent exploring the open arms or in the number of entries into the open arms of the maze (Guscott et al., 2005). Similarly, innate anxiety-like behavior, determined by the light/dark transfer test, was not altered in 5-HT7 knockout mice (Roberts et al., 2004a). No data have been available so far on the effect of selective 5-HT7 receptor antagonists in animal models of anxiety.
Therefore the aim of the present study was to investigate the role of SB 269970, a selective 5-HT7 receptor antagonist (Lovell et al., 2000), administered in a wide range of doses (30 min prior to the test), in behavioral models commonly used to predict anxiolytic- and antidepressant-like activities. We used different experimental procedures to determine the potential anxiolytic-like activity of SB 269970: two conflict procedures (the conflict drinking test in rats and the four-plate test in mice) and one exploratory model (the elevated plus-maze test in rats), since parameters recorded in anxiety models may reflect different emotional states (File, 1992, Belzung and Le Pape, 1994, Rodgers, 1997). The forced swimming test and the tail suspension test in mice were used to evaluate antidepressant-like activity. We applied diazepam and imipramine as reference drugs. Moreover, the effect of SB 269970 on the spontaneous locomotor activity of mice was tested. The time schedule of SB 269970 administration was based on pharmacokinetic results described by Hagan et al. (2000). In fact, the latter authors showed that SB 269970 at a dose of 3 mg/kg i.p. reached the highest concentration (87 nM) in rat brain at 30 min post dose time point which would be predicted to attain a substantial occupancy of 5-HT7 receptors. The doses of SB 269970 (0.25–20 mg/kg) used were based on the literature data obtained in in vivo studies with mice and rats (Guscott et al., 2003, Hedlund et al., 2004, Hedlund et al., 2005), as well as on the findings of a pilot experiment.
SB 269970 was shown to be a potent ligand of human cloned (Lovell et al., 2000) and guinea-pig (Hagan et al., 2000) 5-HT7 receptors (pKi = 8.9 and 8.7, respectively). It had excellent selectivity (>250-fold) over 5-HT1, 5-HT2, 5-HT4, 5-HT6, α1, D2 and D3 receptors, apart from 5-HT5A ones (50-fold). Furthermore, in a commercial screening package, SB 269970 was found to be over 100-fold selective over a total of 50 receptors, enzymes or ion channels (Lovell et al., 2000). Pharmacokinetic studies demonstrated that SB 269970 had good central nervous system penetration (Hagan et al., 2000). SB 269970 showed features of a 5-HT7 receptor antagonist. Indeed, it inhibited the activity of adenylyl cyclase, stimulated by the non-selective 5-HT7 receptor agonist 5-CT (Lovell et al., 2000) or by 5-HT (Mahé et al., 2004). Also in electrophysiological assays, SB 269970 blocked the excitability evoked by 5-CT in the CA1 (Tokarski et al., 2003) and CA3 (Gill et al., 2002) regions of rat hippocampal slices, as well as the 5-HT-induced inward current in neurons of the anterodorsal nucleus of rat thalamus (Chapin and Andrade, 2001). Furthermore, in vivo functional studies indicated that SB 269970 antagonized the 5-CT-evoked hypothermia in guinea-pigs (Hagan et al., 2000) and mice (Guscott et al., 2003).
Section snippets
Subjects
The experiments were performed on male Wistar rats (250–300 g), male Albino Swiss mice (24–28 g), and male C57BL/6J mice (23–24 g). The animals were kept in groups of eight (rats) or twenty (mice) to a cage (60 × 38 × 20 cm) at a temperature of 20 ± 1 °C, and had free access to food (standard laboratory pellets) and water. All the experimental procedures were approved by the Local Bioethics Commission at the Institute of Pharmacology, Polish Academy of Sciences in Kraków.
Experimental procedures
All the experiments were conducted
Conflict drinking test (Vogel test) in rats
Fig. 1 (left panel) shows that SB 269970 in a dose of 1 mg/kg (but not 0.5 or 2.5 mg/kg) exerted anxiolytic-like activity having significantly increased (by 211%, F3,30 = 6.639, P < 0.01) the number of shocks accepted during experimental sessions in the Vogel test. Concurrently diazepam administered in doses of 5 and 10 mg/kg (but not 2.5 mg/kg) significantly (F3,28 = 16.956, P < 0.001) increased number of accepted shocks, by 367 and 448%, respectively (Fig. 1, right panel).
Shock threshold and free-drinking tests in rats
The compounds tested in doses
Discussion
The major finding of the present study was to demonstrate that SB 269970, a 5-HT7 receptor antagonist, produced an anxiolytic-like effect in all the three behavioral models used: rat conflict drinking test (Vogel et al., 1971), rat elevated plus-maze test (Pellow and File, 1986) and mouse four-plate test (Aron et al., 1971). To the best of our knowledge that is the first direct body of evidence for an antianxiety-like effect following administration of a 5-HT7 receptor antagonist. In this study
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