The aim of the present study was to investigate the potential antidepressant-like activity of a group II mGlu receptor antagonist, MGS0039, in the OB model of depression in rats and to assess the mechanism of antidepressant-like activity in the TST in C57BL/6J mice.
A surgical lesion of the olfactory bulbs in animals induces significant behavioral, physiological, endocrine and immune changes, many of which were qualitatively similar to those observed in depressive patients (for review see Kelly et al. 1997
). In animal studies, a variety of OB-related behavioral changes, including hyperactivity in the “open field” and deficiency in passive avoidance, responded selectively to antidepressant treatment. Hyperactivity in the “open field” always responds to chronic treatment with antidepressants, mimicking the clinical lag-time of currently used antidepressant drugs (Harkin et al. 2003
). We used both tests to evaluate a potential antidepressant-like effect of MGS0039 in the OB model of depression in rats. Repeated administration of MGS0039 attenuated the hyperactivity of OB rats in the open field test and attenuated the learning deficit in the passive avoidance experiment, in the manner similar to that seen following chronic treatment with a typical antidepressant, amitriptyline, which was used for positive control. We also found that the administration of group II mGlu receptor antagonist did not result in any behavioral changes in the sham-operated groups, indicating that its effect in OB rats was not due to a stimulant or sedative effect of that compound. MGS0039-induced changes in the behavior of OB rats may be additional evidence for the support of antidepressant-like activity of group II mGlu receptor antagonists. However, it must be remembered, that agonists of group II mGlu receptors were shown to produce a number of desirable effects, such as anti-seizures, neuroprotective, and antipsychotic effects (Conn et al. 2009
; Flor et al. 2002
; Klodzinska et al. 2000
; Ure et al. 2006
). As such, the application of antagonists of mGlu2/3 receptors may carry a risk of undesirable side effects and must be further investigated.
To investigate the mechanism of the antidepressant-like activity of MGS0039, the TST was chosen in this study, since it is widely used to determine behavioral effects of typical antidepressant drugs influencing the serotonergic, noradrenergic, and dopaminergic systems, as well as atypical and new potential antidepressants, including mGlu receptor ligands (Cryan et al. 2005
). Here, we observed the antidepressant-like activity of MGS0039 and another group II mGlu receptor antagonist LY341495, after peripheral administration in the TST in mice. Although the potential antidepressant-like activity of group II mGlu receptor antagonists seems to be well established, the mechanism by which these compounds exhibit such effects is still unclear. However, some data indicates that it may be related to serotonergic system regulation (see “Introduction
”), thus resembling the mechanism of action of typical antidepressants such as SSRIs.
Therefore, we aimed to clarify the role of the serotonergic system in the antidepressant-like action of mGlu2/3 receptor antagonists in the TST. Firstly, we investigated the influence of the pretreatment of mice with specific serotonergic receptors antagonists, such as the 5HT1A receptor antagonist, WAY100635 (Forster et al. 1995
), and 5HT2A/2C receptor antagonist, ritanserin (Hoyer et al. 1994
), on the antidepressant-like activity of MGS0039 or LY341495 in the TST. Secondly, an influence of MGS0039 on the immobility time in the TST was measured in C57BL/6J mice after the pharmacological depletion of serotonin.
Several lines of evidence indicate the crucial role of both 5HT1A and 5HT2A receptors in the regulation of serotonergic neurotransmission and the involvement of these receptors in the mechanism of action of many classes of antidepressant drugs, including SSRI, MAOI, and tricyclics (Hensler 2002
; Middlemiss et al. 2002
). It was also shown that pretreatment with antagonists of 5HT1A and 5HT2A receptors antagonized antidepressant-like activity of SSRIs in the TST in mice (Miyata et al. 2004
), suggesting that activation of both receptors plays a key role in the action of compounds acting via activation of the serotonergic system in this test. The results of our studies show that pretreatment of mice with WAY100635 or ritanserin did not influence an antidepressant-like action of MGS0039 in the TST. Similarly, the antidepressant-like activity of another group II mGlu receptor antagonist, LY341495, was not changed in mice pretreated with WAY100635 or ritanserin. Furthermore, the antidepressant-like activity of LY341495 was not blocked by metergoline, which is an antagonist of a wide spectrum of serotonergic receptors, including 5HT1, 5HT2, 5HT1D, 5HT6, and 5HT7 (Hoyer et al. 1994
). These results suggest that activation of the serotonergic system is not required in the antidepressant-like action of group II mGlu receptor antagonists.
It has been shown that 5HT1A antagonists enhance antidepressant-like activity of antidepressants acting via the activation of the serotonergic system, when given at low, sub-effective doses (Artigas et al. 1996
). This effect resulted probably from a blockade of 5HT1A autoreceptors by 5HT1A antagonists and a subsequent increase in the level of serotonin. Thus, the antagonists of 5HT1A receptors synergistically potentiate the serotonin level, increased by SSRIs (Romero et al. 1996
). Our study shows that the action of the group II mGlu receptors antagonist, MGS0039, was not significantly potentiated by an antagonist of the 5HT1A receptor, WAY100635, further confirming that the mechanism of the antidepressant-like activity of MGS0039 was not serotonin-dependent.
Locomotor activity studies showed that MGS0039, WAY100635, or ritanserin given by themselves did not change this parameter in mice, confirming that the antidepressant-like action of MGS0039 was not due to hyperlocomotion. However, in mice pretreated with WAY100635 or ritanserin, MGS0039 significantly diminished locomotor activity, suggesting that the lack of influence of serotonergic blockers on the action of MGS0039 in the TST was not a false negative effect resulting from their influence on the hyperlocomotion. On the other hand, our locomotor activity results suggest the existence of interaction between group II mGlu receptors and serotonin 5HT1A and 5HT2A receptors. Functional and structural interaction between mGlu2 and 5HT2A receptors is well documented, and it is believed to be implicated in psychosis (González-Maeso et al. 2008
; Kłodzińska et al. 2002
; Marek et al. 2000
). Moreover, the anxiolytic-like activity of mGlu receptor ligands was shown to be dependent on the activation of the serotonergic receptors; e.g., the anxiolytic-like action of mGlu5 receptor antagonist, MTEP, or group III mGlu receptor antagonist, CPPG, were blocked by 5HT2A/2C receptor antagonist, ritanserin, while group III mGlu receptor agonist, ACPT-I, did not display anxiolytic-like activity in mice pretreated with 5HT1A receptor antagonist, WAY100635 (Stachowicz et al. 2007a
), suggesting a functional interaction between group I or III mGlu receptors and serotonergic 5HT1A and 5HT2A/2C receptors in anxiety-related mechanisms. Although our locomotor activity results indicate on interaction between group II mGlu receptors and 5HT1A or 5HT2A/2C receptors, it seems that this interaction did not play a role in the antidepressant-like activity of the glutamatergic and serotonergic antagonists tested in the TST.
It is worth mentioning that our results, showing no changes in the locomotor activity of mice after administration of group II mGlu receptor antagonists, seem to be opposite to other studies where it was reported that the group II mGlu receptor antagonist LY341495 increased the locomotor activity of mice (Bespalov et al. 2007
; O'Neill et al. 2003
). However, in these studies, an influence of the compound on the locomotor activity of habituated animals was investigated. In our studies, animals were given an injection of MGS0039 in a home cage, and 30 min later, they were put into actometers before the locomotor activity was measured. This paradigm showed an influence of tested compounds on spontaneous locomotor activity in a new environment, and the treatment schedule was adequate to that used in the TST, where animals were given an MGS0039 injection, and after 30 min, they were hung for 5 min by their tails.
In order to further investigate the role of serotonin in the mechanism of the antidepressant-like activity of group II mGlu receptor antagonists, we used PCPA to achieve 5HT depletion. PCPA is a selective inhibitor of tryptophan hydroxylase (Koe and Weissman 1966
) and has been shown to decrease the brain 5HT level by about 78%, after IP administration, at a dose of 300 mg/kg, twice daily, for three consecutive days (O'Leary et al. 2007
). This schedule for the administration of PCPA was used to investigate the role of serotonin in the antidepressant activity of SSRIs in the TST in C57BL mice (O'Leary et al. 2007
). The results have clearly shown that citalopram and fluoxetine were not active in the serotonin-depleted mice, while in control groups of animals (not treated with PCPA), both compounds induced a significant decrease in the immobility time in the TST. Thus, in our study, we used the same scheme of PCPA administration. We confirmed that citalopram induced a significant antidepressant effect in vehicle-treated mice, decreasing the immobility time. However, in a group of animals administered with PCPA, citalopram did not change the immobility time of mice compared to that of a respective control group, confirming that PCPA-induced 5HT depletion was enough to inhibit the behavioral effect of a compound selectively acting via serotonergic system modulation. The profile of the action of MGS0039 was therefore opposite to that of citalopram; its antidepressant-like activity in serotonin-depleted animals was not inhibited but significantly enhanced.
We also examined an influence of MGS0039 on locomotor activity. The results clearly show that while MGS0039, in a dose of 3 mg/kg, did not change the locomotor activity of vehicle-treated mice, it induced a significant hyperlocomotion of serotonin-depleted animals in the 30-min test, although it remained without a significant influence during the first 5 min of the locomotor activity test, i.e., the time adequate for the duration of the TST. Thus, the hyperlocomotion induced by MGS0039 in PCPA-pretreated mice seems not to be the reason for the decreased immobility of mice in the tail suspension test.
Our studies confirm the observation of Karasawa et al. 2005
that the antidepressant-like activity of MGS0039 was dependent on AMPA receptor activation, as AMPA receptor antagonist, NBQX, given at a dose of 10 mg/kg, prevented its antidepressant-like activity in the TST. Interestingly, a higher dose (20 mg/kg) of NBQX was necessary to block the antidepressant-like activity of MGS0039 in PCPA-treated mice. It can be speculated that an increased level of AMPA receptors in PCPA-treated animals accounted for the lesser ability of NBQX to block the antidepressant-like effect induced by MGS0039 in PCPA-pretreated mice, which may confirm the data of Shutoh et al. (2000
) showing an increased level of AMPA receptors in PCPA-treated animals. Also, enhanced activity of MGS0039 in the PCPA-pretreated mice is not specifically antidepressant like it seems, but rather a false positive effect, resulting probably from changes in AMPA receptor level.
Altogether, the results of our studies show that MGS0039 was active in the OB model of depression, suggesting that the prolonged blockade of mGlu2/3 receptor may be effective in the treatment of depression. Furthermore, the results of our TST studies show that the antidepressant-like action of group II mGlu receptor antagonists does not depend on serotonergic system activation, indicating that the mechanism of the action of group II mGlu receptor antagonists differs from that of typical antidepressants, such as SSRIs. Moreover, we conclude that the AMPA receptor seems to play a key role in the antidepressant-like action of these compounds.