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Letter
SEROTONIN 5-HT1A RECEPTOR BIASED AGONISTS DISPLAY DIFFERENTIAL ANXIOLYTIC ACTIVITY IN A RAT SOCIAL INTERACTION MODEL Ronan Depoortère, Laurent Bardin, Mark Andrew Varney, and Adrian Newman-Tancredi ACS Chem. Neurosci., Just Accepted Manuscript • DOI: 10.1021/acschemneuro.8b00661 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019
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SEROTONIN 5-HT1A RECEPTOR BIASED AGONISTS DISPLAY DIFFERENTIAL ANXIOLYTIC ACTIVITY IN A RAT SOCIAL INTERACTION MODEL Submitted as a ‘Letter’ to ACS Chemical Neuroscience
Ronan Depoortère1,2, Laurent Bardin L1, Mark A Varney3, Adrian Newman-Tancredi1,2* 1 Pierre 2
Fabre Laboratories, Castres, France
Present address: Neurolixis, Dana Point, CA 92629, USA.
3 Neurolixis,
Dana Point, CA 92629, USA
*Corresponding author: Adrian Newman-Tancredi Neurolixis Inc., 34145 Pacific Coast Highway #504, Dana Point, CA 92629, USA
[email protected] 1
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ABSTRACT When placed in an unfamiliar and brightly-lit open-field, two adult male rats that have not previously interacted display a low level of social interaction (SI) attributed to an anxiety-like state. The SI test has therefore been used to explore anxiolytic/anti-stress activity. Here, we investigated the effects of serotonin 5-HT1A receptor agonists displaying various activity profiles, i.e. partial vs full agonist efficacy, and pre- versus post-synaptic 5-HT1A receptor preferential activation by 'biased agonists'. Adult male Sprague-Dawley rats were housed singly before starting the social interaction session. Thirty min before being placed in an open-field, both rats of the dyad were injected with either vehicle, diazepam (as a reference compound) or one of six 5-HT1A receptor agonists: NLX-101 (a.k.a. F15599), F13714, S15535 flesinoxan, 8-OH-DPAT and buspirone. Time spent in SI (following, sniffing, playing) was recorded for 10 min. Time spent in SI was inversely correlated with light intensity, with values dropping nearly by half (212.6 ± 18.8 vs 113.7 ± 7.0 s) between 10 and 300 lux (measured at floor level). Under the high light intensity conditions (300 lux), diazepam showed a bell-shaped curve, significantly increasing SI (78% increase in interaction time above control) at 1 mg/kg i.p. only. In the case of 5-HT1A receptor ligands, full agonists, whether non-preferential (flesinoxan, (±)8-OHDPAT) or preferential for pre-synaptic receptors (F13714), showed the strongest activity in this model. The preferential pre-synaptic receptor partial agonist, S15535, was also active over a wide dose-range, although with lower efficacy than F13714. In contrast, NLX-101, a high-efficacy biased agonist that preferentially activates post-synaptic 5-HT1A receptors, exhibited little activity. The clinical anxiolytic, buspirone, showed a marked effect likely due to its partial agonist activity at 5-HT1A pre-synaptic receptors. These data support the hypothesis that enhancement of SI in this model is mediated by preferential agonist activation of pre-synaptic 5-HT1A receptors, and confirm previous studies using local microinjections of (±)8-OH-DPAT. They further support the utility of non-invasive administration of biased agonists for exploring the activity of 5-HT1A receptor subpopulations.
Keywords: 5-HT1A receptors; Anxiety; Biased agonist; Rat; Social interaction; Serotonin. Depression Abbreviations: Fore-paw treading, FPT; Flat Body Posture, FBP; Lower Lip Retraction, LLR Running Title: “5-HT1A biased agonists and Social Interaction”
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INTRODUCTION When a dyad of male rats is confronted in a “dominant/submissive neutral” environment, such as an open-field, they engage in social interaction (SI) characterized mainly by sniffing and following one another, and allogrooming. Time spent in SI is greatly reduced when the open-field is both unfamiliar to each rat and brightly lit, and when the rats have never been in contact with one another before. This reduction in SI is counteracted by anxiolytics such as the benzodiazepine chlordiazepoxide or diazepam, administered either acutely or semi-chronically 1, 2. As a consequence, this model of SI under high illumination and unfamiliarity conditions has classically been proposed as a model to detect “anxiolytic-like” activity 3. Other classes of pharmacological agents have gained the status of anxiolytics, mainly antidepressants of the SSRI family 4 and serotoninergic (5-hydroxytryptamine) 5-HT1A receptor agonists such as buspirone or tandospirone 5, 6. Indeed, both classes of agents display “anxiolytic-like” activity in the unfamiliar/high illumination SI model in rats. Thus, antidepressants of the SSRI class (such as paroxetine, administered chronically) or 5-HT1A receptor agonists such as (±)8-OH-DPAT or buspirone increase time spent in SI 1, 7, 8. However, 5-HT1A receptors are widely expressed in the central nervous system, being present as (presynaptic) autoreceptors on serotonergic cell bodies in the raphe nuclei and also as post-synaptic heteroreceptors in a variety of brain regions, including but not restricted to the pre-frontal cortex, the amygdala and the hippocampus 9. Consequently, the role of pre- versus post-synaptic 5-HT1A receptors for inducing anxiolytic-like activity in this SI assay has been a subject of discussion. Several studies investigated this issue using the local microinjection technique. Hence, local microinjections of (±)8OH-DPAT into the dorsal or median raphe nucleus increased SI, suggesting that they produce anxiolytic effects 10, 11. By contrast, local administration of (±)8-OH-DPAT into serotonergic neuron projections areas where post-synaptic 5-HT1A receptors are expressed, such as the amygdala 12 or the dorsal hippocampus 11 are argued to trigger anxiogenic effects. These observations are interpreted to indicate that anxiolytic-like activity of 5-HT1A receptor agonists is mediated through stimulation of somatodendritic autoreceptors (i.e. pre-synaptic receptors), which results in these neurons reducing their firing rate 13. This in turn leads to a subsequent decrease in 5-HT release in the terminal areas of the limbic system, such as the hippocampus 14, 15. Taken together, these observations support the hypothesis that this variant of the SI assay should be able to distinguish between agonists that differentially activate pre- versus post-synaptic 5-HT1A receptors. Such a hypothesis has recently acquired particular relevance because of studies reporting the discovery and characterization of 5-HT1A receptor agonists that display preferential activation of pre- versus post 5-HT1A receptors. Thus, NLX101 (a.k.a. F15599) has been shown to preferentially target post-synaptic (notably cortical) 5-HT1A receptors, whereas its chemical congener, F13714, preferentially targets pre-synaptic 5-HT1A receptors 16. In this context, the present study investigated the effects of six 5-HT1A receptor agonists, displaying various profiles (i.e. partial or full agonist, selective or not for pre or post-synaptic receptors) on social interaction under conditions of low illumination with a pair of rats unfamiliar with both the environment (test open-field) and with one another. The compounds were the prototypical agonist (±)8-OH-DPAT, buspirone, flesinoxan, S15535, F13714 and NLX-101 (Table 1). Two preliminary experiments were conducted before testing the 5-HT1A receptors agonists: first a parametric experiment aimed at assessing the optimal level of illumination of the test open-field, and second a pharmacological validation experiment, with the benzodiazepine anxiolytic diazepam.
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RESULTS Time spent in social interaction is inversely correlated with light intensity
This initial parametric study aimed at defining the level of luminescence that will decrease the time spent in social interaction by about half. Under low illumination, rats spent 212.6 ± 18.8 s interacting (Figure 1). This value significantly dropped as a function of increase in light intensity [F(3,46)=12.87, P
