Role of 5-HT2A and 5-HT2C Receptor Subtypes in the Two Types of Fear Generated by the Elevated T-Maze

doi:10.1016/S0091-3057(97)00057-9

Pharmacology Biochemistry and Behavior

Volume 58, Issue 4, December 1997, Pages 1051-1057

https://doi.org/10.1016/S0091-3057(97)00057-9 Get rights and content

Abstract

To study the role of 5-HT_2A and 5-HT_2C receptor subtypes in anxiety, the behavioral effects of drugs that either block or stimulate these receptors were measured in an animal model of anxiety, the elevated T-maze. One arm of the maze is enclosed by walls and stands perpendicular to the two open arms. Inhibitory (passive) avoidance—representing learned fear—was measured by placing a rat at the end of the enclosed arm and recording the time to leave this arm with the four paws during three consecutive trials. After 30 s, the same animal was placed at the end of one of the open arms and the time to leave this arm with the four paws was recorded. This one-way escape response represents unconditioned fear. The IP injection of the preferential 5-HT_2C receptor agonists mCPP and TFMPP (0.1–0.8 mg/kg), 25 min before the experimental session enhanced inhibitory avoidance. In contrast, the same drugs either tended to impair (mCPP) or significantly inhibited (TFMPP) one-way escape. The preferential 5-HT_2A agonist DOI (0.03–0.3 mg/kg) did not change either inhibitory avoidance or one-way escape. Inhibitory avoidance was impaired by the selective 5-HT_2C antagonists SB 200646A (3.0–30 mg/kg) and SDZ SER 082 (0.1–1.0 mg/kg), by the 5-HT_2A antagonist SR 46349B (1.0–10.0 mg/kg), and by the mixed 5-HT_2A/2C antagonist ritanserin (0.3–3.0 mg/kg). However, it was not affected by the selective 5-HT_2A antagonist RP 62203 (0.25–4.0 mg/kg). All the 5-HT₂ antagonists used were ineffective on one-way escape. Therefore, conditioned fear seems to be tonically facilitated through 5-HT_2C receptor stimulation, although the 5-HT_2A receptor may also participate in its regulation. Unconditioned fear might be phasically inhibited by 5-HT_2C receptor activation.

Section snippets

Animals

Male Wistar rats, 220–260 g in weight, were housed in groups of four or five with food and water freely available. Lights were on from 0700 to 1900 h. Environmental temperature was kept at 22 ± 1°C.

Apparatus

The elevated T-maze was made of wood and had three arms of equal dimensions (50 × 10 cm). One arm, enclosed by 40 cm high walls, was perpendicular to two opposed open arms. The open arms were surrounded by a Plexiglass rim 1 cm high. The whole apparatus was elevated 50 cm above the floor. The

Tfmpp

The upper panel of Fig. 1 shows a dose-dependent increase of avoidance latency caused by TFMPP administration. Two-factor ANOVA detected an effect of trial, F(2, 70) = 29.06, p < 0.001, and of drug, F(4, 35) = 3.36, p = 0.02. No significant drug × trial interaction was found, F(8, 70) = 1.48, p = 0.180. The lower panel of the figure shows that 0.4 mg/kg of TFMPP increased escape latency. One-way ANOVA revealed an overall drug effect, F(4, 35) = 4.19, p = 0.007. Post hoc comparisons with the

Discussion

The present results show that inhibitory avoidance and one-way escape in the elevated T-maze were differentially affected by the drug treatments used. The two preferential 5-HT_2C receptor agonists mCPP and TFMPP facilitated inhibitory avoidance, that is, had an anxiogenic effect in this task. At the same time, escape behavior was impaired, an effect that may be viewed as anxiolytic (see below). On the other hand, the selective 5-HT_2C antagonists SB 200646A and SER 082, the selective 5-HT_2A

Acknowledgements

The experiments described in this manuscript were funded by FAPESP (94/0821-1) and CNPq (520672/96-8). F.G.G. and P.O.M. were recipients of research fellowships from CNPq (Brazil). C.F.N. was a recipient of a research fellowship from CAPES (Brazil). We thank SmithKline Beecham (UK) for the supply of SB 200646A, Sanofi (France) for SR 46349B, Rhone-Poulenc (France) for RP 62203, Jansen (Denmark) for ritanserin, and Sandoz (Switzerland) for SER 082.

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Salchner and Singewald (2006) have shown that MK-212 potentiates escape responses to an airjet in rats, and the antagonist SB-242084 has been shown to produce anxiolytic-like effects in rats (Kennett et al., 1997; Martin et al., 2002). In the elevated T-maze, an apparatus which tests anxiety-like behavior (inhibitory avoidance) and fear-like behavior (one-way escape) in rodents, 5-HT2C agonists facilitate, and antagonists impair, inhibitory avoidance, but only agonists facilitate one-way escape (Mora et al., 1997), suggesting tonic facilitation of responses to potential threat and phasic inhibition of responses to distal or proximal threat. Thus, a conserved role for this receptor appears to be related to phasic and tonic modulation of defensive responses.
Serotonin (5-HT) receptors have been implicated in responses to aversive stimuli in mammals and fish, but its precise role is still unknown. Moreover, since at least seven families of 5-HT receptors exist in vertebrates, the role of specific receptors is still debated. Aversive stimuli can be classified as indicators of proximal, distal, or potential threat, initiating responses that are appropriate for each of these threat levels. Responses to potential threat usually involve cautious exploration and increased alertness, while responses to distal and proximal threat involve a fight-flight-freeze reaction. We exposed adult zebrafish to a conspecific alarm substance (CAS) and observed behavior during (distal threat) and after (potential threat) exposure, and treated with the 5-HT_2C receptor agonists MK-212 or WAY-161503 or with the antagonist RS-102221. The agonists blocked CAS-elicited defensive behavior (distal threat), but not post-exposure increases in defensive behavior (potential threat), suggesting inhibition of responses to distal threat. MK-212 blocked changes in freezing elicited by acute restraint stress, a model of proximal threat, while RS-102221 blocked changes in geotaxis elicited this stressor. We also found that RS-102221, a 5-HT_2C receptor antagonist, produced small effect on behavior during and after exposure to CAS.
Preprint: https://www.biorxiv.org/content/10.1101/2020.10.04.324202; Data and scripts: https://github.com/lanec-unifesspa/5-HT-CAS/tree/master/data/5HT2C
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In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT_2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT_2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT_2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.
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The rat exposure test (RET) is a prey (mouse)–predator (rat) situation that activates brain defensive areas and elicits hormonal and defensive behavior in the mouse. Here, we investigated possible correlations between the spatiotemporal [time spent in protected (home chamber and tunnel) and unprotected (surface) compartments and frequency of entries into the three compartments] and ethological [e.g., duration of protected and unprotected stretched-attend postures (SAP), duration of contact with the rat's compartment] measures (Experiment 1). Secondly, we investigated the effects of systemic treatment with pro- or anti-aversive drugs on the behavior that emerged from the factor analysis (Experiment 2). The effects of chronic (21 days) imipramine and fluoxetine on defensive behavior were also investigated (Experiment 3). Exp. 1 revealed that the time in the protected compartment, protected SAP and rat contacts loaded on factor 1 (defensive behavior), while the total entries and unprotected SAP loaded on factor 2 (locomotor activity). Exp. 2 showed that alprazolam (but not diazepam) selectively changed the defensive factor. Caffeine produced a mild proaversive-like effect, whereas yohimbine only decreased locomotor activity (total entries). Fluoxetine (but not imipramine) produced a weak proaversive-like effect. 5-HT_1A/5-HT₂ receptor ligands did not change any behavioral measure. In Exp. 3, chronic fluoxetine (but not imipramine) attenuated the defensive behavior factor without changing locomotion. Given that the defensive factor was sensitive to drugs known to attenuate (alprazolam and chronic fluoxetine) and induce (caffeine) panic attack, we suggest the RET as a useful test to assess the effects of panicolytic and panicogenic drugs.
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The ETM has undergone both behavioral and pharmacological validation (Zangrossi and Graeff 1997; Graeff et al. 1998). The pharmacological results have shown that inhibitory avoidance is impaired by single administration of several anxiolytic drugs, including benzodiazepines and 5-HT-acting agents, such as buspirone, ipsapirone and ritanserin (Mora et al. 1997; Graeff et al. 1998; Zanoveli et al. 2005), while being enhanced by the anxiogenic drugs mCPP, TFMP and yohimbine (Graeff et al. 1998; Pinheiro et al. 2007). These results indicate that this task is a reliable model of GAD.
The β-adrenergic and 5-HT_1A receptor antagonist pindolol has been used in combination with antidepressant drugs, to shorten the time of onset of clinical efficacy and/or increase the proportion of responders in depressive and anxiety disorders. The aim of this study was to examine the interaction between pindolol and the selective serotonin reuptake inhibitor (SSRI), paroxetine in rats submitted to the elevated T-maze (ETM).
For assessing the drug combination effect, rats were administered with pindolol before paroxetine, using oral or intraperitoneal (i.p.) routes of acute administration, and were submitted to the ETM model.
The highest dose of pindolol used (15.0 mg/kg, i.p.) increased both inhibitory avoidance and escape latencies in the ETM, probably due to nonspecific motor deficit, since locomotion in a circular arena was also significantly decreased. The highest dose of paroxetine (3.0 mg/kg, i.p.) selectively impaired escape, considered a panicolytic effect. Combination of pindolol (5.0 mg/kg, i.p.) with an ineffective dose of paroxetine (1.5 mg/kg, i.p.) impaired escape, indicating a potentiation of the panicolytic effect of paroxetine. By the oral route, neither paroxetine (3.0 mg/kg) nor pindolol (5.0 mg/kg) alone were effective, but the combination treatment had a marked panicolytic effect, again indicating drug potentiation.
The present results show that the combination of the ineffective doses of pindolol and paroxetine significantly increased escape latency, indicating a selective panicolytic effect. These findings give preclinical support for the use of this drug combination in the treatment of panic disorder (PD).
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The light-enhanced startle paradigm (LES) is suggested to model anxiety, because of the non-specific cue and the long-term effect. In contrast, the fear-potentiated startle (FPS) is suggested to model conditioned fear. However, the pharmacological profiles of these two paradigms are very similar. The present study investigated the effects of putative anxiogenic drugs on LES and FPS and aimed at determining the sensitivity of LES for anxiogenic drugs and to potentially showing a pharmacological differentiation between these two paradigms.
Male Wistar rats received each dose of the α₂-adrenoceptor antagonist yohimbine (0.25–1.0 mg/kg), the 5-HT_2C receptor agonist m-chlorophenylpiperazine (mCPP, 0.5–2.0 mg/kg) or the GABA_A inverse receptor agonist pentylenetetrazole (PTZ, 3–30 mg/kg) and were subsequently tested in either LES or FPS.
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ArticlesRole of 5-HT2A and 5-HT2C Receptor Subtypes in the Two Types of Fear Generated by the Elevated T-Maze

Abstract

Section snippets

Animals

Apparatus

Tfmpp

Discussion

Acknowledgements

Binding and second messenger studies. Eur. J. Pharmacol.

Neurosci. Biobehav. Rev.

Pharmacol. Biochem. Behav.

Behav. Brain Res.

Pharmacol. Ther.

Behav. Brain Res.

Eur. J. Pharmacol.

Biol. Psychiatry

Eur. . Pharmacol.

Pharmacol. Biochem. Behav.

Trends Neurosci.

Eur. J. Pharmacol.

Neurosci. Lett.

Pharmacol. Biochem. Behav.

Mediation by serotonin of the antiaversive effect of zimelidine and propranolol injected into the dorsal midbrain central grey

J. Psychopharmacol.

Intra periaqueductal grey administration of mCPP potentiates a chemically induced defence response

Br. J. Pharmacol.

5-HT2 receptors and anxiety

Behav. Pharmacol.

Effects in the X-maze anxiety model of agents acting at 5-HT1 and 5-HT2 receptors

Psychopharmacology (Berlin)

5-HT and mechanisms of defence

J. Psychopharmacol.

Testing theories of anxiety in normal volunteers

Animal models of different anxiety states

Evidence for expression of the 5-HT2B receptor nRNA in rat brain

Br. J. Pharmacol.

Articles
Role of 5-HT_2A and 5-HT_2C Receptor Subtypes in the Two Types of Fear Generated by the Elevated T-Maze

5-HT₂ receptors and anxiety

Effects in the X-maze anxiety model of agents acting at 5-HT₁ and 5-HT₂ receptors

Evidence for expression of the 5-HT_2B receptor nRNA in rat brain