Functional interactions between 5-HT2A and presynaptic 5-HT1A receptor-based responses in mice genetically deficient in the serotonin 5-HT transporter (SERT)

Abstract

Background and purpose: Despite decreased presynaptic 5-HT(1A) and altered 5-HT(2A) receptor function in genetically-deficient serotonin (5-HT) transporter (SERT) mice, the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY 100635) still induced head twitches in these mice, a well-established 5-HT(2A) receptor-mediated response.

Experimental approach: Interactions between 5-HT(1A) and 5-HT(2A) receptors were assessed using the head-twitch response following 5-HT(1A) and 5-HT(2A) receptor agonists and antagonists in SERT wild-type (+/+), heterozygous (+/-), and knockout (-/-) mice. The role of brain 5-HT availability in WAY 100635 induced head twitches was also examined.

Key results: WAY 100635 induced head twitches in a SERT gene-dose dependent manner, inducing 5-fold more head twitches in SERT -/- versus SERT +/+ mice. In SERT -/- mice, inhibition of 5-HT synthesis with p-chlorophenylalanine (PCPA) markedly depleted tissue 5-HT in all five brain areas examined and abolished WAY 100635 induced head twitches. Further, the selective 5-HT reuptake inhibitor fluvoxamine increased WAY 100635 induced head twitches in SERT +/+ and +/- mice. Head twitches following the 5-HT(2A) receptor agonist (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI) were robust in SERT +/+ and +/- mice but much reduced in SERT -/- mice. DOI-induced head twitches were decreased by the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in SERT +/+ and +/- mice. All drug-induced head twitches were blocked by the 5-HT(2A) receptor antagonist a-Phenyl-1-(2-phenylethyl)-4-piperidinemethanol (MDL 11,939).

Conclusions and implications: These data show that indirect activation of 5-HT(2A) receptors via blockade of presynaptic 5-HT(1A) receptors potentiated head-twitch responses, suggesting functional interactions between these receptors, interactions affected by altered 5-HT availability. Our findings strongly support the correlation of WAY 100635 induced head twitches with increased 5-HT availability, induced genetically or pharmacologically.

Figure 1

Head twitches in SERT +/+, +/− and −/− mice following vehicle or the selective 5-HT_1A receptor antagonist WAY 100635 (1 mg·kg⁻¹) (n= 8–10). Data represent the mean ± SEM. +P < 0.05, ++P < 0.01, ++++P < 0.0001 compared with vehicle-treated mice of the same genotype; **P < 0.01 compared with SERT +/+ mice in the same drug condition. SERT, 5-HT transporter; WAY 100635, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt.

Figure 2

Effects of PCPA pretreatment (300 mg·kg⁻¹ twice daily for 3 days) on head twitches induced by WAY 100635 (1 mg·kg⁻¹) in SERT −/− mice (n= 4–5). Data represent the mean ± SEM. ++++P < 0.0001 compared with vehicle-pretreated mice. SERT, 5-HT transporter; WAY 100635, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt; PCPA, p-chlorophenylalanine.

Figure 3

Effects of pretreatment with vehicle or the SSRI fluvoxamine (10 mg·kg⁻¹) 30 min earlier on head twitches induced by WAY 100635 (1 mg·kg⁻¹) in SERT +/+, +/− and −/− mice (n= 5–7). Data represent the mean ± SEM. ++P < 0.01, ++++P < 0.0001 compared with vehicle-pretreated mice of the same genotype; ****P < 0.0001 compared with SERT +/+ mice in the same drug condition. SERT, 5-HT transporter; WAY 100635, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt; SSRI, selective 5-HT reuptake inhibitor.

Figure 4

Head twitches in SERT +/+, +/−, and −/− mice following 8-OH-DPAT (1 mg·kg⁻¹), WAY 100635 (1 mg·kg⁻¹) and DOI (2.5 mg·kg⁻¹) administered alone or in combination (administered at the same time) (n= 4–9). Data represent the mean ± SEM. #P < 0.05, ####P < 0.0001 compared with WAY 100635 alone; ++P < 0.01, ++++P < 0.0001 compared with DOI alone; *P < 0.05, **P < 0.01, ****P < 0.0001 compared with SERT +/+ mice in the same drug condition. SERT, 5-HT transporter; WAY 100635, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt; 8-OH-DPAT, 8-hydroxy-2-(di-n-propylamino)tetralin; DOI, (+/−)-2,5-dimethoxy-4-iodophenyl-2-aminopropane.

Figure 5

Effects of pretreatment 30 min earlier with vehicle or the selective 5-HT_2A receptor antagonist MDL 11,939 (5 mg·kg⁻¹) on head twitches induced (A) by WAY 100635 (1 mg·kg⁻¹) in SERT −/− mice (n= 5), (B) by WAY 100635 (1 mg·kg⁻¹) plus DOI (2.5 mg·kg⁻¹) in SERT +/+, +/− and −/− mice (n= 4–5) or (C) by fluvoxamine (10 mg·kg⁻¹) plus WAY 100635 (1 mg·kg⁻¹) in SERT +/+ and +/− mice (n= 4–5). Data represent the mean ± SEM. +P < 0.05, ++++P < 0.0001 compared with vehicle-pretreated mice [regardless of genotype in panel (C)]; *P < 0.05 compared with SERT +/+ mice in the same drug condition. SERT, 5-HT transporter; WAY 100635, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt; DOI, (+/−)-2,5-dimethoxy-4-iodophenyl-2-aminopropane; MDL 11,939, a-Phenyl-1-(2-phenylethyl)-4-piperidinemethanol.