Agonist- and antagonist-induced plasticity of rat 5-HT(1A) receptor in hippocampal cell culture

We examined the response and regulation of 5-HT(1A) receptor on hippocampal cultured fetal neurons grown in the absence of serotonin and steroids using three experimental designs: 1) functional response using an antibody against phosphorylated cyclic adenosine monophosphate response element binding protein (pCREB); 2) transcriptional regulation using in situ hybridization; and 3) translational expression using antipeptide 5-HT(1A) receptor antibody. Pretreatment of cultured hippocampal cells with the agonist 8-hydroxy-2-(di-N-propylamino)-tetralin (8-OH-DPAT) (10^-8 M) or ipsapirone (IPS) (10^-9 M) for 10 min blocked the forskolin-stimulated increase in pCREB immunoreactivity. In situ hybridization radioautography revealed that IPS (10^-9 M) decreased the 5-HT(1A) receptor mRNA expression (-33%) after a 24-h treatment. The decrease in 5-HT(1A) receptor mRNA was accompanied by a change in protein immunoreactivity using a 5-HT(1A) receptor antipeptide antibody. Computer-assisted morphometric analyses showed a reduction in the 5-HT(1A) receptor immunoreactive (IR) intensity as compared to control 24 h after treatment with 8-OH-DPAT (10^-7-10^-12 M) and IPS (10^-9 M). Thus, fetal hippocampal neurons have a functional 5-HT(1A) receptor that is downregulated at both the transcription and translation levels. In addition, we found increased 5-HT(1A) receptor-IR intensity (+ 17% ~ +39%) 24 h after treatment with the antagonist N-[2-[4-(2-methoxyphenyl)- 1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635) (10^-7-10^-12 M). Our results indicate that the 5-HT(1A) receptor is sensitive to both agonists (downregulation) and antagonists (upregulation) in hippocampal fetal neurons grown in the absence of serotonin and steroids.