The Serotonin 5‐HT_2C Receptor Is a Prominent Serotonin Receptor in Basal Ganglia: Evidence from Functional Studies on Serotonin‐Mediated Phosphoinositide Hydrolysis

Abstract

Serotonin (5-hydroxytryptamine; 5-HT) and 5-HT2receptors belong to the class of phosphoinositide-specific phospholipase C (PLC)-linked receptors. Conditions were established for measuring 5-HT2A-linked and 5-HT2~-linkedPLC activity in membranes prepared from previously frozen rat frontal cortex and caudate. In the presence of Ca 2~(300 nM) and GTPyS (1 ftM), 5-HT increased PLC activity in caudate membranes. Pharmacological analysis using the selective 5-HT 2A antagonist, spiperone, and the nonselective 5-HT2AI2C antagonist, mianserin, demonstrated that over half of the 5-HT-stimulated PLC activity was due to stimulation of 5-HT2~receptors as opposed to 5-HT2A receptors. Radioligand binding assays with [ 3H]RP62203 and [3H]-mesulergine were used to quantify 5-HT 2A and 5-HT2s ites, respectively, in caudate. From these data, the Bmax for caudate 5-HT2A sites and 5-HT2~sites was 165.4 ii 9.7 fmol/mg of protein and 49.7 ±3.3 fmol/mg of protein, respectively. In contrast to that in caudate, PLC activity in frontal cortex was stimulated by 5-HT in a manner that was inhibited by the 5-HT2A-selective antagonists, spiperone and ketanserin. Taken together, the results indicate that 5-HT2A-and 5-HT2~-linkedPLC activity can be discerned in brain regions possessing both receptor subtypes using membranes prepared from previously frozen tissue. More importantly, significant 5-HT2~-mediated phosphoinositide hydrolysis was observed in caudate, despite the relatively low density of 5-HT2~sites. The significance of these observations with respect to the physiological function of 5-HT2~receptors is discussed. Key Words: Serotonin receptor-Phospholipase C-Phosphoinositide hydrolysis-Brain-Rat.

Imaging Brain Phospholipase A2 Activation in Awake Rats in Response to the 5-HT2A/2C Agonist (±)2,5-Dimethoxy-4-Iodophenyl-2-Aminopropane (DOI)

“…The 5-HT 2 receptormediated activation of PLC by DOI, which increases phosphatidylinositol turnover and Ca 2+ mobilization by IP 3 , is also reported to be inhibited by chronic mianserin (Conn and Sanders-Bush, 1986b;Wolf and Schutz, 1997). Inhibition of signaling in both cases is probably due to mianserin-induced neuroplastic changes, rather than to physical blocking of 5-HT 2 receptors by mianserin, as the brain mianserin concentration falls to less than 0.1% of its peak concentration within 24 h after i.p.…”

Metachlorophenylpiperazine (m-CPP) induced intracavernous pressure responses in anaesthetized rats

“…The m-CPP responses in the presence of molar doses of antagonists (70 -100 nmol=kg) known to block 5-HT 2 receptors in vivo and in vitro 11 were similar except for spiperone. Mianserin, ketanserin and spiperone antagonize 5-HT 2a receptor mediated phospholipase C turnover in frontal cortex with K b values of 12, 58 and 6 nM, respectively 16 and block 5-HT 2a mediated responses in vivo over dose ranges (4 -26 mg=kg) 17 -19 encompassing the doses used in the present study. Therefore, if the m-CPP induced ICP responses were mediated by a 5-HT 2a receptor mechanism all of the antagonists might have been expected to diminish, if not abolish, the response at the roughly equimolar doses used in this study.…”

“…Furthermore, mianserin exhibits similar affinity and potency as an antagonist at 5-HT 2a a and 5-HT 2c receptors whereas ketanserin and spiperone exhibit approximately 100 and 1000-fold less affinity and potency at 5-HT 2c receptors. 16,20 Therefore mianserin, but not spiperone, might have been expected to block a 5-HT 2c mediated m-CPP effect on ICP. However, in the present study spiperone completely abolished the m-CPP induced ICP response whereas mianserin was without effect.…”

Chronic olanzapine activates the Stat3 signal transduction pathway and alters expression of components of the 5-HT2A receptor signaling system in rat frontal cortex

“…PLC activity was measured in membrane fractions of rat frontal cortex as previously described using an ED50 concentration of GTPγS and an Emax concentration of serotonin (Damjanoska, et al, 2003;Wolf and Schutz, 1997). 5-HT-stimulated PLC activity in the frontal cortex is almost entirely mediated by 5-HT 2A receptors, as pretreatment with 5-HT 2A receptor antagonists (ketanserin, spiperone, or mianserin) inhibits most of the 5-HT-stimulated PLC activity in the frontal cortex of rats (Wolf and Schutz, 1997).…”

“…5-HT-stimulated PLC activity in the frontal cortex is almost entirely mediated by 5-HT 2A receptors, as pretreatment with 5-HT 2A receptor antagonists (ketanserin, spiperone, or mianserin) inhibits most of the 5-HT-stimulated PLC activity in the frontal cortex of rats (Wolf and Schutz, 1997). Each experiment was repeated at least three times.…”