Activity of 5-HT2ARs in the Prefrontal Cortex is Not Sufficient, but May Be Necessary to Regulate the Head-Twitch Response of Mice to the Agonist DOI

Abstract

Serotonin 2A receptors (5-HT2ARs) mediate the effects of psychedelic drugs. 5-HT2AR agonists, such as (-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI), induce head-twitch responses (HTRs) in rodents. However, it is unknown whether the activity of 5-HT2AR expressing neurons is sufficient to produce the HTR in the absence of an agonist. It is also unclear in which brain region 5-HT2ARs control the HTR. Here, we use an optogenetic approach to examine whether activation of 5-HT2ARs in the mouse prefrontal cortex (PFC) is sufficient to induce HTRs and if inhibition of these neurons prevents HTRs in mice following DOI administration. We crossed Htr2a-Cre mice to Cre-dependent optogenetic lines Ai32 (channelrhodopsin) and Ai39 (halorhodopsin) to selectively activate and inhibit (respectively) 5-HT2AR-expressing neurons. Function was validated by whole cell patch clamp recordings performed on cortical slices from Htr2a-Cre+/-;Ai32+/- and Htr2a-Cre+/-;Ai39+/- mice, confirming that blue (470nm) and yellow (590nm) light activated and inhibited 5-HT2AR+ cells, respectively. Next, fiber optic cannulae were implanted bilaterally into the PFC of male and female Htr2a-Cre+/-;Ai32+/- vs. Htr2a-Cre-/-;Ai32+/- and Htr2a-Cre+/-;Ai39+/- vs. Htr2a-Cre-/-;Ai39+/- mice. Immediately after administration of either DOI (1mg/kg, i.p.) or vehicle, mice were placed into an automated HTR apparatus and monitored for 30 min. All mice were exposed to optogenetic light on and light off conditions. For optogenetic stimulation, Ai32+ mice received 5ms pulses of blue light and Ai39+ mice received continuous yellow light, for the full recording session. Automated HTRs from the latter 15 min. period were analyzed. Results showed that there were no differences in DOI-induced HTRs between any of the genotypes analyzed. Moreover, when exposed to blue light, there DOI-induced more HTRs in female Ai32 mice that express Htr2a-Cre compared to Ai32 female mice that do not. This result was absent in male mice. In both male and female Ai32 mice that received vehicle, there was no difference in HTRs in mice that expressed Htr2a-Cre compared with control mice, indicating that optogenetic activation of 5-HT2AR+ cells in the PFC was not sufficient to produce HTRs in the absence of an agonist. In both male and female Ai39 expressing mice that lack Htr2a-Cre, DOI administration produced a significant increase in HTRs, as expected for wildtype mice. However, in male Ai39 mice that expressed Htr2a-Cre, DOI failed to significantly increase HTRs in the presence of yellow light, indicating that inhibition of 5-HT2AR+ cells in the PFC prevented DOI from inducing significant HTRs in males. Together, these findings suggest that activation of 5-HT2ARs in the PFC is not sufficient to induce HTRs in the absence of a 5-HT2AR agonist but may be necessary for induction of HTRs by a 5-HT2AR agonist.