Differences in potency and efficacy of a series of phenylisopropylamine/phenylethylamine pairs at 5-HT(2A) and 5-HT(2C) receptors

Abstract

The pharmacological profile of a series of (+/-)-2,5-dimethoxy-4-(X)-phenylisopropylamines (X=I, Br, NO(2), CH(3), or H) and corresponding phenylethylamines, was determined in Xenopus laevis oocytes injected with cRNA coding for rat 5-HT(2A) or 5-HT(2C) receptors. The efficacy and relative potency of these drugs were determined and compared to classical 5-HT(2) receptor agonists and antagonists. The rank order of agonist potency at the 5-HT(2A) receptor was: alpha-methyl-5-HT=5-HT>m-CPP>MK-212; at the 5-HT(2C) receptor the order was: 5-HT>alpha-methyl-5-HT>MK-212>m-CPP. All these compounds were full agonists at the 5-HT(2C) receptor, but alpha-methyl-5-HT and m-CPP showed lower efficacy at the 5-HT(2A) receptor. 4-(4-Fluorobenzoyl)-1-(4-phenylbutyl)piperidine (4F 4PP) was 200 times more potent as a 5-HT(2A) antagonist than at 5-HT(2C) receptors. Conversely, RS 102221 was 100 times more potent as a 5-HT(2C) antagonist, confirming their relative receptor selectivities. The phenylisopropylamines were partial agonists at the 5-HT(2A) receptor, with I(max) relative to 5-HT in the 22+/-7 to 58+/-15% range; the corresponding phenylethylamines had lower or undetectable efficacies. All these drugs had higher efficacies at 5-HT(2C) receptors; DOI was a full 5-HT(2C) agonist. 2C-I and the other phenylethylamines examined showed relative efficacies at the 5-HT(2C) receptor ranging from 44+/-10% to 76+/-16%. 2C-N was a 5-HT(2) receptor antagonist; the mechanism was competitive at the 5-HT(2A), but non-competitive at the 5-HT(2C) receptor. The antagonism was time-dependent at the 5-HT(2C) receptor but independent of pre-incubation time at the 5-HT(2A) receptor subtype. The alpha-methyl group determines the efficacy of these phenylalkylamines at the 5-HT(2A) and 5-HT(2C) receptors.

Figure 1

Structural formulae of the PIA/PEA pairs tested. PIAs: R=CH₃; DOI, X=I; DOB, X=Br; DON, X=NO₂; DOM, X=CH₃; 2,5-DMA, X=H. PEAs: R=H; 2C-I, X=I; 2C-B, X=Br; 2C-N, X=NO₂; 2C-D, X=CH₃; 2C-H, X=H.

Figure 2

Relative potency and efficacy of 5-HT and related agonists at 5-HT₂ receptors. Agonist concentration-response curves were obtained using oocytes injected with either the 5-HT_2A (A) or 5-HT_2C (B) receptor cRNAs. The agonist curves were normalized with respect to the maximal response attained by 5-ht in each oocyte. 5-HT (n=83–90); α-methyl-5-HT (n=5), MK-212 (n=5); m-CPP (n=4).

Figure 3

4F 4PP and RS 102221 selectively block 5-HT responses at 5-HT_2A and 5-HT_2C receptors. Upper panels: Representative tracings from a single oocyte showing the selective inhibition of the currents induced by 100 nM 5-HT at 5-HT_2A receptors (A) and 10 nM 5-HT at 5-HT_2C receptors (D) after 1 min pre-incubation with either 100 nM 4F 4PP (open rectangle) or 100 nM RS 102221 (shaded rectangle). Applications of 5-HT are represented by the closed squares. Lower panels: Blockade of the currents induced by 100 nM 5-HT at 5-HT_2A receptors by different concentrations of 4F 4PP (B), or RS 102221 (C), co-applied or pre-incubated for 1 min prior to 5-HT. Blockade of the currents induced by 10 nM 5-HT at 5-HT_2C receptors by different concentrations of 4F 4PP (E), or RS 102221 (F) co-applied or pre-incubated for 1 min prior to 5-HT. A complete antagonist concentration-response curve was obtained per oocyte at the 5-HT_2A (n=7) and at the 5-HT_2C receptor (n=5–6). Symbols indicate the mean values; bars s.e.mean.

Figure 4

Concentration-response curves for DOI and 2C-I at 5-HT₂ receptors. Upper panels: Representative tracings show partial DOI agonism at 5-HT_2A (A) and full agonism at 5-HT_2C receptors (C). Oocytes were challenged with 5-HT concentrations eliciting the maximal current (600 nM at 5-HT_2A receptors and 100 nM at 5-HT_2C receptors) to compare with partial DOI agonism. 5-HT applications are represented by the closed squares; DOI by open squares. Lower Panels: DOI and 2C-I concentration-response curves in oocytes expressing 5-HT_2A (B) and 5-HT_2C (D) receptors. The DOI or 2C-I currents generated in each oocyte were normalized to the maximal 5-HT current attained in each oocyte (n=12 for 5-HT_2A; n=11 for 5-HT_2C receptor), DOI (n=6), 2C-I (n=5–6) for each receptor subtype. Symbols represent mean values; bars, s.e.mean.

Figure 5

DON and 2C-N concentration-response curve at 5-HT₂ receptors. Upper panels: Representative tracings show the lack of efficacy of 2C-N at the 5-HT_2A receptor (A) as compared to its partial agonism at the 5-HT_2C receptor (C). Open squares represent the 10 s 2C-N applications; closed squares indicate the additions of 600 or 100 nM 5-HT, which elicited the maximal 5-HT current at 5-HT_2A and 5-HT_2C receptors respectively. Lower panels: 5-HT, DON and 2C-N concentration-response curves at 5-HT_2A (B) or 5-HT_2C receptors (D). The currents generated by DON or 2C-N were normalised using the maximal 5-HT-evoked current in each oocyte. Four to six separate oocytes were studied per agonist for each receptor subtype. Symbols represent the mean values; bars, s.e.mean.

Figure 6

2C-N is a competitive antagonist at the 5-HT_2A receptor. (A) The co-application of different concentrations of 2C-N plus 100 nM 5-HT blocked the 5-HT-induced currents in a concentration-dependent manner; the sole application of 2C-N did not elicit responses (n=5–6). (B) The inhibition of the 100 nM 5-HT-induced currents by 50 nM 2C-N was time-independent (n=4). (C) Rightward displacements of the control 5-HT concentration-response curve by the co-application of 5-HT plus 0.04 μM (n=4); 1 μM (n=4) or 10 μM 2C-N (n=2). The insert shows a Schild plot. Symbols represent mean values; bars, s.e.mean.

Figure 7

2C-N is a non-competitive 5-HT_2C receptor antagonist. (A) Co-application of different concentrations of 2C-N plus 10 nM 5-HT caused only a partial reduction of the 5-HT-induced currents; 2C-N alone evoked partial agonism (n=4–6 per concentration). (B) The 50 nM 2C-N-induced blockade of the 10 nM 5-HT currents is time-dependent (n=4). (C) Non-competitive blockade of the 5-HT concentration-response curve: control 5-HT concentration-response curve (n=17); following a 3 min pre-incubation with 50 nM (n=6); 100 nM (n=6) or 150 nM 2C-N (n=5). Symbols indicate mean values; bars, s.e.mean. (D) Representative tracings show the 150 nM 2C-N agonist/antagonist effects. Open rectangle represents a 3 min 2C-N application, closed squares represent 10 s 5-HT applications.