Regulation of σ‐Receptors: High‐ and Low‐Affinity Agonist States, GTP Shifts, and Up‐Regulation by Rimcazole and 1,3‐Di(2‐Tolyl)guanidine

Abstract

The regulation of the central sigma-binding site was investigated using both in vitro and in vivo manipulations in conjunction with radioligand binding. The displacement of the binding of R(+)-[3H]3-[3-hydroxyphenyl]-N-(1-propyl)piperidine [R(+)-[3H]3-PPP] to cortical homogenates by a range of drugs was consistent with the site labelled being a sigma-receptor. (+)-SKF 10,047, (-)-SKF 10,047, (+/-)-cyclazocine, phencyclidine, and dexoxadrol displaced R(+)-[3H]3-PPP with pseudo-Hill coefficients of less than 1. Further analysis employing nonlinear curve fitting techniques demonstrated that displacement data for these compounds were described better by a model whereby R(+)-[3H]3-PPP was displaced from two discrete sites; approximately 65% of the total sites were in the high-affinity state. In the presence of 10 mM Mg2+ and 0.3 mM GTP, displacement curves for (+)-SKF 10,047 and (+/-)-cyclazocine were shifted to the right. These findings were due to the shift of some 15% of the high-affinity binding sites to a low-affinity state. Saturation experiments revealed that 0.3 mM GTP acted competitively to decrease the affinity of R(+)-[3H]3-PPP for the sigma sites. The sigma-binding site was thus likely to be linked to a guanine nucleotide regulatory (G) protein. Thus sigma drugs could be subdivided on the basis of their GTP sensitivity and pseudo-Hill coefficients, and by analogy with other receptors R(+)-3-PPP, (+)-SKF 10,047, and (+/-)-cyclazocine, may be putative sigma-agonists. 1,3-Di(2-tolyl)guanidine (DTG), rimcazole, and haloperidol displaced R(+)-[3H]3-PPP with pseudo-Hill coefficients of approximately unity and thus may be sigma-antagonists. Subchronic treatment with rimcazole was characterized by slight sedation and a concomitant up-regulation, with a decrease in the affinity, of sigma-binding sites. The schedule of rimcazole also increased dopamine turnover in the nucleus accumbens; both the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) and the DOPAC/dopamine ratio were elevated. DTG produced similar alterations to the binding parameters of the sigma-binding site; however, changes were not observed in general behavior or accumbal dopamine turnover. sigma-Receptors are likely to be linked to a G protein and are functionally involved in the CNS.

Multiple σ binding sites in guinea‐pig and rat brain membranes: G‐protein interactions

“…Discussion of data is restricted to an indication of the number of sites and whether ligands have relatively higher or lower affinity. The initial observation which suggested the feasibility of such an approach was that under the conditions used in our experiments, and in contrast to the literature at the time (Weber et al, 1986) (Zhou & Musacchio, 1991;Klein & Musacchio, 1992 RI (Zhou & Musacchio, 1991 (Itzhak, 1989;Beart et al, 1989). Reducing the concentration of [3H]-DTG from 3.4 nM to 0.34 nM resulted in a reduction in labelling of the low affinity site, R4.…”

“…It has been suggested that rat brain membranes contain predominantly R3 (Klein & Musacchio, 1991), RI, the other affinity site, being found in much lower density (10%) than in guinea-pig brain membranes. Nevertheless, the use of [3H]-( + )-pentazocine has allowed the presence of al and a2 sites in rat brain to be confirmed (Vilner & Bowen, 1992 (Beart et al, 1989;Itzhak, 1989 (McCann & Su, 1992). The contribution of interactions at this site have not been addressed by our work and require further study.…”

“…agents (Itzhak, 1989;Beart et al, 1989). These studies, which indicate an interaction with G-proteins, are major pieces of evidence for a receptor role for a sites.…”

Blockade by sigma site ligands of high voltage‐activated Ca2+ channels in rat and mouse cultured hippocampal pyramidal neurones

“…Furthermore, the rank order potency of the compounds tested as antagonists of HVA Ca2" channels (Table 1) does not parallel that of their affinities for high-affinity a binding sites. Thus rimcazole, one of the more potent Ca2" channel blockers tested, is a relatively weak displacer of binding to high-affinity a sites (Ferris et al, 1986;Manallack et al, 1988;Beart et al, 1989;Barnes et al, 1992). Conversely, DTG, (+)-3-PPP, L 687,384 and (+)-pentazocine, which possess high affinities for a sites (Manallack et al, 1988;Beart et al, 1989;Musacchio et al, 1989;Ferris et al, 1991;Middlemiss et al, 1991;Barnes et al, 1992;Cagnotto et al, 1994) Ferris et al, 1991), was only a moderately potent Ca2" channel blocker.…”

The effects of sigma ligands and of neuropeptide Y on N‐methyl‐d‐aspartate‐induced neuronal activation of CA3 dorsal hippocampus neurones are differentially affected by pertussin toxin

“…1991b; Riviere et al, 1990), suggesting that it might act on sigma receptors. Guanosine triphosphate (GTP)-binding regulatory G proteins (guanine nucleotide-binding proteins) might be involved in the biological effects of several high affinity sigma ligands (Itzhak & Khouri, 1988;Chattarji et al, 1989;Itzhak, 1989). Indeed, in rat brain membrane preparations, GTP and Gpp(NH)p reduce the high affinity component of the binding of the sigma ligands (+ )-N-allylnormetazocine [(+ )-SKF-10,047], ( + )-3-PPP and pentazocine, this phenomenon being attributed to the conversion of sigma receptors from a high to a low affinity state (Irzhak & Khouri, 1988;Beart et al, 1989). As Gpp(NH)p reduces the slower dissociative component of sigma binding, it has been proposed that, in their high affinity state, at least a subpopulation of sigma receptors are coupled to G proteins (Itzhak, 1989 (Itzhak, 1989), suggesting the coupling of sigma receptors to Gi/O proteins (Gilman, 1987;Fredholm & Lindgren, 1988;Hertting & Allgaier, 1988).…”