The sigma compounds 1,3-di-o-tolylguanidine and N-allylnormetazocine inhibit agonist-stimulated inositol phospholipid metabolism in bovine adrenal medullary cells

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Neuroactive neurosteroids as endogenous effectors for the sigma1 (sigma1) receptor: pharmacological evidence and therapeutic opportunities

Neuroactive neurosteroids, including progesterone, allopregnanolone, pregnenolone and dehydroepiandrosterone, represent steroid hormones synthesized de novo in the brain and acting locally on nervous cells. Neurosteroids modulate several neurotransmitter systems such as gamma-aminobutyric acid type A (GABA(A)), N-methyl-D-aspartate (NMDA) and acetylcholine receptors. As physiologic consequences, they are involved in neuronal plasticity, learning and memory processes, aggression and epilepsy, and they modulate the responses to stress, anxiety and depression. The sigma1-receptor protein was recently purified and its cDNA was cloned in several species. The amino-acid sequences are structurally unrelated to known mammalian proteins, but shared homology with a fungal sterol C8-C7 isomerase. The sigma1-receptor ligands exert a potent neuromodulation on excitatory neurotransmitter systems, including the glutamate and cholinergic systems. Consequently, selective sigma1 agonists show neuroprotective properties and beneficial effects in memory processes, stress and depression. The evidence of a direct interaction between neurosteroids and sigma1 receptors was first suggested by the ability of several steroids to inhibit the binding of sigma1-receptor radioligands in vitro and in vivo. A crossed pharmacology between neurosteroids and sigma1-receptor ligands was described in several physiological tests and behavioral responses. This review will detail the recent evidence for a common mechanism of action between neurosteroids and sigma1-receptor ligands and focus on the potential therapeutic interests of such interaction in the physiopathology of learning and memory impairments, stress, depression and neuroprotection.

Intrasynaptosomal free calcium levels in rat forebrain synaptosomes: modulation by sigma (sigma) receptor ligands

The sigma receptor ligands (+) and (-)pentazocine and BD1008 (1-100 microM) were added to rat forebrain synaptosomes. Their effects on intrasynaptosomal free calcium ([Ca2+(+)]i) levels under basal conditions and after depolarisation with high potassium buffer (45 mM KCl), veratridine (25 microM) and 4-aminopyridine (4-AP, 1 mM) were determined. The sigma ligands elicited significant, concentration-dependent decreases in basal [Ca2+]i levels with an order of potency (-)pentazocine > (+)pentazocine = BD1008. The sigma ligands (at the maximum effective concentrations) also significantly inhibited the rise in [Ca2+]i levels produced by depolarisation with KCl, veratridine and 4-AP. The effect of (+) and (-)pentazocine (100 microM) to inhibit the depolarisation-dependent increase in [Ca2+]i levels was greater when veratridine and 4-aminopyridine were used to depolarise the synaptosomes than with KCl, whereas the effect of BD1008 (100 microM) was approximately equipotent using all three depolarising agents. However, BD1008 was more potent to inhibit the KCl-induced rise in [Ca2+]i compared to (+) and (-)pentazocine. The data demonstrate for the first time that sigma ligands decrease [Ca2+]i levels in rat forebrain synaptosomes and this suggests a possible mechanism for the changes to neuronal protein phosphorylation and neurotransmitter release previously observed with sigma ligands.

Phosphorylation of synapsin I and dynamin in rat forebrain synaptosomes: modulation by sigma (sigma) ligands

The effects of sigma (sigma) ligands on protein phosphorylation were examined in crude, rat forebrain synaptosomes. Synaptosomes were prelabelled with 32P(i) and incubated with the sigma ligands 1,3-di-o-tolylguanidine (DTG), (+)pentazocine and (-)pentazocine (3, 10, 30, 100, 300 microM), or haloperidol, reduced haloperidol, and (+)SKF 10,047 (100 microM). Aliquots were then incubated for 10 s in control (5 mM K+) or depolarising buffer (41 mM K+). All the sigma ligands increased basal phosphorylation of synapsin Ib and other proteins including dynamin, and inhibited the depolarisation-dependent increase in phosphorylation of synapsin Ib in synaptosomes. The effects of these ligands are not directly on protein kinases or protein phosphatases. This indicates that the sigma ligands are mediating their effects via interaction with sigma binding sites, and suggest, for the first time, that protein phosphorylation may be one mechanism through which sigma ligands produce their biological effects.

Sigma binding site ligands inhibit cell proliferation in mammary and colon carcinoma cell lines and melanoma cells in culture

Recent evidence suggests a role for sigma (sigma) binding sites in maintenance of cell growth and/or proliferation. The present study examines, for the first time, the effect of sigma binding site ligands on in vitro growth of tumour cells derived from human mammary adenocarcinoma (MCF-7, MDA) and colon carcinoma (LIM 1215, WIDr), and melanoma (Chinnery). Addition of the sigma ligands haloperidol, reduced haloperidol, 1,3-di(2-tolyl)guanidine (DTG), (+)- and (-)-N-allylnormetazocine (SKF 10,047), (+)- and (-)-pentazocine and rimcazole at 6.25, 12.5, 25, 50, 100 microM at the beginning of culture or 24 h later, inhibited cell proliferation in a dose-dependent manner. Light microscopy revealed cell detachment, rounding and cell death. The potency of sigma ligands on melanoma cells was rimcazole > reduced haloperidol > haloperidol = (+)-pentazocine, whereas DTG and (+)- and (-)-SKF 10,047 and (-)-pentazocine had no effect even at 100 microM. In contrast, in MCF-7 cells, rimcazole > reduced haloperidol > haloperidol > (-)-pentazocine > DTG > (+)-pentazocine > (+)-SKF 10,047 > (-)-SKF 10,047. For colon cancer cells, reduced haloperidol > DTG > haloperidol = (-)-pentazocine = (+)-pentazocine = (+)-SKF 10,047. Of all the ligands tested, rimcazole and reduced haloperidol were the most potent inhibitors of cell proliferation. With the exception of one slow-growing colon cancer cell line (LIM 1215), the order of sensitivity of various cell lines to reduced haloperidol, SFK 10,047, DTG, haloperidol and (+)- and (-)-pentazocine was colon carcinoma > mammary adenocarcinoma > melanoma, whereas to rimcazole, the sensitivities of mammary adenocarcinoma and melanoma cells were comparable. The effect of sigma ligands in MCF-7 and melanoma cells was not due to blockade of dopamine D1 and D2 receptors, serotonin (5-HT2) receptors, N-methyl-D-aspartate (NMDA)/phencyclidine receptors, beta-adrenoceptors or opioid receptors, since 100 microM SCH 23390, raclopride, mianserin, (+)-MK-801, propranolol and 1 microM naloxone respectively, were ineffective. However, mianserin and raclopride were inhibitory to melanoma cells and one colon carcinoma cell line, respectively. Taken together, the results are consistent with the recent observation that sigma binding sites may play a role in cell growth and/or cell proliferation.