Failure of opiates to increase the hydrolysis of GTP in neuroblastoma-glioma 108-15 cells

Simple and rapid determination of Gtpase activity by capillary electrophoresis without radioisotope

In order to determine guanosine-5'-triphosphatase (GTPase) activity, we developed a simple, rapid and reliable method that utilizes capillary electrophoresis without radioisotope. Tubulin-GTPase was used for simple measurement of GTPase activity utilizing capillary electrophoresis. Tubulin, a component of microtubules, was incubated with guanosine-5'-triphosphate (GTP) in 100 mM 2-(N-morpholino) ethanesulfonic acid (MES) buffer (pH 6.5). Guanosine-5'-diphosphate (GDP) was determined as the hydrolyzed product of GTP. Guanosine-5'-monophosphate, GDP and GTP in the filtrate of the mixture were clearly separated using 10 mM MES buffer (pH 6.5) (migration time, 3.8, 5.5 and 7.2 minutes, respectively) with a fused-silica capillary column. The quantification of GDP was based on the peak area, which increased linearly with the concentration of GDP from 1 to 50 microM (r2=0.995). The peak area and migration time had good reproducibility; the intra-assay coefficient of variation (n=6) was 1.3% for peak area and 0.6% for migration time. As an application of this method, we examined the effect of dimethylarsinic acid, an effective antimitotic agent, on tubulin-GTPase. Dimethylarsinic acid inhibited tubulin-GTPase activity in a dose-dependent manner. The inhibition was not complete and the maximum decrease of the activity was about 50% at 200 microM dimethylarsinic acid. Thus, since this method is clean, simple and rapid, its application to the study of various GTPase proteins is expected to be useful.

N-ethylmaleimide blocks the modulatory effects of divalent cations and guanine nucleotides on the brain substance P receptor

The binding of [3H]physalaemin ([3H]PHY) to rat brain substance P receptors is modulated by cations and guanine nucleotides. [3H]PHY binding in the presence of either monovalent or divalent cations (125 mM Na2SO4 or 2.5 mM MnCl2) shows a KD of 5.9 and 5.5 nM and a Bmax of 44.4 and 63.9 fmol/mg protein respectively. In the presence of both, there is a 2-fold increase in the affinity (KD 2.8 nM) and a 25-80% increase in the Bmax (81.6 fmol/mg protein). Addition of 100 microM GTP or Gpp(NH)p in either 125 mM Na2SO4 or 2.5 mM MnCl2 or both decreases the Bmax by 25-55%. However, the receptor affinity for [3H]PHY is not significantly altered by guanine nucleotides. N-Ethylmaleimide (NEM) irreversibly inhibits the receptor binding with an IC50 of 1.0 mM, demonstrating that SH groups play a critical role in the interaction of the ligand with the receptor. If the SP receptors are protected with 1 microM PHY, NEM irreversibly inhibits the effect of divalent cations and guanine nucleotides. Analysis of [3H]PHY binding in 125 mM Na2SO4, 2.5 mM MnCl2 on membranes that were protected with 1 microM PHY and then preincubated with NEM demonstrates a variable decline in receptor number and a 2-fold decrease in the affinity (KD, from 2.8 to 6.9 nM). These observations indicate the existence of a second class of SH groups that are essential for the interaction of divalent cations and guanine nucleotides with the receptor. The blockade of the modulatory effects of divalent cations and guanine nucleotides by NEM treatment further suggests that brain SP receptors are coupled to a guanine nucleotide binding regulatory protein.

The adenylate cyclase rebound response to naloxone in the NG108-15 cells. Effects of etorphine and other opiates

The adenylate cyclase (AC) of the neuroblastoma-glioma hybrid cells (NG108-15), is generally considered to be a model for the study of the biochemical correlates of opiate tolerance and dependence. However, the naloxone-induced rebound response of adenylate cyclase, described in some recent reports, is much smaller than that originally described by Sharma, Klee and Nirenberg (1975). Possible explanations for these discrepancies are: (1) a marked down-regulation of opioid receptors and tolerance produced by the use of delta agonists or (2) the use of etorphine, a relatively hydrophobic drug which has slower dissociation rates than morphine. To test these possibilities, neuroblastoma-glioma hybrid cells were treated cells with morphine, etorphine, [D-Ala2,D-Leu5]enkephalin (DADLE), [D-Ala2]Leu-enkephalinamide (DALAMID) or vehicle. In addition, some of the cells treated with etorphine were washed with DADLE to replace the etorphine without producing the rebound response of adenylate cyclase prior to the addition of naloxone. The cells treated with morphine, DADLE and DALAMID, and incubated with prostaglandin E1 (PGE1) and naloxone showed a significant rebound of adenylate cyclase when compared with control groups and opiate-treated cells, incubated only with PGE1. In contrast, naloxone did not induce any significant rebound response in cells treated with etorphine unless they were previously washed with DADLE. These results demonstrate that the lack of a rebound response in cells treated with etorphine was due to the slow dissociation rates of the opiate and not to tolerance or to down-regulation of opioid receptors produced by agonists of high intrinsic activity.

The effect of tolerance on opiate dependence as measured by the adenylate cyclase rebound response to naloxone in the NG108-15 model system

The naloxone-induced adenylate cyclase rebound response produced by opiates in the NG108-15 cells is quite apparent after 48 h treatment with M but it is undetectable after incubation with the high intrinsic activity agonist etorphine. Replacement of agonists by antagonists in receptors that have been rendered inactive by tolerance does not elicit the AC rebound response, which is considered a biochemical correlate of dependence. This indicates that marked tolerance and receptor down-regulation induced by prolonged treatment with high efficacy agonists may reduce the level of dependence.