Chlorpromazine and QX222 block 5-HT3 receptors in N1E-115 neuroblastoma cells

The effects of chlorpromazine and the lidocaine derivative QX222, which block nicotinic acetylcholine receptors, were examined on 5-HT3 receptors in N1E-115 mouse neuroblastoma cells, using whole cell voltage-clamp and radioligand binding. Electrophysiological studies examining the effects of chlorpromazine and QX222 on 5-HT3 agonist-induced responses revealed IC50s of 0.2 and 8.5 microM respectively. The action was not voltage- or use-dependent and there was no blocking action when chlorpromazine was applied from inside the cell. Chlorpromazine and QX222 inhibited the binding of a radiolabelled 5-HT3 receptor antagonist, [3H]GR65630, with IC50s of 0.9 and 29 microM respectively. Scatchard plots revealed a decrease in affinity (Kd) in the presence of chlorpromazine, but no change in the maximum number of binding sites (Bmax). The results suggest differential actions of the compounds at 5-HT3 and acetylcholine receptors.

Actions of acromelic acid on nervous system L-glutamate receptors

Acromelic acid, a naturally occurring kainoid, isolated from the mushroom Clitocybe acromelalga, is a weak displacer of [3H]L-glutamate binding to cockroach (Periplaneta americana) nerve cord membranes. Acromelic acid (1 mM) displaces approximately 60% of specifically bound [3H]L-glutamate. When applied by bath perfusion to the cell body membrane of the cockroach fast coxal depressor motor neurone, acromelic acid generated slow, prolonged, dose-dependent depolarizations at concentrations of 0.3 microM and above. Thus acromelic acid is among the most potent of the excitatory amino acids tested to date on insect neurones.

Characterization of [3H]meta-chlorophenylbiguanide binding to 5-HT3 receptors in N1E-115 neuroblastoma cells

The binding characteristics of a radiolabelled 5-HT3 receptor agonist, [3H]meta-chlorophenylbiguanide (mCPBG), were examined in membranes from N1E-115 neuroblastoma cells. Scatchard plots of saturation binding data showed the presence of two populations of binding sites, with Kd = 0.03 +/- 0.01 nM and 4.4 +/- 1.2 nM and Bmax = 11.9 +/- 4.2 and 897.9 +/- 184.7 fmol/mg protein respectively. Competition studies with a selection of agonists and antagonists revealed the pharmacological profile expected for a 5-HT3 receptor. The rank order of potency for antagonists was granisetron > quipazine > GR65630 > ondansetron > MDL72222, and for agonists was mCPBG > 5-HT (5-hydroxytryptamine, serotonin) > 2-methyl-5-HT. IC50 values for 5-HT and 2-methyl-5-HT were lower than those observed using radiolabelled antagonists, and combined with functional experiments, the data suggest that [3H]mCPBG may label high affinity desensitized states of the receptor. We conclude that [3H]mCPBG labels 5-HT3 receptors in N1E-115 neuroblastoma cell membranes and may be a useful compound with which to explore 5-HT3 receptors in other systems.

5-HT3 receptors in NG108-15 neuroblastoma x glioma cells: effect of the novel agonist 1-(m-chlorophenyl)-biguanide

The effect of the novel agonist, 1-(m-chlorophenyl)-biguanide (mCPBG) was examined on 5-HT3 receptors in NG108-15 mouse neuroblastoma x rat glioma hybrid cells, using whole-cell voltage-clamp and radioligand binding on intact cells. Electrophysiological studies showed that mCPBG is a partial agonist, with an EC50 of 3.1 microM. Displacement of the selective 5-HT3 receptor antagonist [3H]GR65630 by mCPBG revealed a Ki of 14.2 nM. The study suggests that mCPBG may have a high affinity for desensitized 5-HT3 receptors and also revealed some differences between 5-HT3 receptors in NG108-15 and N1E-115 cells.

The agonist properties of m-chlorophenylbiguanide and 2-methyl-5-hydroxytryptamine on 5-HT3 receptors in N1E-115 neuroblastoma cells

1. The effects of 5-HT3 selective agonists have been studied in whole-cell voltage-clamped N1E-115 neuroblastoma cells. 2. Application of 5-hydroxytryptamine (5-HT) results in the rapid development of a transient inward current at quasiphysiological membrane potentials. This current can be blocked by the 5-HT3 specific antagonists BRL 43694 and GR67330. 3. Application of 2-methyl-5-HT (2-Me5-HT), a 5-HT3 selective agonist, produced a qualitatively similar inward current, but with a maximum response only 20% of that produced by 5-HT. 4. In the presence of 100 microM 2-Me5-HT, the upper part of the 5-HT dose-response curve was shifted to the right but reached the same maximum value as in the absence of 2-Me5-HT. 2-Me5-HT appears therefore to be a partial agonist under these conditions. 5. The novel 5-HT3 agonist, meta-chlorophenylbiguanide (mCPBG) is a full agonist, but has a Hill coefficient (1.5) significantly less than that of 5-HT (2.3). 6. Comparison with radioligand binding data show that mCBPG is 100 times less potent than expected; it may therefore exhibit a high affinity for a desensitized state.

Molecular target size of an L-glutamate receptor from the cockroach central nervous system

L-Glutamate binding to cockroach central nervous system (CNS) membranes is unaffected by freeze-drying and subsequent rehydration. The dissociation constant is similarly unaltered by irradiation at a dose of 3 Mrad which affects only the number of biologically active binding sites. A molecular weight estimate for this insect CNS L-glutamate receptor was performed using irradiation inactivation analysis. By this method a molecular target size of approximately 77.8 kDa was determined. This figure is in agreement with the molecular weight found by other authors for L-glutamate binding sites in the rat brain.