Ondansetron results in improved auditory gating in DBA/2 mice through a cholinergic mechanism.

The 5-HT(3) receptor antagonist, ondansetron, has been shown to correct the auditory gating deficit in medicated schizophrenia patients. Inhibition of 5-HT(3) receptors releases acetylcholine, the endogenous ligand for nicotinic acetylcholine receptors. The schizophrenia-related auditory gating deficit is modulated, in part, by nicotinic acetylcholine receptors, as is the mouse (DBA/2) model of the deficit. The present study assessed the effects of both acute and chronically administered ondansetron on auditory gating in DBA/2 mice. Auditory gating is defined as a decrease in amplitude of response to the second of a paired identical auditory stimulus presented 0.5 s following an initial auditory stimulus. Acute ondansetron administration at the lowest dose (0.1 mg/kg, IP) tested had no effect, while other doses (0.33 and 1 mg/kg, IP) produced improvements in auditory gating. The improvements were produced through both an increase in response to the first auditory stimulus and a decrease in the response to the second auditory stimulus. Co-administration of an alpha7 nicotinic acetylcholine receptor antagonist, alpha-bungarotoxin, or the alpha4beta2 nicotinic acetylcholine receptor antagonist dihydro-beta-erythroidine, with the 0.33 mg/kg dose of ondansetron blocked the improvement in auditory gating produced by ondansetron alone. There was no difference in response between the chronically injected mice and naive mice. Both showed improved auditory gating, thus, demonstrating no "carry over" effect of daily injections. These data demonstrate that indirect stimulation of nicotinic acetylcholine receptors by ondansetron can improve auditory gating parameters in DBA/2 mice.

Stimulation of the alpha4beta2 nicotinic receptor by 5-I A-85380 improves auditory gating in DBA/2 mice.

Abnormal auditory gating is a symptom of schizophrenia which has been proposed to be mediated through the alpha7 nicotinic acetylcholine receptor (nAChR). It has been shown that the non-selective nicotinic agonist nicotine has an influence on auditory gating in part by acting on the alpha4beta2 nAChR. The goal of this study was to determine the effect of 5-I A-85380, an agonist for the alpha4beta2 nAChR, in an inbred mouse model with a deficiency for auditory gating. Anesthetized DBA/2 mice were administered 5-I A-85380 alone and in combination with the alpha4beta2 nAChR antagonist, dihydro-beta-erythroidine, or the alpha7 nAChR antagonist, alpha-bungarotoxin. A recording electrode in the CA3 region of the hippocampus recorded P20-N40 waveforms in response to two auditory stimuli. The amplitudes of the response to the first and second clicks were used to determine TC ratios, the measure of auditory gating. 5-I A-85380 significantly decreased the TC ratios by selectively increasing the response amplitudes to the first click with no significant influence on the response amplitudes to the second click. The effect was blocked by dihydro-beta-erythroidine whereas alpha-bungarotoxin had no effect on response amplitude to either click. Although the alpha7 nAChR may mediate the hippocampal response of DBA/2 mice to the second click, the alpha4beta2 nAChR appears to modulate the response to the first click. Thus, the present study implicates the involvement of more than one subtype of nAChR in the auditory gating of DBA/2 mice, specifically the alpha4beta2 nAChR, and its role in the response amplitude to the first stimulus.

Relative toxicities and neuromuscular nicotinic receptor agonistic potencies of anabasine enantiomers and anabaseine.

Anabasine occurring in wild tree tobacco (Nicotiana glauca) and anabaseine occurring in certain animal venoms are nicotinic receptor agonist toxins. Anabasine lacks the imine double bond of anabaseine; the two possible enantiomers of anabasine occur in N. glauca. A comparision of the relative potencies of S- and R-anabasine has not been previously reported. We separated the enantiomers of anabasine by reaction of the racemic N. glauca natural product with 9-fluorenylmethoxycarbonyl-L-alanine (Fmoc-L-Ala-OH) to give diastereomers, which were separated by preparative reversed phase HPLC. The S- and R-anabasine enantiomer fractions were then obtained by Edman degradation. A mouse bioassay was used to determine the relative lethalities of S- and R-enriched anabasine enantiomers. The intravenous LD50 of the (+)-R-anabasine rich fraction was 11 +/- 1.0 mg/kg and that of the (-)-S-anabasine-rich fraction was 16 +/- 1.0 mg/kg. The LD50 of anabaseine was 0.58 +/- 0.05 mg/kg. Anabaseine was significantly more toxic in the mouse bioassay than S-anabasine (27-fold) and R-anabasine (18-fold). The relative agonistic potencies of these three alkaloids on human fetal nicotinic neuromuscular receptors were of the same rank order: anabaseine>>R-anabasine>S-anabasine.

Nicotinic receptor inhibition by Tetraponera ant alkaloids.

1. Tetraponerines are a group of alkaloids occurring in the venoms of ants belonging to the genus Tetraponera. Eight compounds had been isolated and their structures elucidated, but their mechanisms of action had not yet been reported. We have studied the actions of several of these tetraponerines on vertebrate neuromuscular, ganglionic, and brain nicotinic acetylcholine receptors (nAChRs) using a variety of techniques including muscle contracture, cultured cell functional assays, neuronal patch clamping, and radioligand binding methods. 2. Potency for inhibition of the frog muscle carbachol-elicited contracture increased as the carbon 9 side chain alkyl group was increased in length to 10-12 carbons, then decreased when the chain was 18-carbons long. Potency differences between T-7 and T-8, which differ only in the stereochemistry of the carbon pentyl side chain were rather small. Quaternization of either N atom in a T-8 analog bearing a 10-carbon length alkyl substituent did not greatly affect potency for inhibition of the muscle response; thus the ionized form is an active form of this tetraponerine. 3. T-7 inhibited the nicotine-stimulated efflux of 86Rb from cultured PC12 cells, which primarily express alpha3-beta4 ganglionic type nicotinic receptors. T-8 blockade of BTX-sensitive and insensitive neuronal nAChRs, as studied by patchclamp recordings from cultured rat brain neurons, was also consistent with a noncompetitive type of inhibition. 4. T-7 displaced binding of the nAChR ion channel binding ligand thienylcyclophenidyl (TCP), an analog of PCP, to Torpedo neuromuscular type receptors. The affinity of the TCP binding site for T-7 did not depend upon the desensitization state of the receptor. 5. We conclude that the tetraponerines act at a site on nAChRs different from the ACh binding site which is probably located within the ion channel.

Hydroxy metabolites of the Alzheimer's drug candidate 3-[(2,4-dimethoxy)benzylidene]-anabaseine dihydrochloride (GTS-21): their molecular properties, interactions with brain nicotinic receptors, and brain penetration.

3-[(2,4-dimethoxy)benzylidene]-anabaseine dihydrochloride (DMXBA; GTS-21), an Alzheimer's drug candidate, selectively stimulates alpha7 nicotinic acetylcholine receptors. It rapidly enters the brain after oral administration and enhances cognitive behavior. Less than 1% of orally administered DMXBA is recovered in the urine. We report the identification and characterization of the major phase I metabolites of this drug candidate. Three hydroxy metabolites were generated in vitro by hepatic microsomal O-dealkylation of the two methoxy substituents on the benzylidene ring. They were also found in plasma of rats after oral administration, but at significantly lower concentrations relative to the parent compound. The metabolites displayed similar binding affinities and partial agonist potencies at rat brain alpha7 receptors. However, each displayed a higher efficacy than DMXBA for stimulating rat and human alpha7 receptors. Like DMXBA, the metabolites were weak antagonists at alpha4beta2 receptors. The predicted conformations of the metabolites were nearly identical with that of DMXBA. Ionization of the tetrahydropyridyl nitrogen was essential for high-affinity binding of DMXBA to the alpha7 receptor. The hydroxy metabolites were much more polar than DMXBA, derived from their experimentally estimated octanol/water partition coefficients, and they entered the brain much less readily than DMXBA. Their contributions to the behavioral effects of orally administered DMXBA, if any, would probably be very small during short-term administration. Benzylidene anabaseines pharmacologically similar to the hydroxy metabolites, but which enter the brain more readily, may provide greater stimulation of alpha7 receptors in the whole organism.