Acute and subchronic toxicities of QX100626, a 5-HT4 receptor agonist,in rodents and Beagle dogs

Serotonin 5-hydroxytryptamine 4(5-HT4) receptor agonists have been widely prescribed as a prokinetics drug for patients with gastro-esophageal reflux disease and functional dyspepsia. QX100626, one of the 5-HT4 receptor agonists, has been studied as a promising agent for this clinical use. The objective of the present study was to identify possible target organs of toxicity and propose a non-toxic dose of QX100626 for clinical usage. After single lethal dose oral and intravenous testing in rodents, some signs indicative of adverse CNS effects were observed. The minimum toxic dose of QX100626 for a single oral administration for dogs was 90.0 mg/kg b.w., and the severe toxic dose was more than 300 mg/kg b.w. The No Observed Adverse Effect Level (NOAEL) of QX100626 by daily oral administration for rats and dogs was 20 mg/kg and 10 mg/kg, respectively, whereas the minimum toxic dosages were 67 and 30 mg/kg, respectively. All of the adverse effects suggested that kidney, digestive tract, as well as nervous, hematological, and respiratory systems might be the target organs of toxicity for humans induced by QX100626. The compound could be a safe alternative to other existing prokinetic agents for the treatment of functional bowel disorders.     

Inhibition of brain cell excitability by lidocaine,QX314, and tetrodotoxin: a mechanism for analgesia from infused local anesthetics?

Local anesthetic infusions have been used to provide analgesia in a variety of painful conditions. The mechanism for this drug effect remains unknown. To better define the electrical effects of lidocaine concentrations comparable to those obtained during analgesic infusions, lidocaine (0.05-3 mmol l^-1), QX314 (an obligatorily charged, quaternary lidocaine derivative applied within the cells), and tetrodotoxin (10 nmoll^-1) were applied to rat hippocampal pyramidal cells. The three drugs, which inhibit Na⁺ currents by varying mechanisms, produced tonic increases in (firing) current threshold, and decreases in the amplitude of action potentials measured using an intracellular microelectrode technique. Lidocaine inhibited action potential spikes and increased current threshold in a concentration-dependent fashion. Lidocaine 50 and 100 μmol 1^-1 did not inhibit action potentials, but increased firing threshold by nearly 100%. Lidocaine 1–3 mmol l^-1 significantly inhibited action potential amplitude and increased threshold by as much as 800%. Similarly, QX314 and tetrodotoxin produced greater increases in current threshold than in action potential amplitude. QX314 produced phasic (or frequency-dependent) block during trains of stimuli at 1 Hz, even when almost no tonic block was present. Lidocaine produced less phasic block than QX314, and required both greater tonic block and more frequent stimulation to produce the phenomenon. Tetrodotoxin demonstrated no phasic block. Increases in current threshold occurred in lidocaine concentrations associated with analgesia and toxicity; inhibition of action potentials occurred scarcely at all at these concentrations. Thus, tonic increases in current threshold may underlie analgesia and supplementation of general anesthesia by intravenous lidocaine.     

Activation of protein kinase C and inositol 1,4,5-triphosphate receptors antagonistically modulate voltage-gated sodium channels in striatal neurons

Regulation of voltage-gated sodium channels is crucial to firing patterns that constitute the output of medium spiny neurons (MSN), projecting neurons of the striatum. This modulation is thus critical for the final integration of information processed within the striatum. It has been shown that the adenylate cyclase pathway reduces sodium currents in MSN through channel phosphorylation by cAMP-dependent protein kinase. However, it is unknown whether a phospholipase C (PLC)-mediated signaling cascade could also modulate voltage-gated sodium channels within MSN. Using the whole-cell patch clamp technique, we investigated the effects of activation of two key components in PLC-mediated signaling cascades: protein kinase C (PKC) and inositol-1,4,5-triphosphate (IP(3)) receptors on voltage-dependent sodium current. Cellular dialysis with phorbol 12-myristate 13-acetate, an activator of PKC, significantly reduced peak sodium current amplitude, while adenophostin A, an activator of IP(3) receptors, significantly increased peak sodium current amplitude. This effect of adenophostin was abolished by calcium chelation or by FK506, an inhibitor of calcineurin. These results suggest an antagonistic role of PKC and IP(3) in the modulation of striatal voltage-gated sodium channels, peak current amplitude being decreased through phosphorylation by PKC and increased through dephosphorylation by calcineurin.     

Bilateral dorsolateral thalamic lesions disrupts conscious recollection

In an earlier study we disputed the claim that the mediodorsal thalamic nucleus is critical for familiarity. We reported patient (QX) who showed a severe deficit in conscious recollection, and behavioural problems (disinhibition, emotional lability) with relative sparing of familiarity-aware memory following a left mediodorsal thalamic lesion. More recent MR imaging has revealed that QX's lesions are more extensive than previously reported and involve both dorsolateral thalamic nuclei, and whilst there is evidence of left mediodorsal thalamic damage, it is not the main focus of damage. This paper reports a full analysis of QX's thalamic pathology alongside a more detailed investigation of his recognition memory, using yes/no and forced-choice procedures, and executive function. The results revealed impairments in yes/no recognition and conscious recollection rates of famous, artist and unknown names. In addition to the previously noted behavioural disinhibition and emotional lability, a deficit in spontaneous planning ability was evident on the Zoo Map Test (subtest of the Bahavioural Assessment of the Dysexecutive Syndrome). Forced-choice recognition, familiarity estimates and remote memory showed higher levels of preservation. The findings indicate that the dorsolateral thalamus is part of the extended hippocampal circuit which is causally critical only for recall and conscious recollection of complex associations rather than for the more automatic processes linked with novelty detection.     

Avian metapneumoviruses expressing Infectious Bronchitis virus genes are stable and induce protection

The study investigates the ability of subtype A Avian metapneumovirus (AMPV) to accept foreign genes and be used as a vector for delivery of Infectious bronchitis virus (IBV) QX genes to chickens. Initially the GFP gene was added to AMPV at all gene junctions in conjunction with the development of cassetted full length DNA AMPV copies. After recombinant virus had been recovered by reverse genetics, GFP positions supporting gene expression while maintaining virus viability in vitro, were determined. Subsequently, either S1 or nucleocapsid (N) genes of IBV were positioned between AMPV M and F genes, while later a bivalent recombinant was prepared by inserting S1 and N at AMPV MF and GL junctions respectively. Immunofluorescent antibody staining showed that all recombinants expressed the inserted IBV genes in vitro and furthermore, all recombinant viruses were found to be highly stable during serial passage. Eyedrop inoculation of chickens with some AMPV-IBV recombinants at one-day-old induced protection against virulent IBV QX challenge 3 weeks later, as assessed by greater motility of tracheal cilia from chickens receiving the recombinants. Nonetheless evidence of AMPV/IBV seroconversion, or major recombinant tracheal replication, were largely absent.     

The effects of QX314 on thalamic neurons

The effects of QX314 were studied in cat thalamic neurons recorded in vivo. Besides blocking Na+ spike electrogenesis, QX314 transformed spindle oscillations into a single long-lasting period of hyperpolarization that terminated with a rebound Ca2+ spike. Large sustained injections of the drug resulted in the occurrence of numerous fast prepotentials that had the characteristics of attenuated dendritic Ca2+ spikes. These effects are interpreted as resulting from the blockage by QX314 of a persistent Na+ current and a delayed rectifier K+ current which currents have already been disclosed in thalamic neurons recorded in vitro.     

Attenuation of glutamate-action,excitatory postsynaptic potentials,and spikes by intracellular QX 222 in hippocampal neurons

The effects of intracellular applications of QX 222, a quaternary analogue of lidocaine, were investigated in CA1 neurons of in vitro hippocampal slices of guinea-pig brain. QX 222 produced a strong depression of spontaneous, electrically- (by current injection) or orthodromically-evoked action potentials. These dose-dependent effects were characterized by a reduction in the rate of rise and amplitude of spikes, presumed to be mediated by a Na⁺-conductance. Although resting membrane conductance tended to diminish with prolonged applications of QX 222, marked changes in resting potential generally were not observed. The threshold for eliciting spikes by intracellular injection of depolarizing current was increased greatly by QX 222, reflecting the impairment of Na⁺ -electrogenesis of spikes. The reduction of action potential amplitude by QX 222 may be partly attributable to enhanced inactivation of Na⁺-channels because brief depolarizing pulses preceded by strong tonic hyperpolarization, elicited spikes at a lower threshold and of considerably larger amplitude than in the absence of such tonic hyperpolarization. These observations on recovery are compatible with a removal of sodium inactivation. However, this experimental paradigm of current injection also might be expected to remove QX 222 molecules from their blocking sites at the inner end of Na⁺-channels. When spikes were abolished by QX 222, the depolarization evoked with application of S-glutamate by pressure ejection from an extracellular micropipette positioned close to the neuron was attenuated. This reversible blockade was reproducible in the 14 neurons where the interactions of QX 222 and glutamate were examined systematically. Excitatory postsynaptic potentials, evoked by stimulation of strata oriens or radiatum, were reduced in a similar manner by internal QX 222.These data confirm previous observations that voltage-dependent Na⁺-channels mediating spike genesis in CA1 neurons can be blocked by internal QX 222. However, QX 222 also apparently interferes with the functions of Na⁺ -channels activated by glutamate-receptor interaction or by receptor interactions with neurotransmitter(s) associated with certain excitatory postsynaptic potentials in CA1 neurons.