Agonist and antagonist effects of benzodiazepines on motor performance: influence of intrinsic efficacy and task difficulty.

Previous studies have shown that low-efficacy benzodiazepines may function as full agonists, partial agonists or antagonists, depending upon the sensitivity of the assay to detect a drug's agonist effects. To date, these differential effects have only been observed across tasks, as these drugs rarely produce full agonist and antagonist effects in a single preparation. The purpose of the present study was to examine the agonist and antagonist effects of various benzodiazepines in a motor task in which the sensitivity of the task differed across conditions. To this end, rats were trained to walk on a rotorod apparatus rotating at low (4 rpm), medium (20 rpm) and high (36 rpm) speeds, and the motor-impairing effects of representative low (bretazenil), intermediate (clonazepam) and high (diazepam) efficacy benzodiazepines were examined. Diazepam and clonazepam reduced the latency for rats to fall from the rotorod at all three speeds, with both drugs being more potent at the high speed than at lower speeds. Bretazenil was also effective at decreasing motor performance at the high speed, but was not effective at lower speeds. In drug combination tests, bretazenil antagonized the effects of diazepam and clonazepam under conditions in which it was not effective when administered alone, effectively demonstrating both full agonist and antagonist effects in the same preparation. These data indicate that the sensitivity of a motor task to detect a drug's agonist effects can be manipulated by altering its relative level of difficulty, and that lower-efficacy benzodiazepines are more sensitive than higher-efficacy benzodiazepines to these manipulations.

Detection of influenza a subtypes in community-based surveillance.

A rapid microtitre cell enzyme immuno assay (cell-EIA) was developed for the detection of influenza A subtypes in nasopharyngeal(NPS) swabs taken for surveillance. During the 1997/1998 influenza season in the United Kingdom, cell-EIA was compared to cell culture for the detection and typing of influenza A viruses in NPS obtained by sentinel general practitioners in community surveillance. The cell EIA can also be used to detect different influenza A subtypes (H3N2, H1N1, H5N3, H5N1, H7N7, and H9N2) and was used as a rapid detection assay for the screening of individuals returning from Hong Kong with influenza-like illness suspected to be due to H5N1 in 1997/98, providing a rapid, efficient, inexpensive method for the screening of influenza A cases during an outbreak or pandemic situation. The cell-EIA results reflected the results obtained by traditional virus culture within the age distribution of samples, clinical symptoms, and time between date of illness onset and sampling of cases, indicating its usefulness in surveillance of human and non-human influenza viruses. During two outbreaks of influenza in schools, Directigen Flu-A, a near patient test, the cell-EIA, and tissue culture were compared. The cell-EIA gave higher sensitivity and specificity (74% and 90%) than Directigen Flu-A (65% and 84.6%) in comparison with cell culture.

The use of neurally released agonist in the measurement of antagonism at alpha-adrenoceptors.

Graded contractile responses of the rat vas deferens were obtained to trains containing varying numbers of pulses delivered transmurally at 500 Hz, allowing construction of pulse-response curves. In the presence of postjunctional adrenoceptor blocking drugs, these curves were displaced to the right in a parallel manner, allowing the estimation of pA2 values for the drugs. Activity of drugs at prejunctional adrenoceptors was estimated from the reduction of inhibition of the response to a second pulse delivered 3 s after trains containing different numbers of pulses. It is suggested that the technique is more physiological than others used to determine the antagonistic activity of drugs at adrenoceptors.

Separation of adrenergic and non-adrenergic contractions to field stimulation in the rat vas deferens.

Adrenergic and 'non-adrenergic' nerve-induced contractions in rat vas deferens were separated pharmacologically. Responses to single stimuli comprised two components, an alpha-noradrenergic component (IIs), dominant in the epididymal portion, and a 'non-adrenergic' component (Is), dominant in the prostatic portion. Is but not IIs was blocked by nifedipine. A combination of adrenergic blockade and nifedipine virtually abolished all components. After cocaine, a third component (IIIs) emerged which was abolished by either adrenergic blockade or nifedipine. The response to trains of stimuli consisted of 'twitch' and 'secondary' components. This biphasic time course was modified by adrenergic blockade or nifedipine to reveal the time course of the 'non-adrenergic' and adrenergic components, respectively: these did not correspond to the 'twitch' and 'secondary' components. A combination of adrenergic blockade and nifedipine virtually abolished the whole response. Prejunctional alpha 2-adrenoceptor-mediated inhibition of the contractile responses could be blocked by selective alpha 2-adrenoceptor antagonists. The adrenergic contractile response demonstrated this 'feed-back' even on the second pulse at 0.5 Hz. Endogenous inhibition of the 'non-adrenergic' contraction required higher frequencies or enhancement of the extracellular concentration of noradrenaline by blockade of its neuronal uptake. Contractile responses to exogenous noradrenaline were abolished by nifedipine, at a concentration that did not affect the adrenergic (IIs) neurotransmission. These results reinforce the view that part of the motor transmission in rat vas deferens is non-adrenergic and allow the disentanglement of the various postjunctional and prejunctional elements contributing to the complex response to a train of stimuli.

Evidence to support the hypothesis that ATP is a co-transmitter in rat vas deferens.

Application of exogenous ATP or of noradrenaline (NA) produced responses in bisected rat vas deferens which mimicked the biphasic responses to nerve stimulation, and these actions were modified by nifedipine and verapamil in a manner similar to the modification of the 2 phases of the responses of the vas to nerve stimulation. It is proposed that sufficient evidence now exists to support the hypothesis that in this tissue, ATP is released along with NA from the motor nerves and that ATP may indeed be a co-transmitter.

A comparison of the effects of nifedipine and verapamil on rat vas deferens.

Nifedipine preferentially blocks contractions of the prostatic end of the rat vas deferens to single pulse field stimulation, leaving the epididymal end largely unaffected. This action is not due entirely to antagonism of calcium influx. Verapamil unexpectedly potentiated the responses of the prostatic portion, and antagonized those of the epididymal end. The use of nifedipine may, therefore, allow investigations of adrenergic mechanisms on this tissue to be studied without the complications of non-adrenergic transmission.

The effect of a benzotriazinium salt on in vitro and in vivo arrhythmias in guinea-pigs and mice.

In guinea-pig Langendorff hearts, a benzotriazinium derivative, 2-n-propyl-4-p-tolylamino-1,2,3-benzotriazinium iodide (TnPBI), increased the ventricular refractory period, increased the ventricular fibrillation threshold to electrical stimulation and converted persistent ventricular fibrillation to sinus rhythm. TnPBI also converted aconitine-induced tachyarrhythmias to sinus rhythm in the same preparation. In mice pretreated with TnPBI, both acutely and chronically, the amount of intravenous aconitine necessary to produce cardiac arrhythmias was increased. In guinea-pigs anaesthetised with halothane. TnPBI converted adrenaline-induced arrhythmias to sinus rhythm.

Comparison of the effects of a benzotriazinium iodide and quinidine on guinea-pig heart.

1 The actions of 2-n-propyl-4-p-tolylamino-1,2,3-benzotriazinium iodide (TnPBI) and quinidine were compared on guinea-pig heart preparations. 2 Quinidine had negative inotropic and chronotropic effects on guinea-pig isolated atria and on Langendorff hearts: TnPBI had negative inotropic and chronotropic effects on Langendorff hearts, but showed a positive inotropic effect on isolated atria at low dose levels. 3 Both quinidine and TnPBI increased the effective refractory period of guinea-pig atrial preparations. 4 The effects of TnPBI on transmembrane action potentials indicated a mixed class I and class III action according to the classification of Vaughan Williams (1970). 5 These effects are discussed in relation to the antiarrhythmic actions of TnPBI.

The local anaesthetic activity of a benzotriazinium salt.

1 The local anaesthetic properties of 2-n-propyl-4-p-tolylamino-1,2,3-benzotriazinium iodide (TnPBI) were compared with those of lignocaine hydrochloride on intact and desheathed sciatic nerves of the frog, on the phrenic nerve-hemidiaphragm preparation of the rat, and by the intradermal wheal test in the guinea-pig. 2 Both TnPBI and lignocaine were more potent on desheathed than on intact sciatic nerves. The potency of TnPBI was affected more than that of lignocaine by the presence of the sheath in intact nerves. 3 Both drugs inhibited conduction in the rat phrenic nerve, as shown by the reduction in twitch tension of the diaphragm elicited by nerve stimulation. TnPBI also caused an initial augmentation of the twitch tension of the diaphragm when applied directly to the muscle. 4 TnPBI was shown to be approximately twice as potent as lignocaine by the guinea-pig intradermal wheal test. 5 These results are discussed in view of the known effects of TnPBI on intracellular calcium storage.