Synergism by caffeine and by cocaine of the motor control deficit produced by midazolam

Cocaine's effects on detection, discrimination, and identification of auditory stimuli by baboons

The perceptual effects of cocaine were examined under conditions that required baboons to detect the presence of tones as well as to identify tones of different pitches, and the results compared to the results of prior studies on cocaine's effects on the detection of tones, the discrimination of different tone pitches, and the discrimination of different human vowel sounds of similar pitch. A reaction time procedure was employed in which baboons were trained to press a lever in the presence of a visual "ready" signal, and release the lever only when one tone pitch occurred, but not release the lever when a second, different tone pitch occurred. Changes in the percentage of correct detections and median reaction times for each tone were measured following intramuscular administration of cocaine (0.01-1.0 mg/kg). Cocaine impaired tone identification and shortened reaction times to the tones in all baboons. Cocaine's effects on accuracy, however, were primarily due to elevations in false alarm rates, as opposed to detection of the stimuli themselves. The results demonstrate that cocaine impairs the discriminability of tone pitches in baboons, and that such impairments can depend upon the type of stimuli employed (tones vs. speech sounds) and the type of procedure employed (discrimination vs. identification).

Cocaine's effects on the discrimination of simple and complex auditory stimuli by baboons

The effects of cocaine on tone frequency discriminations by baboons were examined and compared with previous data for more complex acoustic stimuli (speech sounds) to see if cocaine's perceptual effects on these discriminations depends upon the type of stimulus employed (i.e., tones vs. speech sounds). Baboons pressed a lever to produce one repeating "standard" tone and released the lever only when one of four other "comparison" tones occasionally occurred in place of the standard tone. Cocaine's effects were assessed once or twice weekly by giving an intramuscular injection of cocaine hydrochloride (0.01-0.56 mg/kg) immediately prior to performing the task and by examining correct detections and reaction times for each tone following drug administration. Cocaine impaired tone discriminability, with greater impairments occurring for those tones that were more similar in frequency to the standard tone. Cocaine's perceptual effects occurred within 20-70 min following drug administration. Cocaine also impaired or facilitated the speed of responding to auditory stimuli, depending upon the drug dose and subject. The results demonstrate that cocaine can impair auditory discriminations involving simple tones, as well as speech sounds, and further supports the suggestion that cocaine's effects are focused on CNS mechanisms related to the use of pitch cues.

Alprazolam, caffeine and their interaction: relating DRL performance to pharmacokinetics

Three-hour sessions of differential reinforcement of low-rate behavior (DRL 45-s) in rats were used to investigate alprazolam, caffeine, and their interactions at the onset, peak and disappearance of serum alprazolam, while caffeine level remained constant. The dose-response curve (DRC) method of Pöch permitted an extensive evaluation of the kind (additivity or independence) of interactions occurring in combined drug effects. The alprazolam and caffeine DRCs were used to derive theoretical additive and independent relations, and the observed combined effects compared to these functions. Behavior-time profiles of the combined effects were similarly compared. Serum alprazolam and caffeine concentrations correlated with their respective behavior-time profiles. No acute tolerance was observed either for the individual drugs or their combinations. Alprazolam was more potent than caffeine in disrupting DRL behavior. Because alprazolam is much shorter-lived (t1/2 = 32 min) than caffeine (t1/2 = 3 h) in rats, potency ratios between alprazolam and caffeine changed across session time (from 123 to 4), which determined the expression of the combined effects. Although the combined effects were not distinguishable in terms of additivity or independence in both the DRCs and in the behavior-time profiles, they showed neither synergism nor antagonism. The pharmacokinetics of alprazolam were not altered by caffeine, but those of caffeine were affected by alprazolam.

Cocaine's effects on speech sound identification and reaction times in baboons

The effects of cocaine on speech sound discriminations was examined to determine whether cocaine's previously demonstrated effect in reducing speech sound discriminability was dependent upon either the type of stimuli employed (simple tones versus complex speech) or the procedure (stimulus detection versus stimulus discrimination). Because of demonstrated similarities in the way that baboons and humans discriminate speech, and in the way the CNS is thought to encode and process speech sounds in these two species, baboons were trained to perform a choice procedure to identify the occurrence of different synthetic vowel sounds (see text). Animals held down a lever and released the lever only when one of four target vowels sounded, and not when a fifth, standard vowel sounded. Acute IM administration of cocaine (0.0032-1.0 mg/kg) produced dose-dependent decreases in vowel discriminability that were mostly due to elevations in false alarms (i.e., releases to the standard vowel) following cocaine. Cocaine also shortened reaction times to the stimuli in two of three baboons, but to a much lesser extent than observed previously. These results suggest that cocaine may interfere with the ability of the CNS to process the acoustic cues in speech sounds, and that the effects of cocaine on reaction times may depend upon the complexity of the reaction time procedure employed.

Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects

Caffeine is the most widely consumed central-nervous-system stimulant. Three main mechanisms of action of caffeine on the central nervous system have been described. Mobilization of intracellular calcium and inhibition of specific phosphodiesterases only occur at high non-physiological concentrations of caffeine. The only likely mechanism of action of the methylxanthine is the antagonism at the level of adenosine receptors. Caffeine increases energy metabolism throughout the brain but decreases at the same time cerebral blood flow, inducing a relative brain hypoperfusion. Caffeine activates noradrenaline neurons and seems to affect the local release of dopamine. Many of the alerting effects of caffeine may be related to the action of the methylxanthine on serotonin neurons. The methylxanthine induces dose-response increases in locomotor activity in animals. Its psychostimulant action on man is, however, often subtle and not very easy to detect. The effects of caffeine on learning, memory, performance and coordination are rather related to the methylxanthine action on arousal, vigilance and fatigue. Caffeine exerts obvious effects on anxiety and sleep which vary according to individual sensitivity to the methylxanthine. However, children in general do not appear more sensitive to methylxanthine effects than adults. The central nervous system does not seem to develop a great tolerance to the effects of caffeine although dependence and withdrawal symptoms are reported.

Dissociation between the effects of benzodiazepine receptor agonists on behavioral vigilance and responsitivity

The effects of benzodiazepine receptor (BZR) full agonists chlordiazepoxide and midazolam, and the partial agonist beta-carboline ZK 91,296 on the rat's performance in a simple reaction time paradigm were examined. This task required the animals to respond to a rarely and unpredictably occurring brief (50 ms) visual stimulus. Non-parametric measures of signal sensitivity and response bias derived from signal-detection theory were used as a basis for the dissociation between the effects of these drugs on attentional abilities and general responsivity. The dose-dependent effects of midazolam (0.1-3.13 mg/kg) on signal sensitivity and general responsivity occurred in parallel. In contrast, the effects of chlordiazepoxide (1.56-12.5 mg/kg) on signal sensitivity were largely independent from effects on response bias. The partial agonist ZK 91,296 (0.39-25 mg/kg) in general had little effect on performance. The effects of the highest doses of chlordiazepoxide and midazolam were reversed by the co-administration of the BZR antagonist Ro15-1788 (15 mg/kg). Additionally, extension of the stimulus presentation time to 500 ms decreased the magnitude of the effect of chlordiazepoxide on signal sensitivity. These results support the hypothesis that BZR agonist-induced disruption of attentional abilities is not necessarily confounded by effects on general responsivity or sedation, and thus may represent a discrete pharmacological property of BZR-agonists.

Sustained synergism by chronic caffeine of the motor control deficit produced by midazolam

To evaluate the effects of chronic caffeine on the impairment of discriminative fine motor control produced by midazolam, rats were trained to hold a force transducer steady to deliver food pellets. Chronic, daily doses of midazolam (3 mg/kg SC) led to a stable level of motor impairment. Chronic caffeine (20 mg/kg IP) alone usually produced a more moderate deficit or, for one animal, no deficit. Combined, chronic administration of these doses yielded a sustained synergism in motor performance impairment, which contrasted with the antagonism usually found between the benzodiazepines and methylxanthines when performance is evaluated by psychomotor tests not requiring fine motor control. The observed synergism was not explicable in terms of measured disposition of the drugs. The synergistic production of fine motor dyskinesia by the concurrent administration of caffeine and midazolam may be relevant to the triggering of anxiety attacks by caffeine observed in panic disorder patients.