Chronopharmacology and Chronotoxicology of CNS Drugs: Interrelationships with Neuromodulators

Circadian variation in tolerance to the hypothermic action of CNS drugs

Male SAF mice (30-35 g) or male Sprague-Dawley rats (180-250 g) were used to study the circadian variation in tolerance to the hypothermic action of ethanol, apomorphine, and nicotine. Animals were treated for 2 or 3 consecutive days during the light phase (1000, 1400, or 1800 h) or the dark phase (2200, 0200, or 0600 h) and hypothermia produced measured. In one experiment, repeated injections of 20% ethanol (3 g/kg, IP) to mice resulted in varying degrees of hypothermia depending upon the time of injection. Tolerance to hypothermic action was observed only in animals treated during the light phase. On the contrary, the hypothermic response in animals treated during the dark phase increased. In another experiment, apomorphine (15 mg/kg, IP) was used and tolerance to apomorphine-induced hypothermia observed following repeated injections during the light phase with maximum tolerance noticed at 1400 h. In the third experiment, nicotine (2 mg/kg, IP) was repeatedly administered and resulted in tolerance development when given during the light phase. These results indicate that the rapid development of tolerance to CNS drugs studied is a diurnally controlled phenomenon.

Pharmacodynamics of tolerance development to the anesthetic and anticonvulsant effects of phenobarbital in rats

In this study, the potential development of tolerance towards the anesthetic and anticonvulsant effects of phenobarbital after chronic administration to rats was investigated, differentiating between dispositional and functional tolerance. Chronic exposure to phenobarbital by daily ip injections of 20 or 100 mg/kg for 14 days caused a 30% increase in clearance within 1 week. No changes occurred in the total amounts of phenobarbital and para-hydroxyphenobarbital excreted into urine and feces. Pharmacodynamic effects were quantitated after 2 weeks of treatment. The anesthetic effect was measured by slow iv infusion of phenobarbital until onset of loss of righting reflex (LRR), followed by measurement of drug concentrations in serum (both total and free), brain, and cerebrospinal fluid (CSF). With a phenobarbital dose of 100 mg/kg daily, CSF concentrations at the onset of LRR significantly increased from 136 +/- 12 to 176 +/- 21 mg/L (p less than 0.001), indicating that functional tolerance developed for the anesthetic effect. Protection of phenobarbital against convulsions induced by pentylenetetrazol (PTZ), as measured by the elevation of the PTZ plasma threshold concentration necessary to elicit seizures at the EC50 of phenobarbital, was not altered. The discrepancy observed in the development of functional adaptation of the CNS demonstrates that different mechanisms of action are reflected in the different measures of the anesthetic and anticonvulsant effects of phenobarbital that were utilized.

The light-dark cycle modulates the effects of ritanserin on sleep-wakefulness patterns in the rat

The 5-HT2 receptor antagonist ritanserin (0.63 mg/kg IP) produced differential effects on sleep-wakefulness patterns in rats when administered during the light or dark period: an increase in the duration of deep slow wave sleep at the expense of light slow wave sleep, paradoxical sleep and wakefulness when injected during the light period, and no major sleep alteration when given at dark onset. Since circadian variations in serotonin receptor density might modulate the sleep response, we examined the effects of ritanserin on sleep in rats exposed to continuous light for 10 days, and whether 5-HT2 receptors were affected in separate groups of rats exposed to similar conditions. No significant changes in the KD- and Bmax -values of various receptors were found. However, ritanserin produced greater effects in continuous light conditions than when given during the light period in the 12-hr light-dark condition. This suggests a possible role of 5-HT2 receptors in the organization of sleep when the environmental photoperiod is disturbed.

Daily changes in the mouse cingulate cortex responsivity to neurotransmitters and depolarizing medium: an extracellular in vitro study

1. The reaction of the mouse cingulate cortex neurons to the applied substances, measured as changes in the spontaneous activity, was investigated for daily variations in the in vitro slice preparation. 2. Glutamate- and GABA-evoked excitation and inhibition, respectively, were followed by long-lasting after-effects which showed greater potency in slices prepared in the evening. 3. Acetylcholine evoked excitation, inhibition and multiphasic effects. The incidence of an excitatory reaction prevailed in slices prepared in the evening. 4. The majority of cells studied in the evening were inhibited after application of a depolarizing medium, while in the morning the reaction was equally divided between excitation and inhibition. 5. The obtained results suggest that a day-night modulation is imposed upon the reactivity of the cingulate cortex neurons, and that daily changes in the reactivity are observed in vitro.