Hexobarbital (enhexymalum NFN) sleeping times and EEG threshold doses as measurements of tolerance to barbiturates in the rat

The EEG burst suppression threshold test for the determination of CNS sensitivity to intravenous anesthetics in rats

The electroencephalographic (EEG) threshold test was developed to determine the central nervous system (CNS) sensitivity to several depressant drugs, mainly intravenous anesthetic agents. Test drugs were administered to the rats by continuous intravenous infusion until a defined EEG criterion indicating deep anesthesia was reached. The criterion was a burst suppression which lasted 1 s or more, the 'silent second' (SS). The dose of the drug needed to induce the SS, the threshold dose, was the dependent variable. In the intact animal, it is influenced by the potency of the drug and dose administration rate of the infusion (mg/kg/min). The method has several advantages over the commonly used anesthesia times (misnomer: sleeping time) where the duration is influenced by several pharmacokinetic properties such as metabolism and redistribution. With the method it is possible under in vivo conditions to monitor continuously the electrical changes in the CNS. No interruptions are necessary. The pharmacological end-point is well defined, easy to monitor and detect. The threshold doses of the drugs applied remained stable with repetitive testing of the same individual. Thus, it can be used effectively to determine the CNS sensitivity to a number of drugs and to follow changes in this sensitivity after several physiological, pharmacological or pathological interventions.

Anandamides: tolerance and cross-tolerance to delta 9-tetrahydrocannabinol

In this study we examined whether tolerance develops to chronic exposure to anandamides [20:4, n-6 (ANA) and 20:3, n-6 (HLEA)] two of the recently discovered endogenous cannabinoid receptor ligands in brain. Tolerance to ANA and cross-tolerance to delta 9-tetrahydrocannabinol (delta 9-THC) was examined in female Sabra or C57BL/6 mice which had received daily injections (i.p.) of low (0.001-1 mg/kg) or high doses (20 mg/kg) of ANA or HLEA for 2 weeks. Twenty four h after the last injection, the mice were challenged with 20 mg/kg ANA or delta 9-THC. Animals were subjected to a series of tests frequently used to assess cannabinoid-induced effects. The results indicated that the high dose, but not the low doses of anandamides produced tolerance to ANA and cross-tolerance to delta 9-THC for motor activity in an open field, catalepsy on a ring, hypothermia and analgesia on a hot plate. One week after the last ANA treatment, tolerance was not present anymore. No tolerance to ANA was observed for reduced defecation in the open field, a measure of intestinal hypomotility. This phenomenon may possibly be attributed to a difference between activities produced through different types of cannabinoid receptors.

Cholinergic mechanisms in physical dependence on barbiturates, ethanol and benzodiazepines

The aim of this review is to summarize the effects of acute and chronic treatment with barbiturates, ethanol and benzodiazepines on cholinergic mechanisms in the brains of experimental animals. A single dose of each of these substances reduces the turnover of ACh in the brain. Long-term treatment has the opposite effect; complicated interactions including decreased content of ACh are induced. Barbiturates have been shown to bind stereospecifically to muscarinic and nicotinic receptors in the brain, but this has not been observed for ethanol or the benzodiazepines. The effects on the cholinergic system are affected by the length of treatment and choice of treatment regimen. No effect on cholinergic parameters, such as muscarinic receptors, in the brain is observed on withdrawal of ethanol or barbiturate treatment when the animals are still tolerant towards the substances. The increase in the number of muscarinic receptors observed in several brain regions on withdrawal is seen as a sign of cholinergic supersensitivity. The number of receptors returns to normal when abstinence convulsions have occurred. The assumption of a cholinergic influence is supported by the finding that atropine, given as a single dose on the day of withdrawal of barbital, can prevent the muscarinic receptor changes. Furthermore, long-term barbital or ethanol treatment can induce permanent persistent changes in the cholinergic system in the brain. Cognitive defects and a significant permanent reduction in the content of ACh can be measured in rats which have had long-term barbital treatment. Similarly, a reduced number of muscarinic receptors has been measured in different brain regions of chronic alcoholics. Accumulating data support the role of the cholinergic system in expressing symptoms of physical dependence on barbiturates, ethanol and benzodiazepines as well as in the permanent long-term effects observed after end of treatment.

Kinetics of drug action in disease states. XXI. Relationship between drug infusion rate and dose required to produce a pharmacologic effect

Computer simulations were performed to determine if the threshold dose of an infused drug (rather than the drug concentration in the biophase at onset of action) can be a suitable index for pharmacodynamic investigations as proposed by others. A two-compartment pharmacokinetic model with drug elimination from the central compartment was used for the simulations. Drug was administered into the central compartment by a constant-rate infusion, and concentrations in the central and peripheral compartments were calculated as a function of time. The pharmacologic effect was assumed to be reversible and to occur at a defined concentration (the effective concentration) in one or the other compartment. The dose required to produce an effective concentration (threshold dose) was determined as a function of infusion rate. The relationship between infusion rate and the dose required to produce an effective concentration in the peripheral compartment was found to be affected by drug distribution and elimination kinetics and by the effective concentration. The infusion rate-dose relationship showed a dose minimum at an infusion rate which others have designated as the "optimal dose rate" and have used for pharmacodynamic studies. No such minimum occurred for pharmacologic effects associated directly with drug concentrations in the central compartment. Since optimum dose rate and threshold dose are affected by both pharmacokinetic and pharmacodynamic alterations, it is concluded that this method (which avoids determination of drug concentrations) is not generally suitable for quantitative pharmacodynamic investigations.

Effects of castration and testosterone treatment on the hexobarbital sensitivity in male rats

Castration increases the central nervous system (CNS) sensitivity to hexobarbital as measured by an EEG-threshold method. Testosterone treatment in doses from 1 to 5 mg/kg/day decreases CNS-sensitivity and restores threshold values in castrated rats to a level found in normal young adult male rats.

Acute and chronic effects of barbiturates on regional turnover of acetylcholine in brain

Data concerning both the acute and chronic effects of barbiturates on acetylcholine [ACh] turnover in mouse and rat brain are presented. A single anaesthetic dose of pentobarbital [60 mg/kg, i.p.] to mice increased the endogenous content of ACh and decreased the turnover of ACh in the hippo-campus and cortex. No effect was found in the striatum. At sacrifice after chronic treatment with barbital to rats for about 30 weeks no effect was found on the endogenous ACh content. 3 days after the barbital solution was replaced by water [at the day with maximal number of abstinence convulsions] the amount of endogenous ACh was decreased by 35% in the striatum in comparison with control and the decrease was still significant after 30 days of abstinence. The biosynthesis of radioactive ACh from a tracer dose of radioactive choline [Ch] was increased in the cerebellum+midbrain+medulla oblongata of rats receiving barbital until sacrifice and rats abstinent for 3 days. It was also increased in the hippocampus+cortex on the 3rd abstinent day. No significant effect on the ACh formation was found in the striatum. Calculation of the specific radioactivity [SA] of ACh showed an increased SA in all brain regions in the abstinence indicating that the turnover of ACh might be increased during the abstinence after long-term barbital treatment. An increased number of muscarinic binding sites [measured by quinuclidinyl benzilate, QNB, as ligand] was also found in synapotosomes from the striatum of rats abstinent for 3 days.

Interactions between 1,2--propanediol (propylene glycol) and hexobarbital

The interaction between 1,2--propanediol and hexobarbital was studied with an anaesthesia threshold method, where hexobarbital is infused intravenously. The dose needed to obtain a burst suppression of one second or more is determined and given as per cent of a pre-experimental value obtained in the same rat. The effect of varying the interval between the intraperitoneal injection of 1,2--propanediol (2.06 g/kg) and the threshold determination was investigated. In the intervals, 10--50 min., the hexobarbital threshold doses were significantly decreased by 13--27% (n = 7--18). Four different doses between 0.25 and 2.06 g/kg of 1,2--propanediol were then tested after a fixed interval of 30 min. As in the time interval study the thresholds after 2.06 g/kg were significantly decreased. No certain depressant effect was seen with 1.03 and 0.52 g/kg. With the lowest tested dose of 1,2-propanediol (0.25 g/kg) there was instead a significant increase in the threshold dose (13%). This might be an excitatory effect, which could correspond to the well-known effect, seen with low doses of other depressants for instance ethanol.

The effects of atropine on the tolerance and the convulsions seen after withdrawal from forced barbital drinking in the rat

Male rats were forced to drink a barbital solution as their only drinking fluid for 33 weeks. During the last part of the treatment the average dose of barbital was around 200 mg/kg/day. In the abstinence period after barbital treatment, tolerance was recorded with a hexobarbital anaesthesia threshold utilizing an EEG criterium. Convulsions were recorded in jiggle cages. On days 3 and 28 of the abstinence period, i.p. pretreatment with atropine (2, 4, and 8 mg/kg) was given 1.5 h before threshold determinations. The barbital treatment induced a clear tolerance to the hexobarbital, which still could be detected on day 28 of abstinence. Atropine (8 mg/kg) reduced the hexobarbital threshold in tolerant and in control animals at both the time of maximum tolerance (day 3) and later during abstinence (day 28). The magnitude of the response was greater in tolerant animals, but no parallel shift in the dose-response curve was seen. The convulsions during abstinence were reduced for at least 8 h after atropine treatment (8mg/kg) on day 3. These results support the hypothesis that cholinergic mechanisms are involved in the changes induced by chronic barbiturate treatments.

Discriminative properties of pentobarbital after repeated noncontingent exposure in gerbils

Two groups of gerbils were pretreated with pentobarbital (P-barb.) (10 and 20 mg/kg) for 20 days before being subjected to drug discrimination training in a (T-shaped shock-escape maze. The rapidity with which these gerbils acquired the discrimination was compared to that of gerbils that were drug naive until beginning the P-barb.-discrimination training. The acquisition rates of the respective groups did not differ substantially within each dose level (10 and 20 mg/kg), although open-field activity (primarily the rearing scores) differentiated the P-barb.- and vehicle-pretreated animals at both dose levels. The peak effect in rectal temperature, however, was not markedly different after the first and 20th drug exposures, although the temperature effects leveled off earlier during the second recording session. In conclusion, certain parameters (open-field activity) may be changed as a consequence of repeated administrations of P-barb. without a significant parallel loss of the cue or stimulus properties of the drug.

Activity pattern and convulsions in the abstinence period after barbital treatment in the rat

Barbital solution was given as the only drinking fluid to male rats for 50 weeks (daily dose around 200 mg/kg). During the treatment and in the abstinence period activity and convulsive episodes were recorded with jiggle cages. Sensitivity to hexobarbital was tested with a threshold method. A 12:12 HR L:D schedule was used. During the barbital treatment the activity of the treated rats started earlier (prior to light off) and ended earlier (prior to light on) compared with controls. A clear 24-hr pattern remained and the total activity was same as in the controls. In the first part of the abstinence period there was a large increase in activity with loss of the 24-hr pattern. Later in the abstinence period the activity was still increased both during light and darkness but a 24-hr pattern influenced by the L:D cycle was re-established. The changes in activity seemed to occur in a least two phases with the second phase lasting between abstinent Days 20-60. A similar two phase pattern was seen also in the changes of the hexobarbital thresholds. The convulsive episodes had several maxima during the abstinence period. The first one occurred around abstinent Days 3--4 and the second one was seen around Day 12. After Day 37 no convulsions were seen. All recorded changes can tentatively be explained by a common increased excitation in the central nervous system.

The interaction between pilocarpine and hexobarbital in male rats

The interaction between pilocarpine and hexobarbital was studied in male rats. Hexobarbital was infused continously. The dose needed to obtain an EEG criterion (the "silent second") was determined. The ensuing anesthesia times after these equi-anesthetic doses were also recorded. At different times prior to the hexobarbital threshold determination the rats were pretreated with 25-200 mg/kg of pilocarpine. In most experimental series pretreatment with methylatropine (2 mg/kg s.c.) was also given to reduce the effects of pilocarpine on peripheral cholinergic sites. In the dose-response study pilocarpine was given 1 h prior to the hexobarbital threshold determination. Pilocarpine in doses of 25-50 mg/kg increased the amount of hexobarbital needed to obtain the "silent second". With higher doses of pilocarpine, increases in hexobarbital thresholds were seen if no convulsion had been induced by the pilocarpine treatment. If a convulsion was recorded the dose of hexobarbital was reduced. Similar results were obtained in the time-effect studies where more convulsions tended to appear if the time between the dose of pilocarpine and the dose of hexobarbital was increased. In animals without convulsions the effect of pilocarpine on the dose of hexobarbital was counteracted by atropine (8 mg/kg i.p.). The ensuing anesthesia times were increased in the pilocarpine pretreated animals, which could be due to either the pilocarpine dose, the increased dose of hexobarbital needed to obtain the "silent second", or both. No regression between body temperature and dose of hexobarbital was found, but there was a regression with the ensuing anesthesia times. The effects of pilocarpine with an increase in hexobarbital threshold is similar to the changes seen in the threshold in the abstinence after chronic barbital treatments. More important, however, is that both increases are reduced by convulsions. Could pilocarpine be a model for the changes in the abstinence after barbital?

Determination of halothane-induced sleeping time in the rat: effect of prior administration of centrally active drugs

A method is described for the determination of halothane-induced sleeping time in the rat. 2 The sleeping time exhibited a diurnal variation which was due, at least in part, to a change in the sensitivity of the central nervous system (CNS) to the anaesthetic. 3 Tolerance to halothane did not develop in rats repeatedly exposed to the anaesthetic over a period of over 48 hours. 4 Repeated sleeping time determinations have been used to follow changes in the sensitivity of the CNS to the anaesthetic occurring with time. 5 A tolerance to halothane was induced by pretreatment of rats with doses of amylobarbitone, pentobarbitone or meprobamate sufficient to keep animals anaesthetized for approximately 12 hours. This tolerance was followed by a period of halothane-hypersensitivity. 6 Halothane-tolerant animals awakened with higher brain halothane concentrations and were also tolerant to intracerebroventricularly administered pentobarbitone. 7 Halothane-hypertensive rats awakened with lower brain halothane concentrations and were also hypersensitivity to intracerebroventricularly administered pentobarbitone. 8 The possibility that the induction of cross-tolerance to halothane may be indicative of a drug's potential to produce dependence is discussed.

The interaction between electrically induced convulsions and tolerance in the abstinence period after chronic barbital treatments in the rat

The effect of electrically induced convulsions was tested on the tolerance to hexobarbital after chronic barbital treatment in male rats. In two experiments barbital was given in the drinking water for more than 20 weeks. The dose was around 200 mg/kg/day. Tolerance was tested with an EEG threshold method where hexobarbital is infused intravenously to obtain a criterion of burst suppression. If on the third abstinent day an electrical convulsion was induced 1 h prior to the threshold determination then the hexobarbital thresholds were reduced compared with barbital-treated animals where no convulsion had been induced (Fig.2). The distribution of hexobarbital threshold doses tended to be biphasic in the barbital-treated animals where a convulsion had been induced. The animals with the most "normal" thresholds in this distribution did not show any increase in threshold on the 24th abstinent day and had a larger mortality during the observation period after the first barbital treatment. Long-term effects of the convulsion thus cannot be excluded in rats if the tolerance on the third abstinent day after a barbital treatment was influenced. No effect of the convulsion was found in untreated controls. The hypothesis that convulsions are means to reduce the changes of physical dependence in the central nervous system was not refuted by the present experiments. A survey of the literature indicates that acetylcholine might be one central nervous transmittor that is involved in these changes.

Tolerance to hexobarbital and supersensitivity to pilocarpine after chronic barbital treatments in the rat

Tolerance and physical dependence induced by chronic treatments with depressant drugs has been proposed as being due to denervation or disuse supersensitivity in the central nervous system. To illustrate this hypothesis, tolerance to barbiturates and the sensitivity to central cholinergic stimulation were recorded in the same male rats. Tolerance to barbiturates was recorded with a hexobarbital threshold. The dose needed to obtain an EEG criterion was measured. Sensitivity in central cholinergic structures was recorded by a temperature threshold method. A subthreshold dose of pilocarpine (3.5 mg/kg) was given s.c. together with 1.0 mg/kg methylscopolamine i.p. An increase in sensitivity to pilocarpine was recorded as a decrease in body temperature. In the abstinence period after 42 weeks of oral treatment with barbital, tolerance was recorded with the hexobarbital threshold method. The tolerance had two peaks, one after the end of the treatment and one approximately 3 weeks later. An increased sensitivity to pilocarpine was also recorded in the abstinence period. It had the same biphasic time pattern as the tolerance. Cholinergic neurones are thus probably involved in the changes induced by chronic barbital treatment. The denervation supersensitivity hypothesis of tolerance was not refuted by the present experiments.

Barbital alteration of central nervous system sensitivity to hexobarbital in the rat

Alteration in central nervous system (CNS) sensitivity to the hypnotic effect of the hexobarbital was assessed by analytical, electroencephalographic, and pharmacokinetic techniques in barbital-treated rats. Each of these conceptually diverse experimental methodologies yielded similar conclusions concerning the alteration of CNS sensitivity by barbital treatment.