Regional biosynthesis of acetylcholine in brain following forced oral chronic barbitone treatment to rat

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.

Long-term effects on biotransformation of labelled choline in different parts of the rat brain induced by single choline injections

The long-term effects of single choline (Ch) injections on the uptake and metabolism of a tracer dose of 3H-Ch were studied in male rats. Choline was administered as a threshold infusion to obtain convulsions 10 and 4 weeks before sacrifice (group 1). At a single threshold infusion of choline 4 weeks before sacrifice no convulsions were induced in 50% of the animals in a second group (group 2--) whereas convulsions were induced in the remainder of the animals in this group (group 2+). Group 3 contained control animals. One min. after administration of a tracer dose of 3H-Ch the animals were sacrificed and examined for 3H-total activity, 3H-Ch, 3H-acetylcholine (3H-ACh) and 3H-phosphorylcholine (3H-PhCh). These activities were determined in three parts of the brain (cortex, striatum, midbrain + medulla oblongata). In the cortex a significant negative correlation between brain weight and 3H-ACh synthesis was seen in group 1. A comparison between group 2+ and group 2- indicated that induced convulsions were not critical for this effect. In the striatum there was a significant reduction in the total uptake of radioactivity in group 1 and group 2- when values were compared to the control group. Furthermore a significant positive correlation was found between the concentration of radiolabel and 3H-ACh synthesis and a negative relationship with the level of 3H-Ch. In the midbrain preparation the synthesis of 3H-ACh was reduced in group 1 where a significant negative correlation was found between the average threshold dose of choline and both 3H-ACh and 3H-PhCh synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Temperature as a dependent variable in the study of cholinergic mechanisms

1. Change in core temperature over time can be used as a dependent variable when studying the effects of manipulations on neurotransmitter systems. This article focuses on the measurement of core temperature as a strategy for detecting changes in the status of cholinergic systems. 2. Cholinergic neurons participate in the process of thermoregulation and interventions affecting them alter the thermal response to cholinomimetics. For example, chronic treatment with amitriptyline, chronic swim stress and inescapable footshock supersensitize rats to the hypothermic effects of oxotremorine. 3. This is consistent with the hypothesis that the pathophysiologies of tricyclic antidepressant withdrawal phenomena and stress involve supersensitivity of muscarinic mechanisms. 4. Uses of thermoregulation paradigms for investigating the actions of lithium ion, electroconvulsive shock and substances of abuse on muscarinic mechanisms are discussed. Applications to problems in the arena of clinical research are highlighted.

Early phenobarbital-induced alterations in hippocampal acetylcholinesterase activity and behavior

Early exposure to phenobarbital (PhB) causes marked destruction of large neurons which are then forming both in the hippocampus and in the cerebellum. Such exposure to PhB also reduces the achievements of mice in hippocampus-related behaviors such as radial 8-arm maze performance. Experimental evidence suggests that these behaviors are partially mediated by cholinergic transmission. We studied the performance of mice, exposed to PhB prenatally or neonatally, in radial 8-arm maze. Both treatments caused significant impairments in the animals' performance in the maze. Acetylcholinesterase (AChE) and pseudocholinesterase (pChE) activities were studied in the hippocampus and cerebellum of mice who were exposed to PhB prenatally or neonatally. These enzymes are involved both in cholinergic transmission and in neuronal development. A significant decrease (13-16%, P less than 0.01) in hippocampal AChE specific activity was found between days 15 and 22 in animals exposed to PhB neonatally. The total hippocampal activity of AChE was also greatly reduced (25-39%, P less than 0.01) during that period as a result of both the reduction in specific activity and a reduction in hippocampal weight of the treated animals. These alterations were transient and were not detected in adulthood. No changes in hippocampal AChE or pChE activities were found in animals treated prenatally. Cerebellar AChE and pChE activities were not altered after prenatal nor after neonatal exposure to PhB. It is possible that the short-term effect of neonatal treatment on AChE specific activity might mediate the long-term impairments in hippocampus-related behaviors.

Interaction between spontaneous and electrically induced convulsions and their short- and long-term effects in the abstinence after chronic barbital treatment in the rat

Male rats were treated with barbital supplied in their drinking water (daily dose around 200 mg/kg) for 50 weeks. When the treatment was stopped (day 0) spontaneous convulsions were monitored for the first 3 days of the abstinence. On day 3 a convulsion was induced by electricity in half of the rats (controls and barbital-treated) and 1 h later the sensitivity to hexobarbital was determined with a threshold test. Sensitivity to hexobarbital was then tested in the same manner at approximately weekly intervals for the first 110 days of the abstinence. On day 3 of the abstinence a tolerance to hexobarbital (48% increase in threshold above controls) and a reduced threshold to induce convulsions with electricity (-27% compared with controls) was seen in previously barbital-treated animals. Spontaneous or induced convulsions occurring prior to the hexobarbital threshold determination decreased the tolerance to the same extent (-22 to -28%). On day 28 rats with no convulsions up to day 3 had a marked renewal of tolerance to hexobarbital (29% increase above controls), while rats with convulsions recorded up to day 3 had less or no such tolerance. There was a positive correlation (r = 0.63) between the hexobarbital thresholds in barbital-treated rats recorded on day 3 and on day 28. Later in the abstinence, barbital-treated rats with convulsions prior to day 3 tended to have a hexobarbital threshold slightly but significantly elevated compared with the controls (10-15%). This change could be a sign of a long-lasting increased excitation.

Changes in populations of cholinergic binding sites in brain after chronic exposure to barbital in rats

Male rats were treated with barbital in their drinking fluid for about 40 weeks (daily dose 200 mg/kg). The treatment was stopped on day 0 and the number of muscarinic binding sites was measured in 3 brain parts (striatum; cortex and hippocampus; medulla oblongata and midbrain) during the abstinence with labeled quinuclidinyl benzilate (QNB) as ligand. Nicotine-like binding sites were quantified in the cortex with labeled tubocurarine. An increased number of QNB-binding sites was found in the striatum during the early part of the abstinence (days 0--3), found by another increase later in the abstinence (days 12--21). In the cortex preparation a variable increase in QNB binding was measured from days 2--9 of the abstinence, whereas in the midbrain preparation the QNB binding was found to be increased only on day 9 in comparison to controls. A change in the proportion between high and low affinity muscarinic binding sites (measured by agonist-antagonist competition) was found in the cortex preparation during the period when the QNB binding was increased. Prior to the maximum abstinence convulsions the QNB binding was higher in rats without recorded spontaneous convulsions in comparison to rats with convulsions. A negative correlation between the time after the last convulsion and the number of QNB binding sites was observed in data from rats where the maximum convulsions had occurred before sacrifice. In addition, a negative relationship between the number of muscarine- and nicotine-like binding sites was found in the cortex during days 5--9 of the abstinence.

Tolerance, physical dependence and changes in muscarinic receptor binding sites after chronic ethanol treatment in the rat

Rats were treated with ethanol in the drinking water for 73-75 weeks. The daily drinking periods were restricted to only 2x1 h to get significant ethanol blood concentrations. After the end of the ethanol treatment (last day of treatment denoted day 0 of the abstinence) tolerance to sedatives were recorded with a hexobarbital threshold method, convulsions recorded with jiggle cages, and muscarinic receptor binding sites determined with 3H-quinuclidinyl benzilate. Tolerance to hexobarbital was found to have a maximum on day 8 in the abstinence while convulsions recorded from the rats participating in the hexobarbital threshold determinations had a maximum on day 14. Atropine administered as a single dose on day 8 decreased the tolerance to hexobarbital on that day and seemed to reduce the number of convulsions on the following days. Muscarinic receptor sites were significantly increased in the striatum on day 8 of the abstinence at the time of maximal tolerance, This increase in muscarinic receptors was less in rats which had had convulsions during the abstinence before sacrifice. Thus the results indicate that cholinergic mechanisms are involved in the changes seen during the abstinence after chronic ethanol exposure.

The effects of thyrotropin releasing hormone on pentobarbital tolerance

The effects of thyrotropin releasing hormone (TRH) on the responses produced by acute and chronic administration of pentobarbital (PB) were investigated in male Swiss Webster mice. Intraperitoneal injection of TRH antagonized the hypothermic and hypnotic effect of PB (75 mg/kg, i.p.). Pentobarbital hypnosis was also antagonized by TRH in mice rendered tolerant to PB by implantation of a pellet (each containing 75 mg of free acid form of the drug) for two days. Injections of TRH (5 mg/kg) once a day or twice a day for two days did not modify the development of tolerance to the hypothermic and hypnotic effect of PB. It is concluded that TRH can modify only the acute responses of PB in naive and PB-tolerant mice, however, in the dosage regimen used it does not affect the processes involved in the genesis of the tolerant state.

Changes in cholinergic function in rat brain late in abstinence after chronic barbital treatment

The effect of a forced long-term barbital treatment (daily dosage about 200 mg/kg for 32 weeks) on cholinergic brain mechanisms was studied on days 53 and 81 - 83 after withdrawal. Some of the barbital-treated rats were given a single injection of atropine (8 mg/kg intraperitoneally) on the third day of abstinence (group BA), while the other barbital-treated rats received saline (group BS). The sensitivity to a threshold dose of choline was tested on day 53. The rats in the BS group were more sensitive to choline than those in the control groups. When the rats were sacrificed on day 81 - 83 after withdrawal, a significant reduction in brain weight was found in group BS but not in group BA when compared with the controls. At the same time the content of endogenous acetylcholine was reduced in the barbital-treated animals and this reduction seemed to be positively related to the reduction of brain weight. After sacrifice the uptake and biotransformation of a tracer dose of radioactive choline were also studied. A marked difference was seen between the brain regions. In the cortex the biosynthesis of radioactive acetylcholine was significantly lower in the BS group than in the control group; the BA group was in between. In the midbrain no significant changes in radioactive acetylcholine synthesis were seen. In the striatum a significantly lower formation of radioactive acetylcholine was found in the BA group and a lower radioactive phosphorylcholine formation in he BS group. The studied variables indicate that prolonged barbital treatment induces changes in cholinergic function in rat brain extending considerably beyond withdrawal. Treatment with atropine early in the abstinence seemed to reduce these changes.

Atropine in the abstinence after chronic barbital treatment in the rat

Since cholinergic mechanisms seem to be involved in the changes induced by chronic barbital treatments in male rats, various treatments with atropine were given during the abstinence after 32--33 weeks of exposure to barbital (200 mg/kg/day). The effects of the atropine treatment were recorded as a tolerance towards a hexobarbital anaesthesia threshold. In Experiment 1, 1.5 mg/kg/day of atropine was given on Days 23--29 after the end of the barbital treatment. Two weeks after the end of the atropine treatment a significant tolerance (+ 16%, Fig. 1A) was seen in barbital-treated animals given the atropine treatment (group BA), but not in the corresponding control groups (group BS, CA and CS). In Experiment 2, the atropine dose was 4 mg/kg/day and the treatment was given on Days 29--44. A tolerance (+ 20%, Fig. 4) with maximum 2 weeks after the end of the atropine treatment was recorded in the animals given the combined treatment (group BA). In both experiments a single dose of atropine given on Day 3 reduced this tolerance. In Experiment 3 the atropine treatment was 4 mg/kg/day on Days 3--12. A tolerance above that induced by the barbital treatment (+ 16%, Fig. 5) was recorded three weeks after the end of the atropine treatment (group BA). In this group there was also recorded a new tolerance much later (Day 80). Since a tolerance was induced by atropine, only in previously barbital-treated animals, a carry-over of some change in cholinergic mechanisms is probably involved.