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

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.

Lack of influence of brain catecholamines on acute effects of barbiturates

Rats with cerebral electrode implants were tested for sensitivity to EEG burst suppression by intravenously-infused sodium methohexital following manipulation of brain catecholamine function. Although depletion of both norepinephrine (NE) and dopamine (DA) with 6-hydroxydopamine resulted in a slight increase in methohexital sensitivity (MHS), similar depletion with alpha-methyltyrosine did not alter MHS. In addition, desipramine, an agent which selectively blocks uptake of NE did not affect MHS. The results indicate that brain NE and DA exert little, if any, effect on brain responsiveness to the acute effect of barbiturates.

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.