Combined action of thioperamide plus scopolamine, diphenhydramine, or methysergide on memory in mice.

The aim of the present experiments was to test the role played by the interaction of the selective H3 receptor antagonist, thioperamide, with the cholinergic, histaminergic, and serotonergic systems in modifying memory. The behavioral tests used (open-field and passive-avoidance repetition) were selected on the basis of the action displayed by thioperamide in these behavioral situations. Posttrial administration of thioperamide (5 mg/kg) resulted in an improvement in memory consolidation, as tested in the repetition of the open-field test, but repeated posttrial administration of thioperamide (2 or 5 mg/kg) had no effect in the repetition of passive avoidance test. Scopolamine (2 mg/kg) caused a deterioration in the memory processes in both tests: this effect was blocked by 2 mg/kg of thioperamide, which was itself ineffective in the test. These results may suggest that both the improvement in memory due to thioperamide and its antagonism of the amnestic effects of scopolamine are determined by activation of central cholinergic systems, due to thioperamide inhibition of H3 heteroreceptors. Diphenhydramine (2 or 10 mg/kg) was itself ineffective in the tests, but counteracted the memory improvement caused by thioperamide in the repetition of the open-field test. The effect of diphenhydramine is discussed in terms of interactions between histaminergic and cholinergic systems. Methysergide counteracted the effect of thioperamide in the open-field test only at a high dosage (50 mg/kg). The possible implication of serotonergic systems on the effects of the methysergide-thioperamide interaction in the memory process is discussed.

Effects of thioperamide on locomotor activity and on memory processes.

1. Rats were tested in an open field. Thioperamide given i.p. 30 minutes before the tests produced an increase of locomotor activity at 2 mg/kg and no behavioral effect at 5 mg/kg. 2. The repetition of the open field tests caused a reduction of ambulation in three successive tests. This effect was increased by thioperamide 2 and 5 mg/kg given after the tests, suggesting that this compound improved memory consolidation.

Behavioral and neurochemical effects induced by subchronic l-deprenyl administration.

(-)Deprenyl was administered orally to rats for 15 days. In the staircase maze, a reduction of incorrect responses was observed at 0.9 mg/kg/day; higher or lower doses (3.5 or 0.35 mg/kg/day) were ineffective. In the same range of doses, the subchronic administration of (-)deprenyl did not modify the levels of norepinephrine, 5-hydroxytryptamine, 5-hydroxyindolacetic acid or the density and affinity of alpha-noradrenergic receptors in the cortex, olfactory system, hippocampus and striatum. An increase of the dopamine and a reduction of dihydroxyphenylacetic acid levels was observed only at the highest tested doses, at which no behavioral modification was observed. Only at 1.0 mg/kg/day did (-)deprenyl increase the acetylcholine (ACh) levels in the olfactory system, hippocampus and striatum. This neurochemical effect may be correlated to the behavioral effect observed in the same range of doses. We propose that this increase of ACh levels is determined by an activation of dopaminergic systems, resulting from the increase in the levels of PE caused by the inhibition of monoamine oxidase B (MAO-B) by (-)deprenyl.

Behavioral and neurochemical effects of a chronic choline-deficient diet in the rat.

Chronic administration of a choline-deficient diet for 20 days caused no modification in ACh levels in the cortex, hippocampus and olfactory system of the rat and an increase in the density (Bmax) of the muscarinic receptors in the hippocampus. The choline-deficient diet caused no modification in noradrenaline levels and a reduction in the density of alpha1-adrenoceptors in the cortex, hippocampus and olfactory system. This paper discusses the possibility that these neurochemical effects are correlated with the reduction in spontaneous memory decay which was observed in the staircase maze after 20 days of a choline-deficient diet.

Behavioral and neurochemical effects induced by chronic L-DOPA administration.

L-DOPA, in combination with benserazide, in the ratio 4:1 (w/w), was administered orally to rats. In the staircase maze test a low dose of L-DOPA (3 mg/kg/day) reduced the increase in errors caused by 20 days interruption of daily training, while a higher dose (30 mg/kg/day) was ineffective. A decrease in levels of dopamine in the olfactory system and DOPAC in the striatum was seen at all tested doses of L-DOPA, while an increase in 5-HT levels was seen in the hippocampus and in the striatum. 5-HIAA levels did not change. Levels of ACh in the olfactory system were reduced at all doses of L-DOPA, while in the hippocampus this effect was seen only at the dose of 90 mg/kg/day. The density of muscarinic receptors was not altered. All tested doses of L-DOPA caused norepinephrine levels to fall in the hippocampus and increase in the striatum. The density of alpha 1-adrenoceptors was reduced only at the two lower doses of L-DOPA. A comparison of the neurochemical results with the behavioral modifications seen in the staircase maze test suggests that the catecolaminergic systems are implicated in the memory process.

Habituation of exploratory activity in rats: action of N6phenylisopropyladenosine, caffeine and their combination.

1. L-PIA (0.2 mg/kg), caffeine (15 mg/kg) and their combination were given subcutaneously to rats tested in open field. 2. The acute administration of L-PIA reduced ambulation. Caffeine alone was ineffective but increased ambulation in combination with L-PIA. These effects may have been determined by an interaction of L-PIA and caffeine on adenosine receptors. 3. Tolerance to L-PIA was observed after a chronic administration (20 days). The chronic administration of caffeine alone or in combination with L-PIA increased ambulation. 4. In the habituation test the reduction of ambulation in the 3 successive trials was increased by L-PIA. Caffeine did not antagonize the L-PIA effect and, in part, had the same effect. These results suggest that L-PIA and, in part, caffeine improve memory consolidation in open field, and do not support the notion that L-PIA and caffeine action on memory processes is due to an interference on adenosine receptors.

The action of arecoline on retrieval and memory storage evaluated in the staircase maze.

Rats were trained to run on a staircase stopping on the 3rd, 6th, 9th, and 12th steps (correct responses). Stopping on any other step was considered an error. Acute administration of 0.5 mg/kg of arecoline 15 min before the trial improved behavior and 3.5 mg/kg of arecoline caused a reduction of correct responses. An interruption of the daily training for 20 days caused a 15% reduction of correct responses in control animals. Chronic administration of arecoline during the first 15 days of a no-training period caused an increase of errors only at 3.5 and 8 mg/kg/day. The interpretation of these results is that arecoline improves the retrieval process and accelerates the spontaneous decay of memory. The increase of correct responses after the acute administration of 0.5 mg/kg of arecoline was not evident in rats treated for 15 days with arecoline 6.5 mg/kg/day. These results suggest that arecoline improves retrieval only in the absence of tolerance development.

Action of caffeine, L-PIA and their combination on memory retention in the rat.

Rats were trained to run in a staircase and to stop on the 3rd, 6th, 9th and 12th steps. The increase of errors (arrest on any other step) after 2O days of interruption of the daily training was evaluated after a chronic administration (15 days) of L-PIA (N6-(L-phenylisopro-pyl)adenosine) 1 mg/kg/day; caffeine 15,45,8O mg/kg/day and of the combination of L-PIA 1 mg/kg/day with 15, 45 and 80 mg/kg/day of caffeine. In the controls and after caffeine administration, the interruption of the daily training caused a reduction of correct responses. After L-PIA alone or in combination with caffeine there was no reduction of correct responses after 2O days of no daily training. These results indicate that in the behavioral situation of our experiments there was no antagonism between caffeine and L-PIA.

Action of chronic choline administration on behavior and on cholinergic and noradrenergic systems.

Chronic administration of low doses (0.2-0.8 g/kg/day) of choline caused in the rat an increase of errors evaluated in the staircase maze after 20 days of interruption of daily training. An analogous pharmacological treatment caused no modification of the acetylcholine (ACh) and norepinephrine (NE) levels and no consistent modification of the density of muscarinic and alpha 1-adrenergic receptors. Only at higher doses (2.5 g/kg/day) did chronic administration (20 days) of choline cause in several sections of the CNS, an increase of ACh and NE levels and of the muscarinic receptor density. These observations indicate that only at high doses of choline are there consistent modifications of the central cholinergic systems, suggesting that the behavioral modifications observed at low doses of choline are not determined by an upregulation of the central cholinergic system.

The action of scopolamine on retrieval and memory storage in rats evaluated in the staircase maze.

Rats were trained to run on staircase stopping on the 3rd, 6th, 9th, and 12th steps (correct responses). Stopping on any other step was considered an error. The acute administration of scopolamine (1.5 mg/kg) 20 min before the trial caused a reduction of the correct responses. An interruption of the daily training for 20 days caused, in the controls, a 24% reduction of correct responses. A chronic administration of scopolamine, at doses over 10 mg/kg in the first 15 days of the no-training period, nullified the behavioral deterioration observed in the controls. The interpretation of these results is that scopolamine damages the retrieval process and blocks the spontaneous decay of memory, as was observed in the controls after 20 days of interruption of the daily training.

Action of a chronic disulfiram administration on memory decay and on central cholinergic and adrenergic systems.

The chronic administration (15 days) of disulfiram reduced the levels of noradrenaline (NA) in the olfactory system and in the subcortex at all the tested doses (50, 200 and 400 mg/kg/day). No modification of the density (Bmax) and of the dissociation constant (Kd) for alpha 1-adrenergic receptors was observed (radioligand [3H]prazosin). Only in the hippocampus the acetylcholine (ACh) levels and the Bmax for muscarinic receptors (radioligand [3H]QNB) were increased at all doses tested. Modifications of the cholinergic system were observed in the subcortex and in the cortex only at the higher doses of disulfiram. After 20 days interruption of the daily training in the staircase maze, 50, 100 and 200 mg/kg/day of disulfiram accelerated spontaneous decay of memory. It is hypothesized that the modifications of the hippocampal cholinergic system (increase of the ACh levels and of the density of the muscarinic receptors) may be the condition determining the acceleration of the decay of memory caused by disulfiram.

Behavioral and neurochemical modifications caused by chronic alpha-methylparatyrosine administration.

The chronic administration of alpha-methylparatyrosine (AMT) caused a reduction of the noradrenaline levels in the hippocampus (at 150 and 300 mg/kg/day) and in the subcortex (at 30, 150 and 300 mg/kg/day). The acetylcholine levels were reduced in the hippocampus and in the olfactory brain at all the tested doses of AMT. An increase of the Bmax of muscarinic and alpha 1-adrenoceptors was observed at 30 mg/kg/day of AMT; only in the subcortex AMT caused no modification of the density of muscarinic receptors. The degree of increase of the receptors density at 30 mg/kg/day was reduced at the higher doses of AMT. AMT 30 mg/kg/day caused a reduction of the errors in the staircase maze after 20 days of interruption of the daily training. These results might suggest a correlation between the behavioral effect and the increase of density, not only of the adrenoceptors, but also of the muscarinic receptors. It is proposed that the behavioral effects caused by chronic AMT are the consequence of complex neurochemical interactions.

Effect of diisopropylfluorophosphate and atropine or mecamylamine on acetylcholine levels in the central nervous system of the rat.

The effect on acetylcholine levels caused by diisopropylfluorophosphate, either alone or in combination with atropine or mecamylamine, has been studied in the olfactory brain, medulla pons and cortex of the rat. At sublethal doses (0.25 and 0.5 mg/kg), diisopropylfluorophosphate reduced the acetylcholine levels. This effect was completely antagonized by atropine in the medulla pons and by mecamylamine in the medulla pons, olfactory brain and partly in the cortex. These observations suggest that the reduction of acetylcholine levels at low doses of diisopropylfluorophosphate is mediated by a muscarinic system in the medulla pons and by a nicotinic system in the medulla pons, the olfactory brain and also, in part, in the cortex.

Effects of combinations of arecoline and atropine on mouse motor activity.

1. Effects on motor activity were studied after acute administration of arecoline, atropine alone and in combination in the mouse. 2. Atropine from 15 to 45 mg/kg increased motor activity. 3. A reduction in activity was observed at doses of arecoline above 0.2 mg/kg. 4. An antagonism between arecoline and atropine was observed only at low doses of arecoline, while higher doses of arecoline in association with atropine increased activity.

The effect of chronic atropine administration on mouse motility and on ACh levels in the central nervous system.

Changes in mouse motility and CNS cortical and subcortical ACh levels were studied after chronic (20 days) administration of 30, 40 and 60 mg/kg/day atropine. An increase in motility similar to that induced by acute atropine administration was observed, whereas the ACh levels reduction caused by acute administration was not repeated. These results suggest that changes in mouse motility caused by atropine are not correlated to its modification of ACh levels in the CNS.

In vitro and in vivo action of diisopropylfluorophosphate, of atropine and their synergism on acetylcholinesterase activity.

Modifications of acetylcholinesterase (AChE) activity, caused by diisopropylfluorophosphate (DFP) and atropine and their synergism, were determined in vitro and in vivo in the rat. In vitro atropine caused a reduction of the AChE activity, but reduced the enzyme inhibition by DFP. These results are discussed in the light of the evidence that AChE is an allosteric enzyme. In vivo the AChE inhibition by DFP is in part reduced by atropine. The limited protection of AChE by atropine may be one of the factors implied in the reduction of the DFP toxicity by atropine.

Action of a chronic arecoline administration on mouse motility and on acetylcholine concentrations in the CNS.

The modifications of mouse motility and of the levels of acetylcholine (ACh) in two sections of the CNS caused by a chronic administration of 4.5; 9.5; 28.5 and 60 mg kg-1 day-1 of arecoline for 20 days have been studied. At low doses (4.5 and 9.5 mg kg-1 day-1), arecoline caused no modification of the ACh levels and of the motility. The higher doses (28.5 and 60 mg kg-1 day-1) caused a reduction of the mouse motility and an increase of the ACh levels in the subcortical structures of the CNS of the mouse.

Action of diisopropylfluorophosphate and of diacetylmonoxime on acetylcholine levels and on cholinesterase activity in the central nervous system.

The modification of the acetylcholinesterase (AChE) activity and of the acetylcholine (ACh) levels caused by diisopropylfluorophosphate (DFP) alone and in association with 300 mg/kg of diacetylmonoxime (DAM) have been studied in the olfactory brain, in the cortex and in the medulla-pons of the rat. DFP caused a reduction of the levels of ACh at 0.25 and 0.5 mg/kg. An increase of the levels of ACh was observed at 4.0 mg/kg of DFP only in the olfactory brain and in the cortex. A dose-dependent reduction of the AChE activity was observed after DFP. DAM reactivated the AChE activity to the levels of the controls after 0.25 and 0.5 mg/kg of DFP. In this experimental situation, the ACh levels returned to the control levels after 0.25 mg/kg of DFP and, in the medulla-pons, also after 0.5 mg/kg of DFP. DAM caused a partial recovery of the AChE activity but no modification of the ACh levels at 1.0, 2.0 and 4.0 mg/kg of DFP.

Acetylcholine levels in the spinal cord after atropine, diisopropylfluorophosphate and naphthylvinylpyridine.

The decay of ACh levels from the death of the rat to longer time intervals has been estimated in the rat cervical spinal cord. The ACh levels at the beginning of the postmortal decay have been evaluated by extrapolation to t = 0 of the linear regressions log ACh concentrations vs time (t) from the rat decapitation. Atropine, diisopropylfluorophosphate (DFP) and naphthylvinylpyridine (NVP) caused a reduction of the ACh levels evaluated before the beginning of the postmortal decay. The levels of ACh at stabilization of the postmortal decay were increased by atropine and DFP. These effects may be correlated to modifications caused by the studied drugs on the ACh synthesis, release and destruction.

Effects of chronic manipulations of dietary choline on dynamic behavioural situations.

1. The modifications in rat behaviour caused by chronic manipulations of dietary choline were studied in two schedules of operant conditioning. Adult rats were maintained on choline-deficient, low-choline and high-choline enriched diets. 2. In the "periodic conditioning" test, the schedule of reinforcement was changed from a fixed ratio to a fixed interval schedule. In the "reversal" test the contingency for food delivery was switched four times from one lever to the other in a two lever Skinner box. 3. In the "periodic conditioning" test, control and treated groups showed the same reduction of responses/reinforcement from the beginning to the end of trial. The time-course reduction of responses/reinforcement became significant in the high-choline (331 mg/kg/day) and deficient-choline groups earlier than in the low-choline (75 mg/kg/day) enriched and control groups. 4. In the "reversal" test, the low-choline (110 mg/kg/day) enriched diet improved the reinforced responses in the IV reversal; the high-choline (330 mg/kg/day) diet gave a significant impairment of the reinforced responses in the III and IV reversals. The deficient-choline diet caused a reduced number of the total responses and a worsening of the reinforced responses in the II, III and IV reversals.