Effects of p-chlorophenylalanine and scopolamine on retention of habits in rats

Enhancement of hippocampal cholinergic neurotransmission through 5-HT1A receptor-mediated pathways by repeated lithium treatment in rats

Hippocampal cholinergic neuronal activity is reported to be regulated, at least partly, through serotonin1A (5-HT1A) receptors. Chronic lithium treatment has been shown to alter both behavioral and neurochemical responses mediated by postsynaptic 5-HT1A receptors. We investigated whether long-term lithium treatment affects central cholinergic neurotransmission through 5-HT1A receptor-mediated pathways. Changes in acetylcholine (ACh) release induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, in the rat hippocampus were measured using a microdialysis technique and a radioimmunoassay for ACh. Administration of lithium for 21 days resulted in a serum lithium concentration of 1.03 mM and caused little change in density or affinity of [3H]8-OH-DPAT binding sites in the hippocampus. The local application of 8-OH-DPAT into the hippocampus of lithium treated rats increased the ACh efflux in both the absence and the presence of physostigmine, a cholinesterase (ChE) inhibitor, in the perfusion fluid. The basal ACh efflux of lithium treated rats was not different from that of the control rats under normal conditions, but was significantly higher than that of the controls when ChE was inhibited. These results demonstrate that chronic lithium treatment increases spontaneous ACh release in the hippocampus under conditions of ChE inhibition, but not under normal conditions, and enhances cholinergic neurotransmission through 5-HT1A receptor-mediated pathways, and suggest that activation of 5-HT1A receptor function by lithium is related to the enhancement of hippocampal cholinergic neurotransmission.

Key words: Acetylcholine (ACh), hippocampus, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), lithium, serotonin1A (5-HT1A) receptor.

Stimulation of 5-HT1A receptors in the dorsal raphe reverses the impairment of spatial learning caused by intrahippocampal scopolamine in rats

This study investigated the effect of stimulating 5-HT1A receptors in the dorsal raphe on the impairment of learning caused by 4 microg/microL scopolamine injected in the CA1 region of the dorsal hippocampus in rats performing a two-platform spatial discrimination task. At 1 (but not 0.2) microg/0.5 microL administered in the dorsal raphe on each acquisition training day 5 min before bilateral intrahippocampal injection of 4 microg/microL scopolamine, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, had no effect on choice accuracy and latency or errors of omission but completely antagonized the impairment of choice accuracy by intrahippocampal scopolamine. Administered into the dorsal raphe at 0.2 and 1 microg/0.5 microL, WAY 100635, a 5-HT1A receptor antagonist, had no effect on rats' performance or on the impairment caused by intrahippocampal scopolamine but dose-dependently antagonized the effect of 1 microg/0.5 microL 8-OH-DPAT on the scopolamine-induced deficit. The results show that stimulation of presynaptic 5-HT1A receptors in the dorsal raphe reverses the deficit caused by intrahippocampal scopolamine, probably by facilitating the transfer of facilitatory information from the entorhinal cortex to the hippocampus. Together with a previous study showing that blockade of postsynaptic hippocampal 5-HT1A receptors antagonized the effect of intrahippocampal scopolamine in the two-platform spatial discrimination task (Carli et al., 1995b), the results suggest that drugs with presynaptic stimulatory and postsynaptic blocking actions on 5-HT1A receptors, such as partial agonists at these receptors, may be useful in the symptomatic treatment of human memory disturbances associated with loss of cholinergic innervation to the hippocampus.

The decrease of serotonin release induced by a tryptophan-free amino acid diet does not affect spatial and passive avoidance learning

We assessed whether consumption of a diet lacking in tryptophan (TRP) resulted in alteration in learning and memory performance and hippocampal 5-HT release in rats. Two hours after the acute administration of TRP-free (T) and balanced (B) diet rats were trained in a one-trial passive avoidance task. The two groups of rats showed no significant difference in retention latencies. Two other groups of rats, fed with the above diets during the acquisition of a radial-arm maze task, showed no difference in baseline performance. The acute ingestion of the T diet produced a significant and long lasting decrease of hippocampal and cortical 5-HT release in rats when compared to the B diet, while the 12th day of the T diet, 5-HT was not detectable in the dialysate. These data indicate that the diminished brain release of 5-HT induced by a T diet is not sufficient to impair cognitive processes.

Dose-related impairment of spatial learning by intrahippocampal scopolamine: antagonism by ondansetron, a 5-HT3 receptor antagonist

To study the role of hippocampal muscarinic receptors in spatial learning, various doses of scopolamine were injected bilaterally into the CA1 region of the dorsal hippocampus of rats trained in a two-platform spatial discrimination task. Scopolamine administered 10 min before each training session at doses ranging from 3.75 to 15 micrograms/microliter impaired choice accuracy, had no effect on choice latency and increased the errors of omission only with 7.5 micrograms on day 1 and with 15 micrograms on days 1 and 2 of training. No dose affected choice accuracy or latency of a non-spatial visual discrimination task. A subcutaneous dose of 1 microgram/kg ondansetron, a 5-HT3 receptor antagonist, 30 min before each training session prevented the impairment of choice accuracy by intrahippocampal 3.75 micrograms scopolamine but 0.1 microgram/kg ondansetron had no such effect. No dose of ondansetron by itself modified the acquisition of spatial learning. The results suggest that relatively low doses of scopolamine in the dorsal hippocampus selectively impair the acquisition of a spatial discrimination task, and that blockade of 5-HT3 receptors prevents the deficit caused by the muscarinic antagonist. The utility of the deficit of spatial learning induced by intrahippocampal scopolamine for modelling some aspects of memory disturbances in Alzheimer's disease is discussed.

The role of brain noradrenaline in cortical activation and behavior: a study of lesions of the locus coeruleus, medial thalamus and hippocampus-neocortex and of muscarinic blockade in the rat

Local injection of 6-hydroxydopamine in the locus coeruleus resulted in a 90% depletion of noradrenaline (NA) in the cerebral cortex as assessed by high-pressure liquid chromatography. This NA depletion had no effect on scopolamine-resistant hippocampal rhythmical slow activity and only an occasional effect on scopolamine-resistant neocortical low voltage fast activity. However, NE depletion resulted in a slight deficit in a behavioral swim-to-platform test and increased the deficit produced on the test by systemic treatment with scopolamine. Large surgical lesions of the medial thalamus or hippocampal formation plus posterior neocortex greatly increased the behavioral deficit produced by scopolamine. It is concluded that ascending noradrenergic projections play only a modest and possibly indirect role in the control of electrocortical activation and that a number of different brain lesions increase the behavioral impairment produced by central muscarinic blockade.

The role of serotonergic-cholinergic interactions in the mediation of cognitive behaviour

Cholinergic systems have been linked to cognitive processes such as attention, learning and mnemonic function. However, other neurotransmitter systems, such as the serotonergic one, which may have only minor effects on cognitive function on their own, interact with cholinergic function and their combined effects may have marked behavioural actions. Some studies have dealt with serotonergic-cholinergic interactions, but it is unclear whether both systems affect cognition directly or whether interactions at a behavioural level result from additional alterations in non-cognitive factors. This distinction is difficult, since it is possible that the diverse cholinergic and serotonergic systems serve different roles in the mediation of cognitive processes, both at the neuroanatomical and neurochemical level. Nevertheless, it is possible that cholinergic systems primarily alter accuracy in cognitive tasks, whereas serotonergic neurotransmission modulates behaviour by altering bias (motivation, motor processes). Whether serotonin alters accuracy or bias, however, may also depend on the cognitive process under investigation: it is suggested that attention, stimulus processing and/or arousal can be influenced by both cholinergic and serotonergic systems independently from each other. Cholinergic and serotonergic projections to cortex and thalamus may be of importance in the mediation of these cognitive processes. Serotonergic-cholinergic interactions could also be of importance in the mediation of learning processes and trial-by-trial working memory. The data available do not allow an unambiguous conclusion about the role of these interactive processes in the mediation of long-term reference memory. These processes may rely on serotonergic-cholinergic interactions at the hippocampal level. It is concluded that serotonergic-cholinergic interactions play an important role in the mediation of behavioural, including cognitive, performance, but that further studies are necessary in order to elucidate the exact nature of these interactions.

Effects of the cholinomimetic SDZ ENS-163 on scopolamine-induced cognitive impairment in humans

Scopolamine-induced cognitive impairment was used in healthy men to evaluate the central nervous system activity of the new cholinomimetic SDZ ENS-163. Eighteen subjects were treated in a crossover design with oral placebo/intravenous saline, 50 mg of oral SDZ ENS-163/intravenous saline, oral placebo/0.4 mg of intravenous scopolamine, and 50 mg of oral SDZ ENS-163/0.4 mg of intravenous scopolamine. The administration of placebo with scopolamine caused significant cognitive impairment, as assessed by the Computerized Neuropsychological Test Battery (CNTB), and also decreased salivation and heart rate. In contrast, SDZ ENS-163 with saline had no effect on CNTB scores, increased salivation, and increased heart rate. Despite the observed cholinomimetic effects of SDZ ENS-163 when administered with saline, the changes in CNTB scores, heart rate, and salivation were indistinguishable between placebo/scopolamine and SDZ ENS-163/scopolamine. Thus, 50 mg of oral SDZ ENS-163 has cholinomimetic activity in normal men, but this dose is insufficient to reverse the muscarinic effects of 0.4 mg of intravenous scopolamine.

5-HT1A and muscarinic acetylcholine receptors jointly regulate passive avoidance behavior

The present study was designed to investigate the effects of combined stimulation of 5-HT1A or 5-HT2 receptors and blockade of muscarinic acetylcholine receptors on passive avoidance behavior. Administration of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2 receptor agonist, impaired passive avoidance acquisition (pre-training injections) and consolidation (post-training injections) performance. Ketanserin, a 5-HT2 receptor antagonist, blocked the performance-impairing effect of DOI on passive avoidance consolidation. Interestingly, 5-HT receptor agonists may affect passive avoidance consolidation only during the immediate post-training period, as passive avoidance testing performance was not modulated by 8-OH-DPAT or DOI injected 30 min after the training trial. Furthermore, passive avoidance retention (pre-testing injections) performance was impaired only by the highest dose of 8-OH-DPAT, and DOI had no effect on passive avoidance retention. Next, the effects of combined 5-HT and acetylcholine receptor manipulations on passive avoidance behavior were studied. The effects on passive avoidance behavior of a combination of subthreshold doses of scopolamine, a muscarinic acetylcholine receptor antagonist, and 8-OH-DPAT were compared to those of a single high dose of scopolamine. A combination of small doses of scopolamine and 8-OH-DPAT impaired acquisition and consolidation of passive avoidance performance, but a single high dose of scopolamine impaired only acquisition performance. The small dose of 8-OH-DPAT also aggravated medial septal lesion-induced passive avoidance acquisition and consolidation failure. The combination of small doses of scopolamine and DOI had no effect on passive avoidance behavior. Peripherally acting scopolamine methylbromide alone or in combination with 8-OH-DPAT had no effect on passive avoidance performance. Motor activity in a swimming pool was altered by single and combined drug treatments; high doses of 8-OH-DPAT and scopolamine, and the combination of small doses of 8-OH-DPAT + scopolamine increased speed of swimming. Medial septum-lesioning also increased speed of swimming but the speed was not increased further by 8-OH-DPAT. The present data suggest that behavioral defect caused by hypostimulation of muscarinic acetylcholine receptors is aggravated by concurrent 5-HT1A receptor stimulation.

Neurochemical, histopathological and mnemonic effects of combined lesions of the medial septal and serotonin afferents to the hippocampus

Male Long-Evans rats received micro-injections of either N-methyl-D-aspartate (NMDA) in the medial septum/vertical diagonal band (MS/DB), 5,7-dihyroxytryptamine (5,7-DHT) in the fimbria/fornix and cingulate bundle or combined NMDA/5,7-DHT micro-injections. NMDA administration caused considerable damage to the MS and enlarged the lateral ventricles. It reduced the activity of choline acetyltransferase as well as the intensity of acetylcholinesterase staining in the hippocampus. 5,7-DHT selectively reduced the concentration of hippocampal serotonin. The rats were assessed for spatial memory in the Morris water maze and the radial arm maze (reference and working memory version). The 5,7-DHT-induced lesion of hippocampal serotonin had no effect by itself on either task. However, it augmented the reference memory impairment caused by the NMDA-induced lesion and delayed the recovery from NMDA-induced impairment of working memory on the radial maze. Combined damage of hippocampal cholinergic and serotonergic afferents did not severely affect spatial memory.

Combined serotonergic-cholinergic lesions do not disrupt memory in rats

Rats were trained on a delayed nonmatching to position task, divided into four groups and given the following lesions: (a) SHAM (vehicle injection into nucleus basalis magnocellularis (NBM) and raphé nuclei (RN), (b) RN (5,7-dihydroxytryptamine lesions of raphé, vehicle into NBM), (c) NBM (quisqualic acid lesion of NBM, vehicle into RN), and (d) COMB (lesions of both RN and NBM). RN lesions had no effect on performance measures including accuracy (percent correct), errors of omission, bias, latencies, and magazine response rate. NBM lesions produced delay-independent (nonmnemonic) disruptions, but performance improved over the 20 days' test. The effects of COMB lesions were no worse than NBM lesions alone. The results suggest that (a) the serotonergic system is not essential for performance in this task, (b) NBM lesions transiently impair nonmnemonic aspects of performance, and (c) serotonergic-cholinergic interactions may not be essential for some cognitive processes.

Behavior-related cortical activity and swim-to-platform performance in the aged rat

Aged rats (26 months) usually retained normal acetylcholine-dependent and serotonin-dependent forms of neocortical low-voltage fast activity and serotonin-dependent hippocampal rhythmical slow-wave activity. In a simple swim-to-platform test, aged rats (23 and 26 months) performed normally in acquisition and in retention over a 7-day period. The results are discussed in relation to the common assumption that aged rats provide a valid model of human senile dementia.

The effects of p-chlorophenylalanine-induced serotonin synthesis inhibition and muscarinic blockade on the performance of rats in a 5-choice serial reaction time task

The effects of serotonergic dysfunction induced by treatment with p-chlorophenylalanine (PCPA), an inhibitor of serotonin synthesis, and cholinergic dysfunction induced by scopolamine on the performance of adult rats in the 5-choice serial reaction time task measuring selective attention were studied. Food-deprived rats were trained to detect and respond to brief flashes of light presented randomly in one of five locations, until they reached a stable level of performance (about 4 months). Scopolamine 0.2 mg/kg produced a marked variation in the performance but did not, however, induce any consistent impairment in the discriminative accuracy. Other doses of scopolamine (0.05 and 0.1 mg/kg) or N-methyl-scopolamine 0.2 mg/kg, a peripheral muscarinic receptor antagonist, did not affect discriminative accuracy. Furthermore, scopolamine as well as N-methyl-scopolamine produced a number of other performance deficits, such as significantly decreased overall probability of responding and significantly increased response latencies. PCPA treatment induced an almost total depletion (> 99%) of frontal cortical serotonin and its major metabolite 5-HIAA and reduced the frontal cortical concentrations of noradrenaline (-30%) and dopamine (-42%). During baseline testing conditions, there was a trend for the discriminative accuracy to be decreased by PCPA, although this effect failed to reach significance (P = 0.07). Presenting the stimuli at unpredictable intervals or reducing the intensity of the visual stimulus impaired discriminative accuracy in both PCPA-treated and control rats. The decrease in discriminative accuracy induced by PCPA reached statistical significance when the stimuli were presented faster than normally or the intensity of the visual stimulus was reduced. PCPA treatment did not make the rats more susceptible to the effects of scopolamine on discriminative accuracy. However, PCPA treatment also induced a number of other performance deficits, resulting in a decreased overall tendency to respond. In summary, there is a statistically non-significant trend for the discriminative accuracy to be decreased by PCPA treatment under normal testing conditions, and as the discrimination task is made more difficult (stimulus intensity reduction, presentation of the stimuli at faster than normal rates), the deficit in discriminative accuracy produced by PCPA treatment is revealed. The results suggest a role for brain serotonin in the general organization of behavior.

Early post-natal administration of 5,7-dihydroxytryptamine destroys 5-HT neurons but does not affect spatial memory

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) may play an important role in learning and memory. It has also been suggested that 5-HT abnormalities may mediate some aspects of the cognitive disorders associated with Korsakoff syndrome and Alzheimer's Disease. The effect of intracisternally applied 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) on learning and memory in rodents was evaluated. Three-day-old rat pups were treated with pargyline (40 mg/kg, i.p.) followed by 5,7-DHT (50 micrograms/pup) and returned to the dam for a month. At 75 days of age, rats were tested on a learning set problem in the Morris water maze for 5 days followed by 30 days of testing in a 12-arm radial maze with 8 of the 12 arms baited. In the Morris water maze, the latency to locate the hidden platform did not differ significantly for 5,7-DHT treated and control rats (F less than 1.0). Similarly, 5,7-DHT treated rats performed comparably to controls on the 12-arm radial maze (F less than 1.0). At 106 days of age the assay of tryptophan hydroxylase activity in the dorsal raphe nuclei and hippocampus showed marked reduction (86%, 78%, respectively) in 5,7-DHT treated animals compared to vehicle injected controls. Immunocytochemical analysis was consistent with the biochemical results. In 5,7-DHT treated animals there was severe loss of neurons that bind 5-HT antibody in the dorsal and medial raphe nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)

The electrocorticogram in relation to physiology and behavior: a new analysis

Recent research in animals indicates that the generalized regulation of cortical activity that is represented by activation of the electrocorticogram is dependent on ascending cholinergic and serotonergic projections. The effect of the activity of these systems is correlated with concurrent motor activity in a detailed and specific manner. Combined blockade of central cholinergic and serotonergic function results in impaired cerebral control of behavior, i.e., a dementia-like syndrome. Previous concepts, which held that electrocortical activation is mediated via a reticulo-thalamo-cortical pathway and that it is related primarily to arousal, vigilance, and the sleep-waking cycle appear to be incomplete or erroneous.

Effects of raphe stimulation on hippocampal and neocortical activity and behaviour

In chronically prepared rats, electrical stimulation (100 Hz, 0.1 ms pulses) of the dorsal raphe nucleus, some sites in the median raphe nucleus, and adjoining regions of the midbrain produced locomotion accompanied by hippocampal rhythmical slow activity (RSA) and neocortical low voltage fast activity (LVFA). Both the behaviour and the cerebral waveforms persisted after injection of scopolamine HBr (5 mg/kg, s.c.). Median raphe stimulation usually produced behavioural freezing or an unnatural forced movement accompanied by RSA and LVFA. The behavioural response and the LVFA were not affected by scopolamine but scopolamine eliminated the RSA, replacing it with a low amplitude irregular (suppressed) pattern. p-Chlorophenylalanine (PCPA, 500 mg/kg/day x 3, i.p.) reduced the RSA and LVFA normally present during walking after scopolamine but did not reduce the hippocampal suppression produced by median raphe stimulation in scopolamine-treated rats. Hippocampal suppression and LVFA in response to median raphe stimulation were also present in urethane (1.0-1.5 g/kg, i.p.) anesthetized rats, whether pretreated with PCPA or not. Stimulation at most other midbrain sites produced RSA and LVFA in the urethane condition. RSA was abolished in the urethane plus scopolamine condition. The data support the view that scopolamine-resistant RSA and LVFA are dependent on serotonergic projections. The hippocampal suppression produced by median raphe stimulation may be dependent on non-serotonergic neurotransmission.

Animal models of human amnesia and dementia: hippocampal and amygdala ablation compared with serotonergic and cholinergic blockade in the rat

The behavioral effects of combined bilateral hippocampal and amygdala ablation (previously proposed as a model of human global amnesia) were compared to those seen with central blockade of the ascending cholinergic and serotonergic projections (a possible model of human global dementia) in male hooded rats. Rats were prepared with: (a) bilateral surgical lesions of the hippocampus and amygdala; (b) pharmacological blockade of central cholinergic and serotonergic function by systemic injections of scopolamine and p-chlorophenylalanine; and (c) neurotoxic lesions of the rostrally projecting serotonergic nuclei in the brainstem using intracerebral injections of 5,7-dihydroxytryptamine, later combined with scopolamine. The behavioral tests used were: an open field test, a swim-to-platform test, and a Lashley III maze. In all 3 tests, rats with either the neurotoxin lesions plus scopolamine or p-chlorophenylalanine plus scopolamine treatment showed greater impairments in comparison with controls than did the combined lesion group. These results indicate that simultaneous blockade of central serotonergic and cholinergic transmission has a greater effect on some aspects of the organization of behavior than large surgical lesions of the hippocampus and amygdala.