Near-total loss of 'learning' and 'memory' as a result of combined cholinergic and serotonergic blockade in the rat

Effects of selective catechol-O-methyltransferase inhibitors on single-trial passive avoidance retention in male rats

The effects of new selective catechol-O-methyltransferase (COMT) inhibitors entacapone (mainly peripheral effect) and tolcapone (acting also in the brain) on normal and impaired cognitive functions were studied in aversively motivated inhibitory avoidance using a single-trial passive avoidance paradigm in young adult rats. Passive avoidance retention latency was shortened by either scopolamine (1.0 mg/kg) or bilateral lesions to nucleus basalis magnocellularis (NBM) caused by infusions of ethylcholine aziridinium (AF64A). Entacapone (30 mg/kg) administered once before training or before the retention test, 24 h after training, prevented the effect of scopolamine but did not alter extinction in these rats. However, entacapone (30 mg/kg) prolonged lag time when given during the extinction process to intact rats after training. Tolcapone administered once before training (10 mg/kg) counteracted the effect of scopolamine. It prolonged retention latency of the intact rats when given after training (10 mg/kg). Tolcapone (3 mg/kg) also prolonged lag time when given during extinction to rats bearing NBM lesions. The effect of scopolamine on extinction and retrieval was not prevented by tolcapone. Only entacapone improved memory storage. Collectively, the present results indicate that COMT inhibitors prolong retention latencies in a single-trial passive avoidance test assessed at several memory phases.

The effect of several putative cognition enhancers on a water maze acquisition deficit produced by pCPA + scopolamine combination treatment

A combined treatment of a 3-day regimen of pCPA and low-dose scopolamine produced a significant deficit in the acquisition of a water maze task, which has been suggested as a model for the cognitive deficits of Alzheimer's disease. The putative cognition enhancers oxotremorine, captopril, ondansetron, and tacrine were used in attempts to alleviate the water maze impairment. The effects of oxotremorine were difficult to determine due to nonspecific motor effects causing alterations in swimming speed. No evidence for cognition-enhancing properties of captopril was found. Ondansetron showed a cognition-enhancing effect on one of 4 days, but only at a relatively high dose (1 mg/kg i.p.). Tacrine, however, alleviated the pCPA + scopolamine-induced cognitive deficit. This study may thus provide evidence for the usefulness of tacrine in treating spatial deficits in dementia.

Testing hypotheses of spatial learning: the role of NMDA receptors and NMDA-mediated long-term potentiation

The role of NMDA receptors and NMDA-mediated hippocampal long-term potentiation (LTP) in spatial learning was studied in rats using the competitive, systemically administered NMDA receptor antagonists CGS19755 ((+/-)-cis-4-phosphonomethyl-2-piperidine carboxylic acid) and NPC17742 (2R,4R,5S-2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid). CGS19755 caused sensorimotor disturbances and disrupted acquisition of the water maze in naive rats. The sensorimotor disturbances were greatly reduced and maze learning was normal in spite of the blockade of dentate gyrus LTP by CGS19755 in rats that had first been familiarized with the general task requirements by non-spatial pretraining. In a second experiment, antagonism of NMDA receptors caused small, but reliable, impairments in Y-maze and visible platform visual discrimination tasks. The results indicate that NMDA receptors are not crucial for water maze acquisition using a spatial learning strategy, and that NMDA antagonists cause visual and other sensorimotor disturbances in naive rats that could help account for their poor performance in this task.

Contrasting effects of systemic and intracerebral infusions of the 5-HT1A receptor agonist 8-OH-DPAT on spatial short-term working memory in rats

The present study compared the effects of systemic 8-OH-DPAT (0.05, 0.1 and 1.0 mg/kg) with intra-raphe and intra-hippocampal infusions of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (10, 30 100 ng) on delayed non-matching-to-position (DNMP) performance in rats. The highest dose of 8-OH-DPAT administered systemically impaired DNMP performance in a delay-independent manner, increased premature responding and increased response bias. Infusions of 8-OH-DPAT (100 ng) into the median raphe nucleus improved performance accuracy, independent of delay whilst having no effect on any other response measure. Infusions of 8-OH-DPAT into the dorsal raphe nucleus had no effect on performance at any dose tested. Infusions of 8-OH-DPAT into the dorsal hippocampus produced a small impairment in performance which was also independent of delay. However, this decrement in performance accuracy was not accompanied by any changes in other response measures. These findings demonstrate a dissociation between the effects of stimulation of pre- and post-synaptic 5-HT1A receptors on performance of a DNMP task although the changes in performance cannot be accounted for by changes in mnemonic function.

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.

Blockade of 5-HT1A receptors compensates loss of hippocampal cholinergic neurotransmission involved in working memory of rats

NAN-190, a selective 5-HT1A receptor antagonist, had no effect on the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) in the working memory task with a three-panel runway setup, when injected bilaterlly at 0.32 or 1.0 micrograms/side into the dorsal hippocampus. Intrahippocampal administration of the muscarinic receptor antagonist scopolamine at 3.2 micrograms/side or the competitive NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) at 32 ng/side significantly increased the number of working memory errors. The increase in working memory errors induced by intrahippocampal scopolamine (3.2 micrograms/side) was reduced by concurrent infusion of 0.32 and 1.0 microgram/side NAN-190, an effect that reached significance only for the 1.0 microgram/side dose. In contrast, NAN-190 at 1.0 micrograms/side did not affect the increase in working memory errors when infused intrahippocampally together with 32 ng/side CPP. These results suggest that blockade of hippocampal 5-HT1A receptors does not affect impairment of working memory resulting from block of NMDA receptor-mediated neurotransmission, but that it can compensate deficiency of septohippocampal cholinergic activity involved in working memory performance of rats.

The effects of different types of pre-training on the rat's retention performance in a swim-to-platform task following administration of scopolamine

Previous research has found that centrally acting antimuscarinic drugs strongly impair the acquisition of a variety of learned behaviors in rats but have little effect on these same behaviors if training is given prior to drug treatment. We gave groups of rats different types of pre-training followed by treatment with scopolamine hydrobromide and subsequent testing on a simple swim-to-platform test. Factors such as practice in swimming without a platform to escape to, or learning to swim to a platform in a different apparatus or even to the test platform located in a different place did not protect the rats from the behavioral disruption produced by scopolamine. However, five training trials on the specific swim-to-platform task used in the retention test afforded almost complete protection against the effect of scopolamine. It appears that the protective effect of pre-training is highly specific and does not involve acquisition of some type of general rule which might survive antimuscarinic blockade.

Global and serial neurons form A hierarchically arranged interface proposed to underlie memory and cognition

It is hypothesized that the cholinergic and monoaminergic neurons of the brain from a global network. What is meant by a global network is that these neurons operate as a unified whole, generating widespread patterns of activity in concert with particular electroencephalographic states, moods and cognitive gestalts. Apart from cholinergic and monoaminergic global systems, most other mammalian neurons relay sensory information about the external and internal milieu to serially ordered loci. These "serial" neurons are neurochemically distinct from global neurons and commonly use small molecule amino acid neurotransmitters such as glutamate or aspartate. Viewing the circuitry of the mammalian brain within the global-serial dichotomy leads to a number of novel interpretations and predictions. Global systems seem to be capable of transforming incoming sensory data into cognitive-related activity patterns. A comparative examination of global and serial systems anatomy, development and physiology reveals how global systems might turn sensation into mentation. An important step in this process is the permanent encoding of memory. Global neurons are particularly plastic, as are the neurons receiving global inputs. Global afferents appear to be capable of reorganizing synapses on recipient serial cells, thus leading to enhanced responding to a signal, in a particular context and state of arousal.

Combined pCPA and muscarinic antagonist treatment produces a deficit in rat water maze acquisition

A 3-day treatment with p-chlorophenylalanine (pCPA, 100 mg/kg/day) produced a significant decrease (63-89%) in 5-HT levels in both the hippocampus and the cortex of rats, while noradrenaline, adrenaline, and dopamine levels were unaffected. Treatment with pCPA alone did not affect the acquisition of a spatial learning task in the water maze. Treatment with low doses of either scopolamine (0.25 mg/kg) or atropine (10 mg/kg) was also insufficient to cause a significant impairment of water maze acquisition. However, a combined treatment of a 3-day pCPA regimen with the low dose of atropine or scopolamine produced a significant deficit in the acquisition of a water maze task.

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 neuroscience of spatial navigation: focus on behavior yields advances

The development of the water maze as a laboratory approach to the study of spatial navigation has led to a large amount of research on the brain mechanisms of this ecologically important behavior. The procedural simplicity of this task belies its underlying complexity, which can complicate the interpretation of data obtained with the standard water maze procedure. In this review, recent experiments that used novel training procedures and detailed analyses of behavior are evaluated, together with related experiments, to clarify the brain mechanisms involved in this behavior. Pharmacological, lesion, and unit recording experiments demonstrate the existence of forebrain circuits for spatial navigation that are considerably more varied and extensive than was previously proposed, and involve various extrahippocampal structures. The use of novel and specialized procedures, together with a continued detailed focus on the behavior of animals in the maze, appears to be the most promising approach to understanding the mechanisms of spatial navigation.

Cognitive functions of the basal forebrain cholinergic system in monkeys: memory or attention?

The cholinergic hypothesis of memory dysfunction originally proposed that dysfunction of cholinergic neurons in the basal forebrain cholinergic system (BFCS) may be responsible for the memory deficits associated with aging and Alzheimer's disease (AD). This hypothesis directed focus on the BFCS in experimental animal models of AD. In contrast to numerous studies in rodents, fewer investigations have been conducted in monkeys with BFCS lesions. The medical septal nucleus/nucleus of the diagonal band of Broca (MS/NDBB) and the nucleus basalis of Meynert (NBM) may be involved in different cognitive functions in monkeys. Although few investigations have specifically addressed the issue of cognitive functions of the MS/NDBB in monkeys, there is some indication that these regions may be important for memory. In contrast, lesions of the NBM do not consistently disrupt mnemonic functions in monkeys. Recent electrophysiological and lesion studies of monkeys indicate that the NBM may play a more important role in attention functions, impairments of which are an early and significant feature of patients with AD.

Loss of nucleus basalis magnocellularis, but not septal, cholinergic neurons correlates with passive avoidance impairment in rats treated with 192-saporin

Intraventricular injection of 192-saporin, an immunotoxin directed at the low affinity neurotrophin receptor (p75NGFr), selectively destroys cholinergic neurons in the basal forebrain (CBF). In the present study, we sought to determine if there was a correlation between degree of CBF neuron destruction and degree of passive avoidance behavioral impairment. 192-saporin caused a decrease in the number of p75NGFr + neurons in both nucleus basalis magnocellularis (Nbm) and medial septal nucleus/diagonal band of Broca (MS/DBB). All rats had >95% loss of the p75NGFr + cholinergic neurons in the MS/DBB, but there was variation in the extent of the Nbm cell loss. A significant correlation was found between the severity of impairment of passive avoidance learning and the magnitude of the loss in the number of p75NGFr + neurons in the Nbm. Step-through latency also correlated significantly with the magnitude of loss of AChE histochemical staining intensity in dorsolateral neocortex ipsilateral to the injection of 192-saporin. These data show that >95% loss of cholinergic neurons in MS/DBB is not sufficient to impair passive avoidance learning. However, in the presence of severe loss of cholinergic neurons from the MS/DBB, the resulting deficit in passive avoidance behavior is proportional to the degree of cholinergic neuron loss from the Nbm. These results are interpreted as support for the hypothesis that the cholinergic projection from Nbm to neocortex plays a role in passive avoidance behavior.

Hippocampal histamine receptors: possible role on the mechanisms of memory in the rat, II

In this paper it was studied the role of histamine and histamine receptors in the hippocampus of rats on an active avoidance response induced by an ultrasonic tone. The animals had to learn to walk through a swinging door into a safe compartment only after the conditioning ultrasonic tone was on in order to avoid an electric shock to their feet. Trained animals were implanted in the ventral hippocampus with microinjection cannulae and injected twice with 1 microliter of saline solution containing pyrilamine (PYR, H1-HA antagonist), ranitidine (RAN, H2-HA antagonist) or histamine. The histamine antagonists were applied in a dose of 65.5 nmol each while histamine was administered in a dose of 45 nmol. The two variables measured were the time in sec the rats take to present the conditioned avoidance response and the accumulated percentage of conditioned avoidance response (CAR). Results showed that histamine administration significantly increased the latency time to escape and decreased the % CAR. These effects were not blocked by the administration of RAN. However, administration of PYR completely counteracted the HA effects. Present findings confirm our previous findings about the inhibitory effect of histamine on the hippocampal retrieval mechanisms and give further support to the hypothesis that HA acts on the memory processes in the hippocampal formation, by activation of H1-histamine receptors.

Combined treatment with a 5HT1A receptor agonist and a muscarinic acetylcholine receptor antagonist disrupts water maze navigation behavior

The present study was designed to investigate the effects of combined treatment with a serotonin (5-HT)1A receptor agonist, 8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT), and a muscarinic acetylcholine receptor antagonist, scopolamine, on water maze (WM) navigation. Treatment with either 8-OH-DPAT or scopolamine before daily behavioral training disrupted spatial navigation at medium doses and cue navigation at high doses. Pretraining treatment with a combination of subthreshold doses of 8-OH-DPAT and scopolamine impaired WM spatial and cue navigation, but did not impair the WM performance if the drugs were injected post-training. In trained rats, combined injections of subthreshold doses of 8-OH-DPAT and scopolamine given pretraining did not impair the rats' ability to find the platform in a familiar or in a novel position. The combination of 8-OH-DPAT and scopolamine also disrupted WM navigation in rats with central 5-HT depletion. A combination of a peripheral muscarinic acetylcholine receptor antagonist and 8-OH-DPAT had no effect on WM navigation. These data suggest that combined treatment with drugs blocking muscarinic acetylcholine receptors and activating 5-HT1A receptors greatly impairs WM learning/performance, but does not impair spatial memory per se.

Serotonergic modulation of cholinergic function in the central nervous system: cognitive implications

Accumulating evidence suggests that serotonin may modulate cholinergic function in several regions of the mammalian brain and that these serotonergic/cholinergic interactions influence cognition. The first part of this review is an overview of histological, electrophysiological and pharmacological (in vitro, in vivo) data indicating that, in several brain regions (e.g., hippocampus, cortex and striatum), there are neuroanatomical substrates for a serotonergic/cholinergic interaction, and that alterations in serotonergic activity may induce functional changes in cholinergic neurons. In the second part, the review focuses on experimental approaches showing or suggesting that central cholinergic and serotonergic mechanisms are cooperating in the regulation of cognitive functions. These arguments are based on lesion, intracerebral grafting and pharmacological techniques. It is concluded that not all mnesic perturbations induced by concurrent manipulations of the serotonergic and cholinergic systems can be attributed to a serotonergic modification of the cholinergic system. The cognitive faculties of an organism arise from interactions among several neurotransmitter systems within brain structures such as, for instance, the hippocampus or the cortex, but also from influences on memory of other general functions that may involve cerebral substrates different from those classically related to mnesic functions (e.g., attention, arousal, sensory accuracy, etc.).

Metrifonate effects on acetylcholine and biogenic amines in rat cortex

The effect of systemic and local administration of metrifonate (MTF), a long-acting cholinesterase inhibitor (ChEl) on extracellular levels of acetylcholine (ACh), norepinephrine (NE), dopamine (DA) and serotonin (5-HT) was investigated in the rat cortex by using transcortical microdialysis. Metrifonate (20, 40, and 80 mg/kg, s.c.) increased ACh levels in a dose-dependent manner above the baseline. Two consecutive administrations (80 mg/kg) enhanced ACh levels producing two similar patterns of elevation. A significant increase in NE was also seen at 80 mg/kg. Systemic administration (20 mg/kg) of MTF produced a significant increase of DA levels. Local cortical perfusion of MTF through the probe caused a significant but slow increase of ACh as well as an increase of NE levels. Compared to NE, the elevation of DA was more rapid and more long-lasting. The cortical levels of 5-HT were not modified by MTF given by either route. These results support the concept of MTF being a potential drug for treatment of Alzheimer disease (AD).

Synergistic anticholinergic and antiserotonergic effects in humans

Animal research suggests an important interactive role for ascending cholinergic and serotonergic systems in modulation of cerebral function. Employing a randomized, double-blind, crossover design, 11 healthy young adults were tested in each of four conditions: (1) placebo, (2) fenfluramine (a serotonin depleting agent), (3) scopolamine (a muscarinic antagonist), and (4) fenfluramine and scopolamine. P3 latency was slowed by the dual drug treatment to an extent greater than the sum of individual drug effects. EEG mean frequency was decreased by behavioral activation, and this decrease was reversed by the combined drug treatment but not by single drugs. In contrast, verbal memory, EEG alpha power, and P3 amplitude were significantly affected only by scopolamine. No drug effects were found for the N1 and P2 potentials. The results provide the first demonstration of combined anticholinergic and antiserotonergic effects in humans, and offer partial support to the concept of an interactive role of cholinergic and serotonergic systems in cerebral mechanisms.

Effects of tetrahydroaminoacridine and nicotine in nucleus basalis and serotonin-lesioned rats

The present study was designed to investigate the hypothesis that concurrent degeneration of serotonin and acetylcholine cells may decrease the therapeutic effects of cholinergic drugs on cognitive functioning in Alzheimer dementia. Therefore, we compared the effects of pretraining injections of a cholinesterase inhibitor, tetrahydroaminoacridine (1, 3 and 5 mg/kg i.p.), and nicotine (0.03, 0.1 and 0.3 mg/kg i.p.) on spatial navigation (water maze) and passive avoidance in nucleus basalis- and nucleus basalis+p-chlorophenylalanine-lesioned rats. Nicotine (0.1 and 0.3 mg/kg) promoted passive avoidance performance of nucleus basalis-lesioned rats, but nicotine did not improve performance of combined-lesioned rats. Tetrahydroaminoacridine (3 mg/kg) facilitated passive avoidance performance of nucleus basalis- and combined-lesioned rats. However, tetrahydroaminoacridine-treated nucleus basalis+p-chlorophenylalanine-lesioned rats were not performing better than vehicle-treated nucleus basalis-lesioned rats. Spatial navigation of nucleus basalis and nucleus basalis+p-chlorophenylalanine-lesioned rats was slightly impaired during the first training day and tetrahydroaminoacridine 3 mg/kg restored the performance of combined-lesioned rats. Combined-lesioned rats performed as well as the controls during the other training days. The present results suggest that, in Alzheimer's disease, combined degeneration of nucleus basalis cholinergic and brainstem serotonergic cells decreases the therapeutic effect of nicotine, but not that of tetrahydroaminoacridine.

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.

Destruction of the cholinergic basal forebrain using immunotoxin to rat NGF receptor: modeling the cholinergic degeneration of Alzheimer's disease

Degeneration of cholinergic neurons in the basal forebrain (CBF) is a prominent neuropathological feature of Alzheimer's disease and is thought responsible for some cognitive deficits seen in patients. An animal model of pure CBF degeneration would be valuable for analysis of the function of these neurons and testing therapeutic strategies. CBF neurons express receptors for nerve growth factor. In order to selectively destroy these neurons, we developed an immunotoxin using monoclonal antibody (192 IgG) to rat NGF receptor (p75NGFr) armed with the ribosome inactivating protein, saporin. In vitro 192-saporin was highly toxic to neurons expressing p75NGFr. Intraventricular injections of 192-saporin destroyed the CBF and impaired passive avoidance learning. These results indicate that 192-saporin treated rats can be used to model a key feature of Alzheimer's disease and that anti-neuronal immunotoxins are a powerful approach to selective neural lesioning.

Effects of localized histamine microinjections into the hippocampal formation on the retrieval of a one-way active avoidance response in rats

The possible role of histamine (HA) locally applied into the hippocampus on memory mechanisms of the rats was studied. The acquisition of a one-way active avoidance response to an ultrasonic 40 kHz sinus-wave tone anticipating an electric shock was used as experimental model. Learning sessions consisted in placing animals into a two compartment cage were they learnt to escape to the safe compartment after an ultrasonic tone anticipating an electric feet shock. After acquiring the conditioned avoidance response, animals were implanted with microinjection cannulae and injected with 1 microliter of saline, or increasing doses of histamine (9, 22.5, 45, and 90 nmol) into the hippocampus. In the experimental sessions, 4 trials before (PRE) and 4 trials afterward treatment (POST), the percentage of conditioned avoidance responses (% CAR) and the latency time to escape (LT) were measured. Results showed that HA increased significantly the LT and this effect was grossly dose-dependent. % CAR was also affected and the score was significantly inhibited by the imidazolamine administration. Results suggest that HA may be involved in memory retrieval processes in the hippocampus.

Does a history of convulsions increase the amnestic effect of temporal region brain lesions?

Rats with surgical lesions of the hippocampal formation, amygdala, and nearby neocortex and pyriform cortex were impaired on a swim-to-platform test and on a passive avoidance test. These impairments were not increased by presurgical treatment with a series of 21 electroshock convulsions. To the extent that this rat preparation can be compared with human patients, the data suggest that a previous history of epileptic seizures does not increase the amnestic effect of temporal lobe lesions.

The 5-HT2 receptor antagonist ketanserine prevents electroconvulsive shock- and clonidine-induced amnesia

The 5-HT2-selective antagonist ketanserine was examined for its ability to prevent electroconvulsive shock (ECS)- or clonidine-induced performance deficit in the passive avoidance situation (step-down) in rats. The posttrain intraperitoneal injection of ketanserine at doses of 3 and 10 mg/kg prevented the ECS- or clonidine-provoked amnesia upon retention tests given 3 h, 24 h, and 7 days after training. The present data favor the view that the selective modification of 5-HT2 receptors after training can prevent the performance deficit in step-down-trained rats and suggest a role of the 5-HTergic neurotransmitter system in memory. The results of this study further suggest that 5-HTergic receptor antagonists might be useful in treatment of cognitive disorders.

Effects of subchronic pretreatment with D-fenfluramine or p-chloroamphetamine on [3H]inositolmonophosphate accumulation in rat cortical miniprisms

Phosphatidylinositol (PI) breakdown in rat cerebral cortex is stimulated by serotonin (5-HT), acting via 5-HT2 and possibly 5-HT3 receptors and by acetylcholine or carbachol, acting via muscarinic M1 and M3 receptors. Serotoninergic neurons have been described as tonically inhibiting cortical acetylcholine release. We studied the effects of subchronic pretreatment with high doses of D-fenfluramine (10 mg/kg, i.p., daily for 4 days), which releases 5-HT and blocks its reuptake, on 5-HT-and carbachol-stimulated PI breakdown, as measured by [3H]inositolmonophosphate ([3H]IP1) accumulation in cortical miniprisms. This pretreatment decreased 5-HT-stimulated [3H]IP1 accumulation, suggesting that a prolonged increase of 5-HT in the synaptic cleft reduces the activity of the transducing system used by postsynaptic 5-HT receptors. Carbachol-stimulated PI breakdown was unaltered by pretreatment with D-fenfluramine. Pretreatment with a single dose of p-chloroamphetamine (5 mg/kg), a serotoninergic neurotoxin, which depleted cortical 5-HT by 85%, did not change [3H]IP1 accumulation after stimulation by 5-HT or by the muscarinic agonist carbachol. Subchronic pretreatment, which depleted cortical 5-HT by 90%, decreased both 5-HT- and carbachol-stimulated [3H]IP1 accumulation. The mechanism by which p-chloroamphetamine, but not D-fenfluramine, diminishes the PI response to carbachol might involve impairment of the tonic serotoninergic inhibition of acetylcholine release.

The effects of intrahippocampal raphe and/or septal grafts in rats with fimbria-fornix lesions depend on the origin of the grafted tissue and the behavioural task used

Long-Evans female rats sustained electrolytic lesions of the fimbria and the dorsal fornix and, two weeks later, received intrahippocampal suspension grafts of fetal tissue. The grafts were prepared from regions including either the medial septum and the diagonal band of Broca (septal grafts), or the mesencephalic raphe (raphe grafts), or from both these regions together (co-grafts). All rats were submitted to a series of behavioural tests (home cage and open-field locomotion, spontaneous alternation, radial-arm maze and Morris water maze performance) run over two periods after grafting (one to nine weeks and 20-35 weeks). Two weeks after completion of behavioural testing, histological (acetylcholinesterase and Cresyl Violet staining) and/or neurochemical (choline acetyltransferase activity, high-affinity synaptosomal uptake of choline and serotonin, noradrenaline, serotonin and 5-hydroxyindolacetic acid concentrations) verifications were performed on the hippocampus. Compared to sham-operated rats, lesion-only rats exhibited hyperactivity which was transient in a familiar environment (home cage) and lasting in an unfamiliar one (open field), decreased rates of spontaneous T-maze alternation, and impaired memory performance in both the radial-arm maze and the Morris water maze. These rats also showed decreased cholinergic and serotonergic markers with a maximal depletion in the septal two-thirds of the hippocampus. Noradrenaline concentration tended to be increased in the dorsal third of the hippocampus, but was not modified in the other two-thirds. While septal grafts specifically increased the cholinergic markers and raphe grafts the serotonergic ones, neither of these grafts produced a lasting effect on any behavioural variable. Conversely, the co-grafts, which increased both the cholinergic and serotonergic markers in the septal two-thirds of the hippocampus, completely normalized the Morris water maze probe trial performance, but failed to affect any of the other behavioural variables. Our present results confirm that grafts of fetal neurons injected into the denervated hippocampus may induce a neurochemical recovery that depends on the anatomical origin of the grafted cells, and that co-grafting two fetal brain regions allows the combination of their individual neurochemical properties. Furthermore, our results show that these neurochemical effects of the co-grafts may be involved in the recovery of behavioural function observed in the water maze. However, somewhat paradoxically, those effects appear inefficient for inducing any recovery in other behavioural tasks, even in the radial-arm maze; which is assumed to measure similar spatial functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin depletion decreases the therapeutic effect of nicotine, but not THA in medial septal-lesioned rats

The present study compares the effects of systemic pretraining trial injections of a cholinesterase inhibitor, tetrahydroaminoacridine (THA, 1, 3 and 5 mg/kg, i.p.) and nicotine (0.03, 0.1 and 0.3 mg/kg, i.p.) on spatial navigation water maze (WM) and passive avoidance (step-through PA) performance in medial septal (MS)--or MS+p-chlorophenylalanine (PCPA, a serotonin synthesis inhibitor)-lesioned rats. MS-lesion impaired WM and PA acquisition, and serotonin depletion significantly aggravated PA failure of MS-lesioned rats. THA (3 mg/kg) and nicotine (0.1 and 0.3 mg/kg) promoted PA and WM navigation of MS-lesioned rats. THA at a dose of 3 mg/kg improved performance of MS+PCPA-lesioned rats in WM and PA tests, but nicotine did not promote test performance of combined-lesioned rats. This result demonstrates that serotoninergic pathology may decrease the therapeutic effect of nicotine.

The behavioral neurobiology of learning and memory: a conceptual reorientation

Research on the neurobiology of learning and memory has been guided by two major theories: (i) memory as a psychological process and (ii) memory as a change in synaptic neural connectivity. It is not widely recognised that not only are these theories different but, moreover, they are fundamentally incompatible. Confusion concerning basic concepts in the learning and memory field in mammals has lead to the creation of an extensive but often inconclusive experimental literature. However, one important conclusion suggested by recent work in this field is that experience-dependent changes in neural connectivity occur in many different brain systems. Particular brain structures, such as the hippocampus, do not play any uniquely important role in experience-dependent behavior. Research in learning and memory can be best pursued on the basis of biological studies of animal behavior and a cellular approach to brain function.

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.

Possible involvement of central cholinergic system in ameliorating effects of indeloxazine, a cerebral activator, on disturbance of learning behavior in rats

Effects of indeloxazine, a cerebral activator, on passive avoidance learning by disruption of cholinergic transmission were studied in rats. The shortened latency was prolonged when indeloxazine was administered before training in rats subjected to scopolamine-injection, ethylcholine aziridinium ion (AF64A)-treatment and nucleus basalis magnocellularis (NBM) lesion. Indeloxazine administered immediately after training also showed ameliorating effects on passive avoidance in NBM-lesioned rats. Indeloxazine increased extracellular acetylcholine concentration in the frontal cortex of rats. Indeloxazine possesses facilitatory effects on cerebral function in part due to activation of the central cholinergic system.

Hippocampal theta rhythm in behaving rats following ibotenic acid lesion of the septum

The effects of ibotenic acid lesion of the septum were studied in rats implanted with chronically indwelling electrodes and septal cannula. Each rat served as its own control and the properties of the hippocampal theta rhythm were studied before and after ibotenic acid and control saline infusion into the medial septal area. Ibotenic acid preferentially killed neurons in the lateral septum, and significantly attenuated the hippocampal theta rhythm about 50% bilaterally, at both surface and deep electrodes. The coherence and the phase of the theta rhythm at the CA1 apical dendrites, with respect to a superficial electrode, also declined significantly after ibotenic acid lesion. Pilocarpine (25 mg/kg i.p.) induced a theta rhythm of 7-9 Hz during immobility in the lesioned rats that was significantly higher in frequency than that induced in intact rats (4-6 Hz). In lesioned rats, the theta rhythm during tail pinch under urethane anesthesia was largely abolished, and the theta during walking was attenuated by atropine sulfate (50 mg/kg i.p.). Phencyclidine (10 mg/kg i.p.) or parachlorophenylalanine (PCPA) alone, which was inferred to abolish an atropine-resistant theta input, did not affect the power of the walking theta rhythm in either the lesioned or the normal rat. It was concluded that the theta in the behaving rats after ibotenic acid lesion in the septum has a strong atropine-sensitive component, and that it is not predominantly atropine-resistant, as suggested previously. The lack of PCPA effect on the theta phase in intact and lesioned rats also suggested a different view of the atropine-resistant theta in hippocampal region CA1. One possible mechanism of the atropine-resistant theta at the distal dendrites of pyramidal cells may result from rhythmic inhibition by stratum lacunosum-moleculare interneurons which may be activated by either serotonergic or cholinergic inputs.

Tryptophan depletion in normal volunteers produces selective impairments in learning and memory

The amino-acid L-tryptophan is essential in the synthesis of brain serotonin, and its depletion can lead to a widespread reduction in central serotonergic activity. A placebo-controlled cross-over within-subjects design (n = 12) examined the effects of tryptophan depletion on human cognitive performance. A low-tryptophan (low-TRP) drink successfully reduced the levels of plasma and total free tryptophan. Computerized tests of memory, learning and executive function revealed selective and non-sedative impairments on cognitive performance following the active drink. Specifically, low-TRP impaired learning as seen in tests of visual discrimination and paired associates. Furthermore, low-TRP lengthened thinking times during the Tower of London planning task, but only in subjects already familiar with the task, suggesting a retrieval deficit. No evidence was found for an effect of the low-TRP drink on measures sensitive to frontal lobe dysfunction, supporting instead a specific role for the serotonergic system in the processes of memory and learning not directly implicated in frontal lobe function.

Prenatal protein malnutrition effects on the serotonergic system in the hippocampal formation: an immunocytochemical, ligand binding, and neurochemical study

Prenatally protein malnourished rats born to dams maintained on a 6% casein diet during pregnancy and then fostered at birth to females on a 25% casein diet show adult alterations in hippocampal kindling and long-term potentiation and behavioral changes that all suggest dysfunction of hippocampal formation (HF). In the present investigation, compared to well-nourished controls, 220 day malnourished rats exhibited a decrease in the 5-HT fiber density in the dentate gyrus (DG) and CA3 subfield and, a 15-25% decrease 5-HT uptake sites assayed with [3H]-citalopram in CA3 and CA1. In malnourished rats, 5-HT1A receptors assayed with [3H]8-OH-DPAT were decreased by 20% in CA3. Because most hippocampal subfields showed no 5-HT changes, hippocampal 5-HT levels determined via HPLC methods were similar in adult malnourished and control rats. These results suggest that there are localized changes in the 5-HT afferent system in the hippocampal formation of the 220 day prenatally protein malnourished rat. Considering the 5-HT afferent input to inhibitory intrahippocampal neurons, the decreased 5-HT plexus may result in increased inhibition within specific hippocampal subfields despite overall normal levels of 5-HT in the total HF.

The effects of serotonergic drugs on short-term spatial memory in rats

The effects of modulating the serotonergic system on short-term spatial memory were investigated using delayed matching to position and delayed non-matching to position procedures. Rats were trained on one of the two tasks until stability and then administered the drugs 30 min before a session. Fluoxetine (0.625-10 mg/kg), a serotonin uptake inhibitor, fenfluramine (0.313-5 mg/kg), a serotonin release enhancer, and ipsapirone (2.5-10 mg/kg), a serotonin 1A partial agonist, were all injected subcutaneously in saline solution. Apart from fenfluramine, none of the compounds affected accuracy in either procedure, but some effects on other non- mnemonic measures were seen. At 5 mg/kg, fenfluramine significantly affected latency to respond, total responses on the levers and nosepokes in the foodtray as well as accuracy, indicating a non-specific disruption of behaviour rather than a selective effect on memory processes. These data suggest that cognitive effects of serotonergic drugs are difficult to identify in normal animals, irrespective of the mechanism of action of the drug. These data conflict with recent reports indicating memory-enhancing effects of serotonin uptake inhibitors.

Pharmacological consequences of nicotinergic plus serotonergic manipulations

The present study investigates the effects of concurrent manipulations of nicotinic cholinergic receptors (nicotinic cholinergic agonist: nicotine 0.03, 0.1, 0.3 mg/kg, nicotinic cholinergic antagonist: mecamylamine 7.5 mg/kg) and serotonin neurons (p-chlorophenylalanine (PCPA), 400/kg mg on each of 3 days) on spatial navigation (water maze, WM) and passive avoidance (PA) performance. Nicotine did not affect PA performance but at the highest dose slightly impaired WM performance. PCPA did not affect WM navigation or PA performance in saline or nicotine-treated rats. Nicotine restored WM and PA performance defect in mecamylamine pretreated rats. PCPA aggravated the WM defect and decreased the WM performance-improving effect of nicotine in mecamylamine pretreated rats. PCPA did not aggravate the PA performance defect of mecamylamine but completely blocked the PA performance-improving effect of nicotine in mecamylamine pretreated rats. These results suggest that serotonergic and nicotinergic cholinergic systems jointly modulate performance in WM and PA tests.

Structures mediating cholinergic reticular facilitation of cortical responses in the cat: effects of lesions in immunocytochemically characterized projections

Cholinergic afferents to the neocortex controlled by the mesencephalic reticular formation (MRF) are known to transiently facilitate cortical excitability. In an attempt to identify the pathway mediating this effect in the cat visual cortex we combined retrograde tracing techniques with immunocytochemical methods to visualize the acetylcholine-synthesizing enzyme choline acetyltransferase (ChAT). In addition we examined, in acute electrophysiological experiments, whether local neurotoxin injections into nuclei of the basal forebrain interfered with the reticular facilitation of cortical evoked potentials. Cholinergic projections to area 17 originate from different centers in the homolateral substantia innominata/internal capsule, the septal nuclei, and the nuclei of the diagonal band of Broca. No direct cholinergic projection from the MRF to the visual cortex was observed. Retrogradely labelled cells intermingled with ChAT-positive neurons in the brainstem generally revealed immunopositivity for catecholaminergic markers. Local injections of neurotoxins in the substantia innominata blocked reticular facilitation, whereas local lesions of the septal nuclei and the nuclei of the diagonal band had no effect on MRF-induced facilitation. The blockage of the reticular facilitation of cortical evoked responses after unilateral lesions of the substantia innominata was bilateral, suggesting a cooperative interaction between basal forebrain structures of the two hemispheres. The anatomical and physiological data are discussed with respect to possible mechanisms of transient brainstem influences on cortical excitability.

The behavioral effects of serotonin synthesis inhibition and quisqualic acid induced lesions of the nucleus basalis magnocellularis in rats

1. To investigate the role of the cholinergic and serotonergic systems in the regulation of cognitive functions, the effects of concurrent lesioning of nucleus basalis magnocellularis (NB) with quisqualic acid (quis) and inhibition of brain serotonin synthesis by systemic p-chlorophenylalanine (PCPA) treatment on passive avoidance (PA) retention and water maze (WM) spatial navigation performance were studied in rats. 2. Quis NB lesioning induced a marked reduction (-62%) in frontal cortical choline-acetyltransferase activity, impaired retention of PA, and slightly and transiently impaired acquisition of WM spatial navigation. 3. PCPA (400 mg/kg/day x 3, i.p.) treatment depleted frontal cortical concentrations of both serotonin (82% depletion) and its major metabolite 5-HIAA (90% depletion) and slightly affected the noradrenergic and dopaminergic systems. PCPA treatment alone had no effect on WM or PA behavior, but potentiated the PA retention deficit and slightly aggravated the WM deficit in rats subjected to quis NB lesioning. 4. The present results further support the view that serotonergic and NB neurons interact in the regulation of cognitive functions.

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.

Effects of indeloxazine hydrochloride, a cerebral activator, on passive avoidance learning impaired by disruption of cholinergic transmission in rats

The effect of indeloxazine [(+/-)-2-[(inden-7-yloxy)methyl]morpholine hydrochloride, YM-08054], a cerebral activator, on passive avoidance learning by disruption of cholinergic transmission was investigated in rats. Indeloxazine prolonged the latency for stepping out of an illuminated compartment into a dark compartment, in both mature and aged rats. Disruption of cholinergic transmission was induced by injection of scopolamine, ethylcholine, treatment with aridinium ion (AF64A) and by lesioning the nucleus basalis magnocellularis. The shortened latency in these models was prolonged when indeloxazine was administered before training in doses which did not affect spontaneous movement or the response to pain in mature rats and administration of indeloxazine, immediately after training, also had an ameliorating effect on passive avoidance in the lesioned rats. In biochemical studies, indeloxazine increased the extracellular concentration of acetylcholine in the frontal cortex of mature rats. These results suggest that indeloxazine possesses facilitatory effects on cerebral function, in part due to activation of the central cholinergic system.

Independent effects of cholinergic and serotonergic lesions on acetylcholine and serotonin release in the neocortex of the rat

Rats received a unilateral lesion of the nucleus basalis magnocellularis (NBM) by infusion of ibotenic acid. In addition, the dorsal raphe nucleus was lesioned by infusion of 5,7-dihydroxytryptamine (5,7-DHT). The release of acetylcholine (ACh), choline, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was measured in the frontal neocortex by means of microdialysis. Lesions of the NBM, but not the raphe nucleus, reduced the release of ACh significantly (-47%). The release of 5-HT and 5-HIAA was reduced by raphe lesions (-44% and -79%+), but not by NBM lesions. In no case did the combined lesion affect neurotransmitter release more than a single lesion. These results suggest that serotonergic projections from the dorsal raphe nucleus are not involved in tonic inhibition of ACh release in the neocortex.

Effects of grafts containing cholinergic and/or serotonergic neurons on cholinergic, serotonergic and noradrenergic markers in the denervated rat hippocampus

Long-Evans female rats sustained aspirative lesions of the septohippocampal pathways and, 2 weeks later, received intrahippocampal suspension grafts prepared from the regions including either the medial septum and the diagonal band of Broca (group S), or the mesencephalic raphe (group R), or from both these regions together (group S + R). Sham-operated (group SHAM) and lesion-only (group LES) rats were used as controls. Six months after grafting, high affinity synaptosomal uptake of choline (HACU) and serotonin (HASU), choline acetyltransferase (ChAT) activity and, using HPLC, the content of serotonin ([5-HT]), 5-hydroxyindolacetic acid ([5-HIAA]) and noradrenaline ([NA]) were determined in three rostro-caudal segments of the hippocampus (designated hereafter as the dorsal, the 'middle' and the ventral segments). In all three segments of the dorsal hippocampus, septohippocampal lesions decreased HACU, ChAT activity, HASU and [5-HT]; [5-HIAA] was decreased only in the middle and ventral hippocampal segments. The lesions also resulted in an above normal increase of [NA]. Septal grafts increased HACU and ChAT in the three hippocampal regions, had no effect on serotonergic markers and attenuated the lesion-induced increase of [NA] in only the dorsal and middle hippocampal segments. Raphe grafts increased HASU, [5-HT] and [5-HIAA] in the dorsal and middle hippocampal segments, had no effects on cholinergic markers and did not affect the lesion-induced increase of [NA]. Co-grafts increased HACU, ChAT activity, HASU, [5-HT] and [5-HIAA], and attenuated the lesion-induced increase in [NA]. These data demonstrate that grafts of fetal neurons placed into the denervated hippocampus may induce a neurochemical recovery which depends upon the anatomical origin of the grafted cells. They also show that co-grafting allows to combine the neurochemical properties of two fetal brain regions grafted separately. Furthermore, our findings suggest that graft-derived cholinergic reinnervation of the hippocampus prevents the lesion-induced increase of noradrenaline concentration which is likely to result from sympathetic sprouting. Thus, our data confirm the results of a previous experiment carried out at a post-grafting delay of 10-11 months, and show that the graft-induced effects reported previously are already massively present by 6 months after surgery.

Effects of oxotremorine on inhibitory avoidance behaviour in two inbred strains of mice: interaction with 5-methoxy-NN-dimethyltriptamine

The effects of the cholinergic muscarinic agonist, oxotremorine (0.005, 0.01, 0.02 and 0.04 mg/kg), the serotonergic agonist, 5-methoxy-NN-dimethyltriptamine (5-MeODMT) (0.5, 1 and 2 mg/kg), and their combination, were investigated in C57BL/6 and DBA/2 mice using a one-trial inhibitory avoidance task, drug treatment being given immediately after the acquisition trial. Post-trial administration of oxotremorine facilitated, while post-trial administration of 5-MeODMT inhibited memory retention of both strains in a dose-dependent fashion. The DBA/2 strain was more affected by oxotremorine than the C57BL/6 mice; no strain-dependent sensitivity to serotonergic agonist administration was observed. In both strains, the combination of oxotremorine plus 5-MeODMT inhibited the performance improvement shown by the administration of the cholinergic agonist alone. The facilitatory role of cholinergic stimulation on retention performance was confirmed and an inhibitory action of the serotonergic system on memory processes was suggested. Moreover, the present results support a functional interaction between cholinergic and serotonergic systems on memory consolidation.

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.

Serotonin-dependent cerebral activation: effects of methiothepin and other serotonergic antagonists

In scopolamine-treated (5.0 mg/kg, s.c.) rats hippocampal rhythmical slow activity (RSA) and neocortical low voltage fast activity (LVFA) occur only in close correlation with head movements, spontaneous changes in posture, or locomotion (Type I behavior). Previous work indicates that such scopolamine-resistant RSA and LVFA are dependent on ascending serotonergic projections. A test of 9 serotonergic antagonists (methiothepin; ritanserin; ketanserin; pizotifen; mianserin; pirenperone; ICS-205-930; metoclopramide; methysergide) showed that methiothepin produces a partial reduction in RSA and LVFA in scopolamine-treated rats, while the other antagonists are completely inactive over a wide range of doses. It may be that serotonergic cerebral activation depends on both 5-HT1 and 5-HT2 receptors.

Effects of combined acetylcholinesterase inhibition and serotonergic receptor blockade on age-associated memory impairments in rats

We recently reported that post-training administration of serotonergic receptor antagonists attenuated the inhibitory-avoidance memory deficits normally exhibited by aged rats. In the present study, we determined whether a subeffective dose of the serotonergic type-2 receptor antagonist, ketanserin, would augment the facilitative effects produced by the acetylcholinesterase inhibitor, physostigmine, on memory in aged rats using the same task. The drugs were injected intraperitoneally alone, or in combination, immediately following training. Retention testing occurred 24 hours following training. A dose-dependent enhancement of memory was demonstrated as a result of the two treatment conditions (physostigmine 0.01-10.0 micrograms/kg, ketanserin 1.0 mg/kg + physostigmine 0.001-0.01 micrograms/kg). The facilitation of memory produced by the combined treatment was observed at doses well below those required to produce a similar effect when each drug was administered alone. The results provide additional evidence for an interaction between the cholinergic and serotonergic neurotransmitter systems in learning and memory, and may have important implications in the treatment of age-related memory impairments.

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)

Effects of septal and/or raphe cell suspension grafts on hippocampal choline acetyltransferase activity, high affinity synaptosomal uptake of choline and serotonin, and behavior in rats with extensive septohippocampal lesions

At 31 days of age, Long-Evans female rats sustained aspirative lesions of the septohippocampal pathways and, 14 days later, received intrahippocampal suspension grafts prepared from the region including the medial septum and the diagonal band of Broca (Group S, n = 11), from the region including the mesencephalic raphe (Group R, n = 11) or from both regions together (Group S+R, n = 11). Sham-operated (Group Sham, n = 9) and lesion-only (Group Les, n = 11) rats served as non-grafted controls. Seven Sham, 7 Les and 8 rats from each transplant group were tested for home cage activity (6 months after grafting) and radial maze performance (between 7.5 and 8.5 months post-grafting). One month after completion of behavioral testing, the dorsal hippocampi of these rats were prepared for measuring choline acetyltransferase (ChAT) activity and high affinity synaptosomal uptake of both [3H]choline and [3H]serotonin. The remaining rats were used for histological verifications on brain sections stained for acetylcholinesterase (AChE). The lesions increased locomotor activity, impaired radial maze learning and, in the dorsal hippocampus, reduced AChE positive staining, decreased ChAT activity (-73%) as well as high affinity uptake of both choline (-81%) and serotonin (-82%). Neither type of transplant produced any significant behavioral recovery. However, septal transplants increased hippocampal AChE positivity, restored ChAT activity and enhanced choline uptake to 116% and 70% of the values found in sham-operated rats, respectively; they had no significant effect on uptake of serotonin. Transplants from the raphe region had weak effects on hippocampal AChE positivity, increased both the ChAT activity and the choline uptake to 70% ad 38% of the sham-operated rats, respectively, and produced an (over)compensation of the serotonin uptake which reached 324% of the values found in sham-operated rats. The co-transplantation of both regions resulted in restoration of ChAT activity (113% of sham-operated rats values), choline uptake (83% of sham-operated rats) and serotonin uptake (129% of sham-operated rats). Our neurochemical data show that after extensive denervation of the hippocampus, intrahippocampal grafts of fetal neurons may foster a neurotransmitter-specific recovery which depends upon the anatomical origin of the grafted cells: a graft rich in serotonergic neurons overcompensates the serotonergic deficit, a graft rich in cholinergic neurons attenuates the cholinergic deficit, whereas a mixture of both types of grafts produces recovery from both types of deficits. Thereby, both the feasibility and the interest of the co-grafting technique are confirmed.(ABSTRACT TRUNCATED AT 400 WORDS)