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

Combined administration of subthreshold doses of the nitric oxide inhibitor, nitro-L-arginine, and muscarinic receptor antagonist, scopolamine, impairs complex maze learning in rats

Traditionally, research into the neurobiological mechanisms of age-related memory impairments has focused on single neurotransmitter systems. As normal and abnormal age-related declines in memory function probably involve alterations in more than one system, a more effective approach for elucidating underlying neurobiological changes and resulting impairments may be to evaluate the roles of multiple systems simultaneously. This study evaluated the interaction of the cholinergic and nitric oxide systems in rats on acquisition in the 14-unit T-maze. This task requires learning a series of turns to avoid foot shock, and most likely reflects procedural learning. Administration of scopolamine (0.1 mg/kg) or N-nitro-L-arginine methyl ester (30 mg/kg) alone did not impair acquisition, whereas administration of the same doses in combination increased both the latency to complete the maze and number of errors committed. These data suggest that manipulation of learning and memory processes with multiple compounds potentially offers a clinically relevant paradigm for investigating cognitive function in normal and abnormal aging.

Age-related changes in memory and in acetylcholine functions in the hippocampus in the Ts65Dn mouse, a model of Down syndrome

Spatial working memory and the ability of a cholinesterase inhibitor to enhance memory were assessed at 4, 10, and 16 months of ages in control and Ts65Dn mice, a partial trisomy model of Down syndrome, with possibly significant relationships to Alzheimer's disease as well. In addition, ACh release during memory testing was measured in samples collected from the hippocampus using in vivo microdialysis at 4, 10, and 22-25 months of age. When tested on a four-arm spontaneous alternation task, the Ts65Dn mice exhibited impaired memory scores at both 4 and 10 months. At 16 months, control performance had declined toward that of the Ts65Dn mice and the difference in scores across genotypes was not significant. Physostigmine (50 microg/kg) fully reversed memory deficits in the Ts65Dn mice in the 4-month-old group but not in older mice. Ts65Dn and control mice exhibited comparable baseline levels of ACh release at all ages tested; these levels did not decline significantly across age in either genotype. ACh release increased significantly during alternation testing only in the young Ts65Dn and control mice. However, the increase in ACh release during alternation testing was significantly greater in control than Ts65Dn mice at this age. The controls exhibited a significant age-related decline in the testing-related increase in ACh release. With only a small increase during testing in young Ts65Dn mice, the age-related decline in responsiveness of ACh release to testing was not significant in these mice. Overall, these results suggest that diminished responsiveness of ACh release in the hippocampus to behavioral testing may contribute memory impairments in Ts65Dn mice.

Effects of pretraining intrastriatal administration of p-chloroamphetamine on inhibitory avoidance

Pretraining systemic administration of p-chloroamphetamine (PCA) consistently produces retention deficits of inhibitory avoidance. This drug causes a widespread acute release of serotonin from cerebral neuronal terminals, but it is not known where in the brain PCA exerts its disruptive cognitive effects. The present experiment was aimed at determining whether the striatum is a site of action of this drug. PCA (5 microg) was infused, bilaterally, into the striatum of rats at 30, 15, or 5 min before training of inhibitory avoidance, and retention of the task was measured 24 h later. An inversely related time-dependent deficit was found. Together with the results from appropriate control groups, this result strongly suggest that systemic PCA produces its reported cognitive effects, at least in part, through its pharmacological action within the striatum.

Effects of excitotoxic median raphe lesions on scopolamine-induced working memory deficits in inhibitory avoidance

The aim of the present study was to investigate the effects of excitotoxic damage of the serotonergic cell bodies in the median raphe nucleus (MRN) on the scopolamine-induced working memory deficits in a single-trial light/dark inhibitory avoidance task. Rats were given 1 mg/kg of scopolamine hydrobromide (intraperitonal, i.p.) or saline before the inhibitory avoidance training, in which initial preference to the dark compartment (escape latency) was used to measure nonmnemonic behaviors, and response latency to enter the dark compartment immediately after the shock was used to measure working memory. It was found that scopolamine significantly reduced escape latencies in sham-lesioned rats, whereas it had no effect in the rats with MRN lesions. Although MRN lesion per se did not alter response latency, it prevented scopolamine-induced decrease in this parameter. These results suggest that the antagonistic interactive processes between serotonergic projections of the MRN and the muscarinic cholinergic system modulate nonmnemonic attentional component of working memory formation in the inhibitory avoidance.

Effect of tryptophan restriction on short-term memory

Several brain regions are involved in the learning process that is integrated from sensorial inputs. It is thereafter consolidated in short- (STM) or long-term memory. Serotonin is strongly related to both types of memory, and particularly, to STM, however, its regulatory role is still unclear. In this study, the effects of tryptophan (TRY) restriction on learning and STM were evaluated. Ten Sprague-Dawley female rats were fed with a TRY-restricted diet (0.15g/100g) starting from postnatal Day 21. At 21, 40, and 60 days of age, 5 trials per animal were carried out in a "hard-floor"-Biel maze, after 24 h of water abstinence. The number of errors per trial were registered before reaching the goal. At both 40 and 60 days, experimental rats committed less errors than controls. Likewise, the TRY-restricted group learned the task from the second trial on, whereas controls did not solve it until the third trial. TRY restriction, and therefore brain serotonin reduction, could impair normal cholinergic activity in some areas such as the hippocampus and the cerebral cortex, where involvement in learning and memory is well documented. Morphological and neurochemical plastic events could also be related to the more efficient performance of the task by the TRY-restricted rats.

Effect of chronic treatment with piracetam and tacrine on some changes caused by thymectomy in the rat brain

Thymectomized rats, 5 weeks after surgery, showed a significant impairment in learning and memory as shown by deficits in passive avoidance and in the Morris water maze test. The behaviour of the thymectomized rats in the "open field" apparatus was largely unchanged. Following treatment for 20 days with either piracetam (500 mg/kg) or tacrine (3.0 mg/kg), the deficit in passive avoidance learning was largely reversed. Chronic treatment with tacrine also reversed the deficit in the behaviour of the thymectomized rats in the Morris water maze. The effects of thymectomy on the biogenic amines and some of their metabolites in the amygdaloid cortex, hypothalamus, striatum and olfactory bulbs were also determined. Relative to the sham-operated controls, thymectomy resulted in a reduction in the noradrenaline concentration in the amygdala, hypothalamus, and olfactory bulbs. This effect was reversed by chronic piracetam and tacrine treatments. The concentration of dopamine was also reduced in the olfactory bulbs after thymectomy whereas in the striatum the concentration of 5-hydroxytryptamine (5-HT; serotonin) was increased. The concentration of gamma amino butyric acid (GABA) was determined in amygdaloid cortex and hippocampus only. The only significant change occurred following chronic treatment of thymectomized rats with tacrine, when a significant elevation of GABA was found. Neither piracetam nor tacrine produced any change in the amines of their metabolites in the sham-operated controls. Tacrine, however, elevated the dopamine and reduced the 5-HT content of the hypothalamus and increased the 3,4-dihydroxylphenylacetic acid concentration of the striatum of thymectomized rats. Examination of the differential white blood cell count of the thymectomized rats showed that the percentage of lymphocytes was decreased, and the percentage of neutrophils increased, relative to the sham-operated controls. Chronic lacrine, but not piracetam, treatment reversed the lesion-induced changes.

Serotonin1A receptors are expressed by a subpopulation of cholinergic neurons in the rat medial septum and diagonal band of Broca--a double immunocytochemical study

The possible colocalization of 5-hydroxytryptamine1A receptors and choline acetyltransferase in the same neurons of the medial septum and diagonal band of Broca was investigated using double immunocytochemical techniques, either on the same section or on adjacent thin sections of the rat brain. The presence of both antigens in the same neurons was demonstrated at the light and electron microscopic levels. The proportion of cholinergic neurons that express 5-hydroxytryptamine1A receptors was similar in the different parts of the septal complex (around 25%). By contrast, the proportion of 5-hydroxytryptamine1A receptor-positive neurons also exhibiting choline acetyltransferase immunoreactivity was much higher (40-44%) in the dorsal and ventral groups of cholinergic cells, than in the intermediate group (18%). In line with the topographical distribution of cholinergic projections, this result points out the potential involvement of 5-hydroxytryptamine1A receptors in the control of the septohippocampal cholinergic projection by serotonin. This connection might be relevant to learning and memory, and in the appearance of age-dependent or neurodegenerative cognitive deficits, which have been shown to involve alterations in both the serotoninergic and the cholinergic systems.

Tryptophan restriction causes long-term plastic changes in corticofrontal pyramidal neurons

A Golgi study of third-layer pyramidal neurons from the corticofrontal cortex of tryptophan-restricted rats was carried out. At 40 days of age, dendritic arborization from treated rats was less profuse than that seen in control rats, and enlargement of dendritic processes, as well as an increase of the number of dendritic spines, were observed in 60-day-old rats. These plastic responses could be mediated either by a decrease in serotonin, which acts on the serotoninergic receptors of pyramidal neurons, or through an indirect mechanism mediated by cortical interneurons, or by serotoninergic modulation of the activity of other cortical neurotransmitters such as acetylcholine. Also, it could represent compensatory mechanisms underlying behavioral performance in some paradigms related to several cognitive processes.

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.).

Serotonin involvement in the spontaneous alternation ability: a behavioral study in tryptophan-restricted rats

Spontaneous alternation (SA) is controlled by septal cholinergic terminals in the hippocampus. Serotoninergic terminals end on cholinergic nerve endings in the hippocampus, and their possible role in SA was investigated in rats fed with a tryptophan-deficient diet, from weaning to 60 days of age. A T-maze was used for the test. At the age of 40 days, an increase in SA occurred in the tryptophan deficient rats, although this effect disappeared by 60 days of age. A modulatory role of serotonin in the psychoneural control of SA is suggested, and it may be through presynaptic inhibition of hippocampal cholinergic terminals.

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.

Serotonergic drug effects on a delayed conditional discrimination task in the rat; involvement of the 5-HT1A receptor in working memory

We investigated the effects of serotonergic drugs on working memory (WM) in a delayed conditional discrimination task. The 5-HT(1A) receptor full agonist flesinoxan (0.3-3.0 mg/kg) dose- and delay-dependently impaired performance, indicating a specific effect on WM. The 5-HT(1A) receptor partial agonist ipsapirone, the 5-HT( 1B/1D/2C) agonist TFMPP, the 5-HT(1A) antagonist NAN190 and the serotonin re-uptake inhibitor fluvoxamine dose-dependently impaired performance, in a delay-independent manner, indicating no specific effect on WM. The 5-HT( 2) receptor antagonist ketanserin and the 5-HT(3) receptor antagonist ondansetron did not affect performance. It is suggested that the role of central serotonin receptors in WM may be restricted to 5-HT(1A) receptors.

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.