Effects of combined methysergide and mecamylamine/scopolamine treatment on spatial navigation

Systemic injection of nicotinic acetylcholine receptor antagonist mecamylamine affects licking, eyelid size, and locomotor and autonomic activities but not temporal prediction in male mice

Nicotinic acetylcholine receptors are thought to be associated with a wide range of phenomena, such as movement, learning, memory, attention, and addiction. However, the causal relationship between nicotinic receptor activity and behavior remains unclear. Contrary to the studies that examined the functions of muscarinic acetylcholine receptors, the role of the nicotinic acetylcholine receptors on behavior has not been examined as extensively. Here, we examined the effects of intraperitoneal injection of mecamylamine, a nicotinic acetylcholine receptor antagonist, on the performance of male mice in a head-fixed temporal conditioning task and a free-moving open-field task. The head-fixed experimental setup allowed us to record and precisely quantify the licking response while the mice performed the behavioral task with no external cues. In addition, by combining the utility of the head-fixed experimental design with computer vision analysis based on deep learning algorithms, we succeeded in quantifying the eyelid size of awake mice. In the temporal conditioning task, we delivered a 10% sucrose solution every 10 s using a blunt-tipped needle placed within the licking distance of the mice. After the training, the mice showed increased anticipatory licking toward the timing of sucrose delivery, suggesting that the mice could predict the timing of the reward. Systemic injection of mecamylamine decreased licking behavior and caused eye closure but had no effect on learned conditioned predictive behavior in the head-fixed temporal conditioning task. In addition, the injection of mecamylamine decreased spontaneous locomotor activity in a dose-dependent manner in the free-moving open-field task. The results in the open-field experiments further revealed that the effect of mecamylamine on fecal output and urination, suggesting the effects on autonomic activities. Our achievement of successful eyelid size recording has potential as a useful approach in initial screening for drug discovery. Our study paves a way forward to understanding the role of nicotinic acetylcholine receptors on learning and behavior.

Performance of F2 B6x129 hybrid mice in the Morris water maze, latent inhibition and prepulse inhibition paradigms: Comparison with C57Bl/6J and 129sv inbred mice

Assessment of cognition and information processing in mice is an important tool in preclinical research that focuses on the development of cognitive enhancing drugs. Analysis of transgenic (TG) and knockout (KO) mice is usually performed on a F2 B6x 129 background. In the present study, we have compared performance of F2 B6x 129 hybrid mice (F2 mice) with that of the two parental inbred strains (C57Bl/6J and 129sv mice), and a wild-type (WT) strain (with a combined B6x 129 background) in three cognitive/information processing paradigms. It was found that the F2 mice outperformed either of the parental strains and provide a control sample with good baseline performance in the Morris water maze (MWM). Reliable deficits could be obtained in learning and memory in this paradigm following injections with scopolamine (0.16 mg/kg) in the F2 mice, which can potentially be used to test effects of reference and novel compounds in order to develop cognitive enhancing drugs. Furthermore, it was shown that the four genotypes showed normal latent inhibition (LI) using the conditioned taste aversion (CTA) paradigm and exhibited no differences in prepulse inhibition (PPI) levels. Following the setup of these procedures in mice, we are now able to compare the effects of gene knockout/mutations used for target validation with results in the present study as a frame of reference.

Neuropharmacology of cognition and memory: A unifying theory of neuromodulator imbalance in psychiatry and amnesia

The case of HM, a man with intractable epilepsy who became amnesic following bilateral medial temporal lobe surgery nearly half a century ago has instigated ongoing research and theoretical speculation on the nature of memory and the role of the hippocampus. Neuropsychological testing showed that although HM had extensive anterograde memory loss he could still acquire motor and cognitive skills implicitly, but could not remember the context of this learning. This has lead to declarative and procedural descriptions of the memory process. Cholinergic and monoaminergic neurotransmitter systems have also been implicated in the memory process and anticholinergic drugs traditionally have been associated with impairment of declarative memory. The cholinergic hypothesis of Alzheimer's disease is a classic example of an application of these neuropharmacological findings. In schizophrenia, preattentive deficits have been amply demonstrated by unconscious priming studies. Memory processes are also impaired in these patients. Dopamine, glutamate and even cholinergic dysfunction has been implicated in the clinical picture of schizophrenia. The present paper will attempt to bring together both the anatomical and pharmacological data from these disparate fields of research under a cohesive theory of cognition and memory. A hypothesis is presented for an inverse relationship between monoaminergic and cholinergic systems in the modulation of implicit (unconscious) and explicit (conscious) cognitive processes. It is postulated that muscarinic cholinergic receptors and monoaminergic systems facilitate unconscious and conscious processes, respectively, and they disfacilitate conscious and unconscious processes, respectively (the purported inverse relationship). In fact, the muscarinic and monoaminergic modulations of a neural network are proposed to be finely balanced such that, if, the activity of one receptor system is modified then this by necessity has effects on the other system. It takes into account receptor subtypes and their effects mediated through excitatory and inhibitory G-protein complexes. For example, m1/D2 and D1/m4 paired receptor subtypes, colocalized on separate neurons would have opposing functional effects. A theory is then presented that the critical underlying pathophysiology of schizophrenia involves a hypofunctional muscarinic cholinergic system, which induces abnormal facilitation of monoaminergic subsystems such as dopamine (e.g., a decrease in m1R function would potentiate D2R function). This extends the idea of an inverted U function for optimal monoaminergic concentrations. Not only would this impair unconscious preattentive processes, but according to the hypothesis, explicit cognition as well including memory deficits and would underlie the mechanism of psychosis. Contrary to current thinking a different view is also presented for the role of the hippocampus in the memory process. It is postulated that long-term explicit memory traces in the neocortex are laid down by phasic coactivation of forebrain projecting monoaminergic systems above some basal firing rate, such as the rostral serotonergic raphe, which projects diffusely to the cortex and according to a modified Hebbian principle. This is the proposed principal function of the hippocampal theta rhythm. The phasic activation of the cholinergic basal forebrain is mediated by projections from a separate cortical structure, possibly the lateral prefrontal cortex. Phasic muscarinic receptor activation is proposed to strengthen implicit memory traces (at a synaptic level) in the neocortex. Thus, the latter are spared by medial temporal surgery explaining the dissociation of explicit from implicit memory.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

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.

Excitotoxic median raphe lesions aggravate working memory storage performance deficits caused by scopolamine infusion into the dentate gyrus of the hippocampus in the inhibitory avoidance task in rats

The interactions between the median raphe nucleus (MRN) serotonergic system and the septohippocampal muscarinic cholinergic system in the modulation of immediate working memory storage performance were investigated. Rats with sham or ibotenic acid lesions of the MRN were bilaterally implanted with cannulae in the dentate gyrus of the hippocampus and tested in a light/dark step-through inhibitory avoidance task in which response latency to enter the dark compartment immediately after the shock served as a measure of immediate working memory storage. MRN lesion per se did not alter response latency. Post-training intrahippocampal scopolamine infusion (2 and 4 microg/side) produced a more marked reduction in response latencies in the lesioned animals compared to the sham-lesioned rats. Results suggest that the immediate working memory storage performance is modulated by synergistic interactions between serotonergic projections of the MRN and the muscarinic cholinergic system of the hippocampus.

Effects of excitotoxic median raphe lesion on working memory deficits produced by the dorsal hippocampal muscarinic receptor blockade in the inhibitory avoidance in rats

The experiments investigated the interactions between median raphe nucleus (MRN) serotonergic and septo-hippocampal muscarinic cholinergic systems in the modulation of forming and storing performances of working memory. Rats with ibotenic acid-induced MRN-lesion bilaterally received scopolamine (2-4 microg/each side) infusion into the dentate gyrus of the dorsal hippocampus and were tested in a single trial step-through inhibitory avoidance. Initial preference to the dark compartment (escape latency) was taken as the measure of non-mnemonic behaviours and response latency to enter the dark compartment immediately after the foot-shock was used to measure working memory. The high-dose scopolamine infusion 10 min before the training decreased escape latencies in the sham-lesioned rats, whereas had no effect in the MRN-lesioned rats. Although MRN lesion per se did not alter response latency, it alleviated pre-training scopolamine-induced decrease, but aggravated post-training scopolamine-induced reduction in this parameter. These results suggest that the antagonistic interactive processes between MRN-serotonergic and hippocampal cholinergic systems modulate non-mnemonic component of working memory formation, whereas the storing performance of working memory is modulated by the synergistic interactions between these systems in the hippocampus, mainly in the dentate gyrus.

Alzheimer's disease: more than a 'cholinergic disorder' - evidence that cholinergic-monoaminergic interactions contribute to EEG slowing and dementia

The loss of cognitive (particularly mnemonic) abilities constitutes a prominent symptom of Alzheimer's disease (AD). These cognitive symptoms occur in close relation to the slowing of the electroencephalogram (EEG), and it is likely that the inability of cortical circuits to maintain an activated state contributes to the behavioral disorganization in AD. The 'cholinergic hypothesis' of AD suggests that many of the cognitive and EEG symptoms are related to the atrophy of basal forebrain cholinergic neurons, which innervate the neocortex and hippocampus, among others. However, data from behavioral and electrophysiological studies in rats suggest that selective reductions in cholinergic transmission result in relatively small mnemonic impairments, and only a partial reduction in EEG activation. Thus, cholinergic atrophy alone may not be sufficient to cause the marked changes in cognition and cortical activity typical of AD. Cholinergic deficits do, however, make neural circuits susceptible to additional neurodegenerative processes. In rats, lowered serotonergic or noradrenergic activity alone often produces only minor impairments in learning/memory tasks and does not block EEG activation. The same monoaminergic deficits, however, result in severe behavioral impairments, and reduce or abolish EEG activation when they occur in a brain already affected by lowered cholinergic activity. There is an abundance of evidence that monoamines are reduced in AD. These degenerative processes, when occurring in a neural environment compromised by cholinergic atrophy, may then contribute to the disturbances in cortical processing and cognition/behavior in AD. A prediction derived from this theory is that an enhancement of monoaminergic functions may have beneficial effects on behavior and cortical activity. Preliminary experiments support this idea: combined cholinergic-monoaminergic stimulation can be more effective in reversing behavioral (Morris water maze) impairments and EEG slowing in rats with multiple neurotransmitter deficiencies than cholinergic enhancement alone. Thus, a stimulation of monoaminergic activity, in conjunction with cholinergic therapies, may provide an effective treatment option for AD.

Serotonin-dependent maintenance of spatial performance and electroencephalography activation after cholinergic blockade: effects of serotonergic receptor antagonists

The interaction between acetylcholine (ACh) and serotonin (5-hydroxytryptamine, 5-HT) in the control of behavior such as spatial navigation has received considerable attention over the last years. Previous research indicates that while a selective reduction in cholinergic transmission often produces only mild impairments in spatial and other behavioral tests, additional serotonergic blockade results in the appearance of severe behavioral deficits. Consequently, it has been argued that 5-HT plays a role in the maintenance of behavioral capacities in the face of reduced cholinergic transmission. Here, we examined the effects of 5-HT depletion and receptor blockade, alone and in combination with cholinergic-muscarinic antagonism, on spatial navigation of rats in the Morris water maze. Further, electroencephalographic (EEG) recordings were taken to test the hypothesis that a loss of neocortical activation is related to the behavioral deficits apparent after cholinergic-serotonergic blockade. The muscarinic antagonist, scopolamine (1 mg/kg) produced a moderate impairment in navigational performance. The 5-HT depletor, p-chlorophenylalanine (PCPA; 500 mg kg(-1) day(-1)x2) did not impair performance when given alone but strongly potentiated the scopolamine-induced deficit and completely blocked the acquisition of an escape response in the water maze. This effect was mimicked by the non-selective serotonin(1-2) receptor antagonist, methiothepin (0.3 mg/kg), but not by the selective serotonin(1A) antagonist, WAY 100635 (0.1-0.5 mg/kg) or the serotonin(2) antagonist, ketanserin (2-4 mg/kg). None of the 5-HT antagonists impaired performance when given alone. Electrocorticographic recordings in rats treated with scopolamine and serotonergic receptor antagonists showed that during behavioral immobility, scopolamine (1 mg/kg) increased spectral power in all frequency bands between 0.5 and 20 Hz without significantly affecting cortical activity during movement. None of the 5-HT antagonists affected cortical activity when given alone. However, methiothepin, at the same dose that produced behavioral impairments, increased spectral power between 0.5 and 4 Hz and between 8 and 12 Hz during movement when co-administered with scopolamine. The results suggest that a concurrent blockade of multiple 5-HT receptors, but not selective blockade of serotonin(1A) or serotonin(2) receptors alone, mimics the ability of global 5-HT depletion to abolish behavioral capacities that are resistant to muscarinic receptor blockade. The behavioral deficits observed here are accompanied by a reduction in neocortical activation, suggesting that disturbances of processing in cortical networks can contribute to the behavioral disorganization apparent after cholinergic and serotonergic blockade. A focus on concurrent serotonergic-cholinergic deficits may provide a useful framework for the development of novel pharmacological treatments to counteract the behavioral disorganization and loss of EEG activation present in senile dementia and Alzheimer's disease.

Intrahippocampal grafts containing cholinergic and serotonergic fetal neurons ameliorate spatial reference but not working memory in rats with fimbria-fornix/cingular bundle lesions

Three-month-old Long-Evans female rats sustained aspirative lesions of the dorsal septohippocampal pathways and, 2 weeks later, received intrahippocampal suspension grafts containing cells from the mesencephalic raphe, cells from the medial septum and the diagonal band of Broca, or a mixture of both. Lesion-only and sham-operated rats were used as controls. All rats were tested for locomotor activity 1 week, 3 and 5 months after lesion surgery, for spatial working memory in a radial maze from 5 to 9 months, and for reference and working memory in a water tank during the 9th month after lesioning. Determination of hippocampal concentration of acetylcholine, noradrenaline, and serotonin was made after completion of behavioral testing. Compared to sham-operated rats, all rats with lesions, whether grafted or not, exhibited increased levels of locomotor activity and made more errors in the radial maze. The lesioned rats were also impaired in the probe trial (30 first seconds) of the water-tank test made according to a protocol requiring intact reference memory capabilities. While rats with septal or raphe grafts were also impaired, the rats with co-grafts showed performances not significantly different from those of sham-operated rats. With a protocol requiring intact working memory capabilities, all lesioned rats, whether grafted or not, were impaired in the water-tank test. In the dorsal hippocampus of lesion-only rats, the concentration of acetylcholine and serotonin was significantly reduced. In rats with septal grafts or co-grafts, the concentration of acetylcholine was close to normal, as was that of serotonin in rats with raphe grafts or co-grafts. These results confirm previous findings showing that co-grafts enabled the neurochemical properties of single grafts to be combined. Data from the water-tank test suggest that cholinergic and serotonergic hippocampal reinnervations by fetal cell grafts may induce partial recovery of spatial reference, but not working memory capabilities in rats.

Differential effects of three 5-HT receptor antagonists on the performance of rats in attentional and working memory tasks

The effects of three different serotonin (5-HT) receptor antagonists (ketanserin, methysegide, methiothepin) in the modulation of attention, working memory and behavioural activity were investigated in this study by assessing the performance of rats in two separate cognitive models; the 5-choice serial reaction time (5-CSRT) task, which measures attention, and the delayed non-matching to position (DNMTP) task, which measures working memory. Methysergide and methiothepin bind at the 5-HT1 and 5-HT2 as well as the 5-HT5-7 receptors, with varying degrees of selectivity, and ketanserin binds at the 5-HT2A receptors rather selectively. None of these agents bind to any significant extent to 5-HT3 or 5-HT4 receptors. In the 5-CSRT task, neither methiothepin (0.15 mg/kg) nor ketanserin (1.0 and 3.0 mg/kg) impaired the choice accuracy of rats, although they induced sedation. The low doses of methysergide (1.5 and 3.0 mg/kg) slightly increased the behavioural activity of rats, whereas the high dose of methysergide (15.0 mg/kg) reduced behavioural activity and slightly reduced choice accuracy of the rats in the attentional task (monitoring of visual stimuli) under the baseline conditions or curtailed stimulus duration. This effect was not augmented at the reduced stimulus intensity. These findings suggest that the high dose of methysergide did not interfere with the visual discrimination of rats. Furthermore, methysergide did not reduce motivation for this task, since it did not increase food collection latencies. In the DNMTP task, methiothepin (0.15 mg/kg) induced a delay non-dependent deficit in choice accuracy. This could be due to an impaired alternation ability or akinesia, which increases an actual delay between sample and choice. Methiothepin (0.15 mg/kg) also interfered with behavioural activity of rats. Interestingly, ketanserin (1.0 mg/kg and 3.0 mg/kg) and methysergide (3.0-15.0 mg/kg) neither impaired the choice accuracy nor reduced the behavioural activity of rats in the DNMTP task. These results suggest that the blockade of 5-HT2A receptors does not interfere with attention and working memory per se. However, all three serotonin receptor antagonists interfered with behavioural activity of rats in the 5-CSRT task more severely than in the DNMTP task. The possible role of serotonin and non-serotonin receptors underlying the influence of these antagonists on behavioural activity will be 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.

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.).

Modulation of rat neocortical high-voltage spindle activity by 5-HT1/5-HT2 receptor subtype specific drugs

To investigate the role of serotonin (5-hydroxytryptamine; 5-HT) receptors in the modulation of rat thalamocortical oscillations, we studied the effects of 5-HT1/5-HT2 receptor subtype specific drugs on neocortical high-voltage spindle activity in adult male rats. A 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.03, 0.1, 0.3 and 1.0 mg/kg s.c.), had no effect on neocortical high-voltage spindle activity. Furthermore, a mixed 5-HT1/5-HT2 receptor antagonist, methysergide (1.0, 5.0 and 15.0 mg/kg i.p.), had no effect, whereas a non-specific mixed 5-HT1/5-HT2 receptor antagonist, methiothepin (0.2, 1.0 and 5.0 mg/kg i.p.), significantly increased neocortical high-voltage spindles. Of the 5-HT2 receptor antagonists, ritanserin (0.1, 1.0 and 5.0 mg/kg s.c.) had no effect, whereas ketanserin (1.0, 5.0 and 20.0 mg/kg s.c.) increased neocortical high-voltage spindles, but only at the highest dose used. A 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.5, 1.0 and 2.0 mg/kg s.c.), at the two highest doses significantly decreased neocortical high-voltage spindle activity, and this effect was blocked by the 5-HT2 receptor antagonists, ketanserin (1.0, 5.0 and 20.0 mg/kg s.c.) and ritanserin (1.0 and 5.0 mg/kg s.c.), as well as by methiothepin (0.2, 1.0 and 5.0 mg/kg i.p.) and methysergide (1.0, 5.0 and 15.0 mg/kg i.p.). Furthermore, unilateral intrathalamic infusions, but not intrahippocampal control infusions, of DOI (10 and 50 micrograms/1.0 microliters/rat) decreased neocortical high-voltage spindle activity and systemic administration of ketanserin (20.0 mg/kg s.c.) completely blocked this effect. The present results suggest that (1) the serotonergic system modulates rat thalamocortical oscillations as measured by neocortical high-voltage spindle activity, (2) activation of 5-HT2 receptors, possibly located in the thalamus, with a specific 5-HT2 receptor agonist, DOI, causes a reduction in rat neocortical high-voltage spindle activity.

Concurrent blockade of beta-adrenergic and muscarinic receptors disrupts working memory but not reference memory in rats

In order to clarify the interactions between monoaminergic and cholinergic systems in working and reference memory functions, the effects of administration of the alpha, beta-adrenergic, D1-, D2-dopaminergic or serotonergic receptor antagonist together with the muscarinic receptor antagonist scopolamine on this behavior were examined using a three-panel runway task. Both in working and reference memory tasks, the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) was significantly increased by 0.32 mg/kg scopolamine, but not by the doses up to 0.18 mg/kg. The beta-adrenergic antagonist propranolol at 10 mg/kg had no effect on the number of working memory errors. Combined administration of 10 mg/kg propranolol and scopolamine at 0.1 and 0.18 mg/kg significantly increased the number of working memory errors. However, in a reference memory task, propranolol at 10 mg/kg did not affect the number of errors, whether administered alone or together with 0.1 mg/kg scopolamine. Other monoaminergic receptor antagonists, including the alpha-adrenergic antagonist phentolamine (3.2 and 10 mg/kg), D1-antagonist SCH23390 (0.032 and 0.056 mg/kg). D2-antagonist sulpiride (100 mg/kg) and serotonin antagonist cinanserin (10 and 32 mg/kg) had no significant effect on working or reference memory errors, whether they were administered independently or in combination with scopolamine at 0.1 mg/kg. These results suggest that beta-adrenergic/muscarinic interactions play an important role in mediating processes involved in working memory performance of rats.

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.

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.

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.

Interaction between 5-HT1A and nicotinic cholinergic receptors in the regulation of water maze navigation behavior

The interaction between serotonin (5-HT)1A and nicotinic cholinergic receptors in the regulation of spatial navigation behavior in the Morris water maze (WM) test was studied. Pretraining intraperitoneal (i.p.) injections of a combination of subthreshold doses of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (a 5-HT1A receptor agonist) at 30 micrograms/kg and mecamylamine (a nicotinic cholinergic receptor antagonist) a 2500 micrograms/kg greatly impaired WM navigation to a hidden platform and slightly, but not statistically significantly, impaired WM navigation to a visible platform. Post-training i.p. injections of this combination had no effect on WM navigation performance. Serotonin depletion induced by p-chlorophenylalanine (PCPA) increased the performance impairing action of pretraining injected combination of 8-OH-DPAT 30 micrograms/kg and mecamylamine 2500 micrograms/kg. In trained rats combined injections of 8-OH-DPAT 30 micrograms/kg and mecamylamine 2500 micrograms/kg given pretraining had no effect on the navigation to a hidden platform located in a familiar or in a novel position. Pretraining trial injected combination of hexamethonium 2000 micrograms/kg (a peripherally acting nicotinic antagonist) and 8-OH-DPAT 30 micrograms/kg had no effect on navigation. These data suggest that a combined treatment with a 5-HT1a receptor agonist and a nicotinic cholinergic receptor antagonist more severely impair non-mnemonic acquisition performance processes than consolidation and retrieval processes.

Effects of p-chlorophenylalanine and methysergide on the performance of a working memory task

The present study investigated the effects of serotonergic dysfunction on working memory. Therefore, the effects of inhibition of serotonin [5-hydroxytryptamine (5-HT)] synthesis induced by p-chlorophenylalanine (p-CPA) and pharmacological blockade of 5-HT receptors by methysergide on the performance of rats in a delayed nonmatching to position task assessing spatial working memory were studied. Methysergide (1.0, 5.0, or 15.0 mg/kg) significantly disrupted behavioral activity of rats and decreased the percent correct total responses. However, the impairment in the percent correct responses was delay independent, indicating a nonmnemonic disruption of the performance. p-CPA (500 mg/kg/day x 3) induced an almost total depletion (> 97%) of frontal cortical and hippocampal serotonin and its major metabolite 5-hydroxyindoleacetic acid and slightly affected noradrenergic and dopaminergic systems. p-CPA treatment did not affect the percent correct responses. However, the behavioral activity of rats was slightly decreased by p-CPA. The disruptions in behavioral activity and the percent correct responses induced by methysergide (2.0 mg/kg) were not abolished by p-CPA. The present results do not support any important role for the serotonergic system in spatial working memory as assessed using the delayed nonmatching to position task.