Combined 192 IgG-saporin and 5,7-dihydroxytryptamine lesions in the male rat brain: a neurochemical and behavioral study

Adult-born neurons maintain hippocampal cholinergic inputs and support working memory during aging

Adult neurogenesis is reduced during aging and impaired in disorders of stress, memory, and cognition though its normal function remains unclear. Moreover, a systems level understanding of how a small number of young hippocampal neurons could dramatically influence brain function is lacking. We examined whether adult neurogenesis sustains hippocampal connections cumulatively across the life span. Long-term suppression of neurogenesis as occurs during stress and aging resulted in an accelerated decline in hippocampal acetylcholine signaling and a slow and progressing emergence of profound working memory deficits. These deficits were accompanied by compensatory reorganization of cholinergic dentate gyrus inputs with increased cholinergic innervation to the ventral hippocampus and recruitment of ventrally projecting neurons by the dorsal projection. While increased cholinergic innervation was dysfunctional and corresponded to overall decreases in cholinergic levels and signaling, it could be recruited to correct the resulting memory dysfunction even in old animals. Our study demonstrates that hippocampal neurogenesis supports memory by maintaining the septohippocampal cholinergic circuit across the lifespan.  It also provides a systems level explanation for the progressive nature of memory deterioration during normal and pathological aging and indicates that the brain connectome is malleable by experience.

Adult-born neurons maintain hippocampal cholinergic inputs and support working memory during aging

Adult neurogenesis is reduced during aging and impaired in disorders of stress, memory, and cognition though its normal function remains unclear. Moreover, a systems level understanding of how a small number of young hippocampal neurons could dramatically influence brain function is lacking. We examined whether adult neurogenesis sustains hippocampal connections cumulatively across the life span. Long-term suppression of neurogenesis as occurs during stress and aging resulted in an accelerated decline in hippocampal acetylcholine signaling and a slow and progressing emergence of profound working memory deficits. These deficits were accompanied by compensatory reorganization of cholinergic dentate gyrus inputs with increased cholinergic innervation to the ventral hippocampus and recruitment of ventrally projecting neurons by the dorsal projection. While increased cholinergic innervation was dysfunctional and corresponded to overall decreases in cholinergic levels and signaling, it could be recruited to correct the resulting memory dysfunction even in old animals. Our study demonstrates that hippocampal neurogenesis supports memory by maintaining the septohippocampal cholinergic circuit across the lifespan. It also provides a systems level explanation for the progressive nature of memory deterioration during normal and pathological aging and indicates that the brain connectome is malleable by experience.

Intrahippocampal Adeno-Associated Virus-Mediated Overexpression of Nerve Growth Factor Reverses 192IgG-Saporin-Induced Impairments of Hippocampal Plasticity and Behavior

One of the aspects of Alzheimer disease is loss of cholinergic neurons in the basal forebrain, which leads to development of cognitive impairment. Here, we used a model of cholinergic deficit caused by immunotoxin 192IgG-saporin to study possible beneficial effects of adeno-associated virus (AAV)-mediated overexpression of nerve growth factor (NGF) in the hippocampus of rats with cholinergic deficit. Suspension of recombinant AAV carrying control cassette or cassette with NGF was injected into both hippocampi of control rats or rats with cholinergic deficit induced by intraseptal injection of 192IgG-saporin. Analysis of choline acetyltransferase (ChAT) immunostaining showed that NGF overexpression in the hippocampus did not prevent strong loss of ChAT-positive neurons in the septal area caused by the immunotoxin. Induction of cholinergic deficit in the hippocampus led to impairments in Y-maze and beam-walking test but did not affect behavioral indices in the T-maze, open field test, and inhibitory avoidance training. NGF overexpression in the rats with cholinergic deficit restored normal animal behavior in Y-maze and beam-walking test. Recording of field excitatory postsynaptic potentials in vivo in the hippocampal CA1 area showed that induction of cholinergic deficit decreased magnitude of long-term potentiation (LTP) and prevented a decrease in paired-pulse ratio after LTP induction, and NGF overexpression reversed these negative changes in hippocampal synaptic characteristics. The beneficial effect of NGF was not associated with compensatory changes in the number of cells that express NGF receptors TrkA and NGFR in the hippocampus and medial septal area. NGF overexpression also did not prevent a 192IgG-saporin-induced decrease in the activity of acetylcholine esterase in the hippocampus. We conclude that NGF overexpression in the hippocampus under conditions of cholinergic deficit induces beneficial effects which are not related to maintenance of cholinergic function.

Different cholinergic cell groups in the basal forebrain regulate social interaction and social recognition memory

Social behaviour is a complex construct that is reported to include several components of social approach, interaction and recognition memory. Alzheimer's disease (AD) is mainly characterized by progressive dementia and is accompanied by cognitive impairments, including a decline in social ability. The cholinergic system is a potential constituent for the neural mechanisms underlying social behaviour, and impaired social ability in AD may have a cholinergic basis. However, the involvement of cholinergic function in social behaviour has not yet been fully understood. Here, we performed a selective elimination of cholinergic cell groups in the basal forebrain in mice to examine the role of cholinergic function in social interaction and social recognition memory by using the three-chamber test. Elimination of cholinergic neurons in the medial septum (MS) and vertical diagonal band of Broca (vDB) caused impairment in social interaction, whereas ablating cholinergic neurons in the nucleus basalis magnocellularis (NBM) impaired social recognition memory. These impairments were restored by treatment with cholinesterase inhibitors, leading to cholinergic system activation. Our findings indicate distinct roles of MS/vDB and NBM cholinergic neurons in social interaction and social recognition memory, suggesting that cholinergic dysfunction may explain social ability deficits associated with AD symptoms.

Saporin from Saponaria officinalis as a Tool for Experimental Research, Modeling, and Therapy in Neuroscience

Saporin, which is extracted from Saponaria officinalis, is a protein toxin that inactivates ribosomes. Saporin itself is non-selective toxin but acquires high specificity after conjugation with different ligands such as signaling peptides or antibodies to some surface proteins expressed in a chosen cell subpopulation. The saporin-based conjugated toxins were widely adopted in neuroscience as a convenient tool to induce highly selective degeneration of desired cell subpopulation. Induction of selective cell death is one of approaches used to model neurodegenerative diseases, study functions of certain cell subpopulations in the brain, and therapy. Here, we review studies where saporin-based conjugates were used to analyze cell mechanisms of sleep, general anesthesia, epilepsy, pain, and development of Parkinson’s and Alzheimer’s diseases. Limitations and future perspectives of use of saporin-based toxins in neuroscience are discussed.

Nitric oxide donor molsidomine promotes retrieval of object recognition memory in a model of cognitive deficit induced by 192 IgG-saporin

Nitric oxide (NO) plays a leading role in learning and memory processes. Previously, we showed its ability to modify the deleterious effect of immunotoxin 192 IgG-saporin (192-IgG-SAP) in the cholinergic system. The aim of this study was to analyze the potential of a NO donor (molsidomine, MOLS) to prevent the recognition memory deficits resulting from the septal cholinergic denervation by 192 IgG-SAP in rats. Quantification of neuronal and endothelial nitric oxide synthase (nNOS and eNOS, respectively) expression was evaluated in striatum, prefrontal cortex, and hippocampus. In addition, a choline acetyltransferase immunohistochemical analysis was performed in medial septum and assessed the effect of MOLS treatment on the spatial working memory of rats through a recognition memory test. Results showed that 192-IgG-SAP reduced the immunoreactivity of cholinergic septal neurons (41%), compared with PBS-receiving control rats (p < 0.05). Treatment with MOLS alone failed to antagonize the septal neuron population loss but prevented the progressive abnormal morphological changes of neurons. Those animals exposed to 192-IgG-SAP immunotoxin exhibited a reduction of cortical nNOS expression against the control group, whereas expression was enhanced in the 192-IgG-SAP + MOLS group. The most relevant finding was the recovering of the discrimination index exhibited by the 192-IgG-SAP + MOLS group. When compared with the rats exposed to the 192-IgG-SAP immunotoxin, they reached values similar to those observed in the PBS group. Our results show that although MOLS failed to block the cholinergic neurons loss induced by 192-IgG-SAP, it avoided the neuronal damage progression.

Effects of serotonin in the hippocampus: how SSRIs and multimodal antidepressants might regulate pyramidal cell function

The hippocampus plays an important role in emotional and cognitive processing, and both of these domains are affected in patients with major depressive disorder (MDD). Extensive preclinical research and the notion that modulation of serotonin (5-HT) neurotransmission plays a key role in the therapeutic efficacy of selective serotonin reuptake inhibitors (SSRIs) support the view that 5-HT is important for hippocampal function in normal and disease-like conditions. The hippocampus is densely innervated by serotonergic fibers, and the majority of 5-HT receptor subtypes are expressed there. Furthermore, hippocampal cells often co-express multiple 5-HT receptor subtypes that can have either complementary or opposing effects on cell function, adding to the complexity of 5-HT neurotransmission. Here we review the current knowledge of how 5-HT, through its various receptor subtypes, modulates hippocampal output and the activity of hippocampal pyramidal cells in rodents. In addition, we discuss the relevance of 5-HT modulation for cognitive processing in rodents and possible clinical implications of these results in patients with MDD. Finally, we review the data on how SSRIs and vortioxetine, an antidepressant with multimodal activity, affect hippocampal function, including cognitive processing, from both a preclinical and clinical perspective.

Septal serotonin depletion in rats facilitates working memory in the radial arm maze and increases hippocampal high-frequency theta activity

Hippocampal theta activity, which is strongly modulated by the septal medial/Broca׳s diagonal band neurons, has been linked to information processing of the hippocampus. Serotonin from the medial raphe nuclei desynchronises hippocampal theta activity, whereas inactivation or a lesion of this nucleus induces continuous and persistent theta activity in the hippocampus. Hippocampal serotonin depletion produces an increased expression of high-frequency theta activity concurrent with the facilitation of place learning in the Morris maze. The medial septum-diagonal band of Broca complex (MS/DBB) has been proposed as a key structure in the serotonin modulation of theta activity. We addressed whether serotonin depletion of the MS/DBB induces changes in the characteristics of hippocampal theta activity and whether the depletion is associated with learning in a working memory spatial task in the radial arm maze. Sprague Dawley rats were depleted of 5HT with the infusion of 5,7-dihydroxytriptamine (5,7-DHT) in MS/DBB and were subsequently trained in the standard test (win-shift) in the radial arm, while the CA1 EEG activity was simultaneously recorded through telemetry. The MS/DBB serotonin depletion induced a low level of expression of low-frequency (4.5-6.5Hz) and a higher expression of high-frequency (6.5-9.5Hz) theta activity concomitant to a minor number of errors committed by rats on the working memory test. Thus, the depletion of serotonin in the MS/DBB caused a facilitator effect on working memory and a predominance of high-frequency theta activity.

Specific serotonergic denervation affects tau pathology and cognition without altering senile plaques deposition in APP/PS1 mice

Senile plaques and neurofibrillary tangles are major neuropathological features of Alzheimer's Disease (AD), however neuronal loss is the alteration that best correlates with cognitive impairment in AD patients. Underlying neurotoxic mechanisms are not completely understood although specific neurotransmission deficiencies have been observed in AD patients and, in animal models, cholinergic and noradrenergic denervation may increase amyloid-beta deposition and tau phosphorylation in denervated areas. On the other hand brainstem neurodegeneration has been suggested as an initial event in AD, and serotonergic dysfunction, as well as reductions in raphe neurones density, have been reported in AD patients. In this study we addressed whether specific serotonergic denervation, by administering 5,7-dihydroxitriptamine (5,7-DHT) in the raphe nuclei, could also worsen central pathology in APPswe/PS1dE9 mice or interfere with learning and memory activities. In our hands specific serotonergic denervation increased tau phosphorylation in denervated cortex, without affecting amyloid-beta (Aβ) pathology. We also observed that APPswe/PS1dE9 mice lesioned with 5,7-DHT were impaired in the Morris water maze test, supporting a synergistic effect of the serotonergic denervation and the presence of APP/PS1 transgenes on learning and memory impairment. Altogether our data suggest that serotonergic denervation may interfere with some pathological aspects observed in AD, including tau phosphorylation or cognitive impairment, without affecting Aβ pathology, supporting a differential role of specific neurotransmitter systems in AD.

Piperine and curcumin exhibit synergism in attenuating D-galactose induced senescence in rats

Aging is associated with progressive decline in mental abilities and functional capacities. Postmitotic tissues are most vulnerable to alteration due to oxidative damage leading to behavioral and biochemical changes. We hypothesized that the anatomical and functional facets of the brain could be protected with powerful antioxidants such as piperine and curcumin by examining their effects individually and in combination in delaying senescence induced by d-galactose. Young adult male Wistar rats were treated with piperine (12 mg/kg) alone, and curcumin (40 mg/kg) alone; and in combination for a period of 49 days by the oral route with treatment being initiated a week prior to d-galactose (60 mg/kg, i.p.). A control group, d-galactose alone and naturally aged control were also evaluated. Behavioral tests, hippocampal volume, CA1 neuron number, oxidative parameters, formation of lipofuscin like autofluorescent substances, neurochemical estimation, and histopathological changes in CA1 region of hippocampus were established. Our results suggest that the combination exerted a superior response compared to monotherapy as evidenced by improved spatial memory, reduced oxidative burden, reduced accumulation of lipofuscin; improvement in signaling, increase in hippocampal volume and protection of hippocampal neurons. We speculate that the powerful antioxidant nature of both, augmented response of curcumin in the presence of piperine and enhanced serotoninergic signaling was responsible for improved cognition and prevention in senescence.

Acetylcholine and memory: a long, complex and chaotic but still living relationship

Even though "procholinergic" drugs are almost the sole kind of treatments currently used as cognitive enhancers in patients with Alzheimer's disease, the role of acetylcholine (ACh) in learning and memory is still poorly understood. In this short review, we focus on the septo-hippocampal cholinergic system and try to demonstrate that understanding ACh-memory relationships requires taking into account two characteristics of memory function. First, this function is polymorphic and relies on multiple neural systems. It appears that hippocampal ACh may not only modulate specific computational function of the hippocampus but also contributes to the functional coordination of multiple memory systems in a task-dependent manner. Second, memorization implies different phases which are differentially regulated by ACh. Namely, several lines of evidence suggest a "biphasic" involvement with hippocampal ACh facilitating memory encoding but hampering memory consolidation and retrieval, and low hippocampal ACh promoting consolidation of declarative memory. By spotting major determinants of memory modulation by hippocampal ACh, we hope that the present non exhaustive review will help to improve our understanding of the complexity of ACh-memory relationships.

Impact of regional 5-HT depletion on the cognitive enhancing effects of a typical 5-ht(6) receptor antagonist, Ro 04-6790, in the Novel Object Discrimination task

RATIONALE

Selective 5-ht(6) receptor antagonists like Ro 04-6790 prolong memory in many rodent preclinical paradigms, possibly by blocking tonic 5-HT-evoked GABA release and allowing disinhibition of cortico-limbic glutamatergic and cholinergic neurones. If this is the case, behavioural responses to Ro 04-6790 should be abolished by depletion of endogenous 5-HT, and selective lesions of dorsal raphé (DR) or median raphé (MR) 5-HT pathways would allow the neuroanatomical substrates underlying the cognitive effects of 5-ht(6) receptor antagonists to be elucidated.

OBJECTIVES

This study compared the effect of Ro 04-6790 on novel object discrimination (NOD) before and after sham or 5,7-dihydroxytryptamine (5,7-DHT)-induced lesions produced by injection into the lateral ventricles (LV), DR or MR.

MATERIALS AND METHODS

NOD tests used a 4 h inter-trial interval (ITI) and Ro 04-6790 (10 mg kg(-1) i.p.) was administered 20 min before the familiarization trial. Brain region-specific 5-HT depletion was assessed by high performance liquid chromatography with electrochemical detection (HPLC-ED).

RESULTS

Widespread LV or selective MR, but not DR lesions, abolished the ability of Ro 04-6790 to delay natural forgetting. Successful performance of all lesioned rats in subsequent 'drug-free' NOD tests using a 1 h ITI excluded the possibility of any confounding effects on visual acuity or motivation.

CONCLUSIONS

The ability of Ro 04-6790 to prolong object recognition memory requires blockade of MR 5-HT function. Because DR lesions did not produce the expected depletion of striatal 5-HT an additional contribution of DR inputs to this region cannot be completely excluded.

Serotonin depletion in the dorsal and ventral hippocampus: effects on locomotor hyperactivity, prepulse inhibition and learning and memory

We present an overview of our studies on the differential role of serotonergic projections from the median raphe nucleus (MRN) and dorsal raphe nucleus (DRN) in behavioural animal models with relevance to schizophrenia. Stereotaxic microinjection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the MRN or one of its main projections regions, the dorsal hippocampus, induced a marked enhancement of phencyclidine-induced locomotor hyperactivity and a disruption of prepulse inhibition (PPI) in rats. There was no enhancement of locomotor hyperactivity induced by amphetamine or MK-801 or after 5,7-DHT lesions of the DRN or ventral hippocampus. Rats with dorsal hippocampus lesions did not show significant changes in the Y-maze test for short-term spatial memory, the Morris water maze for long-term spatial memory, or in the T-maze delayed alternation test for working memory. These chronic lesion studies suggest a modulatory influence of serotonergic projections from the MRN to the dorsal hippocampus on phencyclidine effects and prepulse inhibition, but not on different forms of learning and memory. The results provide new insight into the role of serotonin in the dorsal hippocampus in aspects of schizophrenia.

Sex does matter: comments on the prevalence of male-only investigations of drug effects on rodent behaviour

Despite abundant evidence of sex differences in the effects of drugs on nonsexual behaviour in rats and mice, most researchers continue to investigate male animals exclusively. This was evident from a survey of all relevant research reports published during the period February 2005-September 2006 (inclusive) in recent issues of five representative behavioural pharmacological journals. Reasons for excluding female animals from most studies are discussed along with attempts to justify the use of either male or female animals only, and the value of including both sexes (especially when a drug effect is poorly understood). Although there are other factors that can influence the effects of drugs, such as strain, age and social density, the sex of experimental animals is the easiest to control and thus is well suited to inclusion in pharmacological investigations. It is accordingly suggested that, as has been recommended many times in the past, animals' sex should play a more important part in future research than is still currently the case.

The behavioral effects of enriched housing are not altered by serotonin depletion but enrichment alters hippocampal neurochemistry

To assess a possible role for serotonin in the mediation of the behavioral changes induced by enriched housing conditions (EC), adult female Long-Evans rats sustaining a serotonin depletion (150 microg of 5,7-dihydroxytryptamine, icv) and sham-operated rats were housed postoperatively for 30 days in enriched (12 rats/large cage containing various objects) or standard housing conditions (2 rats/standard laboratory cage). Thereafter, anxiety responses (elevated plus-maze), locomotor activity (in the home-cage), sensori-motor capabilities (beam-walking task), and spatial memory (eight-arm radial maze) were assessed. Monoamine levels were subsequently measured in the frontoparietal cortex and the hippocampus. Overall, EC reduced anxiety-related responses, enhanced sensori-motor performance and improved the memory span in the initial stage of the spatial memory task. Despite a substantial reduction of serotonergic markers in the hippocampus (82%) and the cortex (74%), these positive effects of EC were not altered by the lesion. EC reduced the serotonin levels in the ventral hippocampus (particularly in unlesioned rats: -23%), increased serotonin turnover in the entire hippocampus (particularly in lesioned rats: +36%) and augmented the norepinephrine levels in the dorsal hippocampus (+68% in unlesioned and +49% in lesioned rats); no such alterations were found in the frontoparietal cortex. Our data suggest that an intact serotonergic system is not a prerequisite for the induction of positive behavioral effects by EC. The neurochemical changes found in the hippocampus of EC rats, however, show that the monoaminergic innervation of the hippocampus is a target of EC.

Effect of selective cholinergic denervation on the serotonergic system: implications for learning and memory

The cholinergic system has been widely implicated in cognitive processes and cholinergic loss is a classical hallmark in Alzheimer disease. Increasing evidence supports a role of the serotonergic system in cognition, possibly through a modulation of cholinergic activity. We compared selective cholinergic denervation by administration of the immunotoxin 192 IgG-saporin in the nucleus basalis of Meynert (NBM) with intracerebroventricular (ICV) lesions of the basal forebrain in male rats 7 days after lesioning. NBM lesions induced significant changes in cholinergic markers in the frontal cortex, whereas ICV lesions produced significant decreases in cholinergic markers both in the frontal cortex and hippocampus. Only ICV lesions lead to memory impairments in passive avoidance and Morris water maze tasks. Both models lead to reductions of serotonin levels in the frontal cortex. Similar changes in 5-hydroxytriptophan levels were observed, suggesting a downregulation of the rate-limiting enzyme for the synthesis of serotonin along with the cholinergic deficit. Neither 5-HT1A nor 5-HT1B receptors seem to mediate this process. These data imply that the serotonergic system in the frontal cortex can compensate for diminished cholinergic function and support the investigation of the serotonergic system as a therapeutic target to treat Alzheimer disease.

Amelioration of water maze performance deficits by topiramate applied during pilocarpine-induced status epilepticus is negatively dose-dependent

Temporal lobe epilepsy is characterized by a progressive loss of memory capacities, due to sclerosis and functional impairment of mesiotemporal brain areas. We have shown recently that topiramate (TPM) dose-dependently protects hippocampal CA1 and CA3 neurons during initial status epilepticus in the rat pilocarpine model of temporal lobe epilepsy by inhibition of mitochondrial transition pore opening. In the present study, in order to evaluate possible positive effects of the treatment on learning and memory, we investigated water maze performance of rats receiving different dosages of TPM (20 and 100 mg/kg) after 40 min and 4 mg/kg diazepam after 160 min of pilocarpine-induced status epilepticus in relation to performance of animals receiving 4 mg/kg diazepam after 40 min of SE, and to performance of sham-treated control animals. Unexpectedly, 20 but not 100 mg/kg TPM significantly extenuated short-term memory deficits. While neuroprotective effects of TPM were observed in hippocampal CA subfields of animals treated with 100 mg/kg TPM, cell loss in rats treated with 20 mg/kg TPM was indistinguishable from animals receiving diazepam only. The present results indicate a negative dose-dependency of memory-saving effects of TPM applied during status epilepticus apparently dissociated from hippocampal neuroprotection.

5,7-DHT lesion of the dorsal raphe nuclei impairs object recognition but not affective behavior and corticosterone response to stressor in the rat

Previous studies with acute tryptophan depletion, leading to transient central 5-HT reductions, showed no effects on affective behavior but impaired object memory. In the present study, the behavioral effects of a 5,7-dihydroxytryptamine (5,7-DHT) lesion in the dorsal raphe were evaluated in animal models of anxiety (open field test), depression (forced swimming test), behavioral inhibition (discrete fixed interval test) and cognition (object recognition task). The corticosterone response to a stress condition was examined at several intervals after 5,7-DHT treatment. The substantial reduction in neuronal 5-HT markers in the dorsal raphe did not affect anxiety-related, depressive-like or impulsive behavior. Compared to the SHAM group, the lesioned rats showed a lower response latency to obtain a reward, indicating a quick and accurate reaction to a stimulus. No differences were found in the progressive ratio test for food motivation. A marked impairment in object recognition was found. The 5,7-DHT treatment did not affect the corticosterone response to a stressful situation. Overall, these results corroborate studies with acute tryptophan depletion suggesting a role of 5-HT in object memory, but not affective behavior.

Long-term effects of immunotoxic cholinergic lesions in the septum on acquisition of the cone-field task and noncognitive measures in rats

In rats, nonspecific mechanical or neurotoxic lesions of the septum impair spatial memory in, e.g., Morris water- and radial-maze tasks. Unfortunately, the lack of specificity of such lesions limits inferences about the role of the cholinergic hippocampal projections in spatial cognition. We therefore tested the effects of septal lesions produced by 192 IgG-saporin in rats, which is highly selective for basal forebrain cholinergic neurons, on home cage activity, noncognitive tests (modified Irwin test, open field and forced swimming tests, and various sensorimotor tasks), and the cone-field spatial learning task. The immunotoxic lesion reduced acetylcholine (ACh) levels in the septum (-61%) and hippocampus (>-75%). Rats with lesions showed mild home-cage hyperactivity at 4 weeks postlesion, but no noncognitive deficits at 13 weeks postsurgery. In the cone-field task, rats with septal lesions made more working- and reference-memory errors than the controls, but acquisition curves were parallel in both groups. The speed of visiting cones was faster in the rats with lesions, indicative of disturbed attention or increased motivation. These data support the growing evidence that involvement of the septohippocampal cholinergic system in spatial learning and memory may have been overestimated in studies that used lesions with poor selectivity.

Ethanol, 3,4-methylenedioxymethamphetamine (ecstasy) and their combination: long-term behavioral, neurochemical and neuropharmacological effects in the rat

This study investigated long-term behavioral, neurochemical, and neuropharmacological effects of ethanol-(+/-)-3,4-methylenedioxymethamphetamine (MDMA, ecstasy) combinations. Over 4 consecutive days, male Long-Evans rats received 1.5 g/kg ethanol and/or 10 mg/kg MDMA, or saline. Rectal temperatures were taken in some rats. Starting 4 days after the last injection, we tested working memory, sensory-motor coordination, and anxiety. Subsequently, we measured cortical, striatal, septal, and hippocampal monoamines (last MDMA injection-euthanasia delay: 20 days), or electrically evoked release of serotonin (5-HT) in cortical and hippocampal slices, and its modulation in the presence of CP 93,129 (3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrollo[3,2-b]pyrid-5-one) or methiotepin (last MDMA injection-euthanasia delays: 3-6 weeks). Ethanol attenuated the MDMA-induced hyperthermia, but only on the first day. In the long-term, MDMA reduced 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) content in most brain regions. The behavioral and neurochemical effects of the ethanol-MDMA combination were comparable to those of MDMA alone; sensory-motor coordination was altered after ethanol and/or MDMA. In hippocampal slices from rats given ethanol and MDMA, the CP 93,129-induced inhibition and methiotepin-induced facilitation of 5-HT release were stronger and weaker, respectively, than in the other groups. This is the first study addressing long-term effects of repeated MDMA and EtOH combined treatments in experimental animals. Whereas the drug combination produced the same behavioral and neurochemical effects as MDMA alone, our neuropharmacological results suggest that MDMA-EtOH interactions may have specific long-term consequences on presynaptic modulation of hippocampal 5-HT release, but not necessarily related to MDMA-induced depletion of 5-HT. Thus, it is likely that the psycho(patho)logical problems reported by ecstasy users drinking alcohol are not solely due to the consumption of MDMA.

Orexin-saporin lesions of the medial septum impair spatial memory

The medial septum and diagonal band of Broca (MSDB) provide a major input to the hippocampus and are important for spatial learning and memory. Although electrolytic MSDB lesions have prominent memory impairing effects, selective lesions of either cholinergic or GABAergic MSDB neurons do not or only mildly impair spatial memory. MSDB neurons are targets of orexin-containing neurons from the hypothalamus. At present, the functional significance of orexin afferents to MSDB is unclear, and the present study investigated a possible involvement of orexin innervation of the MSDB in spatial memory. Orexin-saporin, a toxin that damages neurons containing the hypocretin-2 receptor, was administered into the MSDB of rats. Rats were subsequently tested on a water maze to assess spatial reference memory and a plus maze to assess spatial working memory. At 100 ng/microl, orexin-saporin destroyed primarily GABAergic septohippocampal neurons, sparing the majority of cholinergic neurons. At 200 ng/microl, orexin-saporin almost totally eliminated GABAergic septohippocampal neurons and destroyed many cholinergic neurons. Spatial reference memory was impaired at both concentrations of orexin-saporin with a dramatic impairment observed for 24-h retention. Short-term reference memory was also impaired at both concentrations. Rats treated with 200 ng/microl, but not 100 ng/microl, of orexin-saporin were also impaired on a spontaneous alternation task, showing a deficit in spatial working memory. Our results, together with previous studies, suggest that orexin innervation of the MSDB may modulate spatial memory by acting on both GABAergic and cholinergic septohippocampal neurons.

Basomedial hypothalamic injections of neuropeptide Y conjugated to saporin selectively disrupt hypothalamic controls of food intake

Neuropeptide Y (NPY) conjugated to saporin (NPY-SAP), a ribosomal inactivating toxin, is a newly developed compound designed to selectively target and lesion NPY receptor-expressing cells. We injected NPY-SAP into the basomedial hypothalamus (BMH), just dorsal to the arcuate nucleus (ARC), to investigate its neurotoxicity and to determine whether ARC NPY neurons are required for glucoprivic feeding. We found that NPY-SAP profoundly reduced NPY Y1 receptor and alpha MSH immunoreactivity, as well as NPY, Agouti gene-related protein (AGRP), and cocaine and amphetamine-related transcript mRNA expression in the BMH. NPY-SAP lesions were localized to the injection site with no evidence of retrograde transport by hindbrain NPY neurons with BMH terminals. These lesions impaired responses to intracerebroventricular (icv) leptin (5 microg/5 microl x d) and ghrelin (2 microg/5 microl), which are thought to alter feeding primarily by actions on ARC NPY/AGRP and proopiomelanocortin/cocaine and amphetamine-related transcript neurons. However, the hypothesis that NPY/AGRP neurons are required downstream mediators of glucoprivic feeding was not supported. Although NPY/AGRP neurons were destroyed by NPY-SAP, the lesion did not impair either the feeding or the hyperglycemic response to 2-deoxy-D-glucose-induced blockade of glycolysis use. Similarly, responses to glucagon-like peptide-1 (GLP-1, 5 microg/3 microl icv), NPY (5 microg/3 microl icv), cholecystokinin octapeptide (4 microg/kg ip), and beta-mercaptoacetate (68 mg/kg ip) were not altered by the NPY-SAP lesion. Thus, NPY-SAP destroyed NPY receptor-expressing neurons in the ARC and selectively disrupted controls of feeding dependent on those neurons but did not disrupt peptidergic or metabolic controls dependent upon circuitry outside the BMH.

Septohippocampal acetylcholine: involved in but not necessary for learning and memory?

The neurotransmitter acetylcholine (ACh) has been accorded an important role in supporting learning and memory processes in the hippocampus. Cholinergic activity in the hippocampus is correlated with memory, and restoration of ACh in the hippocampus after disruption of the septohippocampal pathway is sufficient to rescue memory. However, selective ablation of cholinergic septohippocampal projections is largely without effect on hippocampal-dependent learning and memory processes. We consider the evidence underlying each of these statements, and the contradictions they pose for understanding the functional role of hippocampal ACh in memory. We suggest that although hippocampal ACh is involved in memory in the intact brain, it is not necessary for many aspects of hippocampal memory function.

Impaired and spared cholinergic functions in the hippocampus after lesions of the medial septum/vertical limb of the diagonal band with 192 IgG-saporin

To lesion the cholinergic input to the hippocampus, rats received injections of 192 IgG-saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). The lesions produced near-total loss of choline acetyltransferase (ChAT)-positive neurons in the MS/VDB. The loss was accompanied, however, by only partial decreases (to 40% of control levels) in acetylcholine (ACh) release in the hippocampus. Moreover, ACh release in the hippocampus increased when lesioned and control rats were tested on a spontaneous alternation task, indicating that there was significant residual cholinergic function in the hippocampus. The lesions were sufficient to impair spontaneous alternation scores. However, this impairment could be reversed by either systemic or intra-hippocampal injections of the indirect cholinergic agonist, physostigmine, providing additional evidence of residual and effective cholinergic functions in the hippocampus of lesioned rats. Moreover, systemic injections of physostigmine at doses that produced mild tremors in control rats led to more severe tremors in the lesioned rats, suggesting upregulation of cholinergic mechanisms after saporin lesions, likely in brain areas other than the hippocampus. Thus, these findings provide evidence for decreases in cholinergic input to the hippocampus accompanied by deficits on a spontaneous alternation tasks. The findings also provide evidence for considerable residual cholinergic input to the hippocampus after saporin lesions of the MS/VDB. Together, the results suggest that 192 IgG-saporin lesions of the MS/VDB, using methods often employed, do not fully remove septohippocampal cholinergic input to the hippocampus but are nonetheless sufficient to produce impairments on a task impaired by hippocampal lesions.

Delay-dependent working memory impairment in young-adult and aged 5-HT1BKO mice as assessed in a radial-arm water maze

Serotonin (5-HT) plays a modulatory role in mnemonic functions, especially by interacting with the cholinergic system. The 5-HT1B receptor is a key target of this interaction. The 5-HT1B receptor knockout mice were found previously to exhibit a facilitation in hippocampal-dependent spatial reference memory learning. In the present study, we submitted mice to a delayed spatial working memory task, allowing the introduction of various delays between an exposure trial and a test trial. The 5-HT1BKO and wild-type mice learned the task in a radial-arm water maze (returning to the most recent presented arm containing the escape platform), and exhibited a high level of performance at delays of 0 and 5 min. However, at the delay of 60 min, only 5-HT1BKO mice exhibited an impairment. At a delay of 90 min, all mice were impaired. Treatment by scopolamine (0.8 mg/kg) induced the same pattern of performance in wild type as did the mutation for short (5 min, no impairment) and long (60 min, impairment) delays. The 22-month-old wild-type and knockout mice exhibited an impairment at short delays (5 and 15 min). The effect of the mutation affected both young-adult and aged mice at delays of 15, 30, and 60 min. Neurobiological data show that stimulation of the 5-HT1B receptor inhibits the release of acetylcholine in the hippocampus, but stimulates this in the frontal cortex. This dual function might, at least in part, explain the opposite effect of the mutation on reference memory (facilitation) and delay-dependent working memory (impairment). These results support the idea that cholinergic-serotonergic interactions play an important role in memory processes.

Spatial Learning in the 5-HT1B Receptor Knockout Mouse: Selective Facilitation/Impairment Depending on the Cognitive Demand

Age-related memory decline is associated with a combined dysfunction of the cholinergic and serotonergic systems in the hippocampus and frontal cortex, in particular. The 5-HT1B receptor occupies strategic cellular and subcellular locations in these structures, where it plays a role in the modulation of ACh release. In an attempt to characterize the contribution of this receptor to memory functions, 5-HT1B receptor knockout (KO) mice were submitted to various behavioral paradigms carried out in the same experimental context (water maze), which were aimed at exposing mice to various levels of memory demand. 5-HT1BKO mice exhibited a facilitation in the acquisition of a hippocampal-dependent spatial reference memory task in the Morris water maze. This facilitation was selective of task difficulty, showing thus that the genetic inactivation of the 5-HT1B receptor is associated with facilitation when the complexity of the task is increased, and reveals a protective effect on age-related hippocampal-dependent memory decline. Young-adult and aged KO and wild-type (WT) mice were equally able to learn a delayed spatial matching-to-sample working memory task in a radial-arm water maze with short (0 or 5 min) delays. However, 5-HT1BKO mice, only, exhibited a selective memory impairment at intermediate and long (15, 30, and 60 min) delays. Treatment by scopolamine induced the same pattern of performance in wild type as did the mutation for short (5 min, no impairment) and long (60 min, impairment) delays. Taken together, these studies revealed a beneficial effect of the mutation on the acquisition of a spatial reference memory task, but a deleterious effect on a working memory task for long delays. This 5-HT1BKO mouse story highlights the problem of the potential existence of "global memory enhancers."

Hippocampal serotonergic damage induced by MDMA (ecstasy): effects on spatial learning

3,4-Methylenedioxymethamphetamine (MDMA) use has been associated with a decline in various aspects of mnemonic function in humans. We therefore postulated that MDMA-induced damage of serotonergic nerve terminals would alter hippocampal processing. Seven days following treatment with MDMA (2 x 20 mg/kg sc, given 12 h apart), rat spatial learning and memory were tested utilizing the Morris water maze (MWM). No statistical differences were found in MWM platform acquisition latency or pathlength between controls and MDMA-treated animals. Probe trials revealed significantly higher proximity score averages and significantly reduced preference for the target quadrant in the MDMA-treated animals. MDMA treatment resulted in significant reduction (34%) in hippocampal serotonin (5-HT) levels 14 days after initial treatment. The findings of this study demonstrate that hippocampal serotonergic lesions induced by MDMA may be ostensibly linked to a reference memory deficit in rats tested with the MWM.

Does the release of acetylcholine in septal slices originate from intrinsic cholinergic neurons bearing p75ntr receptors? a study using 192 IgG-saporin lesions in rats

In previous studies electrically-evoked release of acetylcholine in septal slices was demonstrated. The present experiment aimed at verifying if this release involved intrinsic neurons bearing p75(NTR) receptors. Long-Evans rats sustained injections of 192 IgG-saporin into the medial septum/diagonal band of Broca (0.8 microg). Sham-operated rats served as controls. Two to 3.5 weeks later, the electrically-evoked release of acetylcholine ([(3)H]ACh) was measured in slices from the lateral septum (LS), medial septum (MS) and diagonal band of Broca (DBB). Choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity, and monoamine concentrations were measured in the septum, cortex and hippocampus. The lesion extent was also assessed by ChAT immunostaining in a separate series of rats. In the septum, the number of ChAT-positive neurons was depleted dramatically (>90% at the level of the injection site). In the hippocampus, the lesions reduced ChAT and AChE activity by 91% and 84%, respectively. In the cortex, this reduction was weaker (-55% and -47%). In the septal region, the reduction was either weak or not significant. The evoked release of acetylcholine in septal slices was not reduced, except in the slices from the LS (-64%). The effects of physostigmine and atropine confirmed the presence of autoreceptors. Our data exclude that a major part of the acetylcholine released by MS and DBB slices derived from intrinsic neurons bearing p75(NTR) receptors. In the LS, part of the released acetylcholine might be from projections of such neurons located in the LS, MS and/or DBB. These data also suggest that the MS and the DBB may be the target of extrinsic cholinergic innervation that does not bear p75(NTR) receptors.

5,7-DHT-induced hippocampal 5-HT depletion attenuates behavioural deficits produced by 192 IgG-saporin lesions of septal cholinergic neurons in the rat

Adult Long-Evans male rats sustained injections of 5,7-dihydroxytryptamine into the fimbria-fornix (2.5 microg/side) and the cingular bundle (1.5 microg/side) and/or to intraseptal injections of 192 IgG-saporin (0.4 microg/side) in order to deprive the hippocampus of its serotonergic and cholinergic innervations, respectively. Sham-operated rats were used as controls. The rats were tested for locomotor activity (postoperative days 18, 42 and 65), spontaneous T-maze alternation (days 20-29), beam-walking sensorimotor (days 34-38), water maze (days 53-64) and radial maze (days 80-133) performances. The cholinergic lesions, which decreased the hippocampal concentration of ACh by about 65%, induced nocturnal hyperlocomotion, reduced T-maze alternation, impaired reference-memory in the water maze and working-memory in the radial maze, but had no effect on beam-walking scores and working-memory in the water maze. The serotonergic lesions, which decreased the serotonergic innervation of the hippocampus by about 55%, failed to induce any behavioural deficit. In the group of rats given combined lesions, all deficits produced by the cholinergic lesions were observed, but the nocturnal hyperlocomotion and the working-memory deficits in the radial maze were attenuated significantly. These results suggest that attenuation of the serotonergic tone in the hippocampus may compensate for some dysfunctions subsequent to the loss of cholinergic hippocampal inputs. This observation is in close concordance with data showing that a reduction of the serotonergic tone, by pharmacological activation of somatodendritic 5-HT(1A) receptors on raphe neurons, attenuates the cognitive disturbances produced by the intrahippocampal infusion of the antimuscarinic drug, scopolamine. This work has been presented previously [Serotonin Club/Brain Research Bulletin conference, Serotonin: From Molecule to the Clinic (satellite to the Society for Neuroscience Meeting, New Orleans, USA, November 2-3, 2000)].