Intermittent working memory training during adulthood protects against age-related long-term spatial reference memory decline in rats

Engagement in cognitive activity in adulthood is one of the factors that enable successful cognitive aging, both in humans and rodents. However, some studies emphasize that the beneficial effect on cognition of such an activity may reflect carry over from one test situation to another, including memory for procedural aspects of the behavioral tasks, and thus question whether this effect can be limited to the trained cognitive domain or whether it can be transferred to an untrained ones. In the current study, we assessed whether adulthood intermittent working memory training has beneficial effect on long-term memory of aged rats using two very different test situations. To this aim, rats trained in a delayed non-matching to position task in operant box at 3 and 15 months of age were tested in a place learning task in water maze when they were 24 months. The two tasks differ with regard to the cognitive domain but also in their spatial ability requirement and the nature of the reinforcer used. During the memory tests, accuracy of the platform search indicated age-related impairment only in the aged-untrained group. Thus, intermittent training during adult life in a task involving working memory protects aged animals from the deleterious effects of aging on spatial reference memory. This result highlights the long-term beneficial effects of training on a working memory task on an untrained cognitive domain.

Behavioural characteristics and sex differences of a treatment-resistant depression model: Chronic mild stress in the Wistar-Kyoto rat

Depression affects 20% of the general population and is a leading cause of disability worldwide, with a strong female prevalence. Current pharmacotherapies have significant limitations, and one third of patients are unresponsive. Male Wistar-Kyoto rats exposed to chronic mild stress (CMS) were recently proposed as a model to study antidepressant resistance. However, sex differences and interindividual vulnerability to stress are yet unexplored in this model. We aimed to investigate these in the context of the behavioural impact of CMS in the sucrose preference test, elevated plus maze (EPM), forced swim test (FST), open field test and daily locomotor activity rhythms, in male and female WKY rats exposed or not to a 4-week CMS protocol. CMS-exposed animals were clustered through K-means into subgroups based on the EPM and FST results. In both sexes, one subgroup behaved similarly to non-stressed animals and was labelled stress-non vulnerable; the second exhibited less open arms exploration in the EPM and higher immobility in the FST and was named stress-vulnerable. Vulnerable males presented phase delay in daily locomotor activity following CMS, but no significant rhythm could be determined in females. CMS-exposed males of both groups showed hyperlocomotion in reaction to novelty and slower weight gain through the course of CMS, while CMS-exposed females showed smaller sucrose intake. Unexpectedly, CMS did not affect sucrose preference. Our findings strengthen the view that in models of psychiatric pathologies based on stress exposure it is important to consider the effect of sex and to differentiate the non vulnerable and vulnerable subpopulations.

Functional brain-wide network mapping during acute stress exposure in rats: Interaction between the lateral habenula and cortical, amygdalar, hypothalamic and monoaminergic regions

Upon stress exposure, a broad network of structures comes into play in order to provide adequate responses and restore homeostasis. It has been known for decades that the main structures engaged during the stress response are the medial prefrontal cortex, the amygdala, the hippocampus, the hypothalamus, the monoaminergic systems (noradrenaline, dopamine and serotonin) and the periaqueductal gray. The lateral habenula (LHb) is an epithalamic structure directly connected to prefrontal cortical areas and to the amygdala, whereas it functionally interacts with the hippocampus. Also, it is a main modulator of monoaminergic systems. The LHb is activated upon exposure to basically all types of stressors, suggesting it is also involved in the stress response. However, it remains unknown if and how the LHb functionally interacts with the broad stress response network. In the current study we performed in rats a restraint stress procedure followed by immunohistochemical staining of the c-Fos protein throughout the brain. Using graph theory-based functional connectivity analyses, we confirm the principal hubs of the stress network (e.g., prefrontal cortex, amygdala and periventricular hypothalamus) and show that the LHb is engaged during stress exposure in close interaction with the medial prefrontal cortex, the lateral septum and the medial habenula. In addition, we performed DREADD-induced LHb inactivation during the same restraint paradigm in order to explore its consequences on the stress response network. This last experiment gave contrasting results as the DREADD ligand alone, clozapine-N-oxide, was able to modify the network.

The Lateral Habenula as a Relay of Cortical Information to Process Working Memory

Working memory is a cognitive ability allowing the temporary storage of information to solve problems or adjust behavior. While working memory is known to mainly depend on the medial prefrontal cortex (mPFC), very few is known about how cortical information are relayed subcortically. By its connectivity, the lateral habenula (lHb) might act as a subcortical relay for cortical information. Indeed, the lHb receives inputs from several mPFC subregions, and recent findings suggest a role for the lHb in online processing of spatial information, a fundamental aspect of working memory. In rats, in a delayed non-matching to position paradigm, using focal microinjections of the GABAA agonist muscimol we showed that inactivation of the lHb (16 ng in 0.2 µL per side), as well as disconnection between the prelimbic region of the mPFC (mPFC/PrL, 32 ng in 0.4 µL in one hemisphere) and the lHb (16 ng in 0.2 µL in the lHb in the contralateral hemisphere) impaired working memory. The deficits were unlikely to result from motivational or motor deficits as muscimol did not affect reward collection or cue responding latencies, and did not increase the number of omissions. These results show for the first time the implication of the lHb in mPFC-dependent memory processes, likely as a relay of mPFC/PrL information. They also open new perspectives in the understanding of the top-down processing of high-level cognitive functions.

Late enrichment maintains accurate recent and remote spatial memory only in aged rats that were unimpaired when middle aged

Exposure of rodents to a stimulating environment has beneficial effects on some cognitive functions that are impaired during physiological aging, and especially spatial reference memory. The present study investigated whether environmental enrichment rescues these functions in already declining subjects and/or protects them from subsequent decline. Subgroups of 17-mo-old female rats with unimpaired versus impaired performance in a spatial reference memory task (Morris water maze) were housed until the age of 24 mo in standard or enriched environment. They were then trained in a second reference memory task, conducted in a different room than the first, and recent (1 d) and remote (10 d) memory were assessed. In unimpaired subgroups, spatial memory declined from 17 to 24 mo in rats housed in standard conditions; an enriched environment during this period allowed maintenance of accurate recent and remote spatial memory. At 24 mo, rats impaired at the age of 17 mo housed in enriched environment learned the task and displayed substantial recent memory, but their performance remained lower than that of unimpaired rats, showing that enrichment failed to rescue spatial memory in already cognitively declining rats. Controls indicated carryover effects of the first water maze training, especially in aged rats housed in standard condition, and confirmed the beneficial effect of enrichment on remote memory of aged rats even if they performed poorly than young adults housed for the same duration in standard or enriched condition.

Attention and executive functions in a rat model of chronic epilepsy

OBJECTIVE

Temporal lobe epilepsy is a relatively frequent, invalidating, and often refractory neurologic disorder. It is associated with cognitive impairments that affect memory and executive functions. In the rat lithium-pilocarpine temporal lobe epilepsy model, memory impairment and anxiety disorder are classically reported. Here we evaluated sustained visual attention in this model of epilepsy, a function not frequently explored.

METHODS

Thirty-five Sprague-Dawley rats were subjected to lithium-pilocarpine status epilepticus. Twenty of them received a carisbamate treatment for 7 days, starting 1 h after status epilepticus onset. Twelve controls received lithium and saline. Five months later, attention was assessed in the five-choice serial reaction time task, a task that tests visual attention and inhibitory control (impulsivity/compulsivity). Neuronal counting was performed in brain regions of interest to the functions studied (hippocampus, prefrontal cortex, nucleus basalis magnocellularis, and pedunculopontine tegmental nucleus).

RESULTS

Lithium-pilocarpine rats developed motor seizures. When they were able to learn the task, they exhibited attention impairment and a tendency toward impulsivity and compulsivity. These disturbances occurred in the absence of neuronal loss in structures classically related to attentional performance, although they seemed to better correlate with neuronal loss in hippocampus. Globally, rats that received carisbamate and developed motor seizures were as impaired as untreated rats, whereas those that did not develop overt motor seizures performed like controls, despite evidence for hippocampal damage.

SIGNIFICANCE

This study shows that attention deficits reported by patients with temporal lobe epilepsy can be observed in the lithium-pilocarpine model. Carisbamate prevents the occurrence of motor seizures, attention impairment, impulsivity, and compulsivity in a subpopulation of neuroprotected rats.

Abstract 298: Chronic Environmental Enrichment Improves Aging-related Endothelial Dysfunction in Rats: Role of Oxidative Stress

Aging is associated with an endothelial dysfunction, which promotes cardiovascular diseases. Physical exercise has beneficial effects on the endothelial function. However, the effect of an enriched environment (EE) promoting physical exercise, on the endothelial function has not been explored during aging. We investigated the endothelial dysfunction in Long-Evans female rats housed in standard conditions (SD, 2 rats per cage) until the age of 18 months. Then, half the rats were transferred to EE (12 rats in one large cage containing various objects, which were changed 5 times per week) for 6 months. A group of 2-month old rats served as young controls. Systolic (SBP), diastolic (DBP) blood pressure and heart rate (HR) were determined by plethysmography. Vascular reactivity was assessed in mesenteric arteries using organ chambers, vascular oxidative stress by dihydroethidine staining, and the proteins expression by immunofluorescence. Aging under SD was associated with increased SBP (by 40 mmHg, P <0.001), DBP (by 12 mmHg, P <0.001) and HR (by 110 bpm, P <0.001) compared to young rats. It was also associated with blunted acetylcholine (ACh)-induced endothelium-dependent nitric oxide (NO)-mediated relaxations (EC50 1.08 10 -8 M vs. 3.08 10 -8 M, P <0.05) and endothelium-derived hyperpolarization (EDH)-mediated relaxation (E max 89.5±11.8% vs. 0.28±0.18%, P <0.001), and the induction of endothelium-dependent contractile responses to ACh (E max 0.004±0.008 g vs. 0.71±0.07 g, P <0.001) in mesenteric artery rings. EE reduced SBP and DBP by 18 and 10 mmHg, respectively ( P <0.05), HR by 30 bpm ( P >0.05), and improved NO-mediated responses to a similar level as in young rats ( P <0.001) without affecting the blunted EDH-mediated relaxation and the increased endothelium-dependent contractile responses in old rats. An excessive oxidative stress and the upregulation of endothelial NO synthase (eNOS), nitrotyrosine and arginase I were observed in the aorta of SD old rats compared to young rats ( P <0.001). EE improved all these effects ( P <0.05). In conclusion, EE retarded the aging-related endothelial dysfunction mostly by improving the NO-mediated relaxation. This effect involves normalization of vascular oxidative stress and possibly also eNOS uncoupling.

Lifelong environmental enrichment in rats: impact on emotional behavior, spatial memory vividness, and cholinergic neurons over the lifespan

We assessed lifelong environmental enrichment effects on possible age-related modifications in emotional behaviors, spatial memory acquisition, retrieval of recent and remote spatial memory, and cholinergic forebrain systems. At the age of 1 month, Long-Evans female rats were placed in standard or enriched rearing conditions and tested after 3 (young), 12 (middle-aged), or 24 (aged) months. Environmental enrichment decreased the reactivity to stressful situations regardless of age. In the water maze test, it delayed the onset of learning deficits and prevented age-dependent spatial learning and recent memory retrieval alterations. Remote memory retrieval, which was altered independently of age under standard rearing conditions, was rescued by enrichment in young and middle-aged, but unfortunately not aged rats. A protected basal forebrain cholinergic system, which could well be one out of several neuronal manifestations of lifelong environmental enrichment, might have contributed to the behavioral benefits of this enrichment.

Detection of histone acetylation levels in the dorsal hippocampus reveals early tagging on specific residues of H2B and H4 histones in response to learning

The recent literature provides evidence that epigenetic mechanisms such as DNA methylation and histone modification are crucial to gene transcription linked to synaptic plasticity in the mammalian brain--notably in the hippocampus--and memory formation. We measured global histone acetylation levels in the rat hippocampus at an early stage of spatial or fear memory formation. We found that H3, H4 and H2B underwent differential acetylation at specific sites depending on whether rats had been exposed to the context of a task without having to learn or had to learn about a place or fear therein: H3K9K14 acetylation was mostly responsive to any experimental conditions compared to naive animals, whereas H2B N-terminus and H4K12 acetylations were mostly associated with memory for either spatial or fear learning. Altogether, these data suggest that behavior/experience-dependent changes differently regulate specific acetylation modifications of histones in the hippocampus, depending on whether a memory trace is established or not: tagging of H3K9K14 could be associated with perception/processing of testing-related manipulations and context, thereby enhancing chromatin accessibility, while tagging of H2B N-terminus tail and H4K12 could be more closely associated with the formation of memories requiring an engagement of the hippocampus.

Contribution of corticosterone to cued versus contextual fear in rats

Several studies have suggested a positive relationship between circulating corticosterone levels and contextual conditioning. However, a positive relationship between circulating corticosterone levels and cued conditioning has also been reported. This study further investigates the relationship between corticosterone and fear conditioning by modulating the predictive value of contextual and discrete tone cues in separate groups of rats. In a first experiment in which training parameters were chosen to induce strong conditioning (five foot-shocks), we used a correlational approach and investigated whether post-training corticosterone levels were related to subsequent expression of contextual and/or tone fear. In a second experiment, in which training parameters were chosen to induce lower conditioning (one and two foot-shocks), we investigated whether a post-training corticosterone injection enhanced the consolidation of contextual and/or tone conditioning. In the first experiment, the highest post-training corticosterone levels were obtained in rats trained with paired tones and shocks. Post-training corticosterone levels tended to be positively correlated with freezing scores during the tone-fear test and were negatively correlated with freezing scores during training although not during the context-fear test. In the second experiment, a post-training injection of corticosterone (3mg/kg) had no effect on subsequent freezing to contextual cues and to a tone that did not predict shock, whereas it was efficient in increasing fear conditioned to a predictive tone. Globally, these results suggest that the predictive value of the conditioned stimulus may be the main determinant of the facilitatory action of acutely enhanced corticosterone in fear conditioning.

Environmental enrichment increases responding to contextual cues but decreases overall conditioned fear in the rat

This study aimed at investigating the effects of environmental enrichment on various aspects of contextual processing in adult female rats. In experiment 1, simple conditioning was studied using either a training procedure allowing overshadowing of the contextual cues by signalling footshock with a discrete tone or a training procedure allowing a reduction of this overshadowing by explicitly unpairing the footshock and the tone. In experiment 2, contextual discrimination and contextual occasion-setting were assessed. Rats were daily exposed to two different contexts. In one context, a footshock was delivered 30s after the offset of a tone, whereas in the other context the same tone was presented alone. Experiment 3 examined familiarization to a new context. Experiment 1 showed that environmental enrichment reduced the overshadowing of contextual cues by the tone and also reduced freezing to the more predictive cue according to the training procedure used. Experiment 2 showed that environmental enrichment increased the ability of rats to discriminate two contexts. Experiment 3 showed that enriched rats familiarized faster to a new context than standard rats. Taken together, these results suggest that environmental enrichment in adult rats enhances learning about contextual cues and reduces overall fear associated with aversive events.

Entorhinal cortex lesions disrupt fear conditioning to background context but spare fear conditioning to a tone in the rat

Recent studies have shown that the integrity of the entorhinal cortex (EC) is not required for simple contextual conditioning. In background contextual conditioning, i.e., when a phasic cue is present during training, the involvement of the EC is still a matter of debate. Therefore, the present work further examines whether the EC is required for background contextual conditioning using a tone as the phasic cue. Rats sustaining either excitotoxic lesions of the EC or sham-lesions were trained with one of two procedures differing with respect to the predictive value of the tone: a paired procedure in which the tone perfectly predicts shock occurrence and overshadows context, and an unpaired procedure in which the predictive value of the tone is reduced. Conditioned fear was assessed by freezing responses during conditioning, reexposure to the training context, and reexposure to the tone in a new context. Postshock freezing was reduced in rats with entorhinal lesions. In all rats trained with the paired procedure, freezing to the context was low and freezing to the tone was high, suggesting that the tone has overshadowed the context during the conditioning session. The reverse pattern was observed with the unpaired procedure in sham-operated rats. In rats with entorhinal lesions trained with the unpaired procedure, freezing responses to the context was markedly reduced. In a new context, however, entorhinal-lesioned rats showed higher freezing scores than those of sham-lesioned rats. Freezing to the tone was unaffected by the lesion irrespective of the tone's predictive value. As a whole, these results support the notion that the EC is required for normal background contextual freezing.

Degeneration of the basalocortical pathway from the cortex induces a functional increase in galaninergic markers in the nucleus basalis magnocellularis of the rat

The present work aimed 1) to evaluate whether an increase in galanin or galanin receptors could be induced in the nucleus basalis magnocellularis (nbm) by degeneration of the basalocortical neurons from the cortex and 2) to analyze the consequences of such an increase on cortical activity. First, a mild ischemic insult to the frontoparietal cortex was performed to induce the degeneration of the basalocortical system; galanin immunoreactivity, galanin binding sites, and cholinergic muscarinic receptors were quantified through immunocytochemistry and autoradiography. Second, galanin infusions in the nbm were undertaken to mimic a local increase of the galaninergic innervation; cortical acetylcholine release, cerebral glucose use, and cerebral blood flow were then measured as indices of cortical activity. As a result of the cortical ischemic lesion, the postsynaptic M1 and presynaptic M2 muscarinic receptors were found to be reduced in the altered cortex. In contrast, galaninergic binding capacity and fiber density were found to be increased in the ipsilateral nbm in parallel with a local decrease in the cholinergic markers such as the muscarinic M1 receptor density. Galanin infusion into the nbm inhibited the cortical acetylcholine release and cerebral blood flow increases elicited by the activation of the cholinergic basalocortical system but failed to affect acetylcholine release, cerebral blood flow, and cerebral glucose use when injected alone in the nbm. These results demonstrate that degeneration of the basalocortical system from the cortex induces an increase in galaninergic markers in the nbm, a result that might suggest that the galaninergic overexpression described in the basal forebrain of patients with Alzheimer's disease can result from a degeneration of the cholinergic basalocortical system from the cortex. Because galanin was found to reduce the activity of the basalocortical cholinergic system only when this one is activated, galanin might exert its role rather during activation deficits than under resting conditions such as the resting cortical hypometabolism, which is characteristic of Alzheimer's disease.

Differentially altered cerebral metabolism in ischemic rats by alpha2-adrenoceptor blockade and its relation to improved limb-placing reactions

The selective alpha2-adrenoreceptor antagonist, atipamezole, improves behavioural performance of rats subjected to focal cerebral ischemia. The aim of the present study was to investigate whether the facilitatory effect of atipamezole on behaviour is related to altered neuronal activity in specific brain areas. The right middle cerebral artery of rats was occluded for 120 min using the intraluminal filament method. Starting on day 2 after induction of ischemia, atipamezole (1mg/kg, s.c.) or 0.9% NaCl was administered to ischemic or sham-operated rats once a day 30 min before the limb-placing test. [14C]Deoxyglucose ([14C]DG) uptake was used to measure neuronal activity 30 min after atipamezole or 0.9% NaCl administration on day 6 after ischemia. Ischemia induced a significant decrease in [14C]DG uptake in several cortical areas ipsilateral and contralateral to the lesion, in the ipsilateral thalamus, and bilaterally in the cerebellum and spinal cord. Administration of atipamezole normalised [14C]DG uptake particularly in the cerebellum and spinal cord both in sham-operated and ischemic rats and to a lesser extent in the thalamus in sham-operated rats. The pattern of altered cerebral [14C]DG uptake following alpha2-adrenoceptor blockade suggests that plasticity in the cerebellum and spinal cord contributes to the improved performance of ischemic rats in tests assessing tactile/proprioceptive limb-placing reactions.

Metabolic alterations in the prefrontal and cingulate cortices are related to behavioral deficits in a rodent model of attention-deficit hyperactivity disorder

Rats with a deficit in selective attention accompanied by impulsivity can be identified using a five-choice serial reaction time task (5-CSRT) and have been proposed to represent a rodent model of attention-deficit hyperactivity disorder (ADHD). The aim of the present study was to investigate which brain areas are important for visuospatial attention and to test the specific hypothesis that dysfunction of the frontal cortex is related to the behavioral deficits observed in poorly performing rats. Therefore, [(14)C]deoxyglucose (DG) uptake, an index of brain metabolic activity, was measured during the performance of a 5-CSRT task in two populations of rats (poorly and well-performing rats) to study the relationships between the regional brain activity and behavioral output. While performing a 5-CSRT task, poorly performing rats exhibited lower DG uptake in the cingulate and ventrolateral orbital cortices than did well-performing rats,. Moreover, there was a positive correlation between choice accuracy and DG uptake in several areas, especially in the frontal and parietal regions, whereas there was an inverse correlation between the percentage of premature responses and DG uptake in the ventrolateral orbital and cingulate cortices. These results, which demonstrated that the poorly performing rats exhibited metabolic dysfunction in the cingulate and prefrontal cortices, provide a basis for the face validity of the rodent model of ADHD. Moreover, they suggest that the neural network of attention in rats is remarkably analogous to that described in primates.

Inhibition of MAO-A activity enhances behavioural activity of rats assessed using water maze and open arena tasks

To determine if the inhibition of MAO-A and/or MAO-B activities can influence cognitive processes in adult rats, we analysed whether chronic treatment with clorgyline, 1-deprenyl and pargyline could modify the performance of adult rats in a modified version of the water maze task. The effects of these treatments on locomotor activity and enzyme activities were also assessed. Rats were treated for 24 days with clorgyline (0.2 mg/kg), 1-deprenyl (0.25 mg/kg) and pargyline (I or 10 mg/kg). The treatments were started two weeks before the water maze experiment and continued until the end of testing. The rats were trained to find a submerged platform (6 days: I trial/day; 7 th day: probe trial). Over the next three days, locomotor activity was assessed in an open arena. Treatments with clorgyline (MAO-A inhibitor), 1-deprenyl (MAO-B inhibitor) and pargyline (non-selective MAO inhibitor) did not improve the finding of the hidden platform, when compared to treatment with saline, but significantly increased the swimming speed of the rats. The different treatments, when compared to saline, failed to modify the distance covered and the number of groomings performed in the open arena. However, clorgyline and pargyline, 10 mg/kg, increased the number of faecal boli and clorgyline enhanced the number of rearings made when compared to saline, 1-deprenyl and pargyline, 10 mg/kg. These results indicate that near total inhibition of MAO-A by clorgyline and pargyline as assessed by MAO activity measurement induces an increase in locomotor activity but that inhibition of MAO-A or MAO-B, either alone or combined, does not facilitate spatial learning in adult rats.

Neurochemical stimulation of the rat substantia innominata increases cerebral blood flow (but not glucose use) through the parallel activation of cholinergic and non-cholinergic pathways

Neurochemical activation of the substantia innominata (SI) in the rat, through the direct injection of the cholinergic agonist carbachol, has been reported to induce large increases in cerebral blood flow (CBF) throughout cortical and subcortical projection regions. The present study aimed to determine whether the vasomotor responses to cholinergic stimulation of the SI were, or were not, the consequence of an increase in metabolic activity. To this end, coupled measurements of CBF and cerebral glucose use (CGU) were undertaken during carbachol-elicited stimulation of the SI. Infusion of carbachol into the basal forebrain induced significant CBF increases in several ipsilateral cortical and subcortical areas including the amygdala. In contrast, CGU increased only in the ipsilateral amygdala and SI. Thus, we tested the hypothesis of a direct neurogenic, rather than metabolic, contribution of the basalocortical system. In this respect, carbachol-elicited stimulation resulted in significant increases in extracellular acetylcholine concentrations in the ipsilateral parietal cortex; systemic pretreatment with the muscarinic receptor antagonist scopolamine completely abolished the increase in cortical CBF elicited by cholinergic stimulation of the SI in the ipsilateral frontoparietal motor cortex while it failed to affect the increase observed in the ipsilateral temporal cortex. Several conclusions can be drawn from the present study. The stimulation of the SI by carbachol induces an increase in CBF that can be dissociated from changes in underlying glucose metabolism. Secondly, these induced changes in cortical CBF are paralleled by an increase in acetylcholine release. Lastly, the failure of scopolamine to block the flow response in all cortical regions would suggest that SI stimulation will evoke the release of vasodilatatory neurotransmitter(s) as well as acetylcholine itself.

Cerebrovascular evidence for a GABAergic modulation of the cholinergic vasodilatatory basalocortical system in the rat

The present work is aimed to study the functional relevance of GABAergic-cholinergic interactions on the modulation of cerebral blood flow (CBF) exerted by the basalocortical system. Injections of GABA into the substantia innominata (SI) induce increases in blood flow in several cortical areas and inhibit partly the increases in cortical blood flow induced by cholinergic activation of this structure. Blockade of local GABAergic receptors by picrotoxin induced almost similar effects. These findings suggest that local GABAergic neurones of the SI exert a complex cortical cerebrovascular modulation at a resting and an activated state.

Synthesis and biological investigations of [18F]MR18445, a 5-HT3 receptor partial agonist

18F Labelled MR18445 (4-[4-(4-[18F]fluorobenzyl)piperazino]-7-methoxypyrrolo++ +[1,2-alpha] quinoxaline), a selective 5-HT3 receptor partial agonist with nanomolar affinity, was synthesized and examined as a potential radioligand for PET imaging of brain 5HT3 receptors. Radiotracer was prepared by N-alkylation of the MR18491 precursor with 4-[18F]fluorobenzyl iodide. This latter was synthesized in a three-step procedure from 4-[18F]fluorobenzaldehyde obtained by 18F-nucleophilic displacement of 4-nitrobenzaldehyde, subsequently reduced to 4-[18F]fluorobenzyl alcohol and converted into reactive 4-[18F]fluorobenzyl iodide. The reduction step was performed on a column filled with NaBH4/Al2O3 and 4-[18F]fluorobenzyl alcohol was obtained with high reproducible yield (82-93% from 4-[18F]fluorobenzaldehyde) if there were traces of water in the system. The radiosynthesis of [18F]MR18445 required approximately 120 min. Semi-preparative HPLC purification followed by formulation gave injectable solutions of [18F]MR18445 with a radiochemical purity > 99%. The overall yield of the synthesis was mainly dependent upon the first step efficiency of aromatic incorporation of 18F- and varied from 12% to 29%. All the synthetic procedure was realized on a ZYMARK robotic system. Biological in vivo studies in rats showed that uptake of [18F]MR18445 in brain was rapid and high. No evidence of radiolabeled metabolites could be observed in the brain as late as 40 min after injection, despite the rapid appearance of metabolites in the plasma. However, neither phosphorimaging autoradiographic studies in rats nor PET experiments in baboons revealed specific binding of the radiotracer in brain, suggesting [18F]MR18445 is not suitable for 5-HT3 receptors PET studies.

Autoradiographic mapping of cerebral blood flow responses to cholinergic stimulation of the rat substantia innominata: modulatory effect of galanin

In order to analyze the precise cerebrovascular effects of a specific cholinergic stimulation of the rat substantia innominata and their modulation by galanin, cerebral blood flow was measured by the [14C]-iodoantipyrine autoradiographic method in anesthetized (urethane and alpha-chloralose), artificially ventilated male Sprague-Dawley rats that received a microinjection into the substantia innominata of saline (n = 7), or 63 pmol of galanin (n = 8), or 50 nmoles of carbachol (n = 6) or a coinjection of carbachol and galanin (n = 8). Significant carbachol-induced cerebral blood flow increases were noted in ipsilateral cortices (+36%, p < 0.01 in the cingulate to +82%, p < 0.01 in the parietal somatosensory cortices), but also in ipsilateral hippocampus and ipsilateral thalamus. These cerebral blood flow increases were abolished by the coinjection of carbachol and galanin, while infusions of galanin alone failed to affect cerebral blood flow. Cholinergic stimulation of the substantia innominata represents thus a good model for the analysis of the detailed pharmacological properties of the cholinergic vasodilatatory basalocortical system. The existence of an inhibitory galaninergic modulation of this system could be of particular interest, in terms of cerebrovascular reactivity, in various neurodegenerative states.

[11C]S21007, a putative partial agonist for 5-HT3 receptors PET studies. Rat and primate in vivo biological evaluation

We recently labeled with carbon-11, a high affinity, selective, 5-HT3 receptor (5-HT3R) ligand, S21007, for potential positron emission tomography (PET) applications. To evaluate the in vivo binding properties of [11C]S21007, its brain regional distribution, tissue and plasma pharmacokinetics and plasma metabolisation were characterized. To circumvent the problem of highly discrete brain localization of the 5-HT3R (area postrema, hippocampus), we designed an original approach combining high-resolution imaging techniques (ex vivo phosphor plate autoradiography and MRI-guided coronal PET in the rat and baboon, respectively). After i.v. injection of trace amounts of [11C]S21007 to rats, phosphorimager autoradiography failed to reveal in vivo specific binding to, nor selectivity for 5-HT3R-rich areas. PET studies in the baboon showed consistent results, i.e., there was no selective accumulation of [11C]S21007 in the area postrema or hippocampus, and neither displacement nor presaturation with cold S21007 resulted in significant changes in tissue distribution or kinetics of [11C]S21007.

Specific in vivo binding in the rat brain of [18F]RP 62203: a selective 5-HT2A receptor radioligand for positron emission tomography

In vivo pharmacokinetic and brain binding characteristics of [18F]RP 62203, a selective high-affinity serotonergic 5-HT2A receptor antagonist, were assessed in the rat following intravenous injection of trace amount of the radioligand. The radioactive distribution profile observed in the brain 60 min after injection was characterized by greater than fourfold higher uptake in neocortex as compared to cerebellum (0.38 +/- 0.07% injected dose/g, % ID/g and 0.08 +/- 0.01 ID/g, respectively), consistent with in vivo specific binding to the 5-HT2A receptor. Furthermore, specific [18F]RP 62203 binding significantly correlated with the reported in vitro distribution of 5-HT2A receptors, but not with known concentration profiles of dopaminergic D2 or adrenergic alpha 1 receptors. Finally, detectable specific binding was abolished by pretreatment with large doses of ritanserin, a selective 5-HT2A antagonist, which resulted in uniform uptakes across cortical, striatal and cerebellar tissues. Thus, [18F]RP 62203 appears to be a promising selective tool to visualize and quantify 5-HT2A brain receptors in vivo with positron emission tomography.

Galanin inhibits the vasodilatatory basalocortical cholinergic system in the anaesthetized rat

In order to test the putative interaction between galanin and the vasodilatatory basalocortical cholinergic system, anaesthetized ventilated rats received a microinjection into the substantia innominata of 0.9% NaCl, 50 nmol carbachol, 50 nmol carbachol and 200 ng galanin, or 200 ng galanin. Cerebral blood flow (CBF) was measured with [14C]iodoantipyrine by the tissue sampling technique immediately following the intracerebral infusions. Under coinjection conditions, the flow increases observed after carbachol microinjection in the ipsilateral temporal and frontoparietal cortices were found to be significantly reduced (-37%, p < or = 0.02 and -25%, p < or = 0.05 respectively) compared with carbachol stimulated rats. The infusion of galanin by itself had no effect on CBF. These results demonstrate that galanin inhibits the vasodilatatory basalocortical cholinergic system and thus may possibly influence CBF by indirect mechanisms.

Regional cerebral blood flow responses to neurochemical stimulation of the substantia innominata in the anaesthetized rat

Since electrical stimulation of neurones may activate not only cell bodies but also neuronal fibres, this study aimed to test a selectively cholinergic neurochemical stimulation of the rat substantia innominata (SI) by the local microinjection of carbachol; the effects of this acetylcholine agonist were compared with glutamate. Cortical and subcortical cerebral blood flow (CBF) were measured in anaesthetized rats with the [14C]iodoantipyrine method by the tissue sampling technique immediately following the intracerebral (SI) microinjection of saline, 50 nmol of carbachol or glutamate. Carbachol microinjection into the SI induced a transient but significant vasodilation in frontoparietal motor (+28%) and temporal (+41%) cortices, that lasted for less than 10 min. Glutamate did not elicit any significant CBF modifications when compared to control rats although a significant interhemispheric asymmetry after microinjection was observed in the frontoparietal motor cortex. This latter observation would suggest that the glutamate-induced cortical response is less pronounced than that elicited by carbachol. Overall, these results demonstrate that a selective cholinergic stimulation of the SI can induce a transient cortical vasodilation and further confirms the hypothesis of a muscarinic modulation of CBF via this basal structure.