Acetylcholine release in the rat hippocampus as studied by microdialysis is dependent on axonal impulse flow and increases during behavioural activation

Regulation of habenular G-protein gamma 8 on learning and memory via modulation of the central acetylcholine system

Guanine nucleotide binding protein (G protein) gamma 8 (Gng8) is a subunit of G proteins and expressed in the medial habenula (MHb) and interpeduncular nucleus (IPN). Recent studies have demonstrated that Gng8 is involved in brain development; however, the roles of Gng8 on cognitive function have not yet been addressed. In the present study, we investigated the expression of Gng8 in the brain and found that Gng8 was predominantly expressed in the MHb-IPN circuit of the mouse brain. We generated Gng8 knockout (KO) mice by CRISPR/Cas9 system in order to assess the role of Gng8 on cognitive function. Gng8 KO mice exhibited deficiency in learning and memory in passive avoidance and Morris water maze tests. In addition, Gng8 KO mice significantly reduced long-term potentiation (LTP) in the hippocampus compared to that of wild-type (WT) mice. Furthermore, we observed that levels of acetylcholine (ACh) and choline acetyltransferase (ChAT) in the MHb and IPN of Gng8 KO mice were significantly decreased, compared to WT mice. The administration of nAChR α4β2 agonist A85380 rescued memory impairment in the Gng8 KO mice, suggesting that Gng8 regulates cognitive function via modulation of cholinergic activity. Taken together, Gng8 is a potential therapeutic target for memory-related diseases and/or neurodevelopmental diseases.

In Vivo Detection of Redox-Inactive Neurochemicals in the Rat Brain with an Ion Transfer Microsensor

Electrochemical tracking of redox-inactive neurochemicals remain a challenge due to chemical inertness, almost no Faraday electron transfer for these species, and the complex brain atmosphere. In this work, we demonstrate a low-cost, simple-making liquid/liquid interface microsensor (LLIM) to monitor redox-inactive neurochemicals in the rat brain. Taking choline (Ch) as an example, based on the difference in solvation energies of Ch in cerebrospinal fluid (aqueous phase) and 1,2-dichloroethane (1,2-DCE; organic phase), Ch is recognized in the specific ion-transfer potential and distinctive ion-transfer current signals. The LLIM has an excellent response to Ch with good linearity and selectivity, and the detection limit is 0.37 μM. The LLIM can monitor the dynamics of Ch in the cortex of the rat brain by both local microinfusion and intraperitoneal injection of Ch. This work first demonstrates that the LLIM can be successfully applied in the brain and obtain electrochemical signals in such a sophisticated system, allowing one new perspective of sensing at the liquid/liquid interface for nonelectrically active substances in vivo to understand the physiological function of the brain.

Identification of vagus nerve stimulation parameters affecting rat hippocampal electrophysiology without temperature effects

BACKGROUND

Recent experiments in rats have demonstrated significant effects of VNS on hippocampal excitability but were partially attributed to hypothermia, induced by the applied VNS parameters.

OBJECTIVE

To allow meaningful preclinical research on the mechanisms of VNS and translation of rodent results to clinical VNS trials, we aimed to identify non-hypothermia inducing VNS parameters that significantly affect hippocampal excitability.

METHODS

VNS was administered in cycles of 30 s including either 0.1, 0.16, 0.25, 0.5, 1.5, 3 or 7 s of VNS ON time (biphasic pulses, 250μs/phase, 1 mA, 30 Hz) and the effect of different VNS ON times on brain temperature was evaluated. VNS paradigms with and without hypothermia were compared for their effects on hippocampal neurophysiology in freely moving rats.

RESULTS

Using VNS parameters with an ON time/OFF time of up to 0.5 s/30 s did not cause hypothermia, while clear hypothermia was detected with ON times of 1.5, 3 and 7 s/30 s. Relative to SHAM VNS, the normothermic 0.5 s VNS condition significantly decreased hippocampal EEG power and changed dentate gyrus evoked potentials with an increased field excitatory postsynaptic potential slope and a decreased population spike amplitude.

CONCLUSION

VNS can be administered in freely moving rats without causing hypothermia, while profoundly affecting hippocampal neurophysiology suggestive of reduced excitability of hippocampal neurons despite increased synaptic transmission efficiency.

Cholinergic modulation of spatial learning, memory and navigation

Spatial learning, including encoding and retrieval of spatial memories as well as holding spatial information in working memory generally serving navigation under a broad range of circumstances, relies on a network of structures. While central to this network are medial temporal lobe structures with a widely appreciated crucial function of the hippocampus, neocortical areas such as the posterior parietal cortex and the retrosplenial cortex also play essential roles. Since the hippocampus receives its main subcortical input from the medial septum of the basal forebrain (BF) cholinergic system, it is not surprising that the potential role of the septo-hippocampal pathway in spatial navigation has been investigated in many studies. Much less is known of the involvement in spatial cognition of the parallel projection system linking the posterior BF with neocortical areas. Here we review the current state of the art of the division of labour within this complex 'navigation system', with special focus on how subcortical cholinergic inputs may regulate various aspects of spatial learning, memory and navigation.

Sodium ion transport participates in non-neuronal acetylcholine release in the renal cortex of anesthetized rabbits

This study examined the mechanism of release of endogenous acetylcholine (ACh) in rabbit renal cortex by applying a microdialysis technique. In anesthetized rabbits, a microdialysis probe was implanted into the renal cortex and perfused with Ringer's solution containing high potassium concentration, high sodium concentration, a Na+/K+-ATPase inhibitor (ouabain), or an epithelial Na+ channel blocker (benzamil). Dialysate samples were collected at baseline and during exposure to each agent, and ACh concentrations in the samples were measured by high-performance liquid chromatography. High potassium had no effect on renal ACh release. High sodium increased dialysate ACh concentrations significantly. Ouabain increased dialysate ACh concentration significantly. Benzamil decreased dialysate ACh concentrations significantly both at baseline and under high sodium. The finding that high potassium-induced depolarization does not increase ACh release suggests that endogenous ACh is released in renal cortex mainly by non-neuronal mechanism. Sodium ion transport may be involved in the non-neuronal ACh release.

Task-based changes in proton MR spectroscopy signal during configural working memory in human medial temporal lobe

PURPOSE

To detect local cholinergic changes in human medial temporal lobe during configural working memory performance.

MATERIALS AND METHODS

Proton magnetic resonance spectroscopy (¹ H-MRS) measurements were acquired at 3T from a 2 × 2 × 3 cm voxel in right medial temporal lobe from 36 subjects during performance of a configural visual working memory task (cWMT). In order to compensate for expected task-based blood oxygenation level-dependent (BOLD) T₂ * effects, resonance signal changes of unbound choline-containing metabolites (Cho) were referenced to an internal standard of creatine + phosphocreatine metabolites (Cre) and compared between four task blocks: rest, memorization, active memory maintenance, and recognition. An unannounced memory retention test was conducted in 21 subjects. Quality assurance analyses examined task-based Cho and Cre individually as well as referenced to resonance signal from N-acetylaspartate (NAA).

RESULTS

Increases from a resting baseline in the Cho/Cre ratio were observed during 60-second blocks of active memory maintenance across the group (P = 0.0042). Behavioral accuracy during task performance correlated with memory retention (r = 0.48, P = 0.027). Quality assurance measures showed task-based changes in Cre resonance signal both individually (P = 0.00099) and when utilized as a noncholinergic internal reference (NAA/Cre, P = 0.00079).

CONCLUSION

Increases in human medial temporal lobe ¹ H-MRS Cho/Cre ratio occur during the maintenance of configural working memory information. However, interpretation of these results as driven by cholinergic activity cannot be assumed, as NAA, a noncholinergic metabolite, shows similar results when utilizing Cre as a reference. Caution is advised when considering Cre as an internal standard for task-based ¹ H-MRS measurements.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:682-691.

Role of the lateral habenula in memory through online processing of information

Our memory abilities, whether they involve short-term working memory or long-term episodic or procedural memories, are essential for our well-being, our capacity to adapt to constraints of our environment and survival. Therefore, several key brain regions and neurotransmitter systems are engaged in the processing of sensory information to either maintain such information in working memory so that it will quickly be used, and/or participate in the elaboration and storage of enduring traces useful for longer periods of time. Animal research has recently attracted attention on the lateral habenula which, as shown in rodents and non-human primates, seems to process information stemming in the main regions involved in memory processing, e.g., the medial prefrontal cortex, the hippocampus, the amygdala, the septal region, the basal ganglia, and participates in the control of key memory-related neurotransmitters systems, i.e., dopamine, serotonin, acetylcholine. Recently, the lateral habenula has been involved in working and spatial reference memories, in rodents, likely by participating in online processing of contextual information. In addition, several behavioral studies strongly suggest that it is also involved in the processing of the emotional valance of incoming information in order to adapt to particularly stressful situations. Therefore, the lateral habenula appears like a key region at the interface between cognition and emotion to participate in the selection of appropriate behaviors.

Scopolamine Impairs Appetitive But Not Aversive Trace Conditioning: Role of the Medial Prefrontal Cortex

The muscarinic acetylcholine receptor is an important modulator of medial prefrontal cortex (mPFC) functions, such as the working memory required to bridge a trace interval in associative leaning. Aversive and appetitive trace conditioning procedures were used to examine the effects of scopolamine (0.1 and 0.5 mg/kg, i.p.) in male rats. Follow-up experiments tested the effects of microinfusion of 0.15 μg of scopolamine (0.075 μg of in 0.5 μl/side) in infralimbic (IL) versus prelimbic regions of rat mPFC, in appetitive trace and locomotor activity (LMA) procedures. Systemic scopolamine was without effect in an aversive trace conditioning procedure, but impaired appetitive conditioning at a 2 s trace interval. This effect was demonstrated as reduced responding during presentations of the conditioned stimulus (CS) and during the interstimulus interval (ISI). There was no such effect on responding during food (unconditioned stimulus, US) responding or in the intertrial interval (ITI). In contrast, systemic scopolamine dose-relatedly increased LMA. Trace conditioning was similarly impaired at the 2 s trace (shown as reduced responding to the CS and during the ISI, but not during US presentations or in the ITI) after infusion in mPFC, whereas LMA was increased (after infusion in IL only). Therefore, our results point to the importance of cholinergic modulation in mPFC for trace conditioning and show that the observed effects cannot be attributed to reduced activity.SIGNIFICANCE STATEMENT Events are very often separated in time, in which case working memory is necessary to condition their association in "trace conditioning." The present study used conditioning variants motivated aversively with foot shock and appetitively with food. The drug scopolamine was used to block muscarinic acetylcholine receptors involved in working memory. The results show that reduced cholinergic transmission in medial prefrontal cortex (mPFC) impaired appetitive trace conditioning at a 2 s trace interval. However, scopolamine was without effect in the aversive procedure, revealing the importance of procedural differences to the demonstration of the drug effect. The finding that blockade of muscarinic receptors in mPFC impaired trace conditioning shows that these receptors are critical modulators of short-term working memory.

Excitatory Transmission to the Lateral Habenula Is Critical for Encoding and Retrieval of Spatial Memory

The lateral habenula (LHb) is viewed as a relay between the limbic system, the basal ganglia (BG), and monoaminergic neurons of the midbrain. If a prominent role has been evidenced in BG-mediated functions such as value-based decision-making, very little is known about the involvement of the LHb in limbic functions such as memory processing. In the present study, we used two pharmacological approaches-LHb reversible inactivation with intra-LHb infusion of muscimol, an agonist of the GABA-A receptor, or blockade of excitatory inputs with intra-LHb infusion of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist of the glutamatergic AMPA receptor-to investigate the involvement of the LHb in encoding, consolidation, and retrieval of spatial memory in the water maze (WM) in rats. We found that intra-LHb infusion of muscimol or CNQX prevented encoding and retrieval, but not consolidation of spatial information. In addition, muscimol but not CNQX induced impairments during a cued version of the WM task, and marked anxiety in the elevated plus maze. These results confirm the involvement of the LHb in higher cognitive functions. They further suggest a dichotomy between the role of glutamatergic and other inputs to the LHb in hippocampus-dependent memory processing, as well as in emotional aspects of goal-directed behaviors.

Cholinergic manipulations bidirectionally regulate object memory destabilization

Consolidated memories can become destabilized and open to modification upon retrieval. Destabilization is most reliably prompted when novel information is present during memory reactivation. We hypothesized that the neurotransmitter acetylcholine (ACh) plays an important role in novelty-induced memory destabilization because of its established involvement in new learning. Accordingly, we investigated the effects of cholinergic manipulations in rats using an object recognition paradigm that requires reactivation novelty to destabilize object memories. The muscarinic receptor antagonist scopolamine, systemically or infused directly into the perirhinal cortex, blocked this novelty-induced memory destabilization. Conversely, systemic oxotremorine or carbachol, muscarinic receptor agonists, administered systemically or intraperirhinally, respectively, mimicked the destabilizing effect of novel information during reactivation. These bidirectional effects suggest a crucial influence of ACh on memory destabilization and the updating functions of reconsolidation. This is a hitherto unappreciated mnemonic role for ACh with implications for its potential involvement in cognitive flexibility and the dynamic process of long-term memory storage.

DREADDing the lateral habenula: a review of methodological approaches for studying lateral habenula function

The lateral habenula (LHb) is part of the habenular complex in the dorsal diencephalon. The LHb is an important regulator of several neurotransmitter systems in the midbrain; disturbances in this regulation may contribute to mood disorders, abnormalities in cognition, drive, and addiction. Owing to the critical role this nucleus plays in modulating activity of midbrain nuclei, there has been a rapid increase in studies targeting the LHb in the recent years. In this review, we describe studies using traditional approaches to elucidate the function of this brain region, such as lesion, electrical and chemical stimulation, electrophysiology and in vivo microdialysis. We have selected a variety of illustrative studies to discuss each of these methods. Next, we describe studies using methods that are based upon recent advances in molecular biology techniques including recent results from our laboratory using the Designer Receptor Exclusively Activated by Designer Drug (DREADD) technology. Using a Gi/o-coupled DREADD, we found that inhibition of the LHb reduces depression-like behavior in the forced swim test in a manner that suggests enhanced serotonergic activity. The emerging picture reveals that the LHb is likely to be a critical node in the network of subcortical nuclei that regulate aversive learning, motivation, stress responses, etc. We describe how recently developed methods have advanced the study of the LHb and are leading research of this brain region in promising new directions. This article is part of a Special Issue entitled Optogenetics (7th BRES).

Acetylcholine and attention

Historically, ACh has been implicated in learning and short-term memory functions. However, more recent studies have provided support for a role of cortical ACh in attentional effort, orienting and the detection of behavioral significant stimuli. The current review article summarizes studies in animals and humans which have investigated the role of ACh in attention and cognition. An attempt has been made to differentiate between brain regions involved in attentional processes versus those important for other cognitive functions. To this purpose, various experimental methods and interventions were used. Animal behavioral studies have injected the selective immunotoxin IgG-saporin to induce specific cholinergic lesions, employed electrochemical techniques such as microdialysis, or have administered cholinergic compounds into discrete parts of the brain. Human studies that give some indication on the link between central cholinergic signaling and cognition are obviously confined to less invasive, imaging methods such as fMRI. The brain areas that are deemed most important for intact attentional processing in both animals and humans appear to be the (pre)frontal, parietal and somatosensory (especially visual) regions, where ACh plays a vital role in the top-down control of attentional orienting and stimulus discrimination. In contrast, cholinergic signaling in the septohippocampal system is suggested to be involved in memory processes. Thus, it appears that the role of ACh in cognition is different per brain region and between nicotinic versus muscarinic receptor subtypes.

Differential neuromodulation of acquisition and retrieval of avoidance learning by the lateral habenula and ventral tegmental area

Several studies suggest an opponent functional relationship between the lateral habenula (LHb) and the ventral tegmental area (VTA). Previous work has linked LHb activation to the inhibition of dopaminergic neurons during loss of reward, as well as to deficits in escape and avoidance learning. We hypothesized that a dopamine signal might underlie the negative reinforcement of avoidance responses and that LHb activation could block this signal and thereby cause avoidance deficits. To test this idea, we implanted stimulating electrodes in either the VTA or LHb of gerbils engaged in two-way active avoidance learning, a task that shows learning-associated dopamine changes and that is acquired faster following LHb lesions. We delivered brief electrical brain stimulation whenever the animal performed a correct response, i.e., when the successful avoidance of foot shock was hypothesized to trigger an intrinsic reward signal. During the acquisition phase, VTA stimulation improved avoidance performance, while LHb stimulation impaired it. VTA stimulation appeared to improve both acquisition and asymptotic performance of the avoidance response, as VTA-stimulated animals reached above-normal performance but reverted to normal responding when stimulation was discontinued. The effects of LHb stimulation during avoidance acquisition were long lasting and persisted even after stimulation was discontinued. However, when given after successful acquisition of avoidance behavior, LHb stimulation had no effect, indicating that LHb stimulation specifically impaired avoidance acquisition without affecting memory retrieval or motivation or ability to perform the avoidance response. These results demonstrate opponent roles of LHb and VTA during acquisition but not during retrieval of avoidance learning.

Modulatory effect of cod-liver oil on Na(+)-K(+) ATPase in rats' brain

Omega-3 fatty acids were used in the treatment of psychiatric diseases such as bipolar disorder. Na(+), K(+)-ATPase is also a well-known target for these fatty acids. In this study, we investigated the impact of cod-liver oil (CLO), docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) on Na(+), K(+)-ATPase, cholinesterase activities, the levels of norepinephrine (NE) and acetylcholine in different regions of rat brain. Our results showed that DHA caused a significant depression in cerebellum Na(+), K( +)-ATPase, whereas CLO activated it. In addition, CLO, EPA and DHA produced a significant activation in Na(+), K(+)-ATPase activity in medulla, midbrain and hypothalamus. There were non-significant changes in the activity of cholinesterase enzyme in cerebellum and medulla, while in midbrain and hypothalamus the CLO, DHA and EPA enhanced the activity by 75%, 100% and 78%, respectively. The content of NE in hypothalamus showed slight increase in different regions of the brain of animals fed CLO, DHA or EPA. In conclusion, CLO, DHA or EPA supplementation had a beneficial effect that associated with a normalization of fatty acids incorporation into phospholipid membranes and a partial restoration of Na(+), K(+)-ATPase activity, suggesting that CLO supplementation may improve fatty acid composition and moderately enhance Na(+), K(+)-ATPase activity.

Neurokinin-2 receptor antagonism in medial septum influences temporal-order memory for objects and forebrain cholinergic activity

In the mammalian brain the neurokinin NK(2) receptors are predominantly located in the hippocampus, thalamus, septum and frontal cortex. It has been shown that administration of the NK(2) receptor agonist, neurokinin A (NKA), into the medial septum of rats increases extracellular levels of acetylcholine (ACh) in the hippocampus and that NK(2) receptor antagonism blocks this increase. Therefore, given the prominent role of hippocampal ACh in information processing, we hypothesized that NK(2) receptor antagonism in the medial septum would negatively affect learning and memory via its influence on the cholinergic neurons of the basal forebrain. We investigated the action of local application of the peptidic NK(2) receptor antagonist, Bz-Ala-Ala-D-Trp-Phe-D-Pro-Pro-Nle-NH (1, 10 and 100pmol), into the medial septum on object memory for temporal order and spatial location using an object novelty paradigm. By means of in vivo microdialysis and HPLC analyses, we also examined the influence of NK(2) receptor antagonism in the medial septum on ACh in major cholinergic projection areas of the basal forebrain, namely, hippocampus, frontal cortex and amygdala.

RESULTS

Injection of vehicle alone into the medial septum impaired memory for temporal order and spatial location of objects. Application of 1pmol of the NK(2) receptor antagonist partially reversed this deficit by reinstating memory for temporal order. Injection of 10pmol of the NK(2) receptor antagonist into the medial septum decreased levels of ACh in the hippocampus (at 30min post-injection), and frontal cortex (at 30 and 80min post-injection) in comparison to vehicle. However, this apparent decrease was the result of the blockade of a saline-induced increase in ACh levels.

Differential effects of systemic and intraseptal administration of the acetylcholinesterase inhibitor tacrine on the recovery of spatial behavior in an animal model of diencephalic amnesia

Several lines of evidence suggest that acetylcholinesterase inhibitors (AChE) have their cognitive enhancing effects by stimulating cholinergic receptors within the medial septum. However, intraseptal administration of cholinergic enhancing drugs produce mixed results that appear to depend on both the integrity of the medial septum as well as task demands. Three experiments were conducted to determine the relationship between increased cholinergic activity within the medial septum and hippocampus and behavioral recovery in a model of diencephalic amnesia produced by pyrithiamine-induced thiamine deficiency (PTD). In Experiment 1, systemic tacrine (0.0, 0.75, 1.5mg/kg) was administered to PTD and pair-fed (PF) rats prior to a spontaneous alternation task. Without tacrine, PF rats alternated at a higher rate than PTD rats. Both doses of tacrine increased alternation in PTD rats to within the range of PF rats. In Experiment 2, three doses of intraseptal tacrine (2.5, 5.0, 12.5microg) were administered to PTD and PF rats and changes in hippocampal acetylcholine efflux were assessed. Both the 5.0 and 12.5microg doses significantly increased hippocampal acetylcholine levels, but the change was greater in the PTD rats. In Experiment 3, despite the fact that both intraseptal doses of tacrine (5.0, 12.5microg) increased hippocampal acetylcholine levels, only 5.0microg significantly improved alternation scores in PTD rats. Thus, when there is basal forebrain cholinergic cell loss in conjunction with diencephalic pathology, the therapeutic range of AChE-I in the medial septum and the effective doses do not directly map onto changes in acetylcholine efflux in the hippocampus.

Effect of voluntary running on adult hippocampal neurogenesis in cholinergic lesioned mice

Background

Cholinergic neuronal dysfunction of the basal forebrain is observed in patients with Alzheimer's disease and dementia, and has been linked to decreased neurogenesis in the hippocampus, a region involved in learning and memory. Running is a robust inducer of adult hippocampal neurogenesis. This study aims to address the effect of running on hippocampal neurogenesis in lesioned mice, where septohippocampal cholinergic neurones have been selectively eliminated in the medial septum and diagonal band of Broca of the basal forebrain by infusion of mu-p75-saporin immunotoxin.

Results

Running increased the number of newborn cells in the dentate gyrus of the hippocampus in cholinergic denervated mice compared to non-lesioned mice 24 hours after injection of bromodeoxyuridine (BrdU). Although similar levels of surviving cells were present in cholinergic depleted animals and their respective controls four weeks after injection of BrdU, the majority of progenitors that proliferate in response to the initial period of running were not able to survive beyond one month without cholinergic input. Despite this, the running-induced increase in the number of surviving neurones was not affected by cholinergic depletion.

Conclusion

The lesion paradigm used here models aspects of the cholinergic deficits associated with Alzheimer's Disease and aging. We showed that running still increased the number of newborn cells in the adult hippocampal dentate gyrus in this model of neurodegenerative disease.

Blocking GABA-A receptors in the medial septum enhances hippocampal acetylcholine release and behavior in a rat model of diencephalic amnesia

Wernicke-Korsakoff syndrome (WKS), a form of diencephalic amnesia caused by thiamine deficiency, results in severe anterograde memory loss. Pyrithiamine-induced thiamine deficiency (PTD), an animal model of WKS, produces cholinergic abnormalities including decreased functional hippocampal acetylcholine (ACh) release and poor spatial memory. Increasing hippocampal ACh levels has increased performance in PTD animals. Intraseptal bicuculline (GABA(A) antagonist) augments hippocampal ACh release in normal animals and we found it (0.50 microg/microl and 0.75 microg/microl) also increased in-vivo hippocampal ACh release in PTD animals. However, the 0.75 microg/microl dose produced a greater change in hippocampal ACh release in control animals. The 0.50 microg/microl dose of bicuculline was then selected to determine if it could enhance spontaneous alternation performance in PTD animals. This dose of bicuculline significantly increased hippocampal ACh levels above baseline in both PTD and control rats and resulted in complete behavioral recovery in PTD animals, without altering performance in control rats. This suggests that balancing ACh-GABA interactions in the septohippocampal circuit may be an effective therapeutic approach in certain amnestic syndromes.

A conductor hidden in the orchestra? Role of the habenular complex in monoamine transmission and cognition

Influences of the habenular complex on electrophysiological and neurochemical aspects of brain functioning are well known. However, its role in cognition has been sparsely investigated until recently. The habenular complex, composed of medial and lateral subdivisions, is a node linking the forebrain with midbrain and hindbrain structures. The lateral habenula is the principal actor in this direct dialogue, while the medial habenula mostly conveys information to the interpeduncular nucleus before this modulates further regions. Here we describe neuroanatomical and physiological aspects of the habenular complex, and its role in cognitive processes, including new behavioral, electrophysiological and imaging findings. Habenular complex lesions result in deficits in learning, memory and attention, some of which decline during repeated testing, while others become worse, consistent with multiple roles in cognition. The habenular complex is particularly responsive to feedback about errors. Electrophysiological studies indicate a role in metaplasticity, the modulation of neuroplasticity. These studies thus reveal important roles of the habenular complex in learning, memory and attention.

Involvement of Tissue Plasminogen Activator-Plasmin System in Depolarization-Evoked Dopamine Release in the Nucleus Accumbens of Mice

Tissue plasminogen activator (tPA), a serine protease, catalyzes the conversion of plasminogen to plasmin. In the present study, we investigated the role of the tPA-plasmin system in depolarization-evoked dopamine (DA) and acetylcholine (ACh) release in the nucleus accumbens (NAc) and hippocampus, respectively, of mice, by using in vivo microdialysis. Microinjection of either tPA or plasmin significantly potentiated 40 mM KCl-induced DA release without affecting basal DA levels. In contrast, plasminogen activator inhibitor-1 dose-dependently reduced 60 mM KCl-induced DA release. The 60 mM KCl-evoked DA release in the NAc was markedly diminished in tPA-deficient (tPA-/-) mice compared with wild-type mice, although basal DA levels did not differ between the two groups. Microinjections of either exogenous tPA (100 ng) or plasmin (100 ng) into the NAc of tPA-/-mice restored 60 mM KCl-induced DA release, as observed in wild-type mice. In contrast, there was no difference in either basal or 60 mM KCl-induced ACh release in the hippocampus between wild-type and tPA-/-mice. Our findings suggest that the tPA-plasmin system is involved in the regulation of depolarization-evoked DA release in the NAc.

Acetylcholine release in the hippocampus during the operant conditioned reflex and the footshock stimulus in rats

The activity of the septo-hippocampal cholinergic pathway was investigated by measuring changes in the extracellular acetylcholine (ACh) levels in the hippocampus, by means of microdialysis, during the operant conditioned reflex and the repeated footshock stimulus. Microdialysis samplings were conducted in a Skinner box where lights were delivered as conditioned stimuli (CS) paired with footshocks as unconditioned stimuli (US). Two groups of rats were used. Extracellular ACh and choline (Ch) in samples collected at 6min intervals were assessed by high-performance liquid chromatography with electrochemical detection. The elevation of hippocampus ACh was observed in the two experimental groups. The increase in ACh during aversive stimulus (footshock) was significantly larger and was probably related to the number of footshocks. There might be moderate increase in the hippocampal ACh release during the retrieval of information. The concentration of choline showed no significant fluctuation in the two groups during the whole process. This experiment explored in more detail hippocampal cholinergic activity in relation to the two different procedures.

Acetylcholine and choline amperometric enzyme sensors characterized in vitro and in vivo

Acetylcholine (ACh) and choline (Ch) are important neuroactive molecules, yet detection of these substances in vivo presents significant analytical challenges. New multienzyme amperometric biosensors are presented here with measurement of physiologically relevant levels of ACh and Ch in vivo. Poly(m-(1,3)-phenylenediamine) (pmPD) electropolymerized on a platinum iridium wire (Pt) served as a template for immobilization of enzymes. A multienzyme layer containing choline oxidase (ChOx) and ascorbic acid oxidase (AAO) for a Ch sensor or ChOx, acetylcholinesterase (AChE), and AAO for a ACh/Ch sensor was immobilized with bovine serum albumin by cross-linking with glutaraldeyhyde. The pmPD enzyme sensors displayed enhanced sensitivity, stability, and selectivity compared to the same multienzyme systems immobilized to solvent cast Nafion and cellulose acetate-modified Pt. Sensor response was linear up to 100 microM ACh or Ch. Detection limits were 0.66 +/- 0.46 microM ACh and 0.33 +/- 0.09 microM Ch, and response times were <1 s. Selectivity for Ch and ACh relative to potential interferences and pharmacological agents commonly used to examine cholinergic physiology was demonstrated. Temperature and pH dependence and the effect of storage conditions on sensor sensitivity and selectivity were determined. Exogenous and endogenous Ch and ACh were measured in the rat brain in vivo.

Stimulation of hippocampal acetylcholine release by hyperforin, a constituent of St. John’s Wort

Extracts of the medicinal plant St. John's Wort (Hypericum perforatum) are widely used in the therapy of affective disorders and have been reported to exert antidepressant, anxiolytic, and cognitive effects in experimental and clinical studies. We here report that hyperforin, the major active constituent of the extract, increases the release of acetylcholine from rat hippocampus in vivo as determined by microdialysis. Hippocampal acetylcholine levels were increased by 50-100% following the systemic administration of pure hyperforin at doses of 1 and 10 mg/kg. The effect was almost completely suppressed by local perfusion with calcium-free buffer or with tetrodotoxin (1 microM). We conclude that hyperforin releases hippocampal acetylcholine by an indirect mechanism of action which is calcium-dependent and requires intact neuronal communication and cell firing. Our findings suggest therapeutic efficacy of St. John's Wort extracts in central cholinergic dysfunction.

Habenula lesions cause impaired cognitive performance in rats: implications for schizophrenia

Cognitive impairment is a prominent feature of schizophrenia. Currently there is no well-accepted explanation of the aetiology of this disorder, but recent evidence indicates that dysfunction of the habenula may be involved. We therefore examined whether habenula lesions in Sprague-Dawley rats cause behavioural changes resembling those of schizophrenia. Rats received either habenula lesions, a sham operation or a small lesion of the overlying dorsal hippocampus as a check that effects observed were not due to incidental damage to this structure. As there are alterations of social behaviour, sensorimotor gating and cognition in schizophrenia, we examined comparable behaviours. Social interaction time was measured during a 5-min encounter with a novel juvenile conspecific. Prepulse inhibition of an acoustic startle response, as an index of sensorimotor gating, was measured with prepulses of various amplitudes, and spatial cognitive performance was assessed in the Morris water maze task. Histological analysis showed that habenula lesions substantially damaged both medial and lateral habenula bilaterally while largely sparing neighbouring structures. Assay of choline acetyltransferase (ChAT) in the interpeduncular nucleus terminal region of the habenulo-interpeduncular tract, showed marked reduction (by 80%) in habenula-lesioned animals. Habenula-lesioned rats, but not the control group with small dorsal hippocampus lesions, showed marked impairment of Morris maze performance compared to the sham-operated control group. Social interaction time and prepulse inhibition were not significantly altered in either lesion group. The results are consistent with a role of the habenula in cognition, and with the view that pathology of the habenula may contribute to the cognitive impairments of schizophrenia.

Changes in acetylcholine extracellular levels during cognitive processes

Measuring the changes in neurotransmitter extracellular levels in discrete brain areas is considered a tool for identifying the neuronal systems involved in specific behavioral responses or cognitive processes. Acetylcholine (ACh) is the first neurotransmitter whose diffusion from the central nervous system was investigated and whose extracellular levels variations were correlated to changes in neuronal activity. This was done initially by means of the cup technique and then by the microdialysis technique. The latter, notwithstanding some technical limitations, makes it possible to detect variations in extracellular levels of ACh in unrestrained, behaving animals. This review summarizes and discusses the results obtained investigating the changes in ACh release during performance of operant tasks, exposition to novel stimuli, locomotor activity, and the performance of spatial memory tasks, working memory, and place preference memory tasks. Activation of the forebrain cholinergic system has been demonstrated in many tasks and conditions in which the environment requires the animal to analyze novel stimuli that may represent a threat or offer a reward. The sustained cholinergic activation, demonstrated by high levels of extracellular ACh observed during the behavioral paradigms, indicates that many behaviors occur within or require the facilitation provided by the cholinergic system to the operation of pertinent neuronal pathways.

α-Sialylcholesterol enhances the depolarization-induced release of acetylcholine and glutamate in rat hippocampus: in vivo microdialysis study

The effects of alpha-sialylcholesterol (alpha-SC), a synthetic ganglioside analogue, on synaptic neurotransmission were studied using in vivo microdialysis technique. Application of alpha-SC through a microdialysis probe enhanced high potassium-evoked release of acetylcholine and glutamate in the hippocampal CA3 region of Wistar rats. The experiments using synaptosomes and FM1-43, a fluorescent styryl dye used for studies of neurotransmitter release mechanisms, showed that alpha-SC increased depolarization-induced loss of dye but it did not evoke the dye loss at resting condition. These results indicate that alpha-SC promotes a depolarization-induced exocytotic neurotransmitter release in the brain under in vivo conditions. Application of alpha-SC increased the level of glutamate but not that of acetylcholine, suggesting that alpha-SC affects spontaneous glutamate release and/or transport system at the brain region.

The effect of walking on regional blood flow and acetylcholine in the hippocampus in conscious rats

Recent studies in our laboratory have demonstrated that stimulation of the septal complex (i.e., the medial septal nucleus and the nucleus of the diagonal band) increases extracellular acetylcholine (ACh) release and, consequently, results in an increase in regional cerebral blood flow in the hippocampus (Hpc CBF) via activation of the nicotinic ACh receptors (nAChRs) [Neurosci. Lett. 107 (1989) 135; Neurosci. Lett. 112 (1990a) 263]. The present study aimed to examine the effects of walking on Hpc CBF, measured by laser Doppler flowmetry, in conscious rats. Walking at a moderate speed (4 cm/s) on a treadmill for 30 s produced increases in Hpc CBF and mean arterial pressure (MAP), reaching 107 +/- 1% and 105 +/- 1% of the prewalking control values, respectively. Walking for 3 min produced an increase in ACh release in the extracellular space of the hippocampus. The increase in Hpc CBF during walking was attenuated by mecamylamine (abbreviated as MEC here; 2 mg/kg, i.v.), a nAChR antagonist permeable to the blood-brain barrier (BBB), but not by hexamethonium (denoted as C6 here; 20 mg/kg, i.v.), a nAChR antagonist impermeable to the BBB, while the walking-induced increase in MAP was abolished by either agent. The response of Hpc CBF and MAP were not altered by atropine (abbreviated as ATR here; 0.5 mg/kg, i.v.), a muscarinic AChR antagonist permeable to the BBB. The increase in Hpc CBF during walking was attenuated by N(omega)-nitro-L-arginine methyl ester (L-NAME, 3 and 30 mg/kg, i.v.), a nitric oxide synthase (NOS) inhibitor, and the reduced responses were reversed following the intravenous (i.v.) administration of a physiological precursor of NO, L-arginine (600 mg/kg). The results suggest that the increase in Hpc CBF during walking is independent of MAP and attributable at least to activation of the nAChRs by the cholinergic vasodilator nerves projecting to the hippocampus and to production of NO in the hippocampus.

Hippocampal brain-derived neurotrophic factor gene regulation by exercise and the medial septum

Brain-derived neurotrophic factor (BDNF) enhances synaptic plasticity and neuron function. We have reported that voluntary exercise increases BDNF mRNA levels in the hippocampus; however, mechanisms underlying this regulation have not been defined. We hypothesized that medial septal cholinergic and/or gamma amino butyric acid (GABA)ergic neurons, which provide a major input to the hippocampus, may regulate the baseline gene expression and exercise-dependent gene upregulation of this neurotrophin. Focal lesions were produced by medial septal infusion of the saporin-linked immunotoxins 192-IgG-saporin or OX7-saporin. 192-IgG-saporin produced a selective and complete loss of medial septal cholinergic neurons with no accompanying GABA loss. Baseline BDNF mRNA was reduced in the hippocampus of sedentary animals, but exercise-induced gene upregulation was not impaired, despite complete loss of septo-hippocampal cholinergic afferents. OX7-saporin produced a graded lesion of the medial septum characterized by predominant GABA neuron loss with less reduction in the number of cholinergic cells. OX7-saporin lesion reduced baseline hippocampal BDNF mRNA and attenuated exercise-induced gene upregulation, in a dose-dependent manner. These results suggest that combined loss of septal GABAergic and cholinergic input to the hippocampus may be important for exercise-dependent BDNF gene regulation, while cholinergic activity on its own is not sufficient. These results are discussed in relation to their implications for aging and Alzheimer's disease.

Endogenous histamine in the medial septum-diagonal band complex increases the release of acetylcholine from the hippocampus: a dual-probe microdialysis study in the freely moving rat

The effects of histaminergic ligands on both ACh spontaneous release from the hippocampus and the expression of c-fos in the medial septum-diagonal band (MSA-DB) of freely moving rats were investigated. Because the majority of cholinergic innervation to the hippocampus is provided by MSA-DB neurons, we used the dual-probe microdialysis technique to apply drugs to the MSA-DB and record the induced effects in the projection area. Perfusion of MSA-DB with high-KCl medium strongly stimulated hippocampal ACh release which, conversely, was significantly reduced by intra-MSA-DB administration of tetrodotoxin. Histamine or the H2 receptor agonist dimaprit, applied directly to the hippocampus, failed to alter ACh release. Conversely, perfusion of MSA-DB with these two compounds increased ACh release from the hippocampus. Also, thioperamide and ciproxifan, two H3 receptor antagonists, administered into MSA-DB, increased the release of hippocampal ACh, whereas R-alpha-methylhistamine, an H3 receptor agonist, produced the opposite effect. The blockade of MSA-DB H2 receptors, caused by local perfusion with the H2 receptor antagonist cimetidine, moderated the spontaneous release of hippocampal ACh and antagonized the facilitation produced by H3 receptor antagonists. Triprolidine, an H1 receptor antagonist, was without effect. Moreover, cells expressing c-fos immunoreactivity were significantly more numerous in ciproxifan- or thioperamide-treated rats than in controls, although no colocalization of anti-c-fos and anti-ChAT immunoreactivity was observed. These results indicate a role for endogenous histamine in modulating the cholinergic tone in the hippocampus.

Effects of novelty and habituation on acetylcholine, GABA, and glutamate release from the frontal cortex and hippocampus of freely moving rats

The involvement of the forebrain cholinergic system in arousal, learning and memory has been well established. Other neurotransmitters such as GABA and glutamate may be involved in the mechanisms of memory by modulating the forebrain cholinergic pathways. We studied the activity of cortical and hippocampal cholinergic, GABAergic and glutamatergic systems during novelty and habituation in the rat using microdialysis. After establishing basal release of the neurotransmitters, the animals were transferred to a novel environment and allowed to explore it twice consecutively for 30 min (60 min apart; exploration I and II). The motor activity was monitored. Samples were collected throughout the experiment and the release of acetylcholine (ACh), GABA and glutamate was measured. During the two consecutive explorations of the arena, cortical and hippocampal, ACh release showed a significant tetrodotoxin-dependent increase which was higher during exploration I than II. The effect was more pronounced and longer-lasting in the hippocampus than in the cortex. Cortical GABA release increased significantly only during exploration II, while hippocampal GABA release did not increase during either exploration. Motor activity was higher during the first 10 min of exploration I and II and then gradually decreased during the further 20 min. Both cortical and hippocampal ACh release were positively correlated with motor activity during exploration II, but not during I. During exploration II, cortical GABA release was inversely correlated, while hippocampal GABA release was positively correlated to motor activity. No change in cortical and hippocampal glutamate release was observed. In summary, ACh released by the animal placed in a novel environment seems to have two components, one related to motor activity and one related to attention, anxiety and fear. This second component disappears in the familiar environment, where ACh release is directly related to motor activity. The negative relationship between cortical GABA levels and motor activity may indicate that cortical GABAergic activity is involved in habituation.

Glucose plus choline improve passive avoidance behaviour and increase hippocampal acetylcholine release in mice

The present study tests the effects of glucose and choline, the biosynthetic precursors of acetylcholine, on passive avoidance behaviour and hippocampal acetylcholine release measured by microdialysis in awake mice. Glucose (10 and 30mg/kg) or choline chloride (6-60mg/kg), given by i.p. injection immediately after training, dose-dependently enhanced retention in an inhibitory avoidance task. Combinations of low doses of glucose (10mg/kg) and choline chloride (20mg/kg) which alone were submaximally effective significantly increased retention latencies in a synergistic manner, an effect which was sensitive to atropine (0.5mg/kg). This beneficial effect vanished when higher doses of glucose or choline were combined. Basal hippocampal acetylcholine release in mice habituated to their environment was not affected by administration of glucose and choline. However, when hippocampal acetylcholine release was stimulated either by infusion of scopolamine (0.3microM) or by transferring the mice into a novel environment, the combination of glucose plus choline further increased acetylcholine release to a significant extent. We conclude that low doses of glucose and choline act synergistically to improve memory storage, an effect which is due to facilitation of acetylcholine release. This finding reinforces the view that central cholinergic functions are influenced under certain conditions by dietary intake of precursors.

Glucocorticoid hypersecretion following intracerebroventricular injection of ethylcholine mustard aziridinium ion in rats

To investigate whether cholinergic hypofunctions in the brain influence hypothalamic-pituitary-adrenal activity, we examined the effects of cholinergic neurotoxin ethylcholine mustard aziridinium ion on basal and stress-induced levels of corticosterone in rats. Blood sampling from rats following intracerebroventricular injection of saline (5 microl, as a control) or this neurotoxin (5 nmol/5 microl) was performed over a day in one series, and was taken before, during and after an immobilization stress exposure in another series. Plasma levels of corticosterone and adrenocorticotropin were determined by the radioimmunoassay. The basal levels of plasma corticosterone and adrenocorticotropin over a day were significantly higher in the neurotoxin-treated rats, compared with the control rats (corticosterone, P<0.001; adrenocorticotropin, P<0.05). Further, relative adrenal gland weight of the neurotoxin-treated rats was significantly greater than that of the control rats (P<0.05). However, responses in plasma corticosterone level caused by the immobilization stress in the neurotoxin-treated rats were not different from those in the control rats. The present study demonstrated that damage to the cholinergic neurons in the brain increased hypothalamic-pituitary-adrenal activity over a day, probably due to freedom from inhibitory influences of the hippocampal cholinergic system, but that this damage did not influence stress-induced changes in plasma glucocorticoid level.

Memory-related acetylcholine efflux from rat prefrontal cortex and hippocampus: a microdialysis study

To investigate the relationship between the prefrontal and hippocampal acetylcholine (ACh) systems and working memory, an in vivo microdialysis study was conducted. A group of rats was trained to perform a working memory task, delayed alternation, in an operant chamber for food reinforcement. The rats had to choose one of two response levers in an alternative manner in each trial, with a certain interval between trials. They had to remember which lever they chose in the previous trial without the assistance of external cues. Another group was trained to perform a reference memory task, cued alternation, in which the behavioral sequence was identical, but an external cue was provided. After stable behavior was established, a dialysis probe was implanted into the prefrontal cortex or the hippocampus of each rat. The extracellular concentration of ACh in the dialysates from the prefrontal cortex increased during performance of the delayed alternation task, while the hippocampal ACh showed a more distinct increase during performance of the cued alternation task. These results suggest that the prefrontal ACh is mainly related to working memory, whereas the hippocampal ACh is mainly related to reference memory.

Application of in vivo microdialysis to the study of cholinergic systems

The application of in vivo microdialysis to the study of acetylcholine (ACh) release has contributed greatly to our understanding of cholinergic brain systems. This article reviews standard experimental procedures for dialysis probe selection and implantation, perfusion parameters, neurochemical detection, and data analysis as they relate to microdialysis assessments of cholinergic function. Particular attention is focused on the unique methodological considerations that arise when in vivo microdialysis is dedicated expressly to the recovery and measurement of ACh as opposed to other neurotransmitters. Limitations of the microdialysis technique are discussed, as well as methodological adaptations that may prove useful in overcoming these limitations. This is followed by an overview of recent studies in which the application of in vivo microdialysis has been used to characterize the basic pharmacology and physiology of cholinergic neurons. Finally, the usefulness of the microdialysis approach for testing hypotheses regarding the cholinergic systems' involvement in cognitive processes is examined. It can be concluded that, in addition to being a versatile and practical method for studying the neurochemistry of cholinergic brain systems, in vivo microdialysis represents a valuable tool in our efforts to better comprehend ACh's underlying role in a variety of behavioral processes.

Acetylcholine release in the hippocampus of the urethane anaesthetised rat positively correlates with both peak theta frequency and relative power in the theta band

The need to achieve a clearer understanding of relations between hippocampal theta characteristics and cholinergic septohippocampal neuron activity, prompted us to re-examine, in the urethane-anaesthetised rat, the statistical relationships between the electrophysiological and neurochemical variables using a procedure which is believed to enhance significantly the degree of confidence with which parameters of theta recorded with classic macroelectrodes can be related to concomitant acetylcholine output measured by high-performance liquid chromatography with electrochemical detection. Firstly, the theta rhythm and the acetylcholine content were derived from the same hippocampus. Secondly, the hippocampal electroencephalogram was quantified using spectral analysis which permits the more objective quantitative evaluation of selected electroencephalogram samples. Thirdly, a larger number of rats than in our previous study was used here, thus enhancing the validity of statistical results. This procedure yielded, in our time-course determination, two main findings. The first finding is that acetylcholine release was positively correlated with frequency at the peak power of the theta band which reflects the frequency of the theta signal. This finding had not been reported yet. The second finding is that hippocampal acetylcholine outflow also covaried with relative power of the theta band which reflects the amplitude of the theta signal. This finding is consistent with our previous study in which EEG was quantified by means of a traditional method. These findings suggest that the cholinergic component of the septohippocampal system, which is the main source of hippocampal acetylcholine, and neurophysiological mechanisms involved in the modulation of both the amplitude and the frequency of theta are functionally related. The possibility that, at least in the urethane-anaesthetised rat, hippocampal acetylcholine is involved in these modulator mechanisms is discussed.

Auditory noise can prevent increased extracellular acetylcholine levels in the hippocampus in response to aversive stimulation

The intent of this study was to investigate neurochemical and behavioural effects of aversive stimulation and the impact of auditory background noise. Using in vivo microdialysis, hippocampal acetylcholine was extracted and subjected to HPLC analysis while male Wistar rats were exposed to aversive stimulation similar to that used in conventional procedures for aversive conditioning. Three groups of animals were used. Animals in the first group were exposed to a single tone/footshock pairing followed by a tone alone 2 h later. Animals in the second group served as controls and were only exposed to the tone without shock. A third group was exposed to the same tone/shock pairing and tone as the first group while being exposed to constant background noise during the whole experiment. The results showed, that the tone/shock combination led to pronounced behavioral and cholinergic activation. In contrast, exposure to background noise prevented the increase in hippocampal ACh levels to tone/shock stimulation. The unconditioned behavioural response, however, was not prevented suggesting that hippocampal ACh is not a necessary correlate of behavioural activation or arousal. A second experiment intended to investigate the effects of background noise in a shuttle box avoidance learning paradigm where rats were trained to avoid an aversive footshock, which was signalled by a tone. There, one group of rats was exposed to background noise during avoidance learning, and the other group was not exposed to noise. Whereas both groups learned to avoid the shock to some degree over training, the noise exposed animals did not show improvement in escape performance over the course of training, indicating that the noise hindered development of an adaptive response to the shock. In summary, our data indicate that background noise can prevent increased extracellular hippocampal ACh levels in response to an aversive stimulus, and can also lead to deficits in learning to escape from shock.

Increasing acetylcholine levels in the hippocampus or entorhinal cortex reverses the impairing effects of septal GABA receptor activation on spontaneous alternation

Intra-septal infusions of the gamma-aminobutyric acid (GABA) agonist muscimol impair learning and memory in a variety of tasks. This experiment determined whether hippocampal or entorhinal infusions of the acetylcholinesterase inhibitor physostigmine would reverse such impairing effects on spontaneous alternation performance, a measure of spatial working memory. Male Sprague-Dawley rats were given intra-septal infusions of vehicle or muscimol (1 nmole/0.5 microL) combined with unilateral intra-hippocampal or intra-entorhinal infusions of vehicle or physostigmine (10 microg/microL for the hippocampus; 7.5 microg/microL or 1.875 microg/0.25 microL for the entorhinal cortex). Fifteen minutes later, spontaneous alternation performance was assessed. The results indicated that intra-septal infusions of muscimol significantly decreased percentage-of-alternation scores, whereas intra-hippocampal or intra-entorhinal infusions of physostigmine had no effect. More importantly, intra-hippocampal or intra-entorhinal infusions of physostigmine, at doses that did not influence performance when administered alone, completely reversed the impairing effects of the muscimol infusions. These findings indicate that increasing cholinergic levels in the hippocampus or entorhinal cortex is sufficient to reverse the impairing effects of septal GABA receptor activation and support the hypothesis that the impairing effects of septal GABAergic activity involve cholinergic processes in the hippocampus and the entorhinal cortex.

Enhancement of cerebral cortical acetylcholine release by intraperitoneal acetic acid and its suppression by analgesics in freely moving rats

Several lines of evidence suggest that central cholinergic neurons play a key role in the perception and control of pain. We investigated the effects of analgesics on the increase in central cholinergic activity and writhing responses elicited by i.p. injection of acetic acid. ACh efflux from the rat cerebral cortex and hippocampus was measured in the absence of a cholinesterase inhibitor using an in vivo microdialysis technique and a highly sensitive and specific radioimmunoassay. ACh efflux from the cerebral cortex was significantly increased during the first 30 min after acetic acid injection and then returned to the control levels. In contrast, acetic acid-induced writhing responses, indicative of the perception of pain, persisted for almost the entire 120 min observation period. No changes in ACh efflux were observed in the hippocampus. The centrally-acting analgesic morphine and the peripherally-acting analgesic indomethacin each completely abolished the enhanced cerebral cortical ACh efflux and the writhing, whereas diazepam, a muscle relaxant, selectively suppressed only the writhing. These results demonstrate that peripheral nociceptive stimulation transiently increases cholinergic activity in the cerebral cortex, but not in the hippocampus, and that analgesics suppress both the enhanced ACh efflux and the writhing induced by acetic acid.

Hippocampal field CA1 interneuronal nociceptive responses: modulation by medial septal region and morphine

A majority (24/32) of the extracellularly recorded dorsal hippocampus field CA1 putative GABAergic interneurons were excited in conjunction with theta activation on formalin injection (5%, 0.05 ml, s.c. into right hind-paw) in urethane (1.0 g/kg, i.p.)-anaesthetized rats. An increase in activity was observed to the 10th minute (n=24) and also at later time-periods at which a few of the neurons were recorded following injection of formalin. The mean peak increase in activity within 5 min of formalin injection was 6.43+/-0.81 Hz over the average background activity for these neurons (6.46+/-1.04 Hz). Of 24 neurons, 14 exhibited an increase in activity which was rhythmically modulated with theta. With a concurrent administration of formalin and morphine (5 mg/kg, i.p.), the presumed interneurons recorded displayed an initial increase in discharge rate (mean peak increase within 5 min of 6.95+/-1.10 Hz) which then declined with a decrease in theta activity. The effect of concurrent morphine was naloxone reversible. Morphine administration alone resulted in an immediate decrease in the interneuronal firing rate. In presence of the medial septal region lesions, formalin did not evoke an excitation of intemeurons or theta activation. Further, such lesions prevented the decrease in intemeuron activity to morphine administration. The above data are consistent with the notion that (i) the field CA1 interneurons participate in a noxious stimulus-induced and medial septal region mediated pyramidal cell suppression, and (ii) morphine affects CA1 nociceptive responses partly in a fashion consistent with the effect of the drug on septohippocampal neural network processing.

Hippocampal BDNF mRNA shows a diurnal regulation, primarily in the exon III transcript

Endogenous expression levels of brain-derived neurotrophic factor (BDNF) mRNA were assessed using in situ hybridization to investigate whether there is a natural diurnal fluctuation in BDNF mRNA expression in the hippocampus of rats housed with a normal (12:12 h) light/dark cycle. BDNF expression was increased during lights out (dark-cycle) to 134%-158% of light-cycle levels in hippocampal regions CA1, CA3, and hilus. In addition, expression levels of the four BDNF transcript forms, exons I-IV, were assessed to evaluate whether expression of specific BDNF transcripts exhibited differential endogenous fluctuation. All exons had lowest levels of expression at either noon or 6 p.m. Significant correlations were found between exon expression level and time, with elevated expression occurring at dark-cycle timepoints. The exon III transcript showed the greatest diurnal change in expression in all hippocampal fields, with dark-cycle expression elevated to 219-419% of light-cycle expression level. In addition to exon III, dark-cycle exon II mRNA levels were elevated in all hippocampal subfields, to 140-180% of light-cycle levels, suggesting that the endogenous fluctuation in BDNF expression results predominantly from activation of the promoters linked to exons II and III. Previously we have shown that physical activity increases BDNF expression. The naturally occurring rise in BDNF expression during the dark-cycle, the time when rats are most physically active, may be due to increased activity and arousal levels. Because BDNF has a role in plasticity, the increase in BDNF expression during the time that a rat is maximally interacting with its surroundings may be part of an ongoing stimulus-encoding mechanism, or may be a mechanism to maximize information storage about the environment.

Serotonin and acetylcholine release response in the rat hippocampus during a spatial memory task

By using in vivo microdialysis we monitored the extracellular levels of acetylcholine and serotonin in the hippocampus of rats performing a spatial memory task. After rats were trained for 10 consecutive days to master a food-reinforced radial-arm maze task, they were implanted with a microdialysis probe in the dorsal hippocampus. On day 12, rats were tested in the maze and acetylcholine and serotonin outputs were monitored before the test, during the waiting phase and while performing the trials. In trained, food-rewarded rats, hippocampal acetylcholine levels increased during the waiting period (181 +/- 90 of baseline) and further increased during the radial-maze performance to 236 +/- 13% of baseline values, while serotonin levels did not change during the waiting period but increased to 142 +/- 3% during the maze performance. To discriminate whether the increase of acetylcholine and serotonin levels during the testing was associated with memory performance or with food consumption, we monitored hippocampal acetylcholine and serotonin release in rats that were trained, but not food rewarded, or in rats that were not trained, but rewarded only on the test day. In the trained, non-rewarded group, acetylcholine release increased during the waiting phase to 168 +/- 6%, but did not increase further during the task performance. In contrast, no change in serotonin release was observed in this group in any phase of the test. In rats which were not trained, but food rewarded, acetylcholine increased only during the maze period (150 +/- 5%). Serotonin increased gradually and become significant at the end of the trials. (130 +/- 3%). While both neurotransmitters could be implicated in feeding behaviour, only activation of cholinergic neurotransmission appears to be associated with memory function. Our results support the following hypotheses: (i) hippocampal acetylcholine could be involved in attentional and cognitive functions underlying motivational processes; (ii) serotonin could be implicated in non-cognitive processes (i.e. in the control of motor and feeding behaviour). Since serotonin and acetylcholine neurotransmission is simultaneously activated during the spatial memory task, this suggests that these neurotransmitter systems regulate behavioural and cognitive functions.

Effects of novelty and pain on behavior and hippocampal extracellular ACh levels in male and female rats

In vivo microdialysis was used to assess the effects of novelty and pain on hippocampal ACh release in male and female rats. Experiments were carried out during the dark phase and consisted of 2 days of tests: on Day 1, after Baseline 1, animals were exposed to a new cage (Novelty) to which, 30 min later, a plastic cylinder (Object) was introduced. On Day 2, after Baseline 2, the Formalin test (50 microl of formalin 10%, s.c. injected in the dorsal hindpaw) was carried out in the animal's home cage. All behaviors were recorded. The extracellular levels of ACh in the dorsal hippocampus were estimated, in 10-min samples, by assay of ACh in the dialysates by HPLC. On Day 1 the raw values of ACh were higher in females than in males, but no sex difference was present when the percentage of change was considered. In both sexes the Novelty and Object tests induced an increase in ACh levels with respect to Baseline. Higher levels of exploration were present in females than males during the first 10 min of Novelty. On Day 2, ACh release increased in both sexes during the Formalin test. No sex difference in either ACh raw values or the percentages of change were found. Females showed higher levels of licking and lower levels of activity than males. The present study shows that novelty and pain induce similar hippocampal cholinergic activation in male and female rats but different behaviors. The results are discussed in light of the several anatomical and functional sex differences present in the hippocampus.