The transsynaptic regulation of the septal-hippocampal cholinergic neurons

Fibrinogen α-chain-derived peptide is upregulated in hippocampus of rats exposed to acute morphine injection and spontaneous alternation testing

Fibrinogen is a secreted glycoprotein that is synthesized in the liver, although recent in situ hybridization data support its expression in the brain. It is involved in blood clotting and is released in the brain upon injury. Here, we report changes in the extracellular levels of fibrinogen α-chain-derived peptides in the brain after injections of saline and morphine. More specifically, in order to assess hippocampus-related working memory, an approach pairing in vivo microdialysis with mass spectrometry was used to characterize extracellular peptide release from the hippocampus of rats in response to saline or morphine injection coupled with a spontaneous alternation task. Two fibrinopeptide A-related peptides derived from the fibrinogen α-chain – fibrinopeptide A (ADTGTTSEFIEAGGDIR) and a fibrinopeptide A-derived peptide (DTGTTSEFIEAGGDIR) – were shown to be consistently elevated in the hippocampal microdialysate. Fibrinopeptide A was significantly upregulated in rats exposed to morphine and spontaneous alternation testing compared with rats exposed to saline and spontaneous alternation testing (P < 0.001), morphine alone (P < 0.01), or saline alone (P < 0.01), respectively. The increase in fibrinopeptide A in rats subjected to morphine and a memory task suggests that a complex interaction between fibrinogen and morphine takes place in the hippocampus.

Brain reinforcement system function is ghrelin dependent: studies in the rat using pharmacological fMRI and intracranial self-stimulation

Ghrelin (GHR) is an orexigenic gut peptide that interacts with brain ghrelin receptors (GHR-Rs) to promote food intake. Recent research suggests that GHR acts as a modulator of motivated behavior, suggesting a direct influence of GHR on brain reinforcement circuits. In the present studies, we investigated the role of GHR and GHR-Rs in brain reinforcement function. Pharmacological magnetic resonance imaging was used to spatially resolve the functional activation produced by systemic administration of an orexigenic GHR dose. The imaging data revealed a focal activation of a network of subcortical structures that comprise brain reinforcement circuits-ventral tegmental area, lateral hypothalamus and nucleus accumbens. We next analyzed whether brain reinforcement circuits require functional GHR-Rs. To this purpose, wild-type (WT) or mutant rats sustaining N-ethyl-N-nitrosourea-induced knockout of GHR-Rs (GHR-R null rats) were implanted with stimulating electrodes aimed at the lateral hypothalamus, shaped to respond for intracranial self-stimulation (ICSS) and then tested using a rate-frequency procedure to examine ICSS response patterns. WT rats were readily shaped using stimulation intensities of 75 µA, whereas GHR-R null rats required 300 µA for ICSS shaping. No differences in rate-frequency curves were noted for WT rats at 75 µA and GHR-R null rats at 300 µA. When current intensity was lowered to 100 µA, GHR-R null rats did not respond for ICSS. Taken collectively, these data suggest that systemic GHR can activate mesolimbic dopaminergic areas, and highlight a facilitative role of GHR-Rs on the activity of brain reinforcement systems.

Mentored and inspired by Mimo: a tribute to Erminio Costa

Throughout his long productive scientific career, Erminio Costa demonstrated several scholarly traits that illustrate a pattern for paths of successful achievement that should guide young scientists. Not only did he seek excellent training, he got and gave good mentoring. That guidance allowed him to ask important questions and to develop the methods necessary to obtain definitive answers by pursuing those questions in depth. Without question, he blazed trails in neuropharmacology that have been an inspiration to many others and me. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Scopolamine- and diazepam-induced amnesia are blocked by systemic and intraseptal administration of substance P and choline chloride

Systemic (IP) and/or intraseptal (IS) administration of scopolamine (SCP) and diazepam (DZP) induce amnesia, whereas IP injection of the neuropeptide substance P (SP) and choline chloride (ChCl) produce memory facilitation. The septohippocampal cholinergic system has been pointed out as a possible site of SCP and DZP-induced amnesia as well as for the mnemonic effects induced by SP and ChCl. We performed a series of experiments in order to investigate the interactions between cholinergic and GABA/benzodiazepine (GABA/BZD) systems with the SPergic system on inhibitory avoidance retention. Male Wistar rats were trained and tested in a step-down inhibitory avoidance task (1.0 mA footshock). Animals received, pre-training, IP (1.0 mg/kg) or IS (1.0 nM/0.5 microl) injection of DZP, SCP (SCP; 1.0 mg/kg - IP or 0.5 microM/0.5 microl--IS) or vehicle (VEH). Immediately after training they received an IP or IS injections of SP 1-11 (50 microg/kg--IP or 1.0 nM/0.5 microl--IS), SP 1-7 (167 microg/kg--IP or 1.0 nM/0.5 microl--IS), ChCl (20 mg/kg--IP or 0.3 microM/0.5 microl--IS) or VEH. Rats pretreated with SCP and DZP showed amnesia. Post-trial treatments with SP 1-11, SP 1-7 or ChCl blocked the amnesic effects of SCP and DZP. These findings suggest an interaction between SPergic and cholinergic mechanisms with GABAergic systems in the modulation of inhibitory avoidance retention and that the effects of these treatments are mediated, at least in part, by interactions in the septohippocampal pathway.

Insights into the Role of Dopamine Receptor Systems in Learning and Memory

It is well established that learning and memory are complex processes involving and recruiting different brain modulatory neurotransmitter systems. Considerable evidence points to the involvement of dopamine in various aspects of cognition, and interest has been focused on investigating the clinical relevance of dopamine systems to age-related cognitive decline and manifestations of cognitive impairment in schizophrenia, Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases. In the past decade or so, in spite of the molecular cloning of the five dopamine receptor subtypes, their specific roles in brain function remained inconclusive due to the lack of completely selective ligands that could distinguish between the members of the D1-like and D2-like dopamine receptor families. One of the most important advances in the field of dopamine research has been the generation of mutant mouse models permitting evaluation of the dopaminergic system using gene targeting technologies. These mouse models represent an important approach to explore the functional roles of closely related receptor subtypes. In this review, we present and discuss evidence on the role of dopamine receptors in different aspects of learning and memory at the cellular, molecular and behavioral levels. We compare evidence using conventional pharmacological, lesion or electrophysiological studies with results from mice with targeted deletions of different subtypes of dopamine receptor genes. We particularly focus on dopamine D1 and D2 receptors in an effort to delineate their specific roles in various aspects of cognitive function. We provide strong evidence, from our own recent work as well as others, that dopamine is part of the network that plays a very important role in cognitive function, and that although multiple dopamine receptor subtypes contribute to different aspects of learning and memory, the D1 receptor seems to play a more prominent role in mediating plasticity and specific aspects of cognitive function, including spatial learning and memory processes, reversal learning, extinction learning, and incentive learning.

Mechanism-Based Approaches for the Reversal of Drug Neurobehavioral Teratogenicity

Understanding the mechanism of neurobehavioral teratogenicity is the primary prerequisite for reversal of the defect. Progress in such studies can be best achieved if the investigation focuses on behaviors related to a specific brain region and innervation. Our model focused on teratogen-induced deficits in hippocampus-related eight-arm and Morris maze behaviors. Different "cholinergic" teratogens, mainly heroin, induced both pre- and postsynaptic hyperactivity in the hippocampal cholinergic innervation that terminated in desensitization of Protein Kinase C (PKC) isoforms to cholinergic receptor stimulation. Understanding this mechanism enabled its reversal with a pharmacological therapy-nicotine infusion. Studies by others provided similar findings by targeting the deficits respective to the model investigated. Consistently, destruction of the A10-septal dopaminergic pathways that downregulate the septohippocampal cholinergic innervation ameliorated maze performance. Grafting of embryonic differentiated cholinergic cells or neural progenitors similarly reversed the biochemical/molecular alterations and the resulting deficits. Reversal therapies offer a model for the understanding of neurobehavioral teratogenicity and, clinically, offer a model for potential treatment of these deficits. Whereas neural progenitor grafting appears to be the most effective treatment, pharmacological reversal with nicotine infusion seems to possess the most feasible and immediate therapy for neurobehavioral birth defects produced by various teratogens, including drugs. This is true even though the effect of pharmacological therapies is diffuse, affecting multiple areas of the brain. "Everybody is talking about the weather but nobody does anything about it." (Mark Twain).

Modulation of memory with septal injections of morphine and glucose: effects on extracellular glucose levels in the hippocampus

The concentration of glucose in the extracellular fluid (ECF) of the hippocampus decreases substantially during memory testing on a hippocampus-dependent memory task. Administration of exogenous glucose, which enhances task performance, prevents this decrease, suggesting a relationship between hippocampal glucose availability and memory performance. In the present experiment, spontaneous alternation performance and task-related changes in hippocampal ECF glucose were assessed in rats after intraseptal administration of morphine, which impairs memory on a spontaneous alternation task, and after co-administration of intraseptal glucose, which attenuates that impairment. Consistent with previous findings, spontaneous alternation testing resulted in a decrease in hippocampal ECF glucose levels in control rats. However, rats that received intraseptal morphine prior to testing showed memory impairments and an absence of the task-related decrease in hippocampal ECF glucose levels. Intraseptal co-administration of glucose with morphine attenuated the memory impairment, and ECF glucose levels in the hippocampus decreased in a manner comparable to that seen in control rats. These data suggest that fluctuations in hippocampal ECF glucose levels may be a marker of mnemonic processing and support the view that decreases in extracellular glucose during memory testing reflect increased glucose demand during memory processing.

Basal forebrain cholinergic system of the anuran amphibianRana perezi: Evidence for a shared organization pattern with amniotes

The organization of the basal forebrain cholinergic system (BFCS) in the frog was studied by means of choline acetyltransferase (ChAT) immunohistochemistry. The BFCS was observed as a conspicuous cholinergic cell population extending through the diagonal band, medial septal nucleus, bed nucleus of the stria terminalis, and pallidal regions. Abundant fiber labeling was also found around the labeled cell bodies. The combination of retrograde tract tracing with dextran amines and ChAT immunohistochemistry revealed intraseptal and intra-BFCS cholinergic connections. In addition, an extratelencephalic cholinergic input from the laterodorsal tegemental nucleus was demonstrated. The possible influence of monoaminergic inputs on the BFCS neurons was examined by means of tyrosine hydroxylase and serotonin immunohistochemistry combined with ChAT immunolabeling. Our results showed that catecholaminergic fibers overlapped the BFCS, with the exception of the medial septal nucleus. Serotoninergic innervation was widespread, but less abundant in the caudal extent of the BFCS. Taken together, our results on the localization of the cholinergic neurons in the basal forebrain and their relationship with cholinergic, catecholaminergic, and serotoninergic afferents have shown numerous common features with amniotes. In particular, anurans and mammals (for which most data is available) share a strikingly comparable organization pattern of the BFCS.

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

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

Involvement of cholinergic neuronal systems in intravenous cocaine self-administration

Recent studies suggest the participation of cholinergic neurons in the brain processes underlying reinforcement. The involvement of cholinergic neurons in cocaine self-administration has been recently demonstrated in studies using muscarinic and nicotinic agonists and antagonists, microdialysis, assessment of choline acetyltransferase activity and acetylcholine (ACh) turnover rates. The present experiment was initiated to identify subsets of cholinergic neurons involved in the brain processes that underlie cocaine self-administration by lesioning discrete populations with a selective neurotoxin. Rats were trained to self-administer cocaine and the cholinergic neurotoxin 192-IgG-saporin or vehicle was then bilaterally administered into the posterior nucleus accumbens (NAcc)-ventral pallidum (VP). The 192-IgG-saporin induced lesions resulted in a pattern of drug-intake consistent with either a shift in the dose intake relationship to the left or downward compared to sham-treated controls. A second experiment used a self-administration threshold procedure that demonstrated this lesion shifted the dose intake relationship to the left compared to the sham-vehicle treated rats. The magnitude and extent of the lesion was assessed by measuring the expression of p75 (the target for 192-IgG-saporin) and choline acetyltransferase (ChAT) in the NAcc, VP, caudate nucleus-putamen (CP) and vertical limb of the medial septal nucleus-diagonal band (MS-DB) of these rats using real time reverse transcriptase-polymerase chain reaction. Significant reductions in gene expression for p75 (a selective marker for basal forebrain cholinergic neurons) and ChAT were seen in the MS-DB and VP while only small decreases were seen in the NAcc and CP of the 192-IgG-saporin treated rats. These data indicate that the overall influence of cholinergic neurons in the MS-DB and VP are inhibitory to the processes underlying cocaine self-administration and suggest that agonists directed toward subclasses of cholinergic receptors may have efficacy as pharmacotherapeutic adjuncts for the treatment of cocaine abuse.

Distribution of polysialylated neural cell adhesion molecule in rat septal nuclei and septohippocampal pathway: Transient increase of polysialylated interneurons in the subtriangular septal zone during memory consolidation

During memory consolidation neuroplastic events in the mediotemporal corticohippocampal pathway are accompanied by transient increases in the frequency of neurons expressing polysialylated neural cell adhesion molecule (NCAM PSA), a posttranslational modification associated with morphofunctional change. As a bidirectional pathway between the hippocampus and the septal nuclei also influences memory processing, we have determined the distribution of NCAM PSA within this system before and after learning in the adult Wistar rat. The most intense NCAM PSA immunoreactivity was observed in the medial and triangular septal nuclei, regions that regulate hippocampal theta rhythm during memory consolidation. Within the fimbria, NCAM PSA was expressed only in a subpopulation of fibres, most likely cholinergic projections from the medial septum to the hippocampus. Grey level analysis or direct cell counting revealed no learning-specific change in NCAM PSA expression in these septal subregions after avoidance conditioning or spatial training. A population of discrete polysialylated neurons in the subtriangular septal zone, however, exhibited a transient twofold frequency increase at 12 hr after training in either task. Immunohistochemical analysis revealed these cells to be gamma-aminobutyric acid (GABAergic) interneurons co-expressing vasoactive intestinal peptide. The unique location of these interneurons is proposed to provide a natural plexus by which bidirectional communication between the septum and hippocampus may be modified during memory consolidation.

Glucose increases hippocampal extracellular acetylcholine levels upon activation of septal GABA receptors

Activation of septal GABA receptors impairs learning and memory and this effect likely involves an influence on the hippocampus. We found previously that the memory-impairing effects of septal infusions of the GABA agonist muscimol are reversed by hippocampal infusions of glucose and suggested that glucose reverses these deficits by increasing hippocampal acetylcholine (ACh). In this study, we report that septal infusions of muscimol produce dose-dependent decreases in ACh levels in hippocampal dialysates. Importantly, increasing glucose levels in the hippocampus elevates hippocampal extracellular ACh levels in rats given septal infusions of muscimol, but not in rats given vehicle. Thus, glucose increases hippocampal extracellular ACh levels when the ACh system is inhibited, an effect that likely contributes to the effects of glucose on memory.

Cholinergic deficits in the septal-hippocampal pathway of the SAM-P/8 senescence accelerated mouse

Senescence accelerated prone mouse strains (SAM-P) and resistant strains (SAM-R) have proven useful in elucidating aspects of the aging process. The senescence accelerated mouse SAM-P/8 strain exhibits severe age-related learning and memory impairments well before the median age of survival. Disruption of the brain cholinergic system produces learning and memory impairments as severe as those seen in aging SAM-P/8 mice. Therefore, we compared the effects of aging on cholinergic parameters in the septal-hippocampal pathway, a region known to play a role in learning and memory, in SAM-P/8 mice and mice of the senescence resistant SAM-R/1 strain. Between 4 and 12 months of age we observed a 40-50% decrease in choline acetyltransferase (ChAT) activity in two of three subregions of the hippocampus in the SAM-P/8, but not the SAM-R/1 strain. Between 4 and 12 months, SAM-P/8 mice also showed a 40-50% decrease in ChAT activity in the septal region that was maximal by 8 months of age. By contrast, these age-related changes were not observed in the control SAM-R/1 mouse strain. The changes in ChAT in the SAMP/8 mouse strain were limited to the septal-hippocampal cholinergic pathway. There were no differences in ChAT activity in the nucleus basalis of Meynert, nor any of several neocortical areas to which it projects. To determine the neurochemical specificity of these alterations, the activity of glutamic acid decarboxylase (GAD), was also measured in the septum and hippocampus of SAM-P/8 mice. There were no age-related alterations in the hippocampus, but a significant 50% increase in GAD activity in the septal nucleus at 12 months of age. There were no age-related alterations in either nicotinic (3H-cytisine) or muscarinic (3H-QNB) cholinergic receptor binding in the cortex or hippocampus of SAM-P/8 mice. However, there were significant strain differences. At 2 months of age, 3H-QNB binding was higher in hippocampus of the SAM-R/1 than in SAM-P/8 mice. Similarly, 3H-cytisine binding in cortex of SAM-R/1 mice was higher at both 2 and 13 months than in SAM-P/8 mice. The results suggest that a compromised septal-hippocampal cholinergic pathway may contribute to the previously reported early onset of impaired learning and memory in the SAM-P/8 mouse strain.

Catecholaminergic innervation of the septum in the frog: a combined immunohistochemical and tract-tracing study

In the present study, we have investigated the distribution and the origin of the catecholaminergic innervation of the septal region in the frog Rana perezi. Immunohistochemistry for dopamine and two enzymes required for the synthesis of catecholamines, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) revealed a complex pattern of catecholaminergic (CA) innervation in the anuran septum. Dopaminergic fibers were primarily present in the dorsal portion of the lateral septum, whereas noradrenergic (DBH immunoreactive) fibers predominated in the medial septum/diagonal band complex. Catecholaminergic cell bodies were never observed within the septum. To determine the origin of this innervation, applications of dextran amines, both under in vivo and in vitro conditions, into the septum were combined with immunohistochemistry for TH. Results from these experiments demonstrated that four catecholaminergic cell groups project to the septum: (1) the group related to the zona incerta in the ventral thalamus, (2) the posterior tubercle/mesencephalic group, (3) the locus coeruleus, and (4) the nucleus of the solitary tract. While the two first groups provide dopaminergic innervation to the septum, the locus coeruleus provides the major noradrenergic projection. Noradrenergic fibers most likely arise also in the nucleus of the solitary tract. The results obtained in Rana perezi are readily comparable to those in mammals suggesting that the role of catecholamines in the septum is well conserved through phylogeny and that the CA innervation of the amphibian septum may be involved in functional circuits similar to those in mammals.

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

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

Involvement of dopamine D(2) receptors in complex maze learning and acetylcholine release in ventral hippocampus of rats

In the current study we focus on the involvement of dopamine D(2) receptors in the ventral hippocampus in memory performance and acetylcholine release. Using the aversively motivated 14-unit T-maze (Stone maze) the injection of raclopride, a D(2) receptor antagonist, into the ventral hippocampus (8 microg/kg) was found to impair memory performance. Co-injection of quinpirole, a D(2) receptor agonist (8 microg/kg), overcame the impairment in performance. Microdialysis study revealed that quinpirole infusion (10-500 microM) into the ventral hippocampus stimulated acetylcholine release in a dose-dependent manner, and systemic injection of quinpirole (0.5 mg/kg, i.p.) also stimulated acetylcholine release in the ventral hippocampus. Infusion of eticlopride, another D(2) receptor antagonist, into the ventral hippocampus suppressed acetylcholine release in the hippocampus induced by systemic injection of quinpirole. Taken together, we suggest that D(2) receptors in the ventral hippocampus are involved in memory performance, possibly through the regulation of acetylcholine.

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.

Permanent and temporary inactivation of the hippocampus impairs T-maze footshock avoidance acquisition and retention

The hippocampus is widely recognized as playing an important role in learning and memory. Lesions of the hippocampus can disrupt spatial navigational learning and memory and injection of drugs into the hippocampus can affect both spatial navigational and nonspatial tasks. In the current studies we tested the effects of bilateral of electrolytic lesions and reversible inactivation of the hippocampus on acquisition and retention of T-maze footshock avoidance conditioning. Electrolytic lesions, which destroyed 31+/-0.04% of the hippocampus, significantly impaired acquisition and retention for T-maze footshock avoidance. No differences were found in motivation to avoid shock, open field activity, or foot shock sensitivity between lesion and control groups. Temporary inactivation of the hippocampus with lidocaine administered immediately before training disrupted acquisition and retention for T-maze footshock avoidance. Temporary hippocampal inactivation performed just prior to retention testing and post-training inactivation in mice trained to first avoidance had no effect on retention. However, temporary post-training inactivation in 'undertrained' (enough trials to remember 1 week later if treated with saline, but not allowed to make the avoidance response) mice impaired retention. The current findings indicate that the hippocampus plays an important role in learning and memory processing in the aversive T-maze paradigm.

The effect of cholinergic, GABAergic, serotonergic, and glutamatergic receptor modulation on posttrial memory processing in the hippocampus

Though the hippocampus is widely recognized as important in learning and memory, most of the evidence for this comes from animal lesion and human pathological studies. Due to the relatively small number of drugs that have been tested in the hippocampus for their ability to alter posttrial memory processing, there is a general impression that memory processing involves only a few neurotransmitters. We have evaluated the effects of cholinergic, GABAergic, serotonergic, and glutamatergic receptor agonists and antagonists for their ability to facilitate or impair retention. CD-1 mice received acute intrahippocampal drug infusion following footshock avoidance training in a T-maze. Retention was tested 1 week after training and drug administration. The results indicate that receptor agonists of acetylcholine and glutamate improved retention, while antagonists impaired retention. However, scopolamine did not impair retention, but M1 and M2 antagonists did. Receptor agonists of serotonin and GABA impaired retention, while antagonists improved retention. Drugs acting on 5-HT-1 and 5-HT-2 as well as GABA(A) and GABA(B) receptor subtypes did not differentially effect retention.

Opioids suppress IPSCs in neurons of the rat medial septum/diagonal band of Broca: involvement of mu-opioid receptors and septohippocampal GABAergic neurons

The medial septum/diagonal band region (MSDB), which provides a major cholinergic and GABAergic input to the hippocampus, expresses a high density of opioid receptors. Behaviorally, intraseptal injections of opioids produce deficits in spatial memory, however, little is known about the electrophysiological effects of opioids on MSDB neurons. Therefore, we investigated the electrophysiological effects of opioids on neurons of the MSDB using rat brain slices. In voltage-clamp recordings with patch electrodes, bath-applied met-enkephalin, a nonselective opioid receptor agonist, decreased the number of tetrodotoxin and bicuculline-sensitive inhibitory synaptic currents in cholinergic- and GABA-type MSDB neurons. A similar effect occurred in brain slices containing only the MSDB, suggesting that opioids decrease GABA release primarily by inhibiting spontaneously firing GABAergic neurons located within the MSDB. Accordingly, in extracellular recordings, opioid-sensitive, spontaneously firing neurons could be found within the MSDB. Additionally, in intracellular recordings a subpopulation of GABA-type neurons were directly inhibited by opioids. All effects of met-enkephalin were mimicked by a mu receptor agonist, but not by delta or kappa agonists. In antidromic activation studies, mu-opioids inhibited a subpopulation of septohippocampal neurons with high conduction velocity fibers, suggestive of thickly myelinated GABAergic fibers. Consistent with the electrophysiological findings, in double-immunolabeling studies, 20% of parvalbumin-containing septohippocampal GABA neurons colocalized the mu receptor, which at the ultrastructural level, was found to be associated with the neuronal cell membrane. Thus, opioids, via mu receptors, inhibit a subpopulation of MSDB GABAergic neurons that not only make local connections with both cholinergic and noncholinergic-type MSDB neurons, but also project to the hippocampus.

Changes in membrane fatty acids and delta-9 desaturase in senescence accelerated (SAMP8) mouse hippocampus with aging

Senescence accelerated mice (SAMP8) exhibit age induced impairments such as loss of memory and learning disabilities by the age of 8-10 months. Analysis of hippocampus of SAMP8 mice revealed that delta 9-desaturase (delta9desaturase) activity reduced up to 44-50% with age. Correspondingly, levels of unsaturated fatty acids are also lowered in the aged animals approximately to the same levels. RNase protection assay showed that delta9specific message decreased similarly with age. As such a decrease is known to cause alterations in membrane fluidity and affect cellular signaling pathways, these results suggest that lowering of delta9gene expression may be partly involved in age induced impairments.

Septo-hippocampal drug interactions in post-trial memory processing

To determine if serotonin and GABA regulate post-trial memory processing of the cholinergic projection from the septum to the hippocampus, mice were trained on footshock avoidance in a T-maze. Immediately after training, drugs were injected into the septum, hippocampus or both. Retention was tested 1 week after training and drug administration. Ketanserin, a serotonin type 2 receptor antagonist at a dose of 0.5 ng, had no measurable effect on retention, but it reduced the dose of bicuculline, in the septum, or arecoline in the hippocampus that was needed to improve retention. DOI, a serotonin type 2 receptor agonist at a dose of 2.5 ng, had the opposite effect of increasing the doses of bicuculline and arecoline needed to improve retention. Bicuculline, a GABA(A) receptor antagonist at a dose of 0.1 pg, did not affect retention when injected alone into the septum, but it reduced the dose of arecoline needed to improve retention in the hippocampus. Muscimol, a GABA(A) receptor agonist at a dose of 5 ng, injected into the septum, increased the dose of arecoline needed to improve retention. The results of this study are compatible with models that propose that serotonin innervation from the median raphe drives GABA interneurons in the medial septum that synapse on cholinergic neurons projecting to the hippocampus.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Cholinergic neurons of the nucleus basalis of Meynert receive cholinergic, catecholaminergic and GABAergic synapses: an electron microscopic investigation in the monkey

An electron microscopic analysis of the nucleus basalis in the macaque monkey was carried out following the immunohistochemical labeling of choline acetyltransferase, either by itself or in conjunction with glutamate decarboxylase or tyrosine hydroxylase. Cholinergic axon varicosities were frequently encountered, and formed large, usually asymmetric, synapses on both choline acetyltransferase-immunopositive and -immunonegative dendrites of nucleus basalis neurons. Catecholaminergic (tyrosine hydroxylase-immunoreactive) axon varicosities formed synapses which in most cases were classified as asymmetric, and glutamate decarboxylase-immunoreactive (GABAergic) axons formed clearly symmetric synapses, each on to choline acetyltransferase-immunopositive or -immunonegative dendrites. These findings indicate that cholinergic cells in the nucleus basalis of the monkey, also known as Ch4 neurons, receive numerous synaptic inputs from cholinergic, catecholaminergic and GABAergic axons.

GABA receptors and benzodiazepine binding sites modulate hippocampal acetylcholine release in vivo

In the present study, the regulation of acetylcholine release from the ventral hippocampus by gamma-aminobutyric acid (GABA) was investigated in vivo. GABA receptor agonists and antagonists were administered locally in the medial septum and the adjacent vertical limb of the diagonal band of Broca, or in the hippocampus by retrograde dialysis. Acetylcholine release was measured in the ventral hippocampus. In addition, the modulation of acetylcholine release via septal benzodiazepine binding sites was assessed by intraseptal administration of an agonists and an antagonist at the benzodiazepine binding site. Intraseptal administration of the GABA(A) receptor agonist muscimol and the GABA(B) receptor agonist baclofen, but not the agonist of the benzodiazepine binding site midazolam, decreased acetylcholine release in the hippocampus. The GABA(A) receptor antagonist bicuculline and the antagonist of the benzodiazepine binding site flumazenil, but not the GABA(B) receptor antagonist 3-N-(3,4,-dichlorobenzyl) aminopropyl-P-diethoxymethyphosphinic acid (CGP 52432) increased acetylcholine release in the hippocampus upon intraseptal administration. The same GABA receptor ligands were administered in the ventral hippocampus. CGP 52432 induced a small increase in acetylcholine release, whereas baclofen, muscimol and bicuculline did not affect local acetylcholine release. Thus, endogenous GABA causes tonic inhibition of acetylcholine release in the ventral hippocampus via septal GABA(A) receptors and, to a lesser extent, via GABA(B) receptors in the medial septum and hippocampus. The GABAergic inhibition in the medial septum is reduced by antagonists of the benzodiazepine binding site.

Age-related changes in septal serotonergic, GABAergic and glutamatergic facilitation of retention in SAMP8 mice

SAMP8/TaJf(P8) mouse strain has an inherited age-related impairment of learning and memory with its onset relatively early in its lifespan. Previously, it was reported that cholinergic and glutamatergic drugs injected into the hippocampus after behavioral training showed considerable shifts in the dose that improved retention in mice at 12 compared to 4 months of age. Cholinergic neurons in the septum supply most of the acetylcholine released in the hippocampus. In the present study, we determined if altered functional status of neurotransmission in the septum might account for the decrease in cholinergic and glutamatergic activity in the hippocampus of older SAMP8 mice. After training on footshock avoidance, P8 mice received a drug injection into the septum. Retention was tested 1 week later. The results indicate that bicuculline, GABA-A, and saclofen, GABA-B, receptor antagonist had to be injected at a higher dose in 12- than in 4-month-old mice to improve retention. The serotonergic antagonists, ketanserin and methiothepin, both showed dose response shifts such that less drug was needed to improve retention in 12- as compared to 4-month-old mice. It required four times more L-glutamate to improve retention in 12- than in 4-month-old mice. Agonists for acetylcholine, dopamine and norepinephrine receptors or an opiate antagonist required little or no change in the dose needed to improve retention in older P8 mice. SAMP8 mice may show an age-related impairment of septohippocampal functioning.

Effect of nicotine use and withdrawal on brain preproenkephalin A mRNA

Although the effect of nicotine on brain neurotransmitters and behavior has been studied, the mechanism(s) by which nicotine contributes to tobacco use remains unclear. One transmitter that may relate to long-term nicotine use and its withdrawal is enkephalin, a five-amino acid opioid peptide derived from the proenkephalin A family. In the present study we determined the effect of acute and chronic nicotine treatment and its withdrawal on preproenkephalin A mRNA levels (PPE mRNA) in specific rat brain regions using Northern blot analysis. Acute treatment with nicotine produced a significant increase in PPE mRNA in striatum and hippocampus. Chronic treatment with nicotine caused a significant decrease in PPE mRNA in these brain regions. In both striatum and hippocampus there was a rebound increase in PPE mRNA 24 h after nicotine cessation which approached the saline level 7 days later. Nicotine withdrawal 24 h following nicotine cessation, caused a significant increase in PPE mRNA in both brain regions. These effects of nicotine were blocked by pretreating rats with the nicotinic antagonist, mecamylamine. These data strongly suggest that brain opioid system(s) are involved in mediating nicotinic responses and its withdrawal and may have clinical implications in treating nicotine addiction.

The pharmacology of post-trial memory processing in septum

The septum is recognized as important in learning and memory, but relatively little is known about the role of specific neurotransmitter receptors in memory processing in the septum. We evaluated the role of the classical neurotransmitters in mice that were prepared for intraseptal microinfusion of drug solution after footshock avoidance training in T-maze. Retention for the footshock training was determined 1 week after training and drug administration. The results indicated that receptor agonists of dopamine, norepinephrine, glutamate and acetylcholine improved retention, while the antagonists impaired retention. Receptor agonists of serotonin, gamma-amino butyric acid (GABA) and opioids impaired retention, while antagonists improved retention.

Involvement of medial septal glutamate and GABAA receptors in behaviour-induced acetylcholine release in the hippocampus: a dual probe microdialysis study

In the present study, the role of medial septal receptors in behaviour-induced increase in acetylcholine (ACh) release in hippocampus was investigated using dual-probe microdialysis in combination with a simple behavioural procedure. gamma-Aminobutyric acid (GABA) and glutamate receptor agonists and antagonists were administered by retrograde dialysis into the medial septal area, while ACh was measured in the ventral hippocampus. Rats were behaviourally activated by immobilization or handling, but only handling was performed during drug-treatment. The GABAA receptor agonist muscimol did not affect ACh release, but blocked the handling-induced increase in ACh release completely. In addition, muscimol administration induced behavioural activity. Administration of the GABAA receptor antagonist bicuculline increased ACh release more than 2-fold. However, handling-induced increase in ACh release, expressed as percentage of drug-induced release, was similar to that of controls. Administration of the glutamate receptor agonists N-methyl-D-aspartate (NMDA) and (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in the septal area increased hippocampal ACh levels, but reduced the handling-induced increase in ACh release. The NMDA antagonist, 3-((R)-2-carboxypiperazin-4-yl) (CPP) increased ACh levels moderately, and reduced handling-induced increase in ACh release. However, similarly to muscimol, CPP administration induced behavioural activity. The AMPA/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) did not affect behaviour or basal ACh levels, but attenuated the handling-evoked ACh release. We conclude that the handling-induced increase in hippocampal ACh levels is mediated via stimulation of septal non-NMDA, and possibly NMDA receptors, whereas GABAA receptors are probably not involved. The feasibility of the experimental design is further discussed.

Learning and memory in the SAMP8 mouse

The SAMP8 (P8) mouse strain develops deficits in learning and memory relatively early in its lifespan. This review provides an overview of the age-related changes that occur in P8 mice. Behavioral studies with P8 mice show impaired acquisition and retention as early as 4 months of age. Deficits in acquisition and retention occur with both aversive and appetitive training tasks. Anatomical studies have detected a number of age-related changes that occur in the central nervous system of P8 mice. The age-related increase in amyloid beta protein is well correlated with the age-related decline in learning and memory. Antibody to amyloid beta protein injected prior to training alleviated impaired acquisition and retention, whereas post-training injections alleviated retention deficits in older P8 mice. Biochemical studies have detected numerous age-related changes with reduced NMDA receptor activity most closely related to impaired learning and memory in P8 mice. Pharmacological studies have found age-related functional changes in the ability of drugs to improve memory processing in P8 mice in the septum and the hippocampus. The specific pattern of pharmacological changes and the inferred change in neurotransmitter activity suggest that age-related impairment in memory processing may be due to impaired septohippocampal interactions. The proposal that P8 mice may be a useful model for studying the early phases of age-related dementia of the Alzheimer type, while still requiring considerable study, seems reasonable.

Microinjection of GABA-A agonist muscimol into the dorsal but not the ventral hippocampus impairs non-mnemonic measures of delayed non-matching-to-position performance in rats

Anatomical studies of afferent and efferent connections suggest that the hippocampus may have more than one information processing role that varies along the septo-hippocampal axis. The present study was conducted to test whether a distinct functional specialization for the dorsal versus ventral extents of the hippocampus could be detected. The effects of a GABA-A receptor agonist, muscimol, microinjected into either the dorsal or ventral hippocampus on an operant, spatial delayed non-matching-to-position (DNMTP) task were measured. A decrease in the number of trials completed per session and disruption of several DNMTP discrimination parameters were produced by muscimol microinjection into the dorsal hippocampus but not into the ventral hippocampus. Muscimol injected into either site did not impair the measure of working memory, delayed choice accuracy. These results are consistent with the view that hippocampal function varies along the septo-temporal axis, and that the dorsal hippocampus is relatively more critical to visual discrimination performance than the ventral hippocampus.

Differential expression of protein kinase C -alpha and -betaII in rat septum and changes following fimbria-fornix lesion

Protein kinase C (PKC)-alpha and -betaII are expressed specifically and differentially in the septal formation. Following unilateral fimbria-fornix lesions, there was a marked reduction in punctiform PKC-alpha immunoreactivity in the lateral septum and in the number of PKC-betaII immunoreactive neurons in the medial septum and the lateral septum, while some PKC-alpha and PKC-betaII immunoreactive glia-like cells were observed in the lateral septum. The response of each PKC isozyme to this lesion supports the suggested role for PKC in the plasticity of the central nervous system.

Intraseptal infusions of muscimol impair spontaneous alternation performance: infusions of glucose into the hippocampus, but not the medial septum, reverse the deficit

As observed with intraseptal injections of opioid receptor agonists, direct infusions of GABAergic receptor agonists into the medial septum impair performance on several tasks that involve spatial or working memory processes in rats. Because the effects of opioid-induced impairments can be reliably reversed by concomitant intraseptal infusions of glucose, the experiments reported here determined whether impairments produced by GABAergic agonists would similarly be reversed by glucose. The findings of Experiment 1 showed, in male Sprague-Dawley rats, that intraseptal infusions of the GABA agonist muscimol (1 or 3 nmol/0.5 microliter) impaired spontaneous alternation performance. The results of Experiment 2 indicated that intraseptal infusions of glucose (8, 17, or 33 nmol) or glutamate (15 or 30 nmol) did not attenuate the muscimol-induced deficit on spontaneous alternation performance, whereas infusions of the GABAergic antagonist bicuculline methiodide (0.1 nmol) did. However, the findings of Experiment 3 indicated that glucose injections (50 nmol/0.5 microliter) into the hippocampus did reverse the impairing effect of the intraseptal muscimol infusions. Combined, these findings suggest that the neurochemical regulation of learning and memory may involve hierarchical interactions between particular neurotransmitter and neuroanatomical systems. Specifically, medial septal GABAergic effects on spontaneous alternation prevail over those of glucose or glutamate in the medial septum, but are overridden by the effects of glucose in the hippocampus.

Effects of RU 52583, an alpha 2-antagonist, on memory in rats with excitotoxic damage to the septal area

The anti-amnesic action of RU 52583, an alpha 2-adrenergic receptor antagonist, was evaluated through performance of spatial tasks in a radial maze by rats with N-methyl-D-aspartic acid (NMDA) lesion of the medial septal (MS) nuclei. Memory performance of lesioned or sham-operated rats was evaluated by measuring reference memory as long-term maintenance of an acquired performance and working memory or memory for recent events. The lesion: a produced significant impairments of the animals' memory performance, b) significantly reduced the sodium-dependent high-affinity choline uptake in the hippocampal formation, and c) deeply disrupted cholinergic hippocampal theta waves. Oral administration of RU 52583 at 1 and 2 mg/kg (tested doses: 1-5 mg/kg) prior to performance of the task markedly reduced memory impairments, whereas idazoxan, another alpha 2-adrenergic receptor antagonist, had no effect at tested doses (2-5 mg/kg). Cholinergic drugs--arecoline at 0.1 and 1 mg/kg (tested doses: 0.05-1 mg/kg) and physostigmine at 0.02 and 0.1 mg/kg (tested doses: 1, 2, and 5 mg/kg)-administered intraperitoneally showed a tendency to alleviate memory deficits. The present results show that the alpha 2-adrenergic antagonist RU 52583 possesses cognition-enhancing properties in rats with damage to the septohippocampal system.

Modulation of cognitive processes by transsynaptic activation of the basal forebrain

Each of the neurotransmitter-specific afferents to the basal forebrain (BF) carry different types of information which converge to regulate the activity of cholinergic projections to telencephalic areas. Brainstem monoaminergic and cholinergic inputs are critical for context-dependent arousal. GABAergic afferents are gated by a variety of ascending and descending systems, and in addition provide an intrinsic control of BF output excitability. Corticofugal glutamatergic inputs represent reciprocal connections from sites to which BF afferents project, and carry information about the current level of cortical processing intensity and capacity. Peptidergic inputs arise from hypothalamic sources and locally modulate BF output as a function of motivational and homeostatic processes. The significance of these afferent systems can be studied by examining the behavioral consequences of infusion into the BF of drugs that act on the specific receptor systems. Although traditional analyses suggest that the BF has many behavioral functions that can be subdivided regionally, an analysis of studies employing transsynaptic approaches lead to the conceptualization of the BF as having a uniform function, that of maximizing cortical processing efficiency. The BF is conditionally active during specific episodes of acquisition and processing of behaviorally significant, externally-derived information, and drives cortical targets into a state of readiness by reducing interference and amplifying the processing of relevant stimuli and associations, thus allowing for more efficient processing. This paper describes the transsynaptic approach to studying BF function, reviews the neurobiological and behavioral consequences of altering neurotransmitter-specific inputs to the BF, and explores the functional significance of the BF.

Effects of novelty, pain and stress on hippocampal extracellular acetylcholine levels in male rats

In vivo microdialysis was used to assess the effects of Novelty, persistent pain (Formalin test) and stress (Restraint) on hippocampal acetylcholine (ACh) release. Experiments were carried out during the dark phase, i.e. during the active period of the animal, and consisted of four experimental phases: Baseline (30 min), Novelty (30 min), Formalin test (90 min) and Restraint (30 min); each animal was consecutively exposed to all phases. The extracellular levels of ACh in the dorsal hippocampus were estimated by measurement of its concentration in the perfusion fluid by high-performance liquid chromatography with electrochemical detection. The introduction to a new environment (Novelty) induced in all rats higher ACh levels than Baseline. Formalin treatment decreased ACh release only in animals considered 'Inactive' during the Novelty phase while no modification in ACh release was observed in the 'Active' ones. Restraint did not produce any modification of ACh release but increased Corticosterone plasma levels both in sham- and formalin-treated animals. Results indicate that Novelty, but not Formalin or Restraint, increases ACh release in the hippocampus and that the type of behavioral state displayed by the animal at the time of formalin injection determines the response of the septo-hippocampal cholinergic pathway.

Amphetamine sensitization enhances regional c-fos expression produced by conditioned fear

Chronically administered amphetamine can result in a paranoid psychosis that can be re-induced in former amphetamine abusers by psychological stressors. In an attempt to investigate the neurobiological correlates of this phenomenon, the present study examined the effects of prior D-amphetamine sensitization on regional c-fos expression induced by a psychological stressor. Rats received intermittent footshock in a distinctive environment for 30 min/day for three days. Three days after the last fear conditioning session, the animals received injections of saline or D-amphetamine (4 mg/kg, i.p.) once every second day for 16 days (eight injections in total). After a 14-day drug abstinent period, the animals were placed in the fear conditioning apparatus but without footshock. The amphetamine sensitization procedure significantly enhanced the effects of conditioned fear on c-fos expression in several brain regions. These included the cingulate cortex area 3, agranular insular cortex (layers 2 and 3), claustrum, piriform cortex, the shell region of the nucleus accumbens, medial striatum, ventral lateral septum, and CA3 and polymorphic layer of the hippocampal formation. These results indicate that D-amphetamine sensitization can have long-lasting effects on the neural circuitries activated by conditioned stressors.

Direct catecholaminergic-cholinergic interactions in the basal forebrain. I. Dopamine-beta-hydroxylase- and tyrosine hydroxylase input to cholinergic neurons

Immunocytochemical double-labeling techniques were used at the light and electron microscopic levels to investigate whether dopamine-beta-hydroxylase and tyrosine hydroxylase-containing axons contact basal forebrain cholinergic neurons. Dopamine-beta-hydroxylase- and tyrosine hydroxylase-positive fibers and terminals were found in close proximity to cholinergic neurons throughout extensive basal forebrain areas, including the vertical and horizontal limb of the diagonal band nuclei, the sublenticular substantia innominata, bed nucleus of the stria terminalis, ventral pallidum, and ventrolateral globus pallidus. Cholinergic cells in some aspects of the globus pallidus appeared to be contacted by tyrosine hydroxylase-positive but not dopamine-beta-hydroxylase-positive fibers, suggesting dopaminergic input to cholinergic neurons in these regions. Direct evidence for the termination of dopamine-beta-hydroxylase and tyrosine hydroxylase-positive fibers on cholinergic neurons was obtained in electron microscopic double-immunolabeling studies. Using high magnification light microscopic screening, both qualitative and quantitative differences were noted in the catecholaminergic innervation of forebrain cholinergic neurons. For example, while many cholinergic neurons were in close proximity to single dopamine-beta-hydroxylase-positive varicosities, others, particularly those located in the substantia innominatabed nucleus of the stria terminalis continuum, were apparently contacted by labeled fibers in repetitive fashion. The findings of the present study, together with our preliminary biochemical experiments (Zaborszky et al. [1993] Prog. Brain Res. 98:31-49) suggest that catecholaminergic afferents can differentially modulate forebrain cholinergic neurons. Such interactions may be important in learning and memory processes, and their perturbations may contribute to the cognitive decline seen in aging and in disorders such as Alzheimer's and Parkinson's diseases.

Oxotremorine infusions into the medial septal area of middle-aged rats affect spatial reference memory and ChAT activity

Age-related spatial memory deficits are correlated with septohippocampal cholinergic system degeneration. The present study examined the effect of intraseptal infusions of the cholinergic agonist, oxotremorine, on spatial reference memory in middle-aged rats using place discrimination in the water maze, and on cholinergic activity using choline acetyltransferase (ChAT) activity. Oxotremorine mildly improved the rate of place discrimination acquisition of middle-aged rats during initial sessions only, but did not affect asymptotic levels of performance achieved. Of the brain regions assayed, ChAT activity increased with age in the temporal cortex and dorsal CA2/3 region of the hippocampus. Oxotremorine significantly decreased ChAT activity in the dorsal hippocampus. In contrast to our previous results in aged rats indicating a more robust effect of oxotremorine on spatial working memory, the present results suggest a modest effect of intraseptal oxotremorine on the acquisition of a spatial reference memory task.

Inescapable stress enhances extracellular acetylcholine in the rat hippocampus and prefrontal cortex but not the nucleus accumbens or amygdala

A number of experimental results has pointed to a cholinergic involvement in the stress response. Recently, analytical techniques have become available to measure acetylcholine release in vivo during exposure to various stressors. In these experiments, microdialysis was used to monitor acetylcholine output every 15 min in the dorsal hippocampus, amygdala, nucleus accumbens and prefrontal cortex before, during and after 1 h of restraint, including a 15-min session of intermittent tail-shock (1/min, 1 mA, 1-s duration) in rats. In response to the stressful event, acetylcholine release was significantly increased in the prefrontal cortex (186%; p < 0.01) and hippocampus (168%; P < 0.01) but not in the amygdala or nucleus accumbens. The sole effects observed in the amygdala and nucleus accumbens occurred upon release from the restrainer, at which point acetylcholine levels were significantly elevated in both areas (amygdala: 150%; P < 0.05; nucleus accumbens: 13%; P < 0.05). An enhanced acetylcholine release was also evident during this sample period in the hippocampus and prefrontal cortex. These data demonstrate an enhancement of cholinergic activity in response to stress in two acetylcholine projection systems (hippocampus and prefrontal cortex) but not in the intrinsic acetylcholine system of the nucleus accumbens or the extrinsic innervation of the amygdala. Moreover, the data showed that relief from stress was accompanied by a more ubiquitous acetylcholine response that extended to each site tested.

Alterations of beta-endorphin-like immunoreactivity in CSF following behavioral training using a passive avoidance procedure

The central opioid system may have an important influence on memory processes. In view of this, the concentration of beta-endorphin-like immunoreactivity (beta-ELIR) in cerebrospinal fluid (CSF) was measured by a radioimmunoassay in rats trained in a passive avoidance procedure. The beta-ELIR in CSF was examined immediately, 2, 5, 10, and 30 min after the learning trial in which rats were exposed to footshock (0, 0.25, or 1.0 mA for 3 s). Avoidance latency and beta-ELIR in CSF were examined 24 and 120 h after the learning trial. The beta-ELIR in CSF was increased at 5 min after the learning trial in rats exposed to footshock of 0.25 mA. The beta-ELIR in CSF was elevated at 5 and 10 min, followed by a significant decrease at 30 min after the learning trial in rats exposed to a footshock of 1.0 mA. Thus, although an increase in beta-ELIR in CSF was not, the duration of the increase was, related to the shock intensity. Interestingly, a decrease followed the increase in beta-ELIR in CSF which was significant only in rats exposed to the high shock intensity. Avoidance latencies were enhanced in a shock intensity-dependent manner at both 24 and 120 h retention tests. No change in beta-ELIR in CSF was found during retention trials. The results suggest that behavioral manipulations alter beta-ELIR in CSF. An increase in beta-ELIR in CSF may be highly associated with stressful and emotional responses during behavioral training.

Electrophysiological actions of GABAB agonists and antagonists in rat dorso-lateral septal neurones in vitro

1. The actions of GABAB-receptor agonists and antagonists on rat dorso-lateral septal neurones in vitro were recorded with intracellular microelectrodes. 2. In the presence of 1 microM tetrodotoxin to prevent indirect neuronal effects caused by action potential-dependent neurotransmitter release, bath application of baclofen (0.1-30 microM) or SK&F 97541 (0.01-3 microM) evoked concentration-dependent hyperpolarizations which reversed close to the potassium equilibrium potential; the EC50S were 0.55 and 0.05 microM, respectively. No significant desensitization was observed during prolonged agonist exposure (< or = 10 min). 3. Hyperpolarizations induced by baclofen were antagonized in a competitive manner by the following GABAB-receptors antagonists (calculated pA2 values in parentheses): CGP 36742 (4.0), 2-OH saclofen (4.2), CGP 35348 (4.5), CGP 52432 (6.7) and CGP 55845A (8.3). Responses to SK&F 97541 were also antagonized by CGP 55845A (pA2 = 8.4). 4. The amplitude of the late, GABAB receptor-mediated inhibitory postsynaptic potential (i.p.s.p.) was reduced by the GABAB antagonists as follows (means +/- s.e.mean): CGP 55845A (1 microM) 91 +/- 5%, CGP 52432 (1 microM) 64 +/- 5%, CGP 35348 (100 microM) 82 +/- 5%, CGP 36742 (100 microM) 76 +/- 8%, and 2-OH saclofen (100 microM) 68 +/- 3%. 5. It is concluded that neurones in the rat dorso-lateral septal nucleus express conventional GABAB receptors, which are involved in the generation of slow inhibitory postsynaptic potentials. CGP 55845A is the most potent GABAB receptor antagonist described in this brain area.

Chronic morphine increases hippocampal acetylcholine release: possible relevance in drug dependence

Previous studies have shown that cocaine and amphetamine stimulate acetylcholine release in the hippocampus via an action of endogenously released dopamine on dopamine D1 and D2 receptors. The present study was aimed at clarifying if the property of stimulating hippocampal acetylcholine release was shared by morphine. The acute administration of morphine (10 mg/kg i.p.) failed to modify acetylcholine release in the hippocampus. However, after repeated administration (10 mg/kg i.p. twice daily) morphine acquired the ability to stimulate hippocampal acetylcholine release. Thus, at days 5 and 7 of chronic morphine treatment, a challenge dose of morphine (10 mg/kg i.p.) increased acetylcholine release by 50 and 100%, respectively. Concomitantly with the development of the stimulant property on acetylcholine release, morphine also acquired that of producing behavioural stimulation and lost that of producing sedation and catalepsy. The morphine-induced increase in acetylcholine output was suppressed by the blockade of dopamine D1 receptors with SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine) (0.1 mg/kg s.c.), which also suppressed the morphine-induced motor stimulation. Moreover, repeated morphine administration markedly potentiated the stimulant effect of the dopamine D1/D2 receptor agonist apomorphine (R(-)-10, 11-dihydroxyaporphine) (0.1 or 0.5 mg/kg s.c.) both on hippocampal acetylcholine release and on behaviour. These results may suggest that the enhancement of hippocampal acetylcholine release as well as the development of behavioural sensitisation after chronic morphine could be related to the development of dopamine receptor supersensitivity. Moreover, increased acetylcholine transmission in the hippocampus may play a role in the 'memory' of the rewarding effects of drugs of abuse.

Alpha 2-adrenoceptor antagonists potentiate acetylcholinesterase inhibitor effects on passive avoidance learning in the rat

The cholinergic hypothesis of Alzheimer's disease (AD) has strongly influenced research on learning and memory over the last decade. However, there has been limited success treating AD dementia with cholinomimetics. Furthermore, there are indications that other neurotransmitter systems affected by this disease may be involved in cognitive processes. Animal studies have suggested that norepinephrine and acetylcholine may interact in learning and memory. The current experiments investigate this interaction in a step-down passive avoidance paradigm after coadministration of acetylcholinesterase inhibitors and alpha 2-adrenoceptor antagonists. Administration of acetylcholinesterase inhibitors heptylphysostigmine (0.625-5.0 mg/kg, IP), tacrine (2.5-10.0 mg/kg, PO), velnacrine (0.312-2.5 mg/kg, SC), and galanthamine (0.312-2.5 mg/kg IP) each enhanced retention of a passive avoidance response at selected moderate doses administered 30-60 min prior to training. The alpha 2-adrenoceptor antagonists idazoxan (0.312-2.5 mg/kg, IP), yohimbine (0.078-0.312 mg/kg, IP) and P86 7480 (0.156-0.625 mg/kg, IP) alone failed to enhance learning in this paradigm. Coadministration of a subthreshold dose of heptylphysostigmine (0.625 mg/kg, IP) with doses of idazoxan, yohimbine or P86 7480 enhanced passive avoidance learning. This synergistic interaction may represent effects of antagonism of presynaptic alpha 2-adrenoceptor since coadministration of heptylphysostigmine and the selective postsynaptic alpha 2-adrenoceptor antagonist SKF 104856 did not result in enhanced learning. Taken together these data suggest noradrenergic activation through pre-synaptic alpha 2-adrenoceptor blockade may potentiate cholinergic activity in the formation of a long-term memory trace. These observations may have implications for the treatment of AD with cholinergic and adrenergic agents.

Medial septal injection of naloxone elevates acetylcholine release in the hippocampus and induces behavioral seizures in rats

The effects of injections of naloxone, a universal opioid receptor antagonist, into the medial septal nucleus on hippocampal acetylcholine (ACh) release and behavior were investigated in freely moving rats by means of the microdialysis method. The injection of naloxone (2, 10 and 20 micrograms) produced a marked increase in hippocampal ACh release in a dose-dependent manner. These effects of naloxone were reversed by the post-injection of [D-Ala2, N-Me-Phe4, Gly-ol]-enkephalin (DAGO; 10 micrograms), an opioid mu receptor agonist. Furthermore, basal release of hippocampal ACh was significantly reduced by the injection of DAGO alone. It was also found that rats given an injection of naloxone showed an increase in motor activity and occasionally exhibited behavioral seizures. These effects of naloxone were also reversed by the post-injection of DAGO. The present results suggest that endogenous opioids ionically inhibit the activity of septo-hippocampal cholinergic neurons via mediation of mu opioid receptors in the medial septal nucleus. They also suggest that endogenous opioids modulate the incidence of seizures, at least in part, through opioid mu receptors in the medial septal nucleus.

Consequences of selective blockade of septal noradrenergic afferents on anxiety and spatial working memory performance in mice

This experiment was designed to investigate the role of septal noradrenergic (NA) afferents in the control of anxiety and spatial working memory. To this end, C57Bl/6 mice were infused bilaterally into the lateral septal nuclei with 500 ng/0.2 microliter of BE 2254, a selective alpha 1 postsynaptic adrenoceptor antagonist. The consequences of this reversible treatment were evaluated 20 min later on the anxiety level measured in an elevated plus-maze and on spatial working memory, evaluated under four different conditions via the learning of a delayed nonmatching to place (DNMTP) rule achieved in an eight-arm radial maze. In these conditions, the BE 2254, as well as the saline-injected control group, showed an elevation of the anxiety level that may be the indirect expression of a nonspecific septal dysfunction induced by the vehicle injection rather than the normal behavioral response produced by the decrease of septal NA activity. This septal dysfunction also impaired spatial working memory but only when mnesic difficulty of the task is increased, suggesting that this impairment expresses a general memory deficit rather than a working memory deficit per se. A lack of spatial working memory deficits in BE 2254 or saline-injected animals was also observed in two other conditions of the behavioral protocol. However, when treatments were applied before the first exposure of animals to the radial maze (exploration session), only the group which received BE 2254 was impaired during the acquisition session for the rule performed 24 h later. This delayed perturbation seems to be linked, at this stage of the learning procedure, to the lack of NA-dependent processes taking place during the exploration session. Taken together, these data suggest that septal NA mechanisms are more essential at initial stage of this learning, when animals process new features of the situation, than during the expression of spatial working memory per se.

Age-related changes in hippocampal drug facilitation of memory processing in SAMP8 mice

SAMP8/TaJf(P8) mouse strain has an inherited age-related impairment of learning and memory, while age-matched subjects of the closely related SAMR1/TaJf(R1) show no impairment. After training on footshock avoidance, P8 and R1 received a drug injection into the hippocampus. Retention was tested 1 week later. The results indicate that bicuculline (GABA antagonist), SKF38393 (DA agonist), and ST587 (NE agonist) facilitated retention with little change in the dose-response curves for P8 mice 4, 8, and 12 months of age. L-glutamate, acting at the NMDA receptor, showed a modest decline in ability to improve retention with increasing age. Arecoline, a muscarinic agonist, had the strongest trend for an age-related decline in potency. The same drug treatments yielded dose-dependent facilitation of retention but no age-related changes in R1 mice. Reduced cholinergic activity in the hippocampus may be, in part, responsible for age-related decline in memory retention in P8 mice.