The transsynaptic regulation of the septal-hippocampal cholinergic neurons

Dopaminergic innervation of the telencephalon of the pigeon (Columba livia): a study with antibodies against tyrosine hydroxylase and dopamine

The dopaminergic structures in the telencephalon of the pigeon were investigated with antisera against glutaraldehyde-conjugated dopamine (DA) and tyrosine hydroxylase (TH). Our goal was to describe the morphological patterns of the labelled axons and to provide a detailed map of the density and regional distribution of the dopaminergic innervation in relation to cytoarchitectonic areas. DA- and TH-like fibers reached their highest density in the paleostriatum augmentatum and the lobus parolfactorius of the basal ganglia. The paleostriatum primitivum was characterized by a dichotomous DA-positive innervation with a diffuse fiber network contacting enpassant granular cells and a more specific input that completely wrapped up large cells, which probably represent relay neurons. Two distinct DA-positive pathways could be followed back from the forebrain leading to the dopaminergic cell groups of the nucleus tegmenti pedunuculopontinus pars lateralis and the area ventralis tegmentalis. The primary sensory areas of the visual, auditory, somatosensory, and trigeminal systems within the forebrain of the pigeon were virtually devoid of DA-like fibers and demonstrated only TH-positive axons, probably of a noradrenergic nature. Among the limbic structures, the neostriatum caudolaterale (a possible equivalent of the mammalian prefrontal cortex), the septum, the nucleus accumbens, and parts of the archistriatum were heavily labelled by DA-like axons. A highly characteristic morphological feature of the catecholaminergic innervation was the presence of "baskets," which are constituted by TH- and DA-positive fibers coiled up around large perikarya, so that the surrounded somata were virtually visible by the presence of labelled axons. The density of basket and nonbasket type innervations seemed to be independently regulated, so that each forebrain structure could be characterized by a mixture consisting of the individual degrees of these two features. Our results demonstrate that the dopaminergic innervation of the forebrain of the pigeon is widespread but shows important regional variations. Similar to mammals, associative and motor structures are heavily innervated by dopaminergic fibers, whereas sensory areas are dominated by their noradrenergic input. The basket and nonbasket type innervations observed in virtually all of these subdivisions of the telencephalon may indicate the presence of two main classes of catecholaminergic afferents with different mechanisms of modulation of forebrain activity patterns.

Septo-hippocampal cholinergic system under the discrimination learning task in the rat: a microdialysis study with the dual-probe approach

To investigate regulation of the septo-hippocampal cholinergic system by dopaminergic inputs to the septum in rats which performed a discrimination learning task, an in vivo microdialysis method with the dual-probe approach was used. Rats were trained to discriminate between lamp-on and -off states under an operant-type learning procedure. After stable discriminative behavior was established, dialysis probes were implanted into the hippocampus and the lateral septum area of each rat. The concentration of dopamine (DA) in the septum rapidly increased within 20 min after the beginning of a learning session. However, another group of rats trained on a similar but non-discriminative task showed no such increase. The concentration of acetylcholine (ACh) in the hippocampus was significantly enhanced during the learning session and rapidly returned to the basal value after the session, but showed a delayed and diminished increase in the non-discrimination group. These results suggest that DAergic inputs to the septum may be involved in control of the septo-hippocampal cholinergic system which is of importance for discrimination learning behavior.

Septohippocampal neurons in the rat septal complex have substantial glial coverage and receive direct contacts from noradrenaline terminals

The ultrastructure of septohippocampal neurons in the septal complex and their relations with catecholamine, principally noradrenaline, terminals were examined in single thin sections. Projection neurons were identified by retrograde transport of wheat-germ agglutinated apo-horseradish peroxidase conjugated to colloidal gold particles (WAHG) following an injection into the hippocampal formation of anesthetized adult rats. After a 1 day survival, sections through the septal complex were labeled with antibodies to tyrosine hydroxylase (TH) or dopamine-beta-hydroxylase (DBH). By light microscopy, numerous processes with TH- and DBH-immunoreactivity were near neurons containing retrogradely transported WAHG. By electron microscopy, most WAHG was associated with lysosomes, multivesicular and 'sequestration' bodies in the cytoplasm of perikarya and large dendrites. WAHG-labeled perikarya (n = 114) had a large amount of astrocytic coverage (> 60% of surface) and a low amount of terminal coverage (< 25%). WAHG-labeled perikarya and dendrites were either directly contacted by TH- or DBH-labeled terminals or abutted glial processes apposed to TH- or DBH-labeled terminals. Immunoreactivity for TH and DBH was found primarily in axons and axon terminals. The morphology and synaptic associations of TH-labeled terminals was similar to that reported previously. DBH-labeled terminals (n = 314; 0.5 +/- 0.2 microns in diameter) contained numerous small clear vesicles and from 0-4 large, dense-core vesicles. DBH-containing terminals: (1) contacted perikarya and dendrites (58%), 10% of which contained WAHG; (2) were closely apposed to other terminals (7%); or (3) were separated by glial processes (35%). DBH-labeled terminals formed chiefly symmetric synapses on perikarya. However, most DBH-containing terminals formed both asymmetric and symmetric synapses on the shafts of small dendrites, suggesting both excitatory and inhibitory functions for noradrenaline terminals on septal neurons. The results demonstrate that septohippocampal neurons (1) are mostly engulfed by astrocytes and have very little terminal coverage; (2) are both directly contacted (synapses) and indirectly contacted (appositions to apposing astrocytes or axon terminals) by catecholamine, particularly noradrenaline, terminals.

Effects of intraseptally injected glutamatergic drugs on hippocampal sodium-dependent high-affinity choline uptake in "naive" and "trained" mice

We have previously reported that spatial reference memory (RM) training-induced alterations in hippocampal cholinergic activity as measured by sodium-dependent high-affinity choline uptake (SDHACU). Each training session was found to induce an immediate (30 s) increase in SDHACU followed (30 s to 15 min posttest) by a deactivation and long-lasting inhibition (15 min to 24 h) of this cholinergic marker. The present experiments were designed to assess the role of septal glutamatergic receptors in this posttraining cholinergic deactivation. In the first experiment, the effects of intraseptal injections of different doses of glutamic acid and glutamatergic antagonists (kynurenic acid, KYN, and AP5) on hippocampal SDHACU were studied in awake but otherwise resting (i.e., naive) mice. The results showed that glutamic acid at the lowest dose used (5 ng) produced a decrease in SDHACU, whereas both glutamatergic antagonists produced a dose-related increase in this cholinergic marker. It was concluded that septal glutamatergic receptors mediate a tonic inhibitory input on the cholinergic cells. Hence, in a second experiment the effect of intraseptal injections of KYN (5 ng) on the training-induced changes in hippocampal cholinergic activity were assessed following variable amounts of radial maze RM training. Trained mice were injected 20 min before the first or the ninth training session and killed 30 s or 15 min posttraining for determination of SDHACU. KYN slowed the posttesting cholinergic deactivation (disinhibition), this effect being more marked in good learners than in bad learners. The present findings suggest that septal glutamatergic receptors mediate an inhibitory input on the cholinergic cells, and that this input could play a role in memory consolidation.

Are the cognitive-enhancing effects of nicotine in the rat with lesions to the forebrain cholinergic projection system mediated by an interaction with the noradrenergic system?

Experiments were conducted to test the hypothesis that the enhancing effect of nicotine on water maze performance in rats with lesions of the forebrain cholinergic projection systems (FCPS) is mediated by an interaction with the noradrenergic system, in particular the ascending dorsal noradrenergic bundle (DNAB) and its projection areas. Three groups of rats received lesions of either: i) the nucleus basalis (NBM) and medial septal area/diagonal band (MSA/DB) by infusion of alpha-amino-3-hydroxy-4-izoxazole propionic acid (AMPA) (FCPS group), ii) DNAB, by infusion of 6-hydroxydopamine (6-OHDA) (NOR group), or iii) both FCPS plus DNAB (COMB group). Control animals received vehicle. Choline acetyltransferase activity was reduced in the cortex and hippocampus of the FCPS and COMB groups and in the hippocampus of the NOR group. NA level was reduced in the cortex and hippocampus of the FCPS and COMB groups, but not the FCPS group. In a reference memory task, the performance of both the NOR and COMB groups, but not the NOR group, was significantly worse than that of controls; there was no effect of nicotine administration (0.1 mg/kg) on escape latency or other measures in this task. In a working memory task, FCPS and COMB rats took longer to find the submerged platform on the second and following trials, and there was a significant enhancement of performance by nicotine in both groups, but not in controls. These results indicate that the enhancing effects of nicotine in rats with FCPS lesions are not mediated by an interaction with the DNAB.

Effects of intraseptally injected noradrenergic drugs on hippocampal sodium-dependent-high-affinity-choline-uptake in 'resting' and 'trained' mice

It has been shown in numerous studies that memory testing can alter presynaptic cholinergic activity within the hippocampus. In the present experiments, the role of the noradrenergic input to the septal cholinergic neurons in the immediate increase in cholinergic activity induced by the first training session of a spatial reference memory task in an 8-arm radial maze was investigated. The effects of bilateral intraseptal injections of noradrenergic drugs on hippocampal sodium-dependent-high-affinity-choline-uptake (SDHACU) were studied in 'resting' animals (basal level) or in 'trained' animals injected 20 min before training and sacrificed immediately after the test. The results showed that: (1) the injection of maprotiline, a noradrenaline reuptake inhibitor (0.06 ng/site), induced an increase in hippocampal SDHACU in 'resting' animals, whereas the alpha 2-adrenoceptor agonist UK 14304 (1.5 ng) significantly reduced the basal level of SDHACU; (2) none of the alpha-adrenoceptor antagonists used (phenoxybenzamine, 10 and 100 ng; BE 2254, 100 and 500 ng; yohimbine, 0.5 and 50 ng) significantly affected the basal level of hippocampal SDHACU, and only the alpha 1-adrenoceptor antagonist BE 2254 (500 ng) significantly reduced the testing-induced activation of SDHACU. Taken together, these findings suggest that noradrenaline may exert a bimodal regulatory influence on the activity of septo-hippocampal cholinergic neurons. The behavior-induced activation of hippocampal SDHACU could be partly mediated by the stimulation of alpha 1-adrenoceptors, whereas postsynaptic alpha 2-adrenoceptors may be important for the maintenance of a tonic inhibition of the steady-state cholinergic activity in the hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel approach for studying septo-hippocampal cholinergic neurons in freely moving rats: a microdialysis study with dual-probe design

In this study, the overflow of acetylcholine (ACh) in the septo-hippocampal system was studied using intracerebral microdialysis in freely moving rats. Dialysis probes were implanted in the ventral hippocampus and in the medial septal area (MS), including a part of the ventral limb of the diagonal band of Broca (VDB). Dialysis samples were analysed 'on-line' using HPLC with post column enzymatic conversion and electrochemical detection. Local perfusion of 1 mumol/l of the sodium-channel blocker tetrodotoxin (TTX) through the probe resulted in 94% and 92% decrease in extracellular levels of ACh in the hippocampus and the septal area, respectively. The effects of septal manipulation on the efflux of ACh in the hippocampus were studied by electrical stimulation of the septal area and by administering drugs via the septal probe. Electrical stimulation of the MS/VDB caused a 336% increase in the output of ACh in the hippocampus. Perfusion of 3 mumol/l TTX through the septal probe caused a maximal decrease of 56% in the output of ACh in the ventral hippocampus. When perfused in the MS/VDB, the excitatory amino-acid agonists N-methyl-D-aspartate (NMDA) (100 mumol/l) and kainic acid (10 mumol/l) caused an increase in the extracellular level of ACh in the hippocampus by 83% and 161%, respectively. Thus, the overflow of ACh in the hippocampus and the septal area both depend on neuronal impulse flow. The extracellular level of ACh in the hippocampus is at least partially dependent on impulse flow in septo-hippocampal fibres. Moreover, the output of ACh in the hippocampus can be manipulated by electrical and pharmacological stimulation of the MS/VDB.

Cognitive functions of cortical ACh: lessons from studies on trans-synaptic modulation of activated efflux

Trans-synaptic modulation of cortical ACh efflux is a useful approach for determining the functions of cortical ACh. Bilateral modulation of basal forebrain GABAergic transmission by benzodiazepine-receptor agonists and inverse agonists decreases and increases, respectively, activated cortical ACh efflux. The determination of behavioral functions which are mediated via activated cortical ACh efflux, and therefore subject to the effects of basal forebrain GABA-cholinergic manipulations, should promote analyses of the functions of cortical ACh. Trans-synaptic approaches to enhance activated cortical ACh efflux offer some potential for the treatment of cognitive dysfunctions associated with impaired cortical cholinergic transmission.

The benzodiazepine receptor antagonist flumazenil increases acetylcholine release in rat hippocampus

The benzodiazepine receptor antagonist flumazenil (2.5-20 mg/kg i.p.) increased acetylcholine (ACh) release by up to 85% in the hippocampus of freely moving rats. In contrast, the benzodiazepine receptor full agonist diazepam (2.5-10 mg/kg i.p.) decreased ACh release up to a maximum of 45% in the same brain area. Injection of flumazenil (10 pmol) or diazepam (10 pmol) into the medial septum increased (95%) or reduced (50%), respectively, ACh release in the hippocampus. The maximum effect produced by those drugs was of the same magnitude as that observed after systemic injection. The changes in hippocampal cholinergic function elicited by activation and blockade of benzodiazepine receptors in the medial septum may thus play a crucial role in the alterations of the cognitive processes elicited by benzodiazepine receptor ligands.

Stimulation of GABAB receptors in the basal forebrain selectively impairs working memory of rats in the double Y-maze

The present experiments were conducted to evaluate the possible contribution of GABAergic inputs to the basal forebrain in the region of the nucleus basalis magnocellularis (nbm) to memory. In two experiments, rats implanted with bilateral intra-nbm guide cannulae were trained in the double Y-maze task to perform working- and reference-memory components. Animals were placed in one of two start arms of the first "Y" and the reference-memory component required travelling to its central stem for food. Access to the second "Y" then was given and the working-memory component for Expt. 1 required travelling to the goal arm diagonally opposite the start arm in the first "Y" of that trial. In Expt. 2, the working-memory component required travelling to the goal arm opposite to the goal arm entered in the second "Y" on the preceding trial, with 0- and 15-s delays between trials. In Expt. 1, pretrained rats (n = 8) received the GABAA agonist, muscimol (0.1 microgram in 0.5 microliter), the GABAB agonist, R(+)-baclofen (0.01, 0.05 and 0.1 microgram), and its less active enantiomer, S(-)-baclofen (0.1 microgram), in a counterbalanced order with retraining to criterion between injections. In Expt. 2, pretrained rats (n = 9) received saline (0.5 microliter), R(+)-baclofen (0.1 microgram), the GABAB antagonist, phaclofen (1 microgram), and R(+)-baclofen+phaclofen. Results of Expt. 1 revealed that intra-nbm muscimol and, in a dose-dependent manner, R(+)-baclofen differentially affected working but not reference memory. In Expt. 2, the differential mnemonic impairment produced by R(+)-baclofen was replicated and co-injection with phaclofen reversed this effect. A 15-s delay between trials significantly impaired working but not reference memory. Results suggest that both GABAA and GABAB receptors may be involved in modulating the possible mnemonic functions of nbm cholinergic neurons.

Baclofen produces dose-related working memory impairments after intraseptal injection

Altering the activity of the septohippocampal pathway can impair spatial memory in rats. Pharmacological manipulation of septal GABA-A receptors with the agonist, muscimol, or the benzodiazepine agonist, chlordiazepoxide, also impairs spatial memory and depresses hippocampal cholinergic activity. The present experiment examined the effects of intraseptal infusion of the GABA-B agonist baclofen on the performance of rats on a working memory radial arm maze (RAM) task. Post-training administration of baclofen (3 nmol, but not 1.5 or 0.75 nmol) produced a significant impairment of RAM performance. Baclofen significantly reduced the number of correct choices and increased the number of errors committed during testing without affecting latency per arm choice or the ability of the rats to navigate the maze and consume food pellets. The data suggest that baclofen impaired retention of the task without producing proactive performance deficits. Furthermore, the present data are consistent with the hypothesis that a GABAergic mechanism in the medial septum modulates the maintenance or retrieval of spatial working memory.

Differential modulation of dopaminergic systems in the rat brain by dietary protein

Rats that consume a diet 50% rich in protein exhibit hyperactivity and hyperresponsiveness to nociceptive stimuli, in which facilitation of dopaminergic activity has been implicated. We studied the regional changes in the concentrations of dopamine (DA) and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brains of rats that were maintained on high-protein (HP, 50% casein), normal-protein (NP, 20% casein), and low-protein (LP, 8% casein) diets for 36 weeks. Brain nuclei that represented different DAergic systems were punch-dissected and analyzed using HPLC. In the substantia nigra, the striatum, and the dentate gyrus, DA concentrations decreased and increased, respectively, with a decrease and increase in dietary protein (p < 0.05 compared to the NP diet). Similar trends in the effect of the HP diet were observed in the ventral tegmental area, amygdala, frontal cortex, subiculum, centromedial nucleus (CM) of the thalamus, and inferior colliculi (IC), although the differences in DA concentrations were not statistically significant. These brain areas also showed a pattern of decreased DA concentration in association with the LP diet, and the differences were statistically significant (p < 0.05) in the CM and IC. DA concentrations in most regions of the midbrain and brainstem were not different between the diet groups, nor were consistent trends observed in those regions. Also, there were no consistent relationships between DOPAC/DA and HVA/DA ratios and dietary protein level. These data suggest that only discrete dopaminergic neuronal circuits in the rat forebrain were sensitive to changes in dietary protein level.

Reversal of early phenobarbital-induced cholinergic and related behavioral deficits by neuronal grafting

The present experiment was performed to assess the possible restoration of normal maze behavior, as well as parallel muscarinic receptor binding capabilities, in mice pre- or neonatally exposed to phenobarbital. Mice were exposed to phenobarbital prenatally by feeding the mother phenobarbital (3 gkg milled food) on gestation days 9-18 (PreB mice), or neonatally, by daily injections of 50 mg/kg Na phenobarbital to the pups on days 2-21 (NeoB). At adulthood, PreB and NeoB mice were 61.3% and 65% deficient, respectively, in the hippocampus-related Morris maze behavior, as compared to control. Both groups had a 58% increase in their hippocampal muscarinic receptors maximal binding (Bmax) (p < 0.001); the dissociation constant (Kd) was not affected by the phenobarbital exposure. Treated animals and their respective controls received septal cholinergic embryonic graft into the hippocampus. The viability of the transplants was confirmed by AChE histochemistry. Nine weeks later the grafted mice showed significant improvement in the Morris maze (52% for both PreB and NeoB (p < 0.001)). Their Bmax was also reduced from early phenobarbital exposed animals' levels by 15% for PreB and by 25% for NeoB (p < 0.001). The results suggest that early phenobarbital-induced behavioral deficit and their related biochemical alterations can be partially corrected by the appropriate neural grafting, and thus provide further support to the apparent relationship between the early phenobarbital-induced septohippocampal cholinergic alterations and the hippocampus-related behavioral deficits.

Basal forebrain grafts in the hippocampus and neocortex: regulation of acetylcholine release

The regulation of acetylcholine (ACh) release from cholinergic neurons transplanted to the hippocampus or neocortex was studied by microdialysis in awake rats. Fetal basal forebrain tissue was implanted as a cell suspension or solid graft into the fimbria-fornix-lesioned hippocampus, or as a cell suspension into the frontal cortex after excitotoxic lesion of the nucleus basalis. Several months after transplantation, microdialysis probes were implanted in areas of the hippocampus or frontal cortex reinnervated by the grafts. The grafts restored lesion-induced deficits in steady-state ACh release up to normal or above normal levels in both hippocampus and frontal cortex. The responses to KCl and tetrodotoxin suggested that the ACh release exhibited normal firing-dependent properties. By applying various behaviorally arousing stimuli that normally activate the basal forebrain projection systems, we wished to investigate the functional integration of the grafts in the host brain. In the hippocampus, sensory stimulation, immobilization stress and motor activity all resulted in increased release of graft-derived ACh amounting to 25-65% of the normal response. Variations in ACh levels during the day-night cycle was, however, not observed in the grafted rats. In the frontal cortex, immobilization enhanced the graft-derived ACh release (60% of normal response), whereas the response to sensory stimulation did not reach significance. Since the activity of the normal basal forebrain projection systems is under influence of monoaminergic brainstem afferents, we investigated the effects of systemic administration of amphetamine or apomorphine on ACh release in the hippocampus. Both drugs produced increases in graft-derived ACh release although the response was variable and less pronounced than normal. In conclusion, the graft-derived ACh release was affected by behavioral manipulations and catecholaminergic drugs that normally modify cholinergic septo-hippocampal and basalo-cortical activity. This strongly suggests a high degree of functional integration of the graft in the host brain allowing for a regulated release of transmitter that can be adjusted during ongoing behavior.

A comparative study of age-related changes in inhibitory processes and long-term potentiation in the lateral septum of mice

Anaesthetized C57 BL/6 mice of different ages (young: 5 months; middle-aged: 15 months; and old: 21 months) were used to determine whether aging alters the efficiency of synaptic inhibition and long-term potentiation (LTP) in the lateral septum (LS). Electrical stimulation of the fimbria induced field potentials in the ipsilateral LS comprising two initial negative components (N2 and N3) followed by a positive wave of low amplitude. Paired-pulse experiments showed a facilitation of the N2 component and a concomitant depression of the N3 components. Facilitation of the N2 component was stronger in both middle-aged and old mice as compared to young mice, whereas an inverse pattern of changes was observed for inhibition of the N3 component. High-frequency stimulation of the fimbria produced a persistent increase in the N3 amplitude. This LTP was of significantly higher amplitude in both young and middle-aged mice as compared to old mice. These results suggest that aging impairs both inhibitory processes and synaptic plasticity in the mouse LS, but that inhibitory processes appear to be affected earlier.

Functional activity of intrahippocampal septal grafts is regulated by catecholaminergic host afferents as studied by microdialysis of acetylcholine

Previous microdialysis experiments have shown that acetylcholine (ACh) release from septal grafts in the hippocampus of awake rats is influenced by the behaviour of the animals, which strongly suggests that the host brain can exert a regulatory control over the activity of the grafted neurons. Since the activity of the normal septo-hippocampal cholinergic system is likely to be regulated, in part, by brainstem catecholaminergic afferents, we wished to study the effect of catecholaminergic drugs on ACh release in the hippocampus reinnervated by septal grafts. Rats were subjected to a unilateral aspirative fimbria-fornix (FF) transection and grafted with tissue from the fetal septal-diagonal band area, either as a cell suspension injection into the depth of the hippocampus or as a solid implant in the FF lesion cavity. Microdialysis of ACh release was carried out 17-20 months after transplantation in awake, freely-moving animals. The reduction in steady-state ACh overflow induced by the FF lesion (-81%) was restored to normal or above normal levels in rats with either solid or suspension grafts. In normal rats, systemic administration of apomorphine (2.0 mg/kg, s.c.) or amphetamine (2.5 mg/kg, i.p.) caused a 3.7 (+189%) or 7.8 (+301%) pmol/15 min increase in ACh overflow compared to the previous baseline level, respectively. The drug-induced increases in ACh levels in the FF-lesioned controls was substantially lower than normal (86-89% reduction). Both apomorphine and amphetamine resulted in an approximately two-fold increase in hippocampal ACh release in rats with suspension grafts. These responses were significantly increased over those seen in rats with FF lesions only, but they tended to be lower and more variable than normal. Rats with solid septal grafts responded significantly stronger than FF lesion controls to amphetamine with two-fold increased ACh overflow, whereas the response to apomorphine was less clear-cut. Pretreatment with the catecholamine synthesis blocker alpha-methyl-p-tyrosine (AMPT; 200 mg/kg x 3) did not affect steady-state or apomorphine-stimulated release of ACh in any of the groups, whereas the effect of amphetamine was abolished in both normal and grafted rats. The results suggest that ACh release derived from septal grafts in the hippocampus, similar to the normal septo-hippocampal system, can be affected by manipulations of the host catecholaminergic systems. This mechanism may, at least in part, underlie the ability of the host brain to influence and control the activity of grafted cholinergic neurons.

Basal forebrain grafts in the rat neocortex restore in vivo acetylcholine release and respond to behavioural activation

Acetylcholine release in the frontal cortex of awake rats after acute or chronic lesions of the nucleus basalis magnocellularis and grafting of cholinergic-rich basal forebrain tissue was studied by in vivo microdialysis. Three to four weeks and five months after a unilateral quisqualic acid lesion of the nucleus basalis, and five months after lesion and cortical implantation of a basal forebrain cell suspension, acetylcholine release was characterized during a range of pharmacological and behavioural manipulations. Neostigmine (5 microM) was added to the perfusion fluid in order to inhibit the degradation of acetylcholine. The extracellular levels of acetylcholine in normal animals increased three- to four-fold when KCl (100 mM) was added to the perfusion medium and was reduced by 80% after addition of tetrodotoxin (1 microM). The nucleus basalis lesion resulted in a 60% reduction in baseline acetylcholine levels compared to normal and the response to KCl-evoked depolarization was significantly reduced. There were no differences between the acute and chronic lesion groups during any of the manipulations performed. Rats with grafts showed baseline levels of acetylcholine about 70% higher than normal, and responded to both KCl (two-fold increased acetylcholine release) and tetrodotoxin (85% reduced levels). All groups showed lower acetylcholine levels during halothane anaesthesia (on average 70-85% reduction). Sensory stimulation by handling resulted in a two-fold increase in acetylcholine release in normal animals, whereas the absolute responses in the lesioned controls were significantly weaker. Rats with grafts increased their acetylcholine release after handling to an extent not different to normal or lesioned controls. Immobilization stress induced an almost two-fold increase in cortical acetylcholine levels in normal rats, whereas the effect in the lesion-only groups was very weak. The grafts responded to the immobilization with an enhanced acetylcholine overflow that was significantly higher than in lesioned controls. The results showed that the reduction in frontocortical acetylcholine release induced by excitotoxic lesions of the nucleus basalis did not recover spontaneously over several months. Intracortical cholinergic-rich grafts obtained from the fetal basal forebrain provided a source of acetylcholine release with firing-dependent properties which could be modulated by behaviourally stressful stimuli. The ability of the grafts to respond to behavioural manipulation strongly suggests that the host brain can functionally influence graft neuronal activity during ongoing behaviour. Host control of graft activity may play a role in the recovery of the lesion-induced deficits seen with these types of grafts.

Hippocampal cholinergic alterations and related behavioral deficits after early exposure to ethanol

The present study was designed to ascertain septohippocampal cholinergic alterations and their related behavioral deficits after early exposure to ethanol. Mouse pups were exposed to ethanol, 3 g/kg by daily subcutaneous injection on postnatal days 2-14. At age 50 days, the ethanol-exposed mice had significant reductions from control levels in eight-arm maze performance. For example, on the fourth testing day, the number of correct entries in the ethanol group was 21% below control levels (P < 0.05) and the number of trials needed to enter all arms was 48% above control (P < 0.001). It took the ethanol-exposed mice twice the time to reach criterion than it did control (P < 0.01). A 33% increase from control level in muscarinic receptor number (Bmax) was found in the treated mice of age 22 days and a 64% increase at age 50 days (P < 0.001). However, no differences between control and treated groups could be detected in the presynaptic component of the cholinergic innervation, choline acetyltransferase activity. The results suggest that early ethanol exposure acts on hippocampal function similarly to phenobarbital, probably via alterations in postsynaptic processes in the septohippocampal cholinergic pathways.

Dimaprit, a histamine H2-agonist, inhibits anaphylactic histamine release from mast cells and the decreased release is restored by thioperamide (H3-antagonist), but not by cimetidine (H2-antagonist)

Whether anaphylactic histamine release from rat peritoneal mast cells is influenced by betahistine, a histamine H1-receptor agonist/H3-antagonist, and dimaprit, an H2-agonist, was examined. Treatment with dimaprit at 6 and 60 microM for 20 min significantly inhibited the anaphylactic histamine release, whereas betahistine at up to 80 microM under the same conditions did not affect it. Treatment with dimaprit at 6 and 60 microM for 1 to 20 min and for 5 to 20 min, respectively, caused a time-dependent inhibition of the release, but up to 30 min treatment with 8 and 80 microM betahistine had no effect. The decreased histamine release induced by dimaprit was recovered by neither mepyramine nor cimetidine. However, thioperamide, an H3-selective antagonist, dose-dependently restored the diminished release. From these results, the inhibition of anaphylactic histamine release by dimaprit is not produced by the stimulation of H2-receptors, but involves the stimulation of H3-like receptors or H3-subtype receptors, which are distinct from the H3-receptors located in brain, and suggests that the receptor plays an important role in the negative feedback regulation of histamine release.

Norepinephrine injections in diagonal band of Broca selectively reduced the activity of vasopressin supraoptic neurons in the rat

In the rat, transient drug-induced elevations of arterial blood pressure, which are sufficient to activate peripheral baroreceptors, produce a brief and selective cessation in the spontaneous activity of vasopressin-secreting cells in the hypothalamic supraoptic nucleus. This response appears to require the noradrenergic innervation of the diagonal band of Broca. The present study evaluated whether injections of norepinephrine into the diagonal band of Broca affect the spontaneous activity of supraoptic vasopressin-secreting neurons. Extracellular recordings were obtained from antidromically identified supraoptic neurons in pentobarbital anesthetized rats using a transpharyngeal approach. Injections of 200 nl of 10 microM norepinephrine into the diagonal band of Broca region arrested the spontaneous activity of 80% (12/15) of vasopressin-secreting neurons but only 7% (1/14) of oxytocin secreting-neurons demonstrated a comparable decrease in excitability. Vehicle injections did not influence the activity of any of the neurons tested. These results are consistent with the hypothesis that the baroreceptor-sensitivity of vasopressin neurons is mediated by a noradrenergic mechanism in the diagonal band of Broca.

Effect of KB-2796, a new diphenylpiperazine Ca2+ antagonist, on glutamate-induced neurotoxicity in rat hippocampal primary cell cultures

The effects of the novel calcium channel antagonist KB-2796, other calcium channel antagonists, an N-methyl-D-aspartate (NMDA) antagonist, non-NMDA antagonists, Mg2+, a Ca(2+)-chelator and a calcium channel agonist on neurotoxicity induced by a 10-min application of 100 microM glutamate were studied in rat hippocampal primary cell cultures. KB-2796 (0.1 and 1 microM), flunarizine (1 microM), nimodipine (10 microM), MK-801 (0.01-1 microM), Mg2+ (10 mM) and EGTA (10 mM) significantly prevented the neurotoxicity, but 6-cyano-7-nitro-quinoxalinedione (CNQX) (10 microM) and 6, 7-dinitro-quinoxalinedione (DNQX) (10 microM) did not. Bay K 8644 (10 and 100 nM) enhanced the neurotoxicity. These findings indicate that KB-2796 protects the neuronal cell from the glutamate-induced neurotoxicity, presumably by blocking the Ca2+ influx into brain neurons.

Septohippocampal cholinergic changes after destruction of the A10-septal dopaminergic pathways

Mice were injected bilaterally into the septum with 6-hydroxydopamine and 6 weeks later the hippocampi were assayed for activity of choline acetyltransferase, muscarinic receptor binding capabilities and for formation of inositol phosphate in response to direct (carbachol) or presynaptically elicited (K+) stimulation of the postsynaptic receptors. Levels of dopamine in the septum were reduced by 70% in the lesioned animals and hippocampal choline acetyltransferase was elevated by the same amount. The Bmax of muscarinic binding was significantly reduced without changes in Kd; nevertheless, carbachol-induced stimulation of formation of inositol phosphate was unaffected. The response to K+ was markedly elevated in the 6-hydroxydopamine-treated animals. Thus, the regulatory effect of A10-septal dopaminergic pathways on the septohippocampal cholinergic innervations is both on the presynaptic and postsynaptic levels.

Intraseptal injection of GABA and benzodiazepine receptor ligands alters high-affinity choline transport in the hippocampus

Injection of GABA and benzodiazepine (BDZ) agonists and antagonists into the medial septum produced bidirectional alterations in hippocampal high-affinity choline transport (HAChT). Male Sprague-Dawley rats were injected in the medial septum with either drug vehicle, a BDZ agonist, antagonist, or inverse agonist, or with a GABA-A or GABA-B agonist or antagonist and sacrificed 1 h later for assessment of HAChT in hippocampal synaptosomes. The GABA-A agonist muscimol, the GABA-B agonist baclofen, and the BDZ agonist chlordiazepoxide (CDP) produced dose-related decreases in HAChT 1 h following injection into the septum. The muscimol-induced decrease in HAChT was prevented by prior intraseptal injection of the GABA-A antagonist, bicuculline. Intraseptal injection of GABA-A (bicuculline) or GABA-B (2-hydroxysaclofen) antagonists did not alter HAChT, whereas the BDZ antagonist flumazenil (RO15,1788) and the BDZ inverse agonist methyl-beta-carboline-3-carboxylate (beta-CCM) increased this measure up to 30% in a dose-dependent manner. These results demonstrate that cholinergic neurons in the medial septum can be modulated in a bidirectional way through the pharmacological manipulation of GABA-A, GABA-B, and BDZ receptors. The potential functional and therapeutic consequences of these interactions are discussed.

Restricted origin and distribution of projections from the lateral to the medial septal complex in rat and guinea pig

The origin and distribution of projections from the lateral to the medial septal complex were studied at the light- and electron-microscopical level in the rat and the guinea pig, with the use of sensitive anterograde tracing techniques. Injections in the lateral septal complex resulted in only weak to moderate labeling in the medial septal nucleus. In contrast, injections in all but the ventral subdivision of the lateral septal complex labeled restricted terminal arborizations in the angular portions of the vertical diagonal band. Analyses at the electron-microscopical level indicated that these fibers form both symmetrical and asymmetrical synapses with dendrites and somata, but not with axons. These findings are discussed in the light of a presumed functional circuit from the hippocampal formation, via the lateral septal complex, to cells in the medial septal complex that originate projections to the hippocampal formation.

Dopaminergic drugs reverse the impairment of radial-arm maze performance caused by lesions involving the cholinergic medial pathway

Pharmacological studies have shown that both cholinergic and dopaminergic transmitter systems are crucial for optimal choice accuracy in the radial-arm maze and that these systems interact in a complex fashion. Lesion studies have provided evidence that the basal nuclear complex of the forebrain, the origin of cholinergic projections to the cerebral mantle, may be critical for the cholinergic modulation of learning and memory. We have shown that knife-cut lesions of the medial cholinergic pathway significantly impair radial-arm maze choice accuracy performance. The current study examined the effectiveness of D1 and D2 ligands in counteracting this lesion-induced deficit. The adverse effects of medial cholinergic pathway lesions were diminished or reversed by daily treatment with a D1 agonist (SKF 38393), a D2 agonist (LY 171555) or a D1 antagonist (SCH 23390), but were not affected by treatment with a D2 antagonist (raclopride). The three beneficial treatments have previously been found to attenuate the adverse effects of nictonic or muscarinic blockade on choice accuracy performance in the radial-arm maze. The finding that these dopaminergic drugs ameliorate the memory deficit caused by lesions involving the cholinergic medial pathway suggests the importance of interactions between cholinergic and dopaminergic systems in radial-arm maze performance. These results may provide leads for the development of novel therapeutic approaches for treating human disorders thought to result from cholinergic hypofunction.

Long-term suppression of the development of complementary memory storage sites in mice: functional interdependence of acetylcholine and dopamine

Bitemporal injections of puromycin consistently induce amnesia of aversive maze learning in mice when given within 3 days after training. These injections consistently fail to induce amnesia when given 6 or more days after training. Consistent with the evidence from other laboratories, we interpret these results to indicate that the initial, temporal memory storage sites are supplemented 6 days after training by the development of complementary storage sites in other cerebral areas. Previous experiments have shown that this process is suppressed for 30-60 days by a single SC injection of scopolamine, a muscarinic antagonist. We now find that this suppressive action of scopolamine can be completely nullified by haloperidol, a dopaminergic antagonist. This finding supports the view that there may be a therapeutic role for dopamine antagonists in the treatment of cognitive dysfunction associated with cholinergic loss.

Septo-hippocampal and nBM-cortical cholinergic neurones exhibit differential time-courses of activation as a function of both type and duration of spatial memory testing in mice

We previously showed that the initial acquisition session of a spatial discrimination (mixed reference/working memory) test in an 8-arm radial maze induced differential activations in the ascending cholinergic septo-hippocampal and nBM-cortical pathways in mice. This data showed that the duration of post-test cholinergic activation was longer in the nBM-cortical pathway than in the septo-hippocampal projection. Moreover, the post-test durations but not the immediate post-test amplitudes of activation in each pathway decreased progressively as a function of repeated daily acquisition sessions. In the present study we have thus tested the hypotheses that the time-courses of post-test cholinergic activation in the septo-hippocampal and nBM-cortical pathways may vary both as a function of the type of memory used (working vs. reference) and according to the duration of repeated daily testing. Cholinergic activity in vivo in the hippocampus or frontal cortex of mice was quantified using measures of sodium-dependent high-affinity choline uptake at two different times (30 s and 15 min) following specific spatial working or reference memory testing in an 8-arm radial maze. The memory tests were administered daily over a 13-day period to attain high levels of performance in each type of task. In comparison to control groups both types of memory testing induced significant post-test cholinergic activations in each brain region on Day 15. However, cholinergic activity remained elevated in frontal cortex at 15 min post-test following reference memory testing, whereas significantly shorter durations of cortical and hippocampal cholinergic activation were observed following working memory testing using short (1 min) retention intervals. The possible significance of these differential modifications to the time-course of the post-test activations in these cholinergic pathways in working and reference memory processes and the putative transsynaptic mechanisms involved are discussed.

GABAergic mediation of indirect transsynaptic control over basal and spatial memory testing-induced activation of septo-hippocampal cholinergic activity in mice

A neurochemical study of the transsynaptic interactions established between septal GABAergic interneurones and cholinergic septo-hippocampal neurones was conducted using mice. The effects of acute in vivo injections of either muscimol (20-500 ng/0.2 microliter), bicuculline (100 ng-1 micrograms/0.2 microliter) or saline vehicle (0.2 microliter) into the medial septum on septo-hippocampal cholinergic activity were evaluated using measures of hippocampal high affinity choline uptake at 30 min post-injection in two main groups of mice. The first (quiet control) remained in their home cages during the post-injection period whereas the second (active) were submitted, 10 min following injection to a 20-min period of spatial working memory testing in an 8-arm radial maze. Intraseptal injections of either muscimol or bicuculline produced significant (25-50%) inhibition of hippocampal cholinergic activity in quiet conditions (basal) as compared to intact or saline-injected mice. In the active groups, whereas memory testing induced significant cholinergic activation (+15-20%) in intact and saline injected mice at 30 s post-test no significant memory testing-induced activation was observed in either muscimol or bicuculline-injected mice at any dose. The role of septal GABAergic interneurones in the indirect transsynaptic control over the basal and activated states of septo-hippocampal cholinergic activity is discussed with respect to the concept that these complex neuronal interactions contribute to the physiological mechanisms involved in the modulation of working memory performance.

The extrinsic modulation of hippocampal theta depends on the coactivation of cholinergic and GABA-ergic medial septal inputs

The long trains of theta field activity recorded from the hippocampal formation of urethane-anesthetized rats are thought to be primarily dependent on cholinergic afferents originating in the medial septum/vertical limb of the diagonal band of Broca (MS/vDBB). Recent anatomical studies have revealed the existence of a septal GABA-ergic input to the hippocampal formation which synapses mainly on intrinsic GABA-ergic interneurons. The present work investigated the possibility that some form of interaction between cholinergic and GABA-ergic MS/vDBB inputs might be required for the generation of hippocampal theta field and cellular activities in urethane-anesthetized rats. Reversible inactivation of the MS/vDBB completely abolished theta field and theta-on cell activities, but "released" theta-off cells. The theta field and theta-on cell activities induced by direct intrahippocampal microinfusions of carbachol were also abolished by MS/vDBB inactivation. We speculated that septal suppression was producing two effects: 1) removing excitatory, cholinergic input; and 2) removing inhibitory control of hippocampal GABA-ergic interneurons, thereby increasing the overall level of hippocampal inhibition. Sequential administration of both carbachol and the GABA-A antagonist, bicuculline, resulted in theta-like oscillations similar to those seen in hippocampal slices bath perfused with carbachol alone. Thus, following MS/vDBB inactivation hippocampal GABA-ergic systems are overactive; this enhances intrinsic inhibition and blocks carbachol theta. By reducing the overall level of inhibition in the hippocampus with bicuculline, it is possible to reinstate its oscillatory properties. Conversely, increasing the level of inhibition in the hippocampus (with muscimol) results in the abolishment of theta field activity and the discharges of both theta-on and theta-off cells. Based on these findings we are proposing that cholinergic and GABA-ergic systems originating in the MS/vDBB act synergistically to modulate hippocampal theta. Cholinergic projections provide the afferent excitatory drive for hippocampal theta-on cells and septal GABA-ergic projections act to reduce the overall level of inhibition by inhibiting hippocampal GABA-ergic interneurons (hippocampal theta-off cells). Both activities must be present for the generation of hippocampal theta field and cellular activities. The balance between the cholinergic and GABA-ergic systems may determine whether hippocampal synchrony (theta) or asynchrony (LIA) occurs.

Alterations in hippocampal cholinergic receptors and hippocampal behaviors after early exposure to nicotine

Mice were exposed to nicotine prenatally by injecting the mother with 1.5 mg/kg nicotine SC twice daily on gestation days 9-18 (PreN mice) or neonatally by daily SC injections of 1.5 mg/kg nicotine on postnatal days 2-21 (NeoN mice). At age 50 days, hippocampal muscarinic receptors Bmax of PreN and NeoN mice were 58% and 79% above control, respectively (p less than 0.01); Kd was unaffected by early nicotine exposure. Eight-arm maze performance of nicotine-exposed animals fell behind control level. Both PreN and NeoN made approximately 10% less correct responses in the first eight trials than controls throughout the test period (p less than 0.01). By the last day of testing, PreN needed 23% and NeoN 31% more trials than controls to enter all arms (p less than 0.001). In addition, PreN needed 35 and NeoN 42% more days than controls to reach criterion (p less than 0.05). Similarly, while 61% of controls reached criterion by day 6 only 17% of PreN and 25% of NeoN reached criterion (p less than 0.01). In the Morris maze, PreN needed from 43-119% more time to reach the platform (p less than 0.001). In the spatial probe test, PreN animals made 35% fewer crosses over the area of the missing platform (p less than 0.001). The study suggests that nicotine administered to the fetus or neonate alters septohippocampal chemistry and induces deficits in hippocampus-related behaviors. The possible reversal of the behavioral changes by manipulating the cholinergic innervations should be the subject of future investigations.

Increased cognitive sensitivity to scopolamine with age and a perspective on the scopolamine model

18 older normal volunteers (mean age = 66.5 +/- 7.9 years) and 46 younger volunteers (mean age = 27.0 +/- 6.1 years) were administered the anticholinergic drug scopolamine (0.5 mg i.v.) followed by a battery of cognitive tests evaluating attention, learning and memory. The older subjects were significantly more impaired than the younger by scopolamine on some tests of learning and memory. This increased sensitivity of the older group to scopolamine is consistent with studies in animals and humans showing decreased cholinergic system function with age. The findings also indicate that age is an important variable to consider in using the scopolamine model of memory impairment. The cognitive impairment caused by scopolamine in younger subjects in this and prior studies is similar to some, but not all aspects of the impairment which occurs in normal aging. Scopolamine also caused impairments on digit span and word fluency tasks, which are not consistent with normal aging changes. In the older group of subjects, scopolamine produced aspects of the cognitive impairment which occurs in AD on tests of episodic memory and learning, vigilance-attention, category retrieval, digit span, and number of intrusions. Other areas of cognition that are of relevance to aging and AD such as psychomotor speed, praxis, concept formation and remote memory were not evaluated in this study. Some of these are being evaluated in ongoing studies, along with additional and more specific tests of retrieval from knowledge memory, implicit memory and attention. The scopolamine model has provided a fruitful pharmacologic starting point for the study of a number of cognitive operations. The idea of dissecting apart aspects of memory systems pharmacologically depends on the availability of neurochemically specific drugs and on the specificity and sensitivity of neuropsychological tests for distinct cognitive operations or domains. Further studies using such tools will aid not only in the understanding of the impairments which occur in aging and in AD, but also of the conceptualization of memory and other cognitive operations and ultimately the physiological mechanisms involved in memory and learning.

Intrahippocampal injections of benzodiazepine and muscimol impair working memory but not reference memory of rats in the three-panel runway task

In a three-panel runway task, the benzodiazepine chlordiazepoxide at 3.2 and 10 mg/kg i.p. significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) in a test of working memory, but it had no effect on errors in a test of reference memory. This effect of 10 mg/kg chlordiazepoxide on working memory was blocked by the benzodiazepine receptor antagonist flumazenil at 10 mg/kg. Intrahippocampal injection of chlordiazepoxide at 10 and 32 micrograms/side significantly increased the number of working memory errors. This effect of intrahippocampal chlordiazepoxide (32 micrograms/side) was attenuated not only by flumazenil at 10 mg/kg but also by the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline at 3.2 mg/kg. Intrahippocampal injection of the GABAA receptor agonist muscimol at 100 and 320 ng/side also significantly increased working memory errors. Neither chlordiazepoxide nor muscimol affected the number of reference memory errors when injected into the hippocampus at doses up to 32 micrograms/side or 320 ng/side, respectively. These results suggest that activation of the GABAA/benzodiazepine receptor complex in the hippocampus impairs working memory, but does not affect reference memory.

Acetylcholine release in the hippocampus: regulation by monoaminergic afferents as assessed by in vivo microdialysis

The role of monoamines in the functional regulation of the septo-hippocampal cholinergic system was studied using in vivo microdialysis of acetylcholine (ACh) release in the hippocampus of awake unrestrained rats. Systemic administration of the dopamine receptor agonist apomorphine (2.0 mg/kg) resulted in a 170% increase in hippocampal ACh overflow. Similarly the catecholamine-releasing agent amphetamine (2.5 mg/kg) produced a 400% increase in ACh overflow. The effect induced by amphetamine, but not that of apomorphine, was blocked in animals pretreated with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (AMPT). The effect of amphetamine on ACh release was reduced by 75% after a 6-hydroxydopamine (6-OHDA) lesion of the ventral tegmental area (VTA) but was not affected by 6-OHDA lesions of the noradrenergic dorsal and ventral bundles. However, baseline ACh overflow was increased by 130% by the dorsal and ventral bundle lesions. The serotonin-releasing agent p-chloroamphetamine (2.5 mg/kg) produced a 160% increase in hippocampal ACh release, and this effect was enhanced after a 5,7-dihydroxytryptamine (5,7-DHT) lesion of the serotonin projection system. The results show that surgical or pharmacological manipulations of the ascending brainstem monoaminergic systems, which innervate wide areas of the forebrain, including the septum and the hippocampal formation, have pronounced effects on septo-hippocampal cholinergic activity. Thus, the present data provide support for the view that information regarding behavioral state and arousal is conveyed to the septo-hippocampal system via ascending monoaminergic systems.

Behaviour-dependent changes in acetylcholine release in normal and graft-reinnervated hippocampus: evidence for host regulation of grafted cholinergic neurons

Grafted neurons obtained from the fetal basal forebrain can provide a functional cholinergic reinnervation of the hippocampal formation in rats with a lesion of the intrinsic septal cholinergic afferents. In the present experiments graft-derived acetylcholine release in the hippocampus was studied by microdialysis in awake rats during different types of behaviours which are known to activate the innate septohippocampal cholinergic system and during different activity periods of the day-night cycle. Two types of basal forebrain grafts were studied: cell suspensions implanted into the hippocampus in rats with an aspirative lesion of the fimbria-fornix, and grafts of solid tissue implanted as a tissue bridge into the fimbria-fornix lesion cavity. Increased acetylcholine overflow was seen in both groups with grafts during sensory stimulation (by handling). The strongest response (50% increase in acetylcholine release) was seen in rats with solid basal forebrain grafts (equivalent to two-thirds of that seen in intact rats). Immobilization stress and motor activity (swimming) also resulted in increased, but more variable, acetylcholine release (+ 30%; about one-third of the normal response). None of these effects was seen in the control rats with fimbria-fornix lesion only. The two-fold difference in hippocampal acetylcholine release in normal animals between day and night was absent in both types of grafted rats. An acute knife-cut, transecting the connections between the solid basal forebrain graft and the host hippocampus, caused an immediate 75% reduction in acetylcholine release (similar to the effect of an acute fimbria-fornix transection in the normal rats) and the response to swimming was no longer evident. The results show that grafted cholinergic neurons can be functionally integrated into the host brain, allowing the grafted neurons to be activated in the correct behavioural contexts, although the changes in acetylcholine overflow were overall smaller and more variable than normal. The ability of the host to influence cholinergic graft activity, most probably mediated via activation of afferent host-graft connections, may contribute to the efficacy of basal forebrain grafts in the amelioration of behavioural impairments in animals with lesions of the forebrain cholinergic system.

Memory-modulatory effects of centrally acting noradrenergic drugs: possible involvement of brain cholinergic mechanisms

Post-training administration of the centrally acting muscarinic agonist oxotremorine (50.0 microgram/kg, ip) facilitated 48-hr retention, in mice, of a one-trial step-through inhibitory avoidance response. Oxotremorine-induced memory facilitation was not prevented by the simultaneous post-training administration of the central beta-adrenoceptor antagonist propranolol (2.0 mg/kg, ip). In contrast, post-training administration of atropine (0.5 mg/kg, ip), but not methylatropine (0.5 mg/kg, ip), completely prevented the facilitatory effects of the central beta-adrenoceptor agonist clenbuterol (30.0 micrograms/kg, ip) on retention. Low subeffective doses of clenbuterol (3.0 micrograms/kg, ip) and oxotremorine (6.25 or 12.5 micrograms/kg, ip) potentiated their effects and facilitated retention when given simultaneously immediately post-training. These results suggest that clenbuterol may induce memory facilitation through an increase of the release of acetylcholine in the brain. Post-training administration of a high dose of clenbuterol (1.0 mg/kg, ip) significantly impaired retention. Clenbuterol (1.0 mg/kg, ip)-induced impairment of retention was completely prevented by simultaneous post-training administration of oxotremorine (6.25, 12.5, or 50.0 micrograms/kg, ip). The centrally acting anticholinesterase physostigmine (21.5 or 68.0 micrograms/kg, ip) partially prevented clenbuterol-induced impairment of memory. The peripherally acting anticholinesterase neostigmine (68.0 micrograms/kg, ip) modified neither retention nor the amnestic effects of clenbuterol. Considered together, these findings are consistent with the view that brain muscarinic cholinergic mechanisms are involved in both the facilitatory and impairing effect of post-training clenbuterol on the modulation of memory storage.

Glucose enhancement of memory in elderly humans: an inverted-U dose-response curve

In animals, enhancement of memory with glucose and many other treatments is characterized by an inverted-U dose-response curve. The present experiment examined the dose-response curve for glucose enhancement of memory in elderly humans. Using a repeated measures, counterbalanced, crossover design, the subjects (60-82 year olds) were tested on four sessions, separated by 1 week or more, for performance on the Wechsler Logical Memory Test after ingestion of a fruit drink sweetened with glucose (0, 10, 25, and 50 g) and saccharin matched to comparable taste. The findings indicate that glucose enhanced performance on this test in an inverted-U dose-response manner, with optimal enhancement obtained at the 25 g glucose dose. These findings provide further demonstration of glucose enhancement of memory in elderly humans and also describe an additional analogy between the characteristics of glucose enhancement of memory in animals and humans.

Chlordiazepoxide-induced working memory impairments: site specificity and reversal by flumazenil (RO15-1788)

The following studies examined the dose and time dependence, site specificity, and reversibility of chlordiazepoxide (CDP)-induced working memory impairments in adult male Sprague-Dawley rats. The rats were tested in a delayed non-match-to-sample radial-arm maze task in which a 1-h delay was imposed between the first four (predelay) and all subsequent (postdelay) arm choices. Intraperitoneal (ip) injection of 2.5 or 5.0 but not 1.25 mg/kg CDP immediately following the predelay session impaired performance in the task. CDP increased the number of errors and decreased the number of correct choices during the postdelay session. The observed working memory impairments also appeared to be site specific since injection of CDP into the medial septum, but not into the anterior amygdala nuclei, immediately following the predelay session also impaired working memory in a dose-related manner. Furthermore, there was a time window for CDP-induced working memory impairments since intraseptal injection of the drug immediately but not 15 min following the predelay session disrupted memory. This observation suggests that the performance deficits reflect disrupted working memory and not proactive effects on performance or the induction of state-dependent learning. In the final experiment, rats were injected ip with either saline or an amnestic dose of CDP (5.0 mg/kg) following the predelay session and then were immediately infused with 10 nmol flumazenil (RO15-1788), a benzodiazepine receptor antagonist or vehicle, into either the medial septum or anterior nuclei of the amygdala. Intraseptal injection of flumazenil prevented the working memory impairments produced by ip injection of CDP. In contrast, intra-amygdala injection of flumazenil did not attenuate, enhance, or modify the CDP-induced working memory impairment. These observations suggest that CDP disrupts working memory by interacting with benzodiazepine receptors in the medial septum.

Antagonism of GABAergic transmission within the septum disrupts working/episodic memory in the rat

Male Sprague-Dawley rats, trained to perform a standard or delayed-non-match-to-sample radial arm maze task, were implanted with a single cannula aimed at the medial septal nucleus. A within-subjects design was utilized to examine the effects of intraseptal administration of the GABAergic antagonist bicuculline on performance of these tasks. Bicuculline (0-0.5 microgram/0.5 microliter) infusion produced dose-dependent impairments when administered prior to performance of a standard radial arm maze task. Post-training infusion of bicuculline (0.-0.25 microgram/0.5 microliter) also induced dose-dependent impairments in the delayed version (4 h) of the task. Further testing indicated that post-training administration of a low dose of bicuculline (0.05 microgram) in the delayed version of the task induced a deficit at a 4-h, but not a 1-h, retention interval. The latter indicates that the impairment varied as a function of bicuculline dose and increasing task difficulty (longer retention intervals). Previous observations indicated that post-training administration of the GABAergic agonist muscimol and the antagonist bicuculline could induce deficits in the performance of the delayed task. The present findings demonstrate that intraseptal bicuculline treatment can disrupt ongoing radial maze performance, as well as the maintenance and/or retrieval of memories necessary for performance of the delayed version of the task. These findings suggest that either activation or blockade of intraseptal GABA receptors is sufficient to disrupt working/episodic memory processes. The role of septum and septohippocampal pathway in working/episodic memory is discussed.

Assessment of a cholinergic contribution to chlordiazepoxide-induced deficits of place learning in the Morris water maze

This investigation sought to characterize the interaction between benzodiazepine and cholinergic systems in place learning in the Morris water maze. In the first experiment, rats were treated with scopolamine (1 mg/kg) alone or concomitantly with one of two doses of flumazenil (15 and 30 mg/kg) or with chlordiazepoxide (5 mg/kg) alone or concomitantly with flumazenil (15 mg/kg). Chlordiazepoxide and scopolamine severely impaired place learning but not cue learning. The low dose of flumazenil completely reversed the impairment produced by chlordiazepoxide and both high and low doses of flumazenil attenuated the place learning deficit produced by scopolamine. Neither dose of flumazenil affected place learning when administered alone. In the second experiment, rats were administered chlordiazepoxide (5 mg/kg) or scopolamine (1 mg/kg) alone or concomitantly with one of four doses of physostigmine (0.05, 0.10, 0.25, and 0.5 mg/kg). Once again, both chlordiazepoxide and scopolamine impaired place but not cue learning. Physostigmine reversed the impairment produced by scopolamine in a dose-dependent manner but failed at every dose to attenuate the impairment produced by chlordiazepoxide. The higher doses of physostigmine impaired place learning when administered alone. None of the drug treatments impaired cue learning. Together, these results suggest that the scopolamine-induced impairment of place learning is due to an increase in benzodiazepine/GABA activity, and contradict the notion that benzodiazepines impair memory by cholinergic mechanisms.

The neurotoxic actions of ibotenic acid on cholinergic and opioid peptidergic systems in the central nervous system of the rat

The neurotoxic effects produced by ibotenic acid (IA) induced chemical lesions of the central nervous system (CNS) cholinergic system were examined on the opioid peptidergic system in adult rats. Forebrain cholinergic systems were bilaterally lesioned by the infusion of IA (1 or 5 micrograms/site) into the nucleus basalis magnocellularis (NBM). One week after the injections, the animals were sacrificed, and activities of acetylcholinesterase (AChE), choline acetyltransferase (ChAT) and concentrations of beta-endorphin (beta-End) and Met-enkephalin (Met-Enk) were measured in different brain regions. Animals treated with IA showed a decrease in the activity of ChAT (-24%), AChE (-36%) and beta-End level (-33%) in the frontoparietal cortex (FC). For the first time we report that these changes were associated with a compensatory increase in the activity of ChAT (+27%), AChE (+25%), beta-End level (+66%) in the remaining part of the cortex, i.e. cortex devoid of frontal cortex (C-FC). Met-enkephalin level increased by 59% in the frontoparietal cortex and did not change in the cortex devoid of frontal cortex upon IA treatment. These results suggest that IA treatment results in changes in the activity of cortical ChAT and AChE, and beta-End level in the same direction. Injection of IA in the NBM did not cause a change in the activity of ChAT or AChE in other brain regions such as hippocampus, striatum or midbrain. In addition to cortex devoid of frontal cortex, midbrain also showed a significant increase in the beta-End level in the IA treated animals. However, pituitary beta-End decreased in the neurotoxin treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic neurons in the rat septal complex: ultrastructural characterization and synaptic relations with catecholaminergic terminals

Physiological and pharmacological studies have suggested that catecholamines modulate cholinergic neurons in the medial septal and diagonal band nuclei (i.e., the septal complex). Thus, the ultrastructural morphology of neurons containing choline acetyltransferase (ChAT), the biosynthetic enzyme for acetylcholine, and their relation to catecholaminergic terminals exhibiting immunoreactivity for the catecholamine synthesizing enzyme tyrosine hydroxylase (TH) were examined in the rat septal complex. Dual immunoautoradiographic and peroxidase anti-peroxidase labeling methods were used to simultaneously localize antibodies raised in rabbits against TH and from rat-mouse hybridomas against ChAT in single sections. At least two types of perikarya with ChAT-immunoreactivity (ChAT-I) were observed. The first type were large (20-30 microns), elongated or round, and contained a small indented nucleus with an abundant cytoplasm and an occasional lamellar body. The second type was also either ovoid or round but was medium-sized (15-20 microns) and contained a larger indented nucleus and a smaller amount of cytoplasm than the first type. Both types of perikarya as well as dendrites with ChAT-I were surrounded by astrocytic processes apposed to most of their plasmalemmal surfaces. The distribution and types of terminal associations (i.e., asymmetric synapses, symmetric synapses and appositions which lacked a membrane specialization in the plane of section analyzed) with ChAT-labeled perikarya and dendrites were quantitatively evaluated. The majority (68% of 197) of the presynaptic terminals were unlabeled; the remaining terminals were immunoreactive for TH (25%) or ChAT (7%). All three types of terminals contacted primarily the shafts of small dendrites and more rarely ChAT-labeled perikarya and large dendrites. ChAT-labeled terminals: (1) formed associations with unlabeled perikarya and dendrites (31% of 176); (2) formed associations with perikarya and dendrites with ChAT-I (7%); (3) contacted the same unlabeled perikarya and dendrite as a TH-containing terminal (21%); (4) were in apposition to TH-labeled terminals (25%); or (5) were either in apposition to unlabeled or ChAT-labeled terminals or lacked associations with any processes. The majority of associations formed by the terminals with ChAT-I were on the shafts of small dendrites. Moreover, most of the associations formed were either symmetric synapses or appositions not separated by astrocytes in the plane of section analyzed. These findings provide cellular substrates in the septal complex (1) for sparse synaptic input relative to astrocytic investment of cholinergic neurons and (2) for direct synaptic modulation of cholinergic and non-cholinergic neurons by catecholamines and/or acetylcholine. These findings have direct relevance to catecholaminergic-cholinergic interactions and to the neuropathological basis for Alzheimer's disease.

Ultrastructural localization of tyrosine hydroxylase immunoreactivity in the rat diagonal band of Broca

The present study sought to establish the cellular basis for the catecholaminergic (i.e., noradrenaline and dopamine) modulation of neurons in the horizontal limb of the diagonal band of Broca (HDB) in the rat brain. The light and electron microscopic localization of antigenic sites for a polyclonal antibody directed against the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), were examined in the HDB using a double-bridged, peroxidase-antiperoxidase method. By light microscopy, numerous punctate, varicose processes with intense TH-immunoreactivity (TH-I) were detected in the HDB. Additionally, a few small, bipolar, or multipolar TH-immunoreactive neurons were observed. Ultrastructural analysis of single sections revealed that the TH-labeled processes were axons and axon terminals. Axons (n = 134) with TH-I were primarily unmyelinated. Terminals with TH-I (n = 169) were 0.3-1.4 microns in diameter and contained many small, clear vesicles and 0-5 larger dense-core vesicles. The types of associations (i.e., asymmetric synapses, symmetric synapses, and appositions which lacked a membrane specialization in the plane of section analyzed) formed by the TH-labeled terminals were quantitatively evaluated. The TH-labeled terminals: (1) formed associations with unlabeled perikarya and dendrites (134 out of 169), (2) were closely apposed without glial intervention to unlabeled and TH-labeled terminals (11 out of 169), or (3) had no neuronal associations in the plane of section analyzed (24 out of 169). The relatively rare (n = 4) associations with unlabeled perikarya were mostly characterized by symmetric synaptic specializations. The majority of the TH-labeled terminals were associated with the shafts of small dendrites (66% of 134). Moreover, most of the associations on dendrites and dendritic spines were further characterized by asymmetric synaptic specializations; however, many were also appositions without any apparent glial intervention in the plane of section analyzed. Additionally, the TH-labeled terminals were often associated with only one dendrite, which, in the same plane of section, was sparsely innervated by other terminals. Astrocytic processes usually surrounded the portions of the terminals and dendrites not involved in the region of association. The TH-immunoreactive perikarya were small (7-12 microns), ovoid, and had an indented nucleus with some heterochromatin. Their scant cytoplasm contained mitochondria, Golgi complexes, and endoplasmic reticulum. A few immunoreactive dendrites, presumably derived from the local neurons, were also detected. Both TH-immunoreactive perikarya and dendrites were associated primarily with unlabeled terminals, although a few terminals with TH-I also contacted them.(ABSTRACT TRUNCATED AT 400 WORDS)

Oxotremorine attenuates retrograde amnesia induced by post-training administration of the GABAergic agonists muscimol and baclofen

These experiments examined the involvement of cholinergic influences in the effects of GABAergic drugs on 24-h retention of an inhibitory avoidance response by mice. A first set of experiments confirmed previous findings indicating that post-training injections (ip) of the GABAergic agonists muscimol (1.0 and 2.0 mg/kg) and baclofen (10.0 and 20.0 mg/kg) impaired retention, as well as previous findings indicating that injections of the cholinergic agonist oxotremorine (5.0 and 10.0 micrograms/kg) enhanced retention. The findings of a second set of experiments indicated that the memory-impairing effects of muscimol and baclofen were attenuated by concurrent injections of a low, and otherwise ineffective, dose of oxotremorine (2.5 micrograms/kg). These findings are interpreted as suggesting that GABAergic drugs affect memory storage through influences on cholinergic systems.

Amygdala and dorsal hippocampus lesions block the effects of GABAergic drugs on memory storage

These experiments examined the effects of posttraining systemic administration of the GABAergic agonist muscimol and the GABAergic antagonist bicuculline on retention in mice with bilateral lesions of the amygdala, dorsal hippocampus or caudate nucleus. Unoperated male CD1 mice and mice with either sham lesions or electrolytically induced lesions of these 3 brain regions were trained in a one-trial inhibitory avoidance task and, immediately after training, received i.p. injections of either muscimol, (1.0, 2.0 or 3.0 mg/kg), bicuculline, (0.25, 0.5 or 1.0 mg/kg), or control solutions. Retention was tested 24 h after training. Lesions of the 3 brain regions produced comparable impairment of retention. In the unoperated controls and sham controls muscimol and bicuculline produced dose-dependent impairment and enhancement, respectively, of retention. The drug effects on retention were blocked by lesions of the amygdala and hippocampus, but were not blocked by lesions of the caudate nucleus. These findings are consistent with other recent evidence suggesting that the amygdala and hippocampus are involved in mediating posttraining neuromodulatory influences on memory storage.

D-1 agonist, SKF 38393, but not a D-2 agonist, produces a cholinergically mediated analeptic effect in rabbits

SKF 38393 (2-15 mg/kg, IV), but not quinpirole, shortened the duration of loss of righting reflex produced in pentobarbital-narcotized rabbits. This effect was blocked by atropine (2-5 mg/kg, IV), but not by atropine methylbromide, suggesting that a central cholinergic mechanism was involved. The analeptic effect was also blocked by SCH 23390 (0.1 mg/kg, IV) or raclopride (5 mg/kg, IV). These results indicate that SKF 38393 activates central cholinergic neurons, which in turn initiate the analeptic effect. However, the fact that raclopride also blocked the SKF 38393 analeptic effect, but quinpirole did not exert any analeptic effect, suggests that a D-1/D-2 modulation of cholinergic systems may be involved in the SKF 38393-induced analeptic effect. These results also support our earlier findings and view that cocaine-induced analeptic activity is mediated by a dopaminergic-cholinergic mechanism.

Intraseptal administration of bicuculline produces working memory impairments in the rat

Male Sprague-Dawley rats, trained to perform a delayed-non-match-to-sample eight-arm radial maze task, were implanted with a single cannula aimed at the medial septal nucleus. A within-subjects design was utilized to examine the effects of intraseptal administration of bicuculline (0.5 micrograms) on performance of this task with 1- and 4-h delay intervals imposed between choices four and five. Administration of bicuculline immediately following the first four choices produced an impairment in maze performance at both a 1- and a 4-h delay interval. This treatment also produced an increase in latency per choice. Bicuculline-induced impairments were not observed when administered 2 h following the predelay session (2 h prior to testing). These data support previous observations that pharmacological manipulation of GABAergic activity within the septum modifies working memory processes.

Interactive effects of D1 and D2 agonists with scopolamine on radial-arm maze performance

Pharmacological blockade of muscarinic cholinergic (ACh) receptors has been found to impair choice accuracy in a variety of tasks including the radial-arm maze. The cognitive impairment caused by the muscarinic antagonist scopolamine is reversed by the dopaminergic (DA) antagonist haloperidol as well as the selective D1 antagonist SCH 23390. In the current study, interactions were studied between scopolamine and selective agonists of D1 (SCH 38393) and D2 (quinpirole) receptors. Surprisingly, the D1 agonist SKF 38393 was found to significantly alleviate the scopolamine-induced choice accuracy deficit. In contrast, the D2 agonist quinpirole was not found to significantly alter the effects of scopolamine on choice accuracy but did have supra-additive effects of increasing choice latency. Both the D1 agonist SKF 38393 and the D1 antagonist SCH 23390 have been found to reverse the choice accuracy deficit caused by scopolamine and the deficit resulting from lesions of the medial projection from the basal forebrain to the cortex. Possible mechanisms for these effects are discussed.