Effects of MK-801 and Pentobarbital on Cholinergic Terminal Damage and Delayed Neuronal Death in the Ischemic Gerbil Hippocampus

The present study covers both the effects of MK-801, a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, and pentobarbital on cholinergic terminal damage and delayed neuronal death (DND) in ischemic gerbil. To study the above effects, in vivo microdialysis, immunohistochemical, and morphological techniques were used. MK-801 (3 mg/kg) or pentobarbital (50 mg/kg) were injected intraperitoneally 1 h or 30 min before 5 min ischemia, respectively. Each estimation was then carried out 4, 7, or 14 days after ischemia. Ischemia induced a significant decrease in acetylcholine (ACh) release and a disappearance of choline acetyltransferase (ChAT)-immunoreactivity in the hippocampus in addition to inducing DND. On day 4, MK-801 protected ischemia-induced DND in the hippocampal CA1 subfield. However, MK-801 had no effect against the decrease in ACh release in spite of protection of the decrease in ChAT-immunoreactivity. On day 7 and 14, no protective effect of MK-801 was observed in all estimations. It became clear that the mechanism of cholinergic terminal dysfunction is different from that involved in pyramidal cell death, i.e., excitative neurotoxicity induced by overabundant extracellular glutamate. Pentobarbital also provided protection against DND. However, protective effects of pentobarbital on the decrease in ACh release and the low ChAT-immunoreactivity were incomplete. Our present study indicated a limitation on the efficacy of NMDA receptor antagonist and barbiturate against cerebral ischemia.     

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.     

Procholinergic and memory enhancing properties of the selective norepinephrine uptake inhibitor atomoxetine

Atomoxetine has been approved by the FDA as the first new drug in 30 years for the treatment of attention deficit/hyperactivity disorder (ADHD). As a selective norepinephrine uptake inhibitor and a nonstimulant, atomoxetine has a different mechanism of action from the stimulant drugs used up to now for the treatment of ADHD. Since brain acetylcholine (ACh) has been associated with memory, attention and motivation, processes dysregulated in ADHD, we investigated the effects of atomoxetine on cholinergic neurotransmission. We showed here that, in rats, atomoxetine (0.3-3 mg/kg, i.p.),--increases in vivo extracellular levels of ACh in cortical but not subcortical brain regions. The marked increase of cortical ACh induced by atomoxetine was dependent upon norepinephrine alpha-1 and/or dopamine D1 receptor activation. We observed similar increases in cortical and hippocampal ACh release with methylphenidate (1 and 3 mg/kg, i.p.)--currently the most commonly prescribed medication for the treatment of ADHD--and with the norepinephrine uptake inhibitor reboxetine (3-30 mg/kg, i.p.). Since drugs that increase cholinergic neurotransmission are used in the treatment of cognitive dysfunction and dementias, we also investigated the effects of atomoxetine on memory tasks. We showed that, consistent with its cortical procholinergic and catecholamine-enhancing profile, atomoxetine (1-3 mg/kg, p.o.) significantly ameliorated performance in the object recognition test and the radial arm-maze test.     

Regional differences in the cerebral blood flow velocity response to hypobaric hypoxia at high altitudes

Symptoms of acute mountain sickness (AMS) may appear above 2,500 m altitude, if the time allowed for acclimatization is insufficient. As the mechanisms underlying brain adaptation to the hypobaric hypoxic environment are not fully understood, a prospective study was performed investigating neurophysiological changes by means of near infrared spectroscopy, electroencephalograpy (EEG), and transcranial doppler sonography at 100, 3,440 and 5,050 m above sea level in the Khumbu Himal, Nepal. Fourteen of the 26 mountaineers reaching 5,050 m altitude developed symptoms of AMS between 3,440 and 5,050 m altitude (Lake-Louise Score ⩾3). Their EEG frontal beta activity and occipital alpha activity increased between 100 and 3,440 m altitude, i.e., before symptoms appeared. Cerebral blood flow velocity (CBFV) in the anterior and middle cerebral arteries (MCAs) increased in all mountaineers between 100 and 3,440 m altitude. During further ascent to 5,050 m altitude, mountaineers with AMS developed a further increase in CBFV in the MCA, whereas in all mountaineers CBFV decreased continuously with increasing altitude in the posterior cerebral arteries. These results indicate that hypobaric hypoxia causes different regional changes in CBFV despite similar electrophysiological changes.     

Effects of simulated high altitude on event-related potential (P300) and auditory brain-stem responses.

OBJECTIVE: This study investigated the effect of hyobaric hypoxia on cognitive function. METHODS: We recorded the auditory brain-stem response (ABR) and auditory-evoked event-related potentials (ERP) in 7 male subjects during a rapid ascent to a simulated 4500 m altitude from their acclimatized altitude of 610 m. The amplitude and latency of each component of ABR and of ERP were assessed. RESULTS: Compared with the values at 610 m, at 4500 m the latencies of both waves I and V of ABR significantly increased, with no change in I-V interpeak latency; and the amplitude of wave I decreased, with no change in the amplitude of wave V. The increase in altitude affected neither the amplitude nor the latency of N100. The P300 latency was prolonged significantly after exposure to hypobaric-hypoxic conditions for 2h, with no significant change in amplitude. At 4500 m, the P300 latency returned to the baseline value after oxygen was inhaled. CONCLUSIONS: Our results suggest it is possible to boost cognitive processing by supplying oxygen even when auditory stimulus intensity decreases under hypobaric and hypoxic conditions, and that P300 latency is affected by hypoxic more than hypobaric conditions. SIGNIFICANCE: This study demonstrated that each component of ABR and the latency of both N100 and P300 are important to record when the effects of hypobaric hypoxia on cognitive function are investigated.     

Effects of carbamazepine on acetylcholine release and metabolism

To clarify the mechanisms of action of carbamazepine (CBZ), we investigated the effects of CBZ on acetylcholine (ACh) release and metabolism in rat striatum and hippocampus. Acute administration of effective dose of CBZ (25 mg/kg) increased both striatal and hippocampal extracellular levels of ACh, whereas a supraeffective dose of CBZ (50 mg/kg) did not affect the levels and a toxic dose of CBZ (100 mg/kg) decreased the extracellular ACh levels in both brain regions. Both acute and chronic administrations of CBZ (25 and 50 mg/kg, mg/kg per day) increased intracellular ACh levels in striatum and hippocampus. The striatal intracellular ACh levels were decreased by both acute and chronic administrations of CBZ (100 mg/kg, mg/kg per day), whereas the hippocampal intracellular ACh levels were not affected. The effective CBZ concentration did not affect cholinesterase activity, whereas supraeffective CBZ concentration reduced it weakly. Effective dose of CBZ enhanced ACh release and synthesis; however, supraeffective doses of CBZ reduced ACh release and synthesis without enhancement of ACh degradation, indicating that CBZ has biphasic effects on ACh release and synthesis. Thus, the present findings, the slight stimulation of ACh function by effective dose of CBZ, are involved, at least partially, in the antiepileptic and mood stabilizing mechanisms of action of CBZ.     

Compensatory elevation of acetylcholine synthesis in vivo by cholinergic neurons surviving partial lesions of the septohippocampal pathway

The present study characterized the effects of partial destruction of the cholinergic septohippocampal pathway on transmitter functions of surviving cholinergic neurons in the hippocampus. Partial and full fimbrial transections were performed, and 3 weeks after lesioning, cholinergic functions were assessed in vivo and in vitro. Hippocampal ChAT activity and the capacity of hippocampal slices to synthesize [3H]ACh in vitro decreased by 35% and 45%, respectively, following partial fimbrial lesions and by 68% and 85%, respectively, following full fimbrial lesions. [3H]ACh release from hippocampal slices in vitro was decreased by 57% and 87%, respectively, following partial and full fimbrial lesions. Partial lesions decreased high-affinity choline uptake into hippocampal synaptosomes by 52%. In contrast to the significant reductions in cholinergic parameters measured in vitro after partial fimbrial lesions, such partial lesions did not significantly alter in vivo measures of hippocampal cholinergic function. Levels of endogenous ACh and choline measured in the hippocampus following partial lesions were similar to that of control values. Also, the hippocampal content of newly synthesized [2H4]ACh and the [2H4]ACh synthesis rate were not significantly different from control values. However, following full fimbrial lesions, in vivo measures of hippocampal cholinergic function were decreased to a degree similar to that observed in vitro. Hippocampal levels of endogenous ACh and [2H4]ACh and the synthesis rate for [2H4]ACh were decreased by 73%, 72%, and 83%, respectively. These results suggest that, following partial destruction of afferent cholinergic fibers that innervate the hippocampal formation, residual cholinergic neurons are able to upregulate their capacity to synthesize and store ACh in vivo, thus compensating for lesion-induced losses of cholinergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of Systemic Administration of 2-(4-Phenyl-Piperidino)-Cyclohexanol (Vesamicol) and an Organophosphate DDVP on the Cholinergic System in Brain Regions of Rats

Vesamicol is known to inhibit the transport of acetylcholine (ACh) into synaptic vesicles in vitro, but much less is known about its effects in the brain in vivo. To assess the effect of vesamicol in vivo, we examined cholinergic parameters, such as the subcellular distribution of ACh, activities of enzymes, uptake of choline, and muscarinic receptor binding in the striatum, hippocampus, and cerebral cortex of rats 30 and 60 min after intraperitoneal injection of vesamicol (3 mg/kg) or of vesamicol in combination with DDVP (5 mg/kg), which was administered 10 min before vesamicol. The levels of cytosolic ACh increased in all regions of the brain after injection of vesamicol, while those of vesicular ACh decreased in all regions except for the striatum. The increase in the levels of extracellular ACh and cytosolic ACh in the striatum induced by DDVP was generally enhanced after injection of vesamicol. Vesamicol did not reduce the level of vesicular ACh when DDVP had been injected previously. Vesamicol did not induce any significant changes in the activities of enzymes, choline uptake, or binding of [³H]quinuclidinyl benzilate to the muscarinic ACh receptors in the three regions. Changes in the cholinergic parameters caused by DDVP were not reversed by the combined administration of DDVP with vesamicol. The present results indicate that vesamicol can inhibit the transport of ACh into synaptic vesicles in the brain tissue in vivo, although it cannot reverse the effects of DDVP that has been injected prior to vesamicol.     

Mild hypoxia preconditioning prevents impairment of passive avoidance learning and suppression of brain NGFI-A expression induced by severe hypoxia

The aim of this work was to study effects of mild preconditioning hypobaric hypoxia (380 Torr for 2 h, repeated 3 or 6 times spaced at 24 h) on brain NGFI-A immunoreactivity and passive avoidance (PA) behavior in rats exposed to severe hypoxia (160 Torr for 3 h). Severe hypobaric hypoxia produced extensive neuronal loss in hippocampal CA1, while the preceding hypoxic preconditioning had clear protective effect on neuronal viability of vulnerable hippocampal cells. Besides, the hypoxic preconditioning prevented impairment of acquisition and retention of PA caused by severe hypoxia. The six-trial hypobaric preconditioning was more effective in protection against PA learning deficits in severe hypoxia exposed rats than the three-trial preconditioning. The preconditioning up-regulated severe hypoxia-suppressed neocortical and hippocampal expression of NGFI-A, suggesting a possible role for NGFI-A in the neuroprotective mechanisms activated by hypoxic preconditioning.     

In vitro presynaptic modulation of cholinergic hippocampal activity by pituitary-adrenocortical hormones

In vivo studies have shown that high blood concentrations of pituitary-adrenocortical hormones can activate the hippocampal cholinergic terminals. Incubation of hippocampal synaptosomal preparations with methylprednisolone, or with ACTH at concentrations comparable to stress-induced high concentrations in plasma, did not have any significant effects on the cholinergic parameters measured under unactivated conditions. In the presence of either high K+ or of ACh, choline uptake was decreased. This decrease was not affected by methylprednisolone. However, methylprednisolone did enhance ACh release both after a previous increase (induced by K+) or a decrease (induced by ACh) of ACh release. In contrast, ACTH had no direct effects on either unactivated or K+-stimulated synaptosomes. Thus, a differential effect was exerted by methylprednisolone on the two presynaptic regulatory mechanisms: choline uptake (no change) and ACh release (increase). We suggest that the activation, observed in vivo, resulted mainly from indirect action of the hormones on the hippocampal cholinergic terminals, in view of the fact that the direct effect in vitro was partial.     

Hypothalamic Cholinergic Activity and 2-Deoxyglucose-Induced Hyperglycemia

To clarify the role of the hypothalamic cholinergic system in the regulation of peripheral glucose metabolism, we investigated hypothalamic cholinergic activities after administration of 2-deoxyglucose (2-DG). Intravenous administration of 2-DG (500 mg/kg) caused neuroglycopenia and marked hyperglycemia; the level of plasma glucose increased to 210% of the initial levels at 20 min. For evaluation of the cholinergic activity, we employed a microwave device and subsequently analyzed the contents of acetylcholine (ACh) and choline after microdissection of the hypothalamic nuclei, ventromedial hypothalamic nucleus (VMH), lateral hypothalamus (LH), and paraventricular nucleus (PVN). In addition, we analyzed fluctuation of extracellular levels of ACh using in vivo brain microdialysis. A decrease in the ACh content, and a corresponding increase in the choline content, was observed in those hypothalamic nuclei 20 min after administration of 2-DG. In the microdialysis perfusate, on the other hand, extracellular level of ACh was increased by 2-DG administration. These data show that ACh release, which is cholinergic activity, was increased after 2-DG administration. Our results suggest the involvement and importance of the hypothalamic cholinergic system in 2-DG-induced hyperglycemia.     

In vivo pharmacological characterization of (+/-)-4-[2-(1-methyl-2-pyrrolidinyl)ethyl]thiophenol hydrochloride (SIB-1553A), a novel cholinergic ligand: microdialysis studies

SIB-1553A ((+/-)-4-[2-(1-methyl-2-pyrrolidinyl)ethyl]thiophenol HCl) is a neuronal nicotinic acetylcholine receptor (nAChR) ligand which is active in rodent and primate models of cognition. In functional assays, SIB-1553A exhibits marked subtype selectivity for nAChRs as compared to nicotine. In addition SIB-1553A also exhibits affinities to histaminergic (H3) and serotonergic (5-HT1 and 5HT2) receptors and sigma binding sites. In the present investigation, we characterized SIB-1553A-induced neurotransmitter release in vivo. Following subcutaneous injection (s.c., 10 mg/kg), SIB-1553A rapidly entered the brain achieving concentration of approximately 20 microM 15 min post-injection and was eliminated from plasma with a terminal half-life of approximately 32 min. In freely moving rats, SIB-1553A (1-40 mg/kg, s.c.), markedly increased ACh release in the hippocampus and prefrontal cortex. In both regions, the magnitude of SIB-1553A-induced ACh release was greater than that seen with the prototypical nAChR agonist, nicotine (0.4 mg/kg, s.c.). Both isomers of SIB-1553A induced similar levels of increase in hippocampal ACh release. Increased hippocampal ACh release was also observed following oral administration of SIB-1553A (40 mg/kg) or after local perfusion into the hippocampus (1 mM). SIB-1553A-induced hippocampal ACh release was significantly attenuated by two nAChR antagonists, mecamylamine (MEC) and dihydro-beta-erythroidine (DHbetaE), and by the dopamine (DA) (D1) antagonist, SCH-23390, arguing that ACh release, in part, involves activation of nAChRs and a permissive DA synapse. In contrast to its robust effects on ACh release, SIB-1553A (40 mg/kg, s.c.) modestly increased striatal DA release (approximately 180% of baseline). Due to the proposed role of cholinergic pathways in learning and memory, the neurochemical profile of SIB-1553A suggests a potential for it to treat cognitive dysfunction.     

Effects of dopaminergic receptor agonists and antagonists on the activity of the neo-striatal cholinergic system.

The effects of various neuroleptics and of apomorphine on the metabolism of ACh were examined in the neostriatum of the rat. For this purpose, a specific radio-enzymatic assay for brain ACh was used. This method is based on the preliminary purification of the choline esters by liquid cation exchange, separation of choline and ACh on thin layer chromatography plates, hydrolysis of ACh then reactylation of the choline moiety with a purified and stabilized rat brain choline acetyltransferase. The rat neostriatal ACh levels were decreased by neuroleptics of the phenothiazine and butyrophenone type and increased by apomorphine. An "in vivo" estimation of the rate of utilization of ACh was obtained by measuring the decline in neostriatal ACh content following the local microinjection of hemicholinium-3. This compound blocked almost totally the synthesis of ACh in these conditions. Chlorpromazine (15 mg/kg) enhanced neo-striatal ACh utilization and apomorphine (10 mg/kg) antagonized this effect. Neuroleptics did not effect ACh levels in the parietal cerebral cortex and the hippocampal formation. The modifications of the activity of neostriatal cholinergic neurons by chlorpromazine and apomorphine were still observed following the degeneration of the nigro-neostriatal dopaminergic fibers induced by the injection of 6-hydroxydopamine into the substantia nigra. The results strongly suggest that dopaminergic receptors as defined by their pharmacological interaction with neuroleptics and apomorphine are localized on neostriatal ACh neurons.     

Raised glucose levels enhance scopolamine-induced acetylcholine overflow from the hippocampus: an in vivo microdialysis study in the rat.

Behavioural studies in both humans and animals have shown that an acute rise in circulating glucose levels at or around the time of training enhances subsequent retention performance and can also afford protection from the amnesia produced by posttraining injections of scopolamine. In an attempt to directly investigate the neurochemical basis for these effects of glucose we have tested the hypothesis that raised glucose levels may enhance acetylcholine (ACh) synthesis and release in the brain during conditions of increased neuronal activity, induced either by training or pharmacological challenge, via a microdialysis study using rats. Microdialysate concentrations of ACh overflow from the hippocampus of fasted rats induced by i.p. injections of scopolamine (1 mg/kg) combined with concurrent s.c. injections of either glucose (2 g/kg) or saline were compared in successive 15-min samples using an on-line HPLC system. Scopolamine injections resulted in an immediate 10-20-fold increase in hippocampal ACh overflow which subsequently progressively declined over a 4-h period to pretreatment baseline levels. The combined injection of glucose with scopolamine resulted in a highly significant enhancement (19.4%; P less than 0.01) in ACh content of the first two samples as compared to saline-injected controls. These results provide the first direct experimental evidence that raised glucose levels, via increased availability of acetyl-coenzyme A (acetyl-coA), transiently facilitates ACh synthesis and release during conditions of increased neuronal activity. This enhancement of ACh availability during states of cholinergic neuronal activation may underlie the previously observed facilitatory effects of glucose on memory performance and its protection from scopolamine-induced amnesia.     

The autonomic nervous system at high altitude

The effects of hypobaric hypoxia in visitors depend not only on the actual elevation but also on the rate of ascent. Sympathetic activity increases and there are increases in blood pressure and heart rate. Pulmonary vasoconstriction leads to pulmonary hypertension, particularly during exercise. The sympathetic excitation results from hypoxia, partly through chemoreceptor reflexes and partly through altered baroreceptor function. High pulmonary arterial pressures may also cause reflex systemic vasoconstriction. Most permanent high altitude dwellers show excellent adaptation although there are differences between populations in the extent of the ventilatory drive and the erythropoiesis. Some altitude dwellers, particularly Andeans, may develop chronic mountain sickness, the most prominent characteristic of which being excessive polycythaemia. Excessive hypoxia due to peripheral chemoreceptor dysfunction has been suggested as a cause. The hyperviscous blood leads to pulmonary hypertension, symptoms of cerebral hypoperfusion, and eventually right heart failure and death.     

Control by tachykinin NK(2) receptors of CRF(1) receptor-mediated activation of hippocampal acetylcholine release in the rat and guinea-pig

In vivo microdialysis was employed to explore the effects of different selective non-peptides NK(1),NK(2) and NK(3) receptor antagonists on the corticotropin releasing factor (CRF)-induced release of acetylcholine (ACh) in the hippocampus of rats and guinea-pigs. In both species, the intracerebroventricular (i.c.v.) administration of CRF produced a time- and dose-dependent increase in hippocampal ACh release that was totally suppressed by an intraperitoneally (i.p.) pretreatment with the selective non-peptide CRF(1) receptor antagonist antalarmin (30 mg/kg). Pretreatment with the selective NK(2) receptor antagonist SR48968 (1mg/kg, i.p.) significantly reduced the increase of ACh induced by CRF. In contrast, its low-affinity enantiomer SR48965 (1mg/kg, i.p.) or the NK(1) receptor antagonist, GR205171 (1mg/kg, i.p.) did not exert any antagonist effect. Moreover, administration of the selective NK(3) receptor antagonist SR142801 (1mg/kg, i.p.) did not significantly reduce the CRF-induced hippocampal ACh release in guinea-pigs (the only species studied). The selective activity of SR48968 versus GR205171 or SR142801 indicates that NK(2) receptors play a major role in the control of CRF-induced hippocampal ACh release. Moreover, in freely moving rats, two sessions of stroking of the neck and back of the rat for 30 min, at 90 min intervals, known to be a stressful stimulus, produced a marked and reproducible increase in hippocampal ACh release. This effect was prevented by the administration of the two selective non-peptide CRF1 and NK(2) receptor antagonists antalarmin (30 mg/kg, i.p.) and SR48968 (1mg/kg, i.p.), respectively. This suggests that stress-induced activation of the hippocampal ACh system may be under the control of both endogenously released CRF and NKA, and opens the possibility of the existence of a functional interplay between the pathways containing these peptides as we observed in our experiments on anaesthetized animals.     

Enhanced hippocampal acetylcholine release in nociceptin-receptor knockout mice

Nociceptin (NOC), an endogenous ligand of the opioid receptor-like 1 receptor, is thought to be involved in learning and memory processes. Since acetylcholine (ACh) is involved in hippocampal function, and the hippocampus plays a critical role on the learning and memory function, hippocampal ACh release in NOC-receptor knockout mice was examined using an in vivo microdialysis method. The release of hippocampal ACh was largely increased in the knockout mice. Furthermore, in the knockout mice, an enhanced hippocampal theta rhythm, which is known to be linked to hippocampal memory function, was also observed. Immunohistochemically, in septum, co-existence of NOC receptor with cholinergic, but not with GABAergic neurons, was verified. The findings demonstrate that the NOC receptor is involved in hippocampal cholinergic function.     

Astrocyte-derived kynurenic acid modulates basal and evoked cortical acetylcholine release

We tested the hypothesis that fluctuations in the levels of kynurenic acid (KYNA), an endogenous antagonist of the α7 nicotinic acetylcholine (ACh) receptor, modulate extracellular ACh levels in the medial prefrontal cortex in rats. Decreases in cortical KYNA levels were achieved by local perfusion of S -ESBA, a selective inhibitor of the astrocytic enzyme kynurenine aminotransferase II (KAT II), which catalyses the formation of KYNA from its precursor l -kynurenine. At 5 m m , S -ESBA caused a 30% reduction in extracellular KYNA levels, which was accompanied by a two-threefold increase in basal cortical ACh levels. Co-perfusion of KYNA in the endogenous range (100 n m ), which by itself tended to reduce basal ACh levels, blocked the ability of S -ESBA to raise extracellular ACh levels. KYNA perfusion (100 n m ) also prevented the evoked ACh release caused by d -amphetamine (2.0 mg/kg). This effect was duplicated by the systemic administration of kynurenine (50 mg/kg), which resulted in a significant increase in cortical KYNA formation. Jointly, these data indicate that astrocytes, by producing and releasing KYNA, have the ability to modulate cortical cholinergic neurotransmission under both basal and stimulated conditions. As cortical KYNA levels are elevated in individuals with schizophrenia, and in light of the established role of cortical ACh in executive functions, our findings suggest that drugs capable of attenuating the production of KYNA may be of benefit in the treatment of cognitive deficits in schizophrenia.     

Bidirectional modulation of stimulated cortical acetylcholine release by benzodiazepine receptor ligands

In vivo microdialysis was used to determine the ability of benzodiazepine receptor (BZR) ligands to modulate stimulated cortical acetylcholine (ACh) efflux in awake, freely-moving Fischer-344/BNNia rats. Cortical ACh efflux was reliably enhanced during presentation of a complex stimulus (exposure to darkness coupled with presentation of a small amount of palatable food) in animals entrained with that stimulus. Administration of the BZR selective inverse agonist ZK 93 426 (5.0 mg/kg, i.p.) potentiated the ability of the darkness/food stimulus to enhance efflux, whereas administration of the BZR full agonist, chlordiazepoxide (5.0 mg/kg, i.p.) blocked the enhancement. The interaction of the BZR ligands with the entrained stimulus in affecting cortical ACh efflux was not secondary to effects on motor activity. These results, combined with results from a previous study, suggest that modulation of cortical ACh efflux by BZR ligands is bidirectional and dependent on the level of activity within cortical cholinergic neurons. This relationship enables the trans-synaptic stimulation of cortical ACh transmission by BZR inverse agonists to be most effective during behavioral activities which recruit the basal forebrain cholinergic system.     

Endogenous acetylcholine lowers the threshold for long-term potentiation induction in the CA1 area through muscarinic receptor activation: in vivo study

Little is known how synaptically released endogenous ACh affects hippocampal synaptic plasticity in vivo. Here, we examined the role of cholinergic drive in the regulation of the induction of long-term potentiation (LTP) at basal dendrites in the CA1 area of the anaesthetized rat hippocampus. The non-subtype selective muscarinic acetylcholine receptor antagonist, scopolamine, (0.3 mg/kg, i.p.) inhibited the induction of LTP by weak, but not strong, high frequency conditioning stimulation. A relatively M1 subtype-selective receptor antagonist, pirenzepine, (50 nmol/5 microL, i.c.v.) also inhibited LTP induction by the weak protocol. As the medial septum (MS) is a major source of endogenous ACh in the hippocampus, we also examined the effect of high frequency pre-conditioning stimulation of the MS on LTP induction. The pre-conditioning MS tetanus reduced the threshold for LTP induction at basal synapses in a narrow time window. Such an effect of MS pre-conditioning was prevented by scopolamine, strong evidence of a direct MS control of LTP threshold through a mechanism dependent on muscarinic receptor activation. These results suggest that the cholinergic drive to the hippocampus is critically involved in the control of the LTP induction threshold in vivo. To the extent that LTP mechanisms may underlie certain types of learning and memory, the septo-hippocampal cholinergic regulation of synaptic plasticity may constitute an important target for the treatment of cognitive disorders associated with ACh deficits.