Systemic and intrabasalis administration of the orexin-1 receptor antagonist,SB-334867, disrupts attentional performance in rats

Rationale  

Orexin neurons project to a number of brain regions, including onto basal forebrain cholinergic neurons. Basal forebrain corticopetal cholinergic neurons are known to be necessary for normal attentional performance. Thus, the orexin system may contribute to attentional processing.     

Orexin/hypocretin role in reward: implications for opioid and other addictions

Addiction is a devastating disorder that affects 15.3 million people worldwide. While prevalent, few effective treatments exist. Orexin receptors have been proposed as a potential target for anti-craving medications. Orexins, also known as hypocretins, are neuropeptides produced in neurons of the lateral and dorsomedial hypothalamus and perifornical area, which project widely throughout the brain. The absence of orexins in rodents and humans leads to narcolepsy. However, orexins also have an established role in reward seeking. This review will discuss some of the original studies describing the roles of the orexins in reward seeking as well as specific works that were presented at the 2013 International Narcotics Research Conference. Orexin signalling can promote drug-induced plasticity of glutamatergic synapses onto dopamine neurons of the ventral tegmental area (VTA), a brain region implicated in motivated behaviour. Additional evidence suggests that orexin signalling can also promote drug seeking by initiating an endocannabinoid-mediated synaptic depression of GABAergic inputs to the VTA, and thereby disinhibiting dopaminergic neurons. Orexin neurons co-express the inhibitory opioid peptide dynorphin. It has been proposed that orexin in the VTA may not mediate reward per se, but rather occludes the ‘anti-reward’ effects of dynorphin. Finally, orexin signalling in the prefrontal cortex and the central amygdala is implicated in reinstatement of reward seeking. This review will highlight recent work describing the role of orexin signalling in cellular processes underlying addiction-related behaviours and propose novel hypotheses for the mechanisms by which orexin signalling may impart drug seeking.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2     

Brain cholinergic vulnerability: relevance to behavior and disease

The major populations of cholinergic neurons in the brain include two "projection" systems, located in the pontine reticular formation and in the basal forebrain. These two complexes comprise, in part, the anatomical substrates for the "ascending reticular activating system" (ARAS). The pontine cholinergic system relays its rostral influences mainly through thalamic intralaminar nuclei, but it also connects to the basal forebrain and provides a minor innervation of cortex. The basal forebrain cholinergic complex (BFCC) projects directly to cortex and hippocampus, and has a minor connection with the thalamus. Recent data reveal that a parallel system of basal forebrain GABAergic projection neurons innervates cortex/hippocampus in a way that seems to complement the BFCC. Generally, the picture developed from more than 50 years of research is consistent with a "global" influence of these two ascending cholinergic projections on cortical and hippocampal regions. Seemingly, the BFCC acts in tandem or in parallel with the pontine cholinergic projection to activate the electro-encephalogram, increase cerebral blood flow, regulate sleep-wake cycling, and modulate cognitive function. There are quite a number and variety of human brain conditions, notably including Alzheimer's disease, in which degeneration of basal forebrain cholinergic neurons has been documented. Whether the corticopetal GABA system is affected by disease has not been established. Studies of degeneration of the pontine projection are limited, but the available data suggest that it is relatively preserved in Alzheimer's disease. Hypotheses of BFCC degeneration include growth factor deprivation, intracellular calcium dysfunction, amyloid excess, inflammation, and mitochondrial abnormalities/oxidative stress. But, despite considerable research conducted over several decades, the exact mechanisms underlying brain cholinergic vulnerability in human disease remain unclear.     

Activating the damaged basal forebrain cholinergic system: tonic stimulation versus signal amplification

The hypothesis that the cognitive decline in senile dementia is related to the loss of cortical cholinergic afferent projections predicts that pharmacological manipulations of the remaining cholinergic neurons will have therapeutic effects. However, treatment with cholinesterase inhibitors or muscarinic agonists has been, for the most part, largely unproductive. These drugs seem to disrupt the normal patterning of cholinergic transmission and thus may block proper signal processing. An alternative pharmacological strategy which focuses on the amplification of presynaptic activity without disrupting the normal patterning of cholinergic transmission appears to be more promising. Such a strategy may make use of the normal GABAergic innervation of basal forebrain cholinergic neurons in general, and in particular of the inhibitory hyperinnervation of remaining cholinergic neurons which may develop under pathological conditions. Disinhibition of the GABAergic control of cholinergic activity is assumed to intensify presynaptic cortical cholinergic activity and to enhance cognitive processing. Although the extent to which compounds such as the benzodiazepine receptor antagonist-carboline ZK 93 426 act via the basal forebrain GABA-cholinergic link is not yet clear, the available data suggest that the beneficial behavioral effects of this compound established in animals and humans are based on indirect cholinomimetic mechanisms. It is proposed that an activation of residual basal forebrain cholinergic neurons can be achieved most physiologically via inhibitory modulation of afferent GABAergic transmission. This modulation may have a therapeutic value in treating behavioral syndromes associated with cortical cholinergic denervation.     

Arousal effects of orexin A on acute alcohol intoxication-induced coma in rats

The key role of the hypothalamic neuropeptides orexins in maintenance and promotion of arousal has been well established in normal mammalian animals, but whether orexins exert arousal effects under pathological condition such as coma was little studied. In this study, a model of unconscious rats induced by acute alcohol intoxication was used to examine the effects of orexins through intracerebroventricular injection. The results revealed that either orexin A or orexin B induced decrease of duration of loss of right reflex in alcohol-induced unconscious rats. In the presence of the selective orexin receptor 1 antagonist SB 334867 and orexin receptor 2 antagonist TCS OX2 29, the excitatory action of orexin A was completely blocked. Our data further presented that orexin A also induced reduction of delta power in EEG in these rats. Single-unit recording experiment in vivo demonstrated that orexin A could evoke increase of firing activity of prefrontal cortex neurons in unconscious rats. This excitation was completely inhibited by an H(1) receptor antagonist, pyrilamine, whereas application of α(1)-adrenoreceptor antagonist prazosin or 5-HT(2) selective receptor antagonist ritanserin partially attenuated the excitatory effects of orexin A on these neurons. Consistently, the results of EEG recordings showed that microinjection of pyrilamine, prazosin, or ritanserin suppressed reduction of delta power in EEG induced by orexin A on unconscious rats. Thus, these data suggest that orexins exert arousal effects on alcohol-induced unconscious rats by the promotion of cortical activity through activation of histaminergic, noradrenergic and serotonergic systems. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.     

High density of benzodiazepine binding sites in the substantia innominata of the rat

In order to study the neuronal basis of the pharmacological interactions between benzodiazepine receptor ligands and cortical cholinergic turnover, we examined the regional distribution of specific benzodiazepine binding sites using in vitro autoradiography. In the basal forebrain, the substantia innominata contained a high density of [3H]lormetazepam (LMZ) binding sites (Bmax = 277 fmol/mg tissue; Kd = 0.55 nM). The label could be displaced by diazepam (IC50 = 100 nM), the benzodiazepine receptor antagonist beta-carboline ZK 93426 (45 nM) and the partial inverse agonist beta-carboline FG 7142 (540 nM). It is hypothesized that the amnesic effects of benzodiazepine receptor agonists are exerted through benzodiazepine receptors which are situated on cholinergic neurons in the substantia innominata and are involved in a tonic inhibition of cortical acetylcholine release. The benzodiazepine receptor antagonist ZK 93426 may exert its nootropic effects via benzodiazepine receptors in the substantia innominata and, consequently, by disinhibiting cortical acetylcholine release.     

Orexin 受体拮抗剂：治疗失眠的新途径

Orexin受体有2种亚型,即orexin-1受体和oerxin-2受体,为下丘脑外侧神经元中的2个G蛋白偶联受体,其内源性配体分别为orexin—A和-B。研究发现,动物或人的orexin神经元损伤后会引起嗜睡症,且orexin受体在调节睡眠-觉醒周期方面发挥重要作用。因此,开发orexin受体拮抗剂,成为改善睡眠和治疗失眠的-条新途径。简介orexin及其受体,综述orexin信号通路对睡眠-觉醒的调控作用与机制以及orexin受体拮抗剂的研究与开发。     

Cholinergic Modulation of General Anesthesia

Acetylcholine in the brain promotes arousal and facilitates cognitive functions. Cholinergic neurons in the mesopontine brainstem and basal forebrain are important for activation of the cerebral cortex, which is characterized by the suppression of irregular slow waves, an increase in gamma (30-100 Hz) activity in the electroencephalogram, and the appearance of a hippocampal theta rhythm. During general anesthesia, a decrease in acetylcholine release and cholinergic functions contribute to the desired outcomes of general anesthesia, such as amnesia, loss of awareness and consciousness, and immobility. Animal experiments indicate that inactivation, lesion, or genetic ablation of cholinergic neurons in the basal forebrain potentiated the effects of inhalational and injectable anesthetics, including isoflurane, halothane, propofol, pentobarbital, and in some cases, ketamine. Increased behavioral sensitivity to general anesthesia, faster induction time, and delayed recovery of a loss of righting reflex have been observed in rodents with basal forebrain cholinergic deficits. Cholinergic stimulation in the prefrontal cortex, thalamus, and basal forebrain hastens recovery from general anesthesia. Anticholinesterase accelerates emergence from general anesthesia, but with mixed success, in part depending on the anesthetic used. Cholinergic deficits may contribute to cognitive impairments after anesthesia and operations, which are severe in aged subjects. We propose a cholinergic hypothesis for postoperative cognitive disorder, in line with the cholinergic deficits and cognitive decline in aging and Alzheimer’s disease. The current animal literature suggests that brain cholinergic neurons can regulate the immune and inflammatory response after surgical operation and anesthetic exposure, and anticholinesterase and α7-nicotinic cholinergic agonists can alleviate postoperative inflammatory response and cognitive deficits.     

Orexins/hypocretins stimulate accumulation of inositol phosphate in primary cultures of rat cortical neurons

BackgroundOrexins A and B (also named hypocretins 1 and 2) are hypothalamic peptides with pleiotropic activity. They signal through two G protein-coupled receptors: OX₁R and OX₂R. We have previously demonstrated that both types of orexin receptors are expressed in cultured rat cortical neurons, and stimulation of the predominant OX₂R inhibits cyclic AMP synthesis. In the present work, we examined effects of orexins on inositol phosphate (IP) accumulation in rat cortical neurons.MethodsExperiments were performed on primary neuronal cell cultures prepared fromWistar rat embryos on day 17 of gestation. Following 1 h incubation with orexins, IP levels were measured using the ELISA IP-One assay kit.ResultsOrexins A and B increased, in a concentration-dependent manner, IP accumulation in primary neuronal cell cultures from rat cerebral cortex. Both peptides acted with a similar potency. The calculated EC₅₀ values were 6.0 nM and 10.4 nM for orexin A and orexin B, respectively.ConclusionThe results indicate that in cultured rat cortical neurons orexin receptors are also coupled to inositol phosphates signaling pathway.     

Orexins in the regulation of the hypothalamic-pituitary-adrenal axis

Orexin-A and orexin-B are hypothalamic peptides that act via two G protein-coupled receptors, named orexin type 1 and type 2 receptors (OX1-Rs and OX2-Rs). The most studied biological functions of orexins are the central control of feeding and sleep, but in the past few years findings that orexin system modulates the hypothalamic-pituitary-adrenal (HPA) axis, acting on both its central and peripheral branches, have accumulated. Orexins and their receptors are expressed in the hypothalamic paraventricular nucleus and median eminence and orexin receptors in pituitary corticotropes, adrenal cortex, and medulla. Whereas the effects of orexins on adrenal aldosterone secretion are doubtful, compelling evidence indicates that these peptides enhance glucocorticoid production in rats and humans. This effect involves a 2-fold mechanism: 1) stimulation of the adrenocorticotropin-releasing hormone-mediated pituitary release of adrenocorticotropin, which in turn raises adrenal glucocorticoid secretion; and 2) direct stimulation of adrenocortical cells via OX1-Rs coupled to the adenylate cyclase-dependent cascade. The effects of orexins on catecholamine release from adrenal medulla are unclear and probably of minor relevance, but there are indications that orexins can stimulate in vitro secretion of human pheochromocytoma cells via OX2-Rs coupled to the phospholipase C-dependent cascade. Evidence is also available that orexins enhance the growth in vitro of adrenocortical cells, mainly acting via OX2-Rs. Moreover, findings suggest that the orexin system may favor HPA axis responses to stresses and play a role in the pathophysiology of cortisol-secreting adrenal adenomas.     

D2-like receptors in nucleus accumbens negatively modulate acetylcholine release in prefrontal cortex

Glutamatergic and dopaminergic inputs converge on medium spiny neurons in nucleus accumbens and regulate the excitability of these projections to target areas including the cholinergic basal forebrain. NMDA receptors situated on these projections are locally modulated by D1- and D2-like receptors. We previously reported that the D1-like positive modulation of NMDA receptor activity is expressed trans-synaptically in the control of basal forebrain cholinergic projections to prefrontal cortex. The present experiments tested the hypothesis that D2-like receptors in accumbens negatively modulate cortical ACh release. Perfusion of NMDA (150 microM) into the shell region of the accumbens produced a sustained increase (150-200%) in ACh release in prefrontal cortex. This increase was completely blocked by co-perfusion with the D2-like agonist quinpirole (100 microM). Perfusion of quinpirole also reduced basal ACh release (approximately 50%) in prefrontal cortex. The contribution of D2 receptors to the quinpirole effect was assessed in two additional studies. The first study revealed that co-perfusion of the D2 antagonist haloperidol (100 microM) blocked the quinpirole-induced attenuation of NMDA mediated ACh release. The second experiment demonstrated that intra-accumbens perfusion of quinelorane (100 microM), a more selective D2 agonist than quinpirole, also attenuated the NMDA mediated ACh release. Collectively, these studies demonstrate that D2 receptors in accumbens negatively modulate basal and NMDA mediated increases in ACh release in prefrontal cortex. This negative modulation may contribute to the integration of normal attentional processing and goal directed behavior and to the therapeutic effects of antipsychotic medication on cognition in psychopathologies such as schizophrenia.     

The hypocretins/orexins: integrators of multiple physiological functions

The hypocretins (Hcrts), also known as orexins, are two peptides derived from a single precursor produced in the posterior lateral hypothalamus. Over the past decade, the orexin system has been associated with numerous physiological functions, including sleep/arousal, energy homeostasis, endocrine, visceral functions and pathological states, such as narcolepsy and drug abuse. Here, we review the discovery of Hcrt/orexins and their receptors and propose a hypothesis as to how the orexin system orchestrates these multifaceted physiological functions.Linked ArticlesThis article is part of a themed section on Orexin Receptors. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-2     

Galanin: neurobiologic mechanisms and therapeutic potential for Alzheimer's disease

The neuropeptide galanin (GAL) is widely distributed in the mammalian CNS. Several lines of evidence suggest that GAL may play a critical role in cognitive processes such as memory and attention through an inhibitory modulation of cholinergic basal forebrain activity. Furthermore, GAL fibers hyperinnervate remaining cholinergic basal forebrain neurons in Alzheimer's disease (AD). This suggests that GAL activity impacts cholinergic dysfunction in advanced AD. Pharmacological and in vitro autoradiographic studies indicate the presence of heterogeneous populations of GAL receptor (GALR) sites in the basal forebrain which bind GAL with both high and low affinity. Interestingly, we have recently observed that GALR binding sites increase in the anterior basal forebrain in late-stage AD. Three G protein-coupled GALRs have been identified to date that signal through a diverse array of effector pathways in vitro, including adenylyl cyclase inhibition and phospholipase C activation. The repertoire and distribution of GALR expression in the basal forebrain remains unknown, as does the nature of GAL and GALR plasticity in the AD basal forebrain. Recently, GAL knockout and overexpressing transgenic mice have been generated to facilitate our understanding of GAL activity in basal forebrain function. GAL knockout mice result in fewer cholinergic basal forebrain neurons and memory deficits. On the other hand, mice overexpressing GAL display hyperinnervation of basal forebrain and memory deficits. These data highlight the need to explore further the putative mechanisms by which GAL signaling might be beneficial or deleterious for cholinergic cell survival and activity within basal forebrain. This information will be critical to understanding whether pharmacological manipulation of GALRs would be effective for the amelioration of cognitive deficits in AD.     

α7 nicotinic acetylcholine receptors in Alzheimer's disease: neuroprotective, neurotrophic or both?

One of the early signs of Alzheimer's disease is the impairment in hippocampus-based episodic memory function, which is improved through the enhancement of cholinergic transmission. Several studies suggest that α7 nicotinic receptor (nAChR) activation represents a useful therapeutic strategy for the cognitive impairments associated with early Alzheimer's disease as the α7 subtype of nicotinic acetylcholine receptors are expressed by basal forebrain cholinergic projection neurons as well as by their targets in the hippocampus. The current model for the cholinergic deficit in Alzheimer's disease posits that inappropriate accumulation of misfolded oligomeric aggregates of β-amyloid peptide leads to the dysfunction of the signaling mechanisms that support the cholinergic phenotype; this is manifested as an altered function of nicotinic acetylcholine receptors and the nerve-growth factor trophic support system that results in the loss of cholinergic markers and eventually cholinergic neurons from the basal forebrain cholinergic system. A view was confounded by the fact that α7 nAChRs and β-amyloid peptides have been shown to interact in vitro and in vivo, including human post-mortem AD brain. This review will begin with a brief overview of the basal forebrain cholinergic system, followed by a discussion of the current knowledge of the cholinergic deficit in Alzheimer's disease, then a summary of the cholinergic phenotype observed in transgenic Alzheimer's disease mouse models. We will also present our recent findings that support our hypothesis that the α7 nicotinic acetylcholine receptor performs both the neurotrophic and neuroprotective roles in the maintenance of the cholinergic phenotype and discusses potential mechanisms and implications for Alzheimer's disease therapy.     

Cholinergic but not monoaminergic denervation increases nerve growth factor content in the adult rat hippocampus and cerebral cortex

Summary We have investigated whether degeneration of basal forebrain cholinergic neurons is a potential trigger for increased NGF production in the adult rat brain. Electrolytic lesions of cholinergic neurons in the septum-diagonal band and in the nucleus basalis of Meynert induced a transient increase in NGF in the ventral hippocampus (+70%) and cerebral cortex (+125%), respectively. In contrast, selective aminergic denervation of the forebrain by electrolytic lesion of the medial forebrain bundle, did not increase NGF levels in hippocampus and cerebral cortex. Thus, a cholinergic mechanism appears to regulate NGF production in adult rat basal forebrain.     

Orexin A and B evoke noradrenaline release from rat cerebrocortical slices.

1. Orexin A and B, recently identified in the rat hypothalamus are endogenous neuropeptide agonists for the G-protein coupled orexin-1 (OX1) and orexin-2 (OX2) receptors. 2. In the present study, we have examined the effects of orexin A, B and raised extracellular K(+) on noradrenaline release from the rat cerebrocortical slice. We have compared this with other sleep-wake-related (excitatory) neurotransmitters; dopamine, glutamate, serotonin and histamine. 3. Neurotransmitter release studies were performed in rat cerebrocortical slices incubated in modified Krebs buffer (with and without Ca(2+)+EGTA 1 mM) with various concentrations of orexin A, B and K(+) for various times. 4. Orexin A and B-evoked (10(-7) M) noradrenaline release was time-dependent reaching a maximum some 10 min after stimulation. K(+) (40 mM) evoked release was also time dependent but reached a maximum after 6 min. Orexin A, B and K(+) stimulation of release was concentration dependent with pEC(50) and E(max) (% of basal) values of 8.74+/-0.32 (1.8 nM) and 263+/-14% and 8.61+/-0.38 (2.4 nM) and 173+/-7% and 1.43+/-0.02 (37 mM) and 1430+/-70%, respectively. Orexin-evoked release was partially extracellular Ca(2+) dependent. 5. Of the other transmitters studied there was a weak orexin A and B stimulation of glutamate release. In contrast K(+) evoked dopamine, glutamate, histamine and serotonin release with pEC(50) and E(max) (% of basal) values of 1.47+/-0.05 (34 mM) and 3430+/-410%, 1.38+/-0.04 (42 mM) and 1240+/-50%, 1.47+/-0.02 (34 mM) and 480+/-10% and 1.40+/-0.05 (40 mM) and 560+/-60% respectively. 6. We conclude that the neuropeptides orexin A and B evoke noradrenaline release from rat cerebrocortical slices.     

Muscarinic receptors in brain stem and mesopontine cholinergic arousal functions

All five muscarinic receptor subtypes and mRNAs are found widely in the brain stem, with M? muscarinic receptors most concentrated in the hindbrain. Three cholinergic cell groups, Ch5: pedunculopontine (PPT); Ch6: laterodorsal tegmental (LDT); Ch8: parabigeminal (PBG), are found in the tegmentum. Ch5,6 neurons are activated by arousing and reward-activating stimuli, and inhibited via M?-like autoreceptors. Ch5,6 ascending projections activate many forebrain regions, including thalamus, basal forebrain, and orexin/hypocretin neurons (via M? receptors) for waking arousal and attention. Ch5,6 activation of dopamine neurons of the ventral tegmental area and substantia nigra (via M? receptors) increases reward-seeking and energizes motor functions. M? receptors on dopamine neurons facilitate brain-stimulation reward, opiate rewards and locomotion, and male ultrasonic vocalizations during mating in rodents. Ch5 cholinergic activation of superior colliculus intermediate layers facilitates fast saccades and approach turns, accompanied by nicotinic and muscarinic inhibition of the startle reflex in pons. Ch8 PBG neurons project to the outer layers of the superior colliculus only, where M? receptors are associated with retinotectal terminals. Ch5,6 descending projections to dorsal pontine reticular formation contribute to M?-dependent REM sleep.     

Connectomics of orexin-producing neurons: analysis using mouse models

Orexin A and orexin B (also known as hypocretin 1 and hypocretin 2) are hypothalamic neuropeptides that we discovered thirteen years ago. Initially, these peptides were recognized as regulators of feeding behavior. Subsequently, several studies suggested that orexin deficiency causes narcolepsy in humans and other mammalian species, highlighting roles of this hypothalamic neuropeptide in the regulation of sleep and wakefulness. Studies of efferent and afferent systems of orexin-producing neurons have shown that the orexin neuronal system has close interactions with systems that regulate emotion, energy homeostasis, reward, and arousal. These observations suggest that orexin neurons are involved in sensing the body's external and internal environments, and regulate vigilance states accordingly.     

The cholinergic deficit in Alzheimer's disease: impact on cognition, behaviour and function

Although the neurodegeneration occurring in Alzheimer's disease (AD) affects multiple neurotransmitters, the cholinergic system has received the greatest attention. Acetylcholine (ACh) is fundamental to mnemonic function, assisting in the septal hippocampal pathway and facilitating cortical activation. One of the earliest pathological events in AD is the degeneration of ACh-synthesizing neurons in the subcortical nuclei of the human basal forebrain. Indeed, the loss of cholinergic function in AD is correlated with the density of histopathological markers of AD, the severity of cognitive dysfunction and disease duration. However, the precise mechanism by which the cholinergic system influences cognition, and behaviour, is unknown. Recent preliminary data from functional imaging and ligand-binding studies implicate a dynamic interaction between the nicotinic-muscarinic cholinergic receptor systems. The relatively preserved thalamic nicotinic system, compared with the dysfunctional cortical muscarinic system, may facilitate thalamocortical metabolic excitation in the failing AD brain. Thus, it is hypothesized that thalamic influence within frontal-subcortical circuits is augmented in AD patients who demonstrate a marked improvement to cholinesterase inhibitor therapy. Understanding the cholinergic basis of the cognitive, functional and behavioural deficits in AD, and the differential treatment response to various agents, will ultimately improve patient care and neuropharmacological insights. This paper reviews the current understanding of the cholinergic influence in cognition, behaviour and, as a result, function in AD patients.     

Sustained effect of metrifonate on cerebral glucose metabolism after immunolesion of basal forebrain cholinergic neurons in rats

To evaluate the influence of cholinergic projections from the basal forebrain on brain metabolism, we measured the cerebral metabolic rate of glucose (CMR(glu)) after unilateral lesioning of cholinergic basal forebrain neurons with the immunotoxin 192 IgG-saporin. CMR(glu) was determined in 24 cortical and 13 sub-cortical regions using the [14C]2-deoxy-D-glucose technique of Sokoloff. Average hemispheric CMR(glu) decreased by 7% (P<0.02) and 5% (P<0.05), 7 and 21 days after lesion, respectively. Regional effects were restricted to parietal and retrosplenial cortices, lateral habenula and the basal forebrain. We have previously shown that metrifonate increased CMR(glu) in intact rats. In lesioned rats, metrifonate (80 mg/kg, i. p.) was still active but the metabolic activation was reduced in terms of both the average hemispheric CMR(glu) and the number of regions significantly affected. Although it is reduced, the sustained effect of metrifonate in lesioned rats makes an argument for the use of this compound as treatment of cholinergic deficit in Alzheimer's disease.