Differential patterns of local cerebral glucose utilization in response to 5-hydroxytryptamine agonists

Molecular and Functional Imaging Studies of Psychedelic Drug Action in Animals and Humans

Hallucinogens are a loosely defined group of compounds including LSD, N,N-dimethyltryptamines, mescaline, psilocybin/psilocin, and 2,5-dimethoxy-4-methamphetamine (DOM), which can evoke intense visual and emotional experiences. We are witnessing a renaissance of research interest in hallucinogens, driven by increasing awareness of their psychotherapeutic potential. As such, we now present a narrative review of the literature on hallucinogen binding in vitro and ex vivo, and the various molecular imaging studies with positron emission tomography (PET) or single photon emission computer tomography (SPECT). In general, molecular imaging can depict the uptake and binding distribution of labelled hallucinogenic compounds or their congeners in the brain, as was shown in an early PET study with N1-([11C]-methyl)-2-bromo-LSD ([11C]-MBL); displacement with the non-radioactive competitor ketanserin confirmed that the majority of [11C]-MBL specific binding was to serotonin 5-HT2A receptors. However, interactions at serotonin 5HT1A and other classes of receptors and pleotropic effects on second messenger pathways may contribute to the particular experiential phenomenologies of LSD and other hallucinogenic compounds. Other salient aspects of hallucinogen action include permeability to the blood-brain barrier, the rates of metabolism and elimination, and the formation of active metabolites. Despite the maturation of radiochemistry and molecular imaging in recent years, there has been only a handful of PET or SPECT studies of radiolabeled hallucinogens, most recently using the 5-HT2A/2C agonist N-(2[11CH3O]-methoxybenzyl)-2,5-dimethoxy- 4-bromophenethylamine ([11C]Cimbi-36). In addition to PET studies of target engagement at neuroreceptors and transporters, there is a small number of studies on the effects of hallucinogenic compounds on cerebral perfusion ([15O]-water) or metabolism ([18F]-fluorodeoxyglucose/FDG). There remains considerable scope for basic imaging research on the sites of interaction of hallucinogens and their cerebrometabolic effects; we expect that hybrid imaging with PET in conjunction with functional magnetic resonance imaging (fMRI) should provide especially useful for the next phase of this research.

Serotonin 5-HT1A Receptor Biased Agonists Induce Different Cerebral Metabolic Responses: A [18F]-Fluorodesoxyglucose Positron Emission Tomography Study in Conscious and Anesthetized Rats

Serotonin 5-HT_1A receptors constitute an attractive therapeutic target for various psychiatric or neurodegenerative disorders. These receptors are expressed in multiple brain regions on different neuronal populations and can be coupled with distinct G-protein subtypes; such functional diversity complicates the use of 5-HT_1A ligands in several pathologies where it would be desirable to stimulate the receptors in a precise region. Therefore, using "biased agonists" able to target specifically certain subpopulations of 5-HT_1A receptors would enable achievement of better therapeutic benefit. Several 5-HT_1A receptor biased agonists are currently in development, including NLX-101 (aka F15599) and NLX-112 (aka F13640, befiradol), with preclinical data suggesting that they preferentially target different populations of 5-HT_1A receptors. However, most previous studies used invasive and regionally limited approaches. In this context, [¹⁸F]-fluorodesoxyglucose (FDG)-positron emission tomography (PET) imaging constitutes an interesting technique as it enables noninvasive mapping of the regional brain activity changes following a pharmacological challenge in conscious animals. We report here the evaluation of cerebral glucose metabolism following intraperitoneal injection of different doses of NLX-112 or NLX-101 in conscious or isoflurane-anesthetized rats. The biased agonists produced different metabolic "fingerprints" with distinct regional preferences, consistent with previous studies. At equal doses, the effect of NLX-101 was less marked than NLX-112 in the piriform cortex, in the striatum (in terms of inhibition), and in the pontine nuclei and the cerebellum (in terms of activation); furthermore, only NLX-112 increased the glucose metabolism in the parietal cortex, whereas only NLX-101 induced a clear activation in the colliculi and the frontal cortex, which may be related to its distinctive procognitive profile. Both agonist effects were almost completely unapparent in anesthetized animals, underlining the importance of studying serotonergic neurotransmission in the conscious state. In this regard, [¹⁸F]FDG-PET imaging seems very complementary with other functional imaging techniques such as pharmacological MRI.

Positron emission tomography assessment of 8-OH-DPAT-mediated changes in an index of cerebral glucose metabolism in female marmosets

As part of a larger experiment investigating serotonergic regulation of female marmoset sexual behavior, this study was designed to (1) advance methods for PET imaging of common marmoset monkey brain, (2) measure normalized FDG uptake as an index of local cerebral metabolic rates for glucose, and (3) study changes induced in this index of cerebral glucose metabolism by chronic treatment of female marmosets with a serotonin 1A receptor (5-HT(1A)) agonist. We hypothesized that chronic treatment with the 5-HT(1A) agonist 8-OH-DPAT would alter the glucose metabolism index in dorsal raphe (DR), medial prefrontal cortex (mPFC), medial preoptic area of hypothalamus (mPOA), ventromedial nucleus of hypothalamus (VMH), and field CA1 of hippocampus. Eight adult ovariectomized female common marmosets (Callithrix jacchus) were studied with and without estradiol replacement. In a crossover design, each subject was treated daily with 8-OH-DPAT (0.1mg/kg SC daily) or saline. After 42-49 days of treatment, the glucose metabolism radiotracer FDG was administered to each female immediately prior to 30 min of interaction with her male pairmate, after which the subject was anesthetized and imaged by PET. Whole brain normalized PET images were analyzed with anatomically defined regions of interest (ROI). Whole brain voxelwise mapping was also used to explore treatment effects and correlations between alterations in the glucose metabolism index and pairmate interactions. The rank order of normalized FDG uptake was VMH/mPOA>DR>mPFC/CA1 in both conditions. 8-OH-DPAT did not induce alterations in the glucose metabolism index in ROIs. Voxelwise mapping showed a significant reduction in normalized FDG uptake in response to 8-OH-DPAT in a cluster in medial occipital cortex as well as a significant correlation between increased rejection of mount attempts and reduced normalized FDG uptake in an overlapping cluster. In conclusion, PET imaging has been used to measure FDG uptake relative to whole brain in marmoset monkeys. Voxelwise mapping shows that 8-OH-DPAT reduces this index of glucose metabolism in medial occipital cortex, consistent with alterations in female sexual behavior.

WITHDRAWN: Is acute tryptophan depletion a valid method to assess central serotonergic function?

This article has been withdrawn at the request of the editor. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.

Chronic treatment with fluoxetine decreases cerebral metabolic responses to the 5-HT1A agonist 8-hydroxy-2(di-N-propylamino)tetralin and increases those to the 5-HT2A/2C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane and to the dopaminergic agonist apomorphine

Fluoxetine is a selective serotonin (5-HT) reuptake inhibitor that, when given chronically, alters different neurotransmitter systems. To assess functional changes occurring in the 5-HT and dopaminergic systems, we investigated the effects of 5-HT(1A) agonist 8-hydroxy-2(di-N-propylamino)tetralin (8-OH-DPAT), of the 5-HT(2A/2C) agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and of the dopamine D(1/2) agonist apomorphine (APO) on behavior and on regional cerebral metabolic rates for glucose (rCMRglc) in rats pretreated for 3weeks with saline or fluoxetine (8mg/kg/day). Behavioral effects were assessed for 8-OH-DPAT by scoring the 5-HT syndrome, for DOI by counting head shakes and for APO with an activity monitor. rCMRglc were measured with quantitative autoradiographic [(14)C]2-deoxyglucose technique in 60 brain regions at 10min after acute administration of 8-OH-DPAT 1mg/kg, at 30min after DOI 5mg/kg or at 10min after APO 1mg/kg. Chronic fluoxetine did not alter the 5-HT syndrome by 8-OH-DPAT, decreased head shakes by DOI and enhanced hyperlocomotion by APO. 8-OH-DPAT produced rCMRglc increases in sensorimotor regions that were unaffected by fluoxetine pretreatment and diffuse metabolic decrements that were attenuated by fluoxetine in limbic and raphe areas (17% and 4% mean decreases, respectively, in saline control and fluoxetine-pretreated rats). DOI produced widespread rCMRglc declines that were intensified by fluoxetine (14% and 20% decreases, in control and fluoxetine rats). APO caused rCMRglc increases in 22 brain regions that were potentiated by fluoxetine in dopaminergic motor areas (10% and 25% increases, in control and fluoxetine rats). In conclusion, fluoxetine enhances 5-HT neurotransmission by blunting responsivity of 5-HT(1A) autoreceptors and increasing that of 5-HT(2A/2C) postsynaptic receptors and enhances dopaminergic D(1/2) receptor neurotransmission.

Phosphodiesterase 2 and 5 inhibition attenuates the object memory deficit induced by acute tryptophan depletion

The underlying mechanism of short-term memory improvement after inhibition of specific phosphodiesterases (PDEs) is still poorly understood. The present study aimed to reveal the ability of PDE5 and PDE2 inhibitors, that increase cyclic guanosine monophosphate (cGMP) and both cyclic adenosine monophosphate (cAMP) and cGMP, respectively, to reverse an object recognition deficit induced by acute tryptophan depletion. Acute tryptophan depletion is a pharmacological challenge tool to lower central serotonin (5-hydroxytryptamine; 5-HT) levels by depleting the availability of its dietary precursor tryptophan. Short-term object memory was tested in male Wistar rats by exposing them to the object recognition task. First, the effects of acute tryptophan depletion upon object recognition 2 h after administration of the nutritional mixture were established. Subsequently, acute tryptophan depletion was combined with the PDE5 inhibitor vardenafil (1, 3 and 10 mg/kg) or with the PDE2 inhibitor BAY 60-7550 (0.3, 1 and 3 mg/kg), 30 min prior to testing. Acute tryptophan depletion significantly lowered plasma tryptophan levels and impaired object recognition performance. Vardenafil (3 and 10 mg/kg) and BAY 60-7550 (3 mg/kg) were able to attenuate the acute tryptophan depletion induced object recognition impairment. Thus, both PDE5 and PDE2 inhibition improved short-term object recognition performance after an acute tryptophan depletion induced deficit. The underlying mechanisms, however, remain poorly understood and further studies are needed to determine whether the present findings can be explained by a direct effect of enhanced cAMP and cGMP levels upon 5-HT activity, or even other neurotransmitter systems, and possibly an interaction with synthesis of nitric oxide or effects upon cerebral blood flow function.

The neurotransmitter basis of cognition: psychopharmacological activation studies using positron emission tomography

The neuromodulatory effect of manipulating monoaminergic receptor function was assessed by combining a psychological and a pharmacological activation during repeated positron emission tomographic (PET) scans. The effects of buspirone (a 5-HT1A receptor partial agonist) on changes in regional cerebral blood flow (rCBF) associated with free word recall were examined. A factorial design was used to demonstrate a significant interaction (changes in rCBF brought about by psychological activation which depend on drug administration) in the left parahippocampal region. This interaction was an attenuation of increases in local neuronal activity (rCBF) related to memory function. Buspirone-induced decreases in rCBF, independent of the memory effect, were seen in the left prefrontal and parietal cortices. We suggest that combined psychological and pharmacological activation is a way of measuring direct (main) drug effects and modulatory effects on neurotransmission associated with cognitive functions (interaction).

Persistent cerebrovascular effects of MDMA and acute responses to the drug

Acutely, 3,4,-methylenedioxymethamphetamine (MDMA) induces cerebrovascular dysfunction [Quate et al., (2004)Psychopharmacol., 173, 287-295]. In the longer term the same single dose results in depletion of 5-hydroxytrptamine (5-HT) nerve terminals. In this study we examined the cerebrovascular consequences of this persistent neurodegeneration, and the acute effects of subsequent MDMA exposure, upon the relationship that normally exists between local cerebral blood flow (LCBF) and local cerebral glucose utilization (LCMRglu). Dark agouti (DA) rats were pre-treated with 15 mg/kg i.p. MDMA or saline. Three weeks later, rats from each pre-treatment group were treated with an acute dose of MDMA (15 mg/kg i.p.) or saline. Quantitative autoradiographic imaging was used to measure LCBF or LCMRglu with [(14)C]-iodoantipyrine and [(14)C]-2-deoxyglucose, respectively. Serotonergic terminal depletion was assessed using radioligand binding with [(3)H]-paroxetine and immunohistochemistry. Three weeks after MDMA pre-treatment there were significant reductions in densities of 5-HT transporter (SERT)-positive fibres (-46%) and [(3)H]-paroxetine binding (-47%). In animals pre-treated with MDMA there were widespread significant decreases in LCMRglu, but no change in LCBF indicating a persistent loss of cerebrovascular constrictor tone. In both pre-treatment groups, acute MDMA produced significant increases in LCMRglu, while LCBF was significantly decreased. In 50% of MDMA-pre-treated rats, random areas of focal hyperaemia indicated a loss of autoregulatory capacity in response to MDMA-induced hypertension. These results suggest that cerebrovascular regulatory dysfunction resulting from acute exposure to MDMA is not diminished by previous exposure, despite a significant depletion in 5-HT terminals. However, there may be a sub-population, or individual circumstances, in which this dysfunction develops into a condition that might predispose to stroke.

The effect of apomorphine and buspirone on regional cerebral blood flow during the performance of a cognitive task-measuring neuromodulatory effects of psychotropic drugs in man

Psychopharmacological activation, in conjunction with positron emission tomographic measurements of regional cerebral blood flow (rCBF), was used to investigate the neurotransmitter basis of a specific cognitive function in man. Monoaminergic neurotransmission was pharmacologically manipulated during performance of auditory - verbal memory tasks. Statistical parametric mapping was used to identify the brain sites of interaction between memory-induced increases in rCBF and active drugs. Memory task-induced increases in rCBF in the left prefrontal cortex were attenuated by apomorphine, a non-selective dopamine agonist, whilst buspirone, a serotonin1A partial agonist, augmented rCBF increases in this area. In addition, apomorphine and buspirone augmented memory-induced increases in rCBF centred in the posterior cingulate cortex, whilst buspirone alone attenuated rCBF increases in the retrosplenial cortex and posterior parahippocampal gyrus. These regionally selective interactions may represent neuromodulatory effects of monoaminergic neurotransmission on a specific cognitive function in man.

Changes in rat cerebral blood volume due to modulation of the 5-HT(1A) receptor measured with susceptibility enhanced contrast MRI

Brain blood volume changes in the rat in response to 5-HT(1A) agonist and antagonist administration were measured using susceptibility contrast enhanced magnetic resonance imaging (MRI). Administration of the 5-HT(1A) agonist 8-OH-DPAT resulted in decreases in fractional brain blood volumes. Administration of the 5-HT(1A) antagonist WAY-100635 following a dose of 8-OH-DPAT resulted in increases in fractional blood volumes greatest in hippocampus and cortex and smallest in thalamus and caudate-putamen. The magnitude of the regional increases in blood volumes paralleled the distribution of 5-HT(1A) receptors in the rat brain. Administration of WAY-100635 alone resulted in decreases in cortical blood volume and increases in cerebellar blood volume.

Serotonin, cerebral blood flow, and cerebral metabolic rate in geriatric major depression and normal aging

While there is substantial evidence for abnormalities in serotonin (5-HT) neurotransmission in major depressive disorder (MDD), almost all of the findings derive from studies of young adults. Moreover, relatively little research has assessed brain 5-HT transmission in vivo. Neuroendocrine studies do not permit evaluation of a range of brain regions, but only the limited circuitry associated with hormone release. Data from autopsy studies are limited by the difficulties of assessment of the acute clinical picture before death, and by post-mortem artifacts. In vivo neuroimaging techniques overcome many of the methodological limitations of both these approaches. There is a large body of imaging data indicating regional cerebral blood flow (rCBF) and cerebral metabolic rate (rCMR) decrements both with aging and in patients with MDD. While the physiological bases for these phenomena are largely unknown, changes in brain 5-HT function may be involved. Neuroanatomical studies have revealed an intricate network of 5-HT-containing neurons within the cerebral microvasculature, with physiological evidence for serotonergic control of both rCBF and rCMR. Acute pharmacological challenges are available to probe brain 5-HT function. Such paradigms, using neuroendocrine responses as endpoints, have been of some utility in predicting outcome with antidepressant treatment. The role of 5-HT dysregulation in geriatric MDD takes on more importance given concerns regarding putative reduced efficacy of serotonin-specific reuptake inhibitors (SSRIs) in this population. If this is due to diminished responsivity of 5-HT systems, then the ability to identify antidepressant nonresponders via 5-HT challenge in combination with neuroimaging measures may have important clinical utility.

Serotonin in the regulation of brain microcirculation

Manipulation of brainstem serotonin (5-HT) raphe neurons induces significant alterations in local cerebral metabolism and perfusion. The vascular consequences of intracerebrally released 5-HT point to a major vasoconstrictor role, resulting in cerebral blood flow (CBF) decreases in several brain regions such as the neocortex. However, vasodilatations, as well as changes in blood-brain barrier (BBB) permeability, which are blocked by 5-HT receptor antagonists also can be observed. A lack of relationship between the changes in flow and metabolism indicates uncoupling between the two variables and is suggestive of a direct neurogenic control by brain intrinsic 5-HT neurons on the microvascular bed. In line with these functional data are the close associations that exist between 5-HT neurons and the microarterioles, capillaries and perivascular astrocytes of various regions but more intimately and/or more frequently so in those where CBF is altered significantly following manipulation of 5-HT neurons. The ability of the microvascular bed to respond directly to intracerebrally released 5-HT is underscored by the expression of distinct 5-HT receptors in the various cellular compartments of the microvascular bed. Thus, it appears that while some 5-HT-mediated microvascular functions involve directly the blood vessel wall, others would be relayed through the perivascular astrocyte. The strategic localization of perivascular astrocytes and the different 5-HT receptors that they harbor strongly emphasize their putative pivotal role in transmitting information between 5-HT neurons and microvessels. It is concluded that the cerebral circulation has full capacity to adequately and locally adapt brain perfusion to changes in central 5-HT neurotransmission either directly or indirectly via the neuronal-astrocytic-vascular tripartite functional unit. Dysfunctions in these neurovascular interactions might result in perfusion deficits and might be involved in specific pathological conditions.

Cerebral metabolic effects of serotonin drugs and neurotoxins

The functional effects of serotonin (5-HT) drugs and toxins on regional cerebral metabolic rates for glucose (rCMRglc) have been determined in rats with the in vivo, quantitative, autoradiographic [14C]2-deoxyglucose technique. Serotonin agents produced rCMRglc patterns different and more specific that one would predict from binding studies. At low doses 5-HT1 agonists reduced rCMRglc in limbic areas and at high doses increased rCMRglc in brain motor regions. The 5-HT2 agonists dose-dependently decreased rCMRglc in proencephalic areas and increased it in thalamic nuclei. 5-HT3 receptor antagonism resulted in rCMRglc decreases in limbic, auditory and visual areas and agents with 5-HT3 receptor activity increased rCMRglc in brain regions with high 5-HT3 receptor densities. Serotonin anxiolytics (e.g. azapirones) and antidepressants (e.g. tryciclic and non-tryciclic 5-HT reuptake inhibitors) reduced rCMRglc selectively in limbic areas and in brainstem monoaminergic nuclei. Dose, time from administration, receptor affinity, behavioral and neurochemical correlates, 5-HT system lesion and circulating glucocorticoid were all relevant factors in determining the rCMRglc effects of 5-HT drugs. Acutely neurotoxic amphetamines markedly increased rCMRglc in brain regions such as the nucleus accumbens that are thought to mediate amphetamine reinforcing properties; on the long term, toxic or electrolytic lesions or chronic treatment with 5-HT agonists produced minimal rCMRglc alterations in spite of marked and persistent changes in 5-HT function. In lesioned or chronically treated rats, acute challanges with 5-HT and non 5-HT agonists demonstrated specific deficits that were not detected in a resting state. Serotonin neuromodulation has been studied in humans by using positron emission tomography with 15O-water. Sequential measurements of regional cerebral blood flow (rCBF) were obtained during combined pharmacological challange with the 5-HT1A agonist buspirone and cognitive activation. Buspirone increased a memory related rCBF activation in task specific regions. This technique can provide a strong theoretical basis for the understanding of 5-HT drug mode of action in normal human brain and in neuropsychiatric diseases. Brain metabolism studies in animals will still be needed to elucidate the factors (e.g. pharmacokinetic and pharmacodynamic) relevant to the cerebral response to 5-HT drugs in humans.

Comparison of the effect of buspirone and 1-(2-pyrimidinyl)-piperazine on cerebral glucose utilization in the rat

The novel anxiolytic and antidepressant, buspirone, and its main metabolite 1-(2-pyrimidinyl)-piperazine (1-PP), which is pharmacologically active, were examined for their effect on regional cerebral glucose utilization in awake rats using quantitative 2-deoxyglucose autoradiography. At a dose of 1 mg/kg, buspirone reduced glucose utilization in the hippocampus by approximately 15%, whilst 1-PP at the same dose had no effect. In comparison, at a higher dose, 10 mg/kg of both buspirone and 1-PP decreased glucose utilization in the hippocampus by 20% and 27%, respectively. In addition, widespread reductions in local cerebral glucose utilization were noted with this higher dose of 1-PP; such generalized effects are compatible with those reported for alpha 2 adrenoceptor antagonists. Buspirone at 1 and 10 mg/kg increased glucose utilization by 40% and 65%, respectively, in the lateral habenular nucleus, whilst 1-PP at 1 and 10 mg/kg had no effect. The findings suggest that buspirone's effects on glucose utilization cannot be attributed to 1-PP, unless high doses of buspirone are administered.

The tricyclic antidepressant clomipramine dose-dependently reduces regional cerebral metabolic rates for glucose in awake rats

The time course and the relation to dose of regional cerebral metabolic rates for glucose (rCMRglc) were measured in awake male Fischer-344 rats after administration of clomipramine (CMI), a serotonin (5-HT) uptake inhibitor and clinical antidepressant. rCMRglc was determined, using the quantitative autoradiographic [14C]2-deoxyglucose technique, in 64 brain regions at 20, 40, 60, 120, and 180 min after administration of CMI 50 mg/kg IP and 120 min after CMI 2 and 10 mg/kg IP. The peak metabolic effect was observed at 120 min after CMI. At that time, CMI 2 and 10 mg/kg IP significantly reduced rCMRglc from control values in 12 (19%) and 14 (22%) brain regions, which correspond to areas with high densities of 5-HT reuptake sites (e.g. visual and limbic areas and raphe nuclei). CMI 50 mg/kg produced widespread rCMRglc reductions in 34 (53%) brain regions, including cortical, hippocampal, raphe and cerebellar areas. The topographic distribution and the relation to time and dose of CMI effects on rCMRglc are different from those of 5-HT1A [8-hydroxy-2(di-N-propylamino) tetralin], 5-HT1B-C (m-chlorophenylpiperazine) and 5-HT3 (quipazine) agonists and resemble those produced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), an agonist of 5-HT2 receptors, suggesting that CMI may prefentially stimulate this 5-HT receptor subtype.

Serotonin-dependent cerebral activation: effects of methiothepin and other serotonergic antagonists

In scopolamine-treated (5.0 mg/kg, s.c.) rats hippocampal rhythmical slow activity (RSA) and neocortical low voltage fast activity (LVFA) occur only in close correlation with head movements, spontaneous changes in posture, or locomotion (Type I behavior). Previous work indicates that such scopolamine-resistant RSA and LVFA are dependent on ascending serotonergic projections. A test of 9 serotonergic antagonists (methiothepin; ritanserin; ketanserin; pizotifen; mianserin; pirenperone; ICS-205-930; metoclopramide; methysergide) showed that methiothepin produces a partial reduction in RSA and LVFA in scopolamine-treated rats, while the other antagonists are completely inactive over a wide range of doses. It may be that serotonergic cerebral activation depends on both 5-HT1 and 5-HT2 receptors.

Adrenalectomy or metyrapone-pretreatment abolishes cerebral metabolic responses to the serotonin agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) in the hippocampus

1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a serotonin type 2 (5-HT2) agonist, elevates plasma corticosterone levels and reduces the cerebral metabolic rate for glucose (rCMRglc) in the hippocampus, a structure which possesses few 5-HT2 receptors but a large number of steroid receptors. To explore the hypothetical interaction between 5-HT and steroid mechanisms in the hippocampus, we measured rCMRglc in intact, adrenalectomized and metyrapone-pretreated rats after saline or DOI administration. Metyrapone pretreatment alone had no significant effect on rCMRglc, but adrenalectomy produced widespread rCMRglc increases in the cortex, hippocampus and monoaminergic brainstem nuclei. In intact rats, DOI 10 mg/kg reduced rCMRglc in limbic areas and increased it in the interanteromedial and paracentral thalamic nuclei. Metyrapone pretreatment and adrenalectomy abolished rCMRglc responses to DOI in hippocampal areas and enhanced those in thalamic nuclei. These results indicate that brain responses to DOI are dependent upon the functional state of the hypothalamus-pituitary-adrenal cortex axis.

Chronic treatment with meta-chlorophenylpiperazine (m-CPP) alters behavioral and cerebral metabolic responses to the serotonin agonists m-CPP and quipazine but not 8-hydroxy-2(di-N-propylamino)tetralin

The effects of the serotonin (5-HT) agonists meta-chlorophenylpiperazine (m-CPP), quipazine and 8-hydroxy-2(di-n-propylamino)tetralin (DPAT) on behavior and on regional cerebral metabolic rates for glucose (rCMRglc) were measured in control rats or in rats pretreated for 2 weeks with continuous infusion of saline or m-CPP (2.5 mg/kg/day, subcutaneously). rCMRglc was measured in 71 brain regions, using the quantitative autoradiographic [14C]2-deoxy-D-glucose technique, at 15 min after acute administration of m-CPP 2.5 mg/kg, 60 min after quipazine 20 mg/kg, or 10 min after DPAT 1 mg/kg. Behavioral effects were assessed for m-CPP with an activity monitor, for quipazine by counting head shakes and for DPAT by scoring the serotonin syndrome. Chronic m-CPP pretreatment produced tolerance to hypolocomotion induced by acute m-CPP and to head shakes caused by acute quipazine, but did not alter the serotonin syndrome produced by DPAT. m-CPP 2.5 mg/kg IP produced widespread rCMRglc reductions in control rats but failed to modify rCMRglc in any region after chronic m-CPP pretreatment. Quipazine increased rCMRglc in 4 regions in control rats, but reduced rCMRglc in 14 brain areas of chronically m-CPP-pretreated animals. DPAT altered rCMRglc to the same degree in control (25 regions affected) and in chronically m-CPP-pretreated rats (28 regions affected). Reduced behavioral and metabolic effects of acute m-CPP in chronically m-CPP-pretreated rats were not due to pharmacokinetic alterations. These results demonstrate that chronic administration of m-CPP produces behavioral and metabolic tolerance to acute administration of m-CPP, but not of DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the 5-HT1A partial agonists gepirone, ipsapirone and buspirone on local cerebral glucose utilization in the conscious rat

The azospirones gepirone (10 mg/kg), ipsapirone (10 mg/kg) and buspirone (10 mg/kg) were examined for their effect on regional cerebral glucose utilization in conscious rats using quantitative 2-deoxy-glucose autoradiography. All three 5-HT1A partial agonists reduced glucose utilization in the hippocampus and dentate gyrus by 20-25% and increased glucose utilization by 38-65% in the lateral habenular nucleus; an important relay between striatal/limbic areas and the mid-brain raphe nuclei. The findings emphasize the potential importance of the hippocampus as a site of action for 5-HT1A receptor active drugs in vivo and also suggest that functional activity in the striatal/limbic-habenular-raphe pathway may be influenced by gepirone, ipsapirone and buspirone.

Serotonergic innervation of the cerebral vasculature: relevance to migraine and ischaemia

Multiple and complex interactions exist between the cerebral circulation and a potent vasoactive (and neurotransmitter) agent, serotonin. The nature and bases of the real and potential relationships are often hotly contested, for example, the serotonergic innervation of brain conducting and resistance vessels. In this review, an attempt is made to reconcile the available literature and to indicate future and possibly fruitful research directions. It appears that, by its very nature, the pattern of the serotonergic innervation is singular to blood vessels of the brain and could provide a neuronal link (or coupling) between functional events within the central nervous system and its perfusion which subserves changes in brain function. Finally, there are sufficient data to suggest an involvement of 5-hydroxytryptamine in different cerebrovascular pathologies.

The pattern of cerebral activity underlying verbal fluency shown by split-dose single photon emission tomography (SPET or SPECT) in normal volunteers

Uptake of 99mTc-Exametazime, a marker of relative regional cerebral blood flow has been determined with Single Photon Emission Tomography (SPET or SPECT) in 20 healthy, elderly female subjects during neuropsychological challenge. Each subject was studied under basal conditions after injection of 125 MBq 99mTc-Exametazime. Without moving the head of the subject, they were scanned again after injection of 375 MBq 99mTc-Exametazime. The second injection was made in 10 subjects during a test of verbal fluency, usually regarded as a test of the integrity of function of the left frontal cortex. In the other 10 subjects the second injection was made during simple verbalization (counting). This method of splitting the normal full dose of 99mTc-Exametazime allows a novel comparison between basal and active conditions for different brain regions. Verbal fluency was associated with reduced uptake bilaterally in the region of the basal ganglia and in left temporal (peri-sylvian) cortex when compared with calcarine cortex, an unstimulated reference sensory area. By contrast, counting produced relative activation, greatest in frontal and parietal areas. Thus, a clinically relevant neuropsychological test can be characterized metabolically by a pattern of regional brain activity, whose localization cannot readily be predicted from classical studies of brain lesions. Reduction of regional uptake may suggest an important role for deactivation or inhibition of function in human cognition. The involvement of basal ganglia and temporal areas is of particular interest in relation to the investigation of functional psychiatric illness.

Cerebrovascular and functional consequences of 5-HT1A receptor activation

Cerebral glucose utilization and blood flow were measured in rats using 2-deoxy-D-[14C]glucose and [14C]iodoantipyrine quantitative autoradiography, respectively, following treatment with the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT). In control and 8-OH-DPAT-treated animals blood flow and glucose use were similarly correlated, but the ratio was increased following 8-OH-DPAT treatment. Since 5-HT1A receptor activation is known to reduce neuronal 5-HT release, these results are consistent with a vasoconstrictor role for endogenous serotonin.

Dose- and time-dependent effects of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a serotonergic 5-HT2 receptor agonist, on local cerebral glucose metabolism in awake rats

The time course and the relation to dose of regional cerebral metabolic rates for glucose (rCMRglc) were measured in awake male Fischer-344 rats after administration of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a selective serotonergic 5-HT2 agonist. rCMRglc was determined, using the quantitative autoradiographic [14C]2-deoxyglucose technique, in 75 brain regions at 5, 15, 30, 60 and 90 min after administration of DOI 10 mg/kg i.p., and at 15 min after DOI 2.5, 25 or 50 mg/kg i.p. In non-hippocampal regions, peak effects were observed at 15-30 min, when rCMRglc in 12% of the regions was significantly different from control. In hippocampal regions rCMRglc effects peaked at 30 min (average rCMRglc reduction 21%) and were sustained for at least 60 min. Higher doses of DOI reduced rCMRglc in most prosencephalic regions (25 mg/kg, 35% of all regions studied; 50 mg/kg, 32%), where 5-HT2 receptors are present in high density. These data suggest that selective 5-HT2 receptor stimulation leads to rCMRglc reduction in areas with high densities of 5-HT2 receptors.

Time courses of behavioral and regional cerebral metabolic responses to different doses of meta-chlorophenylpiperazine in awake rats

The time course and relation to dose of regional cerebral metabolic rates for glucose (rCMRglc) and of motor behavior were measured in awake male adult Fischer-344 rats after administration of meta-chlorophenylpiperazine (MCPP), a serotonin-1B receptor agonist. rCMRglc was determined, using the quantitative autoradiographic [14C]deoxyglucose technique, in 71 brain regions at 5, 15, 30 and 60 min after administration of MCPP 2.5 mg/kg i.p., and at 15 min after MCPP 25 and 40 mg/kg. The time course of performance on a rotating rod was measured periodically for 60 min after MCPP 2.5 mg/kg, a dose which impaired locomotion and reduced rCMRglc maximally at 15-30 min after its administration. At 15 min, rCMRglc declined significantly in 28 (40%) of the areas studied (mean decline 16%). Most regions affected were telencephalic or diencephalic, corresponding to the projection areas of serotonergic fibers arising from the raphe nuclei. After higher doses of MCPP, a behavioral serotonin syndrome was observed with both rCMRglc increases and decreases (25 mg/kg) or only rCMRglc increases (40 mg/kg). Whereas behavioral and metabolic activation induced by high doses of MCPP may result from stimulation at postsynaptic serotonin receptors, rCMRglc reductions and hypomotility produced by MCPP 2.5 mg/kg resemble the effects of serotonin receptor antagonists and suggest that, at this low dose, MCPP acts at modulatory serotonin autoreceptors to reduce endogenous serotonin release.

Chronic effects of the selective serotoninergic neurotoxin, methylenedioxyamphetamine, upon cerebral function

The amphetamine derivative methylenedioxyamphetamine selectively destroys serotoninergic terminals in the brain. We have studied the effects of this toxin upon resting cerebral function, as reflected in rates of glucose utilization. Rats were injected subcutaneously with either 1 ml/kg saline (n = 5) or 20 mg/kg methylenedioxyamphetamine (n = 5) twice daily for four days. Local cerebral glucose utilization was measured between six and nine weeks after treatment using [14C]2-deoxyglucose quantitative autoradiography. Samples of frontal cortex taken from these animals for in vitro [3H]paroxetine binding showed a 64% reduction in 5-hydroxytryptamine uptake sites. In the majority of the 31 functionally diverse brain areas analysed, no significant changes were measured, but significant (P less than 0.05) increases in glucose use were found in neocortical regions e.g. anterior cingulate cortex (+16%) and sensorimotor cortex (+21%). However, the most profound increases were found in globus pallidus (+30%) and hippocampus molecular layer (+34%). It would appear, therefore, that treatment with methylenedioxyamphetamine results in long-lasting alterations in cerebral functional activity.