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

Linking Mitochondria to Synapses: New Insights for Stress-Related Neuropsychiatric Disorders

The brain evolved cellular mechanisms for adapting synaptic function to energy supply. This is particularly evident when homeostasis is challenged by stress. Signaling loops between the mitochondria and synapses scale neuronal connectivity with bioenergetics capacity. A biphasic “inverted U shape” response to the stress hormone glucocorticoids is demonstrated in mitochondria and at synapses, modulating neural plasticity and physiological responses. Low dose enhances neurotransmission, synaptic growth, mitochondrial functions, learning, and memory whereas chronic, higher doses produce inhibition of these functions. The range of physiological effects by stress and glucocorticoid depends on the dose, duration, and context at exposure. These criteria are met by neuronal activity and the circadian, stress-sensitive and ultradian, stress-insensitive modes of glucocorticoid secretion. A major hallmark of stress-related neuropsychiatric disorders is the disrupted glucocorticoid rhythms and tissue resistance to signaling with the glucocorticoid receptor (GR). GR resistance could result from the loss of context-dependent glucocorticoid signaling mediated by the downregulation of the activity-dependent neurotrophin BDNF. The coincidence of BDNF and GR signaling changes glucocorticoid signaling output with consequences on mitochondrial respiration efficiency, synaptic plasticity, and adaptive trajectories.

Affective disorders, antidepressant drugs and brain metabolism

There is increasing evidence that affective disorders are associated with dysfunction of neurotransmitter postsynaptic transduction pathways and that chronic treatment with clinically active drugs results in adaptive modification of these pathways. Despite the close dependence of signal transduction on adenosine triphosphate (ATP) availability, the changes in energy metabolism in affective disorders are largely unknown. This question has been indirectly dealt with through functional imaging studies (PET, SPECT, MRS). Despite some inconsistencies, PET and SPECT studies suggest low activity in cortical (especially frontal) regions in depressed patients, both unipolar and bipolar, and normal or increased activity in the manic pole. Preliminary MRS studies indicate some alterations in brain metabolism, with reduced creatine phosphate and ATP levels in the brain of patients with affective disorders. However, the involvement of the energy metabolism in affective disorders is still debated. We propose direct neurochemical investigations on mitochondrial functional parameters of energy transduction, such as the activities of (a) the enzymatic systems of oxidative metabolic cycle (Kreb's cycle); (b) the electron transfer chain; (c) oxidative phosphorylation, and (d) the enzyme activities of ATP-requiring ATPases. These processes should be studied in affective disorders and in animals treated with antidepressant drugs or lithium.

Effects of medications on cerebral blood flow in late-life depression

Both normal aging and late-life depression (LLD) are associated with reductions in regional cerebral blood flow (rCBF). The impact of medication treatment in baseline rCBF abnormalities in LLD is being investigated via functional imaging research. Some of this work can be informed by pharmacologic challenge studies, which exploit the role of serotonin in regulating rCBF. Preliminary evidence suggests that there may be both state- and trait-level disturbances in rCBF in LLD, and that a common pathway towards clinical response to somatic antidepressant treatments involves reduction in rCBF in critical prefrontal cortical brain regions. Studies of the effects of medications on rCBF in LLD have implications for understanding the neurobiology of treatment resistance in the elderly as well as the mechanisms of action of antidepressant treatments.

Cerebral blood flow and metabolism in late-life depression and dementia

Late-life depression (LLD) is characterized by abnormalities in cerebral blood flow (CBF) and cerebral metabolic rate (CMR) for glucose. Unlike younger adults with major depression, global cortical CBF and CMR reductions have been reported in LLD. Patients with LLD are also characterized by topographic abnormalities, most commonly involving selective prefrontal, superior temporal, and anterior parietal cortex. The fate of these abnormalities with response to antidepressant treatment is highly uncertain, and heterogeneous findings have been reported in younger samples with major depression. The limited data in LLD suggest that response to electroconvulsive therapy or antidepressant medications does not involve reversal of baseline abnormalities but rather accentuation of prefrontal deficits. At minimum, these paradoxical findings suggest that abnormalities in CBF and CMR may be persistent in LLD and a trait characteristic. Characteristic profiles of CBF and CMR abnormalities have also been demonstrated in samples with Alzheimer's disease (AD) and other types of dementia. Functional imaging has shown sensitivity to disease severity and progression. Nonetheless, there is limited information regarding the sensitivity and specificity of the functional imaging modalities in the differential diagnosis of dementias. At present, the evidence does not support the use of functional imaging in isolation as a diagnostic tool. Rather, these imaging modalities may be considered as an adjunct to careful clinical assessment, either to improve diagnosis in early cases or to assist in subtyping difficult cases.

Local cerebral metabolic effects of the novel cocaine analog, WF-31: comparisons to fluoxetine

The effects of the acute administration of the selective serotonin uptake inhibitor, fluoxetine, on rates of local cerebral glucose utilization in rats were compared to those of the novel cocaine analog, [2beta-propanoyl-3beta-(4-isopropylphenyl)-tropane, WF-31, which has greater affinity for serotonin than dopamine transporters, using the quantitative autoradiographic 2-[14C]deoxyglucose method. Locomotor activity was assessed simultaneously. Fluoxetine administration resulted in dose-dependent decreases in locomotor behavior, as well as widespread reductions in rates of metabolic activity in brain areas including raphe nuclei, dorsal and ventral striatum, amygdala, hippocampus, limbic cortex, and thalamus. These effects were largely concentrated in brain regions containing high densities of serotonin transporters as revealed by in vitro autoradiography. In contrast, the acute administration of WF-31 produced more discrete changes in metabolic activity that were localized within the raphe nuclei and in portions of the hippocampal formation. Blockade of WF-31's dopaminergic effects by pretreatment with the antagonist, alpha-flupenthixol, resulted in a pattern of metabolic changes that closely resembled that observed with fluoxetine. These data suggest that the alterations in functional activity produced by both fluoxetine and WF-31 are largely the result of actions on serotonergic systems.

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.