Mechanisms of the placebo effect and of conditioning.

A placebo is a sham treatment, such as a pill, liquid, or injection without biological activity, used in pharmacology to control for the activity of a drug. However, in many cases this placebo induces biological or psychological effects in the human. Two theories have been proposed to explain the placebo effect: the conditioning theory, which states that the placebo effect is a conditioned response, and the mentalistic theory, which sees the patient's expectation as the primary cause of the placebo effect. The mechanisms involved in these processes are beginning to be understood through new techniques of investigation in neuroscience. Dopamine and the endorphins have been clearly shown to be mediators of placebo effects. Brain imaging has demonstrated that placebos can mimic the effect of the active drugs and activate the same brain areas. This is the case for placebo-dopamine in Parkinson's disease, for placebo-analgesics or antidepressants, and for placebo-caffeine in the healthy subject. It remains to be understood how conditioning and expectation are able to activate memory loops in the brain that reproduce the expected biological responses.

[Mechanisms of placebo effect and of conditioning: neurobiological data in human and animals]. / Mécanismes de l'effet placebo et du conditionnement: données neurobiologiques chez l'homme et l'animal.

A placebo is a sham treatment such as pill, liquid, injection, devoid of biological activity and used in pharmacology as a control for the activity of a drug. In many cases, this placebo induces biological or psychological effects in the human. Two theories have been proposed to explain the placebo effect: the conditioning theory which states that the placebo effect is a conditioned response, and the mentalistic theory for which the patient expectation is the primary basis of the placebo effect. The mechanisms involved in these processes are beginning to be understood through new techniques of investigation in neuroscience. Dopamine and endorphins have been clearly involved as mediators of the placebo effect. Brain imaging has demonstrated that the placebo effect activates the brain similarly as the active drug and in the same brain area. This is the case for a dopamine placebo in the Parkinson'disease, for analgesic-caffeine- or antidepressor-placebo in the healthy subject. It remains to be understood how conditioning and expectancy are able to activate, in the brain, memory loops that reproduce the expected biological response.

Dexamethasone downregulates chemokine receptor CXCR4 and exerts neuroprotection against hypoxia/ischemia-induced brain injury in neonatal rats.

OBJECTIVE: Hypoxia/ischemia (H/I) induces rapid and massive brain damage in neonatal rat brain, resulting in long-term consequences on structural and functional maturation of the central nervous system. Inflammatory mediators contribute to these permanent pathological changes, which are sensitive to corticoid treatments. Since the chemokine receptor CXCR4, specific for the SDF-1 alpha/CXCL12 ligand, regulates both apoptotic and neuroregeneration processes, this receptor was quantified 2 days following H/I in neonatal rat brain in relation with dexamethasone (DEX) treatment. METHODS: Seven-day-old male rats were exposed to a 90-min hypoxia following unilateral carotid ligation (H/I) and were sacrificed 48 h later. Glucocorticoid-pretreated animals were injected subcutaneously 5 h prior to hypoxia with 0.5 microg/g DEX. Glial fibrillary acidic protein and cresyl violet staining were used for assessing the extent of brain lesion subdivided into necrotic and penumbra-like areas. The density of CXCR4 receptors was determined by quantitative autoradiography using [(125)I]SDF-1 alpha as a ligand. RESULTS: The H/I resulted in a massive lesion ipsilateral to the carotid ligation, which was extended to cortical, striatal, hippocampal and thalamic areas, while the contralateral hemisphere remained apparently unaffected. DEX decreased the lesion size by reducing mainly the necrotic area. H/I induced a marked increase in CXCR4 receptor binding in the penumbra-like areas. DEX pretreatment decreased CXCR4 receptor density in the penumbra and attenuated astrocytosis. Furthermore, DEX strongly lowered mortality rate and reduced functional recovery time right after hypoxia. CONCLUSION: The rapid enhancement in CXCR4 chemokine receptor binding in the affected brain areas suggests that SDF-1 alpha/CXCR4 may play a role in the hypoxia-induced inflammatory reaction in the neonatal brain. Attenuation of CXCR4 expression and astrogliosis could contribute to the neuroprotective effect of DEX pretreatment via influencing the inflammatory cascade induced by H/I in the neonatal brain.

Characterization and functionality of CXCR4 chemokine receptor and SDF-1 in human corneal fibroblasts.

PURPOSE: The aim of this study was to investigate whether cultured human corneal fibroblasts express functional chemokine CXCR4 receptors on their cell surface and to determine the presence of its specific ligand, SDF-1 (CXCL12), in human corneal fibroblasts. METHODS: Human corneal fibroblast cultures were obtained using human donor corneas. CXCR4 receptors were characterized using binding studies and autoradiography with [125I]SDF-1. The functionality of CXCR4 receptors was assessed by intracellular calcium measurement using a dynamic imaging microscopy system. CXCR4 and SDF-1 mRNA were detected in human corneal fibroblasts using reverse transcriptase polymerase chain reaction (RT-PCR). The CXCR4 protein was detected by western blot analysis. RESULTS: [125I]SDF-1 specifically bound to cultured corneal fibroblasts with a KD value of 8.3+/-1.2 nM. The presence of CXCR4 was confirmed by autoradiography of the radioligand on slices of corneal stroma. SDF-1 induced a rapid and transient intracellular calcium increase in cultured corneal fibroblasts that was blocked by the specific antagonist bicyclam. Moreover, a 48 kDa protein was detected by western blot analysis of corneal fibroblast extracts, using a specific CXCR4 polyclonal antibody. RT-PCR showed the expression of both CXCR4 and SDF-1 mRNAs in human corneal fibroblasts. CONCLUSIONS: These results indicate for the first time that cultured human corneal fibroblasts express the chemokine receptors CXCR4 and its ligand SDF-1. This latter might exert physiological effects on the cornea and could be involved in pathological conditions such as corneal angiogenesis.

Neuroanatomical distribution of CXCR4 in adult rat brain and its localization in cholinergic and dopaminergic neurons.

Accumulating evidence supports a role of chemokines and their receptors in brain function. Up to now scarce evidence has been given of the neuroanatomical distribution of chemokine receptors. Although it is widely accepted that chemokine receptors are present on glial cells, especially in pathological conditions, it remains unclear whether they are constitutively present in normal rat brain and whether neurons have the potential to express such chemokine receptors. CXCR4, a G protein-coupled receptor for the chemokine stromal cell-derived factor-1 (SDF-1/CXCL12) was reported to have possible implications in brain development and AIDS-related dementia. By dual immunohistochemistry on brain sections, we clearly demonstrate that CXCR4 is constitutively expressed in adult rat brain, in glial cells (astrocytes, microglia but not oligodendrocytes) as well as in neurons. Neuronal expression of CXCR4 is mainly found in cerebral cortex, caudate putamen, globus pallidus, substantia innominata, supraoptic and paraventricular hypothalamic nuclei, ventromedial thalamic nucleus and substantia nigra. Using confocal microscopy, a differential distribution of CXCR4 in neuronal perikarya and dendrites can be observed according to the brain structure. Furthermore, this work demonstrates for the first time the coexistence of a chemokine receptor with classical neurotransmitters. A localization of CXCR4 is thus observed in neuronal cell bodies expressing choline acetyltransferase-immunoreactivity in the caudate putamen and substantia innominata, as well as in tyrosine hydroxylase-positive neurons in the substantia nigra pars compacta. In conclusion, the constitutive neuronal CXCR4 expression suggests that SDF-1/CXCL12 could be involved in neuronal communication and possibly linked up with cholinergic and dopaminergic neurotransmission and related disorders.