The melatonin MT1 receptor axis modulates mutant Huntingtin-mediated toxicity

Melatonin mediates neuroprotection in several experimental models of neurodegeneration. It is not yet known, however, whether melatonin provides neuroprotection in genetic models of Huntington's disease (HD). We report that melatonin delays disease onset and mortality in a transgenic mouse model of HD. Moreover, mutant huntingtin (htt)-mediated toxicity in cells, mice, and humans is associated with loss of the type 1 melatonin receptor (MT1). We observe high levels of MT1 receptor in mitochondria from the brains of wild-type mice but much less in brains from HD mice. Moreover, we demonstrate that melatonin inhibits mutant htt-induced caspase activation and preserves MT1 receptor expression. This observation is critical, because melatonin-mediated protection is dependent on the presence and activation of the MT1 receptor. In summary, we delineate a pathologic process whereby mutant htt-induced loss of the mitochondrial MT1 receptor enhances neuronal vulnerability and potentially accelerates the neurodegenerative process.     

Single nucleotide polymorphisms and mRNA expression for melatonin MT(2) receptor in depression

Polymorphisms (rs 4753426 and rs 794837) and expression of the melatonin MT₂ receptor gene were evaluated in 181 patients with recurrent depressive disorder (rDD) and 149 healthy subjects of Polish origin. We found an increased risk for rDD in patients with the C allele and a decreased risk in patients with the T allele (rs4753426). Patients with the AT heterozygote (rs794837) had an increased mRNA level. The significance of the MT₂ receptor gene and the risk of rDD are suggested.     

Rhythms in clock proteins in the mouse pars tuberalis depend on MT1 melatonin receptor signalling

Melatonin provides a rhythmic neuroendocrine output, driven by a central circadian clock that encodes information about phase and length of the night. In the hypophyseal pars tuberalis (PT), melatonin is crucial for rhythmic expression of the clock genes mPer1 and mCry1, and melatonin acting in the PT influences prolactin secretion from the pars distalis. To examine further the possibility of a circadian clockwork functioning in the PT, and the impact of melatonin on this tissue, we assessed circadian clock proteins by immunohistochemistry and compared the diurnal expression in the PT of wild type (WT), and MT1 melatonin receptor-deficient (MT1-/-) mice. While in the PT of WT mice mPER1, mPER2, and mCRY1 showed a pronounced rhythm, mCRY2, CLOCK, and BMAL1 were constitutively present. Despite reported differences in maximal levels and timing of mCry1, mPer1, and mPer2 RNAs, the corresponding protein levels peaked simultaneously during late day, suggesting a codependency for their stabilization and/or nuclear entry. MT1-/- mice had reduced levels of mPER1, mCRY1, CLOCK and BMAL1, consistent with the earlier reported reduction in mRNA expression of these clock genes. Surprisingly, mPER2-immunoreaction was constitutively low, although mPer2 was rhythmically expressed in the PT of MT1-/- mice. This suggests that mPER2 is degraded due to the reduced levels of its stabilizing interaction partners mPER1 and mCRY1. The results show that melatonin, acting through the MT1, determines availability of the circadian proteins mPER1, mPER2 and mCRY1 and thus plays a crucial role in regulating rhythmicity in PT cells.     

Chemokine and chemokine receptor expression in the central nervous system

A decade ago several new cytokines were described that orchestrated the activation and migration of immune cells. These newly described cytokines, of which interleukin-8 (IL-8) was a representative member, defined a novel group of molecules called chemokines (chemotactic cytokines). Chemokines are low molecular weight, 8-12 kDa, basic proteins that have been classified into four distinct families, CXC, CC, C and CX3C, based on the position of their first two conserved cysteine residues. The expression and biological function of chemokines along with their cognate receptors have been well described on various subsets of leukocytes. Only more recently have these molecules been described on various cells within the central nervous system. These pro-inflammatory proteins have been implicated in a variety of diseases within the central nervous system from Multiple Sclerosis to AIDS dementia. While chemokines are likely to enhance the evolution of central nervous system inflammatory disorders they also have other roles in normal brain function and development. This review summarizes the role of chemokines and their receptors in the normal and pathophysiological brain.     

Distribution of MT1 melatonin receptor immunoreactivity in the human hypothalamus and pituitary gland: colocalization of MT1 with vasopressin, oxytocin, and corticotropin-releasing hormone

Melatonin is implicated in numerous physiological processes, including circadian rhythms, stress, and reproduction, many of which are mediated by the hypothalamus and pituitary. The physiological actions of melatonin are mainly mediated by melatonin receptors. We here describe the distribution of the melatonin receptor MT1 in the human hypothalamus and pituitary by immunocytochemistry. MT1 immunoreactivity showed a widespread pattern in the hypothalamus. In addition to the area of the suprachiasmatic nucleus (SCN), a number of novel sites, including the paraventricular nucleus (PVN), periventricular nucleus, supraoptic nucleus (SON), sexually dimorphic nucleus, the diagonal band of Broca, the nucleus basalis of Meynert, infundibular nucleus, ventromedial and dorsomedial nucleus, tuberomamillary nucleus, mamillary body, and paraventricular thalamic nucleus were observed to have neuronal MT1 receptor expression. No staining was observed in the nucleus tuberalis lateralis and bed nucleus of the stria terminalis. The MT1 receptor was colocalized with some vasopressin (AVP) neurons in the SCN, colocalized with some parvocellular and magnocellular AVP and oxytocine (OXT) neurons in the PVN and SON, and colocalized with some parvocellular corticotropin-releasing hormone (CRH) neurons in the PVN. In the pituitary, strong MT1 expression was observed in the pars tuberalis, while a weak staining was found in the posterior and anterior pituitary. These findings provide a neurobiological basis for the participation of melatonin in the regulation of various hypothalamic and pituitary functions. The colocalization of MT1 and CRH suggests that melatonin might directly modulate the hypothalamus-pituitary-adrenal axis in the PVN, which may have implications for stress conditions such as depression.     

The MT2 melatonin receptor subtype is present in human retina and decreases in Alzheimer's disease

The pineal and retinal melatonin regulates endogenous circadian rhythms, and has various physiological functions including neuromodulatory and vasoactive actions, antioxidative and neuroprotective properties. We have previously demonstrated that the melatonin 1a-receptor (MT(1)) is localized in human retinal cells and that the expression of MT(1) is increased in Alzheimer's disease (AD) patients. We now present the first immunohistochemical evidence for the cellular distribution of the second melatonin receptor, MT(2), in the human retina and in AD patients. In elderly controls, MT(2) was localized to ganglion and bipolar cells in the inner nuclear layer, and to the inner segments of the photoreceptor cells. In addition, cellular processes in inner and outer plexiform layers were strongly positive for MT(2). In AD patients the overall intensity of MT(2)-staining was distinctly decreased in all observed cellular localizations. Our results indicate that MT(2) in the humans, similar to MT(1), may indeed be involved in transmitting melatonin's effects in the retina, and AD pathology may impair MT(2) expression. Since our previous results showed an increase in MT(1) expression in AD retina, the two melatonin receptor subtypes appear to be differentially affected by the course of the neurodegenerative disorder.     

MT1 melatonin receptor mRNA expressing cells in the pars tuberalis of the European hamster: effect of photoperiod

Melatonin, secreted only during the night by the pineal gland, transduces the photoperiodic message to the organism. One important target for the hormone is the pars tuberalis (PT) of the adenohypophysis which displays a very high number of melatonin binding sites in mammals and is implicated in the seasonal regulation of prolactin secretion. To gain insight into the mechanism by which the melatonin signal is decoded in the PT, we studied the effect of photoperiod on the PT cells expressing the MT1 melatonin receptor in a highly photoperiodic species, the European hamster. Recently, we showed that, in the rat, the MT1 receptor mRNA is expressed in PT-specific cells characterized by their expression of beta-thyroid stimulating hormone (beta-TSH) along with the alpha-glycoprotein subunit (alpha-GSU). As the cellular composition of the PT shows variability among species, we first identified the cell type expressing the MT1 receptor in the European hamster by combining immunocytochemistry and nonradioactive in situ hybridization for the MT1 receptor mRNA. Our results show that, in the European hamster, as in the rat, the MT1 receptor is only expressed by the PT-specific-cells, beta-TSH and alpha-GSU positive. In a second step, we analysed the effects of photoperiod on the MT1 mRNA, and on beta-TSH and alpha-GSU both at the mRNA and protein levels. Our data show that, compared to long photoperiod, short photoperiod induces a dramatic decrease of MT1, beta-TSH and alpha-GSU expression. Protein levels of beta-TSH and alpha-GSU were also dramatically reduced in short photoperiod. Together, our data suggest that melatonin exerts its seasonal effects in the PT by signalling to PT specific-cells through the MT1 receptor subtype.     

Developmental and regional expression of choline acetyltransferase mRNA in the rat central nervous system

The developmental and regional expression of choline acetyltransferase (ChAT) mRNA was examined in the rat brain and spinal cord by northern blot analysis and in situ hybridization. ChAT mRNA expression in the brain showed a biphasic increase during development, with a first peak at two weeks postnatally, a marked decrease by the third week, and a second increase between the third and fifth week after birth, indicating that emergence of the cholinergic phenotype occurs at different times in different brain regions. In the spinal cord, ChAT mRNA was detected at similar levels from embryonic stage 13 (E13) until birth, increasing thereafter until adulthood. In the adult rat central nervous system, high levels of ChAT mRNA were detected in the spinal cord and brain stem structures. Lower levels were seen in midbrain, septum, striatum, thalamus, and olfactory bulb. ChAT mRNA containing cells were identified by in situ hybridization in the olfactory tubercule, piriform cortex, striatum, several basal forebrain nuclei, and spinal cord. A nearly two-fold increase in adult spinal cord ChAT mRNA levels were seen one week after a bilateral crush lesion of the sciatic nerve, indicating that ChAT mRNA expression is regulated during motoneuron regeneration.     

Retinal dopaminergic receptor affinity and ocular pharmacokinetic profile of piribedil

Journal of Neurology - Binding studies on retinal dopamine receptors have revealed the existence of both D1 and D2 receptors. Human retina micro-autoradiographs confirm the distribution of...     

Effects of sibutramine on the central dopaminergic system in rodents

The effects of sibutramine on central dopaminergic system in rats and mice were examined by neurochemical and behavioral pharmacological methods. Dopamine reuptake inhibition by sibutramine in brain synaptosomes was only 4–5 times stronger than those of amitriptyline and dosulepin, which do not exhibit dopamine uptake inhibition in vivo. Single treatment with sibutramine did not alter the brain content of dopamine and DOPAC. However, similar to methamphetamine and pargyline, sibutramine antagonized methyl-4-phenyl-l, 2,3,6-tetrahydro-pyridine (MPTP) induced dopamine depletion in mouse brain. In forced swimming tests of reserpinized mice, sibutramine shortened the immobilized time, similar to dopaminergic drugs including nomifensine, bupropion (dopamine-reuptake inhibitor), methamphetamine, SKF 38393 (dopamine D1 agonist), quinpirole (dopamine D2 agonist) and apomorphine (dopamine D1/D2 agonist). In addition, sibutramine caused rotational behavior toward the lesioned side in rats with unilateral lesions of the substantia nigra induced by 6-hydroxydopamine. These results suggest that sibutramine exhibits neurochemical and behavioral dopaminomimetic activity in vivo, which is mediated by dopamine reuptake inhibition by the active metabolites of sibutramine.     

Interaction between central GABAA receptor and dopaminergic system on food intake in neonatal chicks: role of D1 and GABAA receptors

Objective: The present study was designed to examine the role of central γ-Aminobutyric acid_A receptors and dopaminergic system on feeding behaviour in neonatal layer-type chicken.

Methods: In this study, six experiments were designed, each with four treatment groups (n = 44 in each experiment). In experiment 1, four groups of 3-h food-deprived chicks received a dose of either the intracerebroventricular injection of (1) control solution, (2) Levo-dihydroxyphenylalanine as precursor of dopamine; 125 nmol, (3) Gaboxadol (γ-Aminobutyric acid_A receptor agonist, 0.2 µg) and (4) Levo-dihydroxyphenylalanine (125 nmol) plus Gaboxadol (0.2 µg). Experiments 2–6 were similar to experiment 1, except that the chickens were intracerebroventricular-injected with 6-hydroxydopamine (is a neurotoxin; 2.5 nmol), SCH23390 (D₁ receptor antagonist, 5 nmol), AMI-193 (D2 receptor antagonist, 5 nmol), NGB2904 (D₃ receptor antagonist, 6.4 nmol) and L-741,742 (D₄ receptor antagonist, 6 nmol) instead of levo-dihydroxyphenylalanine. Then, the cumulative food intake was measured until 120 min post-injection.

Results: According to the results, intracerebroventricular injection of Gaboxadol (0.2 µg) significantly increased the food intake (P < 0.05). Co-injection of the 6-hydroxydopamine + Gaboxadol significantly amplified the food intake (P < 0.05). Intracerebroventricular injection of SCH23390 (5 nmol) + Gaboxadol (0.2 µg) significantly amplified the Gaboxadol-induced hyperphagia (P < 0.05). No significant effect was observed by co-injection of the D₂–D₄ receptor antagonists + Gaboxadol (P > 0.05).

Conclusion: These results suggested the interconnection between central Dopaminergic and γ-Aminobutyric acid_A on the feeding behaviour mediates via D₁ and γ-Aminobutyric acid_A receptors in 3-h food-deprived neonatal layer-type chicken.     

Modulation of dopaminergic receptor sensitivity in the central nervous system: important parameters in synaptic function regulation]

Modulation in sensitivity of dopamine receptors in the central nervous system are reviewed. Three main types differing by their behavioural and biochemical characteristics are described: -- Disuse hypersensitivity to dopamine agonists, induced by an interruption of dopaminergic transmission, seems to depend on an increased number of post-synaptic dopamine receptors; -- Hyposensitivity to dopamine agonists, induced by an overstimulation of dopamine receptors, could depend on their qualitative or quantitative modifications; -- Behavioural facilitation, elicited by a previous administration of dopaminergic agonists in low dosage, could be associated with an hyposensitivity of dopaminergic autoreceptors. They seem to constitute important parameters in adaptation of synaptic efficacy in physiological as well as pathological states.     

Neurodevelopmental expression and localization of the cellular prion protein in the central nervous system of the mouse

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative disorders caused by PrP^Sc, or prion, an abnormally folded form of the cellular prion protein (PrP^C). The abundant expression of PrP^C in the central nervous system (CNS) is a requirement for prion replication, yet despite years of intensive research the physiological function of PrP^C still remains unclear. Several routes of investigation point out a potential role for PrP^C in axon growth and neuronal development. Thus, we undertook a detailed analysis of the spatial and temporal expression of PrP^C during mouse CNS development. Our findings show regional differences of the expression of PrP, with some specific white matter structures showing the earliest and highest expression of PrP^C. Indeed, all these regions are part of the thalamolimbic neurocircuitry, suggesting a potential role of PrP^C in the development and functioning of this specific brain system. J. Comp. Neurol. 518:1879–1891, 2010. © 2010 Wiley‐Liss, Inc.     

Molecular and cellular alterations in the Pitx3-deficient midbrain dopaminergic system

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by loss of midbrain dopaminergic (mDA) neurons in the substantia nigra compacta (SNc). In order to provide insights into adaptive mechanisms of the mDA system in pathology, specific molecular and cellular parameters of the mDA system were studied in Pitx3-deficient Aphakia (ak) mice, which suffer from severe developmental failure of SNc mDA neurons. Here, we demonstrate differential changes in striatal gene expression, reflecting the specific neuronal loss in these mice. In addition, the neuronal activity of remaining mDA neurons in the ventral tegmental area (VTA) was significantly increased in ak mice. In conclusion, ak mice display specific molecular and cellular alterations in the mDA system that provide new insights in compensatory mechanisms present in mDA-associated disorders such as PD.