Activation of extracellular signal-regulated protein kinase is associated with colorectal distension-induced spinal and supraspinal neuronal response and neonatal maternal separation-induced visceral hyperalgesia in rats.

The activation of extracellular signal-regulated protein kinase (ERK) is essential for pain sensation and development of hyperalgesia in chronic pathological pain. Neonatal maternal separation (NMS) could trigger behavioral hyperalgesia and upregulate central neuronal activity in rats. The present study aims to investigate whether ERK associates with the colorectal distension (CRD)-evoked neuronal response and the upregulated central sensitivity to CRD in NMS rats. Male Sprague-Dawley rat pups were either subjected to NMS or not handled (NH) from postnatal day 2 to day 14. The protein expression of phospho-ERK (p-ERK) and c-fos at the spinal and supraspinal levels of adult rats were quantified and their correlation was analyzed. Western blot analysis revealed significant NMS effect on p-ERK expression in the lumbosacral dorsal horn and thalamus. Immunohistochemistry analysis demonstrated that CRD elevated p-ERK and c-fos expression in the dorsal root ganglia (DRG), laminae I-II of the lumbosacral dorsal horn, thalamic nucleus central medial (CM), paraventricular thalamic nucleus (PV), and anterior cingulate cortex (ACC). Significant NMS effect on p-ERK and c-fos expression was observed in the DRG, and laminae I-II, III-IV, and X of the lumbosacral dorsal horn. Furthermore, a significant interactive effect of NMS and CRD on p-ERK expression was observed in laminae III-IV of the lumbosacral dorsal horn. Correlation analysis revealed the positive association between c-fos- and p-ERK-immunoreactive nuclei numbers in the DRG, lumbosacral dorsal horn, and ACC. These results demonstrate that ERK is actively involved in CRD-evoked neuronal activation in both NH and NMS rats. Moreover, ERK is associated with the upregulated central neuronal sensitivity to noxious CRD in NMS rats, which may be responsible for the behavioral hyperalgesia in NMS rat.

Downregulation of glial glutamate transporters after dopamine denervation in the striatum of 6-hydroxydopamine-lesioned rats.

Overactivity of glutamatergic neurotransmission in the basal ganglia is known to be closely related to the onset and pathogenesis of Parkinson's disease. Glutamate homeostasis around glutamatergic synapses is tightly regulated by two groups of glutamate transporters: glial glutamate transporters GLT1 (EAAT2) and GLAST (EAAT1), and neuronal glutamate transporter EAAC1. In order to investigate the changes of glutamate transporters after the onset of Parkinson's disease, unilateral 6-hydroxydopamine-lesioned rat, an animal model of Parkinson's disease, was employed. By immunofluorescence and Western blot analyses, GLT1 and GLAST proteins were significantly reduced in the striatum with lesion. No change in GLT1 and GLAST protein was found in the substantia nigra. The reduction of GLT1 protein in the striatum was more prominent than that of GLAST protein (approximately 40% vs. 20%). In addition, EAAC1 protein was found to be increased in the substantia nigra pars reticulata of the lesioned rats but not in the striatum. The present results indicate that reductions of GLT1 and GLAST may impair glutamate homeostasis around glutamatergic synapses in the striatum and contribute to over-spills of glutamate in the system. An increase in the EAAC1 level in the substantia nigra pars reticulata may increase GABA synthesis and enhance GABAergic neurotransmission. These results indicate that there are differential and distinct modulations of glutamate transporters after dopamine denervation in the 6-hydroxydopamine-lesioned rat.

Visceral hyperalgesia induced by neonatal maternal separation is associated with nerve growth factor-mediated central neuronal plasticity in rat spinal cord.

Neonatal maternal separation (NMS) has been shown to trigger alterations in neuroendocrine, neurochemical and sensory response to nociceptive stimuli along the brain-gut axis. These alterations may be the result of a cascade of events that are regulated by neurotrophic factors. Nerve growth factor (NGF), a member of the neurotrophin family, is essential for the development and maintenance of sensory neurons and for the formation of central pain circuitry. The present study aimed to investigate whether NMS causes changes in neuronal plasticity and the relationship of these changes in plasticity with the expression of NGF and its high affinity tyrosine kinase receptor A (TrkA) in the lumbosacral spinal cord in adult rats. Male Wistar rat pups were either subjected to 180 min daily of NMS or not handled (NH) for 13 consecutive days. The expression of NGF and TrkA was examined in NH and NMS rats with or without colorectal distention (CRD) as determined by Western blot analysis and immunohistochemistry. The present results of Western blot analysis indicated NMS and CRD have a significant effect on NGF protein level in the lumbosacral spinal cord of rats. Assessments of optical densities revealed that NMS enhanced TrkA-ir fiber densities in laminae I-III and laminae V-VI of rats in both conditions with or without CRD. Double immunofluorescence revealed that TrkA co-expressed with calcitonin gene-related peptide (CGRP) in afferent fibers, while no significant difference in terms of the intensity of TrkA-ir in these fibers was found among groups. Quantitative analysis of TrkA-ir neurons indicated a significant interactive effect of NMS and CRD on the mean number of TrkA-ir neurons in laminae V-VI of rats, in which significant difference was found between NMS+CRD and NH+CRD. Double immunofluorescence of TrkA and Fos showed that CRD has a significant effect on TrkA expression in Fos-positive neurons in laminae V-VI and lamina X of rats, while no significant difference was found between NMS+CRD and NH+CRD. These results demonstrate that NMS induced alterations in NGF protein level and TrkA expression in adult rat spinal cord and indicate that NGF is a crucial mediator for the changes in neuronal plasticity that occur in NMS-induced visceral hyperalgesia.

Up-regulation in expression of vesicular glutamate transporter 3 in substantia nigra but not in striatum of 6-hydroxydopamine-lesioned rats.

Overactivity of the glutamatergic system is suggested to be closely related to the onset and pathogenesis of Parkinson's disease. Vesicular glutamate transporters (VGLUT1, T2 and T3) are a group of glutamate transporters in neurons that are responsible for transporting glutamate into synaptic vesicles and they are key elements for homeostasis of glutamate neurotransmission. The present study was aimed to investigate the expression of VGLUT1, T2 and T3 proteins after the onset of Parkinson's disease. A rat model of Parkinson's disease, the 6-hydroxydopamine-lesioned rat, was employed. Immunocytochemistry revealed that VGLUT1, T2 and T3 immunoreactivity was not modulated in the striatum of the lesioned rat. Western blotting analyses also showed that there was no change in the expression of T1, T2 and T3 proteins in the striatum. In contrast, no VGLUT1 protein was detected in the substantia nigra. After the lesion, levels of VGLUT2 immunoreactivity and protein were not modulated. Significant increase of VGLUT3 immunoreactivity was observed in the perikarya of GABAergic substantia nigra pars reticulata neurons (+14.7%) although VGLUT3 protein was not modulated in the nigral tissues. VGLUT3 in GABAergic neurons is suggested to play a role in GABA synthesis. The present results may therefore implicate that VGLUT1 and T2 are not modulated in the striatum and the substantia nigra of the 6-hydroxydopamine-lesioned rat and only VGLUT3 plays a role in pathogenesis of Parkinson's disease.

Differential expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate glutamate receptors in the rat striatum during postnatal development.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)-type glutamate receptors (GluR1-4) are one of the most important ionotropic glutamate receptors in the striatum, a key region of the basal ganglia. The present study investigated the trend of developmental expression of AMPA receptor subunits in the striatum of rats in different developmental stages, i.e., at postnatal day 7 (PND7), PND21 and adult. Perfuse-fixed striatal sections were used. The expression of AMPA subunits was studied by immunofluorescence and reverse transcriptase-polymerase chain reaction (RT-PCR). RT-PCR revealed that the levels of expression of the GluR1 and GluR3 mRNAs were the same among the age groups. The level of GluR2 mRNA was highest in PND21 rats and lowest in adult. The highest level of GluR4 mRNA was detected in rats at PND7. Similar trends of GluR1, GluR2 and GluR2/3 immunoreactivity expression were observed using commercially available specific antibodies. In addition, a large proportion of parvalbumin-immunoreactive GABAergic interneurons in the striatum were found to display GluR1 immunoreactivity in PND21 and adult rats. In contrast, most of the choline acetyltransferase-immunoreactive cholinergic interneurons were found to display GluR2 immunoreactivity but less GluR1 and no GluR2/3 immunoreactivity in PND21 and adult rats. The present study suggests that there is a distinct pattern of expression of AMPA-type receptor mRNAs and proteins in the rat striatum at different stages of development.