Neuropeptides, "Gaseous" messengers and classic transmitters : electrophysiological and histochemical studies on coexistence and interactions in the nervous system
Sammanfattning: Neuropeptides, "gaseous" messengers and classic transmitters: electrophysiologicaland histochemical studies on coexistence and interactions in the nervous system. By Zhi-Qing Xu, 1997. Department of Neuroscience, Karolinska Institute, S-17177Stockholm, Sweden. The distribution and functional role of neuropeptides were studied, with specialreference to galanin (GAL) and the free radical nitric oxide (NO). In developingsystems GAL expression was observed already at E14 in trigeminal and dorsal rootganglion (DRG) neurons and at E15 in sensory epithelia in ear, eye and nose, as wellas at E19 during bone formation. Also GAL-R1 receptor mRNA was expressed in the sensoryganglia of embryos but was detected later than the ligand. Thus GAL may have a developmentalrole in several sensory systems and during bone formation. The GAL-R1 receptor waspresent in CGRP neurons and was down-regulated by peripheral nerve injury and inflammation.GAL did not change the membrane current in DRG neurons from normal rats, but causedan inward current in DRG neurons from sciatic nerve transected (SNT) rats. Similarresults were obtained with CCK-8S. In normal rats neurotensin (NT) receptor mRNAwas expressed in about 25% of the small DRG neuron profiles, and NT evoked an outwardcurrent in neuropeptide Y (NPY)-insensitive C-type (small) neurons, while NPY inducedan outward current in NT-insensitive C-type neurons. In axotomized rats NT only inducedan inward current in C-type neurons. These results give evidence for functional peptidereceptors in the DRG neurons and that nerve injury causes distinct changes in receptorphenotype, perhaps reflecting attempts of the DRG neurons to cope with, and to counteractthe consequences of nerve injury. In the central nervous system, GAL/GMAP-positive fibers in the dorsal and manyin the ventral hippocampal formation were noradrenergic. GAL-R2 receptor mRNA wasexpressed in the granule cell layer in the dentate gyrus. However, especially m theventral hippocampus some GAL/GMAP fibers were DBH-negative, and did not disappearafter 6-OH-DA treatment. This suggests that GAL/GMAP are present in multiple hippocampalsystems and that most GALergic fibers are noradrenergic arising from locus coeruleus(LC). GAL-evoked hyperpolarization of LC neurons, and this was accompanied by a decreasein membrane resistance as recorded in a slice preparation. Binding studies revealedGAL binding sites in the LC, but GAL-R1 and GAL-R2 mRNAs were not detectable. Ultrastructuralimmunocytochemistry showed GAL in many neuronal somata and dendritic processes withinthe nucleus. These findings suggest that GAL exerts its inhibitory effect in theLC, probably via an increase in K+ conductance, and that endogenous GAL, possiblyreleased from the LC cell bodies and dendrites, may act on autoreceptors or receptorson adjacent neurons in LC. In the dorsal raphe (DR), 5-HT/GAL/NO synthase (NOS)-coexistence neurons werefound mainly in the midline. All GAL neuron profiles contained 5-HT, and the proportionof 5-HT/GAL neurons was high while that of 5-HT/NOS neurons was low. Some 5-HT/GAL/NOSneurons were found to project to the striatum. However, only in a few cases couldGAL-LI be shown to coexist with 5-HT in striatal fibers with no NOS/5-HT coexistence.The present findings strongly suggest that many DR 5-HT neurons can synthesize andrelease two additional messenger molecules, GAL and NO. However, little GAL and NOSseem to be transported anterogradely to the nerve terminals in the striatum, supportingthe concept of release at the soma/dendrite level. GAL caused a hyperpolarizationof 5-HT of DR neurons accompanied by a decrease in membrane resistance. The 5-HT-inducedoutward current was enhanced and prolonged by preincubation with low concentrationsof GAL. The dose-response curve to 5-HT was changed by GAL with a shift to the left.These results suggest that GAL exerts its effect in the DR directly by acting atreceptors on the membrane of 5-HT-sensitive neurons via an increase in K+ conductanceand indirectly by enhancing the 5-HT action. A small number of NOS-positive, non-noradrenergic neurons were observed withinthe LC. NOS inhibitors enhanced the EPSP in LC, an effect that was reversed by coadministrationof L-arginine. Application of NO-donors increased levels of cGMP as seen with immunohistochemistryas well as induced a hyperpolarization and reduced the EPSP/EPSC in LC neurons. Priorapplication of hemoglobin prevented the action of NO-donors and enhanced the EPSP/EPSC.Application of the membrane permeable cGMP analog 8-bromo-cGMP mimicked the actionof NO-donors. Preincubation with the guanylate cyclase inhibitor ODQ reduced NO-donor-inducedhyperpolarization. These results suggest a role for NO in synaptic transmission inthe LC, which may act through the NO-cGMP pathway by stimulating guanylyl cyclaseand mcreasing endogenous levels of cGMP. Even if only few NOS-positive neurons arepresent in the LC, NO may after release from cell bodies, dendrites and axonal processesvia diffusion influence many noradrenergic LC neurons. Key words: Dorsal root ganglion, galanin, hippocampus, 5-hydroxytryptamine,locus coeruleus, nerve injury, nitric oxide, noradrenaline. ISBN 91-628-2669-7
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