Experimental studies on the sources, pathways and targets for volume transmission in the rat brain

Abstract

Volume transmission has been defined as the three-dimensional diffusion or convection of transmitters in the extracellular space (ECS) and cerebrospinal fluid (CSF) over a larger distance than the synaptic cleft. The regional localization of dopamine (DA) nerve terminals and dopamine Dl and D2 receptors in nucleus accumbens were studied in detail using double immuno-labelling. In the shell part of the nucleus accumbens of the rat, a rostrocaudally directed tubular network of strongly Dl-immunoreactive (IR) nerve cells was found, surrounded by intense TH-IR nerve terminals. The relationships of 5 hydroxytryptamine (5-HT) nerve terminals and 5-HT receptors, type 2A (5-HT2A), were investigated using double immuno-labelling in the forebrain and brainstem. Both close and short distance relationships between 5-HT-IR nerve terminals and 5-HT2A-IR neuronal profiles were demonstrated in various brain regions. In e.g. the cerebral cortex, the islands of Calleja and in the olfactory bulb, short distance relationships in favour of 5-HT VT were found.

The spread of biotinylated dextran, mw 3000 (dextran), was studied. Nitric oxide synthase inhibition and striatal injections of interleukin-1ß (IL-1ß) decreased the remaining volume of dextran. An increased non-specific uptake was found in diffusion measurements CIMA-method). These findings may indicate an increased clearance over the capillaries induced by NOS inhibition and IL-1ß, suggesting that the extracellular space (ECS) content of VT signals could be modified in this way.

Moreover, striatal injections of IL-1ß induced an increase in the number of FOS-IR nuclei in the hypothalamic paraventricular nucleus that could be attenuated by intraventricular injections of the interleukin antagonist protein. The observed effect could be mediated by IL-lß VT signaling via the CSF.

The periventricular zone of the neostriatum may have role in registering VT via the CSF, as indicated by analysis of the effects of intraventricular injections on the 1 denervated striatum. VT signals modify target cells via receptor-receptor interactions as illustrated for the CCK-8 signal. Thus, evidence was obtained for between cholecystokinin B and dopamine D2 receptor interactions in cell lines cotransfected with the two receptors. An involvement of G-proteins in this interaction was also suggested by results using an Gi-protein inactivating antibody and G-protein activation by GMP-PNP. Intraventricular injections of porcine galanin resulted in the appearance of galanin-IR nerve cell bodies, especially the in hippocampal formation. Therefore, the internalization of gaLanin may also be a target action in the case of galanin VT. In summary, this investigation have resulted in several findings demonstrating sources, pathways and targets of monoaminergic and peptidergic VT in the rat brain.