Cholinergic interneurons are relatively rare neurons in the rat striatum. These sparsely distributed neurons display a synchronous pause in their tonic firing pattern during reward-related learning. It has been hypothesised that a specialised fast-conducting crossed-corticostriatal pathway is involved in synchronising the pause in tonic firing of these interneurons. This study aimed to detail the innervation of cholinergic interneurons by mapping their proximal and distal inputs and to describe the innervation of the crossed-corticostriatal pathway in male Wistar rats. In vivo electrophysiological recording methods were used to label single crossed-corticostriatal neurons but inadequately labeled their axons. Thus, an anterograde neuronal tracing study was conducted. Biotinylated dextran amine (BDA; 1.2 [mu]l) was pressure-injected into the left cerebral hemisphere. Six days later, the rat was perfused-fixed and the brain sectioned. BDA-labelled axons were traced to both the ipsilateral and contralateral striata. Cholinergic interneurons in the right striatum were double-immunolabelled using an optimised protocol including a polyclonal rabbit anti-m2-muscarinic receptor antibody and a monoclonal goat anti-choline acetyltransferase antibody. All sections were processed for transmission electron microscopy. Serial ultrathin sections were montaged and distal (from non BDA-labelled tissue) and proximal synapses were each mapped separately. A reconstructed distal dendrite from a cholinergic interneuron, located 225 [mu]m from the soma, was analysed. It had an average width of 1 .25[mu]m and 0.726 synapses per [mu]m. This was compared to dendrites in the same tissue and from BDA-labelled tissue. Two dendrites were presumed to be distal profiles of either cholinergic or somatostatin interneurons, while the third was thought to belong to another interneuronal cell type. In terms of surface area, there were less somal synapses compared to those made onto the distal dendrite of the cholinergic interneuron. Somal synapse counts were similar to those reported previously from our laboratory, where symmetric synapses were most common. Crossed-corticostriatal BDA-labelled axons were found to course across proximal dendrites and somas of immunolabelled cholinergic interneurons. Varicosities from these axons were found in close proximity to proximal dendrites and somas of cholinergic interneurons. Of all cholinergic interneurons in an adjacent section, 77% showed closely associated proximal varicosities. Of these, 76% of varicosities were associated with the soma, 11% to proximal dendrites and 13% to both locations. Twenty-nine BDA-labeled axons were analysed using transmission electron microscopy. Most were observed making asymmetric synaptic contact with unlabelled spines. In two cases spines were traced to medium spiny projection neurons. Two axon segments were seen touching the proximal regions of separate cholinergic interneurons. At these contact sites interrupted membrane thickenings were observed. It is proposed here that synapses may form at these sites during reward-related learning. However labelling of the contact sites with a postsynaptic marker would be necessary to confirm their synaptic nature. The current study has gathered information about the distal and proximal innervation patterns of these neurons and described the termination pattern of the crossed-corticostriatal pathway in relation to these neurons for the first time. These findings support the crossed-corticostriatal pathway as one possible anatomical substrate for synchronising the pause response on both sides of the brain.
| Identifer | oai:union.ndltd.org:ADTP/256951 |
| Date | January 2009 |
| Creators | Sizemore, Rachel J, n/a |
| Publisher | University of Otago. Department of Anatomy & Structural Biology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Rachel J Sizemore |
Page generated in 0.0015 seconds