Do Serotonin Immunoreactive Terminals Innervate GABAergic Neurones in the Central Autonomic Area of the Mouse Spinal Cord?

Sympathetic preganglionic neurones (SPNs), the origin of spinal sympathetic output, receive input from local interneurones. A recently identified group of presympathetic GABAergic interneurones resides in the central autonomic area (CAA; Deuchars et al., 2005, J.Neurosci. 25 (5):). Descending serotonin (5‐HT) containing terminals heavily innervate the CAA and 5‐HT both excites and indirectly inhibits SPNs in spinal cord slices (Lewis et al., Brain Res. 1993, 610(2):267–75), suggesting actions on local presympathetic interneurones. Thus possible innervation of GABAergic neurones in the CAA by serotonin‐containing inputs was investigated using immunohistochemistry for 5‐HT on spinal cord sections from transgenic mice expressing GFP under control of the promoter for glutamate decarboxylase 65 (GAD65). Fluorescence and confocal microscopy revealed that 5‐HT‐ immunoreactive (IR) terminals were present in a dense plexus in the CAA. Some of these terminals closely apposed GFP cell bodies and proximal dendrites. Of 20 cells examined, 19 (95%) received appositions ranging from 0.09 – 1.82 contacts per 100 μm2 (1–8 appositions per cell body). Thus spinal GABAergic neurones are influenced by descending serotoninergic systems, providing an anatomical substrate for an indirect inhibitory effect of 5‐HT on SPNs. Funded by the British Heart Foundation and Medical Research Council.