Anatomy of Somatostatin-Immunoreactive Fibres and Cell Bodies in the Rat Trigeminal Subnucleus Caudalis

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The distribution of somatostatin-immunoreactive fibres and cells has been analysed in the rat spinal trigeminal subnucleus caudalis. Immunoreactive fibres are most concentrated in lamina II outer but fibres and terminals occur also in lamina I, lamina II inner, and scattered in the magnocellular region and neighbouring lateral reticular area. Immunoreactive cells occur in laminae I and II and in the magnocellular region of the nucleus but are most abundant in lamina II inner. The lamina II immunoreactive cells are morphologically heterogeneous and include types which are similar to cells described in Golgi studies such as stalked and islet cells.

In order to distinguish somatostatin-immunoreactive primary afferents from intrinsic sources of somatostatin such as the lamina II neurons, we have used a monoclonal antibody (LD2) which is specific for primary afferents. Using dual-colour immunofluorescence we have shown that all somatostatin-immunoreactive cells in the trigeminal ganglia express LD2 immunoreactivity. Quantitative immunostaining density profiles indicate that LD2- and somatostatin-immunoreactive fibres overlap mainly in lamina II outer and dual-colour immunofluorescence confirms that this region contains somatostatin and LD2 double-labelled fibres. In contrast, lamina I contains more somatostatin- than LD2-immunoreactive fibres and fewer double-labelled fibres.

The presence of double-labelled fibres in outer lamina II indicates that somatostatin-immunoreactive primary afferents terminate largely in this sublamina. However, the small number of double-labelled fibres found suggests that somatostatin-immunoreactive fibres in laminae I and II are derived mainly from intrinsic sources such as the various types of lamina II neurons.



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