The function of the mammalian superior colliculus (SC) in the subcortical integration of sensorimotor behaviours is modulated by several neuropeptides and neurotransmitters. The SC comprises six laminae, three superficial that receive predominantly visual inputs and three deep layers that receive other sensorimotor inputs and send descending motor fibres to the brainstem and spinal cord. We wished to study the neuronal architecture of the SC of the camel in view of our observation that it was noticeably larger than the inferior colliculus. The brains of 12 adult male camels were obtained from a commercial abattoir, and fixed by immersion in 4% paraformaldehyde within 15 min of death. Using neuron-specific nuclear protein (Neu-N) immunohistochemistry we show that the neuronal architecture of the SC of the camel is similar to that of other mammals. Measurement of neuronal soma sizes after toluidine blue staining demonstrated no significant mean differences between the layers. However, although small neurons predominate in all layers, the deep layers contain numbers of larger neurons. Substance P (SP) and somatostatin (SOM) immunoreactive neuronal somata were limited to the superficial layers. Vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY) were found only in the periaqueductal grey, where SP nerve fibres were also observed. Met-enkephalin immunoreactive (M-enk) neurons, most of which were small, were discernible predominantly in the superficial layers, but some larger M-enk neurons were observed especially in the intermediate grey layer. Projections of NPY, SP, SOM and VIP neurons appeared to be limited to the laminae in which the somata were found, but bridging M-enk fibres were observed between the deep and superficial layers. In addition thick, probably myelinated, M-enk fibres were discernible between the intermediate and deep layers of the colliculus.
This work is supported by grants from the UAE University and the Emirates Natural History Group (Al Ain).