Corneal receptors have not previously been identified in lower vertebrates. The present study describes the properties of trigeminal ganglion corneal receptors in a teleost fish, the rainbow trout (Oncoryhnchus mykiss). Out of 27 receptors, 7 were polymodal nociceptors, 6 were mechanothermal nociceptors, 2 were mechanochemical receptors and the largest group, 12, were only responsive to mechanical stimulation. No cold responsive receptors were found on the trout cornea. Mechanical and thermal thresholds were lower and receptive field diameters smaller than those of cutaneous trigeminal receptors in the trout, demonstrating greater sensitivity in the cornea. The lack of cold sensitive neurons may provide evidence for the evolution of cold nociceptors in vertebrates that is related to the transition from poikilothermy to homeothermy.
Ashley, P. J., Sneddon, L. U., & McCrohan, C. R. (2006). Properties of corneal receptors in a teleost fish. Neuroscience letters, 410(3), 165-168.