The receptive field properties of single units within the nucleus lentiformis mesencephali (LM) of the pigeon were studied using electrophysiological methods. Previous studies have suggested that the avian LM may be homologous to the nucleus of the optic tract (NOT) in mammals. Single units in the pigeon LM are similar to mammalian NOT units in that they are direction-selective, mostly for horizontal directions, velocity-selective, have large visual receptive fields and respond preferentially to large stimuli with many visual contrasts. In contrast to most reports of NOT units of mammals, more than half of pigeon LM units prefer high velocities (greater than 10 degrees/s), a large proportion (0.37) prefer non-horizontal directions, and receptive fields that are retinotopically arranged within the LM. The response properties of pigeon LM units are compared to the response properties of units within the accessory optic nucleus (the nucleus of the basal optic root or nBOR). In the avian brain, nBOR neurons respond at low velocities (0.5-5 degrees/s) and respond predominantly to vertical stimulus movement whereas LM units respond over a broader range of velocities (0.2-80 degrees/s) and respond predominantly to horizontal movements. Thus, the LM and nBOR may play different roles in the control of compensatory eye movements.

PURPOSE. Stimulation to the cornea via noxious chemical and mechanical means evokes tearing, blinking, and pain. In contrast, mild cooling of the ocular surface has been reported to increase lacrimation via activation of corneal cool primary afferent neurons. The purpose of our study was to determine whether menthol induces corneal cool cell activity and lacrimation via the transient receptor potential melastatin-8 (TRPM8) channel without evoking nociceptive responses.METHODS. Tear measurements were made using a cotton thread in TRPM8 wild type and knockout mice after application of menthol (0.05-50 mM) to the cornea. In additional studies, nocifensive responses (eye swiping and lid closure) were quantified following cornea menthol application. Trigeminal ganglion electrophysiologic single unit recordings were performed in rats to determine the effect of low and high concentrations of menthol on corneal cool cells. RESULTS.At low concentrations, menthol increased tear production in TRPM8 wild type and heterozygous animals, but had no effect in TRPM8 knockout mice, while nocifensive responses remained unaffected. At the highest concentration, menthol (50 mM) increased tearing and nocifensive responses in TRPM8 wild type and knockout animals. A low concentration of menthol (0.1 mM) increased cool cell activity, yet a high concentration of menthol (50 mM) had no effect.CONCLUSIONS. These studies indicated that low concentrations of menthol can increase lacrimation via TRPM8 channels without evoking nocifensive behaviors. At high concentrations, menthol can induce lacrimation and nocifensive behaviors in a TRPM8 independent mechanism. The increase in lacrimation is likely due to an increase in cool cell activity. (Invest Ophthalmol Vis Sci. 2012;53:7034-7042)

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