Although atropine does not cause mydriasis, vecuronium bromide does. These results suggest that the parasympathetic system in turtles acts through acetylcholine onto nicotinic receptors to stimulate pupillary light constriction.

Receptive fields of retinal ganglion cells in turtle have excitatory and inhibitory components that are balanced along the dimensions of wavelength, functional ON and OFF responses, and spatial assignments of center and surround. These components were analyzed by spectral light adaptations and by the glutamate agonist, 2-amino-4-phosphonobutyric acid (APB). Extracellular recordings to stationary and moving spots of light were used to map changes in receptive fields. ON spike counts minus OFF spike counts, derived from flashed stationary light spots, quantified functional shifts by calculating normalized mean response modulations. The data show that receptive fields are not static, but rather are dynamic arrangements which depend on linked, antagonistic balances among the three dimensions of wavelength, ON and OFF response functions, and center/surround areas.

When intense adapting lights are turned off, the pupil of the turtle, Pseudemys scripta elegans, enlarges. The recovery functions for pupillary dilation have different time constants that are defined by red-and gree/i-sensitive cones and rods as they are affected by prior light adaptation and time in the dark. Pupillary area related to dilation responds over at least a three-to four-fold range. Following white-\\g\\V adaptation, the course of pupil dilation in the dark shows a three-legged curve of differing time constants. With spectral-light adaptations, the contributions of separate classes of photoreceptors can be isolated. Red-and g/"ee/?-sensitive cones contribute shorter time constants of 3.31 and 3.65 min to prior wMe-light adaptation-4.81 and 4.18 min to prior spectral-light adaptations. Rods contribute a much longer time constant of 6.69 min to prior vWi/te-light adaptation -7.60 min to prior spectral-light adaptation. The ratios are in keeping with the flash sensitivities of photoreceptors in this same animal, as well as with psychophysical visual threshold mechanisms of color sensitivity.

The pond turtle (Trachemys scripta elegans) exhibits a notably sluggish pupillary light reflex (PLR), with pupil constriction developing over several minutes following light onset. In the present study, we examined the dynamics of the efferent branch of the reflex in vitro using preparations consisting of either the isolated head or the enucleated eye. Stimulation of the oculomotor nerve (nIII) using 100-Hz current trains resulted in a maximal pupil constriction of 17.4% compared to 27.1% observed in the intact animal in response to light. When current amplitude was systematically increased from 1 to 400 microA, mean response latency decreased from 64 to 45 ms, but this change was not statistically significant. Hill equations fitted to these responses indicated a current threshold of 3.8 microA. Stimulation using single pulses evoked a smaller constriction (3.8%) with response latencies and threshold similar to that obtained using train stimulation. The response evoked by postganglionic stimulation of the ciliary nerve using 100-Hz trains was largely indistinguishable from that of train stimulation of nIII. However, application of single-pulse stimulation postganglionically resulted in smaller pupil constriction at all current levels relative to that of nIII stimulation, suggesting that there is amplification of efferent drive at the ganglion. Time constants for constrictions ranged from 88 to 154 ms with relaxations occurring more slowly at 174-361 ms. These values for timing from in vitro are much faster than the time constant 1.66 min obtained for the light response in the intact animal. The rapid dynamics of pupil constriction observed here suggest that the slow PLR of the turtle observed in vivo is not due to limitations of the efferent pathway. Rather, the sluggish response probably results from photoreceptive mechanisms or central processing.

A mammalian-like melanopsin (Opn4m) has been found in all major vertebrate classes except reptile. Since the pupillary light reflex (PLR) of the fresh water turtle takes between 5 and 10 min to achieve maximum constriction, and since photosensitive retinal ganglion cells (ipRGCs) in mammals use Opn4m to control their slow sustained pupil responses, we hypothesized that a Opn4m homolog exists in the retina of the turtle. To identify its presence, retinal tissue was dissected from seven turtles, and total RNA extracted. Reverse transcriptase-polymerase chain reactions (RT-PCRs) were carried out to amplify gene sequences using primers targeting the highly conserved core region of Opn4m, and PCR products were analyzed by gel electrophoresis and sequenced. Sequences derived from a 1004-bp PCR product were compared to those stored in GenBank by the basic local alignment search tool (BLAST) algorithm and returned significant matches to several Opn4ms from other vertebrates including chicken. Quantitative real-time PCR (qPCR) was also carried out to compare expression levels of Opn4m in different tissues. The normalized expression level of Opn4m in the retina was higher in comparison to other tissue types: iris, liver, lung, and skeletal muscle. The results suggest that Opn4m exists in the retina of the turtle and provides a possible explanation for the presence of a slow PLR. The turtle is likely to be a useful model for further understanding the photoreceptive mechanisms in the retina which control the dynamics of the PLR.