Brain Photoreceptor Pathways Contributing to Circadian Rhythmicity in Crayfish

Abstract

Freshwater crayfish have three known photoreceptive systems: the compound eyes, extraretinal brain photoreceptors, and caudal photoreceptors. The primary goal of the work described here was to explore the contribution of the brain photoreceptors to circadian locomotory activity and define some of the underlying neural pathways. Immunocytochemical studies of the brain photoreceptors in the parastacid (southern hemisphere) crayfish Cherax destructor reveal their expression of the blue light-sensitive photopigment cryptochrome and the neurotransmitter histamine. The brain photo-receptors project to two small protocerebral neuropils, the brain photoreceptor neuropils (BPNs), where they terminate among fibers expressing the neuropeptide pigment-dispersing hormone (PDH), a signaling molecule in arthropod circadian systems. Comparable pathways are also described in the astacid (northern hemisphere) crayfish Procambarus clarkii. Despite exhibiting markedly different diurnal locomotor activity rhythms, removal of the compound eyes and caudal photoreceptors in both C. destructor and P. clarkii (leaving the brain photoreceptors intact) does not abolish the normal light/dark activity cycle in either species, nor prevent the entrainment of their activity cycles to phase shifts of the light/dark period. These results suggest, therefore, that crayfish brain photoreceptors are sufficient for the entrainment of loco-motor activity rhythms to photic stimuli, and that they can act in the absence of the compound eyes and caudal photoreceptors. We also demonstrate that the intensity of PDH expression in the BPNs varies in phase with the locomotor activity rhythm of both crayfish species. Together, these findings suggest that the brain photoreceptor cells can function as extraretinal circadian photoreceptors and that the BPN represents part of an entrainment pathway synchronizing locomotor activity to environmental light/dark cycles, and implicating the neuropeptide PDH in these functions. (Author correspondence: bbeltz@wellesley.edu)

Chronic electromyographic analysis of circadian locomotor activity in crayfish

“…6). The pattern was consistent with the previous reports [37][38][39]43]. In the free running conditions, the mean periodicity was shorter than 24 h in DD (Fig.8), but longer than 24 h in LL (Fig.9).…”

“…The circadian rhythms of locomotor activity and sensory input, in particular, have been intensively studied to clarify the neuronal mechanisms of the rhythm by using surgical interference, ablation, biochemical and molecular biological techniques [3,38,43]. Although these researches provided evidences that the circadian locomotor activity was attributed by endogenous factors functioning in the brain [3,38,43], it remained unknown how the endogenous factors affect the locomotor control system in the central nervous system. The locomotor behavior can be initiated in general either spontaneously or reflexively [44][45][46].…”

“…We demonstrated by chronic EMG recording that the locomotor behavior was initiated spontaneously, not reflexively by any external stimulus. It has been proposed that the locomotory circadian rhythmicity of P. clarkii is generated by clock proteins and regulated by hormonal and neuromodulator secretion in the brain [3,41,43] and also is regulated by photoreceptors in the brain [37,38,43] or in the 6 th abdominal ganglion [56,57]. Although the details of interaction cascades among those factors remain unknown, the brain should play an important role for the circadian locomotor activity [3,38,43].…”

“…Recent research using Procambarus clarkii and Cherax quadricarinatus suggests that the caudal photoreceptor contributes to the entrainment of circadian locomotor activity [56,57]. Meanwhile, other studies using P. clarkii demonstrated that the brain photoreceptors are necessary for the entrainment of locomotor activity rhythms to light stimuli [38], and that they can function in the absence of the compound eyes and caudal photoreceptors [37,43]. Whatever photoreceptors are the predominant factor for LD entrainment, phonic input sensitivity of any photoreceptors must be strongly coupled to circadian locomotor behavior.…”

“…Crustaceans have been known to show circadian rhythms in hormonal secretion [12][13][14][15], neurosecretion [16], biogenic amine contents and actions in the central nervous system [17][18][19], mechanoreceptor sensitivity [20], photoreceptor sensitivity [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] and locomotor activity [37][38][39][40][41][42][43]. A previous study in the crayfish Procambarus clarkii showed that the circadian rhythm in leg movement disappeared when the circumesophageal commissures were surgically lesioned bilaterally, thus experimentally demonstrating the importance of descending signals from the brain to posterior ganglia [38].…”