Molecules present in an animal's environment can indicate the presence of predators, food, or sexual partners and consequently, induce migratory, reproductive, foraging, or escape behaviors. Three sensory systems, the olfactory, gustatory, and solitary chemosensory cell (SCC) systems detect chemical stimuli in vertebrates. While a great deal of research has focused on the olfactory and gustatory system over the years, it is only recently that significant attention has been devoted to the SCC system. The SCCs are microvillous cells that were first discovered on the skin of fish, and later in amphibians, reptiles, and mammals. Lampreys also possess SCCs that are particularly numerous on cutaneous papillae. However, little is known regarding their precise distribution, innervation, and function. Here, we show that sea lampreys (Petromyzon marinus L.) have cutaneous papillae located around the oral disk, nostril, gill pores, and on the dorsal fins and that SCCs are particularly numerous on these papillae. Tract‐tracing experiments demonstrated that the oral and nasal papillae are innervated by the trigeminal nerve, the gill pore papillae are innervated by branchial nerves, and the dorsal fin papillae are innervated by spinal nerves. We also characterized the response profile of gill pore papillae to some chemicals and showed that trout‐derived chemicals, amino acids, and a bile acid produced potent responses. Together with a companion study (Suntres et al., Journal of Comparative Neurology, this issue), our results provide new insights on the function and evolution of the SCC system in vertebrates.
280 / 350 words) 11 Locomotion is essential for an animal's survival. This behavior can range from directional 12 changes to adapting the motor force to the conditions of its surroundings. Even if speed and 13 force of movement are changing, the relative coordination between the limbs or body 14 segments has to stay stable in order to provide the necessary thrust. The coordinating 15 information necessary for this task is not always conveyed by sensory pathways. Adaptation 16 is well studied in sensory neurons, but only few studies have addressed if and how 17 coordinating information changes in cases where a local circuit within the central nervous 18 system is responsible for the coordination between body segments at different locomotor 19 activity states. 20One system that does not depend on sensory information to coordinate a chain of coupled 21 oscillators is the swimmeret system of crayfish. Here, the coordination of four coupled CPGs 22 is controlled by central Coordinating Neurons. Cycle by cycle, the Coordinating Neurons 23 encode information about the activity state of their home ganglion as burst of spikes, and send 24 it as corollary discharge to the neighboring ganglia. Activity states, or excitation levels, are 25 variable in both the living animal and isolated nervous system; yet the amount of coordinating 26 spikes per burst is limited. 27Here, we demonstrate that the system's excitation level tunes the encoding properties of the 28 Coordinating Neurons. Their ability to adapt to excitation level, and thus encode relative 29 changes in their home ganglion's activity states, is mediated by a balancing mechanism. 30Manipulation of cholinergic pathways directly affected the coordinating neurons' 31 electrophysiological properties. Yet, these changes were counteracted by the network's 32 influence. This balancing may be one feature to adapt the limited spike range to the system's 33 current activity state. 34 35 37 One remarkable feature of animals is their ability to extract useful information from 38 ubiquitous noise to ensure the organism's functionality. For example, sensory neurons adapt 39 to occurring ranges of stimulus intensities to maximize information transfer via their electrical 40 activity (Dean et al., 2005; Laughlin, 1981; Maravall et al., 2007), a process in accordance 41 with Barlow's efficient coding hypothesis (Barlow, 1961). Most studies investigating neural 42 adaptation and gain control focus on perception of environmental stimuli and adaptation of 43 sensory neurons. Less is known about adaptive abilities of interneurons which provide 44 information about internal states. Such interneurons can play important roles in coupling of 45 neural networks that underlie behavior. Coupling networks through interneurons allows for 46 faster information exchange compared to coupling via sensory pathways (LeGal et al., 2017). 47 For example, lamprey increase their respiratory frequency in response to increased activity of 48 the mesencephalic locomotor region even before proprioceptiv...
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