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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.

In vertebrates, stimulation of the mesencephalic locomotor region (MLR) on one side evokes symmetrical locomotor movements on both sides. How this occurs was previously examined in detail in a swimmer using body undulations (lamprey), but in tetrapods the downstream projections from the MLR to brainstem neurons are not fully understood. Here we examined the brainstem circuits from the MLR to identified reticulospinal neurons in the salamander Notophthalmus viridescens. Using neural tracing, we show that the MLR sends bilateral projections to the middle reticular nucleus (mRN, rostral hindbrain) and the inferior reticular nucleus (iRN, caudal hindbrain). Ca(2+) imaging coupled to electrophysiology in in vitro isolated brains revealed very similar responses in reticulospinal neurons on both sides to a unilateral MLR stimulation. As the strength of MLR stimulation was increased, the responses increased in size in reticulospinal neurons of the mRN and iRN, but the responses in the iRN were smaller. Bath-application or local microinjections of glutamatergic antagonists markedly reduced reticulospinal neuron responses, indicating that the MLR sends glutamatergic inputs to reticulospinal neurons. In addition, reticulospinal cells responded to glutamate microinjections and the size of the responses paralleled the amount of glutamate microinjected. Immunofluorescence coupled with anatomical tracing confirmed the presence of glutamatergic projections from the MLR to reticulospinal neurons. Overall, we show that the brainstem circuits activated by the MLR in the salamander are organized similarly to those previously described in lampreys, indicating that the anatomo-physiological features of the locomotor drive are well conserved in vertebrates. J. Comp. Neurol. 524:1361-1383, 2016. © 2015 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc.

The present study was carried out to characterize anatomically the vestibulospinal (VS) system of lampreys. Cobalt-lysine or Texas Red dextran amines were applied in vitro to the rostral spinal cord. Two distinct populations of VS neurons were labeled in the ventral nucleus of the area octavolateralis. The rostral group, comprising the intermediate octavomotor nucleus (ION), contained between 100 and 150 neurons, having somata of variable size and morphology. Intracellular injections of Lucifer Yellow in single neurons revealed ION VS neurons with dendrites extending in the ventrolateral alar plate as well as medially in the basal plate. The caudal group, comprising the posterior octavomotor nucleus (PON), contained approximately 65 neurons, most of which were unipolar with round or oval somata. To study the projections of VS axons, cobalt-lysine was injected into the ION or PON regions in the brainstem. Axons from the ION projected to the ipsilateral spinal cord, whereas PON axons decussated within the basal plate giving out descending and ascending branches. The descending branch projected to the contralateral spinal cord. Injections of two fluorescent dextran-amines, each restricted to one side of the spinal cord, did not double-label VS cells in either octavomotor nuclei, indicating that the projections of each nucleus are restricted to one side. Injections of horseradish peroxidase further caudally in the spinal cord revealed that VS axons from the ION reached past the gill region. Our results indicate that the organization of the VS system of lampreys is similar to that observed in other vertebrates. J. Comp. Neurol. 407:512‐526, 1999. r 1999 Wiley-Liss, Inc.