Modulation of hypercapnic respiratory response by cholinergic transmission in the commissural nucleus of the solitary tract.

The nucleus of the solitary tract (NTS) is an important area of the brainstem that receives and integrates afferent cardiorespiratory sensorial information, including those from arterial chemoreceptors and baroreceptors. It was described that acetylcholine (ACh) in the commissural subnucleus of the NTS (cNTS) promotes an increase in the phrenic nerve activity (PNA) and antagonism of nicotinic receptors in the same region reduces the magnitude of tachypneic response to peripheral chemoreceptor stimulation, suggesting a functional role of cholinergic transmission within the cNTS in the chemosensory control of respiratory activity. In the present study, we investigated whether cholinergic receptor antagonism in the cNTS modifies the sympathetic and respiratory reflex responses to hypercapnia. Using an arterially perfused in situ preparation of juvenile male Holtzman rats, we found that the nicotinic antagonist (mecamylamine, 5 mM), but not the muscarinic antagonist (atropine, 5 mM), into the cNTS attenuated the hypercapnia-induced increase of hypoglossal activity. Furthermore, mecamylamine in the cNTS potentiated the generation of late-expiratory (late-E) activity in abdominal nerve induced by hypercapnia. None of the cholinergic antagonists microinjected in the cNTS changed either the sympathetic or the phrenic nerve responses to hypercapnia. Our data provide evidence for the role of cholinergic transmission in the cNTS, acting on nicotinic receptors, modulating the hypoglossal and abdominal responses to hypercapnia.

Photo-tropotaxis based on projection through the cerebral commissure in the terrestrial slug Limax.

In the terrestrial slug, Limax, eyes are located at the tip of the superior tentacles. This animal has long been believed to show negative phototaxis through tropotaxis, i.e., it compares the two light intensities detected by bilateral eyes to move away from a light source. As one of the possible manifestations of such negative phototaxis, a circling movement has been observed: if one of the superior tentacles is removed, the slugs continuously move in the direction of the removed side. However, there has been no evidence demonstrating that this behavior is actually based on negative phototropotaxis. In this study, we showed that the slugs do not exhibit the circling behavior in the absence of light, and that amputation of the cerebral commissure also diminishes the circling behavior under light. We could detect light-evoked responses during electrical recording from the cut edge of the cerebral commissure. Labeling of the optic nerve with neurobiotin also revealed the presence of the commissural fibers that potentially transmit the light information to the contralateral cerebral ganglion. Our study suggests that the slug's circling behavior is based on phototropotaxis in which the light intensities detected by the bilateral eyes are compared through the cerebral commissure.

Development and plasticity of commissural circuits: from locomotion to brain repair.

Commissural neurons project their axons across the midline of the nervous system to contact neurons on the opposite side. Although their existence has been known for more than a century, the function of brain commissures, as well as their diversity and evolutionary advantage, are far from understood. Recent genetic studies in mammals have led to the identification of subsets of commissural neurons, which, in the hindbrain and spinal cord, control the tuning and bilateral coordination of locomotion. The molecular mechanisms and transcriptional programs which specify axonal laterality during development are also now being elucidated. Finally, new studies have confirmed that axonal laterality is plastic and that facilitating the commissural sprouting of axon collaterals might influence functional recovery after brain injury.