Hypoglossal afferents to lamina I neurons of the cervical spinal cord projecting to the parabrachial nucleus in the cat

Spinal and pontine relay pathways mediating respiratory rhythm entrainment by limb proprioceptive inputs in the neonatal rat

The coordination of locomotion and respiration is widespread among mammals, although the underlying neural mechanisms are still only partially understood. It was previously found in neonatal rat that cyclic electrical stimulation of spinal cervical and lumbar dorsal roots (DRs) can fully entrain (1:1 coupling) spontaneous respiratory activity expressed by the isolated brainstem/spinal cord. Here, we used a variety of preparations to determine the type of spinal sensory inputs responsible for this respiratory rhythm entrainment, and to establish the extent to which limb movement-activated feedback influences the medullary respiratory networks via direct or relayed ascending pathways. During in vivo overground locomotion, respiratory rhythm slowed and became coupled 1:1 with locomotion. In hindlimb-attached semi-isolated preparations, passive flexion-extension movements applied to a single hindlimb led to entrainment of fictive respiratory rhythmicity recorded in phrenic motoneurons, indicating that the recruitment of limb proprioceptive afferents could participate in the locomotor-respiratory coupling. Furthermore, in correspondence with the regionalization of spinal locomotor rhythm-generating circuitry, the stimulation of DRs at different segmental levels in isolated preparations revealed that cervical and lumbosacral proprioceptive inputs are more effective in this entraining influence than thoracic afferent pathways. Finally, blocking spinal synaptic transmission and using a combination of electrophysiology, calcium imaging and specific brainstem lesioning indicated that the ascending entraining signals from the cervical or lumbar limb afferents are transmitted across first-order synapses, probably monosynaptic, in the spinal cord. They are then conveyed to the brainstem respiratory centers via a brainstem pontine relay located in the parabrachial/Kölliker-Fuse nuclear complex.

Parabrachial inputs to Fos-immunoreactive neurons in the lateral central nucleus of amygdala activated by hypotension: a light and electron microscopic study in the rat

Morphological features and functional implications of projections of the parabrachial nucleus to the central nucleus of the amygdala were investigated in the rat. The anatomical study was based on injections of the tracers horseradish peroxidase and biotinylated dextran amine. An extremely dense concentration of labeled fibers was found in the lateral and lateral capsular subdivisions of the central nucleus of the amygdala, originating mainly from the external lateral and ventral lateral subnuclei of the parabrachial nucleus. The parabrachial fibers exhibited the morphological characteristic of forming dense pericellular terminal arborizations. The functional implications of this pathway in cardiovascular functions were verified using Fos protein induction in response to hypotension induced by continuous intravenous administration of hydralazine-hydrochloride. In this paradigm, Fos immunoreactivity was found to be confined to the lateral and lateral capsular subdivisions of the central nucleus of the amygdala. Double immunostaining methods were used to visualize, at the electron microscopic level, terminals labeled by biotinylated dextran amine and Fos cell labeling. With this approach, we were able to confirm that Fos-immunoreactive neurons in the central nucleus of the amygdala receive axosomatic terminals from the parabrachial nucleus. The present findings point out that parabrachial inputs to the central nucleus of the amygdala play a relevant role in regulating cardiovascular function.