Distribution and morphology of baroreceptors in the rat carotid sinus as revealed by immunohistochemistry for P2X3 purinoceptors

A comparative study of bronchopulmonary slowly adapting receptors between rabbits and rats

Pulmonary mechanosensory receptors provide important inputs to the respiratory center for control of breathing. However, what is known about their structure-function relationship is still limited. In these studies, we explored this relationship comparing bronchopulmonary slowly adapting receptor (SAR) units in rabbits and rats. In morphological studies, sensory units in tracheobronchial smooth muscle labeled with anti-Na+ /K+ -ATPase (α3 subunit) were found to be larger in the rabbit. Since larger structures may result from increased receptor size or more numerous receptors, further examination showed receptor size was the same in both species, but more receptors in a structure in rabbits than rats, accounting for their larger structure. In functional studies, SAR units were recorded electrically in anesthetized, open-chest, and artificially ventilated animals and responses to lung inflation were compared at three different constant airway pressures (10, 20, and 30 cmH2 O). At each level of the inflation, SAR discharge frequencies were found to be higher in rabbits than rats. We conclude that a relatively larger number of receptors in a sensory unit may be responsible for higher SAR activities in rabbit SAR units.

Morphology of P2X3-immunoreactive basket-like afferent nerve endings surrounding serosal ganglia and close relationship with vesicular nucleotide transporter-immunoreactive nerve fibers in the rat gastric antrum

We previously reported P2X3 purinoceptor (P2X3)-expressing vagal afferent nerve endings with large web-like structures in the subserosal tissue of the antral lesser curvature, suggesting that these nerve endings were one of the vagal mechanoreceptors. The present study investigated the morphological relationship between P2X3-immunoreactive nerve endings and serosal ganglia in the rat gastric antrum by immunohistochemistry of whole-mount preparations using confocal scanning laser microscopy. P2X3-immunoreactive basket-like subserosal nerve endings with new morphology were distributed laterally to the gastric sling muscles in the distal antrum of the lesser curvature. Parent axons ramified into numerous nerve fibers with pleomorphic flattened structures to form basket-like nerve endings, and the parent axons were originated from large net-like structures of vagal afferent nerve endings. Basket-like nerve endings wrapped around the whole serosal ganglia, which were characterized by neurofilament 200 kDa-immunoreactive neurons with or without neuronal nitric oxide synthase immunoreactivity and S100B-immunoreactive glial cells. Furthermore, basket-like nerve endings were localized in close apposition to dopamine beta-hydroxylase-immunoreactive sympathetic nerve fibers immunoreactive for vesicular nucleotide transporter. These results suggest that P2X3-immunoreactive basket-like nerve endings associated with serosal ganglia are the specialized ending structures of vagal subserosal mechanoreceptors in order to increase the sensitivity during antral peristalsis, and are activated by ATP from sympathetic nerve fibers and/or serosal ganglia for the regulation of mechanoreceptor function.

Distribution and morphology of P2X3-immunoreactive subserosal afferent nerve endings in the rat gastric antrum

The present study investigated the morphological characteristics of subserosal afferent nerve endings with immunoreactivity for the P2X3 purinoceptor (P2X3) in the rat stomach by immunohistochemistry of whole-mount preparations using confocal scanning laser microscopy. P2X3 immunoreactivity was observed in subserosal nerve endings proximal and lateral to the gastric sling muscles in the distal antrum of the lesser curvature. Parent axons ramified into several lamellar processes to form net-like complex structures that extended two-dimensionally in every direction on the surface of the longitudinal smooth muscle layer. The axon terminals in the periphery of P2X3-immunoreactive net-like structures were flat and looped or leaf-like in shape. Some net-like lamellar structures and their axon terminals with P2X3 immunoreactivity were also immunoreactive for P2X2. P2X3-immunoreactive nerve fibers forming net-like terminal structures were closely surrounded by S100B-immunoreactive terminal Schwann cells, whereas axon terminals twined around these cells and extended club-, knob-, or thread-like protrusions in the surrounding tissues. Furthermore, a retrograde tracing method using fast blue dye indicated that most of these nerve endings originated from the nodose ganglia of the vagus nerve. These results suggest that P2X3-immunoreactive subserosal nerve endings have morphological characteristics of mechanoreceptors and contribute to sensation of a mechanical deformation of the distal antral wall associated with antral peristalsis.

Development of a Mouse Reporter Strain for the Purinergic P2X2 Receptor

The ATP-sensitive P2X₂ ionotropic receptor plays a critical role in a number of signal processes including taste and hearing, carotid body detection of hypoxia, the exercise pressor reflex and sensory transduction of mechanical stimuli in the airways and bladder. Elucidation of the role of P2X₂ has been hindered by the lack of selective tools. In particular, detection of P2X₂ using established pharmacological and biochemical techniques yields dramatically different expression patterns, particularly in the peripheral and central nervous systems. Here, we have developed a knock-in P2X₂-cre mouse, which we crossed with a cre-sensitive tdTomato reporter mouse to determine P2X₂ expression. P2X₂ was found in more than 80% of nodose vagal afferent neurons, but not in jugular vagal afferent neurons. Reporter expression correlated in vagal neurons with sensitivity to α,β methylene ATP (αβmATP). P2X₂ was expressed in 75% of petrosal afferents, but only 12% and 4% of dorsal root ganglia (DRG) and trigeminal afferents, respectively. P2X₂ expression was limited to very few cell types systemically. Together with the central terminals of P2X₂-expressing afferents, reporter expression in the CNS was mainly found in brainstem neurons projecting mossy fibers to the cerebellum, with little expression in the hippocampus or cortex. The structure of peripheral terminals of P2X₂-expressing afferents was demonstrated in the tongue (taste buds), carotid body, trachea and esophagus. P2X₂ was observed in hair cells and support cells in the cochlear, but not in spiral afferent neurons. This mouse strain provides a novel approach to the identification and manipulation of P2X₂-expressing cell types.

Vesicular glutamate transporter 2-immunoreactive afferent nerve terminals in rat carotid sinus baroreceptors

Sensory nerve endings respond to various stimuli and subsequently transmit afferent informations to central nervous system, but their responsibility has been suggested to be modulated by glutamate. In the present study, we examined the immunohistochemical localization of vesicular glutamate transporter 1 (vGLUT1) and vGLUT2 in baroreceptor nerve endings immunoreactive for P2X2 and P2X3 purinoceptors in the rat carotid sinus by immunohistochemistry of whole-mount preparations with confocal scanning laser microscopy. P2X3-immunoreactive flat leaf-like axon terminals were immunoreactive to vGLUT2, but not to vGLUT1. Among members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex, immunoreactivities for synaptosomal-associated protein, 25 kDa, Syntaxin1, and vesicle-associated membrane protein 2 (VAMP2) were localized in P2X2- and P2X3-immunoreactive axon terminals. Punctate immunoreactive products for VAMP2 and vGLUT2 were co-localized in axon terminals. These results suggest that vGLUT2 is localized in P2X3-immunoreactive baroreceptor terminals in the carotid sinus, and these terminals may release glutamate by exocytosis in order to modulate baroreceptor function in the carotid sinus.