Chemosensitivity, plasticity, and functional heterogeneity of paraganglionic cells in the rat coeliac-superior mesenteric complex

Cytochrome b558 (p22phox) in the guinea-pig adrenal medulla

Paraganglionic cells are sensitive to hypoxia, and the involvement of a plasmalemmal cytochrome b558-like protein in oxygen sensing by these cells has been suggested, but neither the identity of the immunoreactive protein detected by immunohistochemistry nor its anticipated subcellular (i.e., plasmalemmal) localization were directly proven. Thus, we extended these studies to the largest paraganglion, i.e., the adrenal medulla, in the guinea-pig, which, due to its size and accessibility, allowed us to address both of these issues utilizing antisera raised against synthetic peptides of the small (22 kD) subunit of cytochrome b558, p22phox. Cytochrome b558 was originally identified in granulocytes and macrophages, and antisera against this phagocyte p22phox were utilized. Immunoreactivity to p22phox was observed in all adrenal medullary endocrine cells, and the identity of the immunoreactive protein to the small cytochrome b558-subunit was confirmed by Western blotting. Immuno-electron microscopy of ultrathin cryosections and of resin-embedded tissue demonstrated its subcellular localization in the dense core vesicles of endocrine A-cells but not in the plasma membrane. In conclusion, the present study documents the presence of the small subunit of cytochrome b558 in guinea-pig adrenal medullary cells, but its subcellular vesicular localization does not support the initial interpretation of cytochrome b558 serving as a plasmalemmal oxygen sensor.

Plasticity in the phenotypic expression of catecholamines and vasoactive intestinal peptide in adult rat superior cervical and stellate ganglia after long-term hypoxia in vivo

Sympathetic ganglia in the adult rat contain various populations of nerve cells which demonstrate plasticity with respect to their transmitter phenotype. The plasticity of the neuronal cell bodies and of the small intensely fluorescent cells in the superior cervical and stellate ganglia in response to hypoxia in vivo (10% O2 for seven days) was assessed by studying the expression of catecholamines and vasoactive intestinal peptide. The levels of norepinephrine, dopamine, 3,4-dihydroxyphenylacetic acid and vasoactive intestinal peptide immunoreactivity were determined. In addition, the density of the immunohistochemical staining of cells for tyrosine hydroxylase and vasoactive intestinal peptide was evaluated. In the intact superior cervical ganglion, hypoxia increased the dopamine level as well as the density of small intensely fluorescent cells immunolabelled for tyrosine hydroxylase and vasoactive intestinal peptide. In the axotomized ganglion, hypoxia elicited a twofold rise in the level of the vasoactive intestinal peptide as well as enhancing the density of neuronal cell bodies immunostained for this peptide. Thus, the effect of hypoxia on the expression of vasoactive intestinal peptide expression in neurons was dependent on neural interactions. In the intact stellate ganglion, hypoxia alone induced a 1.5-fold increase in the density of neuronal cell bodies immunostained for vasoactive intestinal peptide. Thus, ganglia-specific factors appeared to play a role in determining changes in neuronal phenotype in response to hypoxia. The present study provides evidence for the involvement of dopamine and vasoactive intestinal peptide in ganglionic responses to long-term hypoxia as well as for differential responses by the two ganglionic cell populations, i.e. neuronal cell bodies and small intensely fluorescent cells. Changes in the expression of the vasoactive intestinal peptide during long-term hypoxia may be of energetic, trophic and/or synaptic significance. Hypoxia may be considered to be a vasoactive intestinal peptide-inducing factor in sympathetic ganglia.

Peripheral chemosensitivity and central integration: neuroplasticity of catecholaminergic cells under hypoxia

The plasticity of catecholaminergic cells within the carotid body, brainstem and sympatho-adrenal system was analyzed in rats subjected to normobaric hypoxia (10% O2) lasting up to 3 weeks. Long-term hypoxia elicited structural, neurochemical and phenotypic changes in carotid body and sympathetic ganglia (SIF cells), and stimulated the norepinephrine turnover in A2 neurons located caudal to the obex, the area where the chemosensory nerve fibers end. Chemodenervation abolished central alterations. Adaptive mechanisms for increasing norepinephrine biosynthesis in hypoxia involved changes in activity of pre-existing tyrosine hydroxylase, the rate-limiting enzyme of catecholamine biosynthesis, and induction of new tyrosine hydroxylase protein. These neurochemical changes occurred after sustained hypoxia only, suggesting that noradrenergic neurons are involved in the central chemoreceptor pathway during sustained hypoxia but are not essential for regulatory responses to acute hypoxia. Acute hypoxia elicited the expression of c-Fos protein in neurons located in nucleus tractus solitarius that were not catecholaminergic. Noradrenaline released under long-term hypoxia could play a neuromodulatory role in ventilatory acclimatization. Cardiovascular responses to hypoxia are mediated by changes in sympatho-adrenal outflow, different according to the target organ. Cardiac sympathetic output and adrenal secretion were stimulated independently of carotid body chemoafferents. Early postnatal hypoxia induced long-term neurochemical changes in carotid body, brainstem and sympathetic efferents that may reveal alterations in development of neurons involved in the chemoreceptor pathway.