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.

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

Chemosensitivity and plasticity of paraganglionic cells in the rat coeliac-superior mesenteric complex (CSMC) were investigated at a basal state of normoxia (21% O2) and after long-term moderate hypoxia (10% O2, 14 days). Chemical sympathectomy previous to hypoxia was performed to destroy principal ganglionic neurons and thus to allow measurement of the norepinephrine and dopamine content of paraganglionic cells. At the basal state, the CSMC contained dopaminergic (TH+/DBH-) and noradrenergic (TH+/DBH+) paraganglionic cells, the majority being of the noradrenergic type. After 14 days of hypoxia, this ratio was reversed and dopaminergic cells predominated, as indicated by a twofold increase of TH+ cells and a twofold decrease of DBH+ cells. Biochemically, hypoxia produced an increase in the content (1.6-fold) and utilization (1.4-fold) of dopamine as well as a smaller increase in the content of norepinephrine, with no change in its utilization rate. The dopaminergic activation induced by hypoxia persisted after sympathectomy with guanethidine. It is concluded that paraganglionic cells in the CSMC display a chemosensitive function. Furthermore, our findings indicate that paraganglionic cells are differentially affected by hypoxia, depending on their distribution and the nature of their neuromodulators. The alterations induced by hypoxia point out the phenotypic plasticity developed by paraganglionic cells in adaptation to hypoxia and further demonstrate the functional heterogeneity of this autonomic cell population in the rat CSMC.

Immunohistochemistry of small intensely fluorescent (SIF) cells and of SIF cell-associated nerve fibers in the rat superior cervical ganglion.

Double-labelling immunofluorescence was applied on single sections of the rat superior cervical ganglion to evaluate neurochemistry and connectivity of intraganglionic SIF cells. The synaptic vesicle membrane protein synaptophysin and secretoneurin, a newly discovered neuropeptide derived from secretogranin II, proved reliable molecular markers of this cell type, whereas serotonin and tyrosine hydroxylase immunoreactivities were observed in slightly incongruent SIF cell subpopulations. Immunolabelling for vasoactive intestinal polypeptide and neuropeptide Y occurred in few SIF cells. None of the above immunoreactivities were visibly altered by preganglionic or postganglionic denervation, while some SIF cells were immunolabelled for galanin or for the neuronal microtubule-associated protein MAP2 after postganglionic denervation. SIF cells were nonreactive for the pan-neuronal marker protein gene product (PGP) 9.5 or neurofilament 160 kD. Intense staining of NADPH-diaphorase in some SIF cells, suggesting catalytic activity of nitric oxide synthase, could not be substantiated by immunoreactivity for this enzyme. SIF cells were approached by nonidentical fiber populations immunoreactive for PGP 9.5, neurofilament, or neuropeptide Y, whereas immunoreactivities for galanin and vasoactive intestinal polypeptide were colocalized in fiber meshes around SIF cells. The findings indicate (1) neurochemical SIF cell heterogeneity, (2) SIF cell plasticity in response to ganglionic perturbation, and (3) a differentiated innervation of SIF cells in the rat superior cervical ganglion.

Preganglionic sympathetic fibres modulate dopamine turnover in rat carotid body during long-term hypoxia.

To assess the influence of sympathetic efferents on the dopamine function of carotid bodies, rats were exposed to long-term hypoxia (10% O2 in nitrogen for 1 or 3 weeks) after unilateral removal of the superior cervical ganglion. In the intact carotid bodies. long-term hypoxia increased the content and turnover of dopamine (DA). The dopaminergic response to hypoxia was reduced but not abolished by the ganglionectomy. To determine whether pre- or postganglionic sympathetic fibres are involved in the control of the dopamine function, rats were exposed to hypoxia either after unilateral transection of the preganglionic cervical trunk or after selective destruction of the postganglionic fibres by guanethidine. The preganglionic transection blunted the dopaminergic response to hypoxia whereas guanethidine had no effect. It is concluded that the sympathetic efferents may activate the synthesis and release of dopamine in glomus cells during long-term hypoxia. The sympathetic efferents responsible for the modulation of dopamine function are probably preganglionic fibres.

Effect of guanethidine on dopamine in small intensely fluorescent cells of the superior cervical ganglion of the rat.

To determine the portion of ganglionic dopamine stored in the small intensely fluorescent (SIF) cells of the superior cervical ganglion, rats were treated chronically with the neurotoxin guanethidine (50 mg/kg i.p. daily for 6, 13 or 18 days) which destroys noradrenergic neurons. The guanethidine effect was assessed in the ganglion using biochemistry of dopamine and norepinephrine and immunocytochemistry of tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH). After 18 days of treatment, the ganglionic norepinephrine content was reduced by 80%, but the dopamine content was reduced by only 20%. Morphologic analysis of ganglia stained to distinguish noradrenergic neurons (TH positive, DBH positive) and SIF cells (TH positive, DBH negative) indicated that guanethidine treatment reduced the number of noradrenergic neurons by 70%, dropping from 19413 +/- 1402 to 6515 +/- 1296 per ganglion, but increased the number of dopaminergic SIF cells by 80% from 578 +/- 150 to 1056 +/- 151 per ganglion. Based on these findings, it is concluded that a substantial portion of the dopamine in the rat superior cervical ganglion is located outside the noradrenergic neurons, i.e. in the SIF cells. Extrapolating the data obtained using guanethidine versus control rat leads to infer that although the proportion of SIF cells in the superior cervical ganglion is small (3 +/- 1% of the SIF and noradrenergic neurons combined), about 40% of the total ganglionic dopamine resides in SIF cells, with the remainder serving as precursor in noradrenergic neurons.