Medullary pathways mediating depressor responses from Na(+)-sensitive sites in nucleus of the solitary tract.

Two series of experiments were done in male Wistar rats to investigate the medullary pathways that mediate the depressor responses from sodium-sensitive sites in the nucleus of the solitary tract (NTS). In the first series, the anterograde tract tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was iontophoresed unilaterally at sites in the NTS at which microinjections (20 nl) of a 154-175 mM NaCl solution elicited depressor responses. PHA-L injection sites were found to be localized within the medial subnucleus of the NTS (Sm). In the medulla, PHA-L-labeled fibers and presumptive terminal boutons were observed bilaterally, but with an ipsilateral predominance, throughout the rostrocaudal extent of the NTS the dorsal motor nucleus of the vagus, area postrema, the ventrolateral medulla (VLM), and nucleus ambiguus. The pontine region, containing the A5 catecholaminergic cell group and the parabrachial nucleus, also received projections from Sm. In the second series of experiments, the effect of blocking synaptic transmission in VLM with cobalt chloride (CoCl2; 5 mM, 100 nl) on the cardiovascular response elicited by microinjection (20 nl) of hypertonic saline (154-175 mM) into the ipsilateral Sm was investigated in the alpha-chloralose-anesthetized, paralyzed, and artificially ventilated rat. Microinjection of CoCl2 into VLM, at sites shown in the previous study to receive efferent projections from Sm, significantly attenuated the depressor (60%) and bradycardic (80%) responses to stimulation of Sm. These data indicate that the sodium-sensitive region of the caudal Sm innervates VLM neurons and suggest that these VLM neurons are involved in mediating the depressor and bradycardic responses elicited by changes in the extracellular concentration of sodium.

Effect of lesions of forebrain circumventricular organs on c-fos expression in the central nervous system to plasma hypernatremia.

Experiments were carried out on conscious adult male Wistar rats to investigate the effect of selective ablation of the subfornical organ (SFO), and/or the anteroventral third ventricular (AV3V) region on the induction of Fos in central structures in response to plasma hypernatremia. Fos induction, detected immunohistochemically, was used as a marker for neuronal activation. Intravenous infusions of hypertonic saline resulted in dense Fos-like immunoreactivity in several forebrain (paraventricular nucleus of the hypothalamus (PVH), supraoptic nucleus (SON), median preoptic nucleus (MnPO), medial preoptic nucleus, organum vasculosum of the laminae terminalis and (SFO) and brainstem (nucleus of the solitary tract, ventrolateral medulla, and parabrachial nucleus) structures. Intravenous infusions of the hypertonic saline solution into animals with lesions of either the SFO, the AV3V or both resulted in a decreased number of Fos-like immunoreactive neurons in the MnPO, PVH and SON. In addition, the number of Fos-labeled neurons in the SON after lesions of both the SFO and the AV3V was significantly greater than that observed in isotonic saline infused controls. Finally, lesions of the forebrain circumventricular structures did not alter the Fos labeling in brainstem structures as a result of the infusion of the hypertonic solution. These data suggest that changes in plasma osmolality and/or concentration of sodium alter the activity of SON and brainstem neurons in the absence of afferent inputs from the SFO and AV3V.

Changes in NADPH diaphorase activity in forebrain structures of the laminae terminalis after chronic dehydration.

The effect of 3-day chronic dehydration on nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-diaphorase) histochemical staining in forebrain circumventricular structures was investigated in the rat. Increased number and/or intensity of staining of NADPH-diaphorase positive neurons was observed in subfornical organ, organum vasculosum of the laminae terminalis, and median preoptic nucleus. In addition, dense punctate NADPH-diaphorase reaction product was found throughout the internal portion of median eminence. These data suggest the involvement of nitric oxide synthase producing neurons in homeostatic mechanisms controlling body fluid balance and the circulation.

Plasma hypernatremia induces c-fos activity in medullary catecholaminergic neurons.

Experiments were done in conscious rats to investigate the effect of intravenous infusion of hypertonic saline on the induction of the phosphoprotein Fos in brainstem catecholaminergic neurons. Fos induction, detected immunohistochemically, was used as a marker for neuronal activation. Infusions of 165 mM or 1.4 M NaCl solutions into the jugular vein resulted in Fos-like immunoreactivity in approximately the caudal two thirds of nucleus of the solitary tract (NTS), the caudal and rostral ventrolateral medulla (VLM), and in the lateral aspects of the parabrachial nucleus (PBN). Within caudal NTS a small number (7.9 +/- 1.8%) of Fos labelled neurons were found also to contain tyrosine hydroxylase (TH) or dopamine beta-hydroxylase (DBH) immunoreactivity. In rostral NTS no Fos labelled cells were found to contain phenylethanolamine N-methyltransferase (PNMT) immunoreactivity, although a few (8.5 +/- 2.3%) were immunoreactive to TH. Similarly, in VLM, most of the Fos labelled cells in caudal VLM (65.9 +/- 2.7%) contained either TH or DBH immunoreactivity, whereas in the rostral VLM, 32.2 +/- 4.6% of the Fos labelled cells were also immunoreactive to TH or DBH. However, no Fos cells were found in either the caudal or rostral VLM that were immunoreactive to PNMT. Little or no Fos-like immunoreactive neurons were detected in the brainstem after intravenous infusions of physiological (143 mM) or hypotonic (106 mM) NaCl solutions. These data suggest that noradrenergic neurons of the caudal NTS and VLM are components of central circuits that are involved in osmoregulation and cardiovascular function.

Effects of plasma hypernatremia on nucleus tractus solitarius neurons.

Experiments were done in chloralose-anesthetized, paralyzed, and artificially ventilated rats to investigate the effect of plasma hypernatremia and baroreceptor activation on the excitability of nucleus tractus solitarius (NTS) neurons. Extracellular, single unit recordings were made from 67 histologically verified neurons in the region of NTS. The firing frequency of 29 (43%) NTS neurons was increased by the intracarotid infusion of hypertonic saline (0.5 M). The acute activation of baroreceptors after the intravenous infusion of phenylephrine resulted in the excitation of seven (31.8%) or the inhibition of five (22.7%) of the units responsive to plasma hypernatremia. Thirteen of the hypertonic saline-responsive neurons were recorded in animals in which the organum vasculosum lamina terminalis and subfornical organ were lesioned and/or the left vagus nerve was cut. These data suggest that within NTS there exists a pool of neurons that alter their discharge rate in response to changes in plasma sodium levels and that these neurons may also function in cardiovascular regulation.

Cardiovascular effects of NaCl microinjections into the nucleus of the solitary tract.

The nucleus of the solitary tract (NTS) was systematically explored in the alpha-chloralose-anesthetized rat for sites that elicited changes in mean arterial pressure (MAP) and heart rate (HR) during microinjections (20 nl) of phosphate-buffered saline (PBS; pH 7.2-7.4) or NaCl solutions containing various concentrations of NaCl (104-326 mM). Decreases in MAP (range 7-83 mmHg) and HR (range 10-70 bpm) were consistently elicited from sites in the caudal medial and commissural subnuclei of NTS. Microinjection of PBS or NaCl into other NTS subnuclei or area postrema did not elicit cardiovascular responses. Microinjection of LiCl in PBS elicited cardiovascular responses that were significantly smaller than those elicited by microinjection of NaCl in PBS at the same NTS site. Injections of either a hyperosmotic (400 mOsm/kg) or a hyposmotic (204 mOsm/kg) solution of mannitol into NaCl-sensitive sites did not elicit cardiovascular responses. Finally, most of the sites in NTS that elicited cardiovascular responses during microinjection of glutamate (1 M) did not respond to microinjections of PBS. Administration of atropine methyl bromide had no effect on the magnitude of the depressor response to injections of PBS into NTS, but significantly attenuated (32%) the HR response. Subsequent administration of the ganglionic blockers hexamethonium bromide or arfonad abolished both the depressor and bradycardic responses. These data suggest that within a restricted region of the caudal NTS there exists a pool of neurons sensitive to changes in extracellular Na+ concentrations that, when activated by the sodium, elicit vasodepressor responses as a result of sympathoinhibition and bradycardia as a result of vagal excitation and sympathoinhibition.

Fos induction in brainstem neurons by intravenous hypertonic saline in the conscious rat.

Experiments were done in conscious rats to investigate the effect of intravenous infusion of hypertonic saline on the induction of the protein Fos, in brainstem neurons. Neurons containing Fos-like immunoreactivity were observed in the caudal nucleus of the solitary tract (NTS), the caudal and rostral ventrolateral medulla, and parabrachial nucleus after an infusion of solutions containing 1.4 M NaCl. Little or no expression of Fos was detected in brainstem neurons after intravenous infusions of either physiological (143 mM) or hypotonic (106 mM) NaCl solutions. These data provide evidence for the involvement of brainstem structures in osmoregulatory functions and suggest that brainstem neuronal circuits that function in cardiovascular regulation may also be shared by those involved in body fluid homeostasis.