House-dust mite allergen and ozone exposure decreases histamine H3 receptors in the brainstem respiratory nuclei.

Allergic airway diseases in children are a common and a growing health problem. Changes in the central nervous system (CNS) have been implicated in contributing to some of the symptoms. We hypothesized that airway allergic diseases are associated with altered histamine H3 receptor expression in the nucleus tractus solitarius (NTS) and caudal spinal trigeminal nucleus, where lung/airway and nasal sensory afferents terminate, respectively. Immunohistochemistry for histamine H3 receptors was performed on brainstem sections containing the NTS and the caudal spinal trigeminal nucleus from 6- and 12-month-old rhesus monkeys who had been exposed for 5 months to house dust mite allergen (HDMA)+O3 or to filtered air (FA). While histamine H3 receptors were found exclusively in astrocytes in the caudal spinal trigeminal nucleus, they were localized to both neuronal terminals and processes in the NTS. HDMA+O3 exposure significantly decreased histamine H3 receptor immunoreactivity in the NTS at 6 months and in the caudal spinal trigeminal nucleus at 12 months of age. In conclusion, exposing young primates to HDMA+O3 changed histamine H3 receptor expression in CNS pathways involving lung and nasal afferent nerves in an age-related manner. Histamine H3 receptors may be a therapeutic target for allergic asthma and rhinitis in children.

A novel postsynaptic group II metabotropic glutamate receptor role in modulating baroreceptor signal transmission.

The nucleus tractus solitarius (NTS) is essential for orchestrating baroreflex control of blood pressure. When a change in blood pressure occurs, the information is transmitted by baroreceptor afferent fibers to the central network by glutamate binding to ionotropic glutamate receptors on second-order baroreceptor neurons. Glutamate also activates presynaptic group II and III metabotropic glutamate receptors (mGluRs), depressing both glutamate and GABA release to modulate baroreceptor signal transmission. Here we present a novel role for postsynaptic group II mGluRs to further fine-tune baroreceptor signal transmission at the first central synapses. In a brainstem slice with ionotropic glutamate and GABA receptors blocked, whole-cell patch-clamp recordings of second-order baroreceptor neurons revealed that two group II mGluR agonists evoked concentration-dependent membrane hyperpolarizations. The hyperpolarization remained when a presynaptic contribution was prevented with Cd(2+), was blocked by a postsynaptic intervention of intracellular dialysis of the G-protein signaling inhibitor, was mimicked by endogenous release of glutamate by tractus solitarius stimulation, and was prevented by a group II mGluR antagonist. Postsynaptic localization of group II mGluRs was confirmed by fluorescent confocal immunohistochemistry and light microscopy. Group II mGluR induced-currents consisted of voltage-dependent outward and inward components, prevented by tetraethylammonium chloride and tetrodotoxin, respectively. In contrast to group II mGluR-induced hyperpolarization, there was no effect on intrinsic excitability as determined by action potential shape or firing in response to depolarizing current injections. The data suggest a novel mechanism for postsynaptic group II mGluRs to fine-tune baroreceptor signal transmission in the NTS.

Exercise reduces GABA synaptic input onto nucleus tractus solitarii baroreceptor second-order neurons via NK1 receptor internalization in spontaneously hypertensive rats.

A single bout of mild to moderate exercise can lead to a postexercise decrease in blood pressure in hypertensive subjects, namely postexercise hypotension (PEH). The full expression of PEH requires a functioning baroreflex, hypertension, and activation of muscle afferents (exercise), suggesting that interactions in the neural networks regulating exercise and blood pressure result in this fall in blood pressure. The nucleus tractus solitarii (NTS) is the first brain site that receives inputs from nerves carrying blood pressure and muscle activity information, making it an ideal site for integrating cardiovascular responses to exercise. During exercise, muscle afferents excite NTS GABA neurons via substance P and microinjection of a substance P-neurokinin 1 receptor (NK1-R) antagonist into the NTS attenuates PEH. The data suggest that an interaction between the substance P NK1-R and GABAergic transmission in the NTS may contribute to PEH. We performed voltage clamping on NTS baroreceptor second-order neurons in spontaneously hypertensive rats (SHRs). All animals were killed within 30 min and the patch-clamp recordings were performed 2-8 h after the sham/exercise protocol. The data showed that a single bout of exercise reduces (1) the frequency but not the amplitude of GABA spontaneous IPSCs (sIPSCs), (2) endogenous substance P influence on sIPSC frequency, and (3) sIPSC frequency response to exogenous application of substance P. Furthermore, immunofluorescence labeling in NTS show an increased substance P NK1-R internalization on GABA neurons. The data suggest that exercise-induced NK1-R internalization results in a reduced intrinsic inhibitory input to the neurons in the baroreflex pathway.

Chronic blockade of hindbrain glucocorticoid receptors reduces blood pressure responses to novel stress and attenuates adaptation to repeated stress.

Exogenous glucocorticoids act within the hindbrain to enhance the arterial pressure response to acute novel stress. Here we tested the hypothesis that endogenous glucocorticoids act at hindbrain glucocorticoid receptors (GR) to augment cardiovascular responses to restraint stress in a model of stress hyperreactivity, the borderline hypertensive rat (BHR). A 3- to 4-mg pellet of the GR antagonist mifepristone (Mif) was implanted over the dorsal hindbrain (DHB) in Wistar-Kyoto (WKY) and BHRs. Control pellets consisted of either sham DHB or subcutaneous Mif pellets. Rats were either subjected to repeated restraint stress (chronic stress) or only handled (acute stress) for 3-4 wk, then all rats were stressed on the final day of the experiment. BHR showed limited adaptation of the arterial pressure response to restraint, and DHB Mif significantly (P

Extended secondhand tobacco smoke exposure induces plasticity in nucleus tractus solitarius second-order lung afferent neurons in young guinea pigs.

Infants and young children experiencing extended exposure to secondhand smoke (SHS) have an increased occurrence of asthma, as well as increased cough, wheeze, mucus production and airway hyper-reactivity. Plasticity in lung reflex pathways has been implicated in causing these symptoms, as have changes in substance P-related mechanisms. Using whole-cell voltage-clamp recordings and immunohistochemistry in brainstem slices containing anatomically identified second-order lung afferent nucleus tractus solitarius (NTS) neurons, we determined whether extended SHS exposure during the equivalent period of human childhood modified evoked or spontaneous excitatory synaptic transmission, and whether those modifications were altered by endogenous substance P. SHS exposure enhanced evoked synaptic transmission between sensory afferents and the NTS second-order neurons by eliminating synaptic depression of evoked excitatory postsynaptic currents (eEPSCs), an effect reversed by the neurokinin-1-receptor antagonist (SR140333). The recruitment of substance P in enhancing evoked synaptic transmission was further supported by an increased number of substance P-expressing lung afferent central terminals synapsing onto the second-order lung afferent neurons. SHS exposure did not change background spontaneous EPSCs. The data suggest that substance P in the NTS augments evoked synaptic transmission of lung sensory input following extended exposure to a pollutant. The mechanism may help to explain some of the exaggerated respiratory responses of children exposed to SHS.

Genetic predisposition to hypertension sensitizes borderline hypertensive rats to the hypertensive effects of prenatal glucocorticoid exposure.

An adverse intrauterine environment can increase the incidence of hypertension and other cardiovascular disease risk factors. However, in clinical and experimental studies the magnitude of the effect is variable. Possibly, the relative influence of the prenatal environment on cardiovascular disease is determined in part by genetic factors that predispose individuals to the development of environmentally induced hypertension. We tested this hypothesis by comparing the effects of prenatal dexamethasone treatment (Dex, 300 microg kg(-1) i.p. on days 15 and 16 of gestation) in borderline hypertensive rats (BHR) and control Wistar-Kyoto (WKY) rats. Blood pressure, heart rate and plasma corticosterone values were measured at rest during the middle of the day, and during 1 h of restraint stress in the adult offspring using indwelling arterial catheters implanted at least 4 days prior to data collection. Compared with the saline (vehicle) control treatment, prenatal dexamethasone significantly (P < 0.05) increased baseline mean arterial pressure in male (123 +/- 2 versus 131 +/- 3 mmHg, saline versus Dex) and female (121 +/- 2 versus 130 +/- 2 mmHg, saline versus Dex) BHR, but not in male (108 +/- 3 versus 113 +/- 2 mmHg, saline versus Dex) or female (112 +/- 2 versus 110 +/- 2 mmHg, saline versus Dex) WKY rats. Relative to saline treatment, prenatal Dex also significantly increased baseline heart rate (328 +/- 6 versus 356 +/- 5 beats min(-1), saline versus Dex) and plasma corticosterone (5 +/- 2 versus 24 +/- 4 microg dl(-1), saline versus Dex), and prolonged the corticosterone response to acute stress, selectively in female BHR. However, prenatal Dex significantly enhanced the arterial pressure response to acute stress only in female WKY, while Dex augmented the elevation in heart rate during stress only in male rats. We conclude that prenatal dexamethasone increased baseline arterial pressure selectively in BHR, and plasma corticosterone only in female BHR. In contrast, prenatal Dex enhanced cardiovascular reactivity to stress in both BHR and WKY rats.

Chronic activation of dorsal hindbrain corticosteroid receptors augments the arterial pressure response to acute stress.

Augmented cardiovascular responses to acute stress can predict cardiovascular disease in humans. Chronic systemic increases in glucocorticoids produce enhanced cardiovascular responses to psychological stress; however, the site of action is unknown. Recent evidence indicates that glucocorticoids can act within the dorsal hindbrain to modulate cardiovascular function. Therefore, we tested the hypothesis that the endogenous glucocorticoid corticosterone can act in the dorsal hindbrain to enhance cardiovascular responses to restraint stress in conscious rats. Adrenal-intact animals with indwelling arterial catheters were treated for 4 or 6 days with 3- to 4-mg pellets of corticosterone or silastic (sham pellets) implanted on the dorsal hindbrain surface. Corticosterone pellets were also implanted either on the surface of the dura or subcutaneously to control for the systemic effects of corticosterone (systemic corticosterone). The integrated increase in arterial pressure during 1 hour of restraint stress was significantly (P<0.05) greater in dorsal hindbrain corticosterone (912+/-98 mm Hg per 60 minutes) relative to dorsal hindbrain sham (589+/-57 mm Hg per 60 minutes) or systemic corticosterone (592+/-122 mm Hg per 60 minutes) rats. The plasma glucose response after 10 minutes of stress was also significantly higher in dorsal hindbrain corticosterone-treated rats relative to both other groups. There were no significant between-group differences in the heart rate or corticosterone responses to stress. There were no differences in baseline values for any measured parameters. We conclude that corticosterone can act selectively in the dorsal hindbrain in rats with normal plasma corticosterone levels to augment the arterial pressure response to restraint stress.

Glucocorticoids act in the dorsal hindbrain to modulate baroreflex control of heart rate.

Systemic corticosterone (Cort) modulates arterial baroreflex control of both heart rate and renal sympathetic nerve activity. Because baroreceptor afferents terminate in the dorsal hindbrain (DHB), an area with dense corticosteroid receptor expression, we tested the hypothesis that prolonged activation of DHB Cort receptors increases the midpoint and reduces the gain of arterial baroreflex control of heart rate in conscious rats. Small (3-4 mg) pellets of Cort (DHB Cort) or Silastic (DHB Sham) were placed on the surface of the DHB, or Cort was administered systemically by placing a Cort pellet on the surface of the dura (Dura Cort). Baroreflex control of heart rate was determined in conscious male Sprague Dawley rats on each of 4 days after initiation of treatment. Plots of arterial pressure vs. heart rate were analyzed using a four-parameter logistic function. After 3 days of treatment, the arterial pressure midpoint for baroreflex control of heart rate was increased in DHB Cort rats (123 +/- 2 mmHg) relative to both DHB Sham (108 +/- 3 mmHg) and Dura Cort rats (109 +/- 2 mmHg, P < 0.05). On day 4, baseline arterial pressure was greater in DHB Cort (112 +/- 2 mmHg) compared with DHB Sham (105 +/- 2 mmHg) and Dura Cort animals (106 +/- 2 mmHg, P < 0.05), and the arterial pressure midpoint was significantly greater than mean arterial pressure in the DHB Cort group only. Also on day 4, maximum baroreflex gain was reduced in DHB Cort (2.72 +/- 0.12 beats x min(-1) x mmHg(-1)) relative to DHB Sham and Dura Cort rats (3.51 +/- 0.28 and 3.37 +/- 0.27 beats x min(-1) x mmHg(-1), P < 0.05). We conclude that Cort acts in the DHB to increase the midpoint and reduce the gain of the heart rate baroreflex function.

Glucocorticoids act in the dorsal hindbrain to increase arterial pressure.

Glucocorticoid receptors (GRs) are present at a high density in the nucleus of the solitary tract (NTS), an area of the dorsal hindbrain (DHB) that is critical for blood pressure regulation. However, whether these receptors play any role in the regulation of blood pressure is unknown. We tested the hypothesis that glucocorticoids act in the DHB to increase arterial pressure using two experimental strategies. In one approach, we implanted pellets of corticosterone (Cort) or sham pellets onto the DHB over the NTS. Compared with rats with sham pellets, rats with DHB Cort pellets had an increased (P < 0.05) mean arterial pressure (111 +/- 2 vs. 104 +/- 1 mmHg) and heart rate (355 +/- 9 vs. 326 +/- 5 beats/min) after 4 days. In the second approach, we implanted subcutaneous Cort pellets to increase the systemic Cort concentration and then subsequently implanted pellets of the GR antagonist mifepristone (Mif; previously RU-38486) or sham pellets onto the DHB. Two days of DHB Mif treatment reduced (P < 0.05) mean arterial pressure in those rats with elevated plasma Cort levels (118 +/- 2 vs. 108 +/- 1 mmHg for sham vs. Mif DHB pellets). Cort and Mif pellets placed on the dura had no effects on arterial pressure or heart rate, ruling out systemic cardiovascular effects of the steroids. DHB Cort treatment had no effects on plasma Cort concentration or adrenal weight, indicating that the contents of the DHB Cort pellet did not diffuse into the systemic circulation or into the forebrain areas that regulate plasma Cort concentration in concentrations sufficient to produce physiological effects. Immunohistochemistry for the occupied GRs demonstrated that the Cort and Mif from the DHB pellets were delivered to the DHB with minimal diffusion to the ventral hindbrain or forebrain. We conclude that glucocorticoids act in the DHB to increase arterial pressure.

Glucocorticoids modulate baroreflex control of heart rate in conscious normotensive rats.

The effect of glucocorticoids on arterial baroreceptor reflex control of heart rate (HR) was determined in conscious rats. Corticosterone (Cort) treatment for 4-6 days doubled plasma Cort in Cort-treated relative to control rats. Cort had no significant effect on mean arterial pressure (MAP) or HR. Ramped changes in MAP were produced using infusions of phenylephrine and nitroprusside. Baroreflex control of HR was analyzed using a four-parameter logistic function. The midpoint of the baseline baroreflex function curve was significantly increased in Cort-treated (n = 14) relative to control (n = 14) rats (112 +/- 2 vs. 98 +/- 2 mmHg, n = 14), and the slope was significantly decreased (0.065 +/- 0.002 vs. 0.091 +/- 0.007). Three hours after the glucocorticoid type II receptor antagonist mifepristone (Mif) was administered to Cort-treated rats (n = 8), the midpoint of the baroreflex function was significantly reduced from 113 +/- 4 to 99 +/- 2 mmHg, and the slope was significantly increased from 0.061 +/- 0.004 to 0.083 +/- 0.005. Mif decreased HR in Cort-treated rats from 355 +/- 17 to 330 +/- 14 beats/min (P = 0.04) but did not alter MAP (111 +/- 2 to 107 +/- 3 mmHg, P = 0.14). Mif had no significant effects on baroreflex function in control rats. Therefore, a moderate elevation in Cort for several days causes pressure-independent modulation of baroreflex control of HR.