Prenatal infection with Mycoplasma pulmonis in rats exaggerates the angiotensin II pressor response in adult offspring.

Exposure to different stressors in utero is linked to adult diseases such as obesity and hypertension. In this study, the impact of prenatal infection (PNI) on adult body weight and cardiovascular function was evaluated using a naturally occurring rodent pathogen, Mycoplasma pulmonis (MP). Pregnant Sprague-Dawley rats were infected with MP on gestationalday 14 and gave birth naturally. Adult PNI offspring weighed more than controls, but resting mean arterial pressure (MAP) was unchanged. Subcutaneous injection of angiotensin II (10 µg/kg) elicited a rise in MAP that was greater in both male and female PNI offspring compared with controls (P < 0.03). The accompanying reflex bradycardia was similar to the controls, suggesting that PNI induced baroreflex dysfunction. Subcutaneous nicotine administration, a potent cardiorespiratory stimulus, also elicited a transient rise in MAP that was generally greater in the PNI group, but the change in MAP from baseline was only significant in the PNI females compared with controls (P < 0.03). Elevated body weight and cardiovascular reactivity in the PNI offspring was associated with an increase in the ratio of hypothalamic corticotrophin-releasing hormone receptors type 1 to type 2 gene expression in both sexes compared with controls. These findings support previous studies demonstrating that PNI induces alterations in cardiovascular function and body weight. Yet, unlike previous studies utilizing other models of PNI (e.g., endotoxin), MP PNI did not induce resting hypertension. Thus, our study provides a foundation for future studies evaluating the cardiovascular risks of offspring exposed to microbial challenges in utero.

DOCA/salt hypertension alters Period1 and orexin-related gene expression in the medulla and hypothalamus of male rats: Diurnal influences.

In peripheral tissues, aldosterone alters expression of multiple genes, including the clock gene Period 1 (Per1), 11 beta-hydroxysteroid dehydrogenase-2 (11-HSD2), and α-ENAC, the epithelial sodium channel subunit. We evaluated the impact of chronic aldosterone exposure (DOCA) and salt intake on nocturnal changes in gene expression in the male Sprague Dawley rat brain. Additionally, genes associated with the orexin (ORX) system were also evaluated based on the role of this neuropeptide in arousal, feeding and hypertension and an interconnection with Per1 expression. DOCA/salt treatment increased saline intake primarily at night, elevated arterial pressure and lowered heart rate. In the medulla oblongata, DOCA/salt upregulated Per1, 11-HSD2, and α-ENAC expression independent of time of day, but did not change ORX receptor type 1 (ORX-R1) or type 2 (ORX-R2) expression. ORX-R1, and ORX-R2 expression in the medulla did however correlate with Per1 expression following DOCA/salt treatment but not in controls. In the hypothalamus, DOCA/salt treatment upregulated Per1, ORX-A, and ORX-R2 expression, in general, and Per1 and ORX-A expression at night. ORX-A, ORX-R1 and ORX-R2 expression in the hypothalamus correlated with Per1 expression following DOCA/salt but not in controls. These findings demonstrate for the first time that DOCA/salt hypertension modulates ORX gene expression in the brain and suggest that changes in expression in the ORX system may occur directly or indirectly via aldosterone-induced changes in Per1 expression. Our findings also build on emerging evidence that monitoring gene expression during both the day and night is critical to understanding the role of specific genes in hypertension.

Cardiorespiratory variability following repeat acute hypoxia in the conscious SHR versus two normotensive rat strains.

A link between exaggerated chemoreceptor sensitivity and hypertension has been documented in the spontaneously hypertensive rat (SHR) but has also been questioned when comparisons with normotensive strains other than the Wistar Kyoto (WKY) rat are made. To further evaluate the link between hypertension and chemoreflex sensitivity, changes in cardiorespiratory variability in response to three successive bouts of 5 min of hypoxia (21%→10%) were evaluated in conscious male SHR, and WKY and Sprague Dawley (SD) rats (n=7-8/group). In response to the first bout of hypoxia, the change in respiratory frequency (RF) was greatest in the SHR, but the increase in mean arterial pressure (MAP) was similar in both SHRs and WKY rats and all strains demonstrated a similar rise in heart rate (HR). All strains showed some level of response accommodation during subsequent bouts of hypoxia. Spectral analysis of HR variability identified a significant difference in high frequency (HF) power between strains during hypoxia, including an increase in HF power in the WKY rats, a decrease in the SHRs and little overall change in the SD rats. Alternatively, all strains demonstrated a rise in systolic arterial pressure (SAP) variability in the low frequency (LF) range in response to hypoxia but the increase was greatest in the SHR. Since SAP LF power is linked to vasosympathetic tone, these results support the hypothesis that essential hypertension is linked to exaggerated sympathetic responses to chemoreceptor stimulation but confirm that estimation of augmented reflex function cannot be determined by quantifying simple changes in MAP or HR.

Prenatal nicotine exposure alters respiratory long-term facilitation in neonatal rats.

Intermittent hypoxia can evoke persistent increases in ventilation (V (E)) in neonates (i.e. long-term facilitation, LTF) (Julien et al., 2008). Since prenatal nicotine (PN) exposure alters neonatal respiratory control (Fregosi and Pilarski, 2008), we hypothesized that PN would influence LTF of ventilation (V (E)) in neonatal rats. An osmotic minipump delivered nicotine 6 mg/kg per day or saline to pregnant dams. V (E) was assessed in unanesthetized pups via whole body plethysmography at post-natal (P) days 9-11 or 15-17 during baseline (BL, 21% O(2)), hypoxia (10 x 5 min, 5% O(2)) and 30 min post-hypoxia. PN pups had reduced BL V (E) (p<0.05) but greater increases in V (E) during hypoxia (p<0.05). Post-hypoxia V (E) (i.e. LTF) showed an agex treatment interaction (p<0.01) with similar values at P9-11 but enhanced LTF in saline (30+/-8%BL) vs. PN pups (6+/-5%BL; p=0.01) at P15-17. We conclude that the post-natal developmental time course of hypoxia-induced LTF is influenced by PN.

Effects of systemic administration of mirtazapine on respiratory muscle activity in sleeping rats.

Mirtazapine (MIRT) is an antidepressant with mixed noradrenergic and serotonergic effects in central nervous system. The present study was undertaken to assess whether MIRT can stimulate genioglossus muscle (GG) activity in the conscious, behaving rat. Nine male rats were chronically instrumented with GG and neck muscle EMG electrodes. EEG electrodes were implanted to acquire sleep stage. Results demonstrated a dose-dependent effect of MIRT on GG activity during sleep, although no changes reached statistical significance. Low dose MIRT (0.1 mg/kg) showed a slight increase in GG phasic activity. In contrast, higher doses of MIRT (0.5-1.0 mg/kg) tended to decrease GG activity relative to vehicle, in addition to decreasing total sleep time.

Disinhibition of the dorsomedial hypothalamus increases the frequency of augmented breaths in the anesthetized rat.

The present study was undertaken to identify if activation of the dorsomedial hypothalamus (DMH) elicits augmented breaths (ABs). DMH disinhibition in urethane anesthetized rats produced both an increase in baseline respiratory rate (RR) and an increase in the number of ABs. The increase in RR was associated with a decrease in both the time of inspiration (T(i)) and expiration (T(e)) and the peak change in RR was observed 5 min post DMH activation. In contrast, the increase in ABs was greatest during the first 1.25 min, and both T(i)s of the ABs did not change significantly from pre-injection values. The T(e) of the ABs did decrease but remained significantly greater than the T(e) of the normal breath during DMH disinhibition. Our results support the hypothesis that the central neural pathway involved in the maintenance of normal respiratory pattern may be distinct from pathways involved in the generation of ABs.

Evidence for alpha-2 adrenoreceptor modulation of arterial chemoreflexes in the caudal solitary nucleus of the rat.

The caudal region of the nucleus of the solitary tract (cNTS) is the primary central termination site for arterial chemoreceptor afferents originating from the carotid body. The purpose of the present study was to investigate the role of endogenous activation of alpha-2 adrenoreceptors in the cNTS on arterial chemoreflex function. Arterial chemoreflex responses to intravenous injections of potassium cyanide (KCN; 75 microg/kg) were recorded before and following blockade of alpha-2 adrenoreceptors in the cNTS of urethane-anesthetized rats. KCN alone elicited a reflex increase in arterial pressure, renal sympathetic nerve activity, and respiration. After bilateral cNTS microinjection of alpha-2 receptor antagonists (2 nmol idazoxan or 0.2 nmol yohimbine), arterial chemoreflex responses were markedly attenuated. Attenuation of chemoreflex function was not accompanied by any significant change in resting blood pressure or respiratory rate. The results suggest that the endogenous activation of alpha-2 adrenoreceptors facilitates central processing of chemoreceptor afferent inputs in the cNTS of the rat.

Arterial chemoreflex in conscious normotensive and hypertensive adult rats.

Evidence from human and animal studies suggests that the arterial chemoreflex may be exaggerated in essential hypertension. In the present study, cardiorespiratory responses to peripheral chemoreceptor stimulation were compared in conscious unrestrained spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) and Sprague-Dawley (SD) rats (13-14 wk old). Chemoreceptors were stimulated by injections of potassium cyanide (30-125 microgram/kg iv). Chemoreceptor stimulation elicited a pressor response and bradycardia. The peak change in mean arterial pressure evoked during chemoreceptor stimulation was not significantly different between SH (n = 18) and WKY (n = 18) rats but was significantly smaller in SD rats (n = 18). An evaluation of respiratory responses to chemoreceptor stimulation in conscious and anesthetized rats also demonstrated no significant difference between SH and WKY rats, but the response of the SD rats tended to be smaller. These results demonstrate that differences in the arterial chemoreflex response of SH vs. normotensive rats are not linked to hypertension but, rather, to differences between rat strains.

Electrophysiological properties of rat lateral parabrachial neurons in vitro.

Anatomical studies have demonstrated that the lateral parabrachial nucleus (LPBN) is composed of at least seven separate subnuclei distinguished by cell morphology, spatial clustering, and afferent and efferent connectivity. We hypothesized that neurons within the subnuclear clusters of the LPBN might have distinct electrophysiological properties that correlate with cellular morphology. An in vitro slice preparation was used to intracellularly record the intrinsic properties of 64 neurons located within the external lateral (EL) and central lateral (CL) subnuclei of the LPBN in adult rats. Analysis of intrinsic properties revealed that neurons in the EL subnucleus had significantly wider action potentials and on the average demonstrated more spike frequency adaptation during 2 s of depolarization compared with CL neurons. The majority of both EL and CL area neurons expressed delayed excitation (DE) after membrane hyperpolarization. DE was eliminated with the A-current blocker 4-aminopyridine (1.5-5 mM). Postinhibitory rebound was also observed in a subpopulation of EL and CL neurons. Morphological analysis of 11 LPBN neurons, which were electrophysiologically characterized and filled with 2% biocytin, failed to demonstrate an association between morphology and the electrophysiological profiles of LPBN neurons. The lack of distinct "type" of neuron within a single subnucleus of the LPBN is in agreement with recent findings reported from the neonatal rat.

Lateral parabrachial nucleus modulates baroreflex regulation of sympathetic nerve activity.

Previous studies have demonstrated that the lateral parabrachial nucleus (LPBN) is an important site for descending modulation of baroreflex control of heart rate. In the present study it was hypothesized that the LPBN neurons may also modulate baroreflex control of arterial pressure and sympathetic nerve activity. In urethan-anesthetized rats, electrical or chemical activation of the LPBN produced a significant reduction in the magnitude of the baroreflex inhibition of mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) elicited by aortic depressor nerve stimulation. Chemical inactivation of the LPBN resulted in a small increase in baroreflex control of MAP, but baroreflex control of RSNA was not affected. The results suggest that LPBN neurons have little tonic influence over baroreflex control of MAP and RSNA in the anesthetized rat. When the LPBN is activated, however, LPBN neurons may function to reduce the capacity of the baroreflex to regulate sympathetically mediated increases in arterial pressure.

Influence of circulating angiotensin II and vasopressin on neurons of the nucleus of the solitary tract.

The central cardiovascular effects of the circulating peptides angiotensin II (ANG II) and arginine vasopressin (AVP) may be mediated through the nucleus of the solitary tract (NTS). In this study, we used single-unit extracellular recording techniques to determine the effects of peripheral infusions of ANG II and AVP on NTS neurons. Barosensitive NTS neurons were characterized by their response to an increase in arterial pressure induced with phenylephrine (PE). Of 74 NTS neurons, 17 were excited by ANG II infusion. Of these, nine were also excited by activation of baroreceptors. ANG II inhibited 15 of 74 neurons, 13 of which were also baroinhibited. Of 77 NTS neurons, 13 were excited by AVP infusion, 7 of which were also baroexcited. One AVP-excited neuron was inhibited by a PE-induced increase in arterial pressure. Of 77 neurons, 21 were inhibited by AVP, 10 of which were also baroinhibited. Out of 21 AVP-inhibited neurons, 2 were excited by an increase in arterial pressure. These results indicate that some barosensitive NTS neurons may be independently influenced by circulating peptides and that separate populations of barosensitive and peptide-sensitive neurons are found in the NTS.

Cardiac rhythmicity among NTS neurons and its relationship to sympathetic outflow in rabbits.

The caudal nucleus tractus solitarius (NTS) is the primary termination site for baroreceptor afferents, but distinct pulse phasic activity is rarely recorded from NTS neurons. In the present study cross-correlation analysis was used to identify pulse phasic activity in NTS neurons in the anesthetized rabbit. Cross-correlation analysis demonstrated that 19 of 38 pressure-sensitive neurons had activity that correlated with the cardiac cycle, including 5 of 11 neurons activated by aortic depressor nerve stimulation, and 12 of 28 pressure-sensitive NTS neurons had activity that correlated with renal sympathetic nerve activity. Autocorrelation analysis identified 17 of 38 pressure-sensitive neurons that had rhythmic activity not related to the cardiac cycle (mean frequency 14.1 +/- 2 Hz). The results indicate that many NTS neurons do have pulse phasic activity consistent with baroreceptor input, but this activity is difficult to detect because it is distributed throughout the cardiac cycle.

Sympathetic responses to stimulation of area postrema in decerebrate and anesthetized rats.

The effects of electrical and chemical stimulation of the area postrema (AP) on mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) were examined in urethan- and pentobarbital sodium-anesthetized rats and in unanesthetized decerebrate rats. The AP was electrically stimulated over a range of frequencies (10-100 Hz) and intensities (10-80 microA) with a pulse duration of 0.2 or 1.0 ms. The excitatory amino acid L-glutamate (100 or 200 mM) was microinjected into the AP to preferentially stimulate neuronal cell bodies. In urethan-anesthetized rats, electrical stimulation of the AP decreased MAP and RSNA. In pentobarbital sodium-anesthetized rats, MAP and RSNA were markedly increased by AP stimulation. In unanesthetized decerebrate rats, increases in MAP and RSNA were also observed during electrical AP stimulation. Microinjection of L-glutamate had no effect on MAP and RSNA in anesthetized or in unanesthetized rats. These results indicate that electrical AP stimulation increases sympathetic output in the unanesthetized decerebrate rat and that anesthesia modifies this sympathetic response. The findings also suggest that peripheral responses to L-glutamate and electrical stimulation of the AP are mediated over different central pathways.

Peripheral chemoreceptor inputs to the parabrachial nucleus of the rat.

In the urethan-anesthetized rat, extracellular recordings were made from 60 neurons within the region of the parabrachial nucleus (PBN). The activity of 37 of 44 neurons was altered by selective stimulation of peripheral chemoreceptors via intracarotid injection of NaH2PO4 or CO2-saturated NaHCO3. Most of these neurons (28 of 37) were excited during chemoreceptor stimulation. Twenty-four of 48 neurons responded to changes in baroreceptor input via changes in blood pressure. Most of these neurons (18 of 24) were inhibited during baroreceptor stimulation. Eleven of 32 neurons were affected by both chemoreceptor and baroreceptor inputs. Seven of these neurons had opposite responses during selective afferent stimulation; that is, they were excited during chemoreceptor activation and inhibited during baroreceptor activation. Our observations reveal that neurons within the medial and lateral PBN are responsive to peripheral chemoreceptor input. A subgroup of PBN neurons was shown to integrate information from chemoreceptors and baroreceptors. These results suggest that both the medial and lateral PBN may play a role in the central integration of cardiovascular inputs.

Monopolar recording of H reflexes at various sites.

Various monopolar recording electrode sites have been used to record H reflexes and M responses. This investigation revealed a decrease in maximum M response amplitude accompanied by an increase in the H/M amplitude ratio as the active recording electrode was positioned more distally, below the gastrocnemii muscle bellies. H and M latencies were also significantly longer at the most distal recording site, but the latency difference is relatively independent of recording site. Serial variation was least at the most proximal recording site for an immobilized ankle. For an unrestrained ankle, serial variation was greater, but was least at the most distal site. The standard recording site, midway between knee and ankle, was not the best site for minimizing serial variation, and it was the least sensitive to vibration-induced reflex excitability changes. For serial testing of H reflex excitability, an immobilized ankle and measured proximal and distal recording sites are recommended.

Tendon vibration-induced inhibition of human and cat triceps surae group I reflexes: evidence of selective Ib afferent fiber activation.

In humans, prolonged vibration of the Achilles tendon produced transient depression or abolition of the soleus H-reflex. Recovery of the electrical reflex threshold to previbration values at a constant lower stimulus intensity usually occurred between 10 to 55 min. Electrical stimulation at higher multiples of the reflex threshold produced reflex EMG amplitudes more immediately comparable to previbration controls. When postvibration H-reflexes were completely abolished, poststimulus averaging of voluntarily maintained tonic EMG activity showed evidence of inhibition at a 46-ms latency in contrast to a 32-ms previbration H-reflex latency. In cat, observation of H-reflexes were rare, but stimulus-evoked changes in EMG activity mimicked the postvibration depression seen in humans. Ventral root postvibration reflexes from triceps surae varied in magnitude but were usually depressed or abolished at 1.0 to 1.2 times the electrical reflex threshold. These responses returned to previbration control amplitudes within 20 to 35 min. Magnitude of depression and time to recovery were dependent on the intensity of the electrical stimulus. In five experiments, depression of postvibration reflex activity and recovery were accompanied by gradual recovery in amplitude of the group I volley to previbration amplitudes. Elevated group Ia axonal electrical thresholds, monitored from seven isolated units, were observed to recover to previbration values in parallel with postvibration reflex recovery to control amplitudes. At electrical stimulus intensities greater than 1.4 times the reflex threshold, postvibration reflex responses were often potentiated, probably reflecting posttetanic potentiation of group Ia pathways activated at their higher axonal thresholds. In two observations, postvibration Ib axonal electrical thresholds did not change. Overall, the findings supported the proposal that postvibration depression of soleus H-reflexes in humans or cats is caused by both disfacilitation and autogenetic inhibition due to withdrawal of Ia afferent activation and increased selectivity of Ib afferent fiber stimulation, respectively.