Effect of alpha-adrenoceptor blockade on the 0.4 Hz sympathetic rhythm in conscious rats

Broad Spectrum Treatment for Ocular Insult Induced by Organophosphate Chemical Warfare Agents

Warfare organophosphates nerve agents constitute one of the prime threats to mankind on the battlefield and in the scenario of civilian terror. Exposure to organophosphate (OP) nerve agents dose-dependently result in incapacitation. They affect multiple organs, but the eye is one of the first and most frequently affected. Ocular OP insult may result in long-term miosis, impaired visual function, and ocular pain thus inducing functional incapacitation. The currently recommended military medical doctrine of using 1% atropine eye drops is far from being the optimal treatment. Although effective in reducing ocular pain and the miotic response, this treatment induces long-term mydriasis and cycloplegia promoting photophobia and restricted accommodation, which may result in further impairment in visual function. An optimal treatment must ameliorate the long-term ocular insult enabling rapid return of normal visual function, while avoiding the induction of mydriasis and cycloplegia side effects, which could possibly worsen the visual performance. Optimal treatment should also keep effects of misuse to a minimum. Work done in recent years examined treatments with various anticholinergic drugs alone or used in combination with oxime treatments and may offer improved efficacy in ameliorating the ocular insult. This review is a summary of the applied research in animals and will discuss clinical implications and possible alterations in treatment protocols following OP exposure. Taken together the data points toward the use of topical low concentrations of potent anticholinergic ophthalmic drops such as atropine or homatropine, which rapidly ameliorate the long-term OP-induced ocular insult.

Assessing low-frequency oscillations in cerebrovascular diseases and related conditions with near-infrared spectroscopy: a plausible method for evaluating cerebral autoregulation?

BACKGROUND

Cerebral autoregulation (CA) is the brain's ability to always maintain an adequate and relatively constant blood supply, which is often impaired in cerebrovascular diseases. Near-infrared spectroscopy (NIRS) examines oxygenated hemoglobin (OxyHb) in the cerebral cortex. Low- and very low-frequency oscillations ( LFOs ≈ 0.1    Hz and VLFOs ≈ 0.05 to 0.01 Hz) in OxyHb have been proposed to reflect CA.

AIM

To systematically review published results on OxyHb LFOs and VLFOs in cerebrovascular diseases and related conditions measured with NIRS.

APPROACH

A systematic search was performed in the MEDLINE database, which generated 36 studies relevant for inclusion.

RESULTS

Healthy people have relatively stable LFOs. LFO amplitude seems to reflect myogenic CA being decreased by vasomotor paralysis in stroke, by smooth muscle damage or as compensatory action in other conditions but can also be influenced by the sympathetic tone. VLFO amplitude is believed to reflect neurogenic and metabolic CA and is lower in stroke, atherosclerosis, and with aging. Both LFO and VLFO synchronizations appear disturbed in stroke, while the former is also altered in internal carotid stenosis and hypertension.

CONCLUSION

We conclude that amplitudes of LFOs and VLFOs are relatively robust measures for evaluating mechanisms of CA and synchronization analyses can show temporal disruption of CA. Further research and more coherent methodologies are needed.

Electroacupuncture suppresses capsaicin-induced secondary hyperalgesia through an endogenous spinal opioid mechanism

Central sensitization, caused either by tissue inflammation or peripheral nerve injury, plays an important role in persistent pain. An animal model of capsaicin-induced pain has well-defined peripheral and central sensitization components, thus is useful for studying the analgesic effect on two separate components. The focus of this study is to examine the analgesic effects of electroacupuncture (EA) on capsaicin-induced secondary hyperalgesia, which represents central sensitization. Capsaicin (0.1%, 20 microl) was injected into the plantar side of the left hind paw, and foot withdrawal thresholds in response to von Frey stimuli (mechanical sensitivity) were determined for both primary and secondary hyperalgesia in rats. EA (2 Hz, 3 mA) was applied to various pairs of acupoints, GB30-GB34, BL40-BL60, GV2-GV6, LI3-LI6 and SI3-TE8, for 30 min under isoflurane anesthesia and then the effect of EA on mechanical sensitivity of paw was determined. EA applied to the ipsilateral SI3-TE8, but to none of the other acupoints, significantly reduced capsaicin-induced secondary hyperalgesia but not primary hyperalgesia. EA analgesic effect was inhibited by a systemic non-specific opioid receptor (OR) antagonist or an intrathecal mu- or delta-OR antagonist. EA analgesic effect was not affected by an intrathecal kappa-OR antagonist or systemic adrenergic receptor antagonist. This study demonstrates that EA produces a stimulation point-specific analgesic effect on capsaicin-induced secondary hyperalgesia (central sensitization), mediated by activating endogenous spinal mu- and delta-opioid receptors.

Correlation of discharges of rostral ventrolateral medullary neurons with the low-frequency sympathetic rhythm in rats

The rostral ventrolateral medulla (RVLM) is critically important in the generation of sympathetic activity. The purpose of this study was to investigate whether discharges of RVLM neurons contribute to low-frequency (LF) sympathetic rhythms. Blood pressure (BP), renal sympathetic nerve activity (SNA), and neuronal activity in the RVLM were simultaneously recorded in seven anesthetized, paralyzed, and artificially ventilated rats. Fifty-one RVLM neurons were recorded and classified into three differential functional groups according to their activities related to baroreceptor input. Those in the category of spike firing inhibited by a BP increase (BP(I)) and which excited sympathetic discharges was the most abundant (24%). Coherence analysis was used to examine the relationship of the firing frequency of RVLM neurons with the LF (0.2-0.8Hz) rhythm of SNA. Forty-one percent of RVLM neurons showed a significant correlation to LF rhythms, and BP(I) neurons with sympathoexcitatory properties were the major contributors. In another 4 baroreceptor-denervated rats, 36 RVLM neurons were recorded. In these rats, RVLM neuronal activities no longer changed with BP fluctuations. Nevertheless, more than 40% of RVLM neurons were sympathoexcitatory, and 36% of RVLM neurons were still correlated with the LF SNA rhythm. Our results suggest that there are RVLM neurons involved in generating the LF rhythm in SNA and that the baroreflex can induce the participation of more neurons in LF rhythm generation.

Exercise training improves aortic depressor nerve sensitivity in rats with ischemia-induced heart failure

Exercise training improves arterial baroreflex control in heart failure (HF) rabbits. However, the mechanisms involved in the amelioration of baroreflex control are unknown. We tested the hypothesis that exercise training would increase the afferent aortic depressor nerve activity (AODN) sensitivity in ischemic-induced HF rats. Twenty ischemic-induced HF rats were divided into trained ( n = 11) and untrained ( n = 9) groups. Nine normal control rats were also studied. Power spectral analysis of pulse interval, systolic blood pressure, renal sympathetic nerve activity (RSNA), and AODN were analyzed by means of autoregressive parametric spectral and cross-spectral algorithms. Spontaneous baroreflex sensitivity of heart rate (HR) and RSNA were analyzed during spontaneous variation of systolic blood pressure. Left ventricular end-diastolic pressure was higher in HF rats compared with that in the normal control group ( P = 0.0001). Trained HF rats had a peak oxygen uptake higher than untrained rats and similar to normal controls ( P = 0.01). Trained HF rats had lower low-frequency [1.8 ± 0.2 vs. 14.6 ± 3 normalized units (nu), P = 0.0003] and higher high-frequency (97.9 ± 0.2 vs. 85.0 ± 3 nu, P = 0.0005) components of pulse interval than untrained rats. Trained HF rats had higher spontaneous baroreceptor sensitivity of HR (1.19 ± 0.2 vs. 0.51 ± 0.1 ms/mmHg, P = 0.003) and RSNA [2.69 ± 0.4 vs. 1.29 ± 0.3 arbitrary units (au)/mmHg, P = 0.04] than untrained rats. In HF rats, exercise training increased spontaneous AODN sensitivity toward normal levels (trained HF rats, 1,791 ± 215; untrained HF rats, 1,150 ± 158; and normal control rats, 2,064 ± 327 au/mmHg, P = 0.05). In conclusion, exercise training improves AODN sensitivity in HF rats.

Role of purinergic cotransmission in the sympathetic control of arterial pressure variability in conscious rats

Previous studies have shown that the sympathetically mediated oscillations of arterial pressure (AP), the so-called Mayer waves, are shifted from 0.4 to 0.6 Hz after acute α-adrenoceptor blockade in conscious rats. This raises the possibility that, under physiological conditions, Mayer waves are mediated by the conjoint action of norepinephrine and other sympathetic cotransmitters. To evaluate the possible role of the cotransmitter ATP in determining the frequency of Mayer waves, AP and renal sympathetic nerve activity (RSNA) were simultaneously recorded in 10 conscious rats with cardiac autonomic blockade before and after acute blockade of P2 receptors with pyridoxal phosphate-6-azophenyl-2′,4′-disulfonic acid. P2 receptor blockade did not alter the mean level and overall variability of AP and RSNA but shifted peak coherence between AP and RSNA from 0.43 ± 0.02 to 0.22 ± 0.01 Hz. A model of the sympathetic limb of the arterial baroreceptor reflex was designed to simulate Mayer waves at 0.2 and 0.6 Hz, with norepinephrine and ATP, respectively, acting as the sole sympathetic cotransmitter. When both cotransmitters acted in concert, a single oscillation was observed at 0.4 Hz when the gain ratio of the adrenergic to the purinergic components was set at 15. The model thus accounted for an important role for ATP in determining Mayer wave frequency, but not in sustaining the mean level of AP or controlling its overall variability.

Miotic Tolerance to Sarin Vapor Exposure: Role of the Sympathetic and Parasympathetic Nervous Systems

O-isopropyl methylphosphonofluoridate, also known as sarin or GB, is a highly toxic organophosphorous compound that exerts its effect by inhibiting the enzyme acetylcholinesterase. While the effects of a single exposure to GB vapor are well characterized, the effects of multiple exposures to GB vapor are less clear. Previous studies in the rat and guinea pig have demonstrated that multiple exposures result in tolerance to the miotic effect of nerve agents. The aim of the present study was to examine potential mechanisms responsible for tolerance to the miotic effect of GB vapor that has been observed in the rat after multiple exposures. Multiple whole-body inhalation exposures to GB vapor were conducted in a dynamic airflow chamber. Exposures lasted 60 min and each of the three exposures occurred at 24-h intervals. The results of the present study demonstrate that the alpha-adrenergic antagonist phentolamine and the beta-adrenergic receptor antagonist propranolol did not affect the development of tolerance to the miotic effect of GB vapor, suggesting that enhanced sympathetic tone to the eye is not responsible for the observed tolerance. Administration of atropine before the first exposure prevented the tolerance to the miotic effect of GB vapor after the third exposure, suggesting that the tolerance is the result of muscarinic receptor desensitization secondary to receptor stimulation. The present study extends the findings of previous studies to strengthen the hypothesis that the miotic tolerance observed in the rat upon repeated exposure to nerve agents is due to desensitization of muscarinic acetylcholine receptors located on the pupillary sphincter.

Baroreflex control of renal sympathetic nerve activity and spontaneous rhythms at Mayer wave's frequency in rats

The effects of sedation with pentobarbital sodium (15 mg/kg followed by 15.9+/-1.2 mg/kg/h, i.v.) on arterial pressure (AP) Mayer waves and accompanying oscillations of renal sympathetic nerve activity (RSNA) were examined in rats (n=8). As compared with values observed in the conscious state, pentobarbital significantly (P<0.05) decreased AP (from 119+/-2 to 93+/-3 mm Hg), heart rate (HR; from 427+/-11 to 355+/-11 beats/min) and RSNA (from 1.20+/-0.27 to 0.62+/-0.13 microV). The baroreflex control of RSNA was analyzed by fitting a sigmoid logistic function to changes in RSNA and AP observed during nitroprusside and phenylephrine administrations. During pentobarbital infusion, the RSNA-AP relationship was reset towards lower AP values, but neither its maximum gain nor its gain at resting AP were significantly altered (from 6.3+/-1.0 to 5.8+/-1.4 and from 3.2+/-0.5 to 3.8+/-1.3 normalized units (n.u.)/mm Hg, respectively). Spectral power in the frequency band containing Mayer waves (0.29-0.73 Hz) was significantly decreased by pentobarbital for both AP (from 4.65+/-0.90 to 0.95+/-0.25 mm Hg2) and RSNA (from 1437+/-245 to 488+/-79 n.u.2). This effect was mainly secondary to the attenuation of strongly coherent oscillations of both variables at approximately 0.4 Hz. Although previous experimental evidence points to a major involvement of the sympathetic limb of the arterial baroreceptor reflex in the genesis of Mayer waves, the present study indicates that the amplitude of these oscillations cannot be used as a quantitative index of sympathetic baroreflex sensitivity.

Steady-State and Dynamic Responses of Renal Sympathetic Nerve Activity to Air-Jet Stress in Sinoaortic Denervated Rats

This study examined the role of arterial baroreceptors in mediating the relationship between changes in the mean level and the amplitude of slow oscillations of renal sympathetic nerve activity (RSNA) during environmental stress. In 7 sham-operated (control) and 7 chronically (2 weeks before study) sinoaortic baroreceptor denervated (SAD) conscious rats, arterial pressure (AP) and RSNA were simultaneously recorded during two 15-minute periods, before and during the application of a mild environmental stressor (jet of air). Air-jet stress induced a similar degree of sympathoexcitation in both groups of rats. During stress in control rats, AP and RSNA spectral power in the mid-frequency (MF) range (0.27 to 0.74 Hz) increased, mainly as a consequence of an amplification of strongly coherent oscillations of ≈0.4 Hz frequency. In SAD rats, MF fluctuations of AP and RSNA were reduced but not abolished before stress, tended to increase during stress, and were linearly related under both experimental conditions. However, in the MF range, there was no well-defined oscillation at any specific frequency. At the peak coherence frequency (≈0.4 Hz), the gain of the transfer function from RSNA to AP did not change during stress in control rats and was similar to that measured in SAD rats, indicating that it mainly reflected the properties of the feedforward effect of RSNA on AP (ie, vascular reactivity). In summary, the parallelism between stress-induced changes in the mean level of RSNA and the amplitude of slow RSNA oscillations requires the functional integrity of the baroreceptor reflex, which is consistent with the hypothesis that slow AP and RSNA rhythms are resonant oscillations within the baroreceptor reflex loop.

Dynamic interactions between arterial pressure and sympathetic nerve activity: role of arterial baroreceptors

The role of arterial baroreceptors in controlling arterial pressure (AP) variability through changes in sympathetic nerve activity was examined in conscious rats. AP and renal sympathetic nerve activity (RSNA) were measured continuously during 1-h periods in freely behaving rats that had been subjected to sinoaortic baroreceptor denervation (SAD) or a sham operation 2 wk before study (n = 10 in each group). Fast Fourier transform analysis revealed that chronic SAD did not alter high-frequency (0.75-5 Hz) respiratory-related oscillations of mean AP (MAP) and RSNA, decreased by approximately 50% spectral power of both variables in the midfrequency band (MF, 0.27-0.74 Hz) containing the so-called Mayer waves, and induced an eightfold increase in MAP power without altering RSNA power in the low-frequency band (0.005-0.27 Hz). In both groups of rats, coherence between RSNA and MAP was maximal in the MF band and was usually weak at lower frequencies. In SAD rats, the transfer function from RSNA to MAP showed the characteristics of a second-order low-pass filter containing a fixed time delay ( approximately 0.5 s). These results indicate that arterial baroreceptors are not involved in production of respiratory-related oscillations of RSNA but play a major role in the genesis of synchronous oscillations of MAP and RSNA at the frequency of Mayer waves. The weak coupling between slow fluctuations of RSNA and MAP in sham-operated and SAD rats points to the interference of noise sources unrelated to RSNA affecting MAP and of noise sources unrelated to MAP affecting RSNA.