Nociceptive inputs into rostral ventrolateral medulla-spinal vasomotor neurones in rats

The impact of sciatic nerve injury and social interactions testing on glucocorticoid receptor expression in catecholaminergic medullary cell populations

Paradoxically, while acute pain leads to transiently elevated corticosterone, chronic pain does not result in persistently elevated corticosterone. In the sciatic nerve chronic constriction injury (CCI) model of chronic pain, we have shown that the same nerve injury produces a range of behavioural outcomes, each associated with distinctive adaptations to the HPA-axis to achieve stable plasma corticosterone levels. We also demonstrated that CRF and GR expression in the paraventricular hypothalamus (PVH) was increased in rats that showed persistent changes to their social behaviours during Resident-Intruder testing ('Persistent Effect' rats) when compared to rats that showed no behavioural changes ('No Effect' rats). In this study, we investigated whether these changes were driven in part by altered sensitivity of the brainstem catecholaminergic pathways (known to regulate the PVH) to glucocorticoids. GR expression in adrenergic (C1,C2) and noradrenergic (A1,A2) cells was determined using immunohistochemistry in behaviourally tested CCI rats and in uninjured controls. We found no differences between Persistent Effect and No Effect rats in (1) the glucocorticoid sensitivity of these cells, or (2) the numbers of adrenergic and noradrenergic cells in each region. However, we discovered an overall reduction in GR expression in the non-catecholaminergic cells of these regions in both experimental groups when compared to uninjured controls, most likely attributable to the repeated Resident-Intruder testing. Taken together, these data suggest strongly that brainstem mechanisms are unlikely to play a key role in the rebalancing of the HPA-axis triggered by CCI, increasing the probability that these changes are driven by supra-hypothalamic regions.

Selective optogenetic activation of rostral ventrolateral medullary catecholaminergic neurons produces cardiorespiratory stimulation in conscious mice

Activation of rostral ventrolateral medullary catecholaminergic (RVLM-CA) neurons e.g., by hypoxia is thought to increase sympathetic outflow thereby raising blood pressure (BP). Here we test whether these neurons also regulate breathing and cardiovascular variables other than BP. Selective expression of ChR2-mCherry by RVLM-CA neurons was achieved by injecting Cre-dependent vector AAV2-EF1α-DIO-ChR2-mCherry unilaterally into the brainstem of dopamine-β-hydroxylase(Cre/0) mice. Photostimulation of RVLM-CA neurons increased breathing in anesthetized and conscious mice. In conscious mice, photostimulation primarily increased breathing frequency and this effect was fully occluded by hypoxia (10% O(2)). In contrast, the effects of photostimulation were largely unaffected by hypercapnia (3 and 6% CO(2)). The associated cardiovascular effects were complex (slight bradycardia and hypotension) and, using selective autonomic blockers, could be explained by coactivation of the sympathetic and cardiovagal outflows. ChR2-positive RVLM-CA neurons expressed VGLUT2 and their projections were mapped. Their complex cardiorespiratory effects are presumably mediated by their extensive projections to supraspinal sites such as the ventrolateral medulla, the dorsal vagal complex, the dorsolateral pons, and selected hypothalamic nuclei (dorsomedial, lateral, and paraventricular nuclei). In sum, selective optogenetic activation of RVLM-CA neurons in conscious mice revealed two important novel functions of these neurons, namely breathing stimulation and cardiovagal outflow control, effects that are attenuated or absent under anesthesia and are presumably mediated by the numerous supraspinal projections of these neurons. The results also suggest that RVLM-CA neurons may underlie some of the acute respiratory response elicited by carotid body stimulation but contribute little to the central respiratory chemoreflex.

The rostral parvicellular reticular formation neurons mediate lingual nerve input to the rostral ventrolateral medulla

In rats that had been anesthetized by urethane-chloralose, we investigated whether neurons in the rostral part of the parvicellular reticular formation (rRFp) mediate lingual nerve input to the rostral ventrolateral medulla (RVLM), which is involved in somato-visceral sensory integration and in controlling the cardiovascular system. We determined the effect of the lingual nerve stimulation on activity of the rRFp neurons that were activated antidromically by stimulation of the RVLM. Stimulation of the lingual trigeminal afferent gave rise to excitatory effects (10/26, 39%), inhibitory effects (6/26, 22%) and no effect (10/26, 39%) on the RVLM-projecting rRFp neurons. About two-thirds of RVLM-projecting rRFp neurons exhibited spontaneous activity; the remaining one-third did not. A half (13/26) of RVLM-projecting rRFp neurons exhibited a pulse-related activity, suggesting that they receive a variety of peripheral and CNS inputs involved in cardiovascular function. We conclude that the lingual trigeminal input exerts excitatory and/or inhibitory effects on a majority (61%) of the RVLM-projecting rRFp neurons, and their neuronal activity may be involved in the cardiovascular responses accompanied by the defense reaction.

Patterning of somatosympathetic reflexes reveals nonuniform organization of presympathetic drive from C1 and non-C1 RVLM neurons

To determine the organization of presympathetic vasomotor drive by phenotypic populations of rostral ventrolateral medulla (RVLM) neurons, we examined the somatosympathetic reflex (SSR) evoked in four sympathetic nerves together with selective lesions of RVLM presympathetic neurons. Urethane-anesthetized (1.3 g/kg ip), paralyzed, vagotomized and artificially ventilated Sprague-Dawley rats (n = 41) were used. First, we determined the afferent inputs activated by sciatic nerve (SN) stimulation at graded stimulus intensities (50 sweeps at 0.5-1 Hz, 1-80 V). Second, we recorded sympathetic nerve responses (cervical, renal, splanchnic, and lumbar) to intensities of SN stimulation that activated A-fiber afferents (low) or both A- and C-fiber afferents (high). Third, with low-intensity SN stimulation, we examined the cervical SSR following RVLM microinjection of somatostatin, and we determined the splanchnic SSR in rats in which presympathetic C1 neurons were lesioned following intraspinal injections of anti-dopamine-β-hydroxylase-saporin (anti-DβH-SAP). Low-intensity SN stimulation activated A-fiber afferents and evoked biphasic responses in the renal, splanchnic, and lumbar nerves and a single peak in the cervical nerve. Depletion of presympathetic C1 neurons (59 ± 4% tyrosine hydroxylase immunoreactivity profiles lesioned) eliminated peak 2 of the splanchnic SSR and attenuated peak 1, suggesting that only RVLM neurons with fast axonal conduction were spared. RVLM injections of somatostatin abolished the single early peak of cervical SSR confirming that RVLM neurons with fast axonal conduction were inhibited by somatostatin. It is concluded that unmyelinated RVLM presympathetic neurons, presumed to be all C1, innervate splanchnic, renal, and lumbar but not cervical sympathetic outflows, whereas myelinated C1 and non-C1 RVLM neurons innervate all sympathetic outflows examined. These findings suggest that multiple levels of neural control of vasomotor tone exist; myelinated populations may set baseline tone, while unmyelinated neurons may be recruited to provide actions at specific vascular beds in response to distinct stressors.

Asymmetrical changes in lumbar sympathetic nerve activity following stimulation of the sciatic nerve in rat

Noxious stimulation of the leg increases hind limb blood flow (HBF) to the ipsilateral side and decreases to the contralateral in rat. Whether or not this asymmetrical response is due to direct control by sympathetic terminals or mediated by other factors such as local metabolism and hormones remains unclear. The aim of this study was to compare responses in lumbar sympathetic nerve activity, evoked by stimulation of the ipsilateral and contralateral sciatic nerve (SN). We also sought to determine the supraspinal mechanisms involved in the observed responses. In anesthetized and paralyzed rats, intermittent electrical stimulation (1 mA, 0.5 Hz) of the contralateral SN evoked a biphasic sympathoexcitation. Following ipsilateral SN stimulation, the response is preceded by an inhibitory potential with a latency of 50 ms (N=26). Both excitatory and inhibitory potentials are abolished following cervical C1 spinal transection (N=6) or bilateral microinjections of muscimol (N=6) in the rostral ventrolateral medulla (RVLM). This evidence is suggestive that both sympathetic potentials are supraspinally mediated in this nucleus. Blockade of RVLM glutamate receptors by microinjection of kynurenic acid (N=4) selectively abolished the excitatory potential elicited by ipsilateral SN stimulation. This study supports the physiological model that activation of hind limb nociceptors evokes a generalized sympathoexcitation, with the exception of the ipsilateral side where there is a withdrawal of sympathetic tone resulting in an increase in HBF.

Reflex sympathetic activity after intravenous administration of midazolam in anesthetized cats

BACKGROUND

Although intrathecal midazolam has been reported to produce antinociceptive effects mediated by gamma-aminobutyric acid type A-benzodiazepine receptor complexes in the spinal cord, the effects of systemic midazolam on nociception remain unclear. We performed this study to examine the effects of IV-administered midazolam on somatosympathetic Adelta and C reflex discharges in brain-intact cats and decerebrate cats (with transection at midbrain level).

METHODS

Somatosympathetic Adelta and C reflexes were elicited in the inferior cardiac sympathetic nerve by electrical stimulation of myelinated (Adelta) and unmyelinated (C) afferent fibers of the superficial peroneal nerve in 28 mature cats. After control somatosympathetic reflex responses were obtained, midazolam was administered IV to four groups of randomly allocated cats as follows: brain-intact cats at a dose of 0.03 mg/kg, brain-intact cats at a dose of 0.1 mg/kg, brain-intact cats at a dose of 0.5 mg/kg, and decerebrate cats at a dose of 0.1 mg/kg.

RESULTS

C reflex discharges were significantly augmented at the dose of 0.03 mg/kg and significantly depressed at the dose of 0.1 and 0.5 mg/kg in brain-intact cats. C reflex discharges were also significantly depressed at the dose of 0.1 mg/kg in decerebrate cats.

CONCLUSIONS

We have demonstrated that IV midazolam produces dose-related effects on somatosympathetic reflex discharges. The clinical implication of these findings is that the effect of midazolam on nociception depends on its dosage. It also appears that the infra-midbrain region plays a major role in mediating the depressive effects of midazolam on somatosympathetic C reflex discharges.

Modulation of cardiovascular responses and neurotransmission during peripheral nociception following nNOS antagonism within the periaqueductal gray

Nitric oxide (NO) within the dorsal periaqueductal gray matter (dPAG) attenuated cardiovascular responses and changes in the concentrations of glutamate during both mechanical and thermal nociceptive stimulation [Ishide, T., Amer, A., Maher, T.J., Ally, A., 2005. Nitric oxide within periaqueductal gray modulates glutamatergic neurotransmission and cardiovascular responses during mechanical and thermal stimuli. Neurosci. Res. 51, 93-103]. Nitric oxide is synthesized from l-arginine via the enzyme, NO synthase (NOS), which exists in 3 isoforms: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS). In this study, we examined the role of nNOS within the dPAG on cardiovascular responses and extracellular glutamate and GABA concentrations during mechanical and thermal nociception in anesthetized rats. The noxious mechanical stimulus was applied by a bilateral hindpaw pinch for 5 s that increased mean arterial pressure (MAP) and heart rate (HR) by 24+/-4 mm Hg and 41+/-7 bpm, respectively (n=10). Extracellular glutamate levels within the dPAG increased by 10.7+/-1.3 ng/mul while GABA concentrations decreased by 1.9+/-0.5 ng/microl. Bilateral microdialysis of a selective nNOS antagonist, 1-(2-trifluoromethylphenyl)-imidazole (TRIM; 10.0 microM), into the dPAG had no effect on MAP, HR, glutamate and GABA values (P>0.05) during a mechanical stimulation. In a separate set of experiments, a noxious thermal stimulus was generated by immersing the metatarsus of a hindpaw in a water-bath at 52 degrees C for 5 s (n=10). Glutamate, MAP, and HR increased by 14.6+/-2 ng/microl, 45+/-6 mm Hg, and 47+/-7 bpm, while GABA decreased by 2.1+/-0.6 ng/microl. Administration of TRIM into the dPAG significantly enhanced the cardiovascular responses and glutamate increases (P<0.05) but further attenuated GABA changes (P<0.05) during subsequent thermal nociception. These results demonstrate that nNOS within the dPAG plays a differential role in modulating cardiovascular responses and glutamatergic/GABAergic neurotransmission during thermal and mechanical nociception.

Effects of Thermal Stimulation, Applied to the Hindpaw via a Hot Water Bath, upon Ovarian Blood Flow in Anesthetized Nonpregnant Rats

The effects of thermal stimulation, applied to the hindpaw via a hot bath set to either 40 degrees C (non-noxious) or 49 degrees C (noxious), upon ovarian blood flow were examined in nonpregnant anesthetized rats. Ovarian blood flow was measured using a laser Doppler flowmeter. Blood pressure was markedly increased following 49 degrees C stimulation. Ovarian blood flow, however, showed no obvious change during stimulation, although a small increase was observed after stimulation. Ovarian blood flow and blood pressure responses to 49 degrees C stimulation were abolished after hindlimb somatic nerves proximal to the stimuli were cut. Heat stimulation (49 degrees C) resulted in remarkable increases in both ovarian blood flow and blood pressure in rats in which the sympathetic nerves supplying the ovary were cut but the hindlimb somatic nerves remained intact. The efferent activity of the ovarian plexus nerve was increased during stimulation at 49 degrees C. Stimulation at 40 degrees C had no effect upon ovarian blood flow, blood pressure or ovarian plexus nerve activity. Electrical stimulation of the distal part of the severed ovarian plexus nerve resulted in a decrease in both the diameter of ovarian arterioles, observed using a digital video microscope, and ovarian blood flow.The present results demonstrate that noxious heat, but not non-noxious warm, stimulation of the hindpaw skin in anesthetized rats influences ovarian blood flow in a manner that is attributed to reflex responses in ovarian sympathetic nerve activity and blood pressure.

Medullary monoamines and NMDA-receptor regulation of cardiovascular responses during peripheral nociceptive stimuli

We have previously reported that AMPA-receptor blockade within the rostral ventrolateral medulla (RVLM) attenuates cardiovascular responses and extracellular concentrations of glutamate during mechanical, but not during thermal stimulation [Gray, T., Lewis III, E., Maher, T.J., Ally, A., 2001. AMPA-receptor blockade within the RVLM modulates cardiovascular responses via glutamate during peripheral stimuli. Pharmacol. Res. 43, 47-54]. In this study, we examined the role of NMDA-receptor blockade within the RVLM on cardiovascular responses and release of biogenic monoamines (serotonin [5HT], dopamine [DA], and norepinephrine [NE]) during both mechanical and thermal nociception using anesthetized Sprague-Dawley rats. Both mechanical and thermal stimulation have been shown to activate peripheral Adelta and C-fiber polymodal nociceptors. Noxious mechanical stimuli were induced by applying a pinch to alternate hindpaw for 5s while the noxious thermal stimuli involved immersion of the metatarsus of alternate hindpaw in a water bath at a temperature of 52 degrees C for 5 s. Mechanical stimulation increased mean arterial pressure (MAP), heart rate (HR), extracellular fluid 5HT, and DA concentrations (n=10). However, extracellular levels of NE were decreased within the RVLM. Furthermore, NMDA-receptor blockade with a competitive antagonist, AP-7 (200 nM), within the RVLM attenuated the cardiovascular responses and changes in 5HT and DA, but had no effect on NE levels. The thermal stimulation elicited similar increases in MAP and HR, however, extracellular levels of 5HT or DA did not change. Concentrations of NE were decreased during a thermal stimulation similar to the levels observed following mechanical stimuli. In contrast to mechanical stimuli, bilateral administration of AP-7 (200-1 mM) into the RVLM had no effect on cardiovascular responses, 5HT, DA or NE concentrations during a thermal stimulation. These results show that NMDA receptors within the RVLM most likely play a role in modulating cardiovascular responses by altering 5HT and DA concentrations within the RVLM during mechanical but not thermal nociception. Overall, the present study delineates the NMDA-receptor mediated central integrative mechanisms within the RVLM that coordinate processing of sensory impulses arising from peripheral noxious stimulation.

A monosynaptic connection between baroinhibited neurons in the RVLM and IML in Sprague-Dawley rats

To date, few studies have examined the relationship between the firing rate of neurons in the rostral ventrolateral medulla (RVLM) and neurons in the intermediolateral cell column (IML) of the spinal cord. In 19 Sprague-Dawley rats, the relationship between 20 pairs of baroinhibited RVLM and IML units was analyzed by cross-correlation. Three criteria were applied before acceptance that the firing rate of a pair of neurons was correlated. First, at an appropriate latency following the firing of an RVLM neuron, as judged from previous studies (4-200 ms), the peak in the firing rate of an IML neuron was approximately double that of the averaged surrounding bin counts. Secondly, the peak grew steadily in the examined period. Thirdly, the peak was restricted to a 1-ms bin. With this approach, a correlation was found between RVLM and IML neurons in 3 pairs in all. In 2 pairs, a correlation was found at basal arterial pressure (AP). When AP was decreased using a caval snare, a correlation was demonstrated in a further pair. A possible potentiation of synaptic strength during hypotensive stimuli is discussed.

Monitoring analgesia

Analgesia (pain relief) amnesia (loss of memory) and immobilisation are the three major components of anaesthesia. The perception of pain, and therefore, the need for analgesia, is individual, and the monitoring of analgesia is indirect and, in essence, of the moment. Under general anaesthesia, analgesia is continually influenced by external stimuli and the administration of analgesic drugs, and cannot be really separated from anaesthesia: the interaction between analgesia and anaesthesia is inescapable. Autonomic reactions, such as tachycardia, hypertension, sweating and lacrimation, although non-specific, are always regarded as signs of nociception or inadequate analgesia. Autonomic monitoring techniques, such as the analysis of heart rate variability, laser Doppler flowmetry, phlethysmographically derived indices and the pupillary light reflex, may help to quantitate reactions of the autonomic nervous system. For the past few years, automated electroencephalographic analysis has been of great interest in monitoring anaesthesia and could be useful in adapting the peroperative administration of opioids. A range of information collected from the electroencephalogram, haemodynamic readings and pulse plethysmography might be necessary for monitoring the level of nociception during anaesthesia. Information theory, multimodal monitoring, and signal processing and integration are the basis of future monitoring.

Nitric oxide within periaqueductal gray modulates glutamatergic neurotransmission and cardiovascular responses during mechanical and thermal stimuli

We have previously reported that nitric oxide (NO) within the rostral ventrolateral medulla (RVLM) attenuates cardiovascular responses and extracellular concentrations of glutamate during thermal, but not during mechanical nociceptive stimulation (Ishide. T., Maher, T.J., Ally, A. 2003. Role of nitric oxide in the ventrolateral medulla on cardiovascular responses and glutamate neurotransmission during mechanical and thermal stimuli. Pharmacol. Res. 47, 59-68). In this study, we examined the role of nitric oxide within the dorsolateral periaqueductal gray matter (PAG), a higher center integrating nociceptive reflexes, on cardiovascular responses and glutamate release during both mechanical and thermal nociception using anesthetized Sprague-Dawley rats. Two types of stimuli were studied, both activating peripheral A(delta) and C fiber polymodal nociceptors. Noxious mechanical stimulus was given by applying a bilateral hindpaw pinch for 5 s. Mechanical stimulation of a hindlimb increased mean arterial pressure (MAP), heart rate (HR), and extracellular fluid glutamate within PAG by 20+/-3 mmHg, 37+/-6 bpm, and 1.7+/-0.3 ng/5 microl, respectively (n=10). Bilateral microdialysis of L-arginine (1.0 microM), a NO precursor, into the PAG significantly attenuated MAP, HR, and glutamate increases during a mechanical stimulation. Subsequent administration of N(G)-methyl-L-arginine (L-NMMA) (1.0 microM), a NO synthase inhibitor, into the PAG blocked the ability of NO within PAG to modulate the cardiovascular responses to mechanical stimulus. The noxious thermal stimulus was generated by immersing the metatarsus of a hindpaw in water-bath at a temperature of 52 degrees C for 5 s. Similar increases were observed following thermal stimulation: 35+/-5 mmHg, 40+/-6 bpm, and 1.14+/-0.4 ng/5 microl (n=10). L-Arginine attenuated both cardiovascular responses and glutamate increase during thermal nociception. These results demonstrate that NO within the dorsolateral PAG plays a role in modulating cardiovascular responses by altering glutamate concentrations during both thermal and mechanical nociception.

Baro-activated neurons with pulse-modulated activity in the rat caudal ventrolateral medulla express GAD67 mRNA

GABAergic neurons in the caudal ventrolateral medulla (CVLM) are believed to mediate the sympathetic baroreceptor reflex by inhibiting presympathetic neurons in the rostral ventrolateral medulla (RVLM). Accordingly, some CVLM neurons are activated by increased arterial pressure (AP; baro-activated), have activity strongly modulated by the AP pulse (pulse-modulated), and can be antidromically activated from the RVLM. This study examined whether baro-activated, pulse-modulated CVLM neurons are indeed GABAergic and examined their structures. We recorded extracellularly from 19 baro-activated, pulse-modulated CVLM neurons in chloralose-anesthetized rats. Most of these cells (13/19) were silenced by decreasing AP with nitroprusside, but some (6/19) remained active at low AP levels. They were also excited by phenyl biguanide (17/17) but inhibited by noxious tail pinch (8/11). Twelve baro-activated cells were filled with biotinamide and examined for expression of GAD67 mRNA. Because adjacent vagal motor neurons are also activated by increased AP, we examined choline acetyltransferase (ChAT) immunoreactivity. Most baro-activated cells (9/12) expressed high levels of GAD67 mRNA, the rest (3/12) displayed lower levels of GAD67 mRNA, but none showed ChAT immunoreactivity. In contrast, adjacent baro-inhibited CVLM cells had no GAD67 mRNA (n = 5) but were instead tyrosine hydroxylase immunoreactive (n = 7). Reconstruction of baro-activated CVLM neurons revealed axons that projected dorsomedially and rostrally with several axon collaterals. These data demonstrate the existence of GABAergic CVLM neurons with the physiological characteristics expected of interneurons that mediate the sympathetic baroreceptor reflex. In addition, baro-activated GABAergic CVLM neurons appear to integrate several types of inputs and provide inhibition to multiple targets.

Role of nitric oxide in the ventrolateral medulla on cardiovascular responses and glutamate neurotransmission during mechanical and thermal stimuli

We have previously reported that alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-receptor blockade within the rostral ventrolateral medulla (RVLM) attenuates cardiovascular responses and extracellular concentrations of glutamate during mechanical, but not during thermal stimulation [Pharmacol. Res. 43 (2001) 47]. In this study, we examined the role of nitric oxide (NO) within the RVLM on cardiovascular responses and glutamate release during both mechanical and thermal nociception using anesthetized Sprague-Dawley rats. Two types of stimuli were studied, both activating peripheral Adelta and C fiber polymodal nociceptors. Noxious mechanical stimuli were given by applying a bilateral hindpaw pinch for 5s. The noxious thermal stimuli were generated by immersing the metatarsus of both hindpaws in a water bath at a temperature of 52 degrees C for 5s. Mechanical stimulation of both hindlimb extremities increased mean arterial pressure (MAP), heart rate (HR), and extracellular fluid glutamate by 14+/-2 mmHg, 35+/-5 bpm, and 1.4+/-0.3 ng/5 microl, respectively (n=8). Similar responses were observed following thermal stimulation: 40+/-4 mmHg, 44+/-6 bpm, and 0.97+/-0.2 ng/5 microl (n=8). Bilateral microdialysis of L-arginine (1.0 microM), a nitric oxide precursor, into the RVLM had no effects on MAP, HR, and glutamate increases during mechanical stimulation. However, L-arginine attenuated these responses during thermal nociception. Subsequent administration of L-NMMA (1.0 microM), a NOS inhibitor, reversed the attenuations. These results show that nitric oxide most likely plays a role in modulating cardiovascular responses by altering glutamate concentrations within the RVLM during thermal but not mechanical nociception. Overall, the present study delineates the differential central integrative mechanisms that regulate processing of sensory impulses arising from peripheral stimulation.

Somatic nociception activates NK1 receptors in the nucleus tractus solitarii to attenuate the baroreceptor cardiac reflex

There is limited information regarding the integration of visceral and somatic afferents within the nucleus of the solitary tract (NTS). We studied the interaction of nociceptive and baroreceptive inputs in this nucleus in an in situ arterially perfused, un-anaesthetized decerebrate preparation of rat. At the systemic level, the gain of the cardiac component of the baroreceptor reflex was attenuated significantly by noxious mechanical stimulation of a forepaw. This baroreceptor reflex depression was mimicked by NTS microinjection of substance P and antagonized by microinjection of either bicuculline (a GABAA receptor antagonist) or a neurokinin type 1 (NK1) receptor antagonist (CP-99994). The substance P effect was also blocked by a bilateral microinjection of bicuculline, at a dose that was without effect on basal baroreceptor reflex gain. Baroreceptive NTS neurons were defined by their excitatory response following increases in pressure within the ipsilateral carotid sinus. In 27 of 34 neurons the number of evoked spikes from baroreceptor stimulation was reduced significantly by concomitant electrical stimulation of the brachial nerve (P < 0.01). Furthermore, the attenuation of baroreceptor inputs to NTS neurons by brachial nerve stimulation was prevented by pressure-ejection of bicuculline from a multi-barrelled microelectrode (n = 8). In a separate population of 17 of 45 cells tested, brachial nerve stimulation evoked an excitatory response that was antagonized by blockade of NK1 receptors. We conclude that nociceptive afferents activate NK1 receptors, which in turn excite GABAergic interneurons impinging on cells mediating the cardiac component of the baroreceptor reflex.

Rostral ventral medulla 5-HT1A receptors selectively inhibit the somatosympathetic reflex

The role of the 5-hydroxytryptamine (5-HT1A) receptors in the rostral ventrolateral medulla (RVLM) on somatosympathetic, baroreceptor, and chemoreceptor reflexes was examined in anesthetized rats. Microinjection of the selective 5-HT1A agonist 8-hydroxy-di-n-propylamino tetralin (8-OH-DPAT) decreased arterial blood pressure and splanchnic sympathetic nerve activity (SNA). Electrical stimulation of the hindlimb evoked early and late excitatory sympathetic responses. Bilateral microinjection in the RVLM of 8-OH-DPAT markedly attenuated both the early and late responses. This potent inhibition of the somatosympathetic reflex persisted even after SNA and arterial blood pressure returned to preinjection levels. Preinjection of the selective 5-HT1A antagonist NAN-190 in the RVLM blocked the sympathoinhibitory effect of 8-OH-DPAT and attenuated the inhibitory effect on the somatosympathetic reflex. 8-OH-DPAT injected in the RVLM did not affect baroreceptor or chemoreceptor reflexes. Our findings suggest that activation of 5-HT1A receptors in the RVLM exerts a potent, selective inhibition on the somatosympathetic reflex.

Glutamatergic and GABAergic inputs to the RVL mediate cardiovascular adjustments to noxious stimulation

Stimulation of cutaneous and muscle afferents induces several cardiovascular adjustments such as hypertension, tachycardia, and muscle vasodilation. Although previous studies have demonstrated that the rostral ventrolateral medulla (RVL) mediates sympathoexcitation and pressor responses to sciatic nerve stimulation (SNS), whether it also mediates blood flow adjustments remains unclear. Therefore, in the present study, we examined the role of the RVL in the vasodilation induced by SNS and the possible neurotransmitters involved. In Urethane-anesthetized, paralyzed, and artificially ventilated rats, SNS (square pulses, 1 ms, 20 Hz, 800--1200 microA, 10 s) produced increases in blood pressure, heart rate, blood flow, and vascular conductance of the stimulated limb. Unilateral microinjection of kainic acid (2 nmol/100 nl) into the RVL contralateral to the stimulated limb abolished cardiovascular adjustments to SNS. Unilateral microinjections of kynurenic acid (2 nmol/100 nl) selectively abolished the pressor response to SNS, whereas bicuculline (400 pmol/100 nl) abolished the increases in blood flow without changing the pressor response. These results suggest that glutamatergic synapses within the RVL mediate pressor responses, whereas GABAergic synapses may mediate the vasodilation to SNS.

AMPA-receptor blockade within the RVLM modulates cardiovascular responses via glutamate during peripheral stimuli

We investigated the effects of AMPA-receptor blockade in the rostral ventrolateral medulla (RVLM) on cardiovascular responses and extracellular concentrations of glutamate during two different types of stimuli that activate peripheral Adelta - and C-fiber polymodal nociceptors using anesthetized rats. First, mechanical stimulation was achieved by applying a bilateral hindpaw pinch for 5 s, and second, thermal stimulation was evoked by immersing bilaterally the hindpaw metatarsi in a 52 degrees C hot water bath for 4 s. Mechanical stimulation increased mean arterial pressure (MAP) by 23 +/- 1 mmHg and heart rate (HR) by 25 +/- 3 bpm (n= 8). Thermal stimuli increased MAP by 32 +/- 3 mmHg and HR by 27 +/- 4 bpm (n= 8). After controlled generation of mechanical or thermal stimulation, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1.0 microM) was microdialysed bilaterally into the RVLM for 30 min. Administration of CNQX attenuated MAP and HR responses during a subsequent mechanical but not during thermal stimulation. Analyses of extracellular concentrations of glutamate within the RVLM bilaterally revealed an increase of this neurotransmitter within the RVLM during mechanical noxious stimulation. Concomitant with attenuation of the cardiovascular responses, glutamate concentrations were also decreased during the mechanical stimulation after administration of CNQX. These results demonstrate that the AMPA-receptor blockade within the RVLM that attenuates cardiovascular responses during mechanical stimulation is associated with a reduction in extracellular levels of glutamate. In addition, it appears that AMPA receptors in the RVLM do not play a role in mediating cardiovascular responses during thermal stimulation.

Reflex patterns in preganglionic sympathetic neurons projecting to the superior cervical ganglion in the rat

Reflex patterns in preganglionic neurons projecting in the cervical sympathetic trunk (CST) were analyzed in response to stimulation of various afferent systems. We focused on the question whether these preganglionic neurons can be classified into functionally distinct subpopulations. Reflex responses were elicited by stimulation of trigeminal and spinal nociceptive, thermoreceptive as well as baroreceptor and chemoreceptor afferents. Multi- and single fiber preparations were studied in baroreceptor intact and sino-aortically denervated animals. Spontaneous activity of 36 preganglionic single neurons ranged from 0.2 to 3.5 imp/s (median= 1.11 imp/s). The degree of cardiac rhythmicity (CR) in the activity of sympathetic neurons was 69.5+/-13% (mean+/-S.D.; N=52; range=39-95%). Noxious stimulation of acral skin activated the majority (67%) of sympathetic preparations by 37+/-25% (N=35) above pre-stimulus activity; 15% were inhibited. In these neurons the response to noxious stimulation of acral skin was significantly correlated with the degree of CR (P<0.001, N=52) in that neurons showing the strongest excitation to noxious stimulation displayed the strongest CR. Noxious mechanical stimulation of body trunk skin (N=60) inhibited the majority (80%) of fiber preparations tested (by 34+/-18% of pre-stimulus activity, N=48); an activation was not observed. Cold stimulation of acral (N=9) and body trunk skin (N=42) activated most fiber preparations. Trigeminal stimulation evoked a uniform reflex activation of preganglionic neurons (+79+/-73% of pre-stimulus activity, N=32). Chemoreceptor stimulation by systemic hypercapnia elicited inhibitory (-31+/-19%, N=8) as well as excitatory (+59+/-5%, N=4) responses. These results show that preganglionic sympathetic neurons projecting to target organs in the head exhibit distinct reflex patterns to stimulation of various afferent systems; however, a clear classification into different functional subgroups did not emerge. Furthermore, reflex patterns showed a segmental organization to noxious cutaneous stimulation of acral parts and body trunk reflecting a differential central integration of spinal afferent input. Compared with the cat the reflex organization of sympathetic neurons projecting to the head seems to be less differentiated in the anesthetized rat.

Synaptically released 5-HT modulates the activity of tonically discharging neuronal populations in the rostral ventral medulla (RVM)

There is substantial evidence for an important modulating role of monoamines (catecholamines and serotonin, 5-HT) in the rostral ventral medulla (RVM), a region which plays an important role in cardiovascular and nociceptive functions. We investigated in slices the role of endogenous monoamines in the synaptic control of the activity of rat RVM neuronal populations using intracellular recordings in the lateral RVM plus lateral aspect of nucleus paragigantocellularis lateralis. A triple-labelling protocol allowed us to identify the location of impaled neurons and their eventual monoaminergic phenotype within the serotonergic and catecholaminergic populations of the RVM. Focal electrical stimulation revealed the existence of a functional monoaminergic input onto RVM neurons which was mediated by endogenous 5-HT acting at inhibitory 5-HT1A receptors but did not involve noradrenergic neurotransmission. The slow 5-HT-mediated inhibitory postsynaptic potential (IPSP) was only observed in the regularly discharging neurons, which were found to be neither catecholaminergic nor serotonergic. The synaptic release of 5-HT was, itself, under an inhibitory control involving GABAA (gamma-aminobutyric acid) receptors. Moreover, we characterized the effect of the 5-HT-releasing agent fenfluramine on this functional 5-HT-mediated synaptic transmission. Our results show that the effect of fenfluramine is biphasic consisting of an initial prolongation of the serotonergic IPSP followed by a decrease in amplitude. Our data provide a basis for the previously reported inhibitory effects of exogenously applied serotonin agonists/antagonists on the autonomic functions controlled by the RVM. This 5-HT pathway, which functionally links the serotonergic and catecholaminergic regions, might play an important role in cardiovascular and nociceptive functions.

Spinal GABA(A) receptors do not mediate the sympathetic baroreceptor reflex in the rat

Activation of baroreceptors causes efferent sympathetic nerve activity (SNA) to fall. Two mechanisms could account for this sympathoinhibition: disfacilitation of sympathetic preganglionic neurons (SPN) and/or direct inhibition of SPN. The roles that spinal GABA and glycine receptors play in the baroreceptor reflex were examined in anesthetized, paralyzed, and artificially ventilated rats. Spinal GABA(A) receptors were blocked by an intrathecal injection of bicuculline methiodide, whereas glycine receptors were blocked with strychnine. Baroreceptors were activated by stimulation of the aortic depressor nerve (ADN), and a somatosympathetic reflex was used as control. After an intrathecal injection of vehicle, there was no effect on any measured variable or evoked reflex. In contrast, bicuculline caused a dose-dependent increase in arterial pressure, SNA, phrenic nerve discharge, and it significantly facilitated the somatosympathetic reflex. However, bicuculline did not attenuate either the depressor response or sympathoinhibition evoked after ADN stimulation. Similarly, strychnine did not affect the baroreceptor-induced depressor response. Thus GABA(A) and glycine receptors in the spinal cord have no significant role in baroreceptor-mediated sympathoinhibition.

Properties of C1 and other ventrolateral medullary neurones with hypothalamic projections in the rat

1. This study compared (i) the properties of C1 cells with those of neighbouring non-C1 neurones that project to the hypothalamus and (ii) the properties of C1 cells that project to the hypothalamus with those of their medullospinal counterparts. 2. Extracellular recordings were made at three rostrocaudal levels of the ventrolateral medulla (VLM) in alpha-chloralose-anaesthetized, artificially ventilated, paralysed rats. Recorded cells were filled with biotinamide. 3. Level I (0-300 microm behind facial nucleus) contained spontaneously active neurones that were silenced by baro- and cardiopulmonary receptor activation and virtually unaffected by nociceptive stimulation (firing rate altered by < 20 %). These projected either to the cord (type I; 36/39), or to the hypothalamus (type II; 2/39) but rarely to both (1/39). 4. Level II (600-800 microm behind facial nucleus) contained (i) type I neurones (n = 3) (ii) type II neurones (n = 11), (iii) neurones that projected to the hypothalamus and were silenced by baro- and cardiopulmonary receptor activation but activated by strong nociceptive stimulation (type III, n = 2), (iv) non-barosensitive cells activated by weak nociceptive stimulation which projected only to the hypothalamus (type IV, n = 9), (v) cells that projected to the hypothalamus and responded to none of the applied stimuli (type V, n = 7) and (vi) neurones activated by elevating blood pressure which projected neither to the cord nor to the hypothalamus (type VI, n = 4). 5. Level III (1400-1600 microm behind facial motor nucleus) contained all the cell types found at level II except type I. 6. Most of type I and II (17/26) and half of type III cells (4/8) were C1 neurones. Type IV-V were rarely adrenergic (2/12) and type VI were never adrenergic (0/3). 7. All VLM baroinhibited cells project either to the cord or the hypothalamus and virtually all (21/23) C1 cells receive inhibitory inputs from arterial and cardiopulmonary receptors.

Activation of the rostral ventrolateral medulla in an acute anesthetized rodent strychnine model of allodynia

After the administration of intrathecal strychnine, allodynia is manifested as activation of supraspinal sites involved in pain processing and enhancement of cardiovascular responses evoked by normally innocuous stimuli. The objective of this study was to investigate the effect of strychnine-induced allodynia on adrenergic neuronal activity in the C1 area of the rostral ventrolateral medulla (RVLM), a major site involved in cardiovascular regulation. The effect of intrathecal strychnine (40 microg) or saline followed by repeated hair deflection to caudal lumbar dermatomes in the urethane-anesthetized rat was assessed by measuring voltammetric changes in the RVLM catechol oxidation current (CA x OC), mean arterial pressure (MAP), and heart rate (HR). After the administration of intrathecal strychnine, hair deflection evoked a significant and sustained increase in the RVLM CA x OC and MAP (peak 146.4%+/-5.6% and 159%+/-18.4% of baseline, respectively; P < 0.05). There was a nonsignificant increase in HR (peak 128%+/-8.2%). In the absence of hair deflection, there was no demonstrable change. Intrathecal saline-treated rats failed to demonstrate changes in RVLM CA x OC, MAP, or HR. In the present study, we demonstrated that, after the administration of intrathecal strychnine, innocuous hair deflection evokes temporally related neuronal activation in the rat RVLM and an increase in MAP. This suggests that the RVLM mediates, at least in part, the cardiovascular responses during strychnine allodynia.

IMPLICATIONS

Neural injury-associated pain, as manifested by allodynia, is resistant to conventional treatment. In a rat model of allodynia, we demonstrated activation of the brain region involved in sympathetic control. Innovative therapies that target this region may be successful in managing this debilitating condition.

Influence of the hypothalamic paraventricular nucleus on cardiovascular neurones in the rostral ventrolateral medulla of the rat

1. The question of whether neurones in the paraventricular nucleus (PVN) of the hypothalamus have an excitatory influence on reticulo-spinal vasomotor neurones of the rostral ventrolateral medulla (RVL) has been addressed in this study using anaesthetized rats. 2. Extracellular microelectrode recordings were made from sixty vasomotor neurones in the RVL, identified by their cardiac cycle-related probability of discharge, by the decrease in activity in response to an increase in arterial blood pressure produced by intravenous phenylephrine and by the increase in activity in response to a decrease in blood pressure produced by intravenous nitroprusside. 3. More than 70 % of these RVL vasomotor neurones were identified as spinally projecting by antidromically activating their axons via a stimulating electrode in the lateral funiculus of the T2 or T10 segment of spinal cord. 4. Activation of neurones at different sites in the PVN with a microinjection of d,l-homocysteic acid (DLH) elicited either pressor or depressor responses. 5. At PVN pressor sites fifteen RVL vasomotor neurones were shown to be activated prior to the blood pressure change. A further twenty RVL vasomotor neurones were observed to decrease activity following the blood pressure rise. At PVN depressor sites twelve RVL neurones were inhibited prior to the blood pressure change whereas another thirteen identified RVL neurones increased their discharge following the fall in blood pressure. 6. In three rats single shock electrical stimulation at a PVN pressor site, first identified with DLH, elicited a single or double action potential in thirteen RVL neurones with a latency of 27 +/- 1 ms. 7. It is concluded that PVN neurones may elicit increases in blood pressure via excitatory connections with RVL-spinal vasomotor neurones, and that other PVN neurones may elicit decreases in blood pressure via inhibitory connections with these RVL neurones.

Catechol activation in the vasomotor center upon emergence from anesthesia: specificity

The rostral ventrolateral medulla (RVLM) controls the vascular system. It may contribute to postoperative hypertension observed upon emergence from anesthesia. This structure contains adrenergic cardiovascular neurons. Therefore, one question was addressed: does a change in RVLM catechol activity occur upon emergence from anesthesia? Halothane-anesthetized, paralyzed rats had their ventilatory, circulatory, and acid-base stability controlled. All pressure points and incisions were infiltrated with local anesthetic. With in vivo electrochemistry, a catechol signal was recorded in the RVLM in the following circumstances: (1) under stable halothane anesthesia for 120 minutes (halothane group), (2) during 120 minutes after halothane discontinuation (saline-emergence group), (3) during 60 minutes after halothane discontinuation followed by 60 minutes after halothane readministration (readministration group), (4) emergence in rats treated with atenolol and nitroprusside to hold blood pressure as close as possible to baseline, (5) emergence after morphine 1 mg.kg(-1) i.v., (6) emergence after decerebration, and (7) emergence upon recording in the mid-brain dopaminergic A10 area. Stable halothane anesthesia (n = 6) led to no change in mean arterial pressure (MAP), heart rate (HR), and catechol signal (CAOC). During emergence from anesthesia (n = 6), MAP, HR, and catechol signal increased and did not return to baseline. By contrast, a return of MAP, HR, and catechol signal to baseline was observed upon readministration of halothane (n = 6). Whereas blood pressure and heart rate were maintained as closely as possible to baseline, a large catechol activation (n = 5) was observed upon emergence from anesthesia. A catechol activation from a lowered baseline was observed upon emergence following morphine administration (n = 5). A minor circulatory activation without RVLM catechol activation was observed upon emergence following decerebration (n = 5). Recordings in the A10 area revealed no increase in the catechol signal following emergence (n = 5). Adrenergic RVLM neurons appear to be responsive upon emergence from anesthesia, possibly being activated by suprapontine afferents impinging on the RVLM.

Nociceptive and thermoreceptive lamina I neurons are anatomically distinct

Pain and temperature stimuli activate neurons of lamina I within the dorsal horn of the spinal cord, and although these neurons can be classified into three basic morphological types and three major physiological classes, earlier studies did not establish a structure/function correlation between their morphology and their physiological responses. We recorded and intracellularly labeled 38 cat lamina I neurons. All 12 fusiform cells were nociceptive-specific, responsive only to pinch and/or heat. All 11 pyramidal cells were thermoreceptive-specific, responsive only to innocuous cooling. Of ten multipolar cells, six were polymodal, responsive to heat, pinch and cold, and four were nociceptive-specific. Five unclassified cells had features consistent with this pattern. These results support the view that central pain and temperature pathways contain anatomically discrete sets of modality-selective neurons.

GABA- and glycine-mediated inhibitory postsynaptic potentials in neonatal rat rostral ventrolateral medulla neurons in vitro

Whole-cell patch recordings were made from rostral ventrolateral medulla neurons of two in vitro preparations: (i) brainstem spinal cords of two- to five-day-old rats, and (ii) coronal brainstem slices of eight- to 12-day-old rats, and the inhibitory synaptic activities in these neurons have been studied. In brainstem spinal cord preparations, Lucifer Yellow was diffused into the recording neurons at the end of experiments. Medullary neurons were characterized as: (i) spinally projecting by the appearance of an antidromic spike following electrical stimulation of the spinal tract between T2 and T3 segments, and (ii) adrenergic by the detection of phenylethanolamine-N-methyltransferase immunoreactivity in Lucifer Yellow-filled neurons. Of the 13 spinally projecting and phenylethanolamine-N-methyltransferase-positive medullary neurons, focal stimulation elicited in the presence of glutamate receptor antagonists an inhibitory postsynaptic potential in nine neurons. Inhibitory synaptic potentials were reversibly eliminated by the GABA(A) receptor antagonist bicuculline (10-20 microM) in six of nine neurons, by the glycine receptor antagonist strychnine (0.1-1 microM) in two and by a combination of bicuculline and strychnine in one neuron. In brainstem slice preparations, focal stimulation elicited three types of synaptic potential: (i) an excitatory postsynaptic potential, (ii) an inhibitory postsynaptic potential and (iii) a biphasic synaptic potential consisting of an excitatory synaptic potential followed by an inhibitory synaptic potential. Inhibitory synaptic potentials had a reversal potential between -70 and -80 mV, reversed their polarity in a low (6.7 mM) Cl- Krebs' solution, and suppressed or blocked by either bicuculline or strychnine or both. Elimination of inhibitory synaptic potentials unmasked in some cells an excitatory synaptic potential or enhanced the excitatory synaptic potential component in medullary neurons with a biphasic response, indicating a marked convergence of excitatory and inhibitory inputs onto a single neuron. A population of medullary neurons appeared to be pacemaker neurons whereby they discharged spontaneously. When discharges were suppressed by membrane hyperpolarization, focal stimulation elicited inhibitory synaptic potentials in 8/23 neurons tested. Our results suggest that inhibitory synaptic potentials in medullary neurons are mediated by either GABA and/or glycine which open primarily Cl- channels. The prevalence of inhibitory synaptic potentials in medullary neurons indicates an essential role of inhibitory transmission in controlling the input and output ratio of these neurons.

On-going and reflex synaptic events in rat superior cervical ganglion cells

1. Synaptic events evoked by brief noxious cutaneous stimuli were recorded in sympathetic neurones in the superior cervical ganglion of anaesthetized rats. 2. On-going excitatory synaptic potentials (ESPs) and/or action potentials (APs) were recorded in 69% of neurones at mean frequencies that varied from 0.01 to 6.3 Hz in different cells. From histograms of ESP amplitude during membrane hyperpolarization, it appears that most cells received one (52%), or two or more (36%), suprathreshold inputs and several subthreshold inputs with overlapping amplitudes. 3. Pinching the skin for 1-3 s evoked either a brief burst of synaptic events (lasting about 300 ms) preceding a few seconds of inhibition (burst-inhibitory (BI) neurones), or simply an excitation (excitatory (E) neurones), or no response (O neurones). In 60% of BI neurones, a second burst occurred after the end of the pinch. 4. BI neurones had a higher frequency of on-going synaptic activity (2.9 +/- 0.5 Hz, n = 15) than E neurones (0.2 +/- 0.1 Hz, n = 5) or O (0.2 +/- 0.1 Hz, n = 5) neurones. Most neurones with two or more suprathreshold inputs were BI neurones. In 20% of neurones (all BI with high rates of synaptic activity), several other inputs had ESPs with amplitudes close to threshold. 5. Subthreshold and suprathreshold inputs responded in the same way in only 45% of neurones, but suprathreshold inputs were excited in 73% of BI and all E neurones. The order of recruitment of different inputs varied from trial to trial. If classification was based only on suprathreshold responses, there were 36% BI, 32% E and 32% O neurones. 6. In the majority of neurones, postganglionic discharge was initiated exclusively by suprathreshold inputs, even during reflex excitation. 7. Qualitatively similar, but smaller, responses were evoked by a puff of air on the abdomen in 71% of cells tested. 8. The data suggest that the natural discharge of SCG neurones is largely determined by the activity of one or two preganglionic inputs with high quantal contents. BI neurones may include vasoconstrictor neurones, whereas the other types include secretomotor, pilomotor and other neurones projecting to targets in the head.

Increased activity of bulbospinal cardiovascular neurons in the rat rostral ventrolateral medulla upon emergence from anaesthesia

The rostral ventrolateral medulla (RVLM) is part of the vasomotor centre which controls the cardiovascular system and may therefore be critical to the genesis of postoperative hypertension. This area is probably a common site of termination of different inputs involved in the baroreflex. It contains at least two classes of neurons exhibiting spontaneous activities and projecting to sympathetic preganglionic neurons located in the intermediolateral cell-column (IML) of the spinal cord. The first class of neurons corresponds to cells with slow axonal conduction velocities (< 0.8 m s-1) and which contain immunoreactive phenylethanolamine-N-methyltransferase (CI cells); the second class, characterized by faster conduction velocities (2.5-8 m s-1), is considered as glutamatergic, although the C1 cells may also release glutamate alongside catecholamine. The purpose of the present study was to investigate the involvement of the "fast-conducting' RVLM barosensitive bulbospinal (RVLM-BB) neurons in the hypertension occurring upon emergence from halothane anaesthesia. Rats were anaesthetized with halothane, paralysed, and their lungs mechanically ventilated. Avoidable pain, distress or discomfort was consistently avoided as required by the fundamental principles of ethical animal research. Hence, all pressure points and surgical wounds, as well as tracheal tube were carefully covered or infiltrated with adequate local anaesthetic. Control experiments have been performed, allowing us to assert that hypertension accompanying halothane withdrawal was not due to suffering (see Discussion). Under halothane anaethesia, fast conducting (2.7 +/- 1.0 m s-1) RVLM-BB neurons (n = 10) exhibited a continuous discharge (8.4 +/- 7.5 Hz). Five minutes after discontinuing halothane, in increase in arterial blood pressure was recorded (AP 19 +/- 6 mmHg), which was accompanied by an increase in the unitary activities (n = 8.43 +/- 23%). Afterwards, both AP and unitary activity frequencies further increased to reach a maximum value at the end of the sequence (34 +/- 9 mmHg and 161 +/- 120% respectively, n = 10). After resumption of halothane administration, both AP and unitary activities fall down to the baseline level within 5 min (n = 10). This study shows that emergence from halothane anaesthesia reversibly induces RVLM-BB units activation, suggesting that a putative glutamatergic bulbospinal pathway may be involved in the genesis of hypertension occurring upon emergence from anaesthesia. These data may therefore contribute to better understanding of postoperative hypertension and to improve its pharmacological treatment in man.

Differential regional sympathetic responses to somatic stimulation in anesthetized dogs

The present study was designed to determine whether regional differences exist in sympathetic responses to somatic nerve stimulation and whether the baroreceptor reflex modulates this somato-sympathetic reflex. The cardiac (CNA), renal (RNA), hepatic (HNA), splenic (SpNA) and adrenal (AdNA) sympathetic postganglionic nerve activities (SNA) were simultaneously recorded in anesthetized dogs with intact (n = 7) or bilaterally sectioned (n = 8) carotid sinus and vagus nerves. In the intact group, electrical stimulation of the left peroneal nerve at low intensity and low frequency (5 V, 5 Hz) produced a fall in mean arterial pressure (MAP) (-9.7 +/- 2.7 mmHg) and a decrease in each SNA with no regional differences RNA (79.2 +/- 8.5%), AdNA (82.7 +/- 5.4%), HNA (89.4 +/- 4.5%), CNA (87.5 +/- 3.5%), SpNA (84.2 +/- 3.2%). In contrast, stimulation at high intensity and high frequency (25 V, 50 Hz) produced a rise in MAP (+21.4 +/- 3.8 mmHg) and increases in SNA with quantitative predominance of RNA (178.6 +/- 13.6%) and AdNA (158.3 +/- 16.1%) over HNA (129.0 +/- 4.2%), CNA (117.7 +/- 7.6%), and SpNA (112.0 +/- 6.2%). Similar responses were observed when the left ulnar nerve was stimulated. The changes in SNA (delta SNA) at 10 s after the start of stimulation were plotted as a function of the changes in MAP (delta MAP) and the regression curves were determined. The best fit regression curve was a logistic sigmoid curve in the intact group and a linear one in the baroreceptor denervated group. Furthermore, delta RNA/delta MAP and delta AdNA/delta MAP during the somato-pressor response were significantly smaller in the intact group than in the denervated group. In conclusion, there are regional differences of sympathetic response during the somato pressor response but not during somato depressor response. The baroreceptor reflex may suppress the somato-sympathetic reflex of RNA and AdNA.

Ethanol inhibits chemoreflex excitation of reticulospinal vasomotor neurons

In anesthetized and ventilated rats, activation of carotid chemoreceptors with intracarotid administration of 100 nmol sodium cyanide rapidly excited the spinal cord-projecting vasomotor neurons in the rostroventrolateral reticular nucleus (RVL) of the medulla oblongata and sympathetic nerves and increased arterial pressure. The chemoreflex sympathoexcitatory pressor responses were attenuated by an acute systemic administration of ethanol at 0.45 g/kg, but not at 45 mg/kg. The ethanol effects were observed at the level of RVL-spinal vasomotor neurons, in attenuating the neuronal responses to the chemoreflex excitation and direct iontophoresis of N-methyl-D-aspartic acid (NMDA) but without altering responses of the carotid sinus nerves to intracarotid cyanide. The effect of ethanol on the RVL neurons was further defined as blocking NMDA-evoked inward current in the corresponding spontaneously active RVL neurons in vitro. The results indicate that acute ethanol intoxication markedly influences NMDA receptor activation and arterial chemoreflexes. The relevance of the type of action to clinical hypertension in chronic and heavy drinkers is discussed.

Excitatory amino acid-mediated chemoreflex excitation of respiratory neurones in rostral ventrolateral medulla in rats

1. In anaesthetized rats, extracellular and intracellular recordings were made from 119 respiratory neurones in the rostroventrolateral reticular nucleus (RVL) of the medulla oblongata. 2. Two types of active respiratory neurones were detected in RVL: expiratory (E) and pre-inspiratory (Pre-I), based on the relationship between their discharge and that of the phrenic nerve. Some Pre-I but none of the E neurones could be antidromically excited from the C(3)-C(4) level of the spinal cord. 3. E and Pre-I neurones of RVL were excited by stimulation of the arterial chemoreceptors by a close arterial injection of sodium cyanide. The reflex excitation of RVL E neurones was preceded by increased phrenic nerve activity, while the excitation of RVL Pre-I neurones preceded the increases in phrenic nerve activity. 4. The chemoreflex excitation of the two types of RVL respiratory neurones as well as their resting discharge was abolished or significantly depressed by microionophoresis of kynurenate, a wide-spectrum antagonist of excitatory amino acid receptors, while xanthurenate, an inactive analogue of kynurenate, was without effect. 5. In ventilated rats, bilateral microinjection into RVL of kynurenate, but not xanthurenate, abolished resting activity and chemoreflex excitation of phrenic nerve activity, whilst in spontaneously breathing rats, kynurenate microinjection into RVL produced apnea and silenced phrenic nerves. 6. We conclude: (a) chemoreflex excitation of the phrenic nerves is mediated by stimulating Pre-I neurones of RVL by excitatory amino acidergic inputs and (b) RVL Pre-I neurones may directly and/or indirectly excite spinal phrenic motor neurones and hence are involved in inspiratory rhythmogenesis.

A comparative study of pre-sympathetic and Bötzinger neurons in the rostral ventrolateral medulla (RVLM) of the rat

The aim of this study was to investigate the degree of functional and anatomical overlap between two major neuronal subpopulations in the rostral ventrolateral medulla: pre-sympathetic (sympathoexcitatory) neurons, and expiratory neurons of the Bötzinger complex. Extracellular recordings were made with dye-filled microelectrodes in pentobarbital anesthetized, paralyzed and artificially ventilated adult Wistar rats. Tests applied included stimulation of baroreceptor afferents, activation of peripheral chemoreceptors and lung stretch receptors, changes in central respiratory drive with hyper- or hypoventilation, nociceptive stimulation, and antidromic stimulation from the T2 segment of the spinal cord or medulla oblongata at obex level. The two groups of neurons showed different patterns of spontaneous activity and generally different responses to these stimuli. The recording positions showed some overlap, but the majority of Bötzinger neurons were dorsolateral to pre-sympathetic neurons. There was a large overlap between the location of pre-sympathetic neurons and the lateral part of the C1 adrenergic group, but only a small overlap between these adrenergic neurons and Bötzinger neurons. These results indicate that the anatomically adjacent pre-sympathetic and Bötzinger expiratory neurons form two functionally distinct neuronal subpopulations.

Distribution of brainstem projections from spinal lamina I neurons in the cat and the monkey

The distribution of terminal projections in the brainstem from lamina I neurons in the spinal dorsal horn was investigated with the anterograde tracer Phaseolus vulgaris-leucoagglutinin in the cat and the cynomolgus monkey. Iontophoretic injections made with physiological guidance were restricted to lamina I or to laminae I-III in the cervical (C6-8) or lumbar (L6-7) enlargement. The distribution of terminal labeling was essentially identical in the cat and the monkey, although consistently of greater intensity in the monkey. Terminations were observed in the solitary nucleus, the dorsomedial medullary reticular formation, the entire rostrocaudal extent of the ventrolateral medulla, the locus coeruleus, the subcoerulear region and the Kölliker-Fuse nucleus, the lateral and medial portions of the parabrachial nucleus, the cuneiform nucleus, the ventrolateral and lateral portions of the periaqueductal gray, and the intercollicular nucleus. Lamina I terminations were generally bilateral in the medulla but more dense contralaterally in the pons and mesencephalon. The density and laterality of labeling in the medulla varied between cases independently from that in the pons and mesencephalon, suggesting that the lamina I projections to these regions may originate from different subsets of neurons. A clear topographic organization was observed only in the lateral column of the periaqueductal gray, where lumbar lamina I terminations were found caudal to cervical terminations. These observations indicate that spinal lamina I neurons project to a variety of brainstem sites involved in autonomic (cardiovascular, respiratory) and homeostatic processing and the control of behavioral state. These projections provide an afferent substrate for spino-bulbo-spinal somatoautonomic reflex arcs activated by nociceptive, thermoreceptive activity and for a spino-bulbo-hypothalamic relay of such activity by cells in the caudal ventrolateral medulla. These observations support the general concept that lamina I projections distribute modality-selective sensory information relevant to the physiological status and maintenance of the tissues and organs of the entire organism.

Internal connections in the rostral ventromedial medulla of the rat

Physiological and pharmacological data suggest that the rostral ventromedial medulla (RVM) is an important site where integration between somatic and visceral functions might occur. The aim of the present study was to describe the interconnections between various nuclei of the rostral ventromedial medulla and thus reveal the possible anatomical basis for such functional interactions. The topography of anterogradely labelled internal projections was examined following iontophoretic microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L). The results revealed that the nuclei of the rostral ventromedial medulla have strong interconnections and, to varying degrees, they also have bilateral projections into the rostral ventrolateral medulla. A particularly dense projection to widespread regions of the ventral medulla was traced from the raphe obscurus. Terminals, originating from the raphe pallidus were similarly dispersed but very low density in comparison. The focus of the projections of the gigantocellular nucleus pars ventralis and pars alpha shifted from the lateral paragigantocellular nucleus towards the RVM in rostral direction. Connections from the raphe magnus were altogether restricted to the RVM and the medial aspects of the lateral paragigantocellular nucleus. The diffuse and dense intramedullary connections of the raphe obscurus suggest that it might have an important role in coordinating the activity of rostral ventral medullary cells. The raphe pallidus and the ventral gigantocellular nuclei, areas that were innervated from widespread regions of the rostral ventral medulla but gave only limited projections there, are more likely to be involved in the direct descending control of spinal activities.

Decerebration does not alter hypoxic sympathoexcitatory responses in rats

In anesthetized, paralyzed and ventilated rats, hypoxia, produced by intratracheal administration of 100% N2 for 20 s, increases sympathetic nerve activity and produces cardiovascular responses. Acute midcollicular decerebration has no effect on these responses in chemo-innervated or chemo-denervated animals. Suprapontine neural structures are, therefore, not required for the rapid sympathetic and cardiovascular responses to acute hypoxia. The results support the view that sympathoexcitatory responses to acute hypoxia depend entirely on the functions of reticulospinal sympathoexcitatory vasomotor neurons of the rostral ventrolateral medulla (RVL).

Noxious heat-evoked Fos-like immunoreactivity in the rat medulla, with emphasis on the catecholamine cell groups

The objectives of the present study were 1) to utilize Fos immunohistochemistry as a marker for neuronal activity in order to examine the population of neurons in the medulla that is engaged by activation of nociceptive peripheral afferents and 2) to determine whether catecholamine-containing neurons in the medulla also express noxious heat-evoked Fos-like immunoreactivity. Noxious heating of the hindpaw evoked specific patterns of Fos-like immunoreactivity in the medulla in regions known to be involved in both nociceptive processing and cardiovascular regulation. Noxious heating of the hindpaw significantly increased the mean number of neurons expressing Fos-like immunoreactivity in the contralateral ventrolateral medulla. Increased numbers of Fos-positive neurons also were observed in both the ipsilateral and the contralateral A1 catecholamine cell groups. Similarly, in the contralateral medullary dorsal reticular fields, noxious heating of the hindpaw significantly increased the mean number of neurons expressing Fos-like immunoreactivity. In contrast, in the paramedian reticular nucleus, noxious heating of the hindpaw resulted in a significant decrease in the mean number of neurons expressing Fos-like immunoreactivity. No significant differences in the mean numbers of neurons expressing Fos-like immunoreactivity were noted in the A2, C1, or C2/C3 medullary catecholamine cell groups. These results suggest that noxious stimuli affect pools of neurons in the medulla with multiple physiological functions.

NMDA receptor-mediated sympathetic chemoreflex excitation of RVL-spinal vasomotor neurones in rats

1. We investigated the role of N-methyl-D-aspartic acid (NMDA) receptors in mediating hypoxic excitation of the reticulospinal vasomotor neurones of the rostroventrolateral reticular nucleus (RVL) of the medulla oblongata in paralysed ventilated rats. 2. Unilateral close arterial injection of sodium cyanide (100 nmol) into the carotid sinus region or ventilation with 100% N2 for 12 s rapidly, reversibly and reproducibly excited the RVL-spinal vasomotor neurones, followed about 1-2 s later by increases in sympathetic nerve activity and arterial pressure, effects abolished by denervation of the ipsilateral carotid sinus nerve. 3. Ionophoresis onto the RVL-spinal vasomotor neurones of kynurenate (a wide-spectrum antagonist of the excitatory amino acid receptors) or of 2-amino-5-monophosphovaleric acid (APV; a selective NMDA receptor antagonist), but not of xanthurenate (an inactive analogue of kynurenate), blocked the excitation elicited by intracarotid cyanide or 12 s of hypoxia. Kynurenate completely and APV partially blocked the excitatory responses to ionophoretically applied L-glutamate. APV, however, did not alter the excitatory responses of the vasomotor neurones to ionophoreses of kainate and quisqualate. 4. Bilateral microinjection of kynurenate (10 nmol, 50 nl per site) or APV (5 nmol, 50 nl per site) into the RVL blocked the increases in arterial pressure elicited by intracarotid cyanide or 12 s of 100% N2 ventilation. 5. Twenty seconds of intratracheal administration of 100% N2 resulted in complex and prolonged elevations of arterial pressure, the late component of which was affected neither by sinus denervation nor by microinjections of kynurenate or APV into the RVL. 6. We conclude that the sympathetic and cardiovascular responses to stimulation of arterial chemoreceptors result from excitation of RVL-spinal vasomotor neurones via activation of the NMDA subtypes of the excitatory amino acid receptors of the neurones. In contrast, the failure of these antagonists to influence the delayed excitation of the RVL-spinal vasomotor neurons by more prolonged exposure to N2 inhalation further supports the view that these neurones are directly stimulated by hypoxia.

Baroreflex failure in a patient with central nervous system lesions involving the nucleus tractus solitarii

Animal studies have shown the importance of the nucleus tractus solitarii, a collection of neurons in the brain stem, in the acute regulation of blood pressure. Impulses arising from the carotid and aortic baroreceptors converge in this center, where the first synapse of the baroreflex is located. Stimulation of the nucleus tractus solitarii provides an inhibitory signal to other brain stem structures, particularly the rostral ventrolateral medulla, resulting in a reduction in sympathetic outflow and a decrease in blood pressure. Conversely, experimental lesions of the nucleus tractus solitarii lead to loss of baroreflex control of blood pressure, sympathetic activation, and severe hypertension in animals. In humans, baroreflex failure due to deafferentation of baroreceptors has been previously reported and is characterized by episodes of severe hypertension and tachycardia. We present a patient with an undetermined process of the central nervous system characterized pathologically by ubiquitous infarctions that were particularly prominent in the nucleus tractus solitarii bilaterally but spared the rostral ventrolateral medulla. Absence of a functioning baroreflex was evidenced by the lack of reflex tachycardia to the hypotensive effects of sodium nitroprusside, exaggerated pressor responses to handgrip and cold pressor test, and exaggerated depressor responses to meals and centrally acting alpha 2-agonists. This clinicopathological correlate suggests that the patient's baroreflex failure can be explained by the unique combination of the destruction of sympathetic inhibitory centers (ie, the nucleus tractus solitarii) and preservation of centers that exert a positive modulation on sympathetic tone (ie, the rostral ventrolateral medulla).