Lateral tegmental field neurons sensitive to muscular contraction: a role in pressor reflexes?

Sciatic nerve stimulation activates the retrotrapezoid nucleus in anesthetized rats

Retrotrapezoid nucleus (RTN) neurons sustain breathing automaticity. These neurons have chemoreceptor properties, but their firing is also regulated by multiple synaptic inputs of uncertain function. Here we test whether RTN neurons, like neighboring presympathetic neurons, are excited by somatic afferent stimulation. Experiments were performed in Inactin-anesthetized, bilaterally vagotomized, paralyzed, mechanically ventilated Sprague-Dawley rats. End-expiratory CO₂ (eeCO₂) was varied between 4% and 10% to modify rate and amplitude of phrenic nerve discharge (PND). RTN and presympathetic neurons were recorded extracellularly below the facial motor nucleus with established criteria. Sciatic nerve stimulation (SNstim, 1 ms, 0.5 Hz) slightly increased blood pressure (6.6 ± 1.6 mmHg) and heart rate and, at low eeCO₂ (<5.5%), entrained PND. Ipsi- and contralateral SNstim produced the known biphasic activation of presympathetic neurons. SNstim evoked a similar but weaker biphasic response in up to 67% of RTN neurons and monophasic excitation in the rest. At low eeCO_2, RTN neurons were silent and responded more weakly to SNstim than at high eeCO₂ RTN neuron firing was respiratory modulated to various degrees. The phasic activation of RTN neurons elicited by SNstim was virtually unchanged at high eeCO₂ when PND entrainment to the stimulus was disrupted. Thus RTN neuron response to SNstim did not result from entrainment to the central pattern generator. Overall, SNstim shifted the relationship between RTN firing and eeCO₂ upward. In conclusion, somatic afferent stimulation increases RTN neuron firing probability without altering their response to CO_2. This pathway may contribute to the hyperpnea triggered by nociception, exercise (muscle metabotropic reflex), or hyperthermia.

Renal sympathetic and circulatory responses to activation of the exercise pressor reflex in rats

We investigated the role played by the exercise pressor reflex in sympathetic regulation of the renal circulation in rats. In mid-collicular decerebrate rats, mean arterial pressure (MAP), heart rate (HR), left renal cortical blood flow (RCBF) and left renal sympathetic nerve activity (RSNA) were recorded before and during 30 s of static contraction of the left triceps surae muscles evoked by electrical stimulation of the tibial nerve, which activates both metabo- and mechanosensitive muscle afferents, and during 30 s of passive stretch of the left Achilles tendon, which selectively activates mechanosensitive muscle afferents. Static contraction (n = 17, +344 +/- 34 g developed tension) significantly (P < 0.05) increased MAP (+14 +/- 3 mmHg), HR (+6 +/- 1 beats min(-1)) and RSNA (n = 11, +19 +/- 5%) and significantly decreased renal cortical vascular conductance (RCVC, n = 11, -11 +/- 2%). Passive stretch (n = 20, +378 +/- 11 g) also significantly increased MAP (+11 +/- 2 mmHg), HR (+7 +/- 2 beats min(-1)) and RSNA (n = 15, +14 +/- 4%) and significantly decreased RCVC (n = 11, -12 +/- 3%). RCBF showed no significant changes during static contraction or passive stretch. Renal denervation abolished the decrease in RCVC during contraction (n = 12) or stretch (n = 13). These data indicate that both the exercise pressor reflex and its mechanically sensitive component, the muscle mechanoreflex, induced renal cortical vasoconstriction through sympathetic activation in rats.

Neurons in and near insular cortex are responsive to muscular contraction and have sympathetic and/or cardiac-related discharge

Insular cortex (IC) is recognized as a potential site for "central command" of cardiorespiratory responses during exercise. Muscular contraction (MC) decreased the discharge rate of most IC neurons. Activity of most contraction sensitive neurons was either not altered by elevating blood pressure or showed a response converse to that of MC. IC may thus have a role in central command but the area is clearly modulated by MC.

Role of the medullary lateral tegmental field in reflex-mediated sympathoexcitation in cats

We tested the hypothesis that blockade of N-methyl-D-aspartate (NMDA) and non-NMDA receptors on medullary lateral tegmental field (LTF) neurons would reduce the sympathoexcitatory responses elicited by electrical stimulation of vagal, trigeminal, and sciatic afferents, posterior hypothalamus, and midbrain periaqueductal gray as well as by activation of arterial chemoreceptors with intravenous NaCN. Bilateral microinjection of a non-NMDA receptor antagonist into LTF of urethane-anesthetized cats significantly decreased vagal afferent-evoked excitatory responses in inferior cardiac and vertebral nerves to 29 +/- 8 and 24 +/- 6% of control (n = 7), respectively. Likewise, blockade of non-NMDA receptors significantly reduced chemoreceptor reflex-induced increases in inferior cardiac (from 210 +/- 22 to 129 +/- 13% of control; n = 4) and vertebral nerves (from 253 +/- 41 to 154 +/- 20% of control; n = 7) and mean arterial pressure (from 39 +/- 7 to 21 +/- 5 mmHg; n = 8). Microinjection of muscimol, but not an NMDA receptor antagonist, caused similar attenuation of these excitatory responses. Sympathoexcitatory responses to the other stimuli were not attenuated by microinjection of a non-NMDA receptor antagonist or muscimol into LTF. In fact, excitatory responses elicited by stimulation of trigeminal, and in some cases sciatic, afferents were enhanced. These data reveal two new roles for the LTF in control of sympathetic nerve activity in cats. One, LTF neurons are involved in mediating sympathoexcitation elicited by activation of vagal afferents and arterial chemoreceptors, primarily via activation of non-NMDA receptors. Two, non-NMDA receptor-mediated activation of other LTF neurons tonically suppresses transmission in trigeminal-sympathetic and sciatic-sympathetic reflex pathways.

Substance P release in the feline nucleus tractus solitarius during ergoreceptor but not baroreceptor afferent signaling

Substance P (SP) is associated with metabo- and mechanoreceptor afferent fibers ('ergoreceptors') in skeletal muscle as well as the afferent fibers from carotid sinus baroreceptors. Afferent activity from each of these are at least partially integrated in the nucleus tractus solitarius (NTS). The purpose of this study was to determine whether SP was released from the NTS during acute reflex-induced changes in blood pressure caused by stimulating these receptors. Both the muscle pressor response and the baroreflex were studied in adult cats anaesthetized with alpha-chloralose. SP antibody-coated microprobes were used to measure the possible release of SP from the NTS. The muscle pressor response caused a release of immunoreactive SP-like substances (irSP) from the rostral medial NTS, as well as the dorsal motor nucleus (DMV) and lateral tegmental field (FTL). This release was not dependent on intact afferent input from the carotid sinus nerve, but was a function of activation of muscle ergoreceptors, since no irSP was released in response to stimulation of the motor nerves after the muscle was paralyzed. There was no detectable release of irSP from the mNTS during carotid artery occlusions (baroreceptor unloading). Baroreceptor activation, induced by the i.v. injection of the vasoconstrictor, phenylephrine, did not cause the release of irSP from the mNTS above resting baseline levels. These data suggest that SP is involved with the mediation of the afferent signal from muscle ergoreceptor fibers in the medial NTS. SP is not involved with the mediation of baroreceptor afferent signaling in the medial NTS. The release of SP in response to ergoreceptors activation may function to excite an inhibitory pathway which inhibits baroreflex signals that would tend to reduce the blood pressure and heart rate during the muscle pressor response.

Interaction between vestibulosympathetic and skeletal muscle reflexes on sympathetic activity in humans

Evidence from animals indicates that skeletal muscle afferents activate the vestibular nuclei and that both vestibular and skeletal muscle afferents have inputs to the ventrolateral medulla. The purpose of the present study was to investigate the interaction between the vestibulosympathetic and skeletal muscle reflexes on muscle sympathetic nerve activity (MSNA) and arterial pressure in humans. MSNA, arterial pressure, and heart rate were measured in 17 healthy subjects in the prone position during three experimental trials. The three trials were 2 min of 1) head-down rotation (HDR) to engage the vestibulosympathetic reflex, 2) isometric handgrip (IHG) at 30% maximal voluntary contraction to activate skeletal muscle afferents, and 3) HDR and IHG performed simultaneously. The order of the three trials was randomized. HDR and IHG performed alone increased total MSNA by 46 +/- 16 and 77 +/- 24 units, respectively (P < 0.01). During the HDR plus IHG trial, MSNA increased 142 +/- 38 units (P < 0.01). This increase was not significantly different from the sum of the individual trials (130 +/- 41 units). This finding was also observed with mean arterial pressure (sum = 21 +/- 2 mmHg and HDR + IHG = 22 +/- 2 mmHg). These findings suggest that there is an additive interaction for MSNA and arterial pressure when the vestibulosympathetic and skeletal muscle reflexes are engaged simultaneously in humans. Therefore, no central modulation exists between these two reflexes with regard to MSNA output in humans.

Sensory modalities conveyed in the hindlimb somatic afferent input to nucleus tractus solitarius

To determine the somatic sensory modalities conveyed by hindlimb somatic afferent inputs, the discharge of neurons in the nucleus tractus solitarius was recorded in anesthetized rats after electrical stimulation of either the contralateral sciatic nerve or L(6) spinal nerve, which innervates the hindlimb. The discharge of seven of eight cells was increased (P < 0.05) by capsaicin injected into the arterial supply of the hindlimb. Discharge was unaltered in 19 neurons tested for sensitivity to nonnoxious (40 degrees C) and noxious (47 degrees C) heating of the hindlimb skin. In contrast, lightly stroking the skin elicited discharge in 2 of 14 cells, whereas noxious pinching increased activity in 4 other cells. Rhythmic (1- to 3-s) muscle contraction (MC) increased (P < 0.05) discharge in >60% of neurons tested (11 of 18). Static (10- to 30-s) MC significantly (P < 0.05) increased discharge in four cells, two of which were also responsive to rhythmic MC. Rhythmic and sustained muscle stretch increased discharge (P < 0.05) in three of eight neurons tested. These data indicate that nucleus tractus solitarius neurons receive input from low- and high-threshold cutaneous mechanoreceptors, respond to capsaicin delivered into the hindlimb arterial supply, lack thermal sensitivity, and respond to activation of mechanosensitive as well as metabosensitive endings in skeletal muscle.

Expression of c-fos-like immunoreactivity in the feline brainstem in response to isometric muscle contraction and baroreceptor reflex changes in arterial pressure

This study compared whether activation of muscle ergoreceptor afferents caused by isometric muscle contraction, activation of baroreceptor afferents induced by i.v. infusion of phenylephrine, or baroreceptor afferent inactivation, caused by carotid artery occlusion, elicit similar patterns of c-Fos induction in brainstem areas. Adult cats were anesthetized with alpha-chloralose, and in each case, the experimental intervention caused an increase in the arterial blood pressure. There were two sets of control experiments: in both, animals underwent the same surgical procedures but then either remained at rest for the entire study, or the tibial nerve was stimulated, as in the contraction group, following muscle paralysis with tubocurarine. Following the procedures, animals rested for 90 min to allow neuronal expression of c-Fos. Control cats showed very little c-Fos immunoreactivity (c-Fos-ir) in the brainstem. Muscle contraction induced c-Fos-ir expression mainly in the nucleus tractus solitarius, lateral reticular nucleus, lateral tegmental field, vestibular nucleus, subretrofacial nucleus, spinal trigeminal tract and in a lateral region of the periaqueductal grey (P 0.5-1.0). The majority of the c-Fos-ir was found in brainstem areas contralateral to the contracted muscle. In addition, muscle contraction induced c-Fos-ir in the dorsal horns of spinal segments L6-S1 on the ipsilateral side of the spinal cord. Phenylephrine infusion caused c-Fos-ir expression in the nucleus tractus solitarius, spinal trigeminal tract, solitary tract, and dorsal motor nucleus of the vagus. No c-Fos-ir was apparent in the periaqueductal grey. Carotid occlusions induced c-Fos-ir expression in the area postrema, nucleus tractus solitarius, solitary tract, and spinal trigeminal tract. Expression was bilateral. Areas that exhibited c-Fos-ir correspond to sites previously reported to release various neuropeptides in response to muscle contraction or carotid occlusions. These results indicate that the exercise pressor reflex and baroreflex activate similar, but not completely identical, sites in the brainstem.

Substance P released in the rostral brainstem of cats interacts with NK-1 receptors during muscle pressor response

The release of immunoreactive substance P-like substances (irSP) was measured from sites in the rostral brainstem (at a level 1.3 mm anterior to the obex) of anesthetized cats in response to fatiguing isometric contractions using SP antibody-coated glass microprobes. The contractions caused a pressor and tachycardic response. irSP were released from sites in the medial subnucleus of the nucleus tractus solitarius (mNTS), the solitary tract and lateral tegmental field at this level of the brainstem. Injections of the specific NK-1 receptor antagonist, GR 82334, bilaterally into the mNTS significantly reduced the muscle pressor response, while bilateral injections of the SP NK-1 agonist, GR 73632, into the mNTS significantly increased the pressor and tachycardic responses during the contractions. Neither the antagonist nor the agonist, at the doses tested, affected the resting arterial pressure or heart rate. These data indicate that irSP are released from sites in the mNTS during the reflex pathways activated by isometric contractions and that they interact with NK-1 receptors in the area of the mNTS to affect the cardiovascular responses during the muscle pressor reflex.