Blockade of non-NMDA receptors attenuates reflex pressor response to static contraction

The exercise pressor reflex: An update

The exercise pressor reflex is a feedback mechanism engaged upon stimulation of mechano- and metabosensitive skeletal muscle afferents. Activation of these afferents elicits a reflex increase in heart rate, blood pressure, and ventilation in an intensity-dependent manner. Consequently, the exercise pressor reflex has been postulated to be one of the principal mediators of the cardiorespiratory responses to exercise. In this updated review, we will discuss classical and recent advancements in our understating of the exercise pressor reflex function in both human and animal models. Particular attention will be paid to the afferent mechanisms and pathways involved during its activation, its effects on different target organs, its potential role in the abnormal cardiovascular response to exercise in diseased states, and the impact of age and biological sex on these responses. Finally, we will highlight some unanswered questions in the literature that may inspire future investigations in the field.

Spinal estrogen attenuates the exercise pressor reflex but has little effect on the expression of genes regulating neurotransmitters in the dorsal root ganglia

Previously, our laboratory showed that estrogen, topically applied to the spinal cord, attenuated the exercise pressor reflex in female cats (Schmitt PM and Kaufman MP. J Appl Physiol 95: 1418-1424, 2003; 98: 633-639, 2005). The attenuation was gender specific and was in part opioid dependent. Our finding that the mu- and delta-opioid antagonist naloxone was only able to partially restore estrogen's attenuating effect on the pressor response to static contraction suggested that estrogen affected an additional pathway, involving the dorsal root ganglion (DRG). Estrogen has been described to stimulate transcription within 10 min of its application to the DRG, raising the possibility that rapid genomic effects on neurotransmitter production may have contributed to estrogen's effect on the exercise pressor reflex. This prompted us to test the hypothesis that estrogen modulated the pressor response to static contraction by influencing gene expression of the neurotransmitters released by the thin-fiber muscle afferents that evoke the exercise pressor reflex. We confirmed in decerebrated female rats that topical application of estrogen (0.01 microg/ml) to the lumbosacral spinal cord attenuated the pressor response to static muscle contraction (from 10+/-3 to 1+/-1 mmHg; P<0.05). DRG were then harvested postmortem, and changes in mRNA expression were analyzed. GeneChip analysis revealed that neither estrogen nor contraction alone changed the mRNA expression of substance P, the neurokinin-1 receptor, CGRP, NGF, the P2X3 receptor, GABAA and GABAB, the 5-HT3A and 5-HT3B receptor, N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors, opioid receptors, and opioid-like receptor. Surprisingly, however, contraction stimulated the expression of neuropeptide Y in the DRG in the presence and absence of estrogen. We conclude that estrogen does not attenuate the exercise pressor reflex through a genomic effect in the DRG.

Role of spinal NMDA and non-NMDA receptors in the pressor reflex response to abdominal ischemia

Abdominal ischemia induces a pressor reflex caused mainly by C-fiber afferent stimulation. Because excitatory amino acids, such as glutamate, bind to N-methyl-D-aspartate (NMDA) and non-NMDA [dl-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)] receptors and serve as important spinal neurotransmitters, we hypothesized that both receptors play a role in the abdominal ischemia pressor reflex. In chloralose-anesthetized cats, NMDA receptor blockade with 25.0 mM dl-2-amino-5-phosphonopentanoate did not alter the pressor reflex (33 +/- 9 to 33 +/- 7 mmHg, P > 0.05, n = 4), whereas AMPA receptor blockade with 4.0 mM 6-nitro-7-sulfamylbenzo(f)quinoxaline-2,3-dione significantly attenuated the reflex (29 +/- 5 to 16 +/- 4 mmHg, P < 0.05, n = 6). Because several studies suggest that anesthesia masks the effects of glutamatergic receptors, this experiment was repeated on decerebrate cats, and in this group, NMDA receptor blockade with 25.0 mM dl-2-amino-5-phosphonopentanoate significantly altered the pressor reflex (36 +/- 3 to 25 +/- 4 mmHg, P < 0.05, n = 5). Our combined data suggest that spinal NMDA and AMPA receptors play a role in the abdominal ischemia pressor reflex.

Dorsal horn administration of muscimol abolishes the muscle pressor reflex

The purpose of this study was to determine the effect of blocking synaptic transmission in the dorsal horn on the cardiovascular responses produced by activation of muscle afferent neurons. Synaptic transmission was blocked by applying the GABA(A) agonist muscimol to the dorsal surface of the spinal cord. Cats were anesthetized with alpha-chloralose and urethane, and a laminectomy was performed. With the exception of the L(7) dorsal root, the dorsal and ventral roots from L(5) to S(2) were sectioned on one side, and static contraction of the ipsilateral triceps surae muscle was evoked by electrically stimulating the peripheral ends of the L(7) and S(1) ventral roots. The dorsal surface of the L(4)--S(3) segments of the spinal cord were enclosed within a "well" created by applying layers of vinyl polysiloxane. Administration of a 1 mM solution of muscimol (based on dose-response data) into this well abolished the reflex pressor response to contraction (change in mean arterial blood pressure before was 47 +/- 7 mmHg and after muscimol was 3 +/- 2 mmHg). Muscle stretch increased mean arterial blood pressure by 30 +/- 8 mmHg before muscimol, but after drug application stretch increased MAP by only 3 +/- 2 mmHg. Limiting muscimol to the L(7) segment attenuated the pressor responses to contraction (37 +/- 7 to 24 +/- 11 mmHg) and stretch (28 +/- 2 to 16 +/- 8 mmHg). These data suggest that the dorsal horn of the spinal cord contains an obligatory synapse for the pressor reflex. Furthermore, these data support the hypothesis that branches of primary afferent neurons, not intraspinal pathways, are responsible for the multisegmental integration of the pressor reflex.

The pressor reflex evoked by static contraction: neurochemistry at the site of the first synapse

Stimulation of somatic sensory neurons activates the sympathetic nervous system, in turn enhancing cardiovascular function. This has been repeatedly demonstrated when afferent fibers arising from skeletal muscle serve as the sensory neurons. Over the past several years, studies have been performed examining the central nervous system (CNS) mechanisms that cause the reflex increases in arterial blood pressure and heart rate when skeletal muscle contracts. These studies have provided insights into how the CNS alters cardiovascular function, and have helped to enhance our understanding of central sensory transduction processes. Using a variety of techniques, several sites have been identified within the brain and spinal cord that are responsible for producing the reflex pressor response to static contraction. However, the purpose of this manuscript is to review the recent developments concerning only one CNS site: the dorsal horn of the spinal cord. This region serves as the first synapse for afferent fibers from skeletal muscle. The release of neurotransmitters, and possibly neuromodulators, into this region initiates the CNS component of this reflex. In addition, the magnitude of the reflex cardiovascular changes can be modulated at this site. The studies described in this review suggest that the dorsal horn of the spinal cord serves as an important site of integration for sensory signals that influence the cardiovascular system.