Intrathecal serotonin attenuates the pressor response to static contraction

Serotonin (5-HT)-mediated activation of 5-HT1 receptors oppositely modulates CaV2.2 currents in rat sensory neurons innervating hindlimb muscle

Serotonin (5-HT) is a multifaceted neurotransmitter that has been described to play a role as a peripheral inflammatory mediator when released in ischemic or injured muscle. Dorsal root ganglia (DRG) neurons are key sensors of noxious stimuli that are released under inflammatory conditions or mechanical stress. Little information is available on the specific 5-HT receptor subtypes expressed in primary afferents that help regulate reflex pressor responses. In the present study, the whole-cell patch-clamp technique was employed to examine the modulation of voltage-gated calcium channel (Ca_V) 2.2 currents by 5-HT and to identify the 5-HT receptor subtype(s) mediating this response in acutely dissociated rat DRG neurons innervating triceps surae muscle. Our results indicate that exposure of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-labeled DRG neurons to 5-HT can exert three modulatory effects on Ca_V currents: high inhibition, low inhibition, and enhancement. Both 5-HT-mediated inhibition responses were blocked after pretreatment with pertussis toxin (PTX), indicating that 5-HT receptors are coupled to Ca_V2.2 via Gα _i/o protein subunits. Application of selective serotonin receptor type 1 (5-HT1) agonists revealed that modulation of Ca_V2.2 currents occurs primarily after 5-HT1A receptor subtype stimulation and minimally from 5-HT1D activation. Finally, the intrathecal administration of the selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), significantly (P < 0.05) decreased the pressor response induced by intra-arterial administration of lactic acid. This suggests that 5-HT1A receptors are expressed presynaptically on primary afferent neurons innervating triceps surae muscle. Our findings indicate that preferential stimulation of 5-HT1 receptors, expressed on thin fiber muscle afferents, serves to regulate the reflex pressor response to metabolic stimuli. SIGNIFICANCE STATEMENT: The monoamine serotonin (5-HT), released under ischemic conditions, can contribute to the development of inflammation that negatively affects the exercise pressor reflex. The 5-HT receptor subtype and signaling pathway that underlies calcium channel modulation in dorsal root ganglia afferents, innervating hindlimb muscles, are unknown. We show that 5-HT can either block (primarily via serotonin receptor type 1 (5-HT1)A subtypes) or enhance voltage-gated calcium channel (Ca_V2.2) currents. Our findings suggest 5-HT exhibits receptor subtype selectivity, providing a complexity of cellular responses.

Glutamatergic receptor dysfunction in spinal cord contributes to the exaggerated exercise pressor reflex in heart failure

Excitatory amino acids (e.g., glutamate) released by contraction-activated skeletal muscle afferents into the dorsal horn of the spinal cord initiate the central component of the exercise pressor reflex (EPR) in physiological conditions. However, the role of glutamate and glutamate receptors in mediating the exaggerated EPR in the chronic heart failure (CHF) state remains to be determined. In the present study, we performed microinjection of glutamate receptor antagonists into ipisilateral L4/L5 dorsal horns to investigate their effects on the pressor response to static contraction induced by stimulation of the peripheral end of L4/L5 ventral roots in decerebrate sham-operated (sham) and CHF rats. Microinjection of glutamate (10 mM, 100 nl) into the L4 or L5 dorsal horn caused a greater pressor response in CHF rats compared with sham rats. Furthermore, microinjection of either the broad-spectrum glutamate receptor antagonist kynurenate (10 mM, 100 nl) or the N-methyl-d-aspartate (NMDA) receptor antagonist dl-2-amino-5-phosphonovalerate (50 mM, 100 nl) or the non-NMDA-sensitive receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (5 mM, 100 nl) into L4/5 dorsal horns decreased the pressor response to static contraction in CHF rats to a greater extent than in sham rats. Molecular evidence showed that the protein expression of glutamate receptors (both non-NMDA and NMDA) was elevated in the dorsal horn of the lumbar spinal cord in CHF rats. In addition, data from microdialysis experiments demonstrated that although basal glutamate release at the dorsal horn at rest was similar between sham and CHF rats (225 ± 50 vs. 260 ± 63 nM in sham vs. CHF rats, n = 4, P > 0.05), CHF rats exhibit greater glutamate release into the dorsal horn during muscle contraction compared with sham rats (549 ± 60 vs. 980 ± 65 nM in sham vs. CHF rats, n = 4, P < 0.01). These data indicate that the spinal glutamate system contributes to the exaggerated EPR in the CHF state.

Spinal cord GABA receptors modulate the exercise pressor reflex in decerebrate rats

Neurotransmitters and neuromodulators released by contraction-activated skeletal muscle afferents into the dorsal horn of the spinal cord initiate the central component of the exercise pressor reflex (EPR). Whether γ-aminobutyric acid (GABA), a major inhibitory neurotransmitter within the mammalian central nervous system, is involved in the modulation of the EPR at the level of dorsal horn remains to be determined. We performed local microinjection of either the GABA(A) antagonist bicuculline or the GABA(B) antagonist CGP 52432 into the ipisilateral L4/L5 dorsal horns to investigate the effect of GABA receptor blockade on the pressor response to either static contraction induced by stimulation of the peripheral end of L4/L5 ventral roots, passive stretch, or hindlimb arterial injection of capsaicin (0.1 μg/0.2 ml) in decerebrate rats. Microinjection of either bicuculline (1 mM, 100 nl) or CGP 52432 (10 mM, 100 nl) into the L4/5 dorsal horns significantly increased the pressor and cardioaccelerator responses to all stimuli. Microinjection of either bicuculline or CGP 52432 into the L5 dorsal horn significantly increased the pressor and cardioaccelerator responses to direct microinjection of l-glutatmate (10 mM, 100 nl) into this spinal segment. The disinhibitory effect of both GABA receptor antagonists on the EPR was abolished by microinjection of the broad-spectrum glutamate receptor antagonist kynurenate (10 mM/100 nl). These data suggest that 1) GABA exerts a tonic inhibition of the EPR at the level of dorsal horn; and 2) that an interaction between glutamatergic and GABAergic inputs exist at the level of dorsal horn, contributing to spinal control of the EPR.

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.

Brief stimulation of the peroneal nerve attenuates the exercise pressor reflex in anaesthetised cats

We recently demonstrated that applying capsaicin to the common peroneal nerve, thereby activating small diameter afferent neurons, caused a substantial rise in mean arterial pressure (MAP) and heart rate (HR) that lasted approximately 20 min. In addition, this application of capsaicin transiently attenuated the exercise pressor reflex (EPR). The purpose of the current study was to test the hypothesis that stimulating the peroneal nerve at an intensity that activated both myelinated and unmyelinated axons for a short duration (1 min) causes a similar attenuation of the EPR. Cats were anaesthetised with alpha-chloralose and urethane, the popliteal fossa was exposed, and static contraction was induced by stimulating the tibial nerve. The ipsilateral peroneal nerve was cut and placed on a stimulating electrode. Prior to peroneal nerve stimulation, static contraction of the triceps surae muscle for 1 min increased MAP 48+/-8 mmHg and HR 16+/-3 bpm. Electrical stimulation of the central end of the cut peroneal nerve for 1 min (100 x motor threshold; 40 Hz; 0.1 ms) increased MAP and HR by 62+/-11 mmHg and 28+/-4 bpm, respectively. These increases returned to prestimulation levels within 1 min. Two minutes after the peroneal stimulation was stopped, the EPR was markedly reduced as muscle contraction increased MAP and HR by 20+/-4 mmHg and 7+/-2 bpm, respectively. Repeating the muscle contraction approximately 25 min after peroneal stimulation increased MAP and HR by 38+/-8 mmHg and 12+/-2 bpm, indicating some recovery of the EPR. These results show that brief (1 min) electrical stimulation of afferent neurons in the peroneal nerve attenuates the EPR. This supports the hypothesis that strong activation of small diameter afferent neurons stimulates a nervous system mechanism that diminishes the sensory input from skeletal muscle involved in cardiovascular regulation.

Spinal modulation of the muscle pressor reflex by nitric oxide and acetylcholine

Static contraction of skeletal muscle activates the sympathetic nervous system, which in turn increases cardiovascular function. These changes are mediated, in part, by a reflex arising from the contracting muscle. This reflex is termed the exercise pressor reflex or, more simply, the muscle pressor reflex (MPR). Over the past few years, studies have been performed investigating the sensory processing that occurs in the dorsal horn of the spinal cord as it pertains to the MPR. Several putative neurotransmitters and receptors have been implicated in mediating the MPR at the level of the dorsal horn. In addition, several receptor systems have been shown to modulate the MPR at the dorsal horn. We have recently performed studies investigating the potential modulatory role of dorsal horn nitric oxide (NO) and acetylcholine (ACH) on the MPR. Along these lines, our experiments suggest that NO enhances the excitability of dorsal horn cells receiving input from muscle afferent neurons, while ACH decreases the MPR when its concentration in the dorsal horn is elevated. The purpose of this manuscript is to review recently published findings from our laboratory and apply this information in an effort to better understand the integration of sensory input that occurs in the dorsal horn as it pertains to cardiovascular regulation. This review is also designed to stimulate questions as to how these two neurochemicals exert their actions and whether or not they represent or can represent important physiological mechanisms involved in regulating the dorsal horn integration of the MPR.

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.