Release of immunoreactive neuropeptide Y from brainstem sites in the cat during isometric contractions

Distribution of neuropeptide Y-immunoreactive neurons in the human brainstem, cerebellum, and cortex during development

1. Neuropeptide Y is found throughout the central nervous system where it appears to play a wide range of often poorly understood functions. In this study, the distribution of neuropeptide Y immunoreactive (NPY-ir) neurons in the brainstem, cerebellum, and cerebral cortex of human fetuses ranging in age from 11 gestational weeks to term was investigated by immunohistochemistry. 2. The NPY-ir cells were detected in the dorsal and ventral rostral midbrain and the interpeduncular nucleus by 21 weeks and 32 weeks of gestation, respectively. Although no positive cells were found in the pons, the NPY-ir fibers were detected there at 32 gestational weeks. 3. The vagal, hypoglossal, and olivary nuclei of the medulla oblongata contained immunoreactive cells by week 21 and the medullary reticular formation by week 25 of gestation. In most of these locations, both the number and size of neuropeptide Y positive cells were greater at birth and reached maximal values of 100-400 cells per 1 mm2 and 2-5 microm in diameter, respectively. 4. In the cerebellum, numerous NPY-ir horizontal and granule cells, as well as the cells within the dentate nucleus were observed as early as 21 weeks of gestation. 5. The NPY-ir cells were also detected in the developing cerebral cortex, with the earliest activity observed within the temporal cortex at 14 weeks of gestation. By week 21, positive cells appeared in the visual, frontal, sensory, and motor cortices. Most of these cells were bipolar or multipolar in morphology but their numbers at birth were relatively low. 6. Our results show a wide distribution of the NPY-ir cells in the developing human brain and offer supporting evidence for the important modulatory role of NPY in both the fetus and adult.

c-Fos expression in the midbrain periaqueductal gray during static muscle contraction

The periaqueductal gray (PAG) of the midbrain is involved in the autonomic regulation of the cardiovascular system. The purpose of this study was to determine if static contraction of the skeletal muscle, which increases arterial blood pressure and heart rate, activates neuronal cells in the PAG by examining Fos-like immunoreactivity (FLI). Muscle contraction was induced by electrical stimulation of the L7 and S1 ventral roots of the spinal cord in anesthetized cats. An intravenous infusion of phenylephrine (PE) was used to selectively activate arterial baroreceptors. Extensive FLI was observed within the ventromedial region (VM) of the rostral PAG, the dorsolateral (DL), lateral (L), and ventrolateral (VL) regions of the middle and caudal PAG in barointact animals with muscle contractions, and in barointact animals with PE infusion. However, muscle contraction caused a lesser number of FLI in the VM region of the rostral PAG, the DL, L, and VL regions of the middle PAG and the L and VL regions of the caudal PAG after barodenervation compared with barointact animals. Additionally, the number of FLI in the DL and L regions of the middle PAG was greater in barodenervated animals with muscle contraction than in barodenervated control animals. Thus these results indicated that both muscle receptor and baroreceptor afferent inputs activate neuronal cells in regions of the PAG during muscle contraction. Furthermore, afferents from skeletal muscle activate neurons in specific regions of the PAG independent of arterial baroreceptor input. Therefore, neuronal cells in the PAG may play a role in determining the cardiovascular responses during the exercise pressor reflex.

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.

Ventrolateral medullary control of cardiovascular activity during muscle contraction

An overview of the role of ventrolateral medulla (VLM) in regulation of cardiovascular activity is presented. A summary of VLM anatomy and its functional relation to other areas in the central nervous system is described. Over the past few years, various studies have investigated the VLM and its involvement in cardiovascular regulation during static muscle contraction, a type of static exercise as seen, for example, during knee extension or hand-grip exercise. Understanding the neural mechanisms that are responsible for regulation of cardiovascular activity during static muscle contraction is of particular interest since it helps understand circulatory adjustments in response to an increase in physical activity. This review surveys the role of several receptors and neurotransmitters in the VLM that are associated with changes in mean arterial pressure and heart rate during static muscle contraction in anesthetized animals. Possible mechanisms in the VLM that modulate cardiovascular changes during static muscle contraction are summarized and discussed. Localized administration of an excitatory amino-acid antagonist into the rostral portion of the VLM (RVLM) attenuates increases in blood pressure and heart rate during static muscle contraction, whereas its administration into the caudal part of the VLM (CVLM) augments these responses. Opioid or 5-HT1A receptor stimulation in the RVLM, but not in the CVLM, attenuates cardiovascular responses to muscle contraction. Furthermore, intravenous, intracerebroventricular or intracisternal injection of an alpha 2-adrenoceptor agonist or a cholinesterase inhibitor attenuates increases in blood pressure and heart rate during static muscle contraction. Finally, the possible involvement of endogenous neurotransmitters in the RVLM and the CVLM associated with cardiovascular responses during static muscle contraction is discussed. An overview of the role of the VLM in the overall cardiovascular control network in the brain is presented and critically reviewed.

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.

Neuropeptide Y-like substances are released from the rostral brainstem of cats during the muscle pressor response

1. In this study, the release of immunoreactive neuropeptide Y (NPY)-like substances (irNPYs) was measured from the rostral brainstem in response to fatiguing isometric contractions of the hindlimb muscle of cats anaesthetized with alpha-chloralose. 2. The irNPYs were measured using a solid phase autoradiographic immunoassay. NPY antibody-coated glass microelectrodes were inserted stereotaxically into the rostral brainstem during rest and contractions. Control procedures such as passive leg flexing, stimulation of the central end of the cut tibial nerve, brief repeated carotid occlusions and carotid sinus nerve stimulations were performed to determine whether patterns of release of irNPYs were specific to the pathways activated during the muscle pressor response. 3. During isometric contractions, the mean arterial blood pressure (MABP) and heart rate (HR) increased by 60-70 mmHg and 15-20 beats min-1, respectively. Only the brief carotid occlusions caused a similar increase in MABP and HR. 4. During isometric contractions, less irNPYs were released from sites in the lateral reticular nucleus, lateral tegmental field and vestibular nuclei at a level in the brainstem 4.0-4.4 mm rostral to the obex. In contrast, irNPYs were released from sites near the lateral tegmental field and nucleus ambiguus at a more caudal level tested (1.3 mm anterior to the obex) in response to contractions. Brief carotid occlusions caused a release of irNPYs from the lateral reticular nucleus on the contralateral side of the brainstem (at 3.5 mm anterior to the obex). None of the other procedures resulted in a release of irNPYs from these sites in the brainstem. 5. These data indicate there are differences in the patterns of release of irNPYs in response to activation of reflex pathways which increase arterial pressure. Given the sites in the rostral brainstem where irNPYs were released, irNPYs may be involved with the central regulation of arterial pressure during the muscle pressor response.

Fatiguing isometric contraction of hind-limb muscles results in the release of immunoreactive neurokinins from sites in the rostral medulla in the anesthetized cat

Antibody-coated microprobes were used to determine whether immunoreactive neurokinins (irNK) were released from sites in the brainstem during fatiguing isometric contractions of the triceps surae muscles in cats anesthetized with alpha-chloralose. Contractions were generated by stimulating the tibial nerve using a microprocessor-controlled stimulator. Microprobes were inserted into the periaqueductal grey (P 0.5-1.0 mm) or the medullary brainstem (either 3.0 or 3.5 mm rostral to the obex) prior to, during and following fatiguing contractions. No release of irNK was detected from the periaqueductal grey as a result of fatiguing isometric contractions. When probes were inserted 3.0 mm rostral to the obex, a basal release of irNK was detected from the medulla but this was inhibited during isometric contractions from sites corresponding to the lateral tegmental field. When probes were inserted into the more rostral site in the medulla (3.5 mm rostral to the obex), irNK were released in response to contractions from sites corresponding to lateral reticular nucleus, ventral regions of the nucleus tractus solitarius and the medial vestibular nucleus. No irNK were released from this site (3.5 mm rostral to obex) either during passive leg flexing, during nerve stimulation following gallamine injection and muscle paralysis or during stimulation of the central end of the cut tibial nerve. These results demonstrate that neurokinins are released from discrete sites in the medulla in response to fatiguing muscle contractions and suggest that tachykinin neurons may be a component of the pathways regulating blood pressure during ergoreceptor activation.

Detection of immunoreactive substance P-like substances from cat brainstem sites during fatiguing isometric contractions

Isometric contractions were generated on the left hindlimb muscles from adult cats (n = 11) anesthetized with alpha-chloralose (75 mg/kg) to determine whether immunoreactive substance P (irSP) was released from either the right periaqueductal grey (PAG) or ventrolateral medulla (VLM), sites shown to be involved with the integration of the muscle pressor response. The release of immunoreactive SP was measured using SP antibody-coated microelectrodes that were inserted into the PAG or the VLM during periods of rest, fatiguing isometric contractions and post-contraction. Mean arterial pressure increased by 78 +/- 11 mmHg during the contractions. There was a release of irSP from sites in the medulla during the contractions compared to the non-contraction periods but none was detected from the PAG in response to muscle stimulation. These results provide further evidence that SP-like substances may be involved with the central integration of the muscle pressor response.