Peripheral factors influencing expression of pressor reflex evoked by muscular contraction.

The effect of evoked muscle tension, active muscle mass, and fiber-type composition on the pressor reflex evoked by muscular contraction was examined in decerebrate and anesthetized cats. Muscular contraction was induced by stimulating the L7 and S1 ventral roots with 0.1-ms duration pulses three times motor threshold at various frequencies. The experiments were designed to isolate the variable under study as much as possible and included the use of selectively denervated preparations to limit contractions to specific muscles. It was found that altering the evoked tension by varying the resting muscle length had commensurate effects on the pressor reflex (greater evoked tension caused a larger reflex). In addition it was found that changing the amount of active muscle mass caused similar changes in the reflex (the smaller the muscle mass, the smaller the reflex). Finally, it was found that contrary to other accounts, pressor reflexes could be evoked by activation of the slow-twitch muscle soleus, composed exclusively of red (type I) fibers.

Caudal ventrolateral medullary cells responsive to muscular contraction.

The pressor reflex evoked by muscular contraction (exercise pressor reflex) is held to be an important mechanism in producing the cardiovascular adjustments to static exercise. Recent experiments using lesioning and metabolic labeling methods have indicated that the caudal ventrolateral medulla may be a key integrative site for the reflex evoked by muscular contraction induced by ventral root stimulation. Therefore, we sought to determine whether cells in this region could be associated with the cardiovascular reflex accompanying muscular contraction through analysis of their discharge characteristics. Eighty cells were characterized as to their response to ventral root stimulus-induced static muscular contraction, intra-arterial capsaicin (selective groups III and IV stimulus), and mechanical probing. The cells' receptive fields were also determined by mechanical probing. The receptive fields were usually large, often including all four limbs and the trunk. Four response patterns were observed to static contractions: a brisk initial discharge followed by a gradual return toward control levels (slowly adapting), a brief onset and cessation response, a brief inhibition followed by a slowly adapting discharge, and inhibition alone. Virtually all cells tested were responsive to capsaicin. Histological analysis verified the position of the recorded cells. It is suggested that the cells most likely to participate in the pressor response to muscular contraction were those cells in the general region of the lateral reticular nucleus which responded with an initial and sustained discharge and the cells that were inhibited in the region of the nucleus ambiguus (possible inhibition of vagal outflow).

Pressor reflex evoked by muscular contraction: contributions by neuraxis levels.

The pressor reflex evoked by muscular contraction (exercise pressor reflex) is one important model of cardiovascular adjustments during static exercise. The central nervous system (CNS) structures mediating this reflex have remained largely obscure. Therefore, we examined the contribution of selected levels of the neuraxis in mediating the pressor reflex evoked by muscular contraction from stimulation of ventral roots. Decerebrate cats exhibited larger pressor reflexes than those found in intact alpha-chloralose-anesthetized cats, a difference more apparent at low (5 Hz or repeated twitch) rather than at high (50 Hz or tetanic) stimulus frequencies. Although a depressor response to 5-Hz stimulation was observed in the intact anesthetized cats, it appeared to be primarily due to anesthetic level, since a depressor response was not observed in decerebrate animals (nonanesthetized). Cerebellectomy produced no changes in the reflexes of the decerebrate animal. Further transection of the neuraxis (caudal to the midcollicular level) attenuated the exercise pressor reflex. The spinal cat demonstrated slight evidence of exercise pressor reflex activity. These results provide clarification as to representation of this pressor reflex within the CNS and establish the reflex's characteristics at several levels of neuraxis integration.

Effects of naloxone on the cardiovascular responses to muscular contraction in decerebrate cats.

The cardiovascular responses to muscular contraction induced by ventral root (L7 and S1) stimulation were studied in unanesthetized decerebrate cats before and after the administration of the opiate antagonist naloxone. Intravenous naloxone (1.0-2.0 mg/kg) did not alter the heart rate or arterial pressure responses to either tetanic or repeated twitch contractions. However, naloxone did increase resting arterial pressure.

Microinjection of GABA antagonists into the posterior hypothalamus elicits locomotor activity and a cardiorespiratory activation.

It is well known that electrical stimulation of an area (subthalamic locomotor region, STLR) of the posterior hypothalamus evokes locomotion as well as increases in cardiorespiratory activity. Uncertainty exists over whether these responses are due to stimulation of STLR neurons or to activation of fibers of passage originating outside this area. The purpose of the present study was to determine if stimulation of STLR neurons alone would elicit the cardiorespiratory and locomotor responses. Neurons were stimulated by microinjections of gamma-aminobutyric acid (GABA) antagonists (picrotoxin and bicuculline) into the posterior hypothalamus of anesthetized cats. Both picrotoxin and bicuculline produced increases in arterial pressure, heart rate and minute ventilation which were accompanied by locomotor movements of the limbs. Increases in arterial pressure, heart rate and phrenic nerve activity were also caused by picrotoxin microinjections in paralyzed, ventilated cats. Microinjections of GABA or a GABA agonist (muscimol) reversed all of these responses. In contrast, microinjection of GABA or muscimol into the STLR without a prior antagonist injection had only small, transient effects upon cardiorespiratory activity. However, microinjection of muscimol prevented the responses to a subsequent injection of bicuculline. These results indicate that: (1) stimulation of cell bodies alone in the subthalamic locomotor region of the hypothalamus produces all the cardiorespiratory and locomotor responses evoked by electrical stimulation; (2) the responses evoked by picrotoxin and bicuculline are due to an interaction with GABA receptors and (3) a GABAergic mechanism exerts a tonic depressive influence over the cardiorespiratory and locomotor systems by an action in the posterior hypothalamus.

The exercise pressor reflex: evidence for an afferent pressor pathway outside the dorsolateral sulcus region.

Pressor reflexes evoked by muscle contraction following stimulation of the cut distal portions of the L7 and S1 ventral roots were studied in decerebrate unanesthetized cats. Reflex responses evoked by this simulated exercise persisted after lesions were made in the dorsolateral sulcus region of the T13-L1 spinal cord, indicating that this area is not essential for mediation of these reflexes. Additional evidence suggested that the pathway responsible for the exercise pressor reflex located in the surviving spinal cord is most likely bilateral.

Cardiovascular reflexes arising from the gallbladder and pancreas in spinal and in decerebrate cats.

Cardiovascular responses to chemical stimulation of thin-fiber afferents from the gallbladder and pancreas were determined before and after C1 transection of the spinal cord in cats. Additional cats were studied before and after decerebration. Stimulation of gallbladder and pancreatic afferents caused significant increases in arterial pressure and heart rate in all groups; however, smaller responses often occurred in spinal cats. These results demonstrate that spinal circuitry alone can generate cardiovascular responses to visceral stimulation and that supraspinal, though not necessarily suprapontine areas, are involved in producing the full response.

Immunoneutralization of substance P attenuates the reflex pressor response to muscular contraction.

We previously reported that subarachnoid injection of a peptide antagonist to substance P attenuated by half the reflex pressor response to static muscular contraction. Subsequently, some of the peptide antagonists to substance P have been found to possess local anesthetic effects. Therefore, we have repeated our experiments using a substance P antiserum, which was shown to be without local anesthetic effect. We found that intrathecal injection of the antiserum attenuated by more than half the reflex pressor response to static contraction of the triceps surae muscles of cats.

Identification of diencephalic and brainstem cardiorespiratory areas activated during exercise.

The purpose of this study was to identify diencephalic and brainstem sites active during exercise (EX) in conscious rats running on a treadmill. Brain areas active during exercise, compared to rest conditions (non-EX), were identified using immunocytochemical labelling of the protein product of the proto-oncogene c-fos. Increased labelling was observed in the 'defence area' or 'hypothalamic/subthalamic locomotor regions' including the posterior and lateral hypothalamic areas. Increased labelling with EX was found in both colliculi, the periaqueductal gray matter, the parabrachial complex and the cuneiform nucleus ('mesencephalic locomotor region'). Increased labelling with EX was also found in the medial portion of n. tractus solitarius, and both the rostral and caudal ventrolateral medulla. Conspicuous by an absence of labelling during EX were cells in thalamic areas associated with somatosensory function, although the dorsal column nuclei were also labelled above control. Thus, areas in which labelling was increased during exercise closely correlate with the brain areas which have been implicated in both autonomic and somatomotor control. These results from awake, exercising rats support those obtained previously in anesthetized animal preparations.

Effects of muscular contraction on discharge patterns of neurons in the medullary raphe nuclei.

Cells of the medullary raphe nuclei were characterized as sympathoinhibitory (SI), sympathoexcitatory (SE) or serotonergic (5-HT). When muscular contraction (MC) was evoked by stimulation of the L7 and S1 ventral roots, putative SI cells were inhibited while putative SE cells were excited. 5-HT cells were unaffected by MC. These data are discussed in relation to integration of somatosensory and cardiovascular reflexes.

Activation of caudal brainstem cell groups during the exercise pressor reflex in the cat as elucidated by 2-[14C]deoxyglucose.

Cell groups of the caudal brainstem were labeled with 2-[14C]deoxyglucose during the pressor response evoked by contraction of hindlimb muscles (exercise pressor reflex). The nuclear groups which were labeled in excess of control levels included: the lateral reticular nucleus, the inferior olive (medial accessory olive), and the lateral tegmental field (adjacent to the lateral reticular nucleus).

Cardiorespiratory responses to chemical stimulation of the caudal most ventrolateral medulla in the cat.

Chemical stimulation of caudal ventrolateral medulla evoked both pressor and depressor responses. The pressor sites were generally located caudal to depressor sites. Effects on heart rate were variable. Significant increases in minute ventilation were also observed, which were primarily due to changes in respiratory frequency.

Localization of tyrosine hydroxylase and phenylethanolamine N-methyltransferase immunoreactive cells in the medulla of the dog.

The tyrosine hydroxylase (TH)- and phenylethanolamine N-methyltransferase (PNMT)-immunoreactive cells of the medulla are closely associated with cardiovascular control in both the cat and rat. Although it is often the species of choice for cardiovascular studies, no previous study had characterized these cell groups in the dog. The TH- and PNMT-immunoreactive cells of the dog were distributed much as they are in both cat and rat but with some species variations, which may be indicative of their functional role.

Distribution of cell bodies for primary afferent fibers from the stomach of the cat.

The distribution of primary afferent cell bodies supplying the stomach of the cat was localized using lectin-conjugated horseradish peroxidase. Labelled cells were found in the nodose ganglia and dorsal root ganglia T4-L2 or T4-L1. The spinal entry levels of the stomach afferents do not overlap extensively with those of the cardiac afferents.

Anatomical distribution of flexor carpi radialis and flexor carpi ulnaris motor nuclei in the cat spinal cord.

The distribution of motoneurons innervating the flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU) muscles was studied utilizing retrograde labeling with horseradish peroxidase (HRP). The motor nuclei showed considerable differences in their longitudinal extents. The FCR nucleus occupied spinal segments C6-T1 while FCU ranged from the C7-C8 junction to T1. Results with localized i.m. injection of HRP suggested a somatotopic distribution of these motoneurons.

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

The medullary lateral tegmental field (LTF) has a major role in sympathetic nerve discharge (SND) rhythmicity, but its role in pressor reflexes generated by hindlimb muscular contraction (MC) is unknown. Therefore, two sets of experiments were performed in 17 chloralose-urethane anesthetized cats. First, responses of single LTF neurons to MC induced by L7-S1 ventral root stimulation were examined. The majority (30 of 47) of LTF neurons increased firing during MC. Most LTF neurons had a basal discharge correlated with the 2-10 Hz rhythm of SND or the cardiac cycle and responded to increases in blood pressure. Only seven neurons were inhibited by MC, most having a respiratory rhythm. Second, pressor responses to MC and to caudal hypothalamic stimulation were examined before and after bilateral LTF microinjections of a synaptic blocker (CoCl2) as well as with lidocaine. Microinjection of CoCl2 or lidocaine significantly attenuating the dominant 2-10 Hz power coefficient of SND had no effect on the pressor responses to MC or caudal hypothalamic stimulation. Therefore, LTF may be important for basal rhythms in SND and may help synchronize SND during MC, but its contribution to basal rhythms is apparently not required for pressor reflexes evoked by hindlimb MC or hypothalamic stimulation.

Cardiovascular responses to chemical stimulation of the inferior olive in the cat.

Previous studies have suggested that electrical stimulation of the inferior olivary nucleus of anesthetized cats does not alter arterial pressure but does inhibit the depressor response to baroreceptor stimulation. However, it was not determined if the observed alteration of the baroreceptor reflex was due to an effect on cell bodies or on fibers of passage. The purpose of the present study was to determine the responses to selective activation of cell bodies in the inferior olive in anesthetized cats. Unilateral microinjections of kainic acid, D,L-homocysteic acid and glutamate were made into the inferior olive at the level of or just rostral to the obex. In addition, the baroreceptor reflex was examined before and after microinjection of kainic acid. The microinjections produced an increase in arterial pressure accompanied by variable effects on heart rate. However, the cardiovascular responses to activation of arterial baroreceptors were not altered by kainic acid microinjections. These results indicate that neurons in the inferior olivary nucleus can exert effects which increase arterial pressure but the inferior olive at this level does not modulate the baroreceptor reflex in anesthetized cats.

Central connections of the ovine olfactory bulb formation identified using wheat germ agglutinin-conjugated horseradish peroxidase.

Pheromonal stimuli elicit rapid behavioral and reproductive endocrine changes in the ewe. The neural pathways responsible for these effects in sheep are unknown, in part, because the olfactory bulb projections have not been examined in this species. Using the anterograde and retrograde neuronal tracer, wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP), we describe the afferent and efferent olfactory bulb connections of the Suffolk ewe. Injections of WGA-HRP limited to the main olfactory bulb resulted in retrograde labeling of cells in numerous telencephalic, diencephalic, and metencephalic regions. Terminal labeling was limited to layer la of ipsilateral cortical structures extending rostrally from the anterior olfactory nucleus (AON), piriform cortex, anterior-, and posterolateral-cortical amygdaloid nuclei to lateral entorhinal cortex caudally. Injections involving the accessory olfactory bulb and AON produced additional labeling of cells within the bed nucleus of the stria terminalis (BNST), medial nucleus of the amygdala, and a few cells in the posteromedial cortical nucleus of the amygdala. Terminal labeling included a small dorsomedial quadrant of BNST and also extended to the far lateral portions of the supraoptic nucleus. A clearly defined accessory olfactory tract and nucleus was not evident, perhaps due to limitations in the sensitivity of the method. With this possible exception, the afferent and efferent olfactory connections in the sheep appear similar to those reported for other species.

A re-evaluation of the effects of gonadal steroids on neuronal activity in the male rat.

Single unit activity (SUA) was recorded from 77 cells located in the arcuate nucleus (ARC) and medial preoptic area (MPA) of anesthetized, intact male rats. Animals were administered vehicle, testosterone (T; 5 or 50 micrograms) or 17 beta-estradiol (E; 0.5 microgram) intravenously and SUA was monitored for 8-12 min. T (50 micrograms) reduced SUA in 50% of ARC units and 44% of MPA units within 2.1 +/- 0.46 and 3.3 +/- 0.92 min, respectively. Inhibition of ARC SUA was more pronounced than MPA SUA. A small percentage (9%) of ARC units were excited by T. E reduced SUA in 29% of ARC units and 27% of MPA units. Single doses of 5 micrograms T did not affect ARC activity. However, when followed within 10 min by an additional dose of 5 or 50 micrograms T, 30% and 43% of ARC units were inhibited, respectively. Doses (10 micrograms) of T produced plasma T concentrations within physiological limits, although 50 micrograms doses produced supraphysiological T levels. Neither dose affected circulating LH concentrations. We conclude that physiological and supraphysiological concentrations of T can rapidly affect SUA within the ARC.

Cardiorespiratory responses to stimulation of the nucleus reticularis gigantocellularis.

The nucleus reticularis gigantocellularis (NGC) has been shown to be involved in somatosensory and somatomotor functions. The purpose of the present study was to determine, in anesthetized cats, the modulatory influence of the portion of the NGC at the ponto-medullary border on respiratory and cardiovascular control. Electrical stimulation (25-100 microA 70 Hz, and 1.0-msec pulse duration) significantly depressed mean arterial pressure, heart rate, breathing frequency, tidal volume and phrenic amplitude. Chemical stimulation of NGC cell bodies (1.0 M L-glutamate or 10(-3) M kainic acid) elicited similar decreases in ventilation, arterial pressure and heart rate. These results show that selective activation of cell bodies in the ponto-medullary NGC can depress, in parallel, respiratory and cardiovascular activity and suggests that the influence of diverse sensory information within this region of the reticular formation must be inhibitory to respiratory and cardiovascular output.