Thesis (M.S.)--University of Missouri-Columbia, 2008. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Includes bibliographical references.

Sudden Infant Death Syndrome (SIDS) is the sudden death of an infant younger than one year of age that remains unexplained after a complete investigation. For these infants, many different reasons have been hypothesized as to the cause of these deaths including: inherent vulnerability and improper hypoxic arousal. Studies done in other laboratories have shown that there seems to be a reduction in the levels of the neurotransmitter serotonin (5-HT) in the neurons of the raphe, extra-raphe, and ventral populations along with projection sites of these neurons. The huge implications of 5-HT in the control of respiration, prompted animal model studies to further investigate a potential connection between 5-HT and SIDS. 5-HT deficient mice were engineered by knocking out the Pet-1 transcription factor so that knockout mice only retained 30-40% of their brainstem 5-HT neurons. By comparing these 5-HT deficient Pet-1 knockout mice to wild-type mice, it was demonstrated that 5-HT deficient mice failed to autoresuscitate themselves after repeated bouts of hypoxia. Intriguingly, these mice only experienced an autoresuscitation deficit during a specific time period during development. To further evaluate the pathological development behind this behavior issue, in the current study we utilized mice that have modified Pet-1-Flpe driver, Egr2-Cre driver, along with a knock-in RC::FPSit allele to observe 5-HT development in the brainstem in a mature adult and across the critical period (postnatal days 8 and 13- P8 and P13). The transgenic mouse model Pet1-Krox20 gives us a way of exploring a specific subset of 5-HT neurons that rise from the developmental rhombomeres r3 and r5. The use of the knock-in RC::FPSit allele allows us to view the axonal projections of these specific 5-HT neurons by utilizing the presynaptic marker synaptophysin-GFP. This model (PKSit) will allow us to target 5-HT neurons that are implicated in respiration. We chose to compare two projection targets of the PKSit 5-HT neuron subtype through the vulnerable period of development and mature adult mouse: the Locus Coeruleus (LC) and the Nucleus Tractus Solitarius (NTS). In this study we tested the amount of colabeling between 5-HT and GFP in the LC and NTS at P8, P13, as well as the mature adult. We hypothesize that the LC undergoes significant serotonergic axon development and increases colocalization with GFP labeled axon projections between the ages of P8 and P13. We sliced mouse brains and ran immunofluorescence before taking confocal images. By utilizing ImageJ software to run colocalization analysis on the images obtained, we were able to quantify the amount of 5-HT labeled axon projections that are colocalized with GFP labeled axon projections. The parameters we used to quantify the amount of colocalization include the Pearson's Coefficient (PC), Mander's Coefficient (M1/M2), Cytofluorograms, Costes' Method, and van Steensel's Cross-Correlation Coefficient (CCF). We found that the LC shows significant changes with age in the colocalization of 5-HT with GFP while the NTS does not exhibit significant changes with age. The significant changes found in the LC 5-HT/GFP expression between the ages of P8 and P13 suggest one possible cause of failure of arousal. At P8, this lack of 5-HT colabeling with GFP projections suggests that there is some development occurring, which prevents the proper function of 5-HT. At P13, there is a significant increase in the colabeling of 5-HT with GFP, which indicates that the Pet1-Krox20 lineage is actively using 5-HT. The colocalization studies demonstrate that as the mouse ages, the amount of 5-HT labeling with GFP-synaptophysin in the NTS stays the same. The lack of overlap even in mature adult mice suggests that the expression of 5-HT in GFP labeled projections is not necessary. This colocalization study shows that there is an effect of age on the development of the serotonergic system in the LC, but no effect of age in the NTS. While this demonstrates that there is a critical period of development in relation to the LC, it is only one aspect of why mice pups failed to respond to repeated bouts of hypoxia.

Oxygen-sensing cells release neurotransmitters, including serotonin, in response to hypoxia in the blood or surrounding air/water. This stimulates the glossopharyngeal and/or vagal nerves, triggering increased ventilation via the respiratory reflex. In the adult, they are located in the carotid body (glomus cells) and lung epithelia (pulmonary neuroendocrine cells) of amniotes, and in the epithelia of the gills and orobranchial cavity (‘neuroepithelial cells’) of anamniotes. Despite their physiological importance, little is known about the molecular mechanisms of their development, while the evolutionary relationships between the various oxygen-sensing cell types are unknown. The chromaffin cells of the mammalian adrenal medulla are hypoxia-sensitive transiently during neonatal life. Both carotid body glomus cells and adrenal chromaffin cells arise from the neural crest and require the transcription factors Phox2b and Ascl1 for their development. Given these similarities, I aimed to test the hypothesis that the same molecular mechanisms underlie their development. Expression analysis of 13 sympathoadrenal pathway genes throughout chicken carotid body development revealed striking similarities with adrenal chromaffin cell development. Analysis of mouse mutants showed that the transcription factors Hand2, Sox4 and Sox11 are required for carotid body development. In addition, loss of the receptor tyrosine kinase Ret or the transcription factor AP-2β, which significantly affects sympathetic ganglion but not adrenal chromaffin cell development, has no effect on the carotid body. Adrenal chromaffin cells differentiate from neurons that migrate into the adrenal gland from ‘primary’ sympathetic ganglia at the dorsal aorta. Carotid body glomus cells were previously proposed to arise from neuronal “émigrés” from neighbouring ganglia: the superior cervical ganglion in mammals and the nodose ganglion in the chick. However, nodose neurons are considered to be nodose placode-derived. Using electroporation and grafting in the chick, I confirmed that the nodose placode does not contribute to the carotid body, identified a small population of autonomic neural crest-derived neurons in the nodose ganglion, and confirmed the existence of “bridges” of neurons between the nodose ganglion and the carotid body. My data suggest that, like adrenal chromaffin cells, carotid body glomus cells differentiate from autonomic neural crest-derived neurons in nearby ganglia, which migrate into the carotid body primordium and down-regulate neuronal markers. The proposed evolutionary relationship between the carotid body glomus cells and the serotonin-positive neuroepithelial cells of anamniote gills has never been tested. Using vital dye labelling, neural fold grafts, genetic lineage-tracing in zebrafish and analysis of zebrafish mutants lacking all neural crest cells, I found that serotonin-positive cells in the gills and orobranchial epithelia of lamprey (jawless fish), zebrafish (ray-finned bony fish) and frog (anamniote tetrapod) are not neural crest-derived, and hence are not homologous to carotid body glomus cells. Genetic lineage-tracing in mouse and neural fold grafts in chick also confirmed that serotonin-positive neuroendocrine cells in the lung are not neural crest- derived, hence must have an endodermal origin (since the lungs are out-pocketings of the gut). My results suggest that the neuroepithelial cells of anamniotes are not related to carotid body glomus cells, but rather are homologous to the oxygen-sensing cells of the lung. Consistent with this hypothesis, I found that many genes expressed during carotid body development are not expressed by the epithelia of either chick lungs or lamprey gills. Taken together, my data suggest that as air-breathing evolved, gut endoderm- derived cells that originally responded to hypoxia in water were maintained in the lungs to monitor oxygen levels in air, while a population of neural crest-derived chromaffin cells near the pharyngeal arch arteries was recruited to monitor oxygen levels in blood.

O ATP mediando a sinalização purinérgica no bulbo ventrolateral rostral contribui para o controle do quimiorreflexo central e periférico regulando a pressão arterial e a respiração, mediante o envolvimento dos neurônios do núcleo retrotrapezóide (RTN). No entanto, as potenciais contribuições da sinalização purinérgica, no RTN, na função cardiorrespiratória em animais não anestesiados ainda não foram testadas. Mostramos que a injeção de ATP no RTN promoveu aumento cardiorrespiratório por um mecanismo dependente de receptores P2. Mostramos também que o bloqueio de receptor P2 não específico (PPADS), mas não de receptores específicos P2Y (MRS2179), reduziu a resposta ventilatória à hipercapnia (7% CO2) e hipóxia (8 % O2) em ratos não anestesiados. Além disso, a adenosina (ADO) no RTN atenuou o aumento da ventilação induzido por hipercapnia in vivo e o disparo dos neurônios in vitro. Estes resultados demonstram que a sinalização mediada por ATP contribui para o controle respiratório do quimiorreflexo central e periférico em ratos acordados e uma vez que o ATP se metaboliza rapidamente em ADO, esta teria ação no balanço da resposta quimiorreceptora no RTN. / ATP-mediated purinergic signaling at the level of the rostral ventrolateral medulla (RVLM) contributes to both central and peripheral chemoreceptor control of breathing and blood pressure within the retrotrapezoid nucleus (RTN). However, potential contributions of purinergic signaling in the RTN to cardiorespiratory function in conscious animals has not been tested. We show that in the absence of functional C1 cells, ATP into the RTN increased cardiorespiratory output by a P2-recepor dependent mechanism. We also show that a non-specific P2 receptor blocker (PPADS) reduced the ventilatory response to hypercapnia (7% CO2) and hypoxia (8% O2) in unanesthetized awake rats. Conversely, a specific P2Y1-receptor blocker (MRS2179) into the RTN had no measurable effect on respiratory responses elicited by hypercapnia or hypoxia. Moreover, adenosine (ADO) into the RTN could attenuate the hypercapnia-induced increase in ventilation in vivo and firing rate in RTN neurons in vitro. These results demonstrate that ATP-mediated purinergic signaling contributes to central and peripheral chemoreflex control of breathing in awake rats and ADO could provide a balance between ATP stimulation and its inhibition in RTN during hipercapnia.

Orientador: Ines Joekes / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-07T21:43:58Z (GMT). No. of bitstreams: 1 Wagner_RitadeCassiaComis_D.pdf: 5300466 bytes, checksum: b220b80949a0b5e00e05ecef6360d5ac (MD5) Previous issue date: 2006 / Resumo: O cabelo é uma estrutura protéica de queratina formada por quatro estruturas principais: as cutículas, o córtex, o cimento intercelular (CMC) e a medula. Esta última nem sempre está presente no fio e, por acreditar-se que sua influência nas propriedades da fibra é negligenciável, foi pouco estudada. Este trabalho almeja esclarecer as estruturas da medula e verificar sistematicamente se ela afeta alguma propriedade na fibra. Os fios medulados e sem medula provenientes do mesmo couro cabeludo foram identificados e separados em mechas utilizando um micro-estereoscópio. Encontraram-se dois tipos de medula (fina e grossa) que foram diferenciadas por microscopia eletrônica de varredura (MEV) e de transmissão (MET). A medula tem uma estrutura esponjosa composta por três unidades principais: a fibrilar (desalinhada com as fibrilas do córtex), os glóbulos e uma camada de CMC na interface com o córtex. Por MET constatou-se que a medula fina é morfologicamente diferente da medula grossa. A medula fina apresenta contraste, interface limitada pelo CMC, nenhum grânulo de melanina e menor diâmetro. A medula grossa apresenta mais glóbulos, bem como maiores dimensões das cavidades, alguns grânulos de melanina disformes e organização gradual das células de fora para dentro. Essas características morfológicas sugerem que a medula seria um córtex em estágio atrasado de formação. Utilizando-se um espectrofotômetro de refletância difusa (ERD), verificou-se que os fios com medula são mais escuros, menos vermelhos e menos amarelos que os fios sem medula oriundos do mesmo couro cabeludo. Sugere-se que as cavidades da medula causem o espalhamento e o confinamento de luz pela diferença de índice de refração com o córtex, diminuindo, então, a reflexão externa da fibra, já que praticamente não possui melanina. Para a mecha estudada, a diferença de cor total está em 4,7 unidades de cor e é visível a olho nu. A medula, então, deve ser considerada em estudos de cor, juntamente com as melaninas e as condições de preservação das cutículas. Já nas propriedades mecânicas, a medula causa uma maior heterogeneidade nas curvas de tensão-deformação, mas não muda os valores médios das propriedades estudadas. Os fios medulados são mais espessos que fios sem medula. Descontando-se o valor do diâmetro da medula do diâmetro total da fibra, têm-se valores aproximados aos dos fios sem medula. A heterogeneidade observada é proveniente da diferença percentual que o diâmetro da medula representa do diâmetro total da fibra. Observaram-se mudanças estruturais quando o cabelo com medula fina é submetido à imersão em SDS 10 % e à 75 °C, transformando-se em medula grossa após os tratamentos / Abstract: Human hair is a keratinous material divided into four main units: cuticles, cortex, intercellular cement (CMC) and medulla. The last one could be present or not in the shafts. There are few studies about it mainly because it is believed to have small or no influence on any hair property. The present work aims to clarify the medulla structure and to systematically verify if mechanical or color properties are affected by its presence in the fiber. Medullated and unmedullated fibers from the same scalp were identified by stereo-microscopy and separated into tresses. Two kinds of medulla were found: thin and thick medulla. Their morphology was characterized by scanning (SEM) and transmission (TEM) electron microscopy. Medulla has a sponge structure composed by three units: fibril (not aligned longitudinally as in the cortex), globular and CMC layer in the interface with the cortex. Thin and thick medullas are different in TEM. Thin medulla has contrast, CMC limited interface, no melanin and smaller diameter. Thick medulla has more globular structures, larger cavities, some elliptical melanin granules and a gradual organization from the outside to the inner side of the medulla. These data suggest that medulla is in an earlier stage of the differentiation process compared to the cortex. Color data obtained using a diffuse reflectance spectrophotometer showed that unmedullated fibers are clearer, redder and yellowier than medullated fibers. Once that medulla presents no melanin, we suggest that the medulla cavities cause scattering and confinement of light by the difference with the refractive index of cortex which decreases the external reflectance of the fiber. For the studied tress, the total color difference was 4.7 (visible to naked eyes). Thus, medulla together with melanin and cuticles must now be considered in studies of hair color. Average values of the mechanical properties are similar for unmedullated and medullated fibers. However, higher dispersion in data for medullated fibers is observed. Unmedulated fibers are more uniform and show smaller diameters. These data indicate that the air cavities in medulla could act as defects but do not interfere in the crystalline character of the fiber. Thus, the heterogeneity observed might be attributed to the difference between the medulla diameter with the whole fiber diameter. Thin medulla regions became thick medulla after immersion in SDS 10 % and after 24 h at 75 °C / Doutorado / Físico-Química / Doutor em Ciências

Both sympathoneural and sympathoadrenal systems are involved in the regulation of arterial blood pressure and in the pathogenesis of hypertension. Spontaneously hypertensive rats (SHR), the mostly used animal model of genetic hypertension, is characterized by multiple molecular, morphological and functional alterations at different levels of sympathoneural and sympathoadrenal systems. The study of young prehypertensive SHR allows to reveal the abnormalities preceding hypertension development, whereas adult SHR with established hypertension offers a better model for the treatment of human essential hypertension. The aim of my PhD Thesis was to describe abnormalities in sympathoneural and sympathoadrenal systems in SHR under different conditions. Firstly, ontogenetic differences which might contribute to hypertension development were determined. Secondly, the effect of chemical sympathectomy induced by guanethidine in adulthood on cardiovascular parameters and on the compensatory mechanisms counteracting the reduction of blood pressure were studied. Thirdly, stress-induced cardiovascular response and stress-induced changes of sympathoneural and sympathoadrenal systems were described in adult SHR. My Thesis brought several important results. The increased adrenal catecholamine content and the...

Spontaneously active primary cultures obtained from dissociated embryonic medial medulla tissue were grown on microelectrode arrays for investigating burst patterns and pharmacological responses of respiratory-related neurons. Multichannel burst rates and spike production were used as primary variables for analysis. Pacemaker-like neurons were identified by continued spiking under low Ca++/high Mg++conditions. The number of pacemakers increased with time under synaptic blocking medium. Sensitivity to CO2 levels was found in some neurons. Acetylcholine changed activity in a complex fashion. Curare, atropine and gallamine modified ACh effects. Eserine alone was ineffective, but potentiated ACh-induced responses. Norepinephrine caused channel-specific increases or decreases, whereas dopamine and serotonin had little effect at 30 μM. GABA and glycine stopped most spiking at 70 μM. Developmental changes in glycine sensitivity (increasing with age) were also observed. It is concluded that pacemaker and chemosensitive neurons develop in medial medulla cultures, and that these cultures are pharmacologically histiotypic.

This work examines somatosensory processing in "giant" neurones of the medial pontomedullary reticular formation (PMRF) in the rat, with particular emphasis on the response to cutaneous thermal stimuli. Thermal test stimuli were employed as these were deemed to be more precisely quantifiable than other forms of cutaneous stimulation. Activity was recorded from 235 PMRF neurones in 94 female Long Evans rats (270 to 320 g) anaesthetised with urethane (1,25g/kg, i.p.). Rectal temperature was closely controlled at 38 ± 0,5°C. Standard stereotactic and extracellular recording techniques were employed. PMRF giant neurones were identified by their stereotactic location, large, stable spike amplitudes of long duration, responses to cutaneous mechanical stimuli and receptive field properties, and spontaneous discharge characteristics. Ramp, step and sine wave cutaneous thermal stimuli (35-48 °C) were applied to the glabrous skin on the hindpaw by means of a computer-controlled Peltier device. The location of the units was confirmed by subsequent histology. One hundred and eleven neurones were located in nucleus reticularis pontis caudalis (NPC), and 124 in nucleus reticularis gigantocellularis (NGC). Mechanical stimulation excited 188 of 235 (80%) PMRF neurones (ON-m cells), and inhibited 40 (17%, OFF-m cells). Seven cells (3%) had mosaic receptive fields of excitation and inhibition (complex responses, CX-m). Twenty-eight percent of neurones were responsive to both weak and intense stimuli (mixed neurones). The remainder (72%) responded only to intense mechanical stimulation of the skin (high threshold neurones). The (excitatory or inhibitory) response of the mixed neurones to intense stimuli was generally greater than to mild stimuli, Receptive fields ranged in size from restricted (hindlimbs only) to very extensive (covering the entire body surface). Neurones with small receptive fields were almost exclusively of the high threshold type, and tended to be located in NGC, while mixed neurones tended to have larger receptive fields, and were located predominantly in NPC. Some portion of the hind limbs were represented in the receptive fields of all but one of the neurones studied, while the tail and/ or trunk were represented in 77%, and the forelimbs and face in 28% of receptive fields. Most of the cells responding to cutaneous mechanical stimulation had bilateral (usually symmetric) receptive fields. Spontaneous (background) activity occurred in the absence of any deliberate sensory stimulation in 72% of PMRF neurones. The frequency of spontaneous discharge rates ranged from O to 47 spikes/ s. The coefficient of variation of the spontaneous discharge rate of a given neurone was generally less than 20% (range O to 85%). Of the 235 identified mechanosensitive PMRF neurones, 203 (86%) also responded to cutaneous thermal stimulation (43-48 °C) of the ipsilateral hind paw. Eighty percent of these responded with increased discharge rates (ON-t cells), and 20% were inhibited (OFF-t cells). The polarities of response of individual PMRF neurones to mechanical and thermal stimuli, and to repeated ipsilateral and contralateral thermal stimuli, did not differ significantly. Following transient thermal stimulation, spontaneous discharge rates largely returned to pre-stimulus levels. The thresholds of response to slow ramp (0,15°C/s) and stepped (2°C/s) thermal stimuli occurred both in the innocuous and noxious temperature ranges (below and above 42°C, respectively). The threshold temperatures showed large variability to repeated identical thermal stimuli. Despite the poor reproducibility of the threshold responses, the distribution of thresholds to thermal ramp stimuli was consistently bimodal, with peaks occurring at 39 and 43°C. The bimodality persisted even when the ipsilateral and contralateral data were pooled. The modes of these threshold distributions conform to the maximum discharge ranges for warm and noxious cutaneous receptors. Thus, it is likely that thermal input to individual PMRF neurones is derived from both types of receptors. The responses of PMRF neurones to repeated thermal stimuli were stable and reproducible with respect to magnitude and time course. The average (static) and maximum (dynamic) responses to thermal stimuli were generally small: for example, the mean of the average responses to ramp stimuli was 5,9 spikes/s ± 11,0 SD, (range -28 to 40 spikes/s), and the mean of the maximum responses was 9,3 spikes/s ± 16,1 SD, (range -46 to 65 spikes/s). The absolute change in firing rate of individual PMRF neurones, and of the population, increased monotonically as a function of the intensity of stepped cutaneous thermal stimuli in the range 40 to 48 °C. However, their resolution, based on their average and maximum responses, was poor. Incorporating the post-stimulus responses into the comparisons between different stimulus intensities marginally increased the resolution of these neurones. Thus, while the majority of PMRF neurones are able to distinguish innocuous from noxious stimuli, few are capable of encoding stimulus intensity within the noxious range (above 43 °C). The majority (70%) of PMRF neurones responded to sustained thermal stimuli with a slow increase or decrease to a new static discharge rate which was maintained with little or no adaptation. Latency to onset of response to stepped thermal stimuli varied from 1 to 50 seconds, and the time to maximal response between 5-60 seconds. Many PMRF neurones also showed marked after-discharge for periods of up to 5 minutes after removal of the stimulus. The thermal receptive fields of over 90% of PMRF neurones were large, incorporating at least both hindlimbs. The extensive receptive field sizes of individual PMRF neurones provides evidence against them having a role in stimulus location. The large number of PMRF neurones showing multimodal convergence, their small magnitude responses, their slow response times, and their large receptive fields strongly suggest that these neurones are not participating in classical sensory discrimination. Rather, they may function as stimulus detectors or alternatively play a role in associative processes.

Cocaine- and amphetamine-regulated transcript (CART) peptide-immunoreactivity was detected in neurons of the rostral ventrolateral medulla (RVLM), but few in the caudal ventrolateral medulla (CVLM). Double-labeling the medullary sections with sheep polyclonal phenylethanolamine N-methyltransferase-antiserum (PNMT) or monoclonal tyrosine hydroxylase-antibody and rabbit polyclonal CART peptide-antiserum revealed that nearly all adrenergic cells in the C1 area were CART peptide-positive and vice versa; tyrosine hydroxylase-positive cells in the A1 area were not. In the thoracolumbar spinal cord, neurons in the intermediolateral cell column (IML) and other sympathetic autonomic nuclei were CART peptide-positive; some of these were contacted by immunoreactive fibers arising from the lateral funiculus. By immuno-electron microscopy, axon terminals containing closely packed agranular CART peptide-immunoreactive vesicles appeared to make synaptic contacts with immunoreactive dendrites and soma in the IML, albeit the incidence of such contacts was low. Microinjection of the retrograde tracer Fluorogold into the lateral horn area of the T1-T3 spinal segments labeled a population of neurons in the C1 area, many of which were also CART peptide-positive. The results indicate that CART peptide-immunoreactivity is expressed in C1 adrenergic neurons, some of which project to the thoracolumbar spinal cord. The presence of this novel peptide in C1 adrenergic neurons underscores the multiplicity of putative transmitters that may be involved in signaling between putative cardiovascular neurons in the medulla oblongata and sympathetic preganglionic neurons (SPNs) in the spinal cord.

Study objective - The aim was to determine whether areas in the periaqueductal grey matter, medial dorsal raphé, or ventrolateral medulla might be involved with the integration of blood pressure and heart rate during isometric exercise.Design - Cats were anaesthetised with α chloralose (75 mg·kg-1) and catheters inserted into the right jugular vein and carotid artery. Isometric contractions were generated using a microprocessor controlled stimulator and sleeve electrode around the tibial nerve. Bilateral lesions were made in the dorsal periaqueductal grey matter (P1.0, LR 2.0, HD + 1.5 mm) or two sites in the ventrolateral medulla (P12.0, RL 2.0, HD -10 mm; or P12.0, RL 2.0, HD -8.5 mm). Lesions were also made in the medial dorsal raphé nuclei (P1.0, RL 0.0, HD +1.5 mm). Clonidine was injected into the cerebral aqueduct to determine whether it would exert an antipressor effect during muscle contraction after the lesions were made. Only one site of lesion was made in a group of animals. Bilateral injections of clonidine (250 ng in 0.5 μl) were made into the intact ventrolateral medulla (P11.5, RL 4.0, HD -8.5 mm) to explore its role further. Fatiguing contractions were performed before and after the lesions were made, or clonidine was injected, and changes in arterial blood pressure and heart rate were measured. Verification of the lesion sites or the microinjection sites, and the extent of the lesion or spread of the clonidine, was made from histological examination of brain tissue after each experiment.Experimental material - Adult cats of either sex, n = 20, weight 2.4 (SD 0.4) kg, were used.Measurements and main results - Fatiguing isometric contractions in control conditions caused mean arterial pressure to increase by 45-50 mm Hg and heart rates by 20-25 beats·min-1. Bilateral lesions in the dorsal periaqueductal grey matter did not alter resting mean arterial pressure but attenuated the pressor response during contractions. Injections of clonidine into the cerebral aqueduct had no further antipressor effects after the lesions. Lesions of the medial dorsal raphé nuclei or injections of clonidine into the intact medial dorsal raphé nuclei did not affect the pressor response to fatiguing isometric contractions. Injections of clonidine into the intact ventrolateral medulla eliminated the pressor response to isometric contractions. Bilateral lesions of the ventrolateral medulla near the rostral lateral border of the inferior olivary tract nuclei (P12.0, LR 2.0, HD -10 mm) also attenuated the muscle pressor response, while subsequent injections of clonidine into the cerebral aqueduct depressed the changes in blood pressure further.Conclusions - Ergoreceptor information may be processed through the periaqueductal grey matter through the ventrolateral medulla to control arterial blood pressure during isometric exercise to fatigue.