The medullary raphe nuclei: a system for integration and gain control in autonomic and somatomotor responsiveness?
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Animals↗
Biomedical subjects
T A Lovick
Publications and source records attributed to T A Lovick.
Inhibitory effect of nitric oxide on neuronal activity in the periaqueductal grey matter of the rat.
Experiments were carried out in urethane-an-anesthetized rats to examine the effect of nitric oxide (NO) on neuronal activity within the dorsolateral sector of the midbrain periaqueductal grey matter (PAG), an area which is rich in NO-synthesizing neurones. NADPH-dependent diaphorase histochemistry revealed small NO synthase-containing perikarya, 15.4 +/- 3.1 microns (mean +/- SEM) in diameter, in a longitudinal column in the dorsolateral sector of the PAG. The labelled cell bodies were surrounded by a dense meshwork of stained fibres and processes in which unlabelled neurones were embedded. In order to establish whether NO was generated when NO donors were ejected iontophoretically from micropipettes, a chemiluminescence method was used to estimate the output of NO in vitro after iontophoresis of two chemically different classes of NO donor: the sydnonimine 3-morpholino-sydnonimin-hydrochloride (SIN 1) and the nitrosothiol S-nitroso glutathione (SNOG). Iontophoresis of both NO donors into 200 microliters aliquots of 165 mM NaCl using ejection currents between 6000 and 18,000 nA.min produced a current-related increase in the concentration of NO. Iontophoresis of SIN 1 in vivo produced a reproducible, current-related inhibition of firing in 40 of 59 neurones in the dorsolateral PAG. In 8 of 10 neurones the effect of SIN 1 was significantly reduced after iontophoresis of methylene blue (10-30 nA for 2.7-5 min). The inhibition took up to 7 min to develop and lasted for up to 13 min. Inhibitory responses to GABA were not affected by methylene blue. Iontophoresis of SNOG also inhibited ongoing activity of 18 of 24 neurones tested in the PAG. The experiments demonstrate firstly that NO donors can be used in vivo to deliver NO in the vicinity of neurones by iontophoresis from micropipettes. Secondly, NO appears to inhibit neuronal activity within the PAG.
Animals↗
Role of nitric oxide in medullary raphe-evoked inhibition of neuronal activity in the periaqueductal gray matter.
In male urethane-anaesthetized rats, activation of neurons in nucleus raphe obscurus and the caudal tip of nucleus raphe magnus by microinjection of 50-100 nl 0.1 M D,L-homocysteic acid produced a 75.6 +/- 5.2% reduction in the firing rate in 25 neurons in the lateral and dorsolateral sectors of the periaqueductal gray matter which lasted for 102.3 +/- 13.3s (mean +/- S.E.M.). The duration of the inhibition was significantly reduced in a dose-dependent manner by intracerebroventricular injection of the inhibitor of nitric oxide synthase, N(w)-nitro-L-arginine methyl ester (50-500 micrograms) but not by N(w)-nitro-D-arginine methyl ester (500 micrograms). In contrast, the magnitude of the raphe-evoked inhibition, i.e. the maximum depression of firing rate, was not significantly affected by either isomer. The results suggest that nitric oxide plays a role in the regulation of the excitability of neurons in the midbrain aversive system by the medullary raphe. The selective effect of the nitric oxide synthase inhibitor on the duration, but not the magnitude, of the raphe-evoked inhibition suggests that nitric oxide is not involved in initiating the inhibition. Rather, its role appears to be in maintaining the raphe-evoked inhibition once it has been initiated by a non-nitrergic mechanism.
Animals↗
Midbrain and medullary regulation of defensive cardiovascular functions.
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Anxiety↗
Distribution of nicotinamide adenine dinucleotide phosphate (NADPH)-dependent diaphorase staining in intraparenchymal blood vessels of the rat brain.
The distribution of nicotinamide adenine dinucleotide phosphate (NADPH)-dependent diaphorase (nitric oxide synthase, NOS) in the endothelial lining of intraparenchymal blood vessels was examined in sections of rat brain prepared from 500 microns thick slices of brain fixed by immersion or from blocks of tissue taken from whole brains fixed by vascular perfusion. In immersion-fixed tissue, a network of stained vessels, many as small as 3 microns in diameter was seen throughout the grey and white matter. In tissue fixed by perfusion small calibre vessels less than 5 microns were less prevalent. The results indicate that NOS is normally present in the endothelial lining throughout the cerebrovascular tree, including the capillaries. Endothelium-derived nitric oxide could have a more widespread role in the regulation of cerebral blood flow than considered previously.
Animals↗
Modulation of the cardiovascular defence response by low frequency stimulation of a deep somatic nerve in rats.
In rats anaesthetised with alphaxalone/alphadolone, electrical stimulation in the dorsal part of the periaqueductal grey matter (PAG) produced a pressor response with tachycardia and vasodilatation in the hind limb, a pattern known as the 'cardiovascular defence reaction' owing to its resemblance to fear-induced hemodynamic changes. Following a 20-min period of stimulation of the peroneal nerve at 10 Hz with current intensities sufficient to recruit group II and III fibres the pressor component of the response was significantly reduced compared to control rats. The maximum decrease of the PAG-evoked pressor response was about 50% (from 32.0 +/- 0.7 to 16.6 +/- 5.9 mmHg). The effect lasted for between 60 and 290 min and was not correlated to baseline blood pressure changes observed after the stimulation of the nerve. In contrast, the tachycardia and hind limb vasodilator components of the defence response as well as their baseline values remained unchanged. Resting blood pressure did not change significantly in control rats but showed a small progressive increase in stimulated rats which reached significance 90-100 min after the stimulation. These results suggest that the afferent input from high threshold fibres in a muscle nerve can produce a selective and long-lasting depression of the vasoconstrictor components of the midbrain-evoked cardiovascular defence response. This effect is discussed in relation to the long-lasting sympathoinhibitory effects of acupuncture-like stimulation or sustained physical exercise.
Action Potentials↗
Dye coupling between dorsal raphe neurones.
Neurones in the dorsal raphe nucleus (DRN) were impaled and filled with biocytin in coronal slices of midbrain taken from young adult rats. The electrophysiological properties and gross morphology of the cells were similar to those reported previously for serotonergic neurones in the DRN. Of 27 cases in which filled neurones were recovered in histological material, almost half (48%) showed labelling of two or three cells, although only one cell had been recorded from. Coupled cells were identified as close or distantly coupled, depending on the distance from the soma of the presumed impaled cell (23.5 +/- 15 microns, n = 7 and 150 +/- 26.5 microns, n = 10 respectively). Whereas close-coupled cells may have been artefactually "coupled" by the penetrating electrode, coupling between distant cells is most likely to be a result of transfer of biocytin through gap junctions. Camera lucida reconstructions of pairs of labelled cells revealed extensive overlap of dendritic fields and numerous crossings between dendrites. When examined at high magnification under a light microscope, many of the crossing dendrites were found to travel in different focal planes. Nevertheless, for each pair of cells, at least one point of close apposition was observed between dendrites or between the axon and a dendrite of the presumed impaled and coupled cell. The incidence of dye coupling between neurones in the DRN may reflect a relatively high level of electronic coupling between the neurones. This form of coupling may be important in determining the synchronous nature of firing of neurones in the DRN.
Action Potentials↗
Attenuation of the midbrain-evoked defense reaction by selective stimulation of medullary raphe neurons in rats.
In rats anesthetized with alfaxalone/alfadolone, electrical stimulation in the dorsal periaqueductal gray matter (PAG) evoked a cardiovascular defense reaction with increases in blood pressure, heart rate, femoral conductance, and respiratory activity but a decrease in renal conductance. These responses were attenuated significantly after microinjection of 200 nl 0.1 M DL-homocysteic acid (DLH) into nucleus raphe magnus (NRM, n = 12) or nucleus raphe obscurus (NRO, n = 22). The maximum reduction in the pressor response was similar after stimulation in NRM (-36.3%) and NRO (-37.3%). The reduction of the tachycardia, however, was greater after stimulation in NRM (-78.1%) compared with NRO (-34.6%). The maximum reduction in the PAG-evoked vasodilation of the hindlimb averaged -55.7% and -55.3% after stimulation of NRM and NRO, respectively, whereas the renal vasoconstriction was reduced by -66.4% and -79.0%. The PAG-evoked increase in respiratory amplitude was attenuated only after stimulation of NRO. It is concluded that neurons in NRM and NRO may be involved in modulating the level of excitability of neurons in the midbrain defense area and/or in its efferent pathway.
Animals↗
Role of the nucleus raphe obscurus in the inhibition of rostral ventrolateral medullary neurones induced by stimulation in the ventrolateral periaqueductal grey matter of the rabbit.
Experiments have been carried out to investigate the pathways which mediate the inhibitory influence of the ventrolateral periaqueductal grey matter (vlatPAG) on neurones in the rostral ventrolateral medulla (RVLM). In anaesthetized rabbits, 20-ms trains of electrical stimulation in the vlatPAG inhibited ongoing activity of neurones in the RVLM to < 30% of the control level. The inhibition was blocked after microinjections of 500-1000 nl local anaesthetic (4% lignocaine, n = 9) or 0.2 M glycine (n = 4) into nucleus raphe obscurus (NRO). We suggest that the inhibitory influence of the vlatPAG on neurones in the RVLM is mediated by a relay in NRO.
Animals↗
Neurones in the medullary raphe nuclei attenuate the cardiovascular responses evoked from the dorsolateral periaqueductal grey matter.
In rats anaesthetised with alphaxalone/alphadolone, electrical stimulation in the dorsolateral part of the periaqueductal grey matter (PAG; 10 s trains of 1 ms pulses at 80 Hz, 40-80 microA) evoked a pressor response accompanied by tachycardia. Both components of the response were attenuated following microinjection of 200 nl 0.1 M D,L-homocysteic acid into the caudal pole of the nucleus raphe magnus (NRM; n = 12) and into the nucleus raphe obscurus (NRO; n = 22) to selectively activate neuronal perikarya. Microinjection of 200 nl 165 mM NaCl into the same region (n = 15) had no effect. The attenuation of the midbrain-evoked cardiovascular responses lasted for 10-20 min and was independent of changes in resting blood pressure and heart rate. The maximum reduction in the pressor component of the midbrain-evoked responses was similar following stimulation in NRM (-35.4%) and NRO (-36.7%). However, the reduction in the midbrain-evoked tachycardia was greater following stimulation in NRM (-62.8%) compared to NRO (-27.2%). These results indicate that neurones in NRM and NRO may be involved in modulating the level of excitability of the midbrain defence area in the PAG and/or its efferent pathway.
Animals↗
Projections from nucleus raphe obscurus to the periaqueductal grey matter in the rat.
Retrograde transport of rhodamine- or coumarin-labelled latex microspheres was used to investigate projections from nucleus raphe obscurus (NRO) to the periaqueductal grey matter (PAG) in rats. Few labelled neurones (3.5 +/- 1.2; mean +/- S.E.M.) were seen in NRO after injections of microspheres into the dorsolateral and lateral PAG (n = 11) but after injections into the ventrolateral sector (n = 9), significant numbers (42.6 +/- 8.1) were present, particularly in the rostral third of NRO (66% of labelled cells). The results suggest that the projection from NRO to the PAG is restricted to the ventrolateral sector. Any influence of NRO on the dorsal PAG must therefore be mediated indirectly.
Animals↗
Influence of the dorsal and median raphe nuclei on neurons in the periaqueductal gray matter: role of 5-hydroxytryptamine.
In rats anaesthetized with urethane, selective activation of neuronal perikarya in the dorsal raphe nucleus evoked inhibitory (n = 17) and excitatory (n = 10) responses in single neurons recorded in the dorsolateral and lateral sectors of the periaqueductal gray matter and in the adjacent tegmental area. A further 11 cells showed biphasic inhibitory/excitatory responses. Ongoing activity of > 85% of the cells was inhibited by iontophoretic application of 5-hydroxytryptamine (1-70 nA). The duration of the inhibitory response evoked from the dorsal raphe nucleus was increased by 66-75% during iontophoretic application (3-10 nA) of the 5-hydroxytryptamine (serotonin) reuptake blocker paroxetine (five of six cells). In contrast, excitatory responses evoked from the dorsal raphe nucleus were either reduced (n = 3) or replaced by inhibitory responses (n = 2) in the presence of paroxetine. Paroxetine also produced a reduction in baseline firing and potentiated the responses of neurons to iontophoretically applied 5-hydroxytryptamine. Stimulation in the median raphe nucleus did not produce any significant changes in the activity of five neurons tested in the periaqueductal gray matter. It is suggested that the inhibitory influence of the dorsal raphe nucleus on cells in the dorsal half of the periaqueductal gray matter is mediated by 5-hydroxytryptamine. This projection may be involved in modulating the level of excitability of neurons in the midbrain aversive system which integrate defence behaviour. In addition, there appears to be a non-serotonergic excitatory projection from the dorsal raphe nucleus to the periaqueductal gray matter. The functional role of this projection remains obscure.(ABSTRACT TRUNCATED AT 250 WORDS)
Action Potentials↗
Inhibitory and excitatory projections from the dorsal raphe nucleus to neurons in the dorsolateral periaqueductal gray matter in slices of midbrain maintained in vitro.
Coronal slices of midbrain from adult rats, maintained in an interface chamber at 33-35 degrees C, were used to investigate projections from the dorsal raphe nucleus to the dorsolateral periaqueductal gray matter. Extracellular recordings were made from neurons in the dorsolateral periaqueductal gray matter. The majority of cells were silent but activity could readily be induced by iontophoretic application of D, L-homocysteic acid from the recording pipette. Neuronal perikarya in the dorsal raphe nucleus were activated either by topical application of 15-100 nl droplets of 1 or 10 mM D,L-homocysteic acid or by iontophoresis into the dorsal raphe nucleus of 0.1 M D,L-homocysteic acid from a five-barrelled glass micropipette. Both forms of stimulation evoked increases or decreases in firing in neurons in the dorsolateral periaqueductal gray matter. The excitatory responses were evoked only from stimulation sites in the dorsal subnucleus of the dorsal raphe nucleus, whilst inhibitory responses could be evoked by stimulating throughout the nucleus. 5-Hydroxytryptamine was applied by iontophoresis to 55 neurons in the dorsolateral periaqueductal gray matter. Of these, 89% were inhibited whilst the remaining 11% were excited. No unresponsive cells were found. In a second series of experiments anterograde transport of biocytin was used to study projections from the dorsal raphe nucleus to the periaqueductal gray matter in vitro. Following injections of pellets of biocytin into the dorsal subnucleus of the dorsal raphe nucleus, two types of labelled axons could be distinguished which had either smooth or beaded profiles. Both types of axon could be followed into the region of the dorsolateral periaqueductal gray matter, from which recordings were made in the electrophysiological experiments. In contrast, following deposits of biocytin into the ventral half of the periaqueductal gray matter, very few labelled axons were found in the dorsolateral periaqueductal gray matter, although the ventrolateral periaqueductal gray matter was heavily labelled. We conclude that both excitatory and inhibitory projections from the dorsal raphe nucleus to the dorsolateral periaqueductal gray matter are present in the coronal midbrain slice preparation. Excitatory projections originate from the dorsal subnucleus of the dorsal raphe nucleus and may be mediated by a direct pathway. The inhibitory effects evoked from the ventral half of the dorsal raphe nucleus are probably mediated indirectly, perhaps by activation of inhibitory interneurons in the ventrolateral periaqueductal gray matter.
Animals↗
The periaqueductal gray-rostral medulla connection in the defence reaction: efferent pathways and descending control mechanisms.
Neuronal systems controlling cardiovascular components of emotional responses must have the capacity to generate different patterns of response and must also be able to modify those patterns in response to changes in environmental circumstances. Using the cardiovascular "defence" response as a model, evidence is presented to show that sympathetic premotor neurons of the rostral ventrolateral medulla (RVLM) possess such properties. Neurones in the RVLM act as relays in the descending efferent pathway to the sympathetic outflows from the dorsal periaqueductal gray matter (dPAG) which integrates the characteristic "defensive" pattern of cardiovascular response that accompanies activation of the midbrain aversive system. Activity in this pathway can be modulated, at the level of the RVLM, by a descending pathway which originates in the ventrolateral PAG. It is suggested that both the dorsolateral and the ventrolateral control systems in the PAG become activated during periods of physical or emotional stress, particularly those which involve sustained motor activity. Activity in the dorsal system initiates cardiovascular components of aversive/defensive behaviour whilst the ventrolateral system plays an important role in initiating the recuperative phase of behaviour characterised by sympathoinhibition, muscular relaxation and immobility which follows a stressful encounter.
Animals↗
Serotonergic influence from nucleus raphe obscurus on neurones in the periaqueductal grey matter in the rat.
In rats anaesthetised with urethane, iontophoretic application of 5-hydroxytryptamine (5-HT, 5-70 nA) produced changes in ongoing activity of 41/44 neurones in the periaqueductal grey matter (PAG). The majority (85%) of responsive cells were inhibited and 15% were excited. The inhibitions were mimicked in 5/7 cells by iontophoresis of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 10-30 nA) whilst excitation was produced in 3/5 cells by iontophoresis of the 5-HT2 agonist alpha-methyl-5-HT (10-30 nA). Selective activation of neuronal perikarya in nucleus raphe obscurus (NRO) by microinjection of 50-100 nl D,L-homocysteic acid (DLH) inhibited ongoing activity of 25/31 neurones tested in the PAG for periods of 30-580 s, mean 183.5 s. The duration of the inhibition was potentiated by between 36 and 300% during iontophoresis of the 5-HT re-uptake blocker paroxetine (1-25 nA, 6/6 cells). The results indicate that there is an extensive inhibitory serotonergic input to the PAG which originates, at least in part, from NRO.
Animals↗
The inhibitory effect of the ventrolateral periaqueductal grey matter on neurones in the rostral ventrolateral medulla involves a relay in the medullary raphe nuclei.
Experiments were carried out in urethane-anaesthetised rats to determine whether the inhibition of neurones in the rostral ventrolateral medulla (RVLM) induced by stimulation in the ventrolateral periaqueductal grey matter (PAG), is mediated via a relay in the medullary raphe nuclei. Electrical stimulation in the ventrolateral part of the PAG (20-ms trains, 7 pulses, 5-100 microA) inhibited ongoing activity of neurones in the RVLM for periods of 10-120 ms (mean 43.6 ms). The duration of the inhibition was reduced by 51.1% after microinjection of GABA (40-160 nmol in volumes of 200-400 nl, 9/12 sites), but not 165 mM NaCl (8/8 sites) in nucleus raphe magnus (NRM) and the rostral half of nucleus raphe obscurus (NRO). In a further series of experiments, activation of neuronal perikarya at 17 sites in NRM or NRO by microinjection of d,l-homocysteic acid (5-40 nmol in volumes of 50-400 nl) inhibited ongoing activity of 9 out of 14 neurones in the RVLM, the other 5 being excited. We suggest that the inhibitory effect on neurones in the RVLM, which can be evoked by stimulation in the ventrolateral PAG, is mediated indirectly by activation of an inhibitory projection to the RVLM from NRM and the rostral half of NRO.
Animals↗
Renal vasodilatation in response to acute volume load is attenuated following lesions of parvocellular neurones in the paraventricular nucleus in rats.
The effect on the cardiovascular response to volume load, of lesioning the paraventriculo-spinal projection to the lower thoracic cord, has been investigated in urethane-anaesthetised rats. In control animals volume loading with 4% Ficoll 70 in 165 mM NaCl at 0.6 ml/min i.v. up to 1% body weight produced a pressor response and tachycardia accompanied by a vasodilatation in the renal vascular bed. Injections of 2-4 micrograms ibotenic acid in 100-200 microliters solution were made into the paraventricular nucleus to lesion parvocellular neurones. When tested 3 days after the lesioning procedure, the renal vasodilator component of the response to volume load was abolished, although blood pressure and heart rate still rose. When the response to volume load was tested 28 days after the lesion, some degree of functional recovery appeared to have taken place. In these rats there was a weak renal vasodilation although the maximal increase in renal vascular conductance (20 +/- 13%) was still smaller than in the control animals. The efficacy of the lesioning procedure was verified by counting the number of fluorescent labelled neurones in PVN after injection of 1-2 microliters of a 1.25% solution of the retrograde marker 4,6 diamidino-2-phenyl indole (DAPI) bilaterally into the cord at T8-10. 1005 +/- 63 neurones were present in each side of PVN in non-PVN-lesioned rats whilst in lesioned animals only 444 +/- 153 cells were present. These results suggest that the paraventriculo-spinal projection to the lower thoracic cord may play an important role in sympathetically mediated changes in renal blood flow during the early phase of the response to an acute volume load.
Animals↗
Integrated activity of cardiovascular and pain regulatory systems: role in adaptive behavioural responses.
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Adaptation, Psychological↗