Subretrofacial Nucleus Research Articles

Actions of carotid chemoreceptors on subretrofacial bulbospinal neurons in the cat

Thirty-one barosensitive bulbospinal neurons were recorded from the subretrofacial (SRF) nucleus in eight chloralose-anaesthetised, paralysed cats. Close arterial injections of CO 2-saturated saline were used to stimulate carotid body chemoreceptors. Seven neurons were abruptly excited and six neurons abruptly inhibited by chemoreceptor stimuli: these were primary responses, independent of changes in blood pressure or central respiratory drive (monitored from the phrenic nerve). A further eight neurons responded only indirectly to chemoreceptor stimuli, showing enhanced modulation of their activity coupled to the enhanced central respiratory drive. The distinction between primary and secondary responses was shown more clearly when central respiratory drive was inhibited by superior laryngeal nerve stimulation. The remaining ten neurons showed no clear response to chemoreceptor stimuli. SRF bulbospinal neurons thus show the same range of responses to chemoreceptor stimuli as the sympathetic neurons they are believed to drive.

Rostrocaudal differences in morphology and neurotransmitter content of cells in the subretrofacial vasomotor nucleus

The rostral ventrolateral medulla (RVLM) contains sympathoexcitatory neurons that exert a powerful control over the sympathetic outflow to the cardiovascular system. In the cat there is a concentration of such neurons (but not neurons subserving other functions) within a narrow longitudinal column in the RVLM termed the subretrofacial (SRF) nucleus. Furthermore, it has been suggested that there are subgroups of cells, located at different rostrocaudal levels of the SRF nucleus, that preferentially or exclusively control different vascular beds (e.g. in the kidney and hindlimb). The aim of this study was to map quantitatively the rostrocaudal distribution within the nucleus of different cell types, defined according to morphological and/or chemical criteria, and to correlate this with the regional vasomotor effects (in hindlimb and kidney) evoked by stimulation of SRF cells at the corresponding rostrocaudal levels. SRF cells were highly heterogeneous with respect to both their morphology and chemical properties. They varied greatly in size (equivalent diameter ranging from 10–40 μm) as well as in shape and orientation. An immunohistochemical examination using the avidin-biotin procedure revealed that many SRF cells (estimated 57% of all SRF cells) were immunoreactive for tyrosine hydroxylase (TH, a marker of catecholamine cells). In addition, there were SRF cells immunoreactive for neuropeptide Y (NPY, 11% of total), enkephalin (ENK, 16% of total), and serotonin (5HT, 10% of total), but not for substance P, galanin or somatostatin. Different cell types, defined according to their morphology and/or chemical properties, were unevenly distributed throughout the nucleus. In the most caudal part of the SRF nucleus, virtually all cells were TH-positive, and the large majority (estimated 80%) were NPY-positive, suggesting that many cells at this level contained both TH and NPY. In contrast, in the most rostral part of the SRF nucleus, only 30% of cells were TH-positive, and no NPY-positive cells were observed. Both 5HT- and ENK-positive cells were found throughout the rostrocaudal extent of the nucleus, but predominantly within its rostral part. Furthermore, TH-positive cells in the rostral SRF nucleus were on average significantly larger (mean equivalent diameter 18–43% greater) than TH/NPY-positive cells in the caudal part of the nucleus, but smaller than 5HT- or ENK-positive cells at the same level. Overall, rostral cells (regardless of their chemical type) were larger than caudal cells within the SRF nucleus (mean equivalent diameter 13–28% greater). Talking into account both morphological and chemical properties, a minimum of four distinct cell subgroups in the SRF nucleus could be defined: caudal TH/NPY-positive cells, rostral TH-positive (but NPY-negative) cells, 5HT-positive cells (possibly also containing ENK), and cells that were not immunoreactive for any of these compounds. Large (> 100%) increases in blood pressure and iliac vascular resistance were elicited by microinjection of picomolar amounts of sodium glutamate into sites throughout the entire rostrocaudal extent of the SRF nucleus. In contrast, large or moderate (> 50%) increases in renal sympathetic nerve activity were evoked only from sites within the most rostral part of the nucleus. Correlation of the functional and immunohistochemical data indicates that NPY/TH-positive cells in the caudal SRF nucleus contribute to the regulation of the hindlimb but not the renal vascular bed. Secondly, they suggest that both catecholamine

Hemorrhage induces c-fos immunoreactivity in spinally projecting neurons of cat subretrofacial nucleus

Neuronal expression of c-fos product in the cat rostral ventrolateral medulla was studied by immunohistochemistry. Spinally projecting neurons of the subretrofacial (SRF) nucleus were pre-labeled by retrograde transport of fluorescent latex microspheres from the lumbar cord. In 3 animals which were bled by approximately 25% blood volume from a carotid cannula, 48-62% of SRF-spinal neurons expressed c-fos immunoreactivity. In 2 control animals, the corresponding values were 2% and 4%. The data show that bulbospinal neurons of presumed vasomotor function express c-fos in response to hemorrhage.

Cardiovascular effects of microinjections of quipazine into nuclei of the medulla oblongata in anaesthetized cats: comparison with L-glutamate

Unilateral microinjections of quipazine (0.9 μg in 50 nl) into the subretrofacial nucleus produced hypertension and a slight tachycardia associated with an increase in renal sympathetic nerve activity. Microinjections of quipazine lateral, caudal or rostral to this nucleus failed to alter blood pressure and heart rate. Similarly, microinjections of l-glutamate (3 nmol in 15 nl) into the subretrofacial nucleus elicited hypertension, tachycardia and renal sympatho-excitation. The magnitude of the pressor response to quipazine was smaller than the response elicited by l-glutamate but its duration was longer. Microinjections of quipazine into the lateral tegmental field at l-glutamate hypertensive sites failed to alter arterial blood pressure and heart rate. In contrast, microinjections of quipazine into the caudal ventrolateral medulla or into the nucleus tractus solitarii produced hypotension and sympatho-inhibition. These effects were prevented by microinjections of the 5-HT 2 receptor antagonists, LY 53857 or BW 501C. The present results indicate that stimulation of 5-HT 2 receptors of the subretrofacial nucleus produces hypertension and sympatho-excitation whereas stimulation of 5-HT 2 receptors in the caudal ventrolateral medulla and in the nucleus tractus solitarii produces hypotension and sympatho-inhibition.

Difference in distribution of glutamate-immunoreactive neurons projecting into the subretrofacial nucleus in the rostral ventrolateral medulla of SHR and WKY: a double-labeling study

Glutamate immunoreactivity was found in 19% and 21% of the neurons of the central autonomic nuclei projecting into the subretrofacial nucleus (SRF) in the rostral ventrolateral medulla of Wistar-Kyoto rat (WKY) and spontaneously hypertensive rat (SHR), respectively, using a double-labeling technique in combination with glutamate immunocytochemistry. Double-labeled neurons were distributed in 22 nuclei or subnuclei in the limbic system, hypothalamus, midbrain, pons and medulla. The average number of glutamate-immunoreactive neurons per thousand in SHR was significantly higher in the ipsilateral lateral parabrachial nucleus ( P < 0.05) and Koelliker-Fuse nucleus ( P < 0.01) than in WKY, while it was significantly lower in the ipsilateral medial subnucleus ( P < 0.05) and the commissure subnucleus ( P < 0.05) of the nucleus tractus solitarii in SHR than in WKY. The results indicate that: (1) glutamate-immunoreactive neurons (possibly glutamatergic) in many central autonomic nuclei project into the sympathetic vasomotor control neurons in the SRF; (2) the large population of glutamate-immunoreactive neurons in the lateral parabrachial nucleus and the Koelliker-Fuse nucleus of SHR is likely to increase excitatory inputs to the SRF vasomotor control neurons, while the smaller population of glutamate-immunoreactive neurons in the medial and commissure subnuclei of the nucleus tractus solitarii is likely to decrease excitatory inputs to the GABAergic neurons intrinsic to the SRF.

Glutamate-immunoreactive neurons of the central amygdaloid nucleus projecting to the subretrofacial nucleus of SHR and WKY rats: A double-labeling study

Glutamate immunoreactivity was found in 9%, in the case of Wistar Kyoto rats (WKY), and 14%, in the case of spontaneously hypertensive rats (SHR), of neurons located in the medial division of the central amygdaloid nucleus (CeM) projecting ipsilaterally to the subretrofacial nucleus (SRF) in the rostral ventrolateral medulla using a double-labeling technique in combination with glutamate immunocytochemistry. The results indicate that possibly-glutamatergic neurons located in the CeM project to the SRF, in which vasomotor neurons are present, suggesting involvement of the CeM in blood pressure regulation. No significant difference was found between the distribution of labeled CeM neurons in SHR and WKY rats.

Difference in sensitivity of cardiovascular and respiratory control neurons in the subretrofacial nucleus to glutamate receptor subtype agonists in SHR, WKY and cats

Using spontaneously hypertensive rats (SHR), Wistar-Kyoto rats (WKY) and cats, either glutamate or a glutamate receptor subtype agonist was injected into the subretrofacial nucleus (SRF) in the rostral ventrolateral medulla at the site where the pressor response had been evoked by electrical stimulation. The sensitivity of SRF neurons to the electrical stimulation or glutamate receptor agonist was estimated by the threshold current or dose required to evoke the pressor response. The threshold of SRF neurons to electrical stimulation was similar in the three animal groups, while that to the glutamate receptor agonist was different. The significance of the difference in threshold between WKY and SHR was calculated as was that between WKY and cats. The threshold for kainate stimulation was ten times lower for SHR (0.016 pmol, P < 0.001) and five times higher for cats (0.78 pmol, P < 0.05); that for quisqualate stimulation was fifty times lower for SHR (0.016 pmol, P < 0.001) but similar for cats; that for NMDA stimulation was twelve times lower for SHR (0.13 pmol, P < 0.001) but seven times higher for cats (11 pmol, P < 0.01); that for glutamate stimulation was ten times lower for SHR (4.2 pmol, P < 0.001) but similar for cats. The heart rate and respiratory responses associated with the pressor response were tachycardiac and hypopneic in SHR and WKY, but bradycardiac and hyperpneic in cats. These responses were less dominant than the pressor response. We suggest that the pathogenesis of hypertension in SHR may be partly due to abnormal properties of glutamate receptor subtypes acting on vasomotor control neurons in the SRF.

The ventrolateral medulla of the cat mediates vestibulosympathetic reflexes

Extracellular recordings were made from 94 neurons located in the ventrolateral medulla (VLM) whose firing rate was affected by vestibular nerve (VN) stimulation; 50 of these units were in the subretrofacial (SRF) nucleus, which contains cells that make direct excitatory connections with sympathetic preganglionic neurons. The sample included 12 SRF cells which were antidromically driven from the upper thoracic spinal cord and had conduction velocities of 10 m/s or less; the effect of VN stimulation on all but one of these units was inhibition. The onset latency of the response to VN stimulation was long [20.3 ± 3.7(S.E.M.) ms, n = 9, for the antidromically activated neurons and 12.1 ± 1.2ms, n = 73, for the others], suggesting that the effects were predominantly polysynaptic. In addition, most of the spontaneously active units tested (33/36) received convergent inputs from the carotid sinus nerve (CSN), as would be expected for neurons which influence sympathetic outflow. Vestibular-elicited inhibition of SRF neurons with projections to the intermediolateral cell column could account for late, long duration inhibition of sympathetic discharges produced by labyrinth stimulation.

CNS Regulation of Cardiac Rhythm and the Potential for Pain‐Induced Arrhythmia

CNS Control of Cardiac Rhythm. The purpose of our studies was to determine brain areas that influence cardiac rhythm. Studies were performed in anesthetized cats and indicate that a yet undetermined site in the hindbrain involving GABAergic mechanisms has a major influence over cardiac rhythm. In addition, a serotonergic mechanism involving 5‐HT2 receptors in the subretrofacial nucleus is responsible for selective activation of sympathetic neurons to the ventricle of the heart. Our data also indicate that neurons in the area postrema exert a major effect on cardiac rhythm. Finally, the question of whether pain‐induced changes in cardiovascular function involves neurons in the subretrofacial nucleus is also addressed.

TONIC CONTROL OF SUBRETROFACIAL VASOMOTOR NEURONS IN THE ROSTRAL VENTROLATERAL MEDULLA

1. Vasomotor pressor neurons in the subretrofacial nucleus of the rostral ventrolateral medulla receive afferent inputs from different sources that utilize different neurotransmitters. This paper briefly reviews recent studies on the role of inputs releasing: (i) GABA, and (ii) angiotensin II (AII) in the subretrofacial nucleus. 2. There are two types of tonic GABAergic inhibitory inputs: one arises from peripheral baroreceptors, while the second is independent of peripheral baroreceptors. 3. Blockade of receptors for AII elicits a decrease in blood pressure and sympathetic vasomotor activity, indicating that subretrofacial neurons are also tonically excited by AII. It is likely that the AII is released from nerve terminals in the subretrofacial nucleus, but the origin of the pathway is unknown.

Viscerotopic organization of neurons subserving hypotensive reactions within the midbrain periaqueductal grey: a correlative functional and anatomical study

Microinjection of the excitatory amino acid d,l-homocysteic acid (40 nmol, in 200 nl) made into the ventrolateral part of the caudal half (A2.5-P1.5) of the midbrain periaqueductal gray (PAG) of the decerebrate cat evoked a hypotensive reaction associated with a slowing of the heart and a decrease in either external iliac or renal vascular resistance. The decrease in iliac vascular resistance was elicited from the pretentorial portion (A2.5-A0.6) of the PAG hypotensive area, whereas the decrease in renal vascular resistance was elicited from the subtentorial portion (A0.6-P1.5). Anatomical experiments using the method of retrograde transport of rhodamine-labelled microspheres or wheat germ agglutinin-horseradish peroxidase demonstrated topographically organized projections from the ventrolateral PAG to the subretrofacial (SRF) pressor nucleus in the rostral ventrolateral medulla. The pretentorial part of the ventrolateral PAG projected mainly to the caudal part of the SRF nucleus, which preferentially controls iliac vascular resistance. The subtentorial part of the ventrolateral PAG projected mainly to the rostral part of the SRF nucleus, which preferentially controls renal vascular resistance. Taken together, these findings suggest: (i) that neurons within the ventrolateral PAG are viscerotopically organized; and (ii) that their hypotensive function may be mediated by an inhibition of SRf pressor neurons. The results are discussed in relation to the recently described PAG hypertensive area which also is viscerotopically organized and projects to the SRF nucleus.

Evidence that glutamate-immunoreactive neurons of the medial division of the central amygdaloid nucleus project to the subretrofacial nucleus of SHR and WKY rats

Evidence that glutamate-immunoreactive neurons of the medial division of the central amygdaloid nucleus project to the subretrofacial nucleus of SHR and WKY rats

Stimulation of serotonin2 receptors in the ventrolateral medulla of the cat results in nonuniform increases in sympathetic outflow.

Topical application of the serotonin2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane or DOI, in a dose of 30 micrograms/side to the intermediate area of the ventrolateral surface of the medulla produced a significant increase in mean arterial pressure with no significant change in heart rate both in intact animals (n = 8) and in cervically vagotomized animals (n = 3). The pressor response of DOI was blocked by pretreatment of the intermediate area with ketanserin, a serotonin2 antagonist (n = 7). Pretreatment with intravenous phentolamine did not block the pressor response of DOI (n = 3). However, this pressor response could be counteracted by intravenous propranolol (n = 5) or by bilateral stellate ganglionectomy (n = 3). These data suggest that sympathoexcitation by centrally applied DOI selectively increased cardiac inotropy but not chronotropy. Further studies indicate that DOI increased contractile force without increasing heart rate and that the positive inotropic effect of DOI could be counteracted by bilateral stellate ganglionectomy. Bilateral microinjections of DOI into the subretrofacial nucleus in a dose of 100 ng (n = 3) and a dose of 300 ng (n = 3) increased mean arterial blood pressure by 23 +/- 2 and 44 +/- 6 mm Hg, respectively, without producing any changes in heart rate. These data suggest that DOI has a central site of action in the ventrolateral medulla, presumably at the subretrofacial nucleus, which leads to selective sympathoexcitation of the cardiac ventricles.

The Subretrofacial Nucleus: Its Pivotal Role in Cardiovascular Regulation

The subretrofacial nucleus is a compact group of cells within the rostral ventrolateral medulla that regulates the sympathetic outflow to the cardiovascular system. This nucleus has distinctive histochemical and receptor-binding properties and is a major site of convergence of inputs arising from peripheral receptors as well as higher brain centres.

Baroreceptor inhibition of subretrofacial neurons: evidence from intracellular recordings in the cat

Neurons of the subretrofacial nucleus in the rostral ventrolateral medulla have been studied by in vivo intracellular recording in the chloralose-anaesthetized cat. Impalements of sufficient quality to demonstrate inhibition by carotid baroreceptor stimulation (blind sac inflation) were obtained for 9 cells. In 8 of these, a clear hyperpolarization followed approximately 100–200 ms after the baroreceptor stimulus, reaching a maximum of 2–9 mV, 200–500 ms later. These findings confirm by more direct methods the inhibition of subretrofacial neurons by arterial baroreceptors.

Ventrolateral Medulla

The purpose of our study was to test the hypothesis that drugs that activate the 5-HT1A receptor lower arterial pressure by interacting with 5-HT1A receptors located in the ventrolateral medulla. For this purpose, cats were anesthetized with alpha-chloralose while arterial pressure, heart rate, and several indices of respiratory activity were being monitored during either topical application or microinjection of 5-HT1A receptor agonist and antagonist drugs in the region of the intermediate area of the ventrolateral medulla. Topical application of drugs that activate 5-HT1A receptors (e.g., 8-OH-DPAT and urapidil) produced decreases in arterial pressure, heart rate, and tidal volume but an increase in respiratory rate. These effects were prevented by prior topical application of the 5-HT1A antagonist drug, spiperone. Intravenous administration of 5-HT1A receptor agonist drugs produced similar cardiorespiratory effects, and these effects were counteracted by topical application of spiperone to the intermediate area of the ventrolateral medulla. Microinjection of 8-OH-DPAT into two sites associated with the intermediate area, namely the subretrofacial nucleus and the bicuculline-sensitive hypotensive site, also resulted in decreases in arterial pressure (-40 +/- 10 mm Hg, n = 5; and -21 +/- 4 mm Hg, n = 3, respectively). These data indicate that hypotension produced by drugs that activate 5-HT1A receptors occurs, at least in part, from an action in the ventrolateral medulla, presumably in the subretrofacial nucleus and/or the bicuculline-sensitive hypotensive site.

Viscerotopic control of regional vascular beds by discrete groups of neurons within the midbrain periaqueductal gray

It is well established that a group of bulbospinal neurons within the rostral ventrolateral medulla plays a crucial role in the tonic and phasic control of arterial pressure. In the cat, these neurons are confined to a discrete region which has been termed the subretrofacial (SRF) nucleus. Recent evidence suggests that this nucleus is viscerotopically organized with respect to its control over different vascular beds. These observations raise the question as to whether functionally different subgroups of SRF pressor neurons receive inputs from supramedullary cell groups that also exert a specific control over particular vascular beds. To answer this question retrogradely transported tracers (i.e. rhodamine or fluorescein-labelled microspheres, wheat germ agglutinin-horseradish peroxidase) were injected into physiologically identified sites within the rostral or caudal parts of the SRF nucleus of the cat. Separate groups of neurons in the midbrain periaqueductal gray region (PAG) were found to project specifically to subgroups of cells within the rostral and caudal parts of the SRF nucleus. These findings, together with the results of recent functional studies of the PAG suggest that these distinct projections from the PAG to the SRF nucleus are involved in the expression of different patterns of emotionally coupled cardiovascular responses.

Relationship between cardiovascular neurones and descending antinociceptive pathways in the rostral ventrolateral medulla of the cat

It has been previously reported that injection of neuroexcitatory compounds into the rostral ventrolateral medulla (RVLM) can produce an inhibition of nociceptive reflexes, often associated with a rise in arterial blood pressure. The aim of this study was to determine whether the subretrofacial (SRF) nucleus, which is a highly circumscribed group of cells within the RVLM known to play a major role in cardiovascular regulation also has an antinociceptive function. In barbiturate-anaesthetised and paralysed cats, unilateral microinjections of the neuroexcitatory compound sodium glutamate (8–20 nl of 0.5 M solution) into the SRF nucleus produced large increases in mean arterial pressure but had only small and inconsistent effects on the simultaneously measured ventral root responses to stimulation of primary afferent C-fibres. On the other hand, glutamate microinjections into RVLM sites closely adjacent to the SRF nucleus, or into the nucleus raphe magnus, produced powerful inhibition of the C-fibre evoked response in the ventral root which was accompanied by no or only small changes in arterial pressure. It is concluded that the SRF pressor cells do not exert any control over nociceptive spinal reflexes, but that such a function may be served by cells in closely adjacent parts of the RVLM. Moreover, the method of recording C-fibre evoked responses in ventral roots as a measure of the magnitude of nociceptive spinal reflexes, combined with the glutamate microinjection procedure, was shown to have a sufficient resolution to allow an accurate mapping of the location of antinociceptive cell groups within the ventrolateral medulla.

Chapter 18 The selectivity of descending vasomotor control by subretrofacial neurons

The notion that neurons in the rostral ventrolateral medulla play an important part in cardiovascular control is now widely accepted. This chapter focuses on a compact group of spinally projecting neurons termed as the “subretrofacial nucleus” (SRF) and argues that this set of neurons is responsible for mediating the region's vasopressor actions. It also presets evidence that SRF neurons selectively drive cardiovascular sympathetic nerves—that is, to heart, blood vessels and, adrenal medulla. The chapter presents data that support the view that within SRF there are neural subpopulations with even greater specificity of action and that these may be functionally dedicated to control vasomotor supply to single tissues. The SRF consists of a compact column of medium-small cells on either side of the medulla, ventral to and separated from the retrofacial nucleus. Electrophysiological recording experiments have provided data to support the idea that SRF neurons form a descending vasomotor pathway. In the cat, studies from two laboratories have localized bulbospinal neurons with cardiovascular properties in a compact group ventral to the retrofacial nucleus.

Angiotensin receptor binding and pressor effects in cat subretrofacial nucleus

Central administration of angiotensin II (ANG II) increases arterial blood pressure via increased sympathetic activity. We have examined the possibility that one site of action of ANG II is the subretrofacial (SRF) nucleus in the rostral ventrolateral medulla, since this nucleus is known to play a critical role in the tonic and phasic control of arterial pressure. In vitro autoradiography, employing 125I-labeled [Sar1, Ile8]ANG II as radioligand, was used to localize binding sites for ANG II in the cat ventrolateral medulla. A high density of ANG II-receptor binding sites was found confined to the SRF nucleus. In a second group of experiments in anesthetized cats, microinjections of ANG II, in doses ranging from 10 to 50 pmol, were made into histologically identified sites within and outside the SRF nucleus. Microinjections into the nucleus resulted in a dose-dependent increase in arterial pressure, which was abolished by systemic administration of the ganglion-blocking drug hexamethonium bromide. In contrast, microinjections just outside the SRF nucleus had no effect on arterial pressure. It is concluded that activation of ANG II-receptor binding sites within the SRF nucleus leads to an increase in arterial pressure via increased sympathetic efferent activity.