Action and specificity of ventral medullary vasopressor neurones in the cat

The supply of vasomotor drive to individual classes of sympathetic neuron

The vasoconstrictor supplies to different tissues show distinct patterns of ongoing and reflex activity, indicating that they are driven by distinct central pathways. Vasomotor tone depends heavily on connections from the brainstem, so class-specific vasomotor drives have been sought amongst the sympathetic premotor neurons which provide those connections. Premotor neurons of the rostral ventrolateral medulla (subretrofacial nucleus) provide most descending vasomotor drive. Together, they drive the sympathetic supplies to heart, blood vessels and adrenal, but not 'non-cardiovascular' sympathetic responses (sweating, pupil dilatation, piloerection, etc.). Individually, they provide preferential or selective drives to particular classes of 'cardiovascular' sympathetic outflow. Subretrofacial neurons are arranged topographically, forming a neural map of the functional class (target tissue), not the body region, of the driven outflows. It is still unknown whether other premotor cell groups are organised this way. Nor are the premotor pathways to 'non-cardiovascular' sympathetic nerves yet well-defined.

2-Deoxy-D-glucose but not 2-mercaptoacetate increases Fos-like immunoreactivity in adrenal medulla and sympathetic preganglionic neurons

2-Deoxy-D-glucose (2DG) and 2-mercaptoacetate (MA) are drugs that competitively inhibit metabolism of glucose and fatty acids, respectively. Both 2DG and MA stimulate food intake. In addition, 2DG-induced glucoprivation is a known stimulus for adrenomedullary secretion. However, very little is known about the effects of MA on the sympathoadrenal system. In the present study, we examined effects of 2DG and MA on the activity of preganglionic neurons and the adrenal medulla, as indicated by expression of Fos-like immunoreactivity (Fos-li). 2DG, MA, or saline was administered using a stress-attenuated paradigm incorporating remote drug infusion. Expression of Fos-like immunoreactivity (Fos-li) was subsequently examined in the adrenal medulla and in preganglionic sympathetic neurons throughout the intermediolateral column (IML) of the thoracic and lumbar spinal cord. We found that 2DG increased Fos-li in the adrenal medulla and in the IML primarily at spinal cord segments T7-T10, where adrenomedullary preganglionic neurons reside. In contrast, MA did not induce Fos-li either in the adrenal medulla or in sympathetic preganglionic neurons at any cord level. Results support the hypothesis that decreased fatty acid oxidation is not a stimulus for adrenal medullary secretion and provide evidence for a highly selective stimulation of adrenal medullary preganglionic neurons by 2DG.

Hypothalamic modulation of laryngeal reflexes in the anaesthetized cat: role of the nucleus tractus solitarii

1. This investigation was initiated because activation of laryngeal afferents, either by electrical stimulation of the superior laryngeal nerve (SLN) or by natural stimulation of receptors in the laryngeal mucosa, results in a cardiorespiratory response comprising bradycardia, hypotension and apnoea (phrenic nerve activity was suppressed). This pattern of response is qualitatively equivalent to the response that is evoked on activation of the arterial baroreceptors. 2. Preliminary studies indicated that the effects of activating the SLN were suppressed during stimulation in the hypothalamic defence area (HDA) at points that also blocked the effects of baroreceptor stimulation. 3. Recordings were taken from seventy-two neurones localized within the ipsilateral nucleus tractus solitarii (NTS) whose activity was modified by SLN stimulation. Sixty neurones responded with an EPSP on SLN stimulation; nine of these had an inspiratory firing pattern. Five neurones were seen to receive an IPSP on SLN stimulation. 4. Five respiratory SLN-activated neurones were unresponsive to stimulation of the other nerve inputs, whilst four received convergent EPSP inputs on sinus nerve (SN) stimulation. One cell of these four also received inputs from the aortic and the vagus nerves. Sixty-one non-respiratory SLN-activated neurones also received convergent inputs from the sinus nerve. Of these, fifty displayed an EPSP, four an IPSP and seven an EPSP-IPSP. Fifteen neurones also received inputs from the aortic nerve and seventeen from the vagus. 5. From the population of neurones affected by SLN stimulation, twenty-four of seventy were also influenced by HDA stimulation (3 were respiratory cells). Sixteen of these responses consisted of an EPSP (2 respiratory cells), five of an IPSP (1 respiratory cell) and three of an EPSP-IPSP. 6. In neurones receiving an IPSP on HDA stimulation, the SLN-evoked excitatory response was reduced throughout the period of HDA-evoked inhibition. These neurones were all shown to receive excitatory inputs from the arterial baroreceptors and laryngeal mechanoreceptors. 7. Additionally, in the thirty-seven neurones that were excited by SLN stimulation but received no direct synaptic input on HDA stimulation, a conditioning stimulus to the HDA evoked a block of SLN-evoked responses without an accompanying change in membrane potential. Several of these neurones were also affected by both baroreceptor and laryngeal mechanoreceptor stimulation. 8. These observations are discussed in the context of the role of the NTS in cardiorespiratory control. The potential importance of these interactions in respiratory distress are highlighted and the implications for the organization of central pathways for the control of autonomic and respiratory function are discussed.

Respiratory-related discharge patterns of caudal raphe neurones projecting to the upper thoracic spinal cord in the rat

Sympathetic activity is modulated by central respiratory drive. Bulbospinal neurones arising in the ventrolateral medulla and A5 region probably contribute to this modulation. In the present investigation the involvement of caudal raphe-spinal neurones in relaying respiratory-related inputs to sympathetic preganglionic neurones was investigated. Experiments were carried out on anaesthetized, vagotomized, paralysed and artificially ventilated rats. Extracellular recordings were made from the cell bodies of 53 caudal raphe neurones activated antidromically by stimulating the spinal cord between T1 and T3. The axonal conduction velocities ranged from 0.7-9.1 m/s (median = 3.8 m/s). Thirty-six of 53 neurones (consisting of neurones with on-going activity and quiescent neurones activated with glutamate) were held long enough for detailed analysis. Of those recorded 26 were in the region of raphe obscurus, nine in raphe pallidus and one in raphe magnus. Twenty-five of 36 neurons had firing patterns related to phrenic nerve discharge. Of the four firing patterns defined: seven neurones had the highest probability of firing during inspiration (inspiratory-related), 10 neurones had the highest probability of firing during expiration (expiratory-related), 3 had the highest probability of firing during post-inspiration (post-inspiratory-related) and 5 had lowest levels of firing during early- and post-inspiratory phases (early and post-inspiratory depressed). Of 27 neurones with axonal projections through or to the region of the intermediolateral cell column in the upper thoracic cord 19 had a respiratory-related discharge pattern. For respiratory-modulated neurones with on-going activity the median of the modal inter-spike intervals was 0.08 s. None of the neurones had an ECG-related firing pattern. The findings of this study also indicate a species difference between rats and cats regarding the physiological properties of some raphe-spinal neurones; i.e., an absence of ECG-related activity in the rats. The characteristics of the neurones recorded in this study are not those of 'typical' 5-HT-containing neurones with reference to axonal conduction velocities and discharge characteristics.

Functional anatomy of sympathetic premotor cell groups in the medulla

Pre- and postganglionic sympathetic neurons are organized into discrete functional channels, according to the target they supply. The activity patterns which they show differ between channels, implying that the CNS pathways driving them are not the same. Premotor neurons probably play a key role. This article focuses on what recent evidence tells us about the organization of premotor neurons which control specific sympathetic outflows. Cells that drive muscle vasoconstrictor (MVC), cutaneous vasoconstrictor (CVC), visceral vasoconstrictor (VVC) and renal sympathetic (RSN) outflows have been identified among the premotor neurons of the rostral ventrolateral medulla (RVLM). Other vasomotor, cardiac or adrenal drives are also represented in that cell group. Neurons driving sudomotor responses have been localized in the rostral ventromedial medulla. Evidence on the specific functions of other premotor cell groups is briefly discussed.

Effects of preoptic warming on subretrofacial and cutaneous vasoconstrictor neurons in anaesthetized cats

1. Sympathetic and subretrofacial neuron responses to preoptic warming were studied in chloralose- or Saffan-anaesthetized, paralysed cats. 2. Warming a thermode in the preoptic region inhibited the activity of cutaneous vasoconstrictor fibres supplying hairy skin. Muscle vasoconstrictor fibre activity recorded at the same time was either unaffected or raised. 3. Small injections of sodium glutamate (5 nl, 0.1 M) were made into the region of the subretrofacial nucleus in the ventrolateral medulla. The part of that region where glutamate injections evoked brisk increases in cutaneous vasoconstrictor fibre activity was chosen for further study. 4. Extracellular single unit recordings were made in that area from seventy-seven subretrofacial neurons, which were identified by their barosensitivity (inhibition by carotid blind sac inflation). Forty-seven of them were antidromically activated by stimulation in the spinal cord. 5. The activity of twenty subretrofacial neurons (twelve proven bulbospinal) was significantly reduced by periods of preoptic warming. Cutaneous vasoconstrictor activity recorded at the same time also fell. Forty-nine subretrofacial neurons (thirty-five proven bulbospinal) were unaffected or excited by periods of preoptic warming that inhibited cutaneous vasoconstrictor fibres. The response of eight neurons was unclear. 6. No difference in either mean firing rate or axonal conduction velocity was found between neurons inhibited by preoptic warming and other subretrofacial neurons. 7. The subretrofacial neurons inhibited by warming were found intermingled with those unaffected or excited. Marked recording sites of warm-inhibited neurons were clustered around the ventromedial border of the subretrofacial nucleus. 8. In two cats, bilateral inhibition of subretrofacial neurons by surface application of 1 M glycine reduced cutaneous vasoconstrictor fibre activity to 32 and 44% of control levels. 9. The results suggest that specific cutaneous vasoconstrictor premotor neurons exist in the subretrofacial nucleus. These apparently provide most of the background excitatory drive to cutaneous vasomotor neurons. Central warming stimuli may act, at least in part, by withdrawing that drive.

Monosynaptic excitation of preganglionic vasomotor neurons by subretrofacial neurons of the rostral ventrolateral medulla

Extracellular single unit recordings were made from barosensitive neurons in the subretrofacial nucleus (SRF) of the rostral ventrolateral medulla in chloralose-anaesthetised cats. At the same time, single preganglionic neuron activity was recorded from filaments of the cervical sympathetic trunk (CST); barosensitive units were selected for study. Evidence for monosynaptic connections between the two neuron groups was sought by cross-correlation analysis of their ongoing activity. Cross-correlograms of 16/16 SRF/CST neuron pairs showed a broad peak (100-200ms wide), reflecting the synchronizing action of arterial baroreceptors on both neurons' activity. Two of the 16 cross-correlograms additionally showed a robust, statistically significant, narrow peak of a single 2 ms bin width, providing the first physiological demonstration that ventrolateral medullary neurons monosynaptically excite preganglionic sympathetic neurons. Deductions are made about the strength, convergence and divergence of the connection.

Annual review prize lecture. Central nervous mechanisms contributing to cardiovascular control

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Immunohistochemical study of catecholamine enzymes and neuropeptide Y (NPY) in the rostral ventrolateral medulla and bulbospinal projection

The purpose of this study was to determine whether neuropeptide Y (NPY) terminals in the intermediolateral spinal cord originate from the rostral ventrolateral medulla (RVLM). Immunohistochemical staining of tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), phenylethanolamine-N-methyltransferase (PNMT), and NPY in the rat brainstem and spinal cord were performed in this study in order to examine consequences of lesions of the RVLM and of intracisternal injections of 6-hydroxydopamine (6-OHDA) on catecholamine and NPY immunoreactivity in the intermediolateral column (IML) of rats. In addition, ricin, a retrograde neurotoxin, was applied in the superior cervical ganglion (SCG) to determine its effect on catecholamine and NPY immunoreactivity in the IML. Computer-aided image analysis was used to quantify the immunohistochemical changes in the RVLM and spinal cord. The results demonstrated that many catecholamine- and NPY-containing neurons and/or fibers existed in the RVLM and their terminals were found in the IML. After administration of 6-OHDA intracisternally, the catecholamine and NPY immunoreactivities were decreased both in the brainstem and IML of the spinal cord. Following unilateral microinjection of 6-OHDA into the RVLM, the number of NPY- and catecholamine-containing neurons decreased and there was a reduction in neuron terminals on the ipsilateral side. After injection of ricin into the SCG, the catecholamine and NPY neurons of the medulla were not affected, whereas their terminals in the IML decreased ipsilaterally. These results indicate that most of the catecholamine- and NPY-immunoreactive terminals found in the IML originated in the RVLM. These terminals appear to project towards the superior cervical ganglia.

Essential organization of the sympathetic nervous system

The sympathetic nervous system consists of efferent neurones supplying the viscera. The cell bodies of preganglionic neurones are located in four areas in the thoracolumbar cord; however, the majority are found in the IML. Various tracing techniques have provided information concerning the location of the cell bodies of sympathetic preganglionic neurones projecting into various nerves and ganglia and regulating the adrenal gland, the kidney and the sympathetic supply to skeletal muscle. Numerous supraspinal neurones project to the neuropil surrounding sympathetic preganglionic neurones and may form synaptic contacts with these neurones. The areas of the brain that project to the IML appear to be part of a network of reciprocally connected supraspinal cell groups. Although much emphasis has been placed on the importance of the RVLM in the mediation of tonic and phasic inputs to sympathetic preganglionic neurones, it appears that other areas are of significant import; the RVLM should not be considered to be 'the vasomotor centre'. Spinal and cranial afferents influence the sympathetic nervous system. Baroreceptor afferents terminate in the NTS and may utilize an excitatory amino acid as their neurotransmitter. However, a number of neuropeptides are also associated with these afferents. Neurones within the NTS project to a number of brain stem areas thought to be involved in the regulation of sympathetic activity; consequently the baroreceptor reflex may be mediated over a number of parallel pathways involving both supraspinal and spinal sites of inhibition. Many neurotransmitters are thought to regulate the activity of sympathetic preganglionic neurons: monoamines, peptides and amino acids. Matching the chemical content of the cell bodies of neurones within a particular cell group with physiological characteristics is a challenging task; some barosensitive neurones of the RVLM do not appear to be adrenergic although they are in the midst of the C1 adrenergic cell group. Besides acetylcholine and noradrenaline, neurotransmission in the periphery appears to involve numerous peptides and ATP.

Characteristics of sympathetic reflexes evoked by electrical stimulation of phrenic nerve afferents

In chloralose-anaesthetized cats, sympathetic reflex responses were recorded in left cardiac and renal nerve during stimulation of afferent fibres in the ipsilateral phrenic nerve. In cardiac nerve, a late reflex potential with a mean onset latency of 75.6 +/- 13.8 ms was regularly recorded which, in 20% of the experiments, was preceded by an early, very small reflex component (latency between 35 and 52 ms). In contrast, in renal nerve only a single reflex component after a mean latency of 122.1 +/- 13.1 ms was observed. Bilateral microinjections of the GABA-agonist muscimol into the rostral ventrolateral medulla oblongata resulted in a nearly complete abolition of sympathetic background activity and in an 88% reduction of the late reflex amplitude with only small effects on the latency of the evoked potentials. Under this condition, an early reflex component was never observed to appear. After subsequent high cervical spinalization, the residual small potentials which persisted after bilateral muscimol injections were completely abolished and in cardiac nerve an early reflex potential with a mean latency of 45 +/- 10 ms was observed in all but one experiment. The early reflex was therefore referred to as a spinal reflex component which, however, is suppressed in most animals with an intact neuraxis. In the renal nerve a spinal response was only observed in one experiment after spinalization. The results suggest that sympathetic reflexes evoked by stimulation of phrenic nerve afferent fibres possess similar spinal and supraspinal pathways as previously described for somato-sympathetic and viscero-sympathetic reflexes.(ABSTRACT TRUNCATED AT 250 WORDS)

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 microns) 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).(ABSTRACT TRUNCATED AT 400 WORDS)

Distinct populations of neurons in the ventromedial periaqueductal gray project to the rostral ventral medulla and abducens nucleus

Following microinjections of a colloidal gold complex into the nucleus paragigantocellularis (PGi) of the ventral medulla, and of latex microspheres into the nucleus abducens (Abd) of the same animal, retrogradely labeled neurons were identified in the region of the contralateral supraoculomotor nucleus (SOM) in the ventromedial periaqueductal gray (PAG). Neurons labeled from the Abd were found in the SOM in the ventromedial PAG throughout the midbrain, as well as scattered ventrally in the oculomotor nucleus. Neurons in the SOM area retrogradely labeled from the PGi were most numerous rostral to the dorsal raphe nucleus and extended throughout the level of the oculomotor nucleus. Direct comparison of the two labels revealed that the neurons that project to the Abd were located slightly more ventrally than PGi-projecting neurons. Almost no doubly labeled neurons were identified, although singly labeled neurons formed adjacent but separate populations. These results indicate that neurons in the SOM area projecting to the PGi are distinct from those projecting to the Abd, and that the PGi-projecting neurons are probably not pre-oculomotor neurons. Given their more dorsal location and the nature of their target neurons in the rostral ventrolateral medulla, PGi-projecting neurons may be related to visceromotor, as opposed to oculomotor functions.

Projections from rabbit caudal medulla to C1 and A5 sympathetic premotor neurons, demonstrated with phaseolus leucoagglutinin and herpes simplex virus

We combined Phaseolus vulgaris leucoagglutinin anterograde tracing and Herpes simplex virus transneuronal retrograde tracing to determine whether neurons in the vasodepressor region of the rabbit caudal ventrolateral medulla project to brainstem neurons containing the virus after its transneuronal transport from the adrenal medulla. Five days after adrenal injection of virus, 764 +/- 159 virus-positive neurons were found bilaterally in the brainstem: 61% in the C1 sympathoexcitatory region of the rostral ventrolateral medulla, 30% in the A5 region, 5% in the parapyramidal region, and 3% in the paraventricular nucleus of the hypothalamus. Many of the virus-positive neurons in the C1 and A5 areas also contained tyrosine hydroxylase and, in the parapyramidal area, many contained 5-hydroxytryptamine. After iontophoretic deposit of leucoagglutinin into the vasodepressor region of the caudal ventrolateral medulla, brain regions containing varicose processes labeled with leucoagglutinin included the regions containing virus-positive neurons. We examined the C1 and A5 regions following injections of both tracers in the same rabbits, leucoagglutinin into the caudal ventrolateral medulla and virus into the adrenal gland. Varicosities containing leucoagglutinin were seen in contiguity with perikarya and dendritic branches of neurons containing HSV1, in both the C1 and A5 regions. Studies also revealed labeled varicosities in contiguity with TH-containing C1 and A5 neurons. The projection from the caudal medulla to presumed sympathetic premotor neurons in the C1 area, including some C1 cells, represents a potential pathway whereby activity of neurons in the caudal medulla could reduce blood pressure by inhibiting sympathoexcitatory neurons in the rostral medulla.

Neurons in the rat medulla oblongata containing neuropeptide Y-, angiotensin II-, or galanin-like immunoreactivity project to the parabrachial nucleus

Projections from the medulla to the parabrachial complex of the rat were examined for their content of neuropeptide Y-, angiotensin II- or galanin-like immunoreactivity using combined retrograde tracing and immunohistochemical techniques. Rhodamine-labelled latex microspheres were stereotaxically injected into discrete nuclei of the parabrachial complex. After survival of two to five days, colchicine (100 micrograms in 10 microliters saline) was injected into the cisterna magna. One day later, rats were perfused and the brainstems were prepared for visualization of the retrograde tracer and immunoreactivity of one of the three peptides. Retrograde labelling verified that the area postrema, nucleus of the tractus solitarius, caudal spinal nucleus of the trigeminal nerve, parvocellular reticular nucleus, and ventrolateral medulla including the rostral ventrolateral medulla and nucleus paragigantocellularis project to the lateral parabrachial and Kölliker-Fuse nuclei. While most projections were primarily ipsilateral, a small proportion of the projections from the ventrolateral medulla was bilateral. Neurons containing neuropeptide Y-like immunoreactivity were found in the caudal and intermediate nucleus of the tractus solitarius, dorsal to the lateral reticular nucleus and in the nucleus paragigantocellularis. After bilateral microsphere injections into the lateral parabrachial and Kölliker-Fuse nuclei, double-labelled neurons were found dorsal to the lateral reticular nucleus of caudal and intermediate medullary levels, at the ventral surface of the medulla at intermediate levels and in the nucleus paragigantocellularis at rostral levels. Neurons with angiotensin II-like immunoreactivity were observed at the dorsomedial border of the caudal and intermediate nucleus of the tractus solitarius, in the area postrema and in the lateral reticular nucleus and nucleus paragigantocellularis. Of these neurons, small numbers in the nucleus of the tractus solitarius and ventrolateral medulla also projected to the lateral parabrachial and Kölliker-Fuse nuclei. Neurons containing galanin-like immunoreactivity were found in the caudal nucleus of the tractus solitarius, the area postrema, the spinal trigeminal nucleus, the raphe nuclei (pallidus and obscurus), the nucleus paragigantocellularis and dorsal to the lateral reticular nucleus. Of these cells, double-labelled neurons were found in the commissural and medial subdivisions of the caudal nucleus of the tractus solitarius and in the rostral ventrolateral medulla including the ventral surface and the nucleus paragigantocellularis. The results suggest that neuropeptide Y, angiotensin II and galanin may serve as neurochemical messengers in pathways from the medulla to the parabrachial complex. The location of double-labelled neurons suggests that the information relayed by these neurons is related to autonomic activity.

Cardiorespiratory changes induced by vertebral artery injection of sodium cyanide in cats

Brain stem hypoxia caused by vertebral artery injection of sodium cyanide (NaCN) (1-20 micrograms) in artificially ventilated cats depressed phrenic and stimulated sympathetic nerve activity with a simultaneous increase in arterial blood pressure. Larger doses of NaCN caused greater effects. Hypercapnia produced by inhalation of 7% CO2 in O2 tended to reduce NaCN-induced responses on phrenic activity but not on blood pressure or sympathetic activity. Infusion into the vertebral artery with hypoxic saline (3% CO2 in N2) altered blood pressure, also affecting phrenic and sympathetic nerves similarly to NaCN administration. However, washout of CO2 by infusion of 100% O2 bubbled saline at high flow rates (3.6 ml/min) depressed phrenic as well as sympathetic activity and blood pressure. Spinal transection at the first cervical level eliminated sympathetic excitatory response to intravertebral cyanide injection. However, a large dose of NaCN (600 micrograms) given intravenously in spinal animal excited sympathetic activity. We conclude that intravertebral injection of NaCN can be used to study the effects of local hypoxia of the brain stem on cardiorespiratory responses and that hypoxia acts at both these sites (brain stem and spinal cord) to stimulate sympathetic excitation.

Differential control of sympathetic activity to kidney and skeletal muscle by ventral medullary neurons

A rise in arterial blood pressure can be evoked by microinjections of D,L-homocysteic acid into localized regions of the ventrolateral medulla of the cat. Three patterns of sympathetic discharge can be identified during the pressor response. A differential pattern consisting of an increase in renal nerve activity and no change in sympathetic activity to skeletal muscle vasculature can be elicited from sites ventromedial to the caudal pole of the facial nucleus. From more lateral and caudal sites, a generalized sympatho-excitation is evoked in the outflow to both the kidney and hindlimb muscle vasculature. A third response consisting of a differential increase in muscle sympathetic activity simultaneous with a small decrease in renal nerve activity could be evoked from caudal sites, lateral to the inferior olives and superficial to the ventral surface. The results show that ventral medullary neurons can selectively activate sympathetic outflow to control specific vascular beds. These data may support the hypothesis that the ventrolateral medulla contains discrete groups of topographically arranged neurons that can differentially control sympathetic tone to various end-organs.

The effects of electrical stimulation of lobule IXb of the posterior cerebellar vermis on neurones within the rostral ventrolateral medulla in the anaesthetised cat

This study was designed to investigate the effects of electrical stimulation of sublobule IXb of the posterial cerebellar vermis (the uvula) on the activity of neurones in the rostroventral medulla (RVLM) of anaesthetised cats. The ongoing activity of 38 RVLM neurones was studied in detail. One group consisted of 22 neurones that had axons projecting to the spinal cord (Group A, 22 neurones), the second had only an excitatory synaptic input from the spinal cord (Group B, 14 neurones) and the last were inhibited by equivalent spinal cord stimulation (Group C, 2 neurones). In Group A 16 neurones were excited, 2 inhibited and the other 5 showed a biphasic response involving excitation followed by inhibition, to lobule IXb stimulation. In Group B, lobule IXb stimulation excited 9 cells, inhibited 2 and evoked a biphasic response in 3 neurones. In Group C both cells were inhibited on cerebellar stimulation. Some cells in Group A and B were affected by electrical stimulation of the carotid sinus nerve--the predominant effect being an excitation. Specific baroreceptor stimulation invariably caused inhibition and several neurones had pulse modulated discharge. These data indicate that stimulation of sublobule IXb has marked influences on RVLM neuronal activity including a proportion of those neurones that have axons descending to the spinal cord that has been described as presympathetic 'vasomotor' neurones. A striking finding is the more widespread action of sublobule IXb, baroreceptor and sinus nerve inputs on other neurones of the RVLM, and the implications of this for the integration of cardiovascular control are discussed.

Responses of cardiovascular neurons in the rostral ventrolateral medulla of the normotensive Wistar Kyoto and spontaneously hypertensive rats to iontophoretic application of angiotensin II

In female pentobarbital-anesthetized Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR), changes in spontaneous discharges of cardiovascular neurons in the rostral ventrolateral medulla (RVL) in response to iontophoretic application of angiotensin II (Ang II) were studied and compared. It was found that iontophoretic application of Ang II to RVL increased the spontaneous neuronal activities of 30% of the cardiovascular neurons in both types of rats and that the increase was significantly greater in SHR than in WKY. In both types of rats, there was an increase in arterial blood pressure in response to iontophoretic release of Ang II to RVL. The pressor response was accompanied by tachycardia, which was significantly greater in SHR than in WKY. The present study provides evidence that Ang II acts directly on cardiovascular neurons in RVL, and in SHR, an enhanced sensitivity and responsiveness of the RVL cardiovascular neurons to Ang II may augment the sympathetic outflow from RVL and contribute to the genesis of hypertension.

Hypertension and components of vascular resistance

1. In renal wrap hypertension in the rabbit and in essential hypertension in humans, non-autonomic components of resistance are the major factors contributing to elevated blood pressure. 2. The central nervous system plays an important role in moment to moment blood pressure control but is not responsible for maintenance of elevated pressure in essential hypertension. 3. Specific regions of the brain-stem maintain vascular tone through the effects of bulbospinal neurones which are organized both topographically and by chemical content. 4. Structural wall thickening in resistance vessels in hypertension is a manifestation of an abnormality of a growth controlling process. 5. Although several structural muscle growth factors have been identified, it is likely that only a few regulatory processes are critical. 6. Future studies in hypertension should examine associations between genetic markers of medial thickening and environmental influences.

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.

Selective control of sympathetic pathways to the kidney, spleen and intestine by the ventrolateral medulla in rats

1. Electrical activity of multifibre renal, splenic, mesenteric and greater splanchnic nerves and 13th thoracic white rami was recorded in artificially respired, urethane-anaesthetized rats. Discharge of neurones in the rostral ventrolateral medulla was blocked by unilateral microinjections of the inhibitory amino acid glycine and effects on the electrical activity of these sympathetic nerves were compared. 2. Blockade of the rostral ventrolateral medulla caused greater decreases in discharge of renal than splenic nerves and had no consistent effect on mesenteric nerves. This blockade also decreased the discharge of the preganglionic white rami more than that of the preganglionic splanchnic nerves. 3. Postganglionic responses to rostral ventrolateral medulla blockade were always greater than preganglionic responses. 4. The arterial pressure and renal nerve responses to rostral ventrolateral medulla blockade in urethane-anaesthetized rats were not different from those in rats anaesthetized with alpha-chloralose. 5. These findings demonstrate that pre- and postganglionic sympathetic pathways to the kidney are more dependent upon excitatory drive from the rostral ventrolateral medulla than pathways directed to the spleen and intestine.

Quantitative analysis of spinally projecting adrenaline-synthesising neurons of C1, C2 and C3 groups in rat medulla oblongata

Spinal projections of the phenylethanolamine N-methyltransferase (PNMT) immunoreactive neurons of the medulla were investigated using a combination of immunohistochemistry and retrograde transport of colloidal gold particles conjugated to cholera toxin B subunit (CTB-gold). The PNMT-containing adrenergic neurons were localised in three groups, the C1 group in the rostral ventrolateral medulla, the C2 group in the nucleus tractus solitarius/dorsal vagal motor complex in the dorsal medulla and the C3 group in the mediodorsal medulla. The C1 group contained 72% of the medullary PNMT-IR neurons, while C2 comprised 13% and C3 15% of the medullary PNMT-IR neuron population. CTB-gold was injected in the area of the intermediolateral cell column in either upper (T2-T4) or lower (T8-T9) thoracic spinal cord and retrogradely labelled cells were found in the areas of the C1, C2 and C3 groups and in other regions of the medulla which did not contain PNMT-IR neurons. After tracer injections bilaterally at levels T2-T4, retrograde labelling suggested that at least 21% of all medullary PNMT-IR neurons projected to these levels. As a proportion of each group, 26% of C1, 9% of C2 and 33% of C3 neurons projected spinally to T2-T4. After tracer injections bilaterally at levels T8-T9, retrograde labelling suggested that at least 17% of all medullary PNMT-IR neurons projected to these levels. As a proportion of each group, 16% of C1, 9% of C2 and 30% of C3 neurons projected spinally to T8-T9. These figures represent minimum numbers since it is impossible to ensure that every neuron has equal access to the tracer. The results demonstrate that contrary to previous belief, the PNMT-IR innervation of the spinal cord derives from PNMT-IR neurons in the dorsal medulla, as well as from the rostral ventrolateral medulla. Indeed 24% of the PNMT-IR neurons terminating at spinal cord levels T2-T4, and 35% of those terminating at levels T8-T9, derive from the dorsal (C2 and C3) medullary cell groups. Since the PNMT-IR projections are directed towards the intermediolateral cell column, it seems likely that all three groups of medullary adrenaline-containing neurons contribute to the regulation of sympathetic outflow.

Stimulation of a bulbospinal 5-HT pathway in the rat brain produces hyperglycaemia

The glucoregulatory role of spinally projecting serotonin (5-HT) neurones near the ventrolateral surface of the medulla oblongata was investigated by stimulating these nerve cells in normal rats and in rats with selective chemical ablation of 5-HT nerves in the spinal cord. Electrical stimulation of the lateral medulla produced hyperglycaemia in normal rats; the increase in blood glucose was proportional to the intensity and frequency of stimulation. Furthermore, microinjection of kainic acid or L-glutamate at the same sites also produced hyperglycaemia. This stimulation-induced hyperglycaemia was significantly reduced by spinal transection or adrenalectomy. Selective destruction of spinal 5-HT nerves produced by intraspinal injection of 5,7-dihydroxytryptamine also reduced the magnitude of the hyperglycaemia response to electrical stimulation of the lateral medulla. This indicates that stimulation of 5-HT nerve cells adjacent to the ventrolateral surface of the medulla oblongata and projecting to the spinal cord increases the adrenal-sympathetic efferent activity and leads to hyperglycaemia in rats.

Vasomotor neurons in the rostral ventrolateral medulla are organized topographically with respect to type of vascular bed but not body region

Microinjections of sodium glutamate (200-800 pmol) were made into sites within the rostral ventrolateral medulla of anaesthetized cats in which the adrenal glands had been removed and the baroreceptors denervated, while arterial pressure, renal sympathetic nerve activity and hindlimb and forelimb blood flows were measured. Hindlimb and forelimb vascular resistances were affected in a parallel fashion by injections at different sites, but there was a clear dissociation between evoked changes in renal nerve activity and either forelimb or hindlimb vascular resistance. The results indicate that ventrolateral medullary vasomotor neurons are organized topographically with respect to the type, rather than body position, of the vascular beds they control.

Regional hemodynamic effects produced following application of L-glutamic acid to the ventral medullary surface of the cat

The neuronal stimulant L-glutamic acid (GA) was applied to the ventral medullary surface and diastolic arterial blood pressure (DBP), heart rate (HR), renal and femoral arterial blood flow were measured in the chloralose-anesthetized cat. GA (1.0 M) increased DAP by 27 +/- 6.6 mmHg (P less than 0.05, n = 6), but heart rate was not affected (P greater than 0.05). Renal arterial resistance (RR) increased by 15 +/- 5.8% (P less than 0.05) and femoral resistance (FR) increased by 46 +/- 10% (P less than 0.05). The difference in the change in RR and FR was statistically significant (P less than 0.05). These effects were dose-dependent and could be blocked by prior intravenous administration of the alpha-blocker prazosin. These data indicate that neurons in the intermediate area of the ventromedullary surface may have a greater control over femoral flow than renal flow.

Contribution between dorsal and ventrolateral regions of medulla oblongata in vasomotor function of cats

In cats anesthetized with alpha-chloralose and urethane, the medulla oblongata was explored with electrical and/or chemical stimulation for vasopressor actions. Chemical stimulation included semimicroinjection of sodium glutamate or microinjection of dl-homocysteic acid (DLH). The dorsal (DM), particularly its dorsomedial (DMM) portion, and the ventrolateral (VLM) regions were found to be most sensitive to both electrical and chemical stimulation. In between these two regions there was an area in the ventral portion of the gigantocellular and the parvocellular reticular nuclei that was sensitive to electrical and somewhat sensitive also to chemical stimulation. The responses to chemical stimulation indicate the existence of perikarya in these three areas for vasopressor actions. The most active response of VLM followed microinjection of DLH into the region of nu. paragigantocellularis lateralis (PGL) and subretrofacial nu. These responses were about 30% greater than those from DM where the most active area was found in the DMM region including the nu. and tractus solitarius. When an extensive area of VLM was lesioned bilaterally with kainic acid (KA, 1 micrograms/100 nl), the resting systemic arterial blood pressure (SAP) fell 42% and the pressor response on DM stimulation fell by more than 80%. In contrast, after similar KA bilateral destruction of DM the resting SAP fell 38% and the pressor responses from 67%. When only a limited area in the PGL/subretrofacial nu. of the rostral VLM (3-4 pressor sites, 1 mm apart) on one side was lesioned, the resting SAP remained unaltered while the pressor response on the ipsilateral DMM decreased 76%. On the other hand, when a similar smaller KA lesion (3-4 sites) was made in the DMM, although the resting SAP did not change, the pressor response on stimulation of the ipsilateral rostral VLM decreased 28%. Further lesioning of the contralateral DMM then decreased the rostral VLM pressor response 62% without much alternation in the resting SAP. Results of the present experiments are in accordance with our previous results that neuronal perikarya for vasopressor action exist not only in VLM but also in DM (18,34) with the function of VLM slightly predominant over DM. It seems highly possible that reciprocal innervation exists between DM and VLM, at least between DMM and the PGL/subretrofacial nu. region.

Evidence for colocalization of substance P and 5-hydroxytryptamine in spinally projecting neurons from the cat medulla oblongata

Substance P (SP)- and 5-hydroxytryptamine (5-HT)-like immunoreactivities were localized in bulbospinal neurons of the raphe nuclei and ventrolateral medulla (VLM). In raphe pallidus and raphe obscurus virtually all of the spinally projecting neurons contained SP and/or 5-HT. Furthermore, SP and 5-HT were colocalized in half of these spinal-raphe neurons. In raphe magnus few spinally projecting neurons contained either SP or 5-HT. Half of the bulbospinal neurons in the caudal VLM contained SP and/or 5-HT and in 50% of these SP and 5-HT were colocalized. However, no SP-containing neurons in the rostral VLM projected to the spinal cord.

Ventrolateral medullary neurones: effects on magnitude and rhythm of discharge of mesenteric and renal nerves in cats

1. Discharge of whole mesenteric and renal nerves was recorded in eighteen chloralose-anaesthetized, artificially respired cats. 2. Inhibition of tonic activity of neurones within the rostral ventrolateral medulla (RVLM blockade) by bilateral application of glycine caused significant reductions in discharge of renal and mesenteric nerves, arterial blood pressure and heart rate. The decrease in discharge of renal nerves was significantly greater than that of mesenteric nerves. 3. During the response to glycine application, the spinal cord was transected at the first cervical segment. The magnitude of renal nerve discharge after transection was not different from that during blockade of the RVLM. On the other hand, mesenteric nerve activity increased following spinal cord transection, returning to control levels. 4. Power spectral analysis revealed that mesenteric and renal nerves discharged with periodicities ranging from 1 to 6 Hz. Application of glycine to the RVLM reduced the slow rhythm in firing of mesenteric and renal nerves similarly. Transection of the spinal cord resulted in further reduction in the rhythmicity in discharge of both nerves. 5. The results indicate that excitatory drive from the RVLM is crucial for the maintenance of on-going discharge of renal, but not of mesenteric nerves. However, such inputs are apparently essential to maintain the slow rhythm in firing of both nerves.

The ventrolateral medulla. A new site of action of the renin-angiotensin system

High-affinity binding sites for angiotensin II (Ang II) in the ventrolateral medulla suggest that Ang II may act at cell groups that are known to modulate blood pressure. This hypothesis was investigated by the topical application of angiotensin I (Ang I), Ang II, the Ang II antagonist [Sar1, Thr8]Ang II, and the Ang I converting enzyme inhibitor MK 422 to a restricted region of the ventral medullary surface known as the glycine-sensitive area. Both Ang I (100 pmol) and Ang II (100 pmol) produced significant (p less than 0.01) increases in blood pressure (+20 +/- 4 and +31 +/- 5 mm Hg, respectively) that were associated with no change in heart rate. Furthermore, the relationship between the peak increases in blood pressure and Ang II was dose-dependent. Blockade of endogenous Ang II by [Sar1, Thr8]Ang II (13 nmol) produced a significant decrease in baseline blood pressure (-8 +/- 1 mm Hg; p less than 0.001). Similarly, topical application of MK 422 prevented the pressor effect of Ang I. Taken together, these experiments indicate that at least some components of the renin-angiotensin system exist in the ventrolateral medulla and they may modulate vasomotor outflow.

Multi- and single-fibre mesenteric and renal sympathetic responses to chemical stimulation of intestinal receptors in cats

1. In cats anaesthetized with alpha-chloralose and artificially respired, stimulation of intestinal receptors with bradykinin caused greater reflex excitation of mesenteric than of renal efferent multifibre nerve activity and significant pressor responses. 2. Activity of all nerve bundles used in this study was inhibited by stimulation of pressoreceptors. Increases in systemic arterial pressure caused inhibition of activity of renal nerves which was significantly greater than that of mesenteric nerves. 3. Spinal transection caused significant decreases in tonic renal nerve activity without altering the ongoing discharge rate of mesenteric nerves. Stimulation of intestinal receptors in spinal cats still caused significant increases is discharge of mesenteric and renal nerves, indicating that this reflex contains a spinal component. 4. Recordings of activity of individual fibres within mesenteric (21) and renal (23) nerves provided information regarding the basis for the multifibre responses to stimulation of intestinal receptors. The same proportion of fibres from both nerves was excited, but the increase in activity of mesenteric fibres was significantly greater than that of renal fibres. 5. Mesenteric fibres could be classified into two groups, based on their sensitivity to pressoreceptor influences. Fibres that exhibited pressoreceptor-independent discharge had the greatest responses to stimulation of intestinal receptors. 6. Following spinal transection the majority of mesenteric fibres continued to fire, whereas most renal fibres became quiescent. 7. The non-uniform pattern of neuronal excitation to chemical stimulation of intestinal receptors was manifest after spinal transection, demonstrating that exclusively spinal pathways can mediate this differential response pattern. 8. These results support the hypothesis that viscero-sympathetic reflexes may be organized to cause preferential excitation of neural activity directed to the organ from which the reflex originates.

Differential control of sympathetic fibres supplying hindlimb skin and muscle by subretrofacial neurones in the cat

1. Simultaneous recordings were made from postganglionic sympathetic fibres supplying hindlimb skin and skeletal muscle in chloralose-anaesthetized, artificially ventilated cats. Single-fibre activity was either isolated by dissection or discriminated from few-fibre preparations of fascicles in the left superficial peroneal or sural nerve (innervating hairy skin) and common peroneal nerve (innervating muscle). Vasoconstrictor fibres were identified by their spontaneous activity as well as their responses to stimulation of the lumbar sympathetic chain and to changes in baroreceptor activity. The baroreceptors were then denervated by bilateral section of the vagi, carotid sinus and aortic nerves. 2. In five cats, neurones in the region of the subretrofacial nucleus were activated chemically by microinjections of 2-10 nl 0.5 M-sodium glutamate from a micropipette inserted into the ventral surface of the medulla. Both skin and muscle vasoconstrictor fibres were activated by glutamate injections into this region on either side of the medulla. Arterial pressure also rose. 3. Glutamate injections at forty-two sites evoked a positive response, defined as an increase in cutaneous and/or muscle vasoconstrictor fibre activity of at least 25%. This response was evoked only in the cutaneous fibre at sixteen of these sites ('skin points'), only in the muscle fibre at seven sites ('muscle points'), and in both fibres in the remainder ('mixed points'). The largest percentage increases in activity of either type of fibre were obtained from mixed points. 4. The blood pressure rises following glutamate stimulation of muscle points were significantly greater than those produced by stimulation of skin points. Analysis of all positive responses showed that the evoked rise in blood pressure was significantly correlated with muscle sympathetic activity but not with cutaneous sympathetic activity. 5. Glutamate stimulation at different sites could evoke differential responses in skin and muscle vasoconstrictor fibres without any detectable change in the pattern of phrenic nerve discharge. 6. Skin points were grouped in the medial part of the subretrofacial region, and muscle points in the lateral part. In addition, for all positive responses there was a highly significant correlation between the ratio of muscle to cutaneous sympathetic activity evoked, and the distance from the mid-line of the corresponding injection site. 7. These results demonstrate a functional differentiation among subretrofacial neurones in their relative control of the sympathetic vasoconstrictor supply to skin and skeletal muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential control of cardiac and vasomotor activity by neurones in nucleus paragigantocellularis lateralis in the cat

1. In cats anaesthetized with chloralose, neuronal cell bodies were excited by micro-injection of DL-homocysteic acid (DLH). 2. Injections made into the ventrolateral medulla in the region of nucleus paragigantocellularis lateralis (p.g.l.) produced a rise in blood pressure accompanied by either an increase or decrease in heart rate according to the precise site of the injection. The tachycardia was blocked by propranolol (0.15-0.5 mg kg-1 I.V.) whilst bradycardia was abolished by vagotomy or atropine (0.2-1.5 mg kg-1 I.V.). 3. Recordings were made of blood flow to the hind-limb, renal and mesenteric vascular beds in order to study the haemodynamic changes separately in each region. 4. Vasoconstriction was seen in the renal and mesenteric beds and either vasodilatation or vasoconstriction could be evoked in hind-limb muscle. The vasodilatation in hind-limb muscle was resistant to atropine but significantly reduced by propranolol (0.15-1.5 mg kg-1 I.V.) and therefore was probably mediated mainly by circulating adrenaline. 5. The relative contribution by each vascular bed to the rise in peripheral resistance produced by micro-injection of DLH at any one site varied according to the site of the injection and there was a degree of topographical organization within p.g.l. The neuronal pools which elicit tachycardia, renal vasoconstriction and hind-limb dilatation were located at the rostral end of the nucleus whilst those which produced vasoconstriction in the hind-limb muscle and mesenteric vascular beds were represented further caudally.

Central respiratory modulation of subretrofacial bulbospinal neurones in the cat

1. Spontaneous activity was recorded from spinally projecting neurones of the subretrofacial nucleus (s.r.f.) in seven chloralose-anaesthetized, vagotomized, paralysed cats. Nineteen such neurones were identified by their antidromic response to stimulation of the ipsilateral dorsolateral funiculus at C5: their axonal conduction velocities were between 2.1 and 9.1 m/s (mean 5.4), and all were silenced by raising the pressure in the ipsilateral carotid sinus (prepared as a blind sac) to 200 mmHg. DL-homocysteic acid was applied ionophoretically to six of these cells, and these were all excited. 2. Activity was recorded simultaneously from the right phrenic nerve and left cervical sympathetic trunk. The phrenic neurogram was taken as an index of central respiratory drive and used to trigger histograms of s.r.f. neurone activity within the respiratory cycle. To eliminate indirect effects via pressure fluctuations, the remaining arterial baroreceptors were disabled during tests by deflation of the ipsilateral carotid sinus and either section of the contralateral sinus nerve (six cats) or occlusion of the carotid artery (one cat). Phrenic-triggered averages of cervical sympathetic activity were also constructed for comparison. 3. Respiratory modulation was detected in the activity of sixteen out of nineteen s.r.f. neurones. Twelve showed an inspiratory peak in activity that was superimposed on a steady discharge maintained throughout the cycle. Five of this group additionally displayed a post-inspiratory dip in activity to below the late expiratory level, and this feature was also seen in one further cell without any detectable inspiratory peak. Three neurones showed an inverse pattern, with reduced or no activity during inspiration, followed by a 'rebound' increased activity level during the post-inspiratory period. The remaining three cells had no discernible respiratory rhythm. 4. Raising end-tidal CO2 levels increased the degree of respiratory modulation in all six inspiratory-firing cells tested as well as one cell showing inspiratory inhibition. Within the range tested, CO2 had no clear effect on the respiratory modulation of two other cells showing inspiratory inhibition and one showing only post-inspiratory depression. No consistent relation was apparent between the overall firing rate of neurones and end-tidal CO2. 5. These results are discussed with reference to the respiratory modulation of sympathetic neurone activity. It is suggested that s.r.f. neurones supply respiratory-related as well as tonic drive to preganglionic neurones.

Identification and properties of sub-retrofacial bulbospinal neurones: a descending cardiovascular pathway in the cat

Experiments were performed on 21 chloralose-anaesthetized, paralyzed cats to identify electrophysiologically the bulbospinal vasomotor neurones of the rostral ventrolateral medulla. Neurones were recorded from a small region close to the ventral medullary surface (sub-retrofacial nucleus), where previous studies had localized points where microinjected excitant amino acids produced large pressor responses. A total of 157 neurones in this region were antidromically activated by electrical stimulation in the spinal cord, 132 from the ipsilateral dorsolateral funiculus at lower cervical level and 25 from the intermediolateral horn at T1. Of the cells tested, 33/34 were excited by iontophoresis of homocysteic acid, indicating that recordings originated from cell bodies rather than axons. The positions of 16 were marked by dye deposition, and located histologically to the sub-retrofacial nucleus. Stimulation of baroreceptors by inflating a carotid sinus blind sac to 200 mm Hg powerfully inhibited 41/48 bulbospinal neurones tested. The threshold for this effect was 80-100 mm Hg. Baroreceptor-sensitive neurones all had axonal conduction velocities between 2 and 8.8 m/s, and within this range, all but two cells were barosensitive. Barosensitive cells were unaffected by low threshold somatosensory stimulation. Baroreceptor stimulation was still able to silence these neurones' activity even when this was raised several-fold by iontophoresis of homocysteic acid. It is argued that sub-retrofacial bulbospinal neurones probably constitute a major descending vasomotor pathway, and that baroreceptors act by inhibiting them postsynaptically.

Location of neurones with cardiovascular and respiratory function, at the ventral surface of the cat's medulla

A study has been made of the ventral surface of the medulla, to identify neurones with cardiovascular and respiratory functions. Experiments were performed on chloralose-anaesthetized, artificially ventilated cats. Ventral medullary neurones were stimulated by microinjections of excitant amino acid (which selectively activates cell bodies), and responses measured in blood pressure, heart rate, renal sympathetic and phrenic nerve activity. A small region of ventral medulla was found, corresponding to the "glycine-sensitive area", from which large increases in blood pressure and renal nerve activity were evoked by amino acid injections. More caudally, another cell group was localized lateral to the hypoglossal nerve roots, and these neurones depressed blood pressure and renal nerve activity. Two distinct regions were found to increase phrenic nerve activity: rostral to the pressor neurones, encroaching on the trapezoid body (roughly corresponding to area "M"), and a caudal group, close to the depressor neurones (i.e. lateral to the hypoglossal roots). No respiratory response could be evoked from medial to the hypoglossal roots (area "L") and stimulation of neurones in area "S" generally depressed phrenic activity. Neurones with cardiovascular and respiratory actions could be distinguished anatomically. Their locations have been mapped and compared with previous studies.