Afferent connections and spinal projections of the pressor region in the rostral ventrolateral medulla of the cat

Distribution of glutamate decarboxylase-containing neurons in rabbit medulla oblongata with attention to intramedullary and spinal projections

Functional studies in the rabbit suggest that GABA is an important inhibitory neurotransmitter in the control of cardiovascular, respiratory and neuroendocrine functions by the medulla oblongata. The present work was undertaken to provide a description of the distribution in the rabbit medulla of neurons containing glutamate decarboxylase, an enzyme present in GABA-synthesizing neurons. Combined retrograde axonal transport and immunohistochemical studies were carried out to determine intramedullary and spinal projections of immunopositive neurons located in regions particularly relevant to the interpretation of functional studies. Neurons containing glutamate decarboxylase, putatively GABA-containing neurons, were found in all nuclei of the rabbit medulla with the exception of somatic cranial nerve nuclei and the lateral reticular nucleus. The immunopositive cells were distributed throughout individual nuclei and their morphological appearance was similar to that of neighbouring immunonegative neurons in the nucleus. An exception was encountered in the dorsal motor nucleus of the vagus where the glutamate decarboxylase-containing neurons belong to a population of small neurons easily distinguished from the larger vagal preganglionic cells. Many immunopositive cells in the raphe nuclei, in the medial reticular formation and in the vestibular nuclei have axonal projections to the spinal cord and presumably represent sources of inhibitory bulbospinal control. Within the medulla there were glutamate decarboxylase-containing neurons in the nucleus tractus solitarius with projections to caudal but not to rostral regions of the ventrolateral medulla. These neurons could provide a GABAergic input to respiratory, cardiovascular and neuroendocrine neurons in the caudal ventrolateral medulla. Immunopositive cells projecting from the caudal to the rostral ventrolateral medulla could form part of the population of inhibitory vasomotor neurons known to be present in the caudal ventrolateral medulla. Some glutamate decarboxylase-containing neurons just medial to the nucleus ambiguous in the rostral medulla, in the region containing the Botzinger group, project to the caudal ventrolateral medulla and could therefore provide an inhibitory input to caudal respiratory cells.

PNMT-containing neurons in the rostral medulla oblongata (C1, C3 groups) are transneuronally labelled after injection of herpes simplex virus type 1 into the adrenal gland

Herpes simplex virus type 1 (HSV1) was injected into the rat adrenal gland. After 3 days the rat CNS was processed immunohistochemically to demonstrate viral antigen. In the lower thoracic spinal cord viral antigen was found in neurons in the intermediolateral column. In the medulla oblongata HSV1-positive neurons were found in the raphe pallidus and in the C1 and C3 regions of the rostral medulla. Approximately 50% of HSV1-positive neurons in the C1 and all the HSV1-positive neurons in the C3 area also contained phenylethanolamine N-methyltransferase (PNMT) and were thus identified as C1 and C3 cells. The HSV1-positive neurons in the C1 region which did not contain PNMT were also negative for tyrosine hydroxylase and were therefore not catecholamine-synthesizing neurons. The HSV1-positive neurons in the medulla oblongata were presumably transsynaptically labelled from the adrenal gland and our study therefore provides neuroanatomical evidence supporting the view that some C1 neurons are involved in controlling the function of the adrenal gland.

Excitatory and inhibitory effects of epinephrine on neonatal rat sympathetic preganglionic neurons in vitro

Current and voltage recordings were made from antidromically identified sympathetic preganglionic neurons (SPNs) in transverse thoracolumbar spinal cord slices removed from neonatal rats. When applied by either pressure ejection or superfusion, epinephrine (Epi) caused a slow depolarization or an inward current in 62 SPNs (42%) and a slow hyperpolarization or an outward current in 21 SPNs (14%). The responses persisted in low calcium- or tetrodotoxin-containing media. The Epi-induced depolarization or inward current was associated with increased membrane resistance; it was reduced by membrane hyperpolarization and nullified at a membrane potential of about -100 mV; a clear reversal however was not observed at more negative potential levels. In a number of SPNs the Epi-induced depolarization was accompanied by small inhibitory postsynaptic potentials. The latter were eliminated by a low calcium solution and by the glycine antagonist strychnine, suggesting that they were caused by glycine or a glycine-like substance released from interneurons subsequent to activation by Epi. The Epi-induced hyperpolarization or outward current was associated with decreased membrane resistance, and nullified around -100 mV. The alpha-adrenergic antagonist, dihydroergotamine, and alpha 1-antagonist, prazosin, reversibly blocked the excitatory, whereas the alpha 2-antagonist, yohimbine, abolished the inhibitory response, respectively. It is concluded that Epi acting on alpha 1- and alpha 2-adrenergic receptors depolarizes and hyperpolarizes the rat SPNs by decreasing or increasing membrane conductances to potassium ions.

Evidence for an excitatory amino acid pathway in the brainstem and for its involvement in cardiovascular control

The source and possible role of excitatory amino acid projections to areas of the ventrolateral medulla (VLM) involved in cardiovascular control were studied. Following the injection of [3H]D-aspartate ([3H]D-Asp), a selective tracer for excitatory amino acid pathways, into vasopressor or vasodepressor areas of the VLM in rats, more than 90% of retrogradely labelled neurones were found in the nucleus of the solitary tract (NTS). Very few of the [3H]D-Asp-labelled cells were immunoreactive for tyrosine hydroxylase, none for phenylethanolamine-N-methyltransferase or gamma-aminobutyric acid. The density of labelled cells in the NTS was similar to that obtained with the non-selective tracers wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and WGA-colloidal gold, but these tracers also labelled other cell groups in the medulla. Furthermore, the decrease in blood pressure, caused by pharmacological activation of neurones in the NTS of rats, or by electrical stimulation of the aortic depressor nerve in rabbits could be blocked by the selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate injected into the caudal vasodepressor area of the VLM. This area corresponds to the termination of [3H]D-Asp transporting NTS neurones. These results provide evidence that a population of NTS neurones projecting to the VLM use excitatory amino acids as transmitters. Among other possible functions, this pathway may mediate tonic and reflex control of blood pressure via NMDA receptors in the VLM.

The cardiovascular actions of clonidine and neuropeptide-Y in the ventrolateral medulla of the rat

1. The cardiovascular responses to neuropeptide-Y (NPY) (25 and 50 pmol) and clonidine (10 and 20 nmol) were examined following microinjection into the rostral ventrolateral medulla (RVLM) and the caudal ventrolateral medulla (CVLM). Mean arterial pressure (MAP) and heart rate (HR) were measured in anaesthetized rats, pre- and post-injection. 2. The alpha 2-adrenoceptor agonist clonidine (10 and 20 nmol) reduced MAP and HR significantly when microinjected into the CVLM and RVLM. 3. NPY (25 and 50 pmol) microinjected into the CVLM decreased MAP and HR. However, in the RVLM neither dose had a significant cardiovascular effect. 4. The possibility of a functional interaction between the adrenergic system and NPY was examined by co-administration of clonidine and NPY in doses that gave submaximal blood pressure responses. In the CVLM this produced hypotension and bradycardia which was similar in magnitude to the sum of their individual responses, indicating that in this area their actions appear to be independent. 5. In the RVLM, where NPY has no significant cardiovascular effects, co-administration with clonidine, did not alter the response to clonidine. 6. It appears that in the areas investigated, there is no functional interaction between NPY and clonidine.

Are there bulbospinal catecholaminergic neurones in the guinea pig equivalent to the C1 cell group in the rat and rabbit?

The C1 cell group in the rat is characterized by neurones which contain both adrenaline and phenylethanolamine-N-methyltransferase, and usually also neuropeptide Y (NPY). The former two substances are lacking in Guinea pig brainstem and spinal cord. We have examined the distribution of NPY- and tyrosine hydroxylase-immunoreactivity in the ventrolateral medulla and thoracolumbar intermediate zone of Guinea pig, as well as the distribution of catecholamine-containing neurone somata and spinal terminals visualized after formaldehyde-glutaraldehyde fixation. The results are compared with comparable immunohistochemical data obtained from rats and rabbits. Catecholaminergic neurones in the Guinea pig with locations and terminations that correspond to those of the C1 cell group in rat and its analogue in the rabbit appear to consist of two subgroups, with only the more caudal group containing NPY. The more rostral group requires pretreatment with monoamine oxidase inhibitor to permit visualization of catecholamine fluorescence, a property previously though to be characteristic of adrenergic neurones. This observation raises the possibility that the catecholaminergic cell group in the C1 region of rabbits may not contain adrenaline either.

Intracisternal naloxone and cardiac nerve blockade prevent vasodilatation during simulated haemorrhage in awake rabbits

1. Acute haemorrhage was simulated in five unanaesthetized rabbits, by inflating a cuff on the inferior vena cava so that cardiac output fell by 8.3% of its resting level per minute. Simulated haemorrhage was performed after sham treatment, after graded doses of intravenous and intracisternal naloxone, and after cardiac nerve blockade with intrapericardial procaine. 2. After sham treatment, the haemodynamic response to simulated haemorrhage was biphasic. During the first phase, systemic vascular conductance fell steadily, heart rate rose steadily, and arterial pressure fell only slightly. A second decompensatory phase began abruptly when cardiac output had fallen to approximately 55% of its resting level. Vascular conductance rose steeply, heart rate fell slowly, and arterial pressure fell precipitately. 3. Treatment with naloxone (intravenous, 0.04-0.4 mg kg-1; intracisternal, 0.2-2 micrograms kg-1) did not affect either phase of the haemodynamic response to simulated haemorrhage. 4. After treatment with larger doses of naloxone (intravenous, 4-8 mg kg-1; intracisternal, 4-69 micrograms kg-1), the first phase was unaffected, but the second phase no longer occurred. Throughout simulated haemorrhage, systemic vascular conductance fell steadily, heart rate rose, and arterial pressure was well maintained. The dose of intracisternal naloxone which prevented the second phase was 90-900 times less than the corresponding intravenous dose. The second phase was also prevented by cardiac nerve blockade. 5. We conclude that an endogenous opiate mechanism is responsible for the haemodynamic decompensation that occurs when cardiac output falls to a critical level. The mechanism is located within the central nervous system. It is triggered by a signal from the heart.

An enkephalin-containing pathway from nucleus tractus solitarius to the pressor area of the rostral ventrolateral medulla of the rabbit

A technique combining retrograde tracing of wheat germ-conjugated gold particles with immunocytochemical demonstration of enkephalin-containing neurons was used to study intramedullary enkephalin-containing pathways to the pressor area of the rostral ventrolateral medulla in rabbits. The rostral ventrolateral medulla represents a main source of bulbospinal sympathoexcitatory neurons, and is critical to the tonic and reflex control of blood pressure. Firstly, the distribution of enkephalin-positive neurons and terminal fibres in rabbit medulla were described, with special reference to a moderately dense terminal plexus in the rostral ventrolateral medulla. Then, retrograde tracing studies were conducted; the rostral ventrolateral medullary pressor region was first localized by injection of L-glutamate (25 nmol in 50 nl). Slow (30-min) injections of wheat germ-gold (1.00 microliter) were then made at the same coordinates, resulting in a restricted injection site corresponding to the C1 pressor area, verified by the presence of tyrosine hydroxylase- and neuropeptide Y-containing neurons. Transported gold was revealed by silver reduction, and enkephalin immunoreactive cells were revealed by immunocytochemistry. Enkephalin-positive gold-containing neurons were found primarily in the nucleus tractus solitarius, especially in the commissural and medial intermediate subnuclei. Cells in the nucleus tractus solitarius containing other transmitters (substance P. galanin, neuropeptide Y and catecholamines) did not show the same degree or pattern of double-labelling, suggesting that the transport was not due to non-specific silver reduction or spread from the pipette track. The potential importance of this endogenous intramedullary opiate system is discussed in terms of medullary control of the cardiovascular system. It is hypothesized that this opiate projection from the nucleus tractus solitarius to the rostral ventrolateral medulla could play an important modulatory function, influencing baroreceptor or other cardiopulmonary reflex pathways involved in the primary regulation of the cardiovascular system. Furthermore, this pathway could represent a central substrate underlying opiate effects on the cardiovascular system during such conditions as hemorrhagic shock, stress or opiate intoxication.

Somatic and vagal afferent convergence on solitary tract neurons in cat: electrophysiological characteristics

Electrophysiological characteristics are described for 67 neurones localized to subnuclei of the solitary tract or the area of the dorsal motor nucleus of the vagus in alpha-chloralose-anesthetized, paralysed cats which received vagal and hindlimb sural or peroneal nerve excitation. The peroneal and sural nerves were stimulated in an exposed hindlimb preparation; the ipsilateral vagus was stimulated at the cervical level. Compound action potentials were recorded from all three nerves. Neurons were recorded with extracellular microelectrodes from the brain stem solitary area contralateral to the stimulated somatic nerves. Ninety-one percent of the recorded neurons were spontaneously active. Eighteen percent and 5% of the neurons received only peroneal or sural excitation, respectively, while 59% of the neurons received convergent peroneal and sural excitation. Thirty-nine of the 67 neurons were also tested for vagal input of which 41% responded with excitation. All of the neurons tested for vagal input also received converging excitation from one or both of the somatic nerves. Thirty-one percent of the vagal-excited neurons received converging input from both the peroneal and sural nerves. The combined mean minimal conduction velocity for peroneal and sural input was 31 +/- 1 m/s (mean +/- 1 S.E., range 9-54 m/s). Thirty-six percent of the peroneal and 31% of the sural afferents were Group II fibers. Significant periods of inhibition of spontaneous neuronal spike activity followed peroneal and sural excitation in 43 and 39% of the neurons, respectively. In many neurons, both excitation and inhibition of spike activity could be elicited at stimulus intensities as low as 1.2 times threshold for the lowest threshold fibers in each nerve. Somatic nerve-induced inhibition of spontaneous neuron activity without prior excitation was also observed. These results suggest that neurons of the solitary tract nuclei receive Group II and Group III somatic afferents which converge on neurons also receiving excitatory vagal input. Consequently, somesthetic and kinesthetic as well as visceral receptor activation may directly modulate solitary tract neurons. A possible conclusion is that the nucleus tractus solitarius is the initial central site of mediation of somatosympathetic reflexes. Modulation of the nucleus tractus solitarius by somatic afferents may then adjust sympathetic tone, via modulation of other medullary centers, in visceral and somatic tissues to match somatic metabolic needs.

The nuclei of origin of monoaminergic, peptidergic, and cholinergic afferents to the cat nucleus reticularis magnocellularis: a double-labeling study with cholera toxin as a retrograde tracer

Using a sensitive double-immunostaining technique with nonconjugated cholera toxin B subunit (CT) as a retrograde tracer, we examined the cells of origin and the histochemical nature of afferents to the cat nucleus reticularis magnocellularis (Mc) of the medulla oblongata. After injections of CT confined to the Mc, we found that the major afferents to the Mc arise from: (1) the lateral part of the bed nucleus of the stria terminalis, the nucleus of the anterior commissure, the preoptic area, the central nucleus of the amygdala, the posterior hypothalamus, and the nucleus of the fields of Forel; (2) the Edinger-Westphal nucleus, the mesencephalic reticular formation, and the ventrolateral part of the periaqueductal grey; (3) the nuclei locus coeruleus alpha (LC alpha), peri-LC alpha, locus subcoeruleus, and reticularis pontis oralis and caudalis; (4) the caudal raphe nuclei; and (5) the nucleus reticularis ventralis of the medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

Origin of tonic GABAergic inputs to vasopressor neurons in the subretrofacial nucleus of the rabbit

The aims of this study were to determine (1) whether the vasomotor effects reflexly elicited by baroreceptor stimulation are dependent upon gamma-aminobutyric acid (GABA) receptors in the subretrofacial (SRF) nucleus in the rostral ventrolateral medulla; (2) the extent to which inputs other than those arising from peripheral baroreceptors, or transmitted via the nucleus tractus solitarius (NTS), contribute to the tonic GABAergic inhibition of SRF vasopressor cells. Following bilateral injection of a mixture of the GABA antagonist bicuculline methiodide (500 pmol) and GABA agonist muscimol (500 pmol) into the SRF nucleus, the sympathoinhibitory response normally evoked by a rise in arterial pressure (induced by inflating an aortic cuff) was abolished in 4 out of 8 rabbits and reduced in the remainder. For the whole group, the mean reduction in this response was 71%. In other experiments, the pressor response produced by injection of bicuculline methiodide into the SRF nucleus was still present after (1) destruction of the intermediate portion of the NTS, and (2) complete removal of the brain rostral to the pons. We conclude that (1) an inhibitory GABAergic input into the SRF nucleus is an important component of the central pathways mediating baroreceptor inhibition of sympathetic vasomotor tone; (2) the SRF nucleus also receives tonic GABAergic inputs that are intrinsic to the lower brainstem and are independent of baroreceptor or other cardiovascular inputs relayed by the NTS.

Anatomical evidence that hypertension associated with the defence reaction in the cat is mediated by a direct projection from a restricted portion of the midbrain periaqueductal grey to the subretrofacial nucleus of the medulla

Wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections were made at sites within a restricted portion of the midbrain periaqueductal grey region (PAG) of the cat at which microinjection of the excitant amino acid, D.L-homocysteic acid, elicits the strongest form of a defence reaction, including a hypertensive response. Among the revealed projections, significant anterograde labelling was found in a discrete region of the rostral ventrolateral medulla, the subretrofacial nucleus (SRF). In the cat, the SRF contains pressor neurones which project to the spinal preganglionic sympathetic outflow. The labelling was most marked ipsilaterally, although substantial contralateral labelling was also observed. To verify that the projection to the SRF originated from the restricted 'defence region' of the PAG, WGA-HRP or rhodamine-labelled microspheres were injected into physiologically-identified sites in the SRF. In all experiments, labelled neurones were found in the same restricted region of the PAG at which DLH injection evokes hypertension and behavioural signs of the defence reaction. The results are consistent with the hypothesis that a discrete cell group within the PAG mediates both somatic and autonomic components of the defence reaction and that the characteristic hypertensive response is mediated by a direct pathway from these PAG cells to pressor neurones in the SRF.

Angiotensin II receptor binding and the baroreflex pathway

Angiotensin II (Ang II) acts centrally to modulate autonomic activity and cardiovascular function. Using in vitro autoradiography we have determined the distribution of putative receptors for ANG II in brain regions associated with the baroreflex pathway. Ang II receptor binding sites were observed in the nucleus of the solitary tract (NTS), the dorsal motor nucleus of vagus, the rostral and caudal ventrolateral medulla and intermediolateral cell column of the spinal cord. Receptor binding sites were also observed in the nodose ganglion and in association with nerve fibres in the heart. Nodose ganglionectomy and vagal ligation studies revealed that Ang II receptor binding sites are produced in the nodose ganglion and transported in the vagus nerve to terminals of vagal afferent neurones. In the NTS these presynaptic receptors could mediate the known baroreflex inhibitory action of Ang II by inhibition of neurotransmitter release. In the cat, Ang II receptors were sharply localized in the rostral ventrolateral medulla to the subretrofacial nucleus. Microinjection of Ang II (10-50 pmoles) into this region induced a sympathetically-mediated pressor response. Together these results demonstrate several regions within the baroreflex arc which contain Ang II receptors, at which Ang II may modulate cardiovascular control and autonomic function.

Angiotensin receptors and the vagal system

Angiotensin II (Ang II) is known to attenuate the vagal component of the baroreflex at both central and peripheral cardiac sites. Ang II receptor binding sites occur in both the nucleus of the solitary tract (NTS), where they are associated with vagal afferent terminals, and in the dorsal motor nucleus of vagus. In this study we have examined the distribution of Ang II binding sites in the cell bodies of vagal afferents in the nodose ganglion, and investigated whether these receptors are transported in the vagus nerve. Dense Ang II receptor binding was observed over neuronal cell bodies in the nodose ganglion and, in streaks, in the vagus nerve. Vagal ligation distal to the nodose ganglion resulted in a marked accumulation of receptor binding sites, proximal to the ligature, with a moderate increase on the distal side. These results demonstrate that Ang II receptor binding sites occur in the nodose ganglion and are transported centrally in the vagus to be located on presynaptic terminals in the NTS and also peripherally where they may occur on terminals of the vagus.

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)