Function of the ventrolateral medulla in the control of the circulation

Control of neurosecretory vasopressin cells by noradrenergic projections of the caudal ventrolateral medulla

Activation of noradrenergic afferents arising from the A1 cell group of the caudal VLM excites neurosecretory AVP cells of both the supraoptic and paraventricular nuclei, thus stimulating the release of this potent vasoconstrictor into the circulation. Although this effect is mimicked by application of alpha 1-adrenoreceptor agonists to AVP cells, the excitatory effects of A1 afferents may not be mediated by activation of post-synaptic alpha 1-receptors. Evidence has also been obtained that the actions of A1 afferents are not dependent upon the release of excitatory amino acids or NPY, although the latter is co-stored with NA in A1 cells and potentiates the actions of low concentrations of NA on AVP cells. Although a projection to AVP and OXY neurosecretory cells from the A2 NA cell group of the NTS has been established, this projection does not appear to contribute directly to the control of SON AVP cell activity. Rather, NTS stimulation excites SON AVP cells via a relay projection through the A1 cell group. This pathway is likely to correspond to that involved in the stimulatory effects of haemorrhage and caval constriction on AVP secretion, although it is uncertain whether the effects of these particular stimuli are contingent upon unloading of arterial baroreceptors and atrial stretch receptors, as commonly presumed, or upon the activation of other receptors such as ventricular mechanoreceptors or chemoreceptors. On balance, current evidence suggests that the A1 projection is unlikely to be critically involved in mediating the effects of arterial baroreceptor, arterial chemoreceptor, or atrial stretch receptor activation on AVP cells.

Cholinergic mechanisms subserving cardiovascular function in the medulla and spinal cord

This investigation was designed to study the role of muscarinic receptor subtypes in the cardiovascular responses elicited by microinjection of cholinergic agonists in the intermediate portion of the NTS, the VLPA and VLDA areas and the IML of the spinal cord. Microinjections of L-glutamate (1.77 nmol in 20-50 nl in 0.9% sodium chloride solution) were used to identify these areas. Bilateral microinjections (0.02-2 nmol/site) of a potent M2 muscarinic receptor agonist, CD, but not those of a relatively selective M1 receptor agonist (McNA343; 3 nmol/site), into the intermediate portion of NTS and the VLDA induced depressor and bradycardic responses. In the VLPA these agonists elicited pressor and tachycardic effects while in the IML at T1-T3 only increase in HR was observed. Previous microinjections of a selective competitive M2 receptor antagonist (AFDX-116; 0.8-1.6 nmol/site), but not those of a potent selective M1 receptor antagonist (PZ; 1.5-2.0 nmol/site), into these areas blocked the effects of CD. These results indicate that the muscarinic receptors of M2 type may play a part in the regulation of cardiovascular function in the above-mentioned cardiovascular areas in a yet unidentified manner.

Organization of ventrolateral medullary afferents to the hypothalamus

In summary, these anatomical and electrophysiological data have provided evidence to support the suggestion that VLM neurons project directly to regions of the hypothalamus that contain magnocellular neurosecretory neurons. In addition, these results support the suggestion that pathways ascending from the VLM to the hypothalamus function, in part, in the control of the release of the neurohypophyseal hormones by PVH and SON magnocellular neurosecretory neurons during activation of peripheral cardiovascular receptors.

Electrophysiological characterization of putative C1 adrenergic neurons in the rat

Recent studies in the rat have demonstrated that at least two populations of sympathoexcitatory reticulospinal neurons reside in the nucleus reticularis rostroventrolateralis. It appears that only one of these populations consists of C1 adrenergic neurons. The present study used both double-labeling (one retrograde tracer and immunohistochemistry) and triple-labeling (two retrograde tracers and immunohistochemistry) to determine if C1 adrenergic neurons, which are immunoreactive for phenylethanolamine N-methyltransferase, exhibit a projection pattern that is sufficiently unique to permit the electrophysiological discrimination between C1 adrenergic and non-adrenergic neurons in the nucleus reticularis rostroventrolateralis. Double-labeling experiments indicated that 71% (range: 53-80) of phenylethanolamine-N-methyltransferase-immunoreactive neurons in the nucleus reticularis rostroventrolateralis could be retrogradely labeled from the thoracic cord, as were 76% (range: 67-94) following tracer injection in the central tegmental tract at pontine levels. Triple-labeling experiments indicated that 88% (range: 82-93) of nucleus reticularis rostroventrolateralis neurons with projections to both spinal cord and central tegmental tract were phenylethanolamine-N-methyltransferase-immunoreactive. Single-unit recording, in nucleus reticularis rostroventrolateralis, was used to identify antidromic potentials elicted from stimulation sites in the spinal cord and/or central tegmental tract. Since clonidine is known to reduce central adrenaline turnover, sensitivity to this drug was used to identify putative adrenergic neurons. Twenty-six nucleus reticularis rostroventrolateralis neurons with axonal projections to both the ipsilateral spinal cord and the central tegmental tract were recorded in halothane-anesthetized rats. All these cells were barosensitive, pulse-modulated, and 16 of the 16 cells tested exhibited a 66 +/- 8% reduction in activity upon the intravenous administration of clonidine (20 micrograms/kg). Most (13 out of 16) exhibited a strong respiratory modulation. The conduction velocity of their spinal collateral was generally low (0.9 +/- 0.1 m/s) and their firing rate moderate (7.4 +/- 1.2 spikes/s). Forty-three nucleus reticularis rostroventrolateralis cells with axonal projections exclusively to the thoracic cord were studied for comparison. These cells were strongly barosensitive and pulse-synchronous, had a high discharge rate (25 +/- 3 spikes/s) and a moderate conduction velocity (3.4 +/- 0.3 m/s). Only one of the 15 cells tested was inhibited by clonidine and only two to these 15 cells exhibited a detectable respiratory modulation. Thus barosensitive nucleus reticularis rostroventrolateralis neurons with axonal projections to both the spinal cord and the central tegmental tract likely belong to the C1 adrenergic cell group. It is concluded that this subgroup of adrenergic neurons probably subserves a vasomotor function.

Coexistence of catecholamine and methionine enkephalin-Arg6-Gly7-Leu8 in neurons of the rat ventrolateral medulla oblongata. Application of combined peptide immunocytochemistry and histofluorescence method in the same vibratome section

An overlapping distribution of catecholamine-containing cells and proenkephaline-A derived peptide-containing neurons have been identified in the rat medulla oblongata. However, it is not evident whether the coexistence of these bioactive substances occurs in the same neurons or not. Therefore, we examined the coexistence of catecholamine and methionine-enkephalin-Arg6-Gly7-Leu8 (MEAGL), a proenkephaline-A derived peptide, using a combination of histofluorescence and peroxidase-anti-peroxidase (PAP) immunohistochemical (modified formaldehyde-glutalaldehyde (Faglu)) methods on the same tissue sections. We found one third of A1/C1 catecholamine fluorescent cells show MEAGL-like immunoreactivity.

Projections of the nucleus of the tractus solitarius in the pigeon (Columba livia)

With the aid of autoradiographic and histochemical (WGA-HRP) tracing techniques, the projections of the nucleus of the tractus solitarius (nTS) in the pigeon have been delineated and related to the viscerotopic organization of the nucleus. As in mammals, nTS projects to both brainstem and forebrain structures. At medullary levels, projections were seen to nTS itself, to the dorsal motor nucleus of the vagus and to the subjacent and more ventral reticular formation. There is a substantial projection to the parabrachial nuclear complex with terminations in all its subnuclei and minor projections to locus coeruleus and several mesencephalic areas, including the ventral area of Tsai, the nucleus of the ascending brachium conjunctivum, and the compact portion of the tegmental pedunculopontine nucleus. At diencephalic levels, projections to the hypothalamus (magnocellular periventricular nucleus, stratum cellulare internum and externum) and dorsal thalamus were seen. Terminal fields within the basal telencephalon included the nucleus of the pallial commissure, the bed nucleus of the stria terminalis, and the nucleus accumbens. The organization of nTS projections in pigeons is correlated with the pattern of inputs to specific nTS subnuclei. Lateral tier subnuclei receiving cardiovascular and pulmonary inputs project upon the ventrolateral reticular formation and the ventrolateral parabrachial complex. Medial tier subnuclei receiving gustatory and gastrointestinal inputs project upon dorsal and medial parabrachial nuclei. Transparabrachial projections arise from nTS subnuclei receiving little or no primary input from the viscera.

Putative neurotransmitters involved in medullary cardiovascular regulation

Experiments were carried out in artificially ventilated pentobarbital-anesthetized male Wistar rats. Following microinjection of muscimol (GABA-mimetic) or kynurenic acid (KYN; glutamate antagonist) into the ventrolateral medullary depressor area (VLDA), microinjection of L-glutamate (GLU; 4.5 nmol) into the NTS elicited a pressor response. This pressor response was attenuated in a dose-dependent manner by microinjection of KYN (0.5-5 nmol) into the ventrolateral medullary pressor area (VLPA). A GLU-induced pressor response could also be elicited from the NTS when GABA receptors in the VLPA were blocked with the microinjection of bicuculline (GABA antagonist, 200 pmol) into this site. The same dose of bicuculline in the VLPA also blocked the depressor responses elicited from the VLDA. With the VLDA or VLPA functionally unimpaired, microinjection of GLU (4.5 nmol) into the NTS elicited a fall in blood pressure and heart rate. This depressor response was attenuated in a dose-dependent manner by the microinjections of KYN (2-20 nmol) into the VLDA. These results indicate that: (1) The NTS sends glutamatergic inputs to both the VLDA and the VLPA; the projection from the NTS to the VLPA mediates pressor responses while that from the NTS to the VLDA represents one component of the pathway mediating the depressor responses elicited from the NTS. (2) The pathway from the VLDA to the VLPA is GABA-ergic and represents another component of the pathway mediating depressor responses evoked from the NTS. (3) The bradycardia evoked from the NTS may involve a pathway from the NTS to the VLDA.

Spinal action of neurokinins producing cardiovascular responses in the conscious freely moving rat: evidence for a NK-1 receptor mechanism

This study was initiated to characterize the receptors which mediate the cardiovascular responses elicited by the intrathecal (i.th.) administration of neurokinins (NK) in the conscious freely moving rat. The dose response profile for substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) was determined over 0.065-65 nmol doses of the peptides. After i.th. administration at the T8-T10 thoracic level, only SP elicited a dose dependent pressor response. However, all NK elicited a dose dependent increase in heart rate (HR), and the following rank order of potency was observed: SP greater than NKA greater than NKB. SP (6.5 nmol) produced cardiovascular responses markedly greater than an equimolar dose of any of the seven SP fragments which were studied. The C-terminal sequences SP (4-11), [pGlu5]SP (5-11), [pGlu6]SP (6-11), and SP (7-11), as a group were slightly more potent than the N-terminal fragments, SP (1-4), SP (1-7) and SP (1-9) which were almost inactive. The NK-1 receptor selective agonists [Pro9, Met(O2)11]SP and [beta-Ala4, Sar9, Met(O2)11]SP (4-11), produced pressor and positive chronotropic responses equal to or greater in intensity than SP. With up to 6.5 nmol of the NK-2 receptor selective agonist [Nle10]NKA (4-10), no dose dependent cardiovascular response was produced and the NK-3 receptor selective agonist senktide (succinyl-[Asp6, MePhe8]SP (6-11], produced neither a cardiac nor pressor response when 6.5 nmol was administered. These results are consistent with the hypothesis that, receptors of the NK-1 subtype mediate the cardiovascular responses evoked by the spinal action of NK.

Respiratory and vasomotor responses to focal cooling of the ventral medullary surface (VMS) of the rat

Experiments were performed on anesthetized, paralyzed and artificially ventilated rats after denervation of the vagus and carotid sinus nerves. The electrical activity of the phrenic and cervical sympathetic nerves (CS) along with the arterial blood pressure (BP) were monitored. Graded unilateral cooling of the ventral lateral surface (VMS) from 37 degrees C to 10 degrees C between 6th and 12th nerve rootlets did not affect the phrenic activity. Whereas, a significant depression or apnea was seen with cooling of an area between 1st cervical and 12th nerve rootlets. Bilateral cooling also produced similar respiratory responses. Respiratory depression could also be obtained during higher respiratory drive (7% CO2 in O2). On the other hand, a significant fall in BP and reduction in CS activity were observed with unilateral cooling in any of these VMS areas. However, the magnitude of BP decrease was less with 7% CO2 in O2 compared to 100% O2 breathing.

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.

Evidence for a sympathoexcitatory pathway from the nucleus tractus solitarii to the ventrolateral medullary pressor area

The role of the ventrolateral medullary pressor (VLPA) and depressor (VLDA) areas in mediating cardiovascular responses evoked from the nucleus tractus solitarius (NTS) was investigated. Male Wistar rats, anesthetized with pentobarbital or urethane, were artificially ventilated and blood pressure (BP) and heart rate (HR) were monitored. The VLPA, VLDA, and the NTS were identified bilaterally with microinjections of L-glutamate. Unilateral microinjections of muscimol or lidocaine into the VLPA or the VLDA blocked the decrease in BP produced by microinjections of L-glutamate (1.77 nmol) into the NTS. These findings indicate that both areas are essential for mediating depressor responses elicited from the NTS. When neuronal activity in the VLDA was depressed unilaterally (leaving the ipsilateral VLPA intact), with the microinjection of muscimol or lidocaine, microinjection of a larger dose (5.0 nmol) of L-glutamate into the ipsilateral NTS elicited a pressor response. This response was blocked by depressing neuronal activity in the ipsilateral VLPA by microinjection of muscimol into this site. This pressor response evoked from the NTS was not due to non-specific effects of L-glutamate since repeated microinjections of L-glutamate (5.0 nmol/site) into the NTS consistently produced decreases in BP and HR. The stimulation of the contralateral NTS by glutamate continued to elicit the usual decreases in BP and HR. Microinjections of either dose (1.77 or 5 nmol) of L-glutamate into the areas adjacent to the NTS (e.g. 1.0 mm rostral or lateral to the NTS, the gracile or cuneate nuclei and area postrema) failed to evoke any cardiovascular responses indicating that the responses were mediated by neurons localized within the intermediate one-third of the NTS. These results indicate that: (1) the depressor responses elicited from the NTS involve the pathways from the NTS to the VLDA and VLDA to VLPA and (2) there may be a pathway from the NTS to the VLPA which is sympathoexcitatory and is unmasked when neuronal activity in the VLDA is depressed.

Distribution of catecholaminergic neuronal systems in the canine medulla oblongata and pons

The distribution of catecholamine-containing neurons, fibers, and varicosities in the brainstem of both adult and juvenile dogs was mapped in detail with glyoxylic acid histofluorescence. Four separate groups of catecholamine-fluorescent neurons were identified within the canine medulla and pons in locations comparable to the A1, A2, A5, and A6 regions reported in other species. However, aspects of the pattern and density of the catecholaminergic neuronal systems appeared to be unique to the dog. The A1 neurons of the caudal ventrolateral medulla were much more scattered than in rats or rabbits, but relatively similar to cats. In the A2 region of the dorsomedial medulla, catecholaminergic cells and fibers were uniquely distributed compared to other species: fluorescent neurons were scattered only within the dorsal motor nucleus of the vagus, and a distinctive pattern of fibers and varicosities outlined the nucleus of the solitary tract and dorsal motor nucleus of the vagus. The A5 neurons of the rostral ventrolateral medulla appeared at the rostral limit of the A1 region. Fluorescent A5 cells were more sparse than in rats or primates, and were patterned similarly to cats and rabbits. The canine A6 region contained the most extensive and dense grouping of catecholamine neurons and was similar in pattern to the rabbits but less extensive than that seen in cats or primates. An ascending catecholaminergic fiber pathway was traced through the central tegmental field of the canine medulla and pons, with features similar to the primate. The present study provides the first description of the catecholaminergic neuronal systems of the canine medulla.

Anatomical specificity of noradrenergic inputs to the paraventricular and supraoptic nuclei of the rat hypothalamus

The distribution of neural inputs to the paraventricular (PVH) and supraoptic (SO) nuclei from the regions of the A1, the A2, and the A6 (locus coeruleus) noradrenergic cell groups was investigated by using a plant lectin, Phaseolus vulgaris leucoagglutinin (PHA-L), as an anterogradely transported tracer. An immunofluorescence double-labeling procedure was used to determine the extent to which individual anterogradely labeled fibers and terminals in the PVH and the SO also displayed immunoreactive dopamine-beta-hydroxylase (DBH), a marker for catecholaminergic neurons. The results may be summarized as follows: (1) Projections from the A1 region were found primarily, and in some experiments almost exclusively, in those parts of the magnocellular division of the PVH and the SO known to contain vasopressinergic neurons. (2) Projections from the A2 region were distributed primarily throughout the parvicellular division of the PVH and were most dense in the dorsal medial part, a region known to contain a prominent population of corticotropin-releasing factor (CRF)-immunoreactive neurons. In addition, a less-dense projection to the magnocellular division of the PVH and to the SO was consistently found. (3) Fibers originating from the locus coeruleus were distributed almost exclusively to the parvicellular division of the PVH, with the most prominent input localized to the periventricular zone, a part of the PVH known to contain dopamine-, somatostatin-, and thyrotropin-releasing-hormone-containing neurons. We found no evidence for a projection from A6 to the SO. (4) The majority of fibers originating from the A1, the A2 or the A6 regions contained DBH immunoreactivity, although an appreciable number did not. These results suggest that each of the three brainstem noradrenergic cell groups that contribute to the innervation of the PVH and/or the SO is in a position to modulate the activity of anatomically and chemically distinct groups of neurosecretory neurons.

Respiratory actions induced by cholecystokinin at the brainstem level

A functional differentiation of the action of cholecystokinin octapeptide (CCK-8) on the respiratory centers was accomplished by the topical application to the ventral surface of the medulla and to the dorso-rostral pontine surface in cats. In the medulla, CCK-8S at doses ranging from 0.09 nmol to 0.88 nmol, stimulated tidal volume in a dose-dependent fashion, with minimal or no changes in frequency. The antagonist proglumide (30 nmol) inhibited specifically the action on the respiratory amplitude. In the pons, CCK-8S did not modify the respiratory activity even at the dose of 8.8 nmol. The results suggest a specific involvement of CCK-8S in the mechanisms controlling respiratory amplitude, which appear mostly restricted to the medullary level. The lack of effect of the peptide in the pons is in agreement with the absence of CCK receptors in the respiration related nuclei located at that level, as evidenced by autoradiographic studies.

Functional mapping of the brainstem during centrally evoked bradycardia: a 2-deoxyglucose study

Autoradiographic 2-deoxy-[14C] glucose (2-DG) procedures were used to map the functional activity of the brainstem during bradycardia elicited in awake rats by stimulation of the deep mesencephalic nucleus of the midbrain reticular formation (MRF). Quantitative determinations of 2-DG uptake in 46 brainstem structures of MRF-stimulated rats were compared to those of control rats without stimulation. This paper is the first 2-DG study to map the brainstem structures involved in a heart rate response evoked by central stimulation. The structures activated in the midbrain, caudal to the stimulation site, are part of the reticular formation and the central gray. The greater focuses of labeling were concentrated on the lateral aspects of the deep mesencephalic nucleus and on the lateral divisions of the midbrain central gray. The remaining structures activated during bradycardia were all located in the caudal medulla. The largest increase was observed in the caudal nucleus ambiguus. Significant increases were also found in the dorsal motor nucleus of vagus and in the nucleus of the solitary tract. The region of the caudal inferior olive showed a small increase in 2-DG uptake, whereas structures like the raphe magnus and parvocellular reticular nucleus showed a tendency to reduce 2-DG uptake levels in the stimulated rat. It was concluded that bradycardia induced centrally by MRF stimulation may be mediated by well-defined brainstem descending pathways, direct and indirect, between the activated regions of the midbrain and the various medullary nuclei known to induce bradycardia upon electrical stimulation. The results suggest that the midbrain central gray and reticular formation may play a role as intermediates in an indirect hypothalamus-medullary circuitry for bradycardia. In addition, descending MRF information and afferent baroreceptor inputs appear to exert their inhibitory influences on heart rate via a common set of neuroanatomical substrates in the medulla.

Ultrastructural characterization of substance P-like immunoreactive neurons in the rostral ventrolateral medulla in relation to neurons containing catecholamine-synthesizing enzymes

Substance P (SP) and catecholamines, particularly adrenaline, have been implicated in cardiovascular responses mediated by neurons in the rostral ventrolateral medulla (RVL). Immunoperoxidase labeling of an antiserum against SP and/or immunoautoradiographic localization of catecholamine (tyrosine hydroxylase-TH)- or adrenaline (phenylethanolamine N-methyltransferase-PNMT)-synthesizing enzymes were examined histologically to determine the cellular basis for a functional interaction involving either synaptic or intracellular relations between these putative transmitters in the adult rat RVL. Peroxidase labeling for SP was localized in perikarya, dendrites, and axon terminals. Most of these perikarya were located medial and ventral to those labeled with TH or PNMT within the same section. However, as others have previously demonstrated by light microscopy, colocalization of SP-like immunoreactivity (SPLI) and PNMT was seen in a few perikarya of colchicine treated animals. Both single- and dual-labeled perikarya contained abundant dense core vesicles. The terminals with SPLI were 0.4-1.4 micron in diameter and contained a few mitochondria, a large population of small, clear vesicles, and from three to 11 large dense core vesicles. In some cases the terminals were seen in continuity with more proximal processes of neurons in the RVL. These terminals formed synapses with a few perikarya and many dendrites, some of which also contained SPLI. In the material dually labeled for TH and SP, terminals with SPLI (n = 32) formed synaptic junctions primarily with TH-labeled dendrites (69%); the remainder were with TH-labeled perikarya (6%) or with unlabeled dendrites (25%). The axosomatic junctions were exclusively symmetric, whereas the majority of axodendritic junctions were primarily asymmetric on small dendrites (0.8-1.0 micron in diameter) or dendritic spines. In sections dually labeled for PNMT and SP, the terminals containing SPLI (n = 37) formed synaptic associations with PNMT-labeled perikarya (11%), PNMT-immunoreactive dendrites (59%), or with perikarya and dendrites lacking PNMT immunoreactivity (30%). The axosomatic junctions were all symmetric and most often associated with the spinous portion of the soma. The axodendritic junctions were primarily asymmetric and were found both on the spinous portion of the PNMT-labeled dendrites. In addition, both TH- and PNMT-labeled somata and dendrites received symmetric and asymmetric contacts from terminals lacking SPLI.(ABSTRACT TRUNCATED AT 400 WORDS)

Primary respiratory rhythm generator in the medulla of brainstem-spinal cord preparation from newborn rat

It has been previously demonstrated that rhythmically firing neurons (Pre-I neurons) preceding cervical root (C4 or C5) inspiratory activity, localized in the rostral ventrolateral medulla (RVL), are important in the generation of the basic respiratory rhythm in brainstem-spinal cord preparations from newborn rats. We examined the effects of single and continuous electrical stimulation applied to the RVL on Pre-I and C4 activities in these preparations. We verified that the phase of respiratory rhythm was reset when Pre-I firing was induced in both right and left RVL by single shock stimulation, whether C4 activity appeared or not. Lower frequency and intensity of continuous pulse train stimulation in the RVL enhanced Pre-I activity, and hence C4 activity, whereas higher frequency and intensity inhibited both. The results suggest that synchronous burst activity between the right and left Pre-I neurons must be above a certain level (in its intraburst firing rate) to trigger C4 inspiratory activity and, therefore, that cooperation among Pre-I neurons is important for induction of rhythmic inspiratory drive. After bilateral lesions of the caudal ventrolateral medulla, Pre-I neurons retained their rhythmic activity, while C4 activity disappeared. Present results further confirmed our hypothesis that Pre-I neurons are the primary generator of respiratory rhythm. We propose a hypothetical model of the generation of rhythmic respiratory activity.

Cholinergic nerve terminals in the ventrolateral medullary pressor area: pharmacological evidence

This investigation was designed to demonstrate the presence of cholinergic nerve terminals in the pressor area of the ventrolateral medulla (VLPA) and to study the effects of the release of endogenous acetylcholine in this area. Bilateral microinjections (0.1-2 nmol)/site) of 3,4-diaminopyridine (DAP), which releases acetylcholine from cholinergic nerve terminals, into the VLPA in anesthetized rats evoked an increase in blood pressure and heart rate which lasted for 20-40 min. Intravenous injections of the same doses of this agent failed to evoke a response. The ganglion blocker, chlorisondamine (3 mg/kg, i.v.) abolished the responses to microinjections of DAP indicating that the responses were mediated by the sympathetic nervous system. Microinjections of scopolamine or a specific M2 muscarinic receptor antagonist (AFDX-116) into the VLPA prevented the pressor and tachycardic responses to subsequent microinjections of DAP at the same sites indicating that the responses were mediated via M2 receptors. Microinjections of hemicholinium (3 nmol/site; which impairs acetylcholine synthesis) attenuated the responses to the subsequent microinjections of DAP at the same sites. These results indicate that the substance released from the terminals in the VLPA may be predominantly acetylcholine which evokes pressor and tachycardic responses via M2 muscarinic receptors. The origin and physiological significance of these cholinergic terminals in the VLPA are not known.

Catecholamine-synthesizing neuronal projections to the nucleus tractus solitarii of the rat

The objective of the present study was to determine the location of the neurons that give rise to catecholamine-containing terminals in the nucleus tractus solitarii. This was done by injecting rhodamine-filled latex microspheres into the nucleus tractus solitarii of rats to retrogradely label neuronal cell bodies and by processing sections from the brains of these animals to determine if the labelled neurons were immunoreactive for the catecholamine-synthesizing enzymes, dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyl transferase (PNMT). Approximately 60% of the DBH-immunoreactive neurons that projected to the nucleus tractus solitarii belonged to the A1/C1 cell group, while an additional 20% belonged to the A5 cell group. Thus, these two ventrolateral rhombencephalic cell groups accounted for nearly 80% of the total number of rhodamine-bead-labelled DBH-immunoreactive neurons in this series of experiments. Only a small number of DBH-immunoreactive neurons of the A2/C2 cell group contained rhodamine-filled latex microspheres. Rarely, DBH-immunoreactive neurons in the locus coeruleus and the nucleus subcoeruleus were found to project to the nucleus tractus solitarii. The majority of the PNMT-immunoreactive neurons that projected to the nucleus tractus solitarii belonged to the C1 cell group. Only small numbers of PNMT-immunoreactive neurons of the C2 and C3 groups were found to contain rhodamine-filled latex microspheres. It is concluded that neurons in the ventrolateral medulla and pons, some of which presumably utilize norepinephrine and/or epinephrine as a transmitter, could regulate autonomic function via direct projections to the nucleus tractus solitarii.

Neuropeptide and serotonin immunoreactive neurons in the cat ventrolateral medulla

The distribution of cell bodies containing serotonin (5-HT)-, substance-P (SP)-, neurotensin (NT)-, and somatostatin (SS)-like immunoreactivity (IR) in ventrolateral medulla (VLM) of the cat was studied immunohistochemically after administration of colchicine into the cisterna magna. Perikarya containing 5-HT-, SP-, NT- or SS-IR were found throughout the rostrocaudal extent of the VLM. Although neurons containing the different neuroactive substances appeared to have an overlapping distribution in VLM, some distinct differences were observed. In the caudal VLM most of the immunoreactive cell bodies observed contained 5-HT-IR. These neurons were found primarily in the region medial to lateral reticular nucleus (LRN) around the exiting intramedullary rootlets of the hypoglossal nerve (12N). In the intermediate region of VLM, perikarya containing 5-HT- and SP-IR were observed primarily near the ventrolateral surface of the medulla in the region around the exiting rootlets of the 12N. In contrast, most of the cells containing NT- and SS-IR were consistently observed to occupy a region in the medullary reticular formation immediately dorsal to that where 5-HT- and SP-IR perikarya were found. Finally, most of the immunoreactive perikarya were found in the rostral VLM; perikarya containing 5-HT- and SP-IR were observed throughout the nucleus paragigantocellularis lateralis (PGL) near the ventrolateral surface of the medulla. These data indicate that neurons immunoreactive to either 5-HT or several different neuropeptides were located in regions of VLM which have previously been implicated in the control of arterial pressure. As regions of VLM containing these neuroactive substances in neuronal perikarya have been shown to have direct connections with spinal sympathetic areas it is likely that these VLM cells are components of neuronal circuits involved in homeostatic mechanisms controlling the circulation.

Brainstem and spinal pathways mediating descending inhibition from the medullary lateral reticular nucleus in the rat

The lateral reticular nucleus (LRN) in the caudal ventrolateral medulla has been implicated in descending monoaminergic modulation of spinal nociceptive transmission. Experiments were undertaken to examine the organization of pontine and spinal pathways mediating inhibition of the tail-flick (TF) reflex from the LRN in rats lightly anesthetized with pentobarbital. Microinjections of the local anesthetic lidocaine ipsilaterally or bilaterally into the dorsolateral pons blocked stimulation-produced inhibition of the TF reflex from the nucleus locus coeruleus/subcoeruleus (LC/SC), but had no effect on descending inhibition produced by microinjection of glutamate into the LRN. Thus, adrenergic modulation of the TF reflex from the LRN is not mediated by activation of spinopetal noradrenergic neurons in the LC/SC. The funicular course of descending inhibition produced by focal electrical stimulation in the LRN was studied in separate groups of rats by reversibly (local anesthetic blocks) or irreversibly (surgical transection) compromising conduction in the dorsolateral funiculi (DLFs) at the level of the cervical spinal cord. Bilateral lidocaine blocks in the DLFs significantly shortened control TF latencies and more than doubled the intensity of electrical stimulation in the LRN necessary to inhibit the TF reflex (153 +/- 29% increase from control); changes in these parameters produced by unilateral blocks of the DLFs were not statistically significant. Ipsilateral or bilateral transections of the DLFs significantly increased the intensity of electrical stimulation in the LRN to inhibit the TF reflex (110 +/- 24% and 265 +/- 46% from control, respectively). Neither lidocaine blocks nor transections of the DLFs completely blocked the descending inhibitory effects of electrical stimulation in the LRN. The DLFs appear to carry fibers mediating LRN stimulation-produced inhibition of the TF reflex as well as tonic descending inhibition of spinal reflexes. The results of the present study indicate that (1) adrenergic modulation of the nociceptive TF reflex from the LRN does not depend on a rostral loop through the pontine LC/SC, and (2) descending inhibitory influences from the LRN are contained in, but not confined to, the dorsal quadrants of the spinal cord.

The C1 area of the brainstem in tonic and reflex control of blood pressure. State of the art lecture

Recent studies have demonstrated that the neurons of the lower brainstem that are responsible for maintaining normal levels of arterial pressure reside in a specific area of the rostral ventrolateral medulla. In rat, the critical zone corresponds to a small region containing a subpopulation of the adrenergic C1 group, defined immunocytochemically by the presence of the epinephrine-synthesizing enzyme phenylethanolamine N-methyltransferase. Neurons of this region (the C1 area), possibly including the adrenergic neurons, directly innervate preganglionic neurons in the spinal cord, and are tonically active and sympathoexcitatory. The excitatory transmitter released into the spinal cord is unknown. The discharge of C1 area neurons is locked to the cardiac cycle and, in turn, leads to firing of sympathetic preganglionic neurons. The C1 area neurons are inhibited by baroceptor input and mediate the vascular component of baroceptor reflexes. They also mediate somato-sympathetic pressor responses from skin and muscle and participate in reflex responses to hypoxia. The neurons are directly innervated by local neurons containing gamma-aminobutyric acid, acetylcholine, enkephalin, and substance P, all of which modulate arterial pressure. The C1 area is the site of the hypotensive actions of clonidine. Clonidine appears to act on imidazole receptors in the C1 area to lower arterial pressure. The natural ligand for these receptors may be a newly defined substance in brain, clonidine-displacing substance. Neurons of the C1 area appear to be the critical neuronal group governing the normal resting and reflex control of arterial pressure. They may play a critical role in the maintenance of elevated arterial pressure in hypertension and as a site of action of antihypertensive drugs.

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.

Brainstem afferents to the magnocellular basal forebrain studied by axonal transport, immunohistochemistry, and electrophysiology in the rat

Brainstem afferents to the magnocellular basal forebrain were studied by using tract tracing, immunohistochemistry and extracellular recordings in the rat. WGA-HRP injections into the horizontal limb of the diagonal band (HDB) and the magnocellular preoptic area (MgPA) retrogradely labelled many neurons in the pedunculopontine and laterodorsal tegmental nuclei, dorsal raphe nucleus, and ventral tegmental area. Areas with moderate numbers of retrogradely labelled neurons included the median raphe nucleus, and area lateral to the medial longitudinal fasciculus in the pons, the locus ceruleus, and the medial parabrachial nucleus. A few labelled neurons were seen in the substantia nigra pars compacta, mesencephalic and pontine reticular formation, a midline area in the pontine central gray, lateral parabrachial nucleus, raphe magnus, prepositus hypoglossal nucleus, nucleus of the solitary tract, and ventrolateral medulla. A similar but not identical distribution of labelled neurons was seen following WGA-HRP injections into the nucleus basalis magnocellularis. The possible neurotransmitter content of some of these afferents to the HDB/MgPA was examined by combining retrograde Fluoro-Gold labelling and immunofluorescence. In the mesopontine tegmentum, many retrogradely labelled neurons were immunoreactive for choline acetyltransferase. In the dorsal raphe nucleus, some retrogradely labelled neurons were positive for serotonin and some for tyrosine hydroxylase (TH); however, the majority of retrogradely labelled neurons in this region were not immunoreactive for either marker. The ventral tegmental area, substantia nigra pars compacta, and locus ceruleus contained retrogradely labelled neurons which were also immunoreactive for TH. Of the retrogradely labelled neurons occasionally observed in the nucleus of the solitary tract, prepositus hypoglossal nucleus, and ventrolateral medulla, some were immunoreactive for either TH or phenylethanolamine-N-methyltransferase. To characterize functionally some of these brainstem afferents, extracellular recordings were made from antidromically identified cortically projecting neurons, mostly located in the HDB and MgPA. In agreement with most previous studies, about half (48%) of these neurons were spontaneously active. Electrical stimulation in the vicinity of the pedunculopontine tegmental and dorsal raphe nuclei elicited either excitatory or inhibitory responses in 21% (13/62) of the cortically projecting neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Central sodium loading modifies norepinephrine content in the ventrolateral medulla and blood pressure in rats

We evaluated the effects of intracerebroventricular (ICV) infusion of hypertonic NaCl on blood pressure (BP) control as well as on NE content in the ventrolateral medulla (VLM). Nine groups of Wistar rats received 10 day's ICV infusion of NaCl solutions containing either norepinephrine (NE, 1.3 micrograms/min) or a synthetic NE precursor, 1-threo-3,4-dihydroxyphenylserine (1-DOPS, 17 micrograms/min) for 3 concentrations (0.15M, 0.8M or 1.5M) of NaCl. On day 9, only the group on ICV infusion of 1.5M NaCl alone had a significant rise in BP (133 +/- 3 mmHg, P less than 0.05 vs control) while other groups remained normotensive. The ICV infusion of 1.5M NaCl reduced NE content, determined by a microdialysis method, in the VLM while the concomitant ICV infusion of NE or 1-DOPS restored it suggesting that the decrease in NE content in the VLM may be a contributing factor in the BP elevation by the central salt loading.

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

Following microinjection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the pressor region of the rostral ventrolateral medulla of the cat, the medulla, pons and hypothalamus were examined for retrogradely labelled cell bodies, while the thoracolumbar segments of the spinal cord were examined for anterogradely labelled axons. Dense groups of labelled cells were found in the following areas: (1) the nucleus of the solitary tract, particularly the medial, ventrolateral and commissural subnuclei; (2) the ambiguous complex and immediately surrounding area; (3) the Kölliker-Fuse nucleus in the pons; (4) the paraventricular nucleus and lateral hypothalamic area. In the spinal cord, labelled axons formed a band extending throughout the dorsolateral and ventrolateral funiculi at thoracic segments, while terminal labelling was observed in the intermediolateral nucleus and to a lesser extent the central autonomic area, but not in other parts of the grey matter. The findings are discussed in relation to the role of the rostral ventrolateral medulla in cardiovascular regulation, particularly the baroreceptor reflex.