Dorsomedial Medulla Research Articles

Potassium induced release of endogenous glutamate, gamma-aminobutyric acid (GABA) and glycine from the caudal dorsomedial medulla of the rat.

Release of endogenous glutamate, gamma-aminobutyric acid (GABA) and glycine was investigated in the caudal dorsomedial (CDM) region of the rat medulla oblongata. High potassium stimulation (50 mM) produced an increase in release of glutamate, GABA and glycine by 579.1, 303.4 and 774.5 (pmol/mg protein), respectively. The release was abolished by Ca2+ deprivation. These results indicate a possible neurotransmitter role for glutamate, GABA and glycine in the CDM medulla of the rat.

Projections between the hypothalamus and the dorsal vagal complex in the cat: An HRP and autoradiographic study

The hypothalamus is known to be intimately involved in the control of autonomic function. This study provides detailed information about pathways between the hypothalamus and the dorsal vagal complex in cat. Injection of horseradish peroxidase into the dorsomedial medulla produced retrograde neuronal labeling in the paraventricular nucleus of the hypothalamus. Injection of 3H-leucine into the paraventricular nucleus produced dense anterograde labeling in the dorsal motor nucleus of the vagus, and lighter labeling in the nucleus of the tractus solitarius, particularly in its medial subnucleus. The subnucleus gelatinosus was virtually free of label, except in its medial and lateral portions. Anterograde labeling was distributed bilaterally, with an ipsilateral predominance. Injection of horseradish peroxidase into the area of the paraventricular nucleus produced retrograde neuronal labeling bilaterally in the nucleus of the tractus solitarius and the reticular formation ventrolateral to the dorsal vagal complex. Anterograde terminal labeling overlapped the distribution of retrogradely labeled neurons. These findings are compared to those in rat, and discussed in relation to their functional implications.

The Distribution of Substance P in the Canine Dorsomedial Medulla

The Distribution of Substance P in the Canine Dorsomedial Medulla

Contribution of the vagus nerve to angiotensin II binding sites in the canine medulla

Specific angiotensin II (Ang II) binding sites are present in the dorsal medulla of several species and dose-related cardiovascular effects are produced by microinjection of the peptide into this region. Because the anatomical location of Ang II binding sites in the area postrema (ap), nucleus tractus solitarii (nTS) and dorsal motor nucleus of the vagus (dmnX) coincides with the topography of vagal afferent fibers and efferent motor neurons, the effect of either nodose ganglionectomy or cervical vagotomy on Ang II binding sites in the dorsomedial medulla was investigated in dogs by in vitro receptor autoradiography. Two weeks after unilateral ganglionectomy, there was a marked reduction in the density of specific Ang II binding sites in the ipsilateral ap, nTS and dmnX and an absence of binding sites in the region where vagal afferent fibers course through the rostral medulla. Unilateral cervical vagotomy, which has been shown to spare central processes of afferent fibers, resulted in a loss of binding only in the ipsilateral dmnX. We also show that Ang II binding sites are present in the nodose ganglion and central and peripheral processes of the vagus nerve. The data indicate that medullary Ang II binding sites are associated with both vagal afferent fibers and efferent motor neurons.

The Distribution of Substance P in the Canine Dorsomedial Medulla

To define the neurochemical substrates underlying cardiovascular regulatory mechanisms in the canine dorsal medulla, we characterized immunocytochemically the organization of substance P (SP) in the dorsomedial medulla. Substance P immunoreactivity was the most dense in the dorsal nucleus tractus solitarii (nTS). Varicose fibers and cells containing this neuropeptide were localized in the dorsal motor nucleus of the vagus (dmnX) and nucleus commissuralis. A few scattered fibers were observed within the area postrema. The distribution of this peptide suggests a topographical organization within the dorsomedial medulla of the dog that is closely related to the cytoarchitectural organization. In addition, the patterns of SP immunoreactivity overlap the distribution of vagal afferent fibers. These observations are discussed in relation to the possible function of SP in the dorsomedial medulla.

The distribution of tyrosine hydroxylase, dopamine-beta-hydroxylase, and phenylethanolamine-N-methyltransferase immunoreactive neurons in the feline medulla oblongata.

The distributions and morphological characteristics of neurons displaying immunoreactivity to the catecholamine synthetic enzymes tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT) were examined in adjacent sections of the feline medulla oblongata. TH-positive neurons were found in two bilaterally symmetrical columns in the ventrolateral and dorsomedial medulla. Within the ventrolateral medulla, TH-positive neurons were found within the lateral reticular formation throughout the entire rostrocaudal extent of the medulla. In the dorsomedial medulla, TH-immunoreactive perikarya were localized to the nucleus of the tractus solitarius including the commissural subnucleus, the dorsal motor nucleus of the vagus, and the area postrema. DBH-positive neurons had distributions and morphologies similar to those of the TH-immunoreactive cells with three exceptions: TH-positive neurons far outnumbered DBH-positive neurons in the area postrema; slightly greater numbers of TH-positive neurons were seen in the commissural nucleus of the tractus solitarius; and, caudal to the obex, only TH-positive neurons were seen within the dorsal motor nucleus of the vagus. PNMT-immunoreactive neurons were found in all the nuclear regions of the medulla where both TH- and DBH-positive neurons were seen. However, the PNMT immunoreactive perikarya had a somewhat more restricted distribution along the rostrocaudal axis. In the ventrolateral medulla, PNMT-positive cells extended rostrally only as far as the retrofacial nucleus and caudally only to the obex. Within the dorsomedial medulla, PNMT immunoreactive cells were found from just rostral to the area postrema to the medullary-spinal cord junction. These findings demonstrate that the distributions of TH, DBH, and PNMT immunoreactive perikarya in the medulla of the cat are generally similar to those seen in the rat insofar as these neurons are arranged in longitudinal columns in both species. However, significant differences exist with regard to the cytoarchitectonic borders within which immunoreactive perikarya can be found and the rostrocaudal extent of the PNMT-positive cell groups in these two species.

Excitatory and inhibitory interactions among renal and cardiovascular afferent nerves in dorsomedial medulla.

Central interactions between renal afferent nerves (RN) and carotid sinus (CSN) and aortic (AN) nerves were investigated by recording extracellular action potentials from single units in and around the nucleus tractus solitarius (NTS) in alpha-chloralose-anesthetized cats. Forty-nine units were activated by electrical stimulation of the renal nerve; twenty-seven of these also were excited by CSN, AN, or both. Summation of inputs was demonstrated during simultaneous subthreshold or suprathreshold stimulation of RN and CSN or AN. In 10 units excited by simultaneous stimulation of RN and AN or CSN, a conditioning stimulus (333 Hz for 10 ms) applied to RN caused prolonged (up to 150 ms) inhibition of responses to stimulation of AN or CSN; reciprocal inhibitory interactions were also shown. These studies show that renal afferent input interacts with cardiovascular afferent input at neurons in the NTS region of dorsal medulla and suggest that the outcome of these interactions at the single cell level may be critically dependent on the temporal relationships among afferent impulses converging on common central synapses.

Relationships between catecholaminergic neurons and cholecystokinin-containing neurons in the caudal part of the dorsomedial medulla oblongata of the rat: light and electron microscopic observations by the ‘mirror technique’

The ‘mirror technique’ was used in immuno-electron microscopy to examine the synaptic relationships between neuronal structures containing catecholamines and cholecystokinin (CCK) in the caudal part of the dorsomedial medulla oblongata in the rat, using antisera against tyrosine hydroxylase and CCK. CCK-immunoreactive axon terminals made synaptic contacts with catecholaminergic neurons. Some of these catecholaminergic neurons contained CCK.

A vasopressor cell group in the rostral dorsomedial medulla of the rabbit

Microinjections of sodium glutamate solution (which excites neuronal cell bodies but not axons of passage) into a circumscribed region within the dorsal reticular formation in the rostral medulla oblongata evoked a large increase in arterial pressure due to widespread vasoconstriction. In spontaneously breathing animals, glutamate stimulation of the pressor region did not affect respiratory activity and evoked only a very small and transient increase in electromyographic activity of axial skeletal muscles. The pressor response was not reduced by decerebration or decerebellation, indicating that the pathway connecting the dorsomedial pressor region to the spinal sympathetic outflow is intrinsic to the lower brainstem and spinal cord. Anatomical observations in the present study, combined with those from previous studies, indicate that neurons in this region do not project directly to the spinal cord and do not receive direct afferent inputs from the nucleus tractus solitarius. It is concluded that there exists a circumscribed group of vasopressor neuronal cell bodies within the rostral dorsomedial medulla. The anatomical connections of these neurons, however, are markedly different from those of a previously described group of vasopressor neurons in the rostral ventrolateral medulla, suggesting that the two groups may have different functional roles in cardiovascular regulation.

Efferent projections from the paraventricular nucleus mediating α 2-noradrenergic feeding

Feeding behavior elicited by central injection of the α-noradrenergic agonists, norepinephrine (NE) and clonidine (CLON), are believed to be mediated via postsynapticα 2-type receptors located in the paraventricular nucleus (PVN). To map the course taken by essential efferent (descending) fibers of this PVN system for noradrenergically-stimulated feeding, the impact of diencephalic and lower brainstem coronal knife cuts, on the responses elicited by PVN-injected NE and CLON, was assessed. Rats that sustained damage in the periventricular gray area of the caudal thalamus and midbrain exhibited significant losses in feeding elicited by PVN injections of these drugs. In the case of animals with midbrain periventricular gray knife cuts, a significant increase in daily food intake was also observed, and this increase was positively correlated in magnitude with the attenuation of NE-induced feeding. This decrease in sensitivity toα 2-noradrenergic stimulation occurred with discrete periventricular knife cuts extending only 0.5 mm lateral to midline. In contrast, large ventral or lateral coronal knife cuts throughout the dorsal and ventral midbrain tegmentum left intact NE- and CLON-induced feeding. These findings provide evidence for localization of anatomical substrates which underlie PVNα 2-noradrenergic feeding. The efferent fibers of this system appear to exit from the PVN in a dorsomedial direction and course through the thalamic periventricular area. As this projection descends into the midbrain, it remains quite medial, maintaining this position throughout the midbrain central gray substance. At the level of the pons, just rostral to the locus coeruleus, this fiber projection appears to course ventrolaterally into the dorsolateral pontine tegmentum and possibly continue towards the dorsal vagal complex of the dorsomedial medulla.

Electrophysiology of the subfornical organ and its hypothalamic connections—an in-vivo study in the rat

The connections of the subfornical organ to a defined population of hypothalamic neurons have been explored with extracellular recording methods in the rat. Electrical stimulation in the subfornical organ has a predominantly excitatory action on a majority of oxytocin and vasopressin-secreting neurosecretory neurons in the supraoptic and paraventricular nuclei. Subfornical organ stimulation also enhances the excitability of paraventricular nucleus neurons that project to the median eminence, and to the dorsomedial medulla. These observations provide initial evidence of functional connectivity of subfornical organ neurons with other hypothalamic cells that are engaged in central regulation of pituitary secretions and autonomic activities.

Properties of renobulbar afferent fibers in rats.

Myelinated renobulbar sensory fibers project directly from the kidney to the dorsomedial medulla. They can be activated by punctate stimulation of the kidney. In these experiments we studied the physiological modality and the intrarenal locations of the receptors for renobulbar fibers. Renobulbar fibers were identified in recordings from the renal nerves of anesthetized rats. Although nonmyelinated renal afferent fibers were activated by increased intrarenal pressure and by chemical stimuli, no renobulbar fiber activated by punctate mechanical stimulation was activated by these other stimuli. Occasionally single fibers could be activated by punctate stimulation of two or more sites on the surface of the kidney, and responses from each site could be selectively blocked with a local anesthetic. Renobulbar fibers could be activated by electrical stimulation of discrete regions deep within the renal parenchyma. Increased intrarenal pressure increased the sensitivity of receptors to punctate stimulation but not to electrical stimulation of the kidney. We conclude that myelinated renal afferent fibers in the rat respond to external mechanical stimuli, lie deep within the renal parenchyma, and occasionally have more than one receptor.

Subfornical organ stimulation excites paraventricular neurons projecting to dorsal medulla.

Electrical stimulation in the subfornical organ (SFO) of pentobarbital-anesthetized Sprague-Dawley rats was noted to influence the excitability of paraventricular nucleus (PVN) neurons antidromically identified as projecting to the dorsomedial medulla. Extracellular recordings indicated that 60% (n = 34) of these caudally projecting PVN neurons increased activity in response to single shock stimuli delivered to the SFO. Short-latency [30.0 +/- 2.7 (SE) ms] and long-latency (162.5 +/- 32.5 ms) responses were observed. The remaining neurons were either unaffected (38%) or inhibited (2%) by SFO stimulation. These data suggest functional connectivity between the SFO and the dorsomedial medulla. It is proposed that such a pathway may mediate pressor responses observed to follow electrical stimulation in the SFO.

Evidence that vasomotor neurons in the rostral ventrolateral medulla project to the spinal sympathetic outflow via the dorsomedial pressor area

Previous work indicates that excitation of spinally projecting neurons in the rostral ventrolateral medulla evokes an increase in arterial pressure. The aim of this study was to test the hypothesis that axons of the ventrolateral neurons project to the spinal cord via a pressor region located in the dorsomedial medulla just rostral to the obex. In one group of experiments, a large quantity of horseradish peroxidase (HRP) was injected bilaterally into the spinal cord of rabbits following placement of a lesion unilaterally in the dorsomedial pressor area. This resulted in a much smaller number of HRP-labeled cells in the rostral ventrolateral medulla on the side of the lesion than on the intact side. In all other brainstem regions examined, however, the numbers of labeled cells were very similar on both sides. In other experiments, microinjections of sodium glutamate (which excites cell bodies but not axons of passage) were made into the dorsomedial pressor area. In contrast to the large pressor response evoked by electrical stimulation of this area, glutamate microinjections evoked a very small or no response. The results of both groups of experiments taken together suggest that the pressor response to electrical stimulation of the dorsomedial medulla arises from excitation of axons of passage which originate, at least in part, from the rostral ventrolateral neurons.

The central projections of carotid baroreceptors and chemoreceptors in the cat: a neurophysiological study.

The medullary projections of afferent neurons with cell bodies in the petrosal ganglion have been investigated using an antidromic mapping technique. Of the ninety-three units studied, fifty-eight were shown to have patterns of discharge indicating that they were baroreceptors and thirty-five showed responses to stimuli indicating that they were arterial chemoreceptors. Twelve baroreceptor and thirteen chemoreceptor afferents had sufficiently stable unitary discharges to permit a detailed estimation of some of their central projections using stimulation through monopolar tungsten micro-electrodes to evoke antidromic spikes. In order to estimate their pattern of projection, depth-threshold contours for each penetration through the dorsomedial medulla and the values of antidromic latency were considered. Baroreceptor afferent fibres with myelinated (six units) and non-myelinated (six units) axons showed similar patterns of central projection. All could be activated from the ipsilateral nucleus of the tractus solitarius (n.t.s.), most often from its lateral divisions rostral to the obex. The dorsolateral and dorsomedial portions of the n.t.s. were most often innervated, with the commissural subnucleus receiving an innervation in seven of the twelve neurones studied. Stimulation of the ventrolateral subnucleus was effective in activating two afferent fibres whilst stimulation of the ventral subnucleus was effective in only one case. All chemoreceptor afferent fibres had calculated conduction velocities less than 4 m/s and all were activated from the dorsomedial and medial subnuclei of the ipsilateral n.t.s. In twelve of the thirteen neurones investigated in detail there was evidence of an innervation of the commissural nucleus both at the level of the obex and behind it. In three cases this extended into the contralateral portion of the commissural nucleus. In four cases a sparse innervation of the lateral subnucleus, comprising its dorsolateral aspects, was seen. The potential significance of these distinctive patterns of projection of arterial baroreceptors and chemoreceptors is discussed in relation to cardiovascular and respiratory control.

Tonic vasomotor control by the rostral ventrolateral medulla: effect of electrical or chemical stimulation of the area containing C1 adrenaline neurons on arterial pressure, heart rate, and plasma catecholamines and vasopressin

We have studied the responses to electrical and chemical stimulation of the ventrolateral medulla in the chloralose-anesthetized, paralyzed, artificially ventilated rat. Locations of most active pressor responses were compared to regions containing neurons labeled immunocytochemically for phenylethanolamine N-methyltransferase (PNMT), the enzyme catalyzing the synthesis of adrenaline. Elevations of arterial pressure (+81.6 +/- 2.5 mm Hg) and cardioacceleration (+73 +/- 13.6 bpm) were elicited with low current (5 times threshold of 9.5 +/- 1.1 microA) electrical stimulation in a region of rostral ventrolateral medullary reticular formation we have termed the nucleus reticularis rostroventrolateralis (RVL). Electrical stimulation of the RVL increased plasma catecholamines (16.8-fold for adrenaline, 5.3-fold for noradrenaline, and 1.9-fold for dopamine) and vasopressin (1.7-fold before spinal transection, 4.7-fold after). The location of the most active pressor region in the ventrolateral medulla corresponded closely with the location of C1 adrenaline-synthesizing (PNMT-containing) neurons. In addition, the location of the most active pressor region in the dorsomedial medulla corresponded with the location of a bundle of PNMT-containing axons. Unilateral injections into the RVL of the excitatory amino acid monosodium L-glutamate (50 pmol to 10 nmol), but not saline, caused transient dose-dependent and topographically specific elevations (maximum +71.6 +/- 4.9 mm Hg) of arterial blood pressure and tachycardia. Injections of the rigid structural analogue of glutamate, kainic acid, caused large, prolonged (at least 15 min) pressor responses and tachycardia. Unilateral injections of the inhibitory amino acid gamma-aminobutyric acid (GABA) into the RVL caused transient dose-dependent hypotension (maximum -40.8 +/- 6.6 mm Hg) and bradycardia, whereas the specific GABA antagonist bicuculline caused prolonged (10 to 20 min) elevations (+64.2 +/- 6.8 mm Hg) of arterial pressure and tachycardia. By contrast, injections of the glycine antagonist strychnine had no significant effect. Bilateral injections of the neurotoxin, tetrodotoxin, dropped arterial pressure to low levels (51.7 +/- 4.7) not changed by subsequent spinal cord transection at the first cervical segment (52.5 +/- 6.2). We propose the following. (1) Neurons within the RVL, most probably C1 adrenaline-synthesizing neurons, exert an excitatory influence on sympathetic vasomotor fibers, the adrenal medulla, and the posterior pituitary. (2) These neurons are tonically active and under tonic inhibitory control, in part via GABAergic mechanisms--perhaps via the nucleus of the solitary tract (NTS).(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of cardiovascular cell groups in the brain stem.

There is now good evidence that there are several distinct groups of cells in the brain stem that are capable of inducing marked changes in cardiovascular function. This paper briefly reviews the results of recent experiments which have identified cardiovascular cell groups in the rostral ventrolateral and dorsomedial medulla, and in the periaqueductal grey and ventral tegmentum of the midbrain. Results of our experiments as well as those of others suggest that excitation of cardiovascular cell groups in the lower brain stem leads to an undifferentiated generalized increase or decrease in sympathetic activity, whereas excitation of certain midbrain cell groups can evoke a highly co-ordinated pattern of autonomic and respiratory changes that closely resembles the pattern associated with certain behaviours in the conscious animal.

Electrophysiological study of paraventricular nucleus neurons projecting to the dorsomedial medulla and their response to baroreceptor stimulation in rats

In male rats anesthetized with urethane, extracellular recordings were made from 415 neurons in the paraventricular nucleus (PVN) and adjacent areas. Of these neurons 64 were excited antidromically by stimulation of the dorsomedial medulla but not by stimulation of the pituitary stalk (first group). Seventy-three neurons were antidromically excited by stimulation of the pituitary stalk but not of the dorsomedial medulla (second group, neurosecretory cells). The other 2 neurons were antidromically excited by stimulation of both the dorsomedial medulla and the pituitary stalk (third group). Latencies of antidromically evoked action potentials by stimulation of the dorsomedial medulla and of the pituitary stalk ranged between 8 and 60 ms ( mean±S.D., 38.5 ± 9.8,n= 66) and from 7 to 24 ms ( mean±S.D., 13.0 ± 3.6,n= 75), respectively, suggesting unmyelinated fiber projections in both instances. PVN neurons of these 3 groups were found to be dispersed throughout the PVN and no differences in specific locations between the neuron groups existed. Their characteristics, however, were different. The first group of neurons discharged at a slower rate and showed no phasic pattern of firing, while 28% of the second group of neurons (‘identified’ neurosecretory cells) showed phasic patterns of firing and their rates of discharge were higher than those of the first group of neurons. The two neurons belonging to the third group showed irregular spontaneous discharges. The areas within the dorsomedial medulla stimulation of which evoked antidromic excitation of PVN neurons were located within and adjacent to the nucleus of the tractus solitarius (NTS) and the dorsal motor nucleus of the vagus (DMV). Among PVN neurons which were antidromically excited by stimulation of dorsomedial medulla, 51 cells were examined for their responses to excitation of baroreceptors. An increase in pressure of the ‘isolated’ carotid sinus excited 2 neurons, and inhibited 7 (14%). On the other hand, 27% (11 out of 41) of neurosecretory cells (second group) were inhibited by baroreceptor stimulation. From these results, it was concluded that essentially separate populations of PVN neurons project to the neurohypophysis and to the NTS, DMV and their vicinities, and that some of the caudally-projecting PVN neurons receive synaptic input from carotid baroreceptor reflex pathway, suggesting the possible involvement of these PVN neurons in central cardiovascular regulation.

EEG effects of microinjection of naloxone in the region of the nucleus tractus solitarius of the rat

In recent years, we have carried out several studies on brainstem mechanisms associated with pain modulation. Of particular interest has been the region of the nucleus tractus solitarius (NTS) at the level of the area postrema (AP) in the dorsomedial medulla. This region has been found to be rich in opiate receptors and to be involved in pain modulation. Having previously demonstrated the EEG and behavioral effects of systemic administration of morphine, we initiated the present study to determine whether microinjection of naloxone directly into the region of the NTS will modify any or all of the EEG effects induced by subsequent systemic administration of morphine. Our results have shown that microinjections of naloxone into the region of the NTS block the cortical EEG synchronizing effects of systemically administered morphine. These results indicate that morphine-induced cortical EEG synchronization is mediated through the region of the NTS. This region may, therefore, comprise part of an opiate sensitive system which can influence cortical EEG activity.

Elevation of blood pressure in hypertensive rats after lesioning serotonin nerves in the dorsomedial medulla oblongata.

The possibility that serotonin (5HT) nerve endings in the dorsomedial medulla oblongata participate in the control of blood pressure was investigated in stroke-prone hypertensive rats and in normotensive rats. 5HT nerves were destroyed by injecting 5,7-dihydroxytryptamine bilaterally into the solitary tract nucleus. Immunohistochemical and biochemical examination revealed a selective loss of 5HT nerves throughout the dorsomedial medulla with little damage in adjacent regions. Arterial blood pressure and heart rate, measured at daily intervals, were similar in lesioned and sham-operated normotensive rats. However, in the hypertensive rats, blood pressure was substantially elevated at 6-9 days after the lesions compared with sham operations. There were no accompanying changes in heart rate.