Effect of structures of the ventral surface of the medulla oblongata on coupled vascular functions of the small intestine

The effect of the ventrolateral medulla electrical stimulation of various intensity on resistance, capacitance and exchange functions was studied in the vascular bed of the small intestine. Brain activation with superliminal current applied to a point 2 mm rostral-wise of the middle of the twelfth cranial nerve rootlets has been shown to produce the strongest effect on the precapillary resistance of the vascular bed, whereas brain stimulation in a point 2 mm caudal-wise of this level produces a much stronger effect on the postcapillary small intestine resistance.     

Stimulation of the caudal ventrolateral medulla decreases total lung resistance in dogs

Although the role played by the caudal ventrolateral medulla in the regulation of the cardiovascular system has been extensively investigated, little is known about the role played by this area in the regulation of airway caliber. Therefore, in alpha-chloralose-anesthetized dogs, we used both electrical and chemical means to stimulate the caudal ventrolateral medulla while we monitored changes in total lung resistance breath by breath. We found that electrical stimulation (25 microA) of 26 sites in this area significantly decreased total lung resistance from 7.1 +/- 0.4 to 5.7 +/- 0.3 cmH2O.1-1.s (P less than 0.001). The bronchodilation evoked by electrical stimulation was unaffected by beta-adrenergic blockade but was abolished by cholinergic blockade. In addition, chemical stimulation of seven sites in the caudal ventrolateral medulla with microinjections of DL-homocysteic acid (0.2 M; 66 nl), which stimulates cell bodies but not fibers of passage, also decreased total lung resistance from 8.3 +/- 1.1 to 6.5 +/- 0.8 cmH2O.l-1.s (P less than 0.01). In contrast, microinjections of DL-homocysteic acid into the nucleus ambiguus (n = 6) increased total lung resistance from 7.5 +/- 0.5 to 9.2 +/- 0.4 cmH2O.l-1.s (P less than 0.05). We conclude that the caudal ventrolateral medulla contains a pool of cell bodies whose excitation causes bronchodilation by withdrawing cholinergic input to airway smooth muscle.     

Substance P in the ventrolateral medulla oblongata regulates ventilatory responses.

Local injection of substance P (SP) into the ventral portion of the nucleus gigantocellularis, nucleus reticularis lateralis, and nucleus retrofacialis of the ventrolateral medulla oblongata (VLM) or direct application on the ventral surface of the medulla oblongata caused marked stimulation of tidal volume (VT) and/or minute ventilation (VE). The ventilatory response to hypoxia was significantly blunted after SP in the VLM but not in the dorsal medulla oblongata (DM) (nucleus tractus solitarius). The SP antagonist [D-Pro2,D-Trp7,9]SP almost completely inhibited this response when applied locally to a wide area of the superficial layer of the VLM but not of the DM. Unilateral or bilateral application of 0.3-1.5 nmol of the SP antagonist in the VLM (corpus trapezoideum and the caudal region extending from the rootlets of the nucleus hypoglossus to the first cervical segment) markedly attenuated the response to a 5% CO2 inhalation. The inhibition of the CO2 response was seen after [D-Pro2,D-Trp7,9]SP in the rostral areas of the medulla oblongata corresponding to the corpus trapezoideum and the caudal region extending from the rootlets of the nucleus hypoglossus to the first cervical segment of the cervical cord. Electric somatosensory-induced ventilatory stimulation could be depressed by approximately 70% by [D-Pro2,D-Trp7,9]SP locally applied on the surface of the VLM. We conclude that SP is involved in the hypoxic, hypercapnic, and somatosensory ventilatory responses in the rat. However, these respiratory reflexes are mediated via different neuronal pools in the medulla oblongata, mainly the VLM.     

Involvement of the ventrolateral medulla in the mediation of pressor responses of the rat to afferent vagal stimulation

Electrical stimulation of afferent vagal fibres evoked a pressor response in rats after transection of the spinal cord. The pressor response was accounted for by an increased release of vasopressin because it was abolished by the intravenous injection of a vasopressin antagonist. Bilateral electrolytic lesions at the sites of the caudal ventrolateral medulla markedly reduced the pressor response to afferent vagal stimulation but not that to carotid occlusion. It is concluded that the area of the caudal ventrolateral medulla is involved in mediation of the vasopressin-induced pressor response to afferent vagal stimulation.     

c-Fos expression in the midbrain periaqueductal gray during static muscle contraction

The periaqueductal gray (PAG) of the midbrain is involved in the autonomic regulation of the cardiovascular system. The purpose of this study was to determine if static contraction of the skeletal muscle, which increases arterial blood pressure and heart rate, activates neuronal cells in the PAG by examining Fos-like immunoreactivity (FLI). Muscle contraction was induced by electrical stimulation of the L7 and S1 ventral roots of the spinal cord in anesthetized cats. An intravenous infusion of phenylephrine (PE) was used to selectively activate arterial baroreceptors. Extensive FLI was observed within the ventromedial region (VM) of the rostral PAG, the dorsolateral (DL), lateral (L), and ventrolateral (VL) regions of the middle and caudal PAG in barointact animals with muscle contractions, and in barointact animals with PE infusion. However, muscle contraction caused a lesser number of FLI in the VM region of the rostral PAG, the DL, L, and VL regions of the middle PAG and the L and VL regions of the caudal PAG after barodenervation compared with barointact animals. Additionally, the number of FLI in the DL and L regions of the middle PAG was greater in barodenervated animals with muscle contraction than in barodenervated control animals. Thus these results indicated that both muscle receptor and baroreceptor afferent inputs activate neuronal cells in regions of the PAG during muscle contraction. Furthermore, afferents from skeletal muscle activate neurons in specific regions of the PAG independent of arterial baroreceptor input. Therefore, neuronal cells in the PAG may play a role in determining the cardiovascular responses during the exercise pressor reflex.     

Role of the solitary tract nucleus and caudal ventrolateral medulla in temperature responses in endotoxemic rats

In experiments on conscious rats it was found that preliminary microinjection of 100 nl 100 microM glutamic acid to the rostral commissural part of the solitary tract nucleus or to the caudal ventrolateral medulla increased a rise in colonic temperature induced by systemically applied endotoxin (3 microg/kg Escherichia coli lipopolysaccharide, i.p.) as compared to animals with intrabulbar injection of vehicle (control group). Preliminary microinjection of glutamate to the caudal commissural part of the solitary tract nucleus levelled the endotoxin-induced temperature response. After glutamate treatment of the caudal ventrolateral medulla there was a significant decrease in the noradrenaline content and decrease in the adrenaline level in the caudal (not significant) and rostral ventrolateral medulla (significant), as well as a small rise in noradrenergic activity at the solitary tract nucleus as compared to control animals. The post-mortem measurement of the optical density of brainstem tissues revealed its significant attenuation at the solitary tract nucleus and caudal ventrolateral medulla after glutamate as compared with these structures after vehicle. The involvement of monoaminergic systems of both structures under study in the initiation and control of temperature responses during endotoxemia is suggested.     

Activation of neurons in the rostral ventrolateral medulla increases bronchomotor tone in dogs.

Previous work from this laboratory has demonstrated that the chemical activation of cell bodies in the caudal ventrolateral medulla of chloralose-anesthetized dogs decreased bronchomotor tone by withdrawing cholinergic input to airway smooth muscle. In the present study we determined the bronchomotor responses to microinjection of DL-homocysteic acid (100 mM; 25-50 nl) into the rostral ventrolateral (RVL) medulla of chloralose-anesthetized dogs. Total lung resistance was used as a functional index of bronchomotor tone. Microinjection of DL-homocysteic acid into the 20 sites located in the lateral aspect of the RVL medulla increased both total lung resistance [from 6.5 +/- 0.4 to 9.1 +/- 0.8 (SE) cmH2O.l-1.s; P less than 0.05] and mean arterial pressure (from 125 +/- 5 to 148 +/- 8 mmHg; P less than 0.05). Microinjection of this amino acid into nine sites located in the medial aspect of the RVL medulla increased mean arterial pressure (from 130 +/- 6 to 153 +/- 6 mmHg; P less than 0.05) but had no effect on total lung resistance. We confirmed in three sites that the increase in total lung resistance evoked by microinjection of DL-homocysteic acid was accompanied by an increase in tracheal smooth muscle tension. The increase in total lung resistance evoked by DL-homocysteic acid was not affected by beta-adrenergic blockade but was abolished by muscarinic blockade.     

Immunocytochemical localization of GABA containing neurons in the ventrolateral medulla oblongata of the rat

Localization of gamma-aminobutyric acid (GABA) in the ventrolateral medulla oblongata of the rat was studied, using antisera directed against GABA molecule fixed to bovine serum albumin. Within the rostral portion of the ventrolateral medulla, GABA-like immunoreactive neurons were found in the lateral wing of the raphe magnus and in the region of the paragigantocellular reticular nucleus. In the caudal portion of the ventrolateral medulla, a lesser number of GABA-stained neurons were found in the region around the nucleus reticularis lateralis. GABA-like immunoreactive punctate structures were also found throughout the ventrolateral medulla. These results provide further evidence for the existence of GABAergic neurons in the ventrolateral medulla oblongata of the rat.     

Immunocytochemical localization of GABA containing neurons in the ventrolateral medulla oblongata of the rat

Summary Localization of -aminobutyric acid (GABA) in the ventrolateral medulla oblongata of the rat was studied, using antisera directed against GABA molecule fixed to bovine serum albumin. Within the rostral portion of the ventrolateral medulla, GABA-like immunoreactive neurons were found in the lateral wing of the raphe magnus and in the region of the paragigantocellular reticular nucleus. In the caudal portion of the ventrolateral medulla, a lesser number of GABA-stained neurons were found in the region around the nucleus reticularis lateralis. GABA-like immunoreactive punctate structures were also found throughout the ventrolateral medulla. These results provide further evidence for the existence of GABAergic neurons in the ventrolateral medulla oblongata of the rat.     

Release by Electrical Stimulation of Endogenous Glutamate, γ-Aminobutyric Acid, and Other Amino Acids from Slices of the Rat Medulla Oblongata

Evidence was obtained for the release of amino acids by electrical stimulation of slices of regions of the rat medulla oblongata: rostral ventrolateral, caudal ventrolateral and caudal dorsomedial. There was a Ca2+-dependent, tetrodotoxin-sensitive increase in the efflux of aspartate, glutamate, gamma-aminobutyric acid (GABA), glycine, and beta-alanine in all regions examined. There were distinct regional differences in the relative amounts of amino acids released. These results provide evidence for the possible neurotransmitter role of aspartate, glutamate, GABA, glycine, and beta-alanine in these regions of the rat medulla oblongata.     

Retrofacial lesions: effects on CO2-sensitive phrenic and sympathetic nerve activity.

We made unilateral chemical (10- or 50-nl microinjections; 4.7 mM kainic acid) or electrolytic (5-15 mA; 15 s) lesions in a region of the rostral ventrolateral medulla (VLM) caudal to the retrotrapezoid nucleus in 10 decerebrate, paralyzed, vagotomized, and servo-ventilated cats. The lesions were 3.0-4.2 mm lateral to the midline, within 2 mm caudal to the facial nucleus, and within 2.5 mm of the VLM surface. Four control injections (mock cerebrospinal fluid and fluorescent beads alone) produced small and inconsistent effects over 3-5 h. The predominant effect of the lesions was a significant decrease in baseline integrated phrenic nerve amplitude (PNA) (apnea in 2 cases), total respiratory cycle duration, and the response to increased CO2 (slope < 15% of control in 3 cases). The respiratory-related peak amplitude of the integrated sympathetic signal, blood pressure, and the sympathetic nerve activity response to CO2 were also decreased after the majority of lesions. Not all lesions produced all effects, and some lesions resulted in increased PNA and respiratory cycle duration. The lesioned region appears functionally to represent a caudal extension of the retrotrapezoid nucleus containing neurons necessary for normal baseline PNA and CO2 sensitivity. In addition, it contains neurons involved in the determination of resting respiratory frequency and normal sympathetic activity and blood pressure. The pattern of mixed responses among animals suggests that a heterogeneity of function is present within a relatively small VLM region.     

c-fos expression in trigeminal spinal nucleus after electrical stimulation of the hypoglossal nerve in the rat

The expression of the immediate early gene, c-fos, was used to determine the distribution of brainstem neurons activated by stimulation of the distal hypoglossal nerve (XIIn) trunk. The traditional view of the XIIn is one of purely motor function; however, stimulation of XIIn excites neurons in the trigeminal spinal nucleus. The rationale for this study was to use c-fos expression as a marker for postsynaptic activity to define the pattern of brainstem neurons excited by XIIn stimulation. It was further hypothesized that if the afferent fibers that course within XIIn supply deep lingual tissues, then c-fos expression after direct stimulation of XIIn should display a pattern similar to that seen after chemical irritant stimulation of the deep tongue muscle. In barbiturate-anesthetized male rats electrical stimulation of XIIn produced a significant increase in Fos-positive neurons in the dorsal paratrigeminal nucleus (dPa5) and laminae I-II of caudal subnucleus caudalis (Vc) and upper cervical dorsal horn. Mustard oil injection into the deep tongue muscle also produced an increase in c-fos expression in dPa5; however, the highest density of expression occurred in laminae I-II at the dorsomedial aspect of rostral Vc. Both electrical stimulation of XIIn and mustard oil stimulation of the deep tongue increased c-fos expression in the caudal ventrolateral medulla, an autonomic relay nucleus. These results suggest that one site of innervation for afferent fibers that travel within the distal trunk of XIIn is to supply the deep tongue muscle and to terminate in the dPa5. A second group of postsynaptic neurons activated only by XIIn stimulation was located in lamina I-II in caudal portions of Vc and upper cervical dorsal horn, a laminar distribution consistent with a role for XIIn afferents in sensory or autonomic aspects of lingual function.     

c-fos expression in trigeminal spinal nucleus after electrical stimulation of the hypoglossal nerve in the rat

The expression of the immediate early gene, c-fos, was used to determine the distribution of brainstem neurons activated by stimulation of the distal hypoglossal nerve (XIIn) trunk. The traditional view of the XIIn is one of purely motor function; however, stimulation of XIIn excites neurons in the trigeminal spinal nucleus. The rationale for this study was to use c-fos expression as a marker for postsynaptic activity to define the pattern of brainstem neurons excited by XIIn stimulation. It was further hypothesized that if the afferent fibers that course within XIIn supply deep lingual tissues, then c-fos expression after direct stimulation of XIIn should display a pattern similar to that seen after chemical irritant stimulation of the deep tongue muscle. In barbiturate-anesthetized male rats electrical stimulation of XIIn produced a significant increase in Fospositive neurons in the dorsal paratrigeminal nucleus (dPa5) and laminae I-II of caudal subnucleus caudalis (Vc) and upper cervical dorsal horn. Mustard oil injection into the deep tongue muscle also produced an increase in c-fos expression in dPa5; however, the highest density of expression occurred in laminae I-II at the dorsomedial aspect of rostral Vc. Both electrical stimulation of XIIn and mustard oil stimulation of the deep tongue increased c-fos expression in the caudal ventrolateral medulla, an autonomic relay nucleus. These results suggest that one site of innervation for afferent fibers that travel within the distal trunk of XIIn is to supply the deep tongue muscle and to terminate in the dPa5. A second group of postsynaptic neurons activated only by XIIn stimulation was located in lamina I-II in caudal portions of Vc and upper cervical dorsal horn, a laminar distribution consistent with a role for XIIn afferents in sensory or autonomic aspects of lingual function.     

The role of the sympathetic nervous system-activating structures of the ventrolateral area of the medulla oblongata in the regulation of cardiac activity]

Localization of sympathoexcitatory neurons regulating in the ventrolateral medulla area participating in the heart rate regulation has been studied. Results suggest, that sympathoexcitatory neurons in the cat are confined to a definite region (middle line of roots of XII nerve and by 4.0 mm more rostral) of rostral ventrolateral medulla. Stimulation of these right regions increases the heart rate, but that of the left regions elevates dp/dt max. Their activity mediated pathways (conduction velocity 10.5 + 0.4 m/s and 6.1 + 0.4 m/s) innervated of "nonclassical" sympathetic neurons of the ventral horn and sympathetic preganglionic neurons of intermediolateral cell column of the spinal cord.     

Substance P and enkephalin in transected axons of medulla and spinal cord

The accumulation of transported materials in cut axons is demonstrated by the light and electron microscopic immunocytochemical localization of substance P and enkephalin in the caudal medulla and cervical spinal cord of adult rat. Two days following unilateral knife-cuts in the caudal medulla or spinal (C2-C3) levels, substance P and enkephalin-like immunoreactivity (SPLI and ELI) are detected in lesioned axons located rostral and caudal to the transection. Rostrally, SPLI and ELI are detected in the lateral reticular region and ventrolateral fasciculus corresponding to the location of previously identified bulbospinal pathways. Caudally, previously unidentified, propriospinal pathways showing SPLI are detected in the dorsal columns and in the dorsolateral fasciculus. In contrast, ELI is found caudal to the transection only in the reticular region of the medulla. For both peptides, immunoreactivity is present throughout axons containing numerous large, dense core, and small clear vesicles. These results support the concept of both particulate and soluble modes of transport for substance P and enkephalin within axons of the central nervous system.     

Role of caudal ventrolateral medulla in reflex and central control of airway caliber.

We investigated the role played by the caudal ventrolateral (CVL) medulla in the reflex and central neural control of airway caliber in chloralose-anesthetized dogs. Changes in total lung resistance were evoked by four different stimuli. These changes were compared before and after bilateral injection of either ibotenic acid (75 nl; 100 mM) or cobalt chloride (75 nl; 50 mM) into the CVL medulla. The four stimuli used to change lung resistance were static muscular contraction, electrical stimulation of thin fiber afferents in the sciatic nerve, electrical stimulation of the posterior diencephalon, and hypoxia. The first three stimuli have been shown to decrease total lung resistance, whereas the latter stimulus has been shown to increase resistance. We found that injection of both ibotenic acid, which destroys cell bodies but not fibers of passage, and cobalt, which prevents synaptic transmission, either abolished or greatly attenuated the decrease in total lung resistance evoked by static contraction, by sciatic nerve stimulation, and by posterior diencephalic stimulation. We also found that injection of ibotenic acid and cobalt attenuated the reflex increase in lung resistance evoked by hypoxia. In control experiments, we found that bilateral injection of ibotenic acid into the dorsal medulla had no effect on the changes in total lung resistance evoked by these four stimuli. We conclude that the CVL medulla plays an important role in the reflex and central control of airway caliber.     

Novel axonal projection from the caudal end of the ventrolateral medulla to the intermediolateral cell column

We used an optical imaging technique to investigate whether axons of neurons in the caudal end of the ventrolateral medulla (CeVLM), as well as axons of neurons in the rostral ventrolateral medulla (RVLM), project to neurons in the intermediolateral cell column (IML) of the spinal cord. Brain stem-spinal cord preparations from neonatal normotensive Wistar-Kyoto and spontaneously hypertensive rats were stained with a voltage-sensitive dye, and responses to electrical stimulation of the IML at the Th2 level were detected as changes in fluorescence intensity with an optical imaging apparatus (MiCAM-01). The results were as follows: 1) depolarizing responses to IML stimulation during low-Ca high-Mg superfusion were detected on the ventral surface of the medulla at the level of the CeVLM, as well as at the level of the RVLM, 2) depolarizing responses were also detected on cross sections at the level of the CeVLM, and they had a latency of 24.0 +/- 5.5 (SD) ms, 3) antidromic action potentials in response to IML stimulation were demonstrated in the CeVLM neurons where optical images were detected, and 4) glutamate application to the CeVLM increased the frequency of excitatory postsynaptic potentials (EPSPs) and induced depolarization of the IML neurons. The optical imaging findings suggested a novel axonal and functional projection from neurons in the CeVLM to the IML. The increase in EPSPs of the IML neurons in response to glutamate application suggests that the CeVLM participates in the regulation of sympathetic nerve activity and blood pressure and may correspond to the caudal pressor area.     

Effects of 5-HT3 receptor blockade on visceral nociceptive neurons in the ventrolateral reticular field of the rat medulla oblongata

The caudal ventrolateral reticular formation of the medulla oblongata is the first layer of visceral nociceptive processing. In experiments on rats, neuronal responses in this zone to nociceptive stimulation of the large intestine were examined and the effects of selective blockade of 5-HT3 receptors on these responses were assessed. By the character of responses to nociceptive colorectal stimulation (CRS), the recorded medullary neurons were divided into three groups—excited, inhibited and indifferent. Intravenous injection of 5-HT3 antagonist granisetron (1 and 2 mg/kg) as well as local application of this agent on the surface of the medulla oblongata (1.25 and 2.5 nmole) suppressed the background and evoked firing of CRS-excited reticular neurons in a dose-dependent manner but did not exert as pronounced influence on the cells inhibited by visceral nociceptive stimulation. Spike activity in the group of CRS-indifferent neurons under similar conditions was 5-HT3-independent. The results obtained provide evidence that 5-HT3 receptors mediate the facilitating effect of serotonin on supraspinal transmission of the abdominal nociceptive stimulus which, at least in part, is realized via selective activation of visceral medullary nociceptive neurons. A shutdown of this mechanism may underlie the analgesic effect of 5-HT3 antagonists in abdominal pain syndromes.     

Effect of exogenous cortisone on amino acid metabolism in rats

1. The effect of exogenous cortisone on concentration of free amino acids in serum, skeletal muscle, kidney, small intestine and liver was studied. 2. The amino acid pool in serum, skeletal muscle and small intestine decreased significantly. 3. Glutamine synthesis increased significantly in skeletal muscle. 4. Levels of branched amino acids increased in serum and small intestine. 5. Levels of alanine increased in kidney and liver.