The ventral surface of the brain stem: a scarcely explored region of pharmacological sensitivity

Hyperglycaemia: imitating Claude Bernard's piqûre with drugs

Hyperglycaemia lasting for hours, has been produced in unanesthetized cats, rabbits and rats by injection into the cerebral ventricles or the cisterna magna of a variety of drugs (morphine, etorphine, pethidine, beta-endorphin, enkephalin, bombesin, TRH, cholecystokinin, naloxone, propranolol, phentolamine, chloralose, magnesium chloride and GABA). These drugs probably act at the ventral surface of the brainstem and initiate a sympathetic discharge to the adrenals which results in a prolonged release of relatively small amounts of adrenaline. When adrenaline is released in this way hyperglycaemia may be the only effect. The mechanism of the piqûre hyperglycaemia of Claude Bernard may be the same, although Bernard assumed that it resulted from an effect on the floor of the fourth ventricle, i.e. on the dorsal surface of the brainstem. However, it is clear from his description that his trochar not only pricked the floor of the fourth ventricle but penetrated to the ventral surface of the brainstem. Release of adrenaline from the adrenals is usually regarded as a stress response, as in fight, flight, fear or rage when it is suddenly released in large amounts and produces its typical cardiovascular and ocular reactions. The results now obtained with drugs injected intraventricularly or intracisternally suggest an additional physiological role for adrenaline when it is released over prolonged periods and in relatively small amounts producing only hyperglycaemia. Such a release may play a role in the day-to-day control of blood glucose, and its disturbance might underlie non-insulin-dependent diabetes.

The medullary cardiovascular effects of imidazolines and some GABA analogues: a review

A study of the localizations and mechanisms of the hypotensive action of clonidine-like substances and GABA analogues. It is confirmed that these agents act on the ventral surface of the medulla oblongata and are involved in the central control of the cardiovascular system. Suggestions are offered relative to the development of new centrally acting antihypertensive agents.

Mapping of cholinoceptive(nicotinoceptive)neurons in the lower brainstem: with special reference to the ventral surface of the medulla

The distribution of cholinoceptive neurons in the lower brainstem of the rat was investigated by means of a histochemical method for specific acetylcholinesterase. Nicotinoceptive neurons were characterized using an alpha-bungarotoxin-horseradish peroxidase conjugate for the detection of nicotinic acetylcholine receptors. For the first time a nearly complete mapping of the location of cholinoceptive (nicotinoceptive) neurons of the lower brainstem was achieved. Special attention was focused on the organization of the cholinoceptive neuronal matrix of the ventral surface of the medulla, where regulative centers for vasomotor and respiratory control are located.

Excitatory projections from hypothalamic and midbrain defense regions to nucleus paragigantocellularis lateralis in the rat

Electrical stimulation of the hypothalamic and midbrain defense regions evoked convergent excitatory responses in neurons in nucleus paragigantocellularis lateralis (PGL) in rats anesthetized with urethane. Stimulation in PGL activated neurons in the periaqueductal gray matter (PAG) and in the hypothalamus antidromically. In the PAG the projecting neurons were localized in the defense region but in the hypothalamus they were more widely distributed lateral and dorsal to it, mainly in the lateral hypothalamic region and the zona incerta. We conclude that there are independent excitatory pathways from the midbrain and hypothalamic defense regions which relay on neurons in PGL. Stimulation in the midbrain defense region may activate the monosynaptic projection to PGL as well as polysynaptic pathways. In contrast, stimulation of the hypothalamic defense region probably excites mainly fibers of passage: the integrated pattern of autonomic changes produced by electrical stimulation may be due to activation of the efferent outflow from widely dispersed cell bodies whose axons run through the defense region.

2-Deoxyglucose uptake in the central nervous system during systemic hypercapnia in the peripherally chemodenervated rat

Changes in 2-deoxyglucose (2-DG) uptake in the central nervous system during systemic hypercapnia were determined by the [3H]2-DG autoradiographic method in peripherally chemodenervated rats. Autoradiographs were made from serial transverse sections of the brain and analyzed by a computer-based interactive image processing system for areas having increases or decreases in metabolic activity compared with control animals. The most pronounced change shown by autoradiographs of the hypercapnic animals was a generalized decrease in the metabolism of the gray matter throughout the central nervous system with respect to the normocapnic controls. However, several central structures showed evidence of either no change or an increased metabolism in the hypercapnic animals. In the brain stem these areas were localized to the ventrolateral region of the nucleus of the solitary tract rostral to the obex, around the region of the nucleus retroambiguus, in a region of the ventrolateral medullary reticular formation extending rostrally from the obex to the level of the intramedullary rootlets of the facial nerve, in the region of the ventral nucleus raphe pallidus, and in the region of the lateral parabrachial nucleus. In the diencephalon these regions included the supraoptic nucleus and the dorsal hypothalamic area, extending into the caudal portion of the paraventricular nucleus. The thoracolumbar cord showed activation of the lateral aspects of the dorsal horns, the region of lamina X and the region of the intermediolateral nucleus. These data may be interpreted as a functional map of the central structures activated in hypercapnia in the peripheral chemodenervated rat. It appears likely that these structures are involved in mediating the cardiorespiratory responses associated with the activation of central chemoreceptors by the increased carbon dioxide concentrations.

A study of the substance P innervation of the intermediate zone of the thoracolumbar spinal cord

Immunocytochemical procedures have been used to examine the distributions of substance P (SP)-positive fibres within the intermediate zone of the thoracolumbar spinal cords of rabbits, cats, and monkeys. In all three species SP fibres were concentrated in areas known to contain sympathetic preganglionic neurones. These included the intermediolateral nucleus and the funiculus just lateral to it, the medial gray matter in the area of the nucleus intercalatus, and the paracentral region. The density of the SP innervation varied in a characteristic way both between these subpopulations of sympathetic neurones and in its overall input to different segmental levels. Generally the greatest accumulations of SP fibres were found in the T3-T5 and L2-L4 regions and these were concentrated in the intermediolateral nucleus (ILN). The highest densities of SP fibres in the lateral funiculus were in the upper thoracic and upper lumbar segments whereas SP fibres forming transverse bands, possibly in association with neurones in the nucleus intercalatus, were most prominent in T5-T8. Substance P fibres adjacent to the midline were more or less equally dense throughout the segments examined. Substance P-positive cell bodies situated immediately lateral to the central canal were present at a density of 200-300 per segment throughout the cat thoracolumbar cord. These neurones may be the cells of origin of at least some of the SP fibres in the intermediate zone. The close association of sympathetic preganglionic neurones with SP fibres, many of which are thought to be derived from cells in the medulla, suggests a role for SP-containing fibres in the modulation of sympathetic activity. The variation in input to different segments and classes of sympathetic neurones further suggests a specificity which may be related to the different functions of the neurones innervated.

Topographic principles in the spinal projections of serotonergic and non-serotonergic brainstem neurons in the rat

The spinal projections from the raphe-associated brainstem areas containing serotonergic neurons were studied with aldehyde-induced fluorescence in combination with the retrograde fluorescent tracer True Blue in the rat. This technique makes it possible to determine simultaneously the projections of individual neurons and to detect whether serotonin is present in the same neurons. After tracer injections into the spinal cord retrogradely labeled serotonergic and non-serotonergic neurons were found in the medullary raphe nuclei and adjacent regions and to a lesser extent in association with the dorsal and median raphe nuclei in the mesencephalon. Large True Blue injections that covered one side of the spinal cord at mid-cervical level labeled about 60% of the ipsilaterally situated serotonergic neurons in the medullary raphe regions while the corresponding figure contralaterally was about 25%. On both sides a larger number of labeled non-serotonergic neurons were found; these were sometimes located dorsal to, but often intermingled with, the serotonergic cells. While the serotonergic projection from the mesencephalon could not be labeled from injections below cervical levels, the labeling in more caudal brainstem regions exhibited only minor variations depending on the rostrocaudal level of the spinal segment injected. Furthermore, quantitative data from injections at different levels indicate that the majority of the spinal-projecting neurons traverse most of the length of the cord. Summarizing the results obtained from small injections restricted to subregions of the cord we feel that it is possible to distinguish three fairly distinct pathways for spinal projections from the medullary raphe and adjacent regions: The dorsal pathway originates mainly from cells in the caudal pons and rostral medulla oblongata (rostral part of nucleus raphe magnus, nucleus raphe magnus proper, nucleus reticularis gigantocellularis pars alpha and nucleus paragigantocellularis). This pathway, which contains a large non-serotonergic component, descends through the dorsal part of the lateral funiculus and terminates mainly in the dorsal horn at all spinal cord levels. The intermediate pathway is largely serotonergic with its cell bodies located within the arcuate cell group (situated just ventral and lateral to the pyramids very close to the ventral surface of the brainstem) and in the nucleus raphe obscurus and pallidus and terminates in the intermediate grey at thoracolumbar and upper sacral levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Monoamine distribution on the ventral surface of the rat medulla oblongata

The distribution of monoamine transmitters in the area near the ventral surface of the rat medulla oblongata was studied using the Falck-Hillarp histofluorescence method. Histological examination and scanning electron microscopy of these regions were also performed. It was found that there is a wide area dense with catecholamine terminals in the external layer of the ventral medulla oblongata. 5-Hydroxytryptamine-containing terminals and nerve cell bodies on and near the surface were also found. Due to their superficial localization these monoamines may influence the content of cerebrospinal fluid and in this way have effects on cardiovascular and other physiological functions.

Projections from the diencephalon and mesencephalon to nucleus paragigantocellularis lateralis in the cat

The distribution of labelled cells in the diencephalon and mesencephalon has been mapped following injections of horseradish peroxidase into nucleus paragigantocellularis lateralis in the cat. Most of the labelled cells were found ipsilateral to the injection site. A group of small and medium-sized labelled perikarya (11-40 microns in diameter) was present in the caudal part of the periaqueductal grey matter (A3-P2) and in the adjacent tegmentum. Small, round or fusiform cells (8-25 microns were labelled in the tuberal region of the hypothalamus in the dorsomedial hypothalamus and in the lateral hypothalamic area. It is suggested that the cardiovascular responses which can be elicited by stimulation in these regions of the periaqueductal grey and hypothalamus are mediated via a relay on to spinally projecting neurones in nucleus paragigantocellularis lateralis which synapse on sympathetic preganglionic neurones in the intermediolateral cell column.

Evidence for a capsaicin-sensitive vasomotor mechanism in the ventral medullary chemosensitive area of the cat

The effects of capsaicin applied to the exposed ventral surface of the medulla were studied on the mean arterial blood pressure, heart rate, respiration and sympathetic efferent nerve activity in chloralose-urethane-anaesthetized cats. The application of capsaicin produced a marked increase in the mean arterial blood pressure and sympathetic nerve activity, but not in the heart rate. The "intermediate area" proved to be the most sensitive to capsaicin. Pressor responses could be elicited repeatedly; tachyphylaxis was not noted provided a time interval of 30 min elapsed between consecutive applications. Repeated applications of capsaicin at intervals of less than 30 min led to tachyphylaxis. However, pressor responses evoked by either topical application of glutamate or pentamethylene-tetrazole or bilateral carotid occlusion could invariably be demonstrated during this period of tachyphylaxis. Histological studies revealed the existence of a hitherto unrecognized termination of capsaicin-sensitive nerve endings within the ventral medullary chemosensitive area of the cat. The results provide both functional and morphological evidence for the presence of a capsaicin-sensitive vasomotor mechanism in the ventral medullary chemo-sensitive area of the cat. It is suggested that the pressor effects of capsaicin applied to the ventral medullary chemo-sensitive area may be mediated by an activation of capsaicin-sensitive primary sensory afferents terminating in this area. Accordingly, capsaicin-sensitive neuronal mechanisms located in the ventral medullary chemosensitive area may play an important role in the central nervous regulation of blood pressure.

Caudal ventrolateral medulla. A region responsible for the mediation of vasopressin-induced pressor responses

We localized glutamate-sensitive sites in the ventrolateral medulla of the rat with the spinal cord cut at C. When unilaterally injected into a circumscribed region of the caudal ventrolateral medulla, L-glutamate (30-300 ng) elicited a dose-dependent increase in arterial pressure. 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 microinjections of kainic acid (50 ng) into the ventrolateral glutamate-sensitive area markedly reduced a vasopressin-induced pressor response to kainic acid (30 ng), injected bilaterally into the nucleus tractus solitarii. It is concluded that the glutamate-sensitive neurons in the caudal ventrolateral medulla are involved in mediation of the vasopressin-induced pressor response arising from the nucleus tractus solitarii.

Bulbospinal substance P and sympathetic regulation of the cardiovascular system: a review

The neurotransmitter role of substance P in mediating sympathoexcitatory effects in the spinal cord and cardiovascular effects elicited from the ventral medulla is presented. SP neurons located in the ventral medulla project to the intermediolateral cell column (IML) of the thoracic spinal cord. Intrathecal administration of a SP analog excites sympathetic outflow to the cardiovascular system. Likewise, activation of the ventral medulla results in sympathetically mediated increases in blood pressure and heart rate which are blocked with SP antagonists. The IML contained a high density of SP binding sites through which the peptide likely exerts its sympathoexcitatory influence on the cardiovascular system.

Glutamic acid decarboxylase immunoreactivity is present in perikarya of neurons in nucleus tractus solitarius of rat

Immunohistochemical procedures reveal perikarya containing glutamic acid decarboxylase immunoreactivity in the nucleus tractus solitarius of the rat. After colchicine pretreatment, neurons were observed in all subdivisions of the nucleus with a particular concentration in the ventrolateral and intermediate subdivisions.

Blockade of central nervous system GABAergic tone causes sympathetic-mediated increases in coronary vascular resistance in cats

Picrotoxin, an antagonist of gamma-aminobutyric acid, produces an increase in coronary vascular resistance, S-T segment elevation, and ventricular arrhythmias after an intravenous injection of 2 mg/kg in chloralose-anesthetized cats. To determine whether these responses were due to blockade of central nervous system GABAergic mechanisms leading to an increase in sympathetic outflow to the coronary vasculature, several types of experiments were performed. First, picrotoxin was injected directly into the brain in a dose of 600 micrograms while coronary blood flow and S-T segment changes were monitored. Central nervous system administration of this agent resulted in a significant increase in coronary vascular resistance, S-T segment elevation, and arrhythmias. Second, animals were pretreated with the gamma-aminobutyric acid receptor agonist drug, muscimol, prior to central administration of picrotoxin. Pretreatment prevented the usual increase in coronary vascular resistance, S-T segment elevation, and arrhythmias. Third, animals were subjected to acute bilateral cardiac sympathetic denervation prior to picrotoxin administration. Denervation attenuated the picrotoxin-induced increase in coronary vascular resistance (mean = 11.6 +/- 2.1% vs. 26.1 +/- 7.1%, P less than 0.05) and elevation in S-T segment (mean = 0.09 +/- 0.03 mV vs. 0.29 +/- 0.04 mV, P less than 0.05), and prevented arrhythmias. Pretreatment with the alpha-receptor blocking agent, phentolamine, produced even more pronounced antagonistic effects. These results suggest that blockade of central nervous system GABAergic tone leads to enhanced sympathetic outflow to the coronary vasculature, resulting in an increase in coronary vascular resistance of sufficient intensity to cause S-T segment elevation and arrhythmias.

A cholinergic link in the reflex release of vasopressin by hypotension in the rat

Inhalation of amyl nitrite in the water-loaded rat under ethanol anaesthesia produced a brief fall of blood pressure followed by a prolonged antidiuretic response. The antidiuretic response to amyl nitrite was accompanied by increased urinary excretion of vasopressin, it was blocked by a specific vasopressin antagonist and by a barbiturate and it was absent in the Brattleboro rat with congenital diabetes insipidus. These results show that the antidiuretic response to the hypotension induced by amyl nitrite is due to the release of vasopressin and that this release is mediated by a neuroendocrine reflex acting through the brain stem. Carbachol and nicotine produced an antidiuretic response on injection into a lateral cerebral ventricle (i. vent.). Carbachol was almost ineffective, but nicotine much more effective, when injected into the cisterna magna (i.cist.) from which in the rat there is no access to the ventricles. Carbachol therefore acts at a site reached from the ventricles, possibly the paraventricular nucleus. Nicotine acts at a more distal site reached from the subarachnoid space. This site may correspond with the nicotine-sensitive area on the ventral surface of the brain stem which has been described in the cat. Atropine blocked the antidiuretic response to carbachol but not that to amyl nitrite. Hexamethonium blocked the antidiuretic response to amyl nitrite as well as that to nicotine and was more effective on i.cist. than i.vent. injection. These results reveal a cholinergic link with a nicotinic but not a muscarinic receptor in the neural pathways controlling the release of vasopressin in response to hypotension. A hypothetical model is presented in which the release of vasopressin is stimulated by a pathway arising from chemoreceptors and inhibited by a second pathway arising from stretch- and baroreceptors. Hypotension acts by suppressing the normally predominant inhibitory pathway and stimulating the excitatory pathway. Hexamethonium is presumed to block transmission at a synapse in the excitatory pathway at the ventral surface or, less probably, at the paraventricular and supraoptic nuclei.

Cardiovascular response to injections of enkephalin in the pressor area of the ventrolateral medulla

The cardiovascular effects of the injection of an enkephalin analogue, [D-ala2-met5]enkephalinamide (DAME) into the pressor area of the rostral ventrolateral medulla were studied in urethane-anesthetized and decerebrate rats. The excitatory amino acid L-glutamate was used to identify the ventrolateral medulla. The pressor responses to L-glutamate were elicited from an area that included the nucleus reticularis gigantocellularis, the medial aspect of the nucleus reticularis parvocellularis and the dorsal portion of the nucleus reticularis lateralis. Injection (0.1 microliter volume) of DAME (2.5-500.0 ng/site) into the ventrolateral medulla elicited a dose-related decrease in arterial blood pressure and heart rate and attenuated the carotid occlusion response (COR). Control injections (0.1-0.2 microliter vol) of saline into the same area failed to produce any response. The specificity of this opiate response was tested with naloxone HCl, an opiate antagonist, which prevented, as well as reversed, the action of DAME both by intravenous (i.v.) administration and by injection into the ventrolateral medulla. It was concluded that the ventrolateral medulla plays a role in the generation of vasomotor tone and that stimulation of opiate receptors in this area by an enkephalin analogue produced hypotension, bradycardia and modification of cardiovascular reflexes.

Spinally projecting neurones near the ventral surface of the medulla in the cat

In anaesthetized cats antidromic responses were recorded in nucleus paragigantocellularis lateralis following stimulation of the ventrolateral funiculus in the ipsilateral thoracolumbar cord. Conduction velocities ranged from 5.4-120 m/s but 50% of the fibres conducted at less than 20 m/s. About half of the neurones were facilitated by stimulation in the hypothalamic or midbrain defence areas. These neurones are superficially located at the site where application of glycine or a lesion blocks the effects of defence area stimulation and reduces blood pressure. They could therefore be involved in the regulation of vasomotor activity.

The ventral surface of the medulla in the rat: pharmacologic and autoradiographic localization of GABA-induced cardiovascular effects

Experiments were done to evaluate a rat model for studying the cardiovascular effects of pharmacological manipulations of the ventral surface of the medulla. GABAergic drugs were used because of their well-characterized actions at the ventral surface of the medulla in the cat. GABA and muscimol, applied to the exposed ventral surface with filter paper pledgets, produced dose-dependent decreases in heart rate (HR) and mean arterial pressure (MAP) which were reversed with bicuculline but not with strychnine. Bicuculline alone raised HR and MAP. The GABA- or bicuculline-induced cardiovascular effects were mediated primarily by inhibition of sympathetic outflow. The most sensitive site was localized to an intermediate area on the ventral surface of the medulla, between the trapezoid body and exits of the hypoglossal nerves and just lateral to the pyramids. Topical application of [3H]GABA to the intermediate area resulted in labeling that was concentrated at the site of application, and which penetrated the parenchyma 1 mm dorsally. The heaviest labeling was found primarily in the ventral halves of the lateral paragigantocellular nuclei. No tritium was detected in peripheral blood. These data provide evidence for a neuronal system at the ventral medullary surface of the rat which influences sympathetic outflow and is modulated by GABA.

Hypertensive response following stimulation of opiate receptors in the caudal ventrolateral medulla

In urethane-anesthetized rats, vasodepressor neuron pools were located bilaterally in and adjacent to the A1 area of the ventrolateral medulla by injecting the neuroexcitatory amino acid, L-glutamate. Ventrolateral vasodepressor areas included the caudalateral part of the nucleus reticularis gigantocellularis, the rostrolateral part of the nucleus reticularis ventralis, and the dorsal nucleus reticularis lateralis. In the ventrolateral vasodepressor areas L-glutamate elicited a transient fall in blood pressure (BP) and heart rate (HR). The opiate agonist (D-ala2-met5)-enkephalinamide (DAME) was used to stimulate opiate receptors in vasodepressor sites, identified with L-glutamate. In these sites, bilateral injections (0.1 microliter/site) of DAME caused a dose-related (2.5-500.0 ng) increase in blood pressure and heart rate, as well as exaggeration of the response to occlusion of the carotid. The effects of DAME on blood pressure were completely abolished by alpha-adrenergic blockade (phentolamine, 2 mg/kg, i.v.) and all effects of DAME were reversed by the administration of naloxone HCl (1 mg/kg, i.v.). Naloxone reversal was accompanied by an unexpected "rebound" hypertension. Saline had no significant effects when injected, or administered intravenously, in the absence or presence of DAME. It was concluded that stimulation of opiate receptors in the ventrolateral vasodepressor areas activated sympathetic outflow. An enkephalinergic system in this area of the brain stem may serve to modulate blood pressure, heart rate and cardiovascular reflexes.

Cardiovascular responses evoked from the nicotine-sensitive area on the ventral surface of the medulla oblongata in the cat

Experiments were carried out in cats anaesthetized with chloralose, (a) to examine the effect on blood pressure, heart rate and respiratory frequency produced by topical application of leptazol, nicotine and sodium pentobarbitone to the ventral surface of the medulla at an area around the rootlets of the XII cranial nerve, and (b) to study the role of this area in some cardiovascular reflexes. Leptazol applied uni- or bilaterally to this area produced hypotension, bradycardia and bradypnoea. The area from which leptazol produced these effects was localized 3-6 mm lateral to the mid line and 5-9 mm caudal to the lower border of the trapezoid bodies. When comparing the effects of leptazol and nicotine applied to this area it was found that in concentrations that produced similar falls in arterial blood pressure and heart rate leptazol produced a much stronger bradypnoea than nicotine. The hypotension produced by leptazol was mainly due to inhibition of sympathetic vasomotor tone since it was little affected by section of the vagi and by atropine given intravenously. Bilateral application of sodium pentobarbitone produced a small hypertension, tachycardia and pronounced tachypnoea. Unilateral application of sodium pentobarbitone had no effect by itself but inhibited the effects of leptazol applied to the same site. Cardiovascular reflexes produced by sinus nerve stimulation, by increased sinus pressure or by injections of veratridine into a vein or into the left ventricle of the heart were potentiated by topical application of leptazol to the ventral surface and depressed by the topical application of sodium pentobarbitone. The chemoreceptor reflex, produced by retrograde injections of lobeline into the lingual artery, was partially affected by topical application of sodium pentobarbitone: the evoked bradycardia was attenuated but the tachypnoea and hypertension were not affected. These results suggest that this medullary area on the ventral surface of the medulla plays an important role in normal cardiovascular regulation.

Arterial pressure and plasma vasopressin: regulation by neurons in the caudal ventrolateral medulla of the rabbit

Experiments in the anesthetized rabbit have demonstrated that the caudal ventrolateral medulla oblongata contains tonically active vasodepressor neurons which are themselves inhibited by GABAergic and glycine-like inputs. Alteration of neuronal function in this region also elevates plasma vasopressin levels, contributing to, but not completely accounting for, the hypertension and pulmonary edema observed after destruction of neurons in the region.

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.

Direct pathway from cardiovascular neurons in the ventrolateral medulla to the region of the intermediolateral nucleus of the upper thoracic cord: an anatomical and electrophysiological investigation in the cat

Horseradish peroxidase (HRP) and single unit recording experiments were done in cats to identify neurons in the ventrolateral medulla (VLM) projecting directly to the intermediolateral nucleus (IML) of the thoracic cord and relaying cardiovascular afferent information from the buffer nerves and hypothalamus. In the first series, HRP was allowed to diffuse from a micropipette into the region of the IML at the level of T2. After a survival period of 30-138 h, transverse and horizontal sections of the brainstem were processed according to the tetramethyl benzidine method. Labeled neurons were found in the VLM 1-5 mm rostral to the obex, bilaterally, but with an ipsilateral predominance. The majority were observed in sections 2-4 mm rostral to the obex, clustered in an area lateral to the inferior olivary nucleus around the intramedullary rootlets of the hypoglossal nerve. Additional labeled neurons were found scattered along the ventral surface of the medulla; most of these neurons were oval in shape, 15-30 micron in diameter, and had dendritic processes which lay parallel to the ventral surface. In the second series, the region of the VLM shown to contain labeled neurons was systematically explored for single units antidromically activated by electrical stimulation of the IML in chloralosed, paralyzed and artificially ventilated animals. These antidromically identified units were then tested for their responses to electrical stimulation of the carotid sinus (CSN) and aortic depressor (ADN) nerves, and the paraventricular nucleus (PVH). Ninety-four single units in the VLM were antidromically activated with latencies corresponding to a mean conduction velocity of 19.1 +/- 1.5 m/s. Of these units 52% (49/94) were orthodromically excited by stimulation of buffer nerves; 12 by stimulation of the CSN only (mean latency, 16.0 +/- 3.6 ms), 5 by stimulation of the ADN only (mean latency, 9.5 +/- 2.0 ms), 7 by both buffer nerves, and the remaining 25 units responded to at least one of the buffer nerves and to PVH. Stimulation of PVH excited orthodromically 42 of the 94 units (45%), of which 17 responded only to stimulation of PVH (mean latency, 17.9 +/- 3.5 ms). These experiments provide anatomical and electrophysiological evidence for the existence of a direct cardiovascular pathway from the VLM to the region of the IML and suggest that neurons in the VLM are involved in the integration of cardiovascular afferent inputs from buffer nerves and the hypothalamus to provide an excitatory input to vasoconstrictor neurons in the IML.

Cardiovascular afferent inputs to neurons in the ventrolateral medulla projecting directly to the central autonomic area of the thoracic cord in the cat

Experiments were done in chloralosed, paralyzed and artificially ventilated cats to identify single units in the ventrolateral medulla (VLM) projecting directly to the central autonomic area of the thoracic cord (CA) and responding to peripheral and central inputs carrying cardiovascular information. Forty-three single units were antidromically activated in the VLM to stimulation of either ipsilateral or contralateral CA with latencies corresponding to conduction velocities of 27.5 +/- 2.0 m/s. Of these 43 units, only 14 (33%) responded orthodromically to stimulation of either the carotid sinus nerve (CSN) or of pressor sites in the paraventricular nucleus of the hypothalamus (PVH) or both. These experiments have demonstrated a bilateral projection of VLM neurons to the CA and have provided evidence for their role in integrating and mediating cardiovascular information from the CSN and PVH directly to spinal sympathetic centers.

Aortic baroreceptor reflex pathway: a functional mapping using [3H]2-deoxyglucose autoradiography in the rat

The organization of pathways within the central nervous system which are activated by aortic baroreceptor input was studied in the urethane anesthetized rat using the 2-deoxyglucose method. [3H]2-deoxyglucose was administered i.v. while either the aortic nerve was electrically stimulated or aortic baroreceptors were physiologically activated by pulse increases in arterial pressure in animals with bilateral denervation of the carotid sinus. Autoradiographs of transverse sections of the central nervous system were developed and analyzed for changes in metabolic activity in discrete regions compared to control animals, as indicated by the density of the photographic emulsion. Electrical stimulation of the aortic nerve resulted in all animals in an increase in the uptake of deoxyglucose in a number of sites throughout the central nervous system, primarily ipsilateral to the site of stimulation. In the brainstem, structures previously implicated in cardiovascular reflexes were labeled. These included the nucleus of the solitary tract, the solitary tract, the dorsal motor nucleus of the vagus, and the nucleus ambiguus. In addition, the inferior olivary nucleus, the parabrachial nuclei and the ventrolateral reticular formation showed increased labeling. In the hypothalamus, increased labeling was observed only in the paraventricular and supraoptic nuclei.

Hypothalamic pathways involved in metabolic regulatory functions, as identified by track-tracing methods

The present review of the fiber connections of the hypothalamus has been concerned basically with recent data obtained by the aid of the autoradiographic and HRP tracer techniques. Evidence presented has shown that, besides confirming many of the older data, recent studies have resulted in the introduction of several conceptual modifications into the classic picture of hypothalamic hodological relationships. Among these conceptual modifications, the following can be mentioned: (1) the medially placed nuclei of the hypothalamus have a great number of long efferent and afferent connections with many extrahypothalamic structures; (2) many hypothalamic nuclei send direct projections to cell territories in the brainstem and spinal cord that contain preganglionic autonomic motor neurons; (3) several neural districts that lie caudal to the mesencephalon send direct projections to the hypothalamus; (4) in addition to the olfactory channel, other sensory pathways (including interoceptive and gustatory conduction lines) have a relatively direct access to hypothalamic mechanisms; (5) the hypothalamus sends fibers to several brainstem territories that give rise to widespread monoaminergic projections; and (6) there are anatomical pathways that establish reciprocal connections between the hypothalamus and the basal ganglia. Some of the possible physiological correlates of these anatomical findings in the context of metabolic regulatory functions have been briefly indicated.

Central chemosensitivity and the reaction theory

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Substance P neurons project from the ventral medulla to the intermediolateral cell column and ventral horn in the rat

The descending substance P projections from the ventral medulla were studied in the rat. Electrolytic lesions which included the nucleus interfascicularis hypoglossi decreased the substance P-like immunoreactivity (SP-I) in both the intermediolateral cell column and the ventral horn of the spinal cord. Lesions of other ventral medullary areas and midbrain hemisections did not change spinal cord SP-I levels. Intracerebroventricular administration of the serotonin neurotoxin, 5,7-dihydroxytryptamine, reduced the SP-I content of the ventral horn but not of the intermediolateral cell column.

Development of raphe-spinal connections in the North American opossum

The Falck-Hillarp technique, serotonin (5-HT) immunohistochemistry and the retrograde transport of horseradish peroxidase (HRP) were utilized to investigate the development of raphe-spinal connections in the pouch-young opossum. The brainstem raphe and adjacent reticular formation contain 5-HT immunoreactive neurons in the newborn opossum (12 days after conception) and processes from these cells can be visualized in the marginal zone of the spinal cord. Between eight and 15 days after birth 5-HT immunoreactive varicosities begin to grow into the presumptive deep layers of the dorsal horn, the intermediolateral cell column and the ventral horn. In the latter region some of them approximate presumed motor neurons. Between 40-50 days after birth 5-HT immunoreactive varicosities appear in presumptive laminae I and II of the dorsal horn.

Neuroanatomical localization of substance p: implications for central cardiovascular control

The presence of substance P (SP) neurons in pathways known to be involved in the central control of the cardiovascular system has been studied with neuroanatomical and neurochemical techniques. SP-immunoreactive (SP-I) neurons are found in afferent baro- and chemoreceptor pathways which transmit information from peripheral receptors to the nucleus tractus solitarius. In addition, SP-I neurons located in the nucleus interfascicularis hypoglossi of the ventral medulla innervate the intermediolateral cell column, the site of origin of preganglionic sympthetic nerves. The role of these SP-I neurons in cardiovascular control remains to be determined.

Initiation and inhibition of shivering in the rat: interaction between peripheral and central factors

1. Unanaesthetized, female white laboratory rats were subjected to almost square-wave changes in ambient temperature, from 20 degrees C to 35 degrees C, in a small climatic chamber. 2. Intracerebroventricular injections of serotonin and methysergide were made in shivering, unanaesthetized rats. 3. The integrated electromyogram of m. suprascapularis was recorded as a quantitative measure of shivering activity to monitor the effects of the above procedures. 4. Both warm transients and serotonin inhibited shivering, but methysergide blocked only the effect of serotonin.

Serotonergic projections from the ventral medulla to the intermediolateral cell column in the rat

Serotonergic neurons of the ventral medulla (B1 and B3 cell groups are retrogradely labeled after HRP injection in the T1-T2 levels of the spinal cord. Following stereotaxic injections of 3H-labeled amino acids in this region, a direct projection to th intermediolateral cell column and ventral horn can be demonstrated.

Descending pathways to sympathetic and parasympathetic preganglionic neurons

In this review a summary of some of the neural pathways that appear to be involved in central cardiovascular control is given. The efferent connections of the nucleus tractus solitarius are described. Particular emphasis is placed on those projections that go to nuclei that have direct connections with the intermediolateral cell column (viz. paraventricular hypothalamic nucleus, Kõlliker-Fuse nucleus, A5 catecholamine cell group, the chemosensitive region of the ventral medulla, and possibly the region of the A1 catecholamine cell group). In addition, some fo these nuclei also have direct or indirect connections with the paraventricular and/or supraoptic hypothalamic nuclei. This suggests that some of the pathways that project to the cardiovascular preganglionic neurons may also influence the release of vasopressin.

The nucleus paragigantocellularis lateralis in the rat. Demonstration of afferents by the retrograde transport of horseradish peroxidase

Injections of horseradish peroxidase (HRP) were placed in the middle or caudal portion of the nucleus paragigantocellularis lateralis (PGCL) and 24 h later the entire spinal cord and brain were processed and examined for labeled neurons. Spinal afferents arise from all levels of the cord. Rexed's lamination scheme was adapted to the spinal cord of the rat and labelled neurons were localized to laminae IV, V, VII, VIII and X mainly on the side contralateral to the injection. At cervical levels, labeled neurons were consistently found bilaterally. The medial reticular nuclei of the medulla and pons contained HRP-labelled perikarya, which were concentrated most heavily in the nuclei reticularis medullae oblongatae ventralis, gigantocellularis, and pontis caudalis predominantly ipsilateral to the injection. The medial vestibular nucleus was consistently labeled. HRP-labeled perikarya were found bilaterally within the commissural portion and in the medial part of the nucleus of the solitary tract on the side of the injection. The rostral portion of the PGCL receives afferents from some secondary auditory nuclei: the ipsilateral inferior colliculus and the posterior ventral cochlear nucleus bilaterally. Thus, the rostral PGCL may be involved in auditory feedback loops. The caudal raphe nuclei are a major source of afferents to the caudal PGCL. The lateral hypothalamic area, paraventricular nucleus, and zona incerta also contain labeled neurons when injections are centered in the caudal portion of the nucleus.

The nucleus paragigantocellularis lateralis in the rat. Conformation and cytology

The nucleus paragigantocellularis lateralis (PGCL) is located in the ventral portion of the rostral medulla. Serial sections of the rat brainstem were examined in the three cardinal planes and the boundaries of the PGCL were determined. In order to visualize the shape and extent of the nucleus, a three-dimensional reconstruction of the PGCL was made from a series of coronal sections. Measurements of neuronal areas, lengths, and widths indicate that a number of neuronal types are present. Small neurons measure less than 150 micron2 and large neurons greater than 250 micron2. Some neuronal types are distributed preferentially throughout the PGCL, and on this basis the nucleus may be divided into caudal and rostral subgroups. Most large neurons (greater than 250 micron2) are found in the caudal portion. Certain neurons contain intranuclear rods, and these neurons are often disposed in small groups, especially common the caudal PGCL. Two morphologically distinct neuronal types incorporate 3H-serotonin when this marker is infused into the ventricular system; the other neurons not marked by this method probably contain other, different transmitters. On the basis of neuronal measurements and staining qualities, it is ascertained that the PGCL is a parvocellular reticular nucleus characterized by many neuronal types.

The nucleus reticularis lateralis: a region highly sensitive to clonidine

Slow bilateral microinjections of a low dose of clonidine (75 ng/kg) in the cat's nucleus reticularis lateralis (NRL) lead to significant hypotension and bradycardia. This finding confirms the existence of a ventromedullary highly sensitive site of action of clonidine. It is suggested that clonidine inhibits some vasopressive and cardioacceleratory structures within the NRL region.

The respiratory role of the ventral surface of the medulla studied in the anaesthetized rat

1. The respiratory role of the ventral surface of the medulla was studied in rats anaesthetized with a urethane-chloralose mixture. 2. In fifty-eight studies on twelve animals, direct superfusion of the medullary surface with artificial c.s.f. made acid by the reduction of bicarbonate content or by the increase of PCO2 produced no significant stimulation of respiration provided that the temperature of the brain surface was unaltered. 3. Superperfusion of the medullary surface with c.s.f. of low bicarbonate content produced an inhibition of respiration in fourteen of thirty-eight experiments. 4. Electrical stimulation on the surface revealed a localized area lateral to the pyramids and rostral to the XIIth nerve where stimulation at low intensity produced an increase in the frequency and depth of respiration. 5. The application of carbachol to a similar region increased both the frequency and amplitude of ventilation at lower concentrations than were required to obtain effects from surrounding areas. 6. Sudden switching between perfusates at different temperatures produced changes of ventilation within 1-2 sec of a change of surface temperature. The Q10 for the ventilation/temperature relationship was approximately 6. 7. The experiments confirm that the ventral surface of the medulla contains neural elements which, at least during urethane-chloralose anaesthesia, have a significant effect on respiration. The stimulus for these effects in the rat does not appear to be a change in H+ concentration. It appears more probable that the primary role of the area lies in the link between thermal and respiratory regulation.

Medullary cardiovascular effects of tetrodotoxin in anaesthetized cats

Tetrodotoxin injected in the cat nucleus tracus solitarii produced hypertension without bradycardia, whereas in the nucleus reticularis lateralis it led to hypotension and bradycardia. Tetrodotoxin acts by blocking neuronal activity; it is a useful tool for studying the central organization of cardiovascular regulation. The present data confirm that the nucleus reticularis lateralis is a vasopressive centre.

Diabetes: the genetic connections

Insulin dependent (IDD) and non-insulin dependent diabetes (NIDD) are separate disorders. Twin studies show that IDD cannot be entirely due to genetic causes as concordance is no more than about 50%, but there is some inherited predisposition to it as shown by HLA patterns. NIDD, on the other hand, is predominantly due to genetic causes since identical twins are nearly always concordant. Many cases of NIDD show chlorpropamide alcohol flushing (CPAF), a dominantly inherited feature which may precede the appearance of diabetes and thus act as a genetic marker for this type of diabetes. Diabetics who show chlorpropamide acohol flushing are less likely to develop retinopathy than those who do not. Genetic factors must therefore affect the incidence and severity of diabetic retinopathy. Chlorpropamide alcohol flushing is due to sensitivity to enkephalin. Enkephalin and other opioids affect carbohydrate metabolism and insulin release. It is possible therefore that they act as neurotransmitters and cause NIDD by a sympathetically mediated effect on the liver and pancreas--in other words, that as far as NIDD is concerned Claude Bernard's views on the cause of diabetes may have been right.

Sensitivity to enkephalin as a cause of non-insulin dependent diabetes

Non-insulin-dependent diabetes is associated with facial flushing after alcohol in patients on chlorpropamide (chlorpropamide alcohol flushing, C.P.A.F.) especially when there is a family history of diabetes. C.P.A.F. in three subjects (two diabetics, one non-diabetic) was blocked by the specific opiate antagonist naloxone. In nine subjects (six diabetics) C.P.A.F. was reproduced by the enkephalin analogue with opiate-like activity [D-Ala2, MePhe4, Met (O)-ol] enkephalin (DAMME). C.P.A.F. thus may be due to increased sensitivity to endogenous opiates. DAMME and other substances with opiate-like activity, such as morphine and beta-endorphin, affect carbohydrate metabolism and insulin secretion. Increased sensitivity to endogenous opiates such as enkephalin may thus give rise to non-insulin-dependent diabetes associated with C.P.A.F.

Topography of the respiratory and circulatory responses to acetylcholine and nicotine on the ventral surface of the medulla oblongata

1. Acetylcholine and nicotine were superfused on the ventral medullary surface between the ponto-medullary border and C1 in anaesthetized cats in order to determine the topical distribution of their actions on respiration and circulation. 2. Acetylcholine (10(-4) g . ml-1 = 5.5 . 10(-4) mMol . ml-1) produced an increase in respiration and a lowering of blood pressure. The magnitude and the time course of the responses varied according to the points of superfusion on the surface. 3. Nicotine (10(-4) g . ml-1 = 6.2 . 10(-4) mMol . ml-1) elicited hyperventilation and more often an increase in arterial pressure on unilateral superfusion of the surface. In some cases, however, a drop in blood pressure was also observed. 4. The responsive regions of the surface on which nicotine acted and elicited hyperventilation, bear a close resemblance to the regions responsive to acetylcholine. 5. The topographical distribution of the respiratory effects elicited by the above-mentioned drugs were similar to the distribution of the responses to changes in pH on the ventral medullary surface or to electrical stimulation. 6. Procaine (2 . 10(-2) g . ml-1 = 7.3 . 10(-2) mMol . ml-1) applied bilaterally in the intermediate zone (S) caused profound inhibition of respiration and of arterial pressure. Procaine at this concentration also inhibited respiratory hyperventilation caused by nicotine (10(-4) g . ml-1 = 6.2 . 10(-4) mMol . ml-1) applied to the caudal and rostral areas.

Elimination of central chemosensitivity by coagulation of a bilateral area on the ventral medullary surface in awake cats

Breathing and respiratory response to CO2 were observed in 6 awake cats and 1 control before and after bilateral coagulation of the formerly described area S (Schläfke and Loeschcke, 1967) on the ventral medullary surface under hyperoxic conditions. Ventilation decreased, PCO2 rose and CO2 response was almost or completely abolished in 4 cats, and moderately reduced in 2 cats. Inhalation of CO2 had an inhibitory effect on ventilation in two cases. In some instances the respiratory frequency was increased by CO2. Periodic breathing as well as spontaneous hyperventilation elicited by 'arousal' indicate parallels to the Pickwickian or Ondine's curse syndrome. No respiratory changes were produced by a lesion on the pyramidal tract medial to the area S. It is concluded that central chemosensitivity can be eliminated within the superficial layer of the area S. The loss of CO2 response seems to be correlated with complete destruction of the superficial nerve cells located within the area S (Petrovický, 1968) and degeneration within the ventral part of the nucleus paragigantocellularis.

The paratrigeminal nucleus. II. Identification and inter-relations of catecholamine axons, indoleamine axons, and substance P immunoreactive cells in the neuropil

The localization of three different putative neurotransmitters -- indoleamine, catecholamine, and Substance P -- was studied in the paratrigeminal nucleus of the rat and rhesus monkey at the light and electron microscope level by autoradiography following administration of [3H]5-hydroxytryptamine, or [3H]norepinephrine, and by immunocytochemistry using the unlabelled anti-Substance P antiserum peroxidase--antiperoxidase technique. The paratrigeminal neurons are not monoaminergic but certain cells exhibit Substance P-like immunoreactivity. These cells receive a rich plexus of indoleamine afferents, a sparse catecholamine input, and a rich plexus of fibres with Substance P-like immunoreactivity. Of the entire monoaminergic population of labelled axons, more than 60% are synaptic and less then 40% nonsynaptic, and this proportion is the same for indoleamines as for catecholamines. Indoleamine axons form a heterogeneous population with at least four different morphological types that are synaptic and three that are nonsynaptic. They bear distinctive collections of small, clear, tubular or large granular vesicles, which distinguish one category of axon from another. These axons engage in numerous axo--somatic, axo--spinous, axo--dendritic, and possibly axo--axonic relations with paratrigeminal neurons. The catecholamine axons are also heterogeneous in axoplasmic morphology but their terminal contacts are distributed to more peripheral portions of dendrites. The significance of the inter-relations between the monaminergic and peptidergic elements in the paratrigeminal nucleus is discussed in relation to the possible functions of this nucleus as a nociceptive, chemosensitive, or pressure-sensitive centre on the lateral medullary surface.

Respiratory and circulatory effects of saxitoxin in the cerebrospinal fluid

1 In cats anaesthetized with pentobarbitone, saxitoxin and, on a few occasions, tetrodotoxin were injected into a lateral cerebral ventricle or into the subarachnoid space of the lower brain stem. Observations were made on frequency and tidal volume of breathing, on CO(2) responsiveness and on electrical responsiveness of the respiratory centre. Effects on the blood pressure were observed simultaneously.2 A single large dose of toxin, e.g., 250 ng, produced within minutes apneustic breathing and a rise in blood pressure which were converted rapidly to respiratory failure and hypotension. In contrast, repeated small doses, e.g., 25 ng, yielded only progressive slowing of the respiration together with circulatory hypotension. Bulbar depression was produced as effectively by subarachnoid injection as by intraventricular injection of the toxins. Onset of action was detectable within seconds.3 Slowing of the respiration occurred independently of change in tidal volume and whether or not the vagus nerves were cut. The reduction in breathing frequency is attributed to direct toxin-induced depression of the central respiratory oscillator.4 Steady-state measurements of tidal volume at controlled levels of alveolar CO(2) pressure in intermediate stages of respiratory depression showed that the toxins produced an increase in CO(2) stimulation threshold as well as a reduction in gain of CO(2) responsiveness, whether or not the vagus nerves were cut. Carotid arterial chemoreceptor reactivity to O(2) was demonstrable when central sensitivity to CO(2) was depressed. These effects are attributed to a direct influence of the toxins upon the brainstem CO(2)-tidal volume controller.5 Responsiveness of the medullary inspiratory centre to electrical stimulation persisted after the failure of spontaneous breathing was caused by the toxins. Conversely, restitution of electrical responsiveness preceded the reappearance of spontaneous respiratory activity in the recovery phase of toxic depression. Circulatory effects paralleled the changes in respiratory behaviour.6 On the basis of the relatively prompt and discrete alterations in the central respiratory and circulatory control mechanisms produced by saxitoxin and tetrodotoxin placed in the cerebrospinal fluid, it is concluded that the agents rapidly penetrated to deep target loci in the lower brain stem.