J. Czachurski

Bulbospinal projections to the intermediolateral cell column; a neuroanatomical study

The location of those neurones in the brain stem that project to the intermediolateral column (ILC) from which preganglionic sympathetic neurones have their origin was studied by the method of retrograde transport of horseradish peroxidase (HRP). In cats 30--50 nl of a 30% HRP solution was injected into the region of the ILC at T3 or L1 on one side. After a survival period of 72 h the lower brain stem from C1 to the inferior colliculi was sectioned and prepared for histological study under brightfield illumination. Neurones stained with exogenous HRP were found in three regions: (a) in the ipsilateral, dorsomedial part of the nucleus of the solitary tract (NTS) (43% of all labeled neurones), in the cranial part of the NTS, and also on the contralateral side (7%); (b) in the ventrolateral reticular formation beginning at the level of the obex up to 8 mm cranial to the obex (25% ipsilateral, 3% contralateral); and (c) in the ventral part of the raphe nuclei (postpyramidal and inferior central nucleus) from 2 to 9 mm cranial to the obex (22%).

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

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

An intracellular study of the synaptic input to sympathetic preganglionic neurones of the third thoracic segment of the cat

In chloralose anaesthetized, paralyzed and artificially ventilated cats intracellular recordings were obtained from sympathetic preganglionic neurones (SPN) of the third thoracic segment of the spinal cord identified by antidromic stimulation of the white ramus T3. The synaptic input to SPNs was assessed, in cats with intact neuraxis or spinalized at C3, by electrical stimulation of segmental afferent fibres in intercostal nerves and white rami of adjacent thoracic segments and by stimulation of the ipsi- and contralateral dorsolateral funiculus and of the dorsal root entry zone of the cervical spinal cord. In both preparations SPNs showed on-going synaptic activity which predominantly consisted of excitatory post-synaptic potentials (EPSPs). Inhibitory post-synaptic potentials (IPSPs) were rarely observed. EPSPs were single step (5 mV) or, less frequently, large (up to 20 mV) summation EPSPs. The proportion of SPNs showing very low levels of on-going activity was markedly higher in spinal than in intact cats. Stimulation of somatic and sympathetic afferent fibres evoked early EPSPs (amplitude 3 mV, latency 5-22.3 ms), and late, summation EPSPs (amplitude up to 20 mV, latency 27-55 ms). Early and late EPSPs were evoked in nearly all SPNs in which this synaptic input was tested in the intact preparation (from 79-93% of the SPNs). In spinal cats, early EPSPs were evoked in 88% of the SPNs, whereas late EPSPs were recorded only in half of the neurones. No evidence for a monosynaptic pathway from these segmental afferent fibres to SPNs was obtained. In both intact and spinal cats, stimulation of the dorsolateral funiculus evoked early and late EPSPs in SPNs. Late EPSPs were recorded in 70% and 37% of the SPNs in intact and spinal cats, respectively. Early EPSPs, however, were evoked in all neurones. The early EPSPs evoked by stimulation of the dorsolateral funiculus had several components which are suggested to arise from stimulation of descending excitatory pathways with different conduction velocities. The following conduction velocities were calculated in intact (spinal) cats: 9.5-25 m/s (7.8-13.2 m/s), 5.7-9.5 m/s (5.5-7.8 m/s), 3.8-5.7 m/s (3.2-5.5 m/s), and 2.6-3.8 m/s (2.1-3.2 m/s). EPSPs of these various groups were elicited in a varying percentage in SPNs. EPSPs of the most rapidly conducting pathway were subthreshold for the generation of action potentials; some EPSPs of this group had a constant latency suggesting a monosynaptic pathway to SPNs. Stimulation of the dorsal root entry zone at the cervical level yielded essentially the same results as stimulation of the dorsolateral funiculus.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurones within the "chemosensitive area" on the ventral surface of the brainstem which project to the intermediolateral column.

SummaryWith the method of retrograde transport of horseradish peroxidase it has been demonstrated that neurones within the “chemosensitive area” of the brainstem project to the thoracic intermediolateral column. The function of these neurones is discussed in regard to the regulation of blood pressure.

Effects of nitric oxide on sympathetic baroreflex transmission in the nucleus tractus solitarii and caudal ventrolateral medulla in cats

We have previously shown that nitric oxide (NO) attenuates baseline sympathetic tone in the rostral ventrolateral medulla (RVLM), while having no effects on baroreflex transmission in this region in cats. In the present study, we tested the effects of microinjections (500 nl) of NG-nitro-L-arginine (L-NNA, 0.3 mM) or the NO donor S-nitroso-N-acetylpenicillamine (SNAP, 10 microM) in the nucleus tractus solitarii (NTS) and in the caudal ventrolateral medulla (CVLM) which are the two other relays of the sympathetic baroreflex within the brainstem. Neither L-NNA nor SNAP significantly changed the length of inhibition of renal sympathetic nerve activity (SNA) evoked by electrical stimulation of the ipsilateral carotid sinus nerve. In contrast, glutamate (1 mM) in the NTS markedly increased baroreflex inhibition of SNA and the glutamate receptor antagonist kynurenate (5 mM) in the CVLM significantly decreased baroreflex transmission in the same experiments. These results suggest that sympathetic baroreflex function is preserved during both impaired endogenous synthesis and excess exogenous supply of NO in the brainstem.

Tonic descending inhibition of the spinal somato-sympathetic reflex from the lower brain stem

In chloralose-anaesthetized cats the spinal and supraspinal components of the somato-sympathetic reflex were evoked in the white ramus at T3 and/or L2 by stimulation of intercostal and spinal nerves. A reversible blockade of all ascending and descending spinal pathways was performed by cooling the spinal cord between the second and third cervical segment. Total blockade of conduction was produced at temperatures below 8.5 degrees C (281.5 K). The spinal blockade produced the following reversible effects. (1) Mean arterial pressure fell to 30-50 mm Hg (4.0-6.7 kpa) and the tonic background activity in the white ramus was reduced to 0-24% of control (mean 12.1 +/- 10.0%). (2) The amplitude of the early spinal reflex was increased from 100% to 111-316% (mean 200.9 +/- 49.5%, n = 49) at the thoracic level and to 125-342% (mean 181.4 +/- 74.4%, n = 7) at the lumbar level. The onset latency of the spinal reflex at T3 (range 8-21 msec) was shortened by 0.5-3.0 msec (mean 1.7 +/- 0.9 msec). (3) Supraspinal components were completely abolished. (4) Neither baroreceptor denervation nor midcollicular decerebration altered these effects. (5) The cold block induced increase of the amplitude of the spinal reflex was reduced by the alpha-adrenoceptor agonist clonidine; this effect was reversed by the alpha-adrenoceptor antagonist yohimbine. Selective cooling of the dorsolateral funiculus caused the same effects on the spinal and supraspinal reflexes as cold block of the whole spinal cord. From these findings it is concluded that in the anaesthetized cat the spinal component of the somato-sympathetic reflex is modulated by a descending tonic inhibition. This inhibition acts at both the thoracic and the lumbar level and its origin is in the medulla oblongata. This inhibition is, however, independent of baroreceptor inputs. The pathways descends in the dorsolateral funiculus. It is suggested that noradrenaline or adrenaline might be involved in the transmission of this inhibitory influence.

Three types of sympathetic preganglionic neurones with different electrophysiological properties are identified by intracellular recordings in the cat

AbstractIntracellular recordings were obtained from sympathetic preganglionic neurones (SPN) of the third thoracic segment in cats. Based on differences in their active and passive electrophysiological properties, three different types of SPNs were discerned:1)Type A neurones had a high resting membrane potential (RMP) (−60 to −86 mV) and a low input resistance (RN) 12–23 MΩ). Action potentials of these neurones had a pronounced IS-SD inflexion and a prominent shoulder in their falling phase. Spikes were rarely generated from the on-going synaptic activity.2)Type B neurones had a lower RMP (−48 to −65 mV) and a higher RN (21–37 MΩ). Their action potentials were characterized by an after-depolarization; they showed a slight IS-SD inflexion and a less pronounced shoulder in their falling phase. The after-depolarization was abolished by membrane hyperpolarization in a time dependent way. A hyperpolarization of at least 50 ms duration was required for its abolition. The after-depolarization was also abolished during repetitive discharges. In most of these neurones spikes were generated at irregular intervals and low rates (0.06–4.6 spikes/s) from the synaptic activity.3)Type C neurones were similar to type B neurones, but their action potentials did not show the after-depolarization. Additionally, spikes were generated at fairly regular intervals and rather high rates (0.8–6.5 spikes/s). The rate of spike repolarization of all neurones was markedly increased by hyperpolarization and decreased by membrane depolarization. Current-voltage curves of some type B and C neurones showed a marked rectification upon membrane hyperpolarization. In all type B and C neurones the presence of the A-current is indicated by the voltage trajectory at the end of hyperpolarizing current pulses. Current-frequency curves of type B and C neurones revealed discharge rates of SPNs as high as 100 spikes/s.

Tonic catecholaminergic inhibition of the spinal somato-sympathetic reflexes originating in the ventrolateral medulla oblongata.

In chloralose-anesthetized cats activity of the spinal and supraspinal components of the somato-sympathetic reflex were evoked in the white ramus at T3 by stimulation of the corresponding intercostal nerve. A blockade of all spinal pathways by means of a reversible cold blockade of the spinal cord at C2-C3 produced the following effects: (1) mean arterial blood pressure fell to 30-50 mm Hg and the tonic background activity in the white ramus was markedly reduced; (2) the amplitude of the spinal reflex was significantly increased and the supraspinal reflex was completely abolished; (3) localized cold block of the dorsolateral funiculus produced the same effect as cold block of the whole spinal cord; (4) neither baroreceptor denervation nor midcollicular decerebration altered these effects; and (5) the alpha-adrenoceptor agonist clonidine reduced the increased amplitude of the spinal reflex during cold blockade; this effect was reversed by the alpha-adrenoceptor antagonist yohimbine. Bilateral cold blockade of areas on the ventrolateral surface of the brain stem between the rootlets of the hypoglossal nerve and the trapezoid body caused the same effect on background and reflex activity in the white ramus as did spinal cord blockade. A mapping of the catecholaminergic (CA) neurons in the lower brain stem of the cat by means of the fluorescence method showed CA neurons in the ventrolateral medulla at two levels: (1) one group of neurons in the caudal medulla, which lies ventral and dorsal to the lateral reticular nucleus (corresponding to area A1 in the rat); and (2) a second group found more cranially and located ventrally to the facial nucleus (corresponding to area A5 in the rat). CA nerve terminals in the spinal cord mainly innervate the intermediolateral cell column. From these findings it is concluded that in the anesthetized cat the spinal component of the somato-sympathetic reflex is modulated by a descending tonic inhibition. This inhibition is independent of baroreceptor input. The pathways descend in the dorsolateral funiculus of the spinal cord, and it is suggested that they originate either in the cranial part of area A1 and/or area A5.

Localization of neurones with baroreceptor input in the medial solitary nucleus by means of intracellular application of horseradish peroxidase in the cat.

Abstract The location and morphology of second order baroreceptor neurones was investigated in cats. Using intracellular recordings neurones were identified by their discharge pattern in phase with the pulse pressure wave, and horseradish peroxidase (HRP) was injected by iontophoresis. The second order baroreceptor neurones were located in close vicinity to the medial boundary of the solitary tract, from 400 μm caudal to 200 μm cranial to the obex. The neurones were spindle shaped with a diameter of 6–8 μm in the short axis and 25–30 μm in the long axis.

Somato-sympathetic reflex transmission in the ventrolateral medulla oblongata: spatial organization and receptor types

Tonic sympathetic activity in vivo is continuously modulated by inhibitory and excitatory reflex mechanisms. We studied the properties of somato-sympathetic excitatory reflex transmission in the rostral ventrolateral medulla (RVLM) of baroreceptor-denervated and vagotomized chloralose-anesthetized cats. Electrical stimulation of the left intercostal nerve of the 4th thoracic segment (IC-T4) elicited an early spinal and a late supraspinal reflex in the ipsilateral white ramus T3 from which recordings were made. Bilateral cooling of the ventral surface of the RVLM reversibly reduced the supraspinal reflex amplitude to 18.0 +/- 3.1% of control (100%). The spinally evoked reflex was enhanced to maximally 154.7 +/- 5.3%. Cooling of only the ipsilateral side of the RVLM was nearly equieffective in both, suppressing the supraspinal and enhancing the spinal reflex component. In contrast, cooling of the contralateral side had no significant effects on supraspinal reflex transmission but caused slight increases of the spinal reflex amplitudes. Similar effects were obtained by microinjection (RVLM) of the glutamate antagonist kynurenic acid (5 x 10(-3) M, n = 7) and the specific non-NMDA receptor antagonist CNQX (4 x 10(-3) M, n = 4) which, however, blocked the supraspinal reflex less effectively. These results demonstrate that the RVLM represents an essential relay in the transmission of both somatosympathetic reflex components. The experiments further suggest an almost completely ipsilateral neuronal pathway for the supraspinal reflex component which projects from the RVLM to the intermediolateral cell column (IML). The descending inhibition of the spinal reflex, however, receives neuronal inputs from the contralateral side.