Central respiratory modulation of subretrofacial bulbospinal neurones in the cat.

Abstract

SUMMARY1. Spontaneous activity was recorded from spinally projecting neurones of the subretrofacial nucleus (s.r.f.) in seven chloralose-anaesthetized, vagotomized, paralysed cats. Nineteen such neurones were identified by their antidromic response to stimulation of the ipsilateral dorsolateral funiculus at C5: their axonal conduction velocities were between 2X1 and 9-1 m/s (mean 5 4), and all were silenced by raising the pressure in the ipsilateral carotid sinus (prepared as a blind sac) to 200 mmHg. DL-homocysteic acid was applied ionophoretically to six of these cells, and these were all excited.2. Activity was recorded simultaneously from the right phrenic nerve and left cervical sympathetic trunk. The phrenic neurogram was taken as an index of central respiratory drive and used to trigger histograms of s.r.f. neurone activity within the respiratory cycle. To eliminate indirect effects via pressure fluctuations, the remaining arterial baroreceptors were disabled during tests by deflation of the ipsilateral carotid sinus and either section of the contralateral sinus nerve (six cats) or occlusion of the carotid artery (one cat). Phrenic-triggered averages of cervical sympathetic activity were also constructed for comparison.3. Respiratory modulation was detected in the activity of sixteen out og nineteen s.r.f. neurones. Twelve showed an inspiratory peak in activity that was superimposed on a steady discharge maintained throughout the cycle. Five of this group additionally displayed a post-inspiratory dip in activity to below the late expiratory level, and this feature was also seen in one further cell without any detectable inspiratory peak. Three neurones showed an inverse pattern, with reduced or no activity during inspiration, followed by a 'rebound' increased activity level during the postinspiratory period. The remaining three cells had no discernible respiratory rhythm.4. Raising end-tidal CO2 levels increased the degree of respiratory modulation in all six inspiratory-firing cells tested as well as one cell showing inspiratory inhibition. Within the range tested, CO2 had no clear effect on the respiratory modulation of two other cells showing inspiratory inhibition and one showing only post-inspiratory depression. No consistent relation was apparent between the overall firing rate of neurones and end-tidal CO2.5. These results are discussed with reference to the respiratory modulation of sympathetic neurone activity. It is suggested that s.r.f. neurones supply respiratoryrelated as well as tonic drive to preganglionic neurones. R. M. MOALLEN

Medullary Respiratory Network Drives Sympathetic Overactivity and Hypertension in Rats Submitted to Chronic Intermittent Hypoxia

“…22,52 These observations clearly indicate the existence of a central coupling between respiratory network and sympathetic nervous system. 22,[52][53][54][55] Considering that the mechanisms generating high levels of sympathetic activity and arterial hypertension in CIH rats are complex and involve several mechanisms, it is reasonable to take into consideration that alterations in the functioning of central respiratory network may impose an additional excitatory drive to the presympathetic neurons. This hypothesis is supported by the fact that CIH produce substantial changes in the mechanisms controlling baseline respiratory activity.…”

“…64 The respiratory-sympathetic coupling has been suggested to be primarily generated by interactions between medullary central respiratory generator and presympathetic neurons (Figure 1). [52][53][54][55] In fact, important nuclei of brain stem controlling the respiratory activity are anatomically intermingled with neurons involved in the generation and regulation of sympathetic activity. 22 However, direct recordings of the neuronal activity revealed that presympathetic neurons located in the rostral ventrolateral medulla (RVLM) exhibit distinct patterns of activity entrained with the respiratory cycle.…”

“…54 Thereby, it has been proposed that the ventral surface of medulla is one of the most important areas in the brain stem responsible for integration of respiratory and sympathetic neuronal activities. 22,55,65,66 The inspiratory and expiratory brain stem neurons, especially those from ventral respiratory column, the pons, or the retrotrapezoid nucleus, 67-69 may establish interactions with the RVLM neurons and drive excitatory or inhibitory inputs. 69 These connections, acting directly or indirectly via projections to neurons located in the caudal ventrolateral medulla, 70 provide excitatory and inhibitory inputs for the generation of respiratory rhythmicity of sympathetic activity (Figure 1).…”

Intermittent hypoxia-induced sensitization of central chemoreceptors contributes to sympathetic nerve activity during late expiration in rats

“…Central coupling of respiratory and sympathetic neurons may occur at the level of the ventrolateral medulla, where many of the neurons involved in the generation of respiratory and sympathetic activities are located (Barman and Gebber 1980;Dampney 1994;Häbler et al 1994;Haselton and Guyenet 1989;Koshiya and Guyenet 1996;McAllen 1987;Richter and Spyer 1990;Zhong et al 1997). Specifically in this region, the inspiratory and expiratory neurons of the ventral respiratory column (VRC) interact with the presympathetic neurons of rostral ventrolateral medulla (RVLM) as well as with inhibitory interneurons of caudal ventrolateral medulla (CVLM) Haselton and Guyenet 1989;Mandel and Schreihofer 2006;Richter and Spyer 1990;Sun et al 1997).…”

Effects of hypothalamic thermal stimuli on sympathetic neurones innervating skin and skeletal muscle of the cat hindlimb.

“…Respiratory modulation of activity in postganglionic CVC neurones supplying hairy and hairless skin of the cat hindlimb (Boczek-Funcke et al 1992b) and in thoracic preganglionic neurones with reflex patterns of CVC neurones (Boczek-Funcke et al 1992a) is different from that of muscle vasoconstrictor activity (for review see Hiibler et al 1994). Furthermore, coupling between central respiration and sympathetic vasoconstrictor systems occurs in the medulla oblongata, notably the rostral ventrolateral medulla (McAllen, 1987;Haselton & Guyenet, 1989;Guyenet, Darnall & Riley, 1990). We therefore postulate that MVC and CVC presympathetic systems in the rostral ventrolateral medulla are not only differently organized with respect to their baroreceptor, chemoreceptor and respiratory inputs but that they are also different with respect to their inputs from thermoregulatory centres.…”