Daniel Marlot

Somatotopy in the phrenic motor nucleus of the cat as revealed by retrograde transport of horseradish peroxidase

Cats received unilateral or bilateral horseradish peroxidase (HRP) injections into various portions of the diaphragm. In two experiments one of the cut cervical roots of the phrenic nerve was immersed in HRP. The phrenic motoneurons located in the fifth and occasionally the fourth cervical segment send their axons, via the upper phrenic root, to pars sternalis and pars costalis of the diaphragmatic dome whereas the neurons of the sixth segment innervate preferentially the dorsal portion both crura and dome. No evidence of contralateral innervation of the diaphragm was obtained.

Spinal localization of the intercostal motoneurones innervating the upper thoracic spaces

At the rostral level of the thorax, the intercostal muscles participate both in postural and respiratory functions to a variable degree depending upon the considered muscle: external intercostal, intercartilaginous, internal intercostal, and triangularis sterni. In order to determine if these physiological properties are related to a special organization at the spinal cord level, we have used the retrograde transport of horseradish peroxidase as a tool for studying the spinal distribution of intercostal motor cells in the adult cat. Results suggest that the intercostal motoneurones could be distributed, in the ventral grey horn, among two areas according to the respiratory or postural muscle specialization.

Regulation by protein kinase-C of putative P-type Ca channels expressed in Xenopus oocytes from cerebellar mRNA.

Xenopus oocytes injected with rat cerebellar mRNA expressed functional voltage‐dependent Ca channels detected as an inward Ba current (I Ba). The pharmacological resistance to dihydropyridines and ω‐conotoxin together with the blockade obtained with Agelenopsis aperta venom suggest that these channels could be somehow assimilated to P‐type Ca channels. The precise nature of the transplanted Ca channels was assessed by hybrid‐arrest experiments using a specific oligonucleotide antisense‐derivated from the recently cloned α1‐subunit of P channels (BI‐1 clone). In addition, we demonstrate that exogenous Ca channel activity was enhanced by two different PKC activators (a phorbol ester and a structural analog to diacylglycerol). The general electrophysiological and pharmacological properties of the stimulated Ca channels remain unchanged. This potentiation induced by PKC activators is antagonized by a PKC inhibitor (staurosporine) and by a monoclonal antibody directed against PKC. It is concluded that P‐type Ca channels are potentially regulated by PKC phosphorylation and the functional relevance of this intracellular pathway is discussed.

Connections between retrotrapezoid nucleus and nucleus tractus solitarii in cat.

The retrotrapezoid nucleus (RTN), a part of the rostral ventrolateral medulla, is involved in the control of breathing. The mechanisms by which the RTN modulate the activity of respiratory neurons during chemoreceptor stimulation are not fully understood. This electrophysiological study performed in the cat demonstrates that 18 out of 22 RTN neurons receive inputs from the commissural subnucleus of the solitary tract (cNTS), the peripheral chemoreceptor afferents projection site. Moreover, six RTN neurons are found to present interconnection between RTN and the ventrolateral subnucleus of the solitary tract, an area containing mainly bulbo-spinal respiratory neurons. The present data suggest that RTN play a key role by concomitantly integrating chemosensitive informations relayed by the cNTS and providing an influence on the respiratory network.

Segmental motor innervation of the cat diaphragm

In ten anaesthetized adult cats, bipolar recording electrodes were inserted in different muscular bundles of each hemi-diaphragm. Both stimulation and section of the phrenic cervical branches were made. The upper phrenic cervical branch innervates both the sternal and lateral portions of the diaphragm whereas the lower phrenic cervical branch innervates both the lateral and dorso-caudal portions.

Projection of phrenic afferents to the external cuneate nucleus in the cat

The present study, performed on anesthetized, spontaneously breathing cats, deals with the projection of group I and II muscle afferents of the phrenic nerve (PN) to the external cuneate nucleus (ECN). Stimulation of the central end of the PN evoked a complex response in the ipsilateral ECN. Two principal components could be distinguished in this potential from the respective absolute refractory periods (ARP) and from the effect of antidromic stimulation in the ECN. Thus, the early group of waves may correspond to recordings of direct fibers and the later group to postsynaptic activations within the ECN. Similar to the forelimb nerves and intercostal nerves of the upper intercostal spaces, the larger muscle afferents of the PN project to the ECN.

Effect of hypoxia on the activity of respiratory and non-respiratory modulated retrotrapezoid neurons of the cat.

The retrotrapezoid nucleus (RTN), a part of the rostral ventrolateral medulla, is involved in the control of breathing. A recent immunohistological study suggested a possible involvement of the RTN in hypoxic chemoreflex loop. The present electrophysiological study performed in the cat demonstrates that 23 out of 24 RTN neurons were stimulated during the biphasic respiratory response to hypoxia, which consists of a reinforcement followed by a depression of respiratory activity. This confirms the previous immunohistological study. While 15 RTN neurons might integrate either phase I (n = 7) or phase II (n = 8) O2-chemosensitive inputs, the remaining eight RTN neurons stimulated by hypoxia are susceptible to integrate both phase I and phase II O2-chemosensitive inputs. In conclusion, our results suggest that the different subsets of RTN neurons may influence respiratory output by conveying signals originating from peripheral and/or central chemoreceptors stimulated during hypoxia.

Phrenic afferent input to the lateral medullary reticular formation of the cat

The afferent inputs from phrenic nerve stimulation to the lateral reticular formation of the lower brain stem were studied in anesthetized spontaneously breathing cats. The activity of reticular neurons was recorded by means of extracellular tungsten microelectrodes. Electrical stimulation of the central end of the right phrenic nerve evoked excitatory or inhibitory responses in the lateral reticular nucleus (LRN), in the nucleus ambiguus (AMB) and in a region dorsal to the AMB of ipsi- and contralateral sides. Phrenic afferents belonging to the flexor reflex afferent group were involved in these responses. The discharge pattern of the respiratory related units (RRU) of the AMB were exceptionally affected by phrenic nerve stimulations. It is concluded that high threshold phrenic afferents relay in the LRN before projecting to the cerebellar cortex. The overlapping of respiratory and non-respiratory afferents in the reticular formation may participate to the adaptations of respiratory and somatomotor functions during specific behaviors.

Positive regulation by protein kinase C of slow Na current in Xenopus oocytes

The slow inward Na current observed during sustained depolarization of the Xenopus oocyte membrane is due to a complex mechanism described as the induction of the channels. The present work investigates the role of protein phosphorylation in Na channel function. Injection of alkaline phosphatase in the oocytes decreased inward current. Therefore, the possible involvement of protein kinase in Na channel induction was explored. Treatment of oocytes with two activators of protein kinase C (PKC) resulted in enhanced Na current amplitude, whereas the treatment of oocytes with two potent PKC inhibitors decreased the inward current. These results imply that PKC phosphorylation is a fundamental step of Na channel induction. The possibility that the depolarization of the oocyte membrane may be the factor involved in PKC activation is discussed.

Effects of stimulation of phrenic afferent fibers on medullary respiratory neurons in cat

The effects of electrical stimulation of both cervical branches (C5 and C6) of the right phrenic nerve on medullary respiratory neuron activity were studied in anesthetized, spontaneously breathing cats. In 14 cats, the stimulation of the thin phrenic afferents had no effect on the inspiratory duration and evoked excitatory or inhibitory responses in only 3/86 inspiratory neurons tested. In 3 cats, the stimulation decreased the inspiratory duration and 26/26 inspiratory neurons showed a shortened discharge without modification of their discharge frequency. Although the effects of the stimulation were not analysed by averaging techniques, it is concluded that phrenic afferents do not exert an important control on the medullary respiratory neuron discharge.