Nonadrenergic inhibitory innervation to the airways of the newborn cat

Vagal, nonadrenergic inhibitory system (NAIS) innervation to airway smooth muscle has been demonstrated in adults of several species, including humans. However, the functional status of this system in newborns is not known. The NAIS of intestinal smooth muscle has been demonstrated in newborns and develops in parallel with cholinergic innervation (14). Since the lung is derived embryologically from the foregut and cholinergic innervation is operative at birth, we tested the hypothesis that NAIS innervation to the airways is functional in newborn cats. Nineteen cats (2-11 days of age) were anesthetized with chloralose-urethan, and a tracheal cannula was inserted. The chest was opened and the animals were mechanically ventilated. The cervical vagus nerves were separated from the sympathetics, cut, and placed on stimulating electrodes. Mean inspiratory resistance (RL, I) and dynamic compliance (Cdyn, L) were measured on a breath-by-breath basis. Atropine and propranolol were administered (2 mg/kg iv) to block cholinergic and adrenergic pathways, respectively. Subsequently, serotonin infusion was used to increase RL, I approximately 150%. Stimulation (10 s) at frequencies ranging from 2 to 20/s caused a slow-onset (30 s to peak) long-lasting decrease in RL, I and a much smaller increase in Cdyn, L. The magnitude and duration of the bronchodilation increased with stimulus frequency to a plateau at approximately 15/s. At a stimulus frequency of 2/s, RL, I decreased 11 +/- 1.9 vs 36 +/- 4.8% (SE) at 20/s, whereas Cdyn, L increased 2 +/- 1.1 vs. 6 +/- 1.7%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of substance P on neurally mediated contraction of rabbit airway smooth muscle

The neuromodulatory action of substance P (SP) was investigated in isolated rabbit tracheal smooth muscle (TSM) segments contracted with electrical field stimulation (ES). The tissues were placed in organ baths containing modified Krebs-Ringer solution and stimulated at a constant voltage (8 V; 24.5 mA) and pulse duration (2 ms) with ES frequencies ranging from 1 to 100 Hz. In the presence of SP, there occurred a dose-dependent augmentation of the TSM contractile response to any given ES, with the maximal effect of SP obtained at a dose of 10(-7) M. Accordingly, with the administration of 10(-7) M SP, the ES frequency-response relationship was altered so that 1) the mean (+/- SE) maximal tension (Tmax) induced by ES significantly increased (P less than 0.02) from a base-line value of 273 +/- 53 to 402 +/- 45 g/g TSM; and 2) the mean (+/- SE) log ES frequency producing 50% of Tmax (ES50) significantly decreased from a base-line value of 1.278 +/- 0.069 to 1.102 +/- 0.070 Hz (P less than 0.01). In contrast to these effects on ES-induced contraction, SP administration did not affect the TSM contractile response to administered methacholine chloride (10(-8) to 10(-3) M). On the other hand, the effects of SP on ES-induced contraction were independently blocked by the cholinergic antagonist, atropine (10(-6) M); the neurotoxin, tetrodotoxin (10(-6) g/ml); and the SP antagonist, D-Arg1,D-Pro2,D-Trp7,9,Leu11-SP (10(-5) M).(ABSTRACT TRUNCATED AT 250 WORDS)     

Dynamic cerebral autoregulation is preserved in neurally mediated syncope.

To test whether cerebral autoregulation is impaired in patients with neurally mediated syncope (NMS), we evaluated 15 normal subjects and 37 patients with recurrent NMS. Blood pressure (BP), heart rate, and cerebral blood velocity (CBV) (transcranial Doppler) were recorded at rest and during 80 degrees head-up tilt (HUT). Static cerebral autoregulation as assessed from the change in cerebrovascular resistance during HUT was the same in NMS and controls. Properties of dynamic cerebral autoregulation were inferred from transfer gain, coherence, and phase of the relationship between BP and CBV estimated from filtered data segments (0.02-0.8 Hz). During the 3 min preceding syncope, dynamic cerebral autoregulation of subjects with NMS did not differ from that of controls nor did it change over the course of HUT in patients with NMS or in control subjects. Dynamic cerebral autoregulation was also unaffected by the degree of orthostatic intolerance as inferred from latency to onset of syncope. We conclude that cerebral autoregulation in patients with recurrent syncope does not differ from that of normal control subjects.     

Distribution of myelinated nerves in ascending nerves and myenteric plexus of cat colon

The parts of the colon differ in motor function and in responses to extrinsic and intrinsic nerve stimulation. The distribution of myelinated nerve fibers in the colonic myenteric plexus is not known. Because these fibers might be largely extrinsic in origin, their distribution might indicate the domain of influence of extrinsic nerves and help to explain the different behaviors of the different parts of the colon. Myelinated fibers were examined by electron microscopy in cross sections of the ascending nerves and in myelin-stained whole-mount preparations in the colon. The ascending nerves are much like one another. They have the structure of peripheral nerves, not that of myenteric plexus. The proportion of myelinated fibers in the ascending nerves declines rostrad with no uniform change in total nerve fiber number. Cross-sectional areas of ascending nerves, 3,304 to 7,448 microns 2; total number of nerve fibers per profile, 703-2,651; and mean myelin coat thickness, 0.45 +/- 0.01 micron, do not change uniformly along the ascending nerves. Myelinated fibers are about 2% of total fibers in the extramural colonic nerves, 7-9% in the ascending nerves in the sigmoid colon, and 2-3% at the rostrad ends of the ascending nerves in the transverse colon. Blood vessels lie at the core of each ascending nerve and on the nerve sheath. Myelinated fibers in the ascending nerves degenerate after section of colonic branches of the pelvic plexus and after section of the pudendal nerves, indicating that myelinated nerves reach the colon through both pathways.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dysfunctional inhibitory muscarinic receptors mediate enhanced histamine release in isolated human bronchi

In human airways mucosal mast cells are under the control of inhibitory muscarinic receptors. The described experiments tested, whether the inhibitory potency of two muscarinic receptor agonists (oxotremorine, acetylcholine) becomes impaired in advanced chronic obstructive pulmonary disease (COPD). Isolated human bronchi obtained from 26 patients with lung cancer were separated into two groups. Group 1 patients suffered from moderate COPD (mean FEV1 56%; range 34-71%; mean pack years of cigarette smoking 50, range 20-96; one non-smoker). Group 2 patients had no or only a mild form of COPD; mean FEV1 was 82% (62-97%) and the number of pack years 22 (6-45; 3 non-smoker). The calcium ionophore A23187 induced a maximal histamine release of 4100+/-870 pmol/g/5 min in group 1 bronchi, in contrast to only 1730+/-240 pmol/g/5 min in group 2 bronchi (p<0.02). Oxotremorine (1 nmol/L) reduced the stimulated histamine release by 81+/-5% in group 2 bronchi, but did not produce a significant effect in group 1 bronchi (11+/-14%). In conclusion, the present experiments show an enhanced histamine release in advanced COPD, which can be explained by a dysfunction of inhibitory muscarinic receptors.     

Botzinger complex region role in phrenic-to-phrenic inhibitory reflex of cat

Neuronal recordings, microstimulation, and electrolytic and chemical lesions were used to examine the involvement of the B?tzinger Complex (B?tC) in the bilateral phrenic-to-phrenic inhibitory reflex. Experiments were conducted in decerebrate cats that were paralyzed, ventilated, thoracotomized, and vagotomized. Microelectrode recordings within the B?tC region revealed that some neurons were activated by phrenic nerve stimulation (15 of 69 expiratory units, 9 of 67 inspiratory units, and 19 nonrespiratory-modulated units) at average latencies similar to the onset latency of the phrenic-to-phrenic inhibition. In addition, microstimulation within the B?tC caused a short latency transient inhibition of phrenic motor activity. In 17 cats phrenic neurogram responses to threshold and supramaximal (15 mA) stimulation of phrenic nerve afferents were recorded before and after electrolytic B?tC lesions. In 15 animals the inhibitory reflex was attenuated by bilateral lesions. Because lesion of either B?tC neurons or axons of passage could account for this attenuation, in eight experiments the phrenic-to-phrenic inhibitory responses were recorded before and after bilateral injections of 5 microM kainic acid (30-150 nl) into the B?tC. After chemical lesions, the inhibitory response to phrenic nerve stimulation remained; however, neuronal activity typical of the B?tC could not be located. These results suggest that axons important in producing the phrenic-to-phrenic reflex pass through the region of the B?tC, but that B?tC neurons themselves are not necessary for this reflex.     

PAF-induced contraction of canine trachea mediated by 5-hydroxytryptamine in vivo

We studied the secretory correlates of tracheal smooth muscle contraction caused by platelet-activating factor (PAF) in nine mongrel dogs in vivo. In five dogs, dose-response curves were generated by rapid intra-arterial injection of 10(-10) to 10(-6) mol PAF into the isolated tracheal circulation; tracheal contractile response was measured isometrically in situ. To examine the mechanism by which PAF elicits contraction of canine trachealis, concentrations of serotonin (5-HT) and histamine were assayed in the venous effluent as the arteriovenous difference (AVd) in mediator concentration across the airway for each level of contraction. PAF caused dose-related active tracheal tension to a maximum of 37.2 +/- 5.4 g/cm (10(-6) mol PAF). The AVd in 5-HT increased linearly from 0.20 +/- 0.05 (10(-9) mol PAF) to 3.5 +/- 0.3 ng/ml (10(-6) mol PAF) (P less than 0.005). In contrast, the AVd in histamine was insignificant and did not change with increasing doses of PAF. A positive correlation was obtained between the AVd in 5-HT and active tracheal tension (r = 0.92, P less than 0.001); there was no correlation between AVd in histamine and active tension (r = -0.16). PAF-induced parasympathetic activation was not mediated by 5-HT; contraction elicited by exogenous 5-HT was not affected by muscarinic blockade. We conclude that nonparasympathetically mediated contraction elicited acutely by PAF in dogs results at least in part from secondary release of serotonin and is not mediated by histamine.     

Survival dependency of intramuscular ICC on vagal afferent nerves in the cat esophagus

Interstitial cells of Cajal (ICC) have been proposed as stretch receptors for vagal afferent nerves in the stomach based on immunohistochemical studies. The aim of the present study was to use electron microscopy and the anterograde degeneration technique to investigate ultrastructural features and survival dependency of ICC associated with vagal afferent innervation of the cat esophagus. This is the first report on the ultrastructural characteristics of ICC in the cat esophagus. Intramuscular ICC (ICC-IM) were identified throughout the musculature, whereas ICC in the myenteric plexus were rare. ICC-IM were particularly numerous in septa aligned with smooth muscle bundles. They were in synapse-like contact with nerve varicosities and in gap junction contact with smooth muscle cells. Smooth muscle cells also made contact with ICC through peg and socket junctions. Precision damage through small-volume injection of saline in the center of the nodose ganglion from the lateral side, known to selectively affect sensory nerves, was followed within 24 h by degeneration of a subset of nerve varicosities associated with ICC-IM, as well as degeneration of the associated ICC-IM. Smooth muscle cells were not affected. Nerves of Auerbachs plexus and associated ICC were not affected. In summary, ICC-IM aligning the esophageal muscle bundles form specialized synapse-like contacts with vagal afferent nerves as well as gap junction and peg-and-socket contacts with smooth muscle cells. This is consistent with a role of ICC-IM as stretch receptors associated with vagal afferent nerves; the ICC-vagal nerve interaction appears essential for the survival of the ICC.     

Postnatal development of node-paranode regions in two hind limb nerves of the cat

Node-paranode regions of large myelinated axons from the nerves to the lateral gastrocnemius muscle (ankle extensor) and the anterior tibial muscle (ankle flexor) were studied in the cat during postnatal development and examined with regard to the occurrence of paranodal Schwann cell Marchi-positive bodies and mitochondria. It was found, in newborn kittens with respect to both parameters, that paranodes of flexor nerve fibers, being part of the functionally more developed ankle flexor reflex arc [cf. Mellström, A. (1971). Acta Physiol. Scand., 82, 477–489], appeared more mature than did those of extensor nerve fibers, which are part of the less developed ankle extensor reflex arc. It is concluded that the maturation of large feline hind limb muscle nerve fibers runs through a “nodalization” process similar to that described earlier for feline lumbar spinal root fibers [cf. Berthold, C.-H. (1973). Neurobiology, 3, 339–352] and that this normally occurring, rather striking remodeling of the node-paranode regions is likely to be functionally significant.     

Prejunctional inhibitory muscarinic receptors on cholinergic nerves in human and guinea pig airways

We have investigated whether prejunctional inhibitory muscarinic receptors ("autoreceptors") exist on cholinergic nerves in human airways in vitro and whether guinea pig trachea provides a good model for further pharmacological characterization of these receptors. Pilocarpine was used as a selective agonist and gallamine as a selective antagonist of these autoreceptors. Acetylcholine (ACh) release from postganglionic cholinergic nerves was elicited by electrical field stimulation (EFS) (40 V, 0.5 ms, 32 Hz). In human bronchi, pilocarpine inhibited the contractile response to EFS in a dose-related fashion; the dose inhibiting 50% of the control contraction was 2.2 +/- 0.4 x 10(-7) (SE) M (n = 22), and the inhibition was 96% at 3 x 10(-5) M. The inhibitory effects of pilocarpine were antagonized by gallamine in a dose-related fashion. The results were qualitatively the same in the guinea pig. Gallamine significantly enhanced the contractile response to EFS in the guinea pig, whereas pirenzepine failed to do so, which suggests that M2-receptors are involved. We conclude that prejunctional muscarinic receptors that inhibit ACh release are present on cholinergic nerves in human airways and that guinea pig trachea is a good model for further pharmacological characterization of these receptors, which appear to belong to the M2-subtype.     

Distribution and origins of VIP-immunoreactive nerves in the cephalic circulation of the cat

VIP-immunoreactive (IR) nerves were visualized in whole mounts and sections of cephalic arteries and cranial nerves of cats with indirect immunofluorescence. Perivascular VIP-IR nerves were very widely distributed in arteries and arterioles supplying glands, muscles and mucous membranes of the face. Within the cerebral circulation, perivascular VIP-IR nerves were most abundant in the Circle of Willis and the proximal portions of the major cerebral arteries and their proximal branches supplying the rostral brain stem and ventral areas of the cerebral cortex. VIP-IR nerves were absent from arterial branches supplying the posterior brain stem, cerebellum and dorsal cerebral cortex. Cerebral perivascular VIP-IR nerves probably arise from VIP-IR perikarya within microganglia found in the cavernous plexus and external rete. Extracerebral perivascular VIP-IR nerves probably arise from VIP-IR perikarya in microganglia associated with the tympanic plexus, chorda tympani, lingual nerve and Vidian nerve as well as from cells in the otic, sphenopalatine, submandibular and sublingual ganglia. It seems likely, therefore, that each major segment of the cephalic circulation is supplied by local VIP-IR neurons.     

Origin and colocalization of CGRP- and SP-reactive nerves in cat airway epithelium.

A combination of neuroanatomic techniques was used to examine the origin and neuropeptide content of nerve fibers in the airway epithelium of adult cats. By the use of immunocytochemical methods, the peptides substance P (SP) and calcitonin gene-related peptide (CGRP) were colocalized in airway epithelial nerve fibers. Two days after wheat germ agglutinin (WGA) was injected into the nodose ganglion, fibers containing WGA immunoreactivity (IR) were detected in the airway epithelium. SP-like immunoreactivity (LI) and CGRP-LI were demonstrated separately in the WGA-IR fibers, establishing their origin from nerve cell bodies of nodose ganglion. Vagal transection inferior to the nodose ganglion reduced the number of SP- and CGRP-IR fibers by greater than 90% in ipsilateral airways. In contralateral airways, SP-IR fibers were substantially reduced, whereas the effect on CGRP-IR fibers was not statistically significant. Vagotomy superior to the nodose ganglion did not alter the density of peptide-IR fibers. The results prove that SP- and CGRP-IR nerve fibers of cat airway epithelium originate from nerve cell bodies in the nodose ganglion and that SP- and CGRP-like peptides may be stored together in some nerve fibers of the airway epithelium.     

Cobalt injections into the pedunculopontine nuclei attenuate the reflex diaphragmatic responses to muscle contraction in rats.

Previous studies have suggested that neurons in the pedunculopontine nucleus (PPN) are activated during static muscle contraction. Furthermore, activation of the PPN, via electrical stimulation or chemical disinhibition, is associated with increases in respiratory activity observed via diaphragmatic electromyogram recordings. The present experiments address the potential for PPN involvement in the regulation of the reflex diaphragmatic responses to muscle contraction in chloralose-urethane anesthetized rats. Diaphragmatic responses to unilateral static hindlimb muscle contraction, evoked via electrical stimulation of the tibial nerve, were recorded before and subsequent to bilateral microinjections of a synaptic blockade agent (CoCl2) into the PPN. The peak reflex increases in respiratory frequency (9.0 +/- 1.0 breaths/min) and minute integrated diaphragmatic electromyogram activity (14.6 +/- 3.3 units/min) were attenuated after microinjection of CoCl2 into the PPN (2.6 +/- 0.9 breaths/min and 4.6 +/- 2.1 units/min, respectively). Consistent diaphragmatic responses were observed in the subset of animals that were barodenervated. Control experiments suggest no effects of PPN synaptic blockade on the cardiovascular responses to muscle contraction. The results are discussed in terms of a potential role for the PPN in modulation of the reflex respiratory adjustments that accompany muscular activity.