Effect of Substance P Injection into the Nucleus Tractus Solitarius of Rats on Cricothyroid and Thyroarytenoid Motor Activity and Cardiovascular and Respiratory Systems

Identification of central neurotransmitters that mediate laryngeal adductor and/or tensor activity may prove useful in managing pathological laryngeal adduction as occurs in laryngospasm or apparent life-threatening events. The putative transmitter substance P (SP) is found in the nucleus tractus solitarius (NTS), in which laryngeal afferents terminate. Therefore, we studied the laryngeal, cardiovascular, and respiratory effects of SP injected into the NTS of rats. We completed bilateral stereotactic injections of 20 nL of SP (15 μmol) or control solution into the region of the NTS, the dorsal motor nucleus (DMN), or the nucleus gracilis (GR) in 30 anesthetized rats. Changes in diaphragm, cricothyroid (CT), and thyroarytenoid (TA) electromyography (EMG), as well as blood pressure (BP), were compared. The injection sites were verified histologically. Injection of SP into the NTS altered CT and/or TA EMG activity in all animals. The change ranged from complete inhibition, to a phasic increase, to a tonic increase. No change in laryngeal adductor EMG activity was seen in 8 of 9 animals after SP injections into the DMN (4/5) or GR (4/4), but 1 animal demonstrated brief inhibition of CT and TA EMG activity after SP injection into the DMN. Injection of SP into the NTS induced central apnea and a significant decrease in BP in all animals. The duration of apnea tended to be longer after NTS injections than after DMN or GR injections (p < .10 and p < .05, respectively). We conclude that stereotactic injections of putative neurotransmitters in rats may be accomplished to identify effects on laryngeal motor activity. Direct application of SP into the NTS consistently elicits a change in CT and/or TA EMG activity, ranging from inhibition to excitation. This model may prove useful in evaluating pharmacological targets of central reflex activity to manage life-threatening laryngeal reflex activity.

Nitric oxide and nitrosocysteine mimic nonadrenergic, noncholinergic hyperpolarization in canine proximal colon

Previous evidence suggests that nonadrenergic, noncholinergic (NANC) inhibitory neurotransmission in visceral muscles may be mediated by nitric oxide (NO). We have demonstrated that NO and the NO carrier S-nitrosocysteine can mimic the hyperpolarization in colonic muscle caused by nerve stimulation. The finding that S-nitrosocysteine breaks down fast enough to cause inhibitory junction potential (IJP)-like hyperpolarizations suggests that NO could be stored as a nitrosothiol in secretory vesicles in nerve terminals. Oxyhemoglobin blocked hyperpolarization responses to NO and S-nitrosocysteine and NANC IJPs. These findings suggest that NO is a biologically active transmitter substance in NANC inhibitory neurotransmission. NO enhanced the open probability of Ca(2+)-activated K+ channels in isolated colonic muscle cells. These channels may mediate the hyperpolarization response to NANC neurotransmission in colonic muscles.

GABA: history and perspectives

In 1957, factor I, a brain agent I had discovered earlier, was chemically identified as GABA in a collaboration between myself and Alva Bazemore at the Montréal Neurological Institute (MNI) in the Neurochemistry Laboratory then headed by K. A. C. Elliott. A personally biased excursion into the history of neurobiology illuminates the development of methods and concepts that led to this event, and recounts the early days at the MNI, when Hugh McLennan and I applied factor I to the exposed surface of the spinal cord and to sympathetic ganglia of cats and rabbits. It also tells of earlier studies at Graz, Naples, and elsewhere that prompted the experiments at the California Institute of Technology in which factor I was discovered as the agent in nerve extracts causing inhibition of isolated crayfish stretch receptor neurons, and in which it was found that this inhibition could be prevented by picrotoxin. There was justified doubt that GABA is indeed the transmitter substance of inhibitory neurones. Later studies, however, resolved the controversy. The functional role of GABA in brain and spinal cord and its mechanism of action are still far from being fully understood. Special problems are the extent and significance of spontaneous quantal and nonquantal release, the functional role and the mechanism of excitatory actions of GABA, its release from glial cells, and the energetics of its metabolic turnover.Key words: factor I, GABA, glia, convulsants, inhibition.

Cystic fibrosis, vasoactive intestinal polypeptide, and active cutaneous vasodilation

The transmitter substance for the active cutaneous vasodilation that accompanies sweating during hyperthermia in humans is unknown. Hokfelt et al. (Nature Lond. 284: 515-521, 180) hypothesized that it is vasoactive intestinal polypeptide (VIP) that is cotransmitted with acetylcholine. Heinz-Erian et al. (Science Wash. DC 229: 1407-1408, 1985) reported that VIP innervation is sparse in the skin of persons with cystic fibrosis (CF). A corresponding attenuation of active vasodilation in these subjects would be evidence that VIP is involved in this effector mechanism of human thermor-regulation. Immunocytochemical analysis of skin biopsies from four men with CF confirmed that VIP innervation was sparse. We also analyzed immunoreactivity for calcitonin gene-related peptide (CGRP; normal), substance P (normal), and neuropeptide Y (low). VIP-immunoreactive Merkel cells were abnormal. Despite sparse VIP-immunoreactive innervation, our CF subjects' cutaneous vascular responses to hyperthermia were normal. Because VIP was not completely absent, this evidence is insufficient to rule out VIP as the vasodilator transmitter. However, the CGRP and substance P innervation we observed could mean that release of one or both of these peptides was the mechanism of the fully developed active cutaneous vasodilation.

Couplings, EJSR, Gap junctions, ECC, in Ratite Heart Conduction Fibers

The ultrastructure of avian hearts has provided seminal information regarding striated muscle function. Extended junctional SR (EJSR) in avian hearts suggests that excitation-contraction coupling (ECC) is accomplished through a diffusible transmitter substance, but raises questions regarding the function of “couplings” in general. Other information gleaned from small avian hearts has elucidated adaptations of the individual and appositional geometry of the conduction fibers (CF) to functional demands imposed by different heart sizes and rates, as well as the relationship between the size of gap junctions (GJ), cell input resistance and coupling resistance between cells. The hearts of ratites (ostrich, emu, rhea) are very large (> 1 kg) and beat at about 50/min. Their working myocytes are almost identical to those found in chickens.We investigated the following questions: 1. Do ratite CF contain “couplings”, EJSR and corbular SR? 2. Are there differences in geometry between the CF of small, fast beating, and large, slow beating hearts as is the case in mammals?

Receptor kinetics pertaining to blockade of nerve-released transmitter at synapses

The question is raised as to whether competitive inhibitors should block responses of tissue to nerve-released neurotransmitter to the same extent as they block equivalent responses to exogenous agonist. From a simple dynamic model of synaptic events, which takes into account non-constancy of transmitter concentration in space and time, it is deduced that equal blockade of responses to nerve-released and exogenous transmitter substance will occur if: (i) there are locally many more receptor molecules than transmitter molecules; (ii) the active agonist–receptor complex, A n R, has n = 1 ; and (iii) tissue response is insensitive to spatial or temporal inhomogeneity of AR. In such a case there will also be equal sensitivity of responses to other modes of inhibition: irreversible competitive, uncompetitive, and non-competitive. Equal blockade of responses to equi-effective endogenous and exogenous agonist will also occur if nerve stimulation gives rise to a steady uniform concentration of agonist, so that equilibrium kinetics are applicable. When n > 1 and/or when tissue responses reflect local peak A n R, response to nerve-released transmitter will be relatively insensitive to receptor blockade by a competitive inhibitor. The same is true for irreversible competitive blockade or for modulation of receptor density. However, an uncompetitive inhibitor (e. g. a ‘channel blocker’) may be more effective against nerve-released agonist than against exogenous agonist.

The Anatomy of the Sarcoplasmic Reticulum in Vertebrate Skeletal Muscle: Its Implications for Excitation Contraction Coupling

The sarcoplasmic reticulum in situ is an intricate tubular network that surrounds the contractile material in striated muscle cells. Its topographical relationship to other intracellular components, especially the myofibrils, is rather rigidly mainiained by a cytoskeleton which enmeshes Z line material and sarcoplasmic reticulum and, ultimately, is anchored at the plasmalemma. As a result, the two main components of the sarcoplasmic reticulum, the junctional SR and the free SR, retain their typical location in the A band region and in the I band region, respectively. The junc­tional SR, which is thought to be the site for calcium storage and release for contraction, is, thus, always well within one micron of the regulatory proteins associated with the actin filaments. The junctional SR, a synonym for terminal cisterna applying to both skeletal and cardiac muscle, is generally held to be involved in the translation of the action potential into calcium release, mainly because of the close topographic apposition between the junctional SR and the plasmalemma, especially in skeletal muscle. This attractive structure-function correlation is challenged by the observation that in bird cardiac muscle 80% of the junctional SR is spacially far removed from plas­malemma, the site of electrical activity. This anomalous topography is not in conflict with the notion that translation of the action potential into calcium release may be accomplished by a dif­fusible transmitter substance, e.g. calcium. Any hypothesis dealing with this problem must ac­ count for the anatomy of the bird heart.