tetrodotoxin and Apnea

1. To examine the functional importance of the pre-Bötzinger complex for breathing we micro-injected, under in vivo conditions, the calcium channel blocker omega-conotoxin GVIA and the sodium channel blocker tetrodotoxin (TTX) into the ventrolateral medulla of adult cats, while monitoring respiratory rhythmic motor output in the phrenic nerve. 2. omega-Conotoxin GVIA caused a highly localized synaptic ablation by blocking presynaptic N-type calcium channels. When injecting 5-60 fmol omega-conotoxin GVIA unilaterally, the amplitude of phrenic nerve activity decreased bilaterally and sometimes disappeared, indicating central apnoea. These effects were reversible and could only be induced in a very localized area of the pre-Botzinger complex. By injecting omega-conotoxin GVIA several times during an experiment and analysing the areas where injections affected respiratory activity, it was possible to map exactly the anatomical extent of the area critical for respiratory rhythm generation. 3. Following the precise localization of the pre-Bötzinger complex with omega-conotoxin GVIA, we injected TTX to induce an irreversible inactivation of this region. TTX injected unilaterally into the pre-Bötzinger complex irreversibly reduced the amplitude of phrenic nerve activity. Bilateral TTX injections eliminated respiratory rhythmic activity, causing a persistent central apnoea. 4. After bilateral lesioning of the pre-Bötzinger complex, it was still possible to induce gasping during hypoxia or asphyxia, indicating that respiration and gasping are generated by two different neuronal networks. 5. We propose that omega-conotoxin GVIA as employed in this study to investigate the functional role of the pre-Bötzinger complex can also be used as a general pharmacological approach to map other neuronal networks. We call this the 'omega-conotoxin GVIA tracing' method.

Topics: Animals; Apnea; Asphyxia; Brain Mapping; Calcium Channel Blockers; Cats; Female; Male; Medulla Oblongata; Motor Neurons; omega-Conotoxin GVIA; Peptides; Phrenic Nerve; Respiration; Respiratory Center; Sodium Channel Blockers; Tetrodotoxin

To determine the cardiovascular, autonomic, and neuromuscular effects of an IV infusion of tetrodotoxin (TTX) when ventilation is supported.. TTX was infused in 18 anesthetized beagles during conventional mechanical ventilation. TTX infusion continued at a rate of 9.3 micrograms/kg/hr until apnea occurred with 1 minute of ventilator disconnection. Measurements included intravascular pressures, heart rate (HR), cardiac output, blood gases, displacements of the rib cage and abdomen, O2 delivery, and responses to train-of-four and tetanic peripheral nerve stimulation. Results are expressed as mean +/- SD.. During TTX infusion, all the dogs had discoordinate movements of the rib cage, abdomen, and limbs. Vomiting, urination, defecation, and increased salivation occurred. Nicotinic and muscarinic effects, neuromuscular blockade, and cardiovascular depression were produced by TTX. Apnea occurred in 72.0 +/- 17.0 minutes when a total of 119.0 +/- 17.4 micrograms of TTX was infused. At apnea, decreases in arterial pressure, cardiac index, HR, O2 delivery, and systemic vascular resistance occurred, while pulmonary artery pressure and pulmonary vascular resistance increased. Loss of response to tetanic stimulation was closely correlated with the dose of TTX that produced apnea.. The clinical symptoms and signs of TTX poisoning are similar to those of anticholinesterase poisons, and TTX dosing as described by this model may serve as a surrogate for organophosphorus poisoning. The model may be useful to determine optimum therapies for TTX poisoning and, since TTX prevents sodium influx into cells, to investigate enhanced survival in animals suffering from ischemia.

Topics: Animals; Apnea; Autonomic Nervous System; Cholinesterase Inhibitors; Disease Models, Animal; Dogs; Drug Evaluation, Preclinical; Hemodynamics; Infusions, Intravenous; Muscles; Respiration, Artificial; Tetrodotoxin