Yohimbine antagonism of the vasodepression elicited by organophosphates applied on ventral medulla oblongata

Toxic doses of paraoxon alter the respiratory pattern without causing respiratory failure in rats

Respiratory failure, through a combination of muscarinic, nicotinic, and central effects, is the primary cause of death in acute organophosphate poisoning. However, the mechanisms inducing respiratory failure remain unclear. In rats poisoned subcutaneously with paraoxon at doses near the LD(50), we studied the pattern of respiration using whole body plethysmography and the occurrence of respiratory failure using arterial blood gases. Subsequently, we studied the effects of atropine on paraoxon-induced modification of ventilation and arterial blood gases. Fifty and 75%, but not 10% of the subcutaneous LD(50) of paraoxon induced marked and sustained signs and symptoms. At 30min post-injection and throughout the study, there was a significant decrease in the respiratory frequency (34% (50% versus solvent), and 29% (75% versus solvent)) and a significant increase in the expiratory time (72% (50% versus solvent) and 60% (75% versus solvent)) with no modifications of the inspiratory time. The tidal volume was significantly increased for the 75% but not for the 50% dose. Apnea was never detected. Even at the 75% dose, paraoxon had no effects on PaO(2), PaCO(2) or HCO(3)(-); however, a significant decrease in arterial pH was observed at 30min (7.34+/-0.07 versus 7.51+/-0.01, p=0.03). Atropine completely reversed the paraoxon-induced respiratory alterations. We conclude that paraoxon, at doses equal to 50 and 75% of the LD(50), alters ventilation at rest without inducing respiratory failure during the study period.

Function of the ventrolateral medulla in the control of the circulation

The CNS control of the cardiovascular system involves the coordination of a series of complex neural mechanisms which integrate afferent information from a variety of peripheral receptors and produce control signals to effector organs for appropriate physiological responses. Although it is generally thought that these control signals are generated by a network of neural circuits that are widely distributed in the CNS, over the last two decades a considerable body of experimental evidence has accumulated suggesting that several of these circuits involve neurons found on or near the ventral surface of the medulla oblongata. Neurons in the VLM have been shown to be involved in the maintenance of vasomotor tone, in baroreceptor and chemoreceptor (central and peripheral) reflex mechanisms, in mediating the CIR and somatosympathetic reflexes and in the control of the secretion of vasopressin. These physiological functions of VLM neurons have been supported by neuroanatomical and electrophysiological studies demonstrating direct connections with a number of central structures previously implicated in the control of the circulation, including the IML, the site of origin of sympathetic preganglionic axons, and the SON and PVH, the site of origin of neurohypophyseal projecting axons containing AVP. Considerable suggestive evidence has also been obtained regarding the chemical messengers involved in transmitting information from VLM neurons to other central structures. There have been developments suggesting a role for monoamines and neuropeptides in mediating the neural and humoral control of SAP by neurons in the VLM. This review presents a synthesis of the literature suggesting a main role for VLM neurons in the control of the circulation.