Capsaicin-sensitive afferents and blood pressure regulation during pentobarbital anaesthesia in the rat

Summary

(1) Maintenance of blood pressure was investigated during induction of pentobarbital anaesthesia in rats after elimination of capsaicin-sensitive afferent neurons (capsaicin-denervated rats) as compared to vehicle-treated controls. The catecholamine content of heart and adrenals and the rise in blood pressure following electrical excitation of the spinal adrenergic nerves (pithed rat preparation) was also compared between both groups. (2) Capsaicin-denervated rats and their controls had equal amounts of catecholamines in heart and adrenals as well as equal pressor responses to electrical stimulation of spinal sympathetic nerves, thus excluding an influence of capsaicin on efferent pathways. In the state of consciousness, both groups showed the same blood pressure. (3) In capsaicin-denervated rats and in their controls, pentobarbital-induced anaesthesia (50 mg/kg i.p.) was characterized by a decline in blood pressure during the first 6 min. In the controls, this fall in blood pressure was followed by a slow compensatory rise to a level slightly higher than before anaesthesia, and this level was maintained during the following 60 min. This compensation was completely absent in capsaicin-denervated rats, indicating a role for capsaicin-sensitive nerves in this mechanism. An injection of pentobarbital (50 mg/kg i.p.) in pithed rats reduced the pressor response to electrical stimulation of spinal sympathetic nerves by about 40% in capsaicin-denervated rats and in their controls. This inhibitory effect of pentobarbital might be involved in the initial fall in blood pressure in intact animals. (4) The blood pressure which had reached a steady level 20 min after induction of the pentobarbital anaesthesia in intact rats responded to various factors as follows: (A) Bilateral adrenalectomy hardly decreased the blood pressure in the controls and not at all in the capsaicin-denervated rats; thus a substantial influence of the adrenals in the maintenance of blood pressure was excluded. (B) a-Adrenoceptor blockade by phentolamine caused a greater fall of blood pressure in the controls than in the capsaicin-denervated rats; the latter seem therefore under a much lower peripheral adrenergic vasoconstrictor tone. (C) Blockade of the angiotensin-converting enzyme by captopril had a more pronounced and persistent depressor effect in the capsaicin-denervated rats than in the controls; this indicates that the capsaicin-denervated rats have an increased dependence on an angiotensin II-mediated blood pressure regulation. (D) Sodium nitroprusside caused an equally pronounced fall in blood pressure in both groups; its vasodilatory potency thus seems to overrun all other compensatory reflex regulations. It is concluded that the continuous blood pressure regulation under pentobarbital anaesthesia depends upon signals conveyed to the central nervous system via capsaicin-sensitive afferent neurons which activate efferent adrenergic mechanisms. These regulations are regarded supplementary or auxiliary to the baro-and chemoreceptor reflexes.