Excitatory and inhibitory effects of tricaine (MS-222) on fictive breathing in isolated bullfrog brain stem

Abstract

This study examined the direct effects of tricaine methanesulfonate (MS-222), a sodium-channel blocking local anesthetic, on respiratory motor output using an in vitro brain stem preparation of adult North American bullfrogs (Rana catesbeiana). Bullfrogs were anesthetized with halothane, and the brain stem was removed and superfused with artificial cerebrospinal fluid containing MS-222 at concentrations ranging from 0.1 to 1,000 micro M. At the lowest concentration of MS-222, respiratory frequency (fR) increased significantly (P < 0.05), but at higher concentrations, fR progressively decreased and was abolished in all preparations at 1,000 micro M (P < 0.01). Respiratory burst amplitude and burst duration were not affected by MS-222. The frequency of nonrespiratory neural activity did not significantly change with the addition of MS-222 below 1,000 micro M. These data indicate that MS-222 has a significant, direct effect on respiratory motor output from the central nervous system, producing both excitation and inhibition of fictive breathing. The results are consistent with other studies demonstrating that low concentrations of anesthetics generally cause excitation followed by depression at higher concentrations. Although the mechanisms underlying the excitatory effects of MS-222 in this study are unclear, they may include increased excitatory neurotransmission and/or disinhibition of inputs to the respiratory central pattern generator.

Development of the respiratory response to hypoxia in the isolated brainstem of the bullfrogRana catesbeiana

“…Respiratory-related neural discharges were defined by previously described criteria for fictive gill and lung bursts (Reid and Milsom, 1998;Torgerson et al, 1998;Hedrick and Winmill, 2003;Winmill and Hedrick, 2003a,b). Burst frequency is defined as neural bursts per minute.…”

“…Lung episodes are defined as two or more bursts occurring in succession separated by a pause not longer than the average duration of two ventilatory cycles. Non-respiratory bursts were defined as bursts with duration longer than 1·s and having a burst shape not resembling that of normal lung breaths (see Hedrick and Winmill, 2003). Burst duration was measured from the onset of deviation from the baseline to the return to baseline in the integrated neural trace.…”

“…Adults were anesthetized by placing them in a sealed container (1.5·l) with a cotton gauze that held approximately 1·ml of the volatile anesthetic 2-bromo-2-chloro-1,1,1-trifloroethane (halothane; Webster Veterinary Supply, Inc., Sterling, MA, USA). In a previous study with adult bullfrogs, we found that halothane results in the rapid induction of a surgical plane of anesthesia, and fictive breathing is produced much faster than with MS-222 anesthesia (Hedrick and Winmill, 2003). When breathing movements ceased and the withdrawal and corneal reflexes were abolished (adults: 15-20·min; tadpoles: 2-5·min), animals were removed from anesthesia.…”

“…In contrast to the response of pre-metamorphic tadpoles, post-metamorphic tadpole and adult preparations exposed to hypoxia within 5-15·min became very episodic (Fig.·1B,C), and exhibited an increase in the number of 'non-respiratory' neural bursts ( Fig.·1B) (cf. Hedrick and Winmill, 2003); this was followed by a complete cessation of respiratory activity after 20-40·min. The cessation of respiratory activity was sustained throughout the period of hypoxic exposure, but was reversible upon reoxygenation with control aCSF (Fig.·1B,C).…”

“…An example of a non-respiratory lung burst is provided in Fig.·1 for a post-metamorphic tadpole (see also Hedrick and Winmill, 2003). Non-respiratory bursts are those that do not conform to the criteria established for normal lung bursts (see Materials and methods) and are typically long duration, high amplitude bursts and make up about 10% of the total bursts (Hedrick and Winmill, 2003).…”