Participation of nitric oxide in the nucleus isthmi in CO2-drive to breathing in toads

Abstract

The nucleus isthmi (NI) is a mesencephalic structure of the amphibian brain. It has been reported that NI plays an important role in integration of CO 2 chemoreceptor information and glutamate is probably involved in this function. However, very little is known about the mechanisms involved. Recently, it has been shown that nitric oxide synthase (NOS) is expressed in the brain of the frog. Thus the gas nitric oxide (NO) may be involved in different functions in the brain of amphibians and may act as a neurotransmitter or neuromodulator. We tested the hypothesis that NO plays a role in CO 2 -drive to breathing, specifically in the NI comparing pulmonary ventilation, breathing frequency and tidal volume, after microinjecting 100 nmol/0.5 µl of L-NAME (a nonselective NO synthase inhibitor) into the NI of toads (Bufo paracnemis) exposed to normocapnia and hypercapnia. Control animals received microinjections of vehicle of the same volume. Under normocapnia no significant changes were observed between control and L-NAME-treated toads. Hypercapnia caused a significant (P<0.01) increase in ventilation only after intracerebral microinjection of L-NAME. Exposure to hypercapnia caused a significant increase in breathing frequency both in control and L-NAME-treated toads (P<0.01 for the control group and P<0.001 for the L-NAME group). The tidal volume of the L-NAME group tended to be higher than in the control group under hypercapnia, but the increase was not statistically significant. The data indicate that NO in the NI has an inhibitory effect only when the respiratory drive is high (hypercapnia), probably acting on tidal volume. The observations reported in the present investigation, together with other studies on the presence of NOS in amphibians, indicate a considerable degree of phylogenetic conservation of the NO pathway amongst vertebrates.

Effect of nitric oxide in the nucleus isthmi on the hypoxic and hypercarbic drive to breathing of toads

“…The animal chamber was continuously flushed with humidified air (1.5 l/min). In one group (n ϭ 7), experimental animals received one microinjection of L-NAME (Sigma; 100 nmol/0.5 l) dissolved in mock cerebrospinal fluid (mCSF) of a composition described in previous studies (5,6,9). The vehicle group (n ϭ 7) was treated with intracerebral microinjections of mCSF of the same volume.…”

Nitric Oxide Synthase and NADPH Diaphorase Distribution in the Bullfrog <i>(Rana catesbeiana)</i> CNS: Pathways and Functional Implications

“…The importance of NO to the activity of a variety of CPGs is emerging as a critical role of the transmitter [Scholz et al, 2001;Alford et al, 2003]. Additionally, in a toad, NO in the nucleus isthmi has an inhibitory effect on respiration during hypercapnia [Gargaglioni and Branco, 1999]. Anuran respiratory patterns can thus be altered by NO activity at multiple points in the control pathway.…”