ATP, astrocytes and central respiratory control in the lamprey

The paratrigeminal respiratory group (pTRG) is a brainstem region responsible for the respiratory rhythm generation in the lamprey, a lower vertebrate that has proved to be highly useful to demonstrate that the basic features of rhythmogenic networks have been conserved throughout vertebrate evolution. Experiments were performed on the isolated brainstem of the adult lamprey to investigate the respiratory role of ATP and astrocytes. The vagal motor output was used to monitor respiratory activity. Bath application and microinjections (30-50 nl) of several drugs were employed. Bilateral microinjections of 1 mM ATP-γ-S, a nonhydrolyzable ATP analog, performed into the pTRG caused marked increases in respiratory frequency. Bath application of the P2 receptor antagonist PPADS (100 μM) did not alter respiratory activity, but prevented the increases in respiratory frequency in response to microinjections of ATP-γ-S into the pTRG. We investigated the possible involvement of astrocytes in the ATP-induced effects. The contribution of astrocytes to the modulation of the respiratory activity was ascertained by using the gliotoxin aminoadipic acid (AAA). Bath application of 1 mM AAA caused increases in the frequency and amplitude of vagal bursts followed by progressive decreases in both these respiratory variables and abolished the responses to ATP-γ-S microinjected into the pTRG, indicating that pTRG astrocytes play a key respiratory role. Consistently with the glial function of providing glutamine to neurons for glutamate synthesis, bath application of 5 mM glutamine (Gln) caused a rapid recovery of baseline respiratory variables. In addition, to ascertain whether ATP and astrocytes contribute to acidification-induced increases in respiratory activity the pH of the perfusing solution was reduced from 7.4 to 7.0. Marked low pH-induced increases in the respiratory motor output were still present after bath application of PPADS, but were completely abolished after AAA application. However, bath application of Gln in the presence of AAA restored the low pH-induced responses. The results show that astrocytes are involved in the modulation of respiratory activity and that their role is highly conserved throughout vertebrate evolution. The results also reveal the existence of a central, ATPindependent, pH sensitivity that requires astrocyte metabolic support.