Role of ATP and astrocytes in the lamprey respiratory network

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. Since no information is available on the role of ATP and astrocytes within the lamprey respiratory network, experiments were performed on isolated brainstems of lampreys. 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--S, a nonhydrolyzable ATP analog, performed into the pTRG caused marked increases in respiratory frequency. Bath application of the P2 receptor antagonist PPADS (100 M) did not alter respiratory activity, but prevented the increases in respiratory frequency in response to microinjections of ATP--S into the pTRG. The contribution of astrocytes to the modulation of the respiratory activity was investigated by using the gliotoxin aminoadipic acid. Bath application of 1 mM aminoadipic acid caused increases in the frequency and amplitude of vagal bursts followed by progressive decreases in both these respiratory variables. Furthermore, the responses to ATP--S microinjected into the pTRG were abolished, indicating that pTRG astrocytes play a key respiratory role. However, consistently with the glial function of providing glutamine to neurons for glutamate synthesis, bath application of 5 mM glutamine caused a rapid recovery of baseline respiratory variables. In addition, the pH of the perfusing solution was reduced from 7.4 to 7.0 to ascertain whether ATP and astrocytes contribute to acidification-induced increases in respiratory activity. Marked low pH-induced increases in the respiratory motor output were still present after bath application of PPADS, but were completely abolished after aminoadipic acid application. The results show that astrocytes are involved in rhythm generation as well as in ATP- and acidification-induced increases of respiratory activity. The role of astrocytes in rhythmic networks appears to be phylogenetically conserved.