Respiration-modulated membrane potential and chemosensitivity of locus coeruleus neurones in the in vitro brainstem-spinal cord of the neonatal rat

1. The activity of locus coeruleus (LC) neurones (n=126) was examined in wh ole-cell (conventional and amphotericin B-perforated patch) recordings, and the relationship of this activity to the respiratory discharge recorded on the C4 or C5 phrenic nerve roots was determined at different CO2 concentra tions (2 and 8%; bath pH 7.8 and 7.2) in the in vitro brainstem-spinal cord preparation of the neonatal rat (1-5 days old). 2. In most neurones (n=105) ongoing activity was modulated at respiratory f requency. Typically, this consisted of a phase of depolarization and increa sed discharge frequency synchronous with the phrenic burst, followed by a p hase of hyperpolarization and inhibition of discharge (n=94 of 105). The in cidence of respiratory modulation decreased from 91% on P1 to 57% on P5. 3. Bath application of the non-NMDA receptor antagonist 6-cyano-7-nitroquin oxaline-2,3-dione (CNQX; 5 mu M) or the NMDA receptor antagonist DL-2-amino -5-phosphonovaleric acid (APV; 100 mu M) abolished both phases of respirato ry modulation. The hyperpolarizing phase alone was abolished by the adrenoc eptor antagonists idazoxan (5 mu M) or phentolamine (0.8 mu M). These resul ts indicate that excitatory amino acid pathways are involved in the transmi ssion of both the excitatory and inhibitory components and that the latter involves in addition an alpha(2)-adrenoceptor-mediated pathway. 4. Increasing the CO2 concentration from 2 to 8% resulted in a shortening o f expiratory duration and weakening or loss of respiratory-phased inhibitio n; this was accompanied by depolarization, increased discharge frequency an d, in those neurones where they were initially present (60%), an increase i n the frequency of subthreshold membrane potential oscillations. The depola rizing response was retained in the presence of tetrodotoxin (TTX, 0.2-1.0 mu M). 5. These results indicate that in this neonatal preparation LC neurones for m part of the synaptically connected brainstem respiratory network, aad tha t the LC constitutes a site of CO2- or pH-dependent chemoreception.