Effects of bicuculline on inspiratory activities in piglets

GABA(A) receptor-mediated inspiratory termination evoked by vagal stimulation in decerebrate cats

To identify the GABAergic inhibitory mechanisms involved in inspiratory termination or off-switching (IOS), the effects of a specific enhancer of GABA(A) receptors, midazolam, and an antagonist, bicuculline, on vagally evoked inspiratory inhibitions and IOS were investigated in decerebrate cats. Stimulation of vagal afferents at late inspiration provoked either reversible inspiratory inhibition or IOS, depending on the stimulus intensity. Each response occurred at a constant latency (phase 1). The reversible response was triphasic, consisting of an early (phase 2) inhibition, a brief (phase 3) excitation and a late (phase 4) inhibition in the phrenic neurogram, and early (phase 2) IPSPs, brief (phase 3) EPSPs and late (phase 4) IPSPs in bulbar inspiratory (I) neurones. With an increasing stimulus intensity, phase 4 inhibitions were increased in amplitude and duration, leading to IOS. Midazolam (0.1 mg/kg i.v.) increased more selectively phase 4 IPSPs than phase 2 IPSPs in I neurones, and decreased the threshold for evoking IOS by producing an earlier and larger phase 4 IPSPs. Bicuculline (1.0 mg/kg i.v.) had an opposite effect. These results suggest that the late inhibitory response evoked by vagal stimulation in the I neuronal pool organizes an initial phase of IOS which is mediated by GABA(A) receptors.

Blockade of synaptic inhibition within the pre-Bötzinger complex in the cat suppresses respiratory rhythm generation in vivo

1. The role of synaptic inhibition in respiratory rhythm generation was analysed by microinjections of GABAA and glycine receptor antagonists into the bilateral pre-Botzinger complex (PBC) of anaesthetized cats. Central respiratory activity was monitored by phrenic nerve recordings. 2. Bilateral injections of bicuculline (50 or 100 microM) irreversibly slowed respiratory frequency and induced apneustic patterns. 3. Bilateral injections of strychnine (50 or 100 microM) greatly reduced phrenic burst amplitudes leading to increased burst frequency or irreversibly blocked rhythmic phrenic discharges. After unilateral tetrodotoxin (TTX) blockade in the PBC, strychnine injection into the contralateral PBC blocked rhythmic phrenic discharges. 4. Bilateral blockade of both GABAergic and glycinergic inhibition abolished rhythmic burst discharges and only tonic phrenic activity remained. Such tonic activity was blocked only by TTX (1 microM). 5. Potentiation of synaptic inhibition by the serotonin 1A receptor agonist 8-hydroxydipropylaminotetralin (8-OH-DPAT; 50 microM) restored rhythmic activity only when given shortly after strychnine and bicuculline applications. It was, however, ineffective after blockade of synaptic inhibition was complete. 6. The study demonstrates the significance of synaptic inhibition in the process of respiratory generation in the adult cat in vivo.