Figure 3. Endogenous activity of GABA_BRs is stimulus frequency dependent.

A, Average IPSC traces of one NL neuron (E16–17) in response to train stimulations at 5 Hz (5 pulses) under the conditions of control, GABA_BR antagonist CGP52432 (10 µM), and washout. B, IPSC peak amplitude normalized to the control showed that CGP52432 did not have significant effects on the IPSCs elicited at 5 Hz (n = 7), indicating lack of endogenous GABA_BR activity under this stimulation condition. C & D, Blocking GABA uptake by NNC 711 (20 µM) reduced IPSCs elicited at 5 Hz. In the presence of NNC 711, CGP52432 (10 µM) increased IPSC amplitude significantly (n = 7), revealing endogenous GABA_BR activity. E & F, In response to blockade of GABA_BRs by CGP52432 (10 µM), a significant increase in IPSC amplitude was observed at the stimulus frequency of 200 Hz (n = 8). The inset in panel E shows six superimposed individual IPSCs obtained under control conditions, at enlarged scales. Only the responses to the first five stimulus pulses (without blanking the stimulus artifacts) are shown to indicate low noise levels of the recordings.