Figure 12.
Whole-cell recordings of two connected pairs in which the presynaptic neuron was an SPN and the postsynaptic neuron was a Type I neuron. A–D, Presynaptic and postsynaptic neurons recorded with 140 mm CsMeSO4 internal solution. A, Postsynaptic EGFP–TH+ Type I neuron (red arrow, post) is fluorescent and shows typical response to depolarizing current pulses. Presynaptic SPN are not fluorescent and cannot be seen (pre) and were patched under DIC visualization. B1, Depolarizing current pulse evokes three spikes in a Type I neuron (top black trace). B2, Single action potential elicited in the SPN (red trace) causes an IPSP (−45 mV) in the Type I neuron held at −48 mV (black trace) that was sufficient to completely block the second of three spikes evoked in the Type I neuron. Mean IPSP amplitude, 0.58 ± 0.14 mV; latency, 2.6 ± 0.13 ms; p = 100%. C, Single spike (top red trace) in the SPN evokes a hyperpolarizing IPSP in a Type I neuron held at −46 mV. Black trace is average, and gray traces are individual trials. Note that the IPSP failure rate is zero. The IPSP is reversed at −80 mV (middle) and is completely blocked by 10 μm bicuculline (bottom), indicating a GABAA IPSP. D, Comparison of a spontaneous IPSP and one evoked from the SPN shows them to have identical amplitudes and time courses, suggesting that some of the spontaneous IPSPs in Type I neurons arise from surrounding SPNs. E–G, Another connected pair in which the presynaptic neuron was an SPN and the postsynaptic neuron was a Type I. Both presynaptic and postsynaptic neurons recorded with normal internal solution. E, Postsynaptic Type I neuron (red arrow, post) is fluorescent. Presynaptic SPN is not fluorescent and cannot be seen (pre) and was patched under DIC visualization. F, Train of spikes in the presynaptic SPN elicited by a current pulse (red traces) elicit IPSPs (green arrows) in the postsynaptic Type I neuron (black traces). The IPSPs delay the onset of the subsequent spike. G, Single depolarization-evoked spikes in the SPN elicit IPSPs that delay the occurrence of the subsequent spike in the postsynaptic Type I neuron depolarized to threshold by current injection. H2, Bicuculline at 10 μm blocks the IPSPs and the subsequent postsynaptic spikes occur with a decreased latency. H3, The IPSP returns after washout, and the latency to the subsequent postsynaptic spike increases. Scale bars: A, E, 20 μm.