Different inhibitory inputs onto neostriatal projection neurons as revealed by field stimulation

This work investigated if diverse properties could be ascribed to evoked inhibitory postsynaptic currents (IPSCs) recorded on rat neostriatal neurons when field stimulation was delivered at two different locations: the globus pallidus (GP) and the neostriatum (NS). Previous work stated that stimulation in the GP could antidromically excite projection axons from medium spiny neurons. This maneuver would predominantly activate the inhibitory synapses that interconnect spiny cells. In contrast, intrastriatal stimulation would preferentially activate inhibitory synapses provided by interneurons. This study shows that, in fact, intensity-amplitude experiments are able to reveal different properties for IPSCs evoked from these two locations (GP and NS). In addition, while all IPSCs evoked from the GP were always sensitive to ω-conotoxin GVIA (Ca_V2.2^2.2 or N-channel blocker), one-half of the inhibition evoked from the NS exhibited little sensitivity to ω-conotoxin GVIA. Characteristically, all ω-conotoxin GVIA-insensitive IPSCs exhibited strong paired pulse depression, whereas ω-conotoxin GVIA-sensitive IPSCs evoked from either the GP or the NS could exhibit short-time depression or facilitation. ω-Agatoxin TK (Ca _V2.1^2.1 or P/Q-channel blocker) blocked IPSCs evoked from both locations. Therefore 1) distinct inhibitory inputs onto projection neostriatal cells can be differentially stimulated with field electrodes; 2) N-type Ca²⁺ channels are not equally expressed in inhibitory terminals activated in the NS; and 3) synapses that interconnect spiny neurons use both N- and P/Q-type Ca²⁺ channels.