SURROUND INHIBITION AMONG PROJECTION NEURONS IS WEAK OR NONEXISTENT IN THE RAT NEOSTRIATUM

1. Antidromic activation of striatal spiny projection neurons by subst antia nigra stimulation in vivo did not evoke inhibitory postsynaptic potentials (IPSPs) in the antidromically activated neurons, or in neig hboring spiny neurons. 2. More generalized activation of projection ce lls by stimulation of the efferent pathway in slices did not evoke IPS Ps in spiny neurons. Inhibitory mechanisms were operative in these sli ces, as indicated by the presence of an IPSP component in the orthodro mic response to local stimulation. 3. Dual intracellular recordings, o btained from cells located within 50-200 mu m of each other in striata l slices also failed to demonstrate any inhibition among striatal spin y cells. Spikes triggered in one spiny neuron by current injection fai led to produce any postsynaptic potential in nearby spiny cells at res ting or depolarized membrane potentials. Excitatory postsynaptic poten tials (EPSPs) evoked by electrical stimulation of the cortex were not affected by spiking of neighboring cells. 4. It is impossible to rule out the presence of inhibition among striatal spiny neurons in all cir cumstances. However, the absence of demonstrable IPSPs in these experi ments argue against the common view that mutual inhibition among spiny neurons is a central organizing principle of striatal function.