The striatum can be divided into the DLS (dorsolateral striatum) and the VMS (ventromedial striatum), which includes NAcC (nucleus accumbens core) and NAcS (nucleus accumbens shell). Here, we examined differences in electrophysiological properties of MSSNs (medium-sized spiny neurons) based on their location, expression of DA (dopamine) D1/D2 receptors and responses to the μ-opioid receptor agonist, DAMGO {[D-Ala(2)-MePhe(4)-Gly(ol)(5)]enkephalin}. The main differences in morphological and biophysical membrane properties occurred among striatal sub-regions. MSSNs in the DLS were larger, had higher membrane capacitances and lower Rin (input resistances) compared with cells in the VMS. RMPs (resting membrane potentials) were similar among regions except for D2 cells in the NAcC, which displayed a significantly more depolarized RMP. In contrast, differences in frequency of spontaneous excitatory synaptic inputs were more prominent between cell types, with D2 cells receiving significantly more excitatory inputs than D1 cells, particularly in the VMS. Inhibitory inputs were not different between D1 and D2 cells. However, MSSNs in the VMS received more inhibitory inputs than those in the DLS. Acute application of DAMGO reduced the frequency of spontaneous excitatory and inhibitory postsynaptic currents, but the effect was greater in the VMS, in particular in the NAcS, where excitatory currents from D2 cells and inhibitory currents from D1 cells were inhibited by the largest amount. DAMGO also increased cellular excitability in the VMS, as shown by reduced threshold for evoking APs (action potentials). Together the present findings help elucidate the regional and cell-type-specific substrate of opioid actions in the striatum and point to the VMS as a critical mediator of DAMGO effects.
Keywords: ACSF, artificial cerebrospinal fluid; AHP, after hyperpolarization; AP, action potential; AP-5, dl-2-amino-5-phosphonovaleric acid; BIC, bicuculline; CNQX, 6-cyano-7-nitroquinoxaline-2,3-dione; CsMeth, Cs-methanesulfonate; D1/D2 receptors; DA, dopamine; DAMGO, [d-Ala2-MePhe4-Gly(ol)5]enkephalin; DLS, dorsolateral striatum; EGFP, enhanced green fluorescent protein; EPSC, excitatory postsynaptic current; IPSC, inhibitory postsynaptic current; KGluc, K-gluconate; MSSN, medium-sized spiny neuron; NAcC, nucleus accumbens core; NAcS, nucleus accumbens shell; RMP, resting membrane potential; Rin, input resistance; TBST, TBS containing 0.1% Tween 20; TTX, tetrodotoxin; UCLA, University of California at Los Angeles; VMS, ventromedial striatum; VTA, ventral tegmental area; electrophysiology; mEPSC, miniature EPSC; mIPSC, miniature IPSC; nucleus accumbens; opioid receptors; sEPSC, spontaneous EPSC; sIPSC, spontaneous IPSC; striatum.