H.B. FERNANDES1, G. TOMBAUGH1, J. S. PADILLA1, S.GELMAN1, K.KRETSCHMANNOVA1, J.PALMA1, A.GHAVAMI1, V.BEAUMONT2, R.CACHOPE2
1PsychoGenics, Inc., Paramus, NJ, USA; 2Biology, CHDI Mgmt. / CHDI Foundation, Los Angeles, CA
Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder resulting from an expanded number of CAG repeats in the Huntingtin (Htt) gene. HD patients exhibit both cognitive and affective symptoms, as well as uncontrolled movements (chorea) which are thought to reflect pathological changes in striatal medium spiny projection neurons (SPN) from both the direct (dSPN) and indirect (iSPN) pathways. The Q175 knock-in mouse model of HD has been useful in revealing functional alterations in intrinsic membrane and synaptic properties of striatal SPNs. Q175 mice have been successfully crossed with mice expressing GFP under control of the promoters of either the dopamine D1 or D2 receptor genes. To the extent that 1) D1- and D2- driven GFP expression putatively labels dSPNs and iSPNs, respectively, and 2) that unlabeled SPNs represent the complementary pathway, comparisons of GFP+ and GFP- cells in either model should be equally valid. However, testing this assumption by directly comparing independent datasets obtained from these two mouse lines has not yet been reported. In this study, we performed whole cell patch-clamp recordings from GFP+ and GFP- striatal SPNs in brain slices from 6-month old WT and heterozygous (Het) mice in D1-GFP and D2-GFP lines of Q175 mice. In cells from WT mice, membrane resistance values of putative dSPNs and iSPNs in the D1 line (i.e. GFP+ and GFP- cells, respectively) were nearly identical to those of the cognate cell types in the D2 line. Moreover, both dSPNs and iSPNs in Het mice from each line showed markedly and comparably elevated membrane resistance relative to WT controls. Similarly, both cell types in Het mice from each line exhibited significantly reduced rheobase compared to WT controls. Resting membrane potential and access resistance remained uniform between genotypes and cell types in both lines. The frequency of miniature excitatory synaptic currents (mEPSCs) in iSPNs from Het mice was selectively decreased relative to that seen in WT mice in both D1-GFP and D2-GFP lines. The average mEPSC amplitude was unchanged across cell types in both lines and no genotypic differences were observed. These symmetrical findings help confirm that both D1-GFP and D2-GFP lines of Q175 mice can be used to assess the electrophysiological properties of putative direct and indirect pathway SPNs.