mice, no SSTR1 expression was detected, while wt mice exhibited receptor-like immunoreactivity (Figure 4a). Like SSTR1, in R6/2 and SSTR1/52/2 mice striatum, SSTR2 like immunoreactivity was increased as determined by immunohistochemistry and Western blot analysis (Figure 4b). Interestingly, SSTR3 like immunoreactivity in R6/2 and SSTR1/52/2 mice exhibited distinct distribution pattern. As illustrated in Figure 4c, increased expression of SSTR3 was seen in R6/2 mice in comparison to wt, whereas SSTR1/52/2 displayed significant loss in SSTR3 likes immunoreactivity when compared with wt. Conversely, SSTR4- like immunoreactivity was comparable in both R6/2 and SSTR1/ 52/2 mice without any discernable changes (Figure 4d). R6/2 transgenic mice exhibited increased expression of SSTR5 like immunoreactivity when compared with wt (Figure 4e). Although no SSTR5 expression was seen in SSTR1/52/2 mice, SSTR5 like receptor expression was well expressed in wt. As illustrated in supplemental Figure S1, quantitative analysis of SSTR subtypes indicates receptor specific changes in R6/2 and SSTR1/52/2. In R6/2 transgenic mice SSTR1, SSTR2, SSTR3 and SSTR5 positive immunoreactive neurons were increased by 25, 18, 19 and 17%, respectively, in comparison to wt. Conversely, SSTR4 positive neurons were not changed significantly in R6/2 and SSTR1/52/2 in comparison to wt. However, in SSTR1/52/2 mice SSTR2-positive neurons increased by 25%, whereas SSTR3 positive neurons declined by 38% when compared to wt. Comparable changes in signaling cascades in striatum of SSTR1/52/2 and R6/2 mice The physiological response of neurons upon activation or inhibition of GPCR or ionotropic receptors is intimately associated with the modulation of signaling pathways. Whether the ablation of SSTR modulates signaling cascades similar to the model of neurodegenerative disease has not been studied yet. Several previous studies have shown significant changes in mitogen activated protein kinases (MAPK); extracellular regulated kinase (ERK1/2) and several other key regulators associated with neuronal degeneration and pathogenesis of neurodegenerative diseases. To ascertain the underlying mechanism and functional consequences of the changes seen in SSTR and NMDAR subtypes here we determined the expression levels of pERK1/2, PKC-a, PI3K, calcineurin, calpain, synapsin-IIa, and enkephalin in striatum of R6/2 and SSTR1/52/2 mice brain by Western blot analysis. As shown in Figure S2a, the expression levels of pERK1/ 2 were not altered in R6/2 mice whereas the status of pERK1/2 was increased in SSTR1/52/2 mice but not significantly when compared to wt. The expression level of cell survival pathway PI3K was increased in SSTR1/52/2 mice, whereas, it decreased significantly in R6/2 transgenic mice (Supplemental Figure S2b). In contrast, decreased expression of PKC-a (Figure S2c) was observed in SSTR1/52/2 mice and R6/2 mice when compared to wt. Previous studies have shown that the inhibitors of calcineurin accelerate HD neurological phenotype in R6/2 mice [47]. Accordingly, we determined the expression levels of calcineurin in R6/2 and SSTR1/52/2 mouse brains. As illustrated in Figure 5a, calcineurin expression decreased in R6/2 and SSTR1/52/2 mice, when compared to wt mice. Studies using YAC transgenic mice models have shown increased calpain expression, which is involved in neuronal apoptosis [48]. However, no data is presently available for calpain expression in R6/2 mice. As shown in Figure 5b, increased expression of calpain in R6/2 mice as well as in SSTR1/52/2 mice was observed. Consistent with previous studies, these data strongly suggest the possible Somatostatin Receptor 1/5 Knockout and R6/2 Mice interaction between activation of NMDARs and increased expression of calpain in YAC and R6/2 transgenic mice. Decreased expression of synapsin-IIa mRNA in striatum of R6/ 2 mice without significant changes in protein expression has been described earlier [43]. In further extension to our study, we next determined the expression level of synapsin-IIa and enkephalin in striatum of R6/2 and SSTR1/52/2 mice and compared with wt using Western blot analysis. As shown in Figure 5c, synapsin-IIa expression was increased in both R6/2 and SSTR1/52/2 mice, whereas, no significant change in enkephalin protein expression was observed in R6/2. However, increased expression was seen in SSTR1/52/2 mice striatum in comparison to wt (Figure 5d). In HD transgenic mice, namely YAC and R6/2, increased apoptosis has been described in brain due to the toxic effect of mutated Htt [49]. Here we determined whether occurrence of apoptosis is comparable in R6/2 and SSTR1/52/2 mice using Hoechst 33258 staining. As shown in Figure 5e, in the absence of SSTR1/5 no apoptosis was observed. In comparison, R6/2 mice exhibited increased number of apoptotic cells when compared to their respective wt. These data strongly suggest that lack of SSTR1/5 does not induce excitotoxicity and the changes seen in several markers might be associated with the physiological and pharmacological effects of SSTR subtypes. Discussion Receptor specific ablation of SSTR subtypes have shown behavioral changes, impaired cognitive function, loss of analgesic effect and sustained pain in a receptor-specific manner [50,51]. Whether SSTR ko or any other GPCRs ko mice mimic the neurochemical changes similar to the neurodegenerative disorders has not been studied yet. In the present study, we compared the expression levels of key molecular markers that exert a determining role in the pathogenesis of HD in SSTR1/52/2 and R6/2 mice striatum. We demonstrate that, SSTR1/52/2 and R6/2 mice display comparable changes in distribution pattern of SST, NMDA receptor (NR1, NR2A and NR2B), DARPP-32, calbindin and bNOS, as well as the downstream signaling pathways including pERK1/2, calpain, synapsin IIa, PKC-a, and calcineurin in striatum. In contrast, SSTR subtypes, enkephalin, and PI3K are modulated in strain specific manner. This is the first comprehensive description showing comparable neurochemical and biochemical changes in a transgenic model of HD and SSTR1/52/2 mice brains. The most coherent finding in HD pathology as well as in transgenic mice is the loss of DARPP-32 [43]. Consistent with this study, we describe comparable changes in DARPP-32 like immunoreactivity in R6/2 and SSTR1/52/2 mice, exhibiting .60% loss of DARPP-32. For the first time, we have demonstrated a similar pattern of DARPP-32 loss in the striatum of SSTR1/5 ablated mice and HD transgenic mice R6/2 indicating that SSTRs might be involved in regulation of DARPP-32. The DARPP-32 mediated downstream signaling pathway is modulated by dopamine receptors (DRs), another prominent member of GPCR family [52,53]. SSTRs share ,30% structural homology with DRs and constitute functional hetero- dimers with distinct pharmacological properties and enhanced signaling [36,54]. In agreement with this information, SSTRs, directly or indirectly, via interaction with DR, may modulate DARPP-32. This concept is further strengthened by our recent studies showing receptor- and region-specific colocalization of DARPP-32 and DRs in rat brain [55]. DARPP-32 becomes phosphorylated in response to the activation of D1R [43]. Conversely, D2R activation inhibits DARPP-32 phosphorylation. PLoS ONE | www.plosone.org 6 September 2011 | Volume 6 | Issue 9 | e24467