In Huntington’s disease (HD) mouse models, spontaneous inhibitory synaptic activity is enhanced in a subpopulation of medium-sized spiny neurons (MSNs), which could dampen striatal output. We examined the potential source(s) of increased inhibition using electrophysiological and optogenetic methods to assess feedback and feedforward inhibition in two transgenic mouse models of HD. Single whole-cell patch-clamp recordings demonstrated that increased GABA synaptic activity impinges principally on indirect pathway MSNs. Dual patch recordings between MSNs demonstrated reduced connectivity between MSNs in HD mice. However, while connectivity was strictly unidirectional in controls, in HD mice bidirectional connectivity occurred. Other sources of increased GABA activity in MSNs also were identified. Dual patch recordings from fast spiking (FS) interneuron–MSN pairs demonstrated greater but variable amplitude responses in MSNs. In agreement, selective optogenetic stimulation of parvalbumin-expressing, FS interneurons induced significantly larger amplitude MSN responses in HD compared with control mice. While there were no differences in responses of MSNs evoked by activating single persistent low-threshold spiking (PLTS) interneurons in recorded pairs, these interneurons fired more action potentials in both HD models, providing another source for increased frequency of spontaneous GABA synaptic activity in MSNs. Selective optogenetic stimulation of somatostatin-expressing, PLTS interneurons did not reveal any significant differences in responses of MSNs in HD mice. These findings provide strong evidence that both feedforward and to a lesser extent feedback inhibition to MSNs in HD can potentially be sources for the increased GABA synaptic activity of indirect pathway MSNs.

A rodent cortex-enriched mRNA, RC3, was identified by screening a rat brain cDNA library with a cortex-minus-cerebellum subtracted cDNA probe. Conceptual translation of RC3 cDNA sequences indicates that the rat and mouse mRNAs encode identical, novel 78 amino acid proteins. The RC3 protein amino terminus contains a cysteine-rich domain similar to those found in snake venom neurotoxins; the carboxyl terminus contains a collagen-like motif that may function in the assembly of RC3 subunits into a multimeric protein. Western blot experiments with an antiserum to a synthetic peptide corresponding to 27 residues of the 78 residue sequence identify an immunoreactive polypeptide with 18 kDa gel mobility that is likely to correspond to the RC3 protein. Northern blot analysis and in situ hybridization experiments show that RC3 mRNA is highly enriched in rat brain, with restricted expression in neuronal subsets primarily in the cortex, striatum, and hippocampus as well as certain nuclei within the thalamus, hypothalamus, the olfactory bulb.

Accumulation of amyloid beta-peptides (Abeta) in the brain has been linked with memory loss in Alzheimer's disease and its animal models. However, the synaptic mechanism by which Abeta causes memory deficits remains unclear. We previously showed that acute application of Abeta inhibited long-term potentiation (LTP) in the hippocampal perforant path via activation of calcineurin, a Ca2+ -dependent protein phosphatase. This study examined whether Abeta could also inhibit Ca2+/calmodulin dependent protein kinase II (CaMKII), further disrupting the dynamic balance between protein kinase and phosphatase during synaptic plasticity. Immunoblot analysis was conducted to measure autophosphorylation of CaMKII at Thr286 and phosphorylation of the GluR1 subunit of AMPA receptors in single rat hippocampal slices. A high-frequency tetanus applied to the perforant path significantly increased CaMKII autophosphorylation and subsequent phosphorylation of GluR1 at Ser831, a CaMKII-dependent site, in the dentate area. Acute application of Abeta1-42 inhibited dentate LTP and associated phosphorylation processes, but was without effect on phosphorylation of GluR1 at Ser845, a protein kinase A-dependent site. These results suggest that activity-dependent CaMKII autophosphorylation and AMPA receptor phosphorylation are essential for dentate LTP. Disruption of such mechanisms could directly contribute to Abeta-induced deficits in hippocampal synaptic plasticity and memory.

Most forms of Parkinson's Disease (PD) are sporadic in nature, but some have genetic causes as first described for the α-synuclein gene. The α-synuclein protein also accumulates as insoluble aggregates in Lewy bodies in sporadic PD as well as in most inherited forms of PD. The focus of the present study is the modulation of synaptic plasticity in the corticostriatal pathway of transgenic (Tg) mice that over express the human α-synuclein protein throughout the brain (ASOTg). Paired-pulse facilitation was detected in vitro by activation of corticostriatal afferents in ASOTg mice, consistent with a presynaptic effect of elevated human α-synuclein. However basal synaptic transmission was unchanged in ASOTg, suggesting that human α-synuclein could impact paired-pulse facilitation via a presynaptic mechanism not directly related to the probability of neurotransmitter release. Mice lacking α-synuclein or those expressing normal and A53T human α-synuclein in tyrosine hydroxylase-containing neurons showed, instead, paired-pulse depression. High-frequency stimulation induced a presynaptic form of long-term depression solely in ASOTg striatum. A presynaptic, NMDA receptor-independent form of chemical long-term potentiation induced by forskolin (FSK) was enhanced in ASOTg striatum, while FSK-induced cAMP levels were reduced in ASOTg synaptoneurosome fractions. Overall the results suggest that elevated human α-synuclein alters presynaptic plasticity in the corticostriatal pathway, possibly reflecting a reduction in glutamate at corticostriatal synapses by modulation of adenylyl cyclase signaling pathways. ASOTg mice may recapitulate an early stage in PD during which over expressed α-synuclein dampens corticostriatal synaptic transmission and reduces movement. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Author ManuscriptNeuroscience. Author manuscript; available in PMC 2010 March 17. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptMost forms of Parkinson's Disease (PD) are sporadic in nature, but a small percentage have genetic causes as first described for dominant, single base pair changes in the α-synuclein gene (Polymeropoulos et al., 1997;Kruger et al., 1998;Zarranz et al., 2004). The α-synuclein gene encodes a 140 amino acid, mostly unfolded protein that is expressed ubiquitously throughout the brain and is enriched in presynaptic terminals at synapses (Maroteaux and Scheller, 1991;Hsu et al., 1998;Clayton and George, 1998). The α-synuclein protein accumulates as insoluble aggregates in Lewy bodies in sporadic forms of PD (S...

The rodent protein RC3 is expressed mainly by forebrain neurons during postnatal development and maturity. RC3 and its bovine homolog neurogranin/B-50 inmmunoreactive C-kinase substrate (BICKS) contain overlapping sites for protein kinase C phosphorylation and calmodulin binding that resemble those of the presynaptic 43-kDa growthassociated protein (GAP-43). However, morphological evidence suggests that RC3 has a postsynaptic localization. To test this hypothesis, we used two polyclonal antisera against synthetic peptides corresponding to nonoverlapping sequences within RC3 and compared cellular distributions of their binding in neostriatum of adult rats by immunohistochemistry, Golgi impregnation/gold toning, and correlative light/electron microscopy. Somatic and punctate patterns of RC3 immunoreactivity were observed. Somatic RC3 was found in cytoand nucleoplasmic compartments of aW neuronal phenotypes (medium spiny, medium aspiny, and large aspiny cells). Punctate RC3 was found mostly in dendritic spines. In contrast to the 43-kDa growth-associated protein, RC3 was seen infrequently in axons. We conclude that RC3 accumulates postsynaptically in dendritic spines of neostriatal neurons. We propose that RC3 acts as a "third messenger" substrate of protein kinase C-mediated molecular cascades during synaptic development and remodeling.RC3 is a 78-amino acid protein that was first identified in rodent forebrain by screening a cDNA library with a neocortex-minus-cerebellum subtracted cDNA probe (1). RC3 is found principally in neostriatum, neocortex, and hippocampus. The bovine protein kinase C (PKC) substrate neurogranin and calmodulin-binding protein B-50 immunoreactive C-kinase substrate (BICKS) are structural homologs (75/78 amino acids) of RC3 (2, 3). The cellular localization and function of these proteins are largely unknown. Part of RC3 resembles overlapping PKC phosphorylation and calmodulin-binding sites of the 43-kDa growth-associated protein (GAP-43) (1-9). However, RC3 and GAP43 have distinct properties. GAP43 is located along the entire neuraxis and accumulates mostly in presynaptic profiles offorebrain axons during early synaptic formation or after injury (9, 10). RC3 is located mostly in the telencephalon and is believed to accumulate in neuronal cell bodies and dendrites during development and maturity (1, 11).Previous hybridization and immunohistochemical studies show that RC3 mRNA and RC3 protein are expressed in the same characteristic kinds of neuronal cell bodies in neostriatum, neocortex, and hippocampus (1, 11). However, a dendritic localization of RC3 protein is uncertain because of potential cross-reactivity between GAP43 and RC3 antibodies. This problem is acute in forebrain regions with heterogeneous cell compositions. The relationship between RC3 and synaptic junctions is also uncertain because earlier studies are based on light microscopy, a morphological technique of inadequate precision to resolve subcellular sites of proteins. We now address these two issues with antisera t...