This profound cell type specificity of G a GLP complex

This profound cell-type specificity of G9a/GLP complex modulation of neuronal plasticity is accentuated by studies employing genetic ablation of G9a in dopamine 1 receptor (Drd1)- and dopamine 2 receptor (Drd2)-expressing medium spiny neurons (MSNs) in the nucleus accumbens (NAc). Conditional G9a knockout (KO) in Drd1 MSNs led to reduced behavioural responses to D1 receptor agonist, SKF81297 (as measured by decreased locomotor and exploratory activity compared to SKF-treated WT mice); whereas cell-specific loss of G9a in Drd2 MSNs resulted in heightened behavioural response to A2a-receptor antagonists, caffeine (as shown by significantly increased locomotor activity and exploration as compared to caffeine-treated WT animals) (). Additionally, selective G9a KO in Drd2 increased cocaine-mediated conditioned place preference, whereas G9a ablation in Drd1 neurons downregulated cocaine CPP (). These experiments suggest that cell type-specific KO of G9a in either Drd1- or Drd2-MSNs differentially changes their response to environmental stimuli and is associated with altered behaviour in the animal. At the cellular level, G9a-deficient Drd2-MSNs displayed higher excitability as compared to WT neurons (, ). In addition, G9a knockout led to a reduction in the threshold for first spike induction and an increase in membrane input resistance in Drd2 MSNs but not in Drd1 MSNs. Interestingly, Drd2 MSNs lacking G9a exhibited Drd1 MSN-like K channel-mediated current response to D1 receptor agonist, dihydrexidine hydrochloride, implying the presence of functional D1 receptors that are normally not expressed in Drd2 MSNs. These cell type-specific functional shifts in electrophysiological properties in G9a-deficient Drd2-MSNs are paralleled by a shift in transcriptional profile. Selective G9a KO in Drd2 MSNs downregulated genes that are enriched in WT Drd2 neurons, and upregulated genes enriched in WT Drd1 MSNs, suggesting that developmental G9a ablation led to an unsilencing of a Drd1-like transcriptional profile in Drd2 MSNs (). The above results suggest that G9a/GLP may be involved in maintaining neuronal subtype identity. They also highlight that the role of G9a in modulating neuronal function and associated behaviour in response to environmental stimuli can be remarkably alpha adrenergic blockers region- and cell type-specific. Furthermore, recent publications suggest that G9a/GLP function could also be dependent on the form of environmental stimuli (, , , , ). The following section highlights recent studies on the role of G9a/GLP in synaptic plasticity in the hippocampus, in which G9a/GLP activity appears to differ in response to distinct patterns of electrical stimuli. Evidence suggests that G9a/GLP complex functions as a bidirectional, activity-dependent regulator of synaptic plasticity. Bath application of G9a/GLP inhibitor BIX (1 μM) prevented the maintenance of late-LTP induced by high-frequency stimulation (HFS) at the SC-CA1 synapses in the hippocampus – the increase in synaptic strength returned to baseline levels 30 min after HFS, in a manner similar to early-LTP. On the other hand, G9a/GLP inhibition did not affect L-LTP induction and maintenance in the temporoammonic pathway synapses in the hippocampal area CA1. G9a/GLP inhibition did not affect paired-pulse facilitation in both SC-CA1 and TA-CA1 pathways, indicating minimal effects on presynaptic release probability (). These results suggest that G9a/GLP plays a role in positively regulating the expression of plasticity-related genes in an activity-dependent, neuronal pathway-specific manner. In contrast, an increase in paired-pulse facilitation was observed in CA1 neurons, implying altered presynaptic function. Interestingly, haploinsufficiency did not impair LTP induced by theta-burst stimulation in SC-CA1 synapses (). This may be due to compensatory mechanisms during development. Alternatively, this might indicate that a single copy of is sufficient to maintain its function in regulating the expression of plasticity-related genes. Along similar lines, inhibition of G9a/GLP complex using lower inhibitor concentrations (500 nM BIX and 150 nM UNC) did not affect the induction of transient early-LTP in SC-CA1 synapses; instead, it reinforced early-LTP into late-LTP, which lasted for four hours, in an N-methyl-D-aspartate (NMDA) receptor- and protein synthesis-dependent manner (). Furthermore, repression of G9a/GLP facilitated synaptic tagging and capture (STC), a prominent hypothesis of the cellular mechanisms of the formation of associative memory: G9a/GLP inhibition-reinforced late-LTP in the first synaptic pathway transformed early-LTP into L-LTP in another independent pathway, even when the inhibitors had been washed out (, ). The above results suggest that slight G9a/GLP inhibition enhanced the synthesis of plasticity-related products and facilitated LTP maintenance; hence, they point to the possible role of G9a/GLP in negatively regulating synaptic plasticity-related genes. Incidentally, pharmacological inhibition of G9a/GLP complex exhibited dosage-dependent effects in regulating peripheral nerve injury-induced neuropathic hypersensitivity. Intrathecal administration of certain threshold doses of BIX (10 μg) and UNC (80 μg) aggravated allodynia, as demonstrated by a significant decrease in pain withdrawal threshold in a mouse model of spared nerve injury. On the other hand, injection of below- or above-threshold doses of G9a/GLP inhibitors significantly increased the pain threshold in these animals (). Although the dosage dependence of G9a/GLP inhibition on its regulatory function in LTP and synaptic plasticity has not been directly tested, the fact that different groups report disparate effects of G9a/GLP inhibition on LTP underscores the importance of maintaining a fine balance of transcription and translation in synaptic plasticity (, , ).