Gα_i2 inhibition of adenylate cyclase regulates presynaptic activity and unmasks cGMP-dependent long-term depression at Schaffer collateral-CA1 hippocampal synapses

Abstract

Cyclic AMP signaling plays a central role in regulating activity at a number of synapses in the brain. We showed previously that pairing activation of receptors that inhibit adenylate cyclase (AC) and reduce the concentration of cyclic AMP, with elevation of the concentration of cyclic GMP is sufficient to elicit a presynaptically expressed form of LTD at Schaffer collateral-CA1 synapses in the hippocampus. To directly test the role of AC inhibition and G-protein signaling in LTD at these synapses, we utilized transgenic mice that express a mutant, constitutively active inhibitory G protein, G␣ i2 , in principal neurons of the forebrain. Transgene expression of G␣ i2 markedly enhanced LTD and impaired late-phase LTP at Schaffer collateral synapses, with no associated differences in input/output relations, paired-pulse facilitation, or NMDA receptor-gated conductances. When paired with application of a type V phosphodiesterase inhibitor to elevate the concentration of intracellular cyclic GMP, constitutively active G␣ i2 expression converted the transient depression normally caused by this treatment to an LTD that persisted after the drug was washed out. Moreover, this effect could be mimicked in control slices by pairing type V phosphodiesterase inhibitor application with application of a PKA inhibitor. Electrophysiological recordings of spontaneous excitatory postsynaptic currents and two-photon visualization of vesicular release using FM1-43 revealed that constitutively active G␣ i2 tonically reduced basal release probability from the rapidly recycling vesicle pool of Schaffer collateral terminals. Our findings support the hypothesis that inhibitory G-protein signaling acts presynaptically to regulate release, and, when paired with elevations in the concentration of cyclic GMP, converts a transient cyclic GMP-induced depression into a long-lasting decrease in release.Long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength are reciprocal, activity-dependent mechanisms that are thought to mediate synaptic competition during development and store information in mature networks. cAMP and its major effector kinase, cyclic AMP-dependent protein kinase (PKA), play key roles in the induction of LTP (Frey et al. 1993;Impey et al. 1996;Nguyen and Kandel 1997;Otmakhova et al. 2000;Matsushita et al. 2001). Evidence also suggests that inhibition of adenylate cyclase (AC) and reduced PKA activity promote the induction of LTD. Inhibiting PKA enhances the induction of LTD at Schaffer collateral-CA1 synapses (Santschi et al. 1999(Santschi et al. , 2006, and simultaneous elevation of the concentration of intracellular cGMP and inhibition of PKA is sufficient to elicit LTD at these synapses in the absence of afferent stimulation (Santschi et al. 1999;Stanton et al. 2001). This chemically induced form of LTD (CLTD) is presynaptically expressed and occludes stimulusevoked LTD (SLTD), suggesting convergence of key mechanisms between CLTD and SLTD (Santschi et al. 1999;Stanton et al. 2001Stanton et al. , 200...

Persistent modifications of hippocampal synaptic function during remote spatial memory

“…There is ample evidence that presynaptic function is key for the induction of synaptic plasticity and the expression of memory (Dumas et al, 2004; Ferguson et al, 2004; Lisman and Raghavachari, 2006; Bailey et al, 2008; Lin et al, 2010). This presynaptic function is usually measured using paired-pulse facilitation analyses that report on the efficacy of probability of presynaptic vesicle release (Schulz et al, 1995).…”

Role of presynaptic metabotropic glutamate receptors in the induction of long-term synaptic plasticity of vesicular release

“…Transient inhibition of glutamate release is thought to be due to modulation of voltage-dependent calcium channels, particularly N-type presynaptic calcium channels that are downstream targets of GPCRs (Tedford and Zamponi, 2006), but whether the same signal transduction and expression mechanisms are involved in any forms of LTD was unknown. Transgenic mice that express a mutant, constitutively active Gα i2 which inhibits adenylate cyclase, exhibit enhanced stimulus-induced LTD at both mossy fiber-CA3 (MF-CA3; Nicholls et al, 2006) and Schaffer collateral-CA1 (SCH-CA1; Bailey et al, 2008) synapses without significant changes in basal synaptic transmission. Interestingly, pharmacological elevations in [cGMP] which elicit a transient presynaptic depression in wildtype rodent slices (Boulton et al, 1994) evokes a presynaptic LTD in slices from these Gα i2 transgenic animals (Bailey et al, 2008; Nicholls et al, 2006).…”

“…Transgenic mice that express a mutant, constitutively active Gα i2 which inhibits adenylate cyclase, exhibit enhanced stimulus-induced LTD at both mossy fiber-CA3 (MF-CA3; Nicholls et al, 2006) and Schaffer collateral-CA1 (SCH-CA1; Bailey et al, 2008) synapses without significant changes in basal synaptic transmission. Interestingly, pharmacological elevations in [cGMP] which elicit a transient presynaptic depression in wildtype rodent slices (Boulton et al, 1994) evokes a presynaptic LTD in slices from these Gα i2 transgenic animals (Bailey et al, 2008; Nicholls et al, 2006). Furthermore, LFS-induced LTD at both MF-CA3 and SCH-CA1 synapses is also enhanced in slices from these animals (Bailey et al, 2008), implying a decreased threshold for chemically and stimulus-evoked LTD.…”

“…Interestingly, pharmacological elevations in [cGMP] which elicit a transient presynaptic depression in wildtype rodent slices (Boulton et al, 1994) evokes a presynaptic LTD in slices from these Gα i2 transgenic animals (Bailey et al, 2008; Nicholls et al, 2006). Furthermore, LFS-induced LTD at both MF-CA3 and SCH-CA1 synapses is also enhanced in slices from these animals (Bailey et al, 2008), implying a decreased threshold for chemically and stimulus-evoked LTD. Overall, these studies suggest that activation of inhibitory Gα i subunits by presynaptic G-protein coupled receptors contribute to inhibition of presynaptic adenylate cyclase/suppression in cyclic AMP production in the presynaptic terminal that is essential for induction of a presynaptic form of LTD.…”

“…Similarly, constitutively active Gα i2 leads to suppression of MF-CA3 and SCH-CA1 LTP, which was rescued by exogenous application of S P -adenosine 3',5'-monophosphorothioate (SP-cAMPS), implying that the cAMP-PKA pathway is important for presynaptic LTP, at least at these synapses (Bailey et al, 2008; Nicholls et al, 2006). Previous studies have shown LTP at MF-CA3 synapses to be expressed presynaptically and depend on cAMP and PKA (Huang et al, 1994; Villacres et al, 1998), consistent with presynaptic group II mGluRs acting via inhibitory Gα i2 to suppress activity in the cAMP-PKA pathway.…”