Atrial Natriuretic Peptide Blocks Synaptic Transmission And Its Signal Transduction Cascade

Synaptic transmissions are dynamically gated, filtered, or amplified by external modulators such as neuropeptides. Presynaptic modulators are often thought to act on receptors that belong to the G protein-coupled receptor (GPCR) family. In addition to GPCRs, a unique family of receptors known as membrane guanylyl cyclases (GCs) can be activated by neuropeptides such as natriuretic peptides to catalyze the intracellular production of cyclic guanosine monophosphate (cGMP). In animals across taxa, cGMPs signals influence cellular physiology by acting on cGMP-stimulated protein kinase G (PKG), cyclic nucleotide-gated (CNG) channels, or cGMP-sensitive phosphodiesterases (PDEs). In C. elegans, a membrane GC acts on the presynaptic terminals of olfactory neurons to induce a behavioral switch. Several membrane GCs and their associated peptide ligands are expressed in the mammalian brain. For example, GC-C is activated by gut peptide hormones guanylin and uroguanylin to amplify postsynaptic responses of midbrain dopamine neurons. The heart peptide hormone atrial natriuretic peptide (ANP) regulates blood pressure by stimulating guanylyl cyclase-A (GC-A) to produce cGMPs. ANP and GC-A are also expressed in many brain areas, but their physiological functions and downstream signaling pathways remain enigmatic.One of the most prominent brain areas expressing GC-A is the projection from the the medial habenula (MHb) in the epithalamus to the interpeduncular nucleus (IPN) in the midbrain. This neural pathway links forebrain limbic areas with midbrain modulatory systems and regulates a diverse array of behaviors including pain, anxiety, sleep, and nicotine addiction. Abundant GC-A mRNA is detected in MHb neurons. In addition, strong ANP binding is observed in the IPN, which receives dense innervation from the MHb. Thus GC-A is likely expressed in the axonal terminals of MHb neurons and may regulate neurotransmitter release from MHb neurons to IPN neurons. Here we investigated the physiological functions of ANP signaling in the neural pathway from the MHb to the IPN.We took advantage of ChAT-ChR2-EYFP mice, which allowed us to selectively stimulate the axonal terminals of MHb neurons with light and evoke fast glutamatergic responses in IPN neurons. Biochemical assays indicate that ANP increases cGMP accumulation in the IPN of mouse brain slices. Using optogenetic stimulation and electrophysiological recordings, we show that both ANP and brain natriuretic peptide (BNP) profoundly block glutamate release from MHb neurons. Pharmacological applications reveal that this blockade is mediated by phosphodiesterase2A (PDE2A) but not PKG or CNG channels. In addition, focal infusion of ANP into the IPN enhances stress-induced analgesia, and the enhancement is prevented by PDE2A inhibitors. PDE2A is richly expressed in the axonal terminals of MHb neurons and its activation by cGMP depletes cyclic adenosine monophosphates (cAMPs). The inhibitory effect of ANP on glutamate release is reversed by selectively activating protein kinase A (PKA). These results demonstrate strong presynaptic inhibition by natriuretic peptides in the brain and suggest important physiological and behavioral roles of PDE2A in modulating neurotransmitter release by negative crosstalk between cGMP and cAMP signaling pathways. Data from this study provide so far the clearest picture of signal transduction cascades underlying the ANP effect of presynaptic inhibition.