Many cortical synapses exhibit spike timing-dependent plasticity (STDP) in which the precise timing of presynaptic and postsynaptic spikes induces synaptic strengthening [long-term potentiation (LTP)] or weakening [long-term depression (LTD)]. Standard models posit a single, postsynaptic, NMDA receptor-based coincidence detector for LTP and LTD components of STDP. We show instead that STDP at layer 4 to layer 2/3 synapses in somatosensory (S1) cortex involves separate calcium sources and coincidence detection mechanisms for LTP and LTD. LTP showed classical NMDA receptor dependence. LTD was independent of postsynaptic NMDA receptors and instead required group I metabotropic glutamate receptors and calcium from voltage-sensitive channels and IP 3 receptor-gated stores. Downstream of postsynaptic calcium, LTD required retrograde endocannabinoid signaling, leading to presynaptic LTD expression, and also required activation of apparently presynaptic NMDA receptors. These LTP and LTD mechanisms detected firing coincidence on ϳ25 and ϳ125 ms time scales, respectively, and combined to implement the overall STDP rule. These findings indicate that STDP is not a unitary process and suggest that endocannabinoid-dependent LTD may be relevant to cortical map plasticity.

The capsaicin receptor, VR1 (also known as TRPV1), is a ligand-gated ion channel expressed on nociceptive sensory neurons that responds to noxious thermal and chemical stimuli. Capsaicin responses in sensory neurons exhibit robust potentiation by cAMP-dependent protein kinase (PKA). In this study, we demonstrate that PKA reduces VR1 desensitization and directly phosphorylates VR1. In vitro phosphorylation, phosphopeptide mapping, and protein sequencing of VR1 cytoplasmic domains delineate several candidate PKA phosphorylation sites. Electrophysiological analysis of phosphorylation site mutants clearly pinpoints Ser116 as the residue responsible for PKA-dependent modulation of VR1. Given the significant roles of VR1 and PKA in inflammatory pain hypersensitivity, VR1 phosphorylation at Ser116 by PKA may represent an important molecular mechanism involved in the regulation of VR1 function after tissue injury.

Protein kinase C (PKC) modulates the function of the capsaicin receptor transient receptor potential vanilloid 1 (TRPV1). This modulation manifests as increased current when the channel is activated by capsaicin. In addition, studies have suggested that phosphorylation by PKC might directly gate the channel, because PKC-activating phorbol esters induce TRPV1 currents in the absence of applied ligands. To test whether PKC both modulates and gates the TRPV1 function by direct phosphorylation, we used direct sequencing to determine the major sites of PKC phosphorylation on TRPV1 intracellular domains. We then tested the ability of the PKC-activating phorbol 12-myristate 13-acetate (PMA) to potentiate capsaicin-induced currents and to directly gate TRPV1. We found that mutation of S800 to alanine significantly reduced the PMA-induced enhancement of capsaicin-evoked currents and the direct activation of TRPV1 by PMA. Mutation of S502 to alanine reduced PMA enhancement of capsaicin-evoked currents, but had no effect on direct activation of TRPV1 by PMA. Conversely, mutation of T704 to alanine had no effect on PMA enhancement of capsaicin-evoked currents but dramatically reduced direct activation of TRPV1 by PMA. These results, combined with pharmacological studies showing that inactive phorbol esters also weakly activate TRPV1, suggest that PKC-mediated phosphorylation modulates TRPV1 but does not directly gate the channel. Rather, currents induced by phorbol esters result from the combination of a weak direct ligand-like activation of TRPV1 and the phosphorylation-induced enhancement of the TRPV1 function. Furthermore, modulation of the TRPV1 function by PKC appears to involve distinct phosphorylation sites depending on the mechanism of channel activation. P rotein kinase C (PKC) in peripheral sensory afferents plays a prominent role in hypersensitivity to thermal and mechanical stimuli after tissue injury. PKC sensitizes heat responses and potentiates peptide release from cultured dorsal root ganglion neurons (1, 2) and sensitizes nociceptive afferent neurons to thermal and mechanical stimuli in intact peripheral nerve preparations (3, 4). Diabetic neuropathic hyperalgesia and epinephrine-induced hyperalgesia are attenuated by PKC inhibitors in vivo (5, 6). Recently, several studies have focused on the role of the PKC isoform. Specific blockade of PKC diminishes PKCmediated enhancement of heat currents in sensory neurons and epinephrine-induced hypersensitivity in vivo (7,8). PKC knockout mice exhibit reduced hyperalgesia after intracutaneous injection of epinephrine and nerve growth factor (8). Whereas a role of PKC in peripheral sensitization is well established, PKC-mediated phosphorylation and modulation of specific substrates during peripheral sensitization is not fully understood.Transient receptor potential vanilloid 1 [TRPV1; formerly known as vanilloid receptor 1 (VR1)] is an attractive PKC effector in peripheral nociceptors. TRPV1 was cloned as a capsaicin receptor and is a ligand-gated ion channel, which ...

Many aspects of synaptic development, plasticity, and neurotransmission are critically influenced by NMDA-type glutamate receptors (NMDARs). Moreover, dysfunction of NMDARs has been implicated in a broad array of neurological disorders, including schizophrenia, stroke, epilepsy, and neuropathic pain. Classically, NMDARs were thought to be exclusively postsynaptic. However, substantial evidence in the last 10 years demonstrates that NMDARs also exist presynaptically, and that presynaptic NMDA receptors (preNMDARs) modulate synapse function and have critical roles in plasticity at many synapses. Here we review current knowledge of the role of preNMDARs in synaptic transmission and plasticity, focusing on the neocortex. We discuss the prevalence, function, and development of these receptors, and their potential modification by experience and in brain pathology.

Presynaptic NMDA receptors (NMDARs) modulate release and plasticity at many glutamatergic synapses, but the specificity of their expression across synapse classes has not been examined. We found that non-postsynaptic, likely presynaptic NR2B-containing NMDARs enhanced AMPA receptor-mediated synaptic transmission at layer 4 (L4) to L2/3 (L4 -L2/3) synapses in juvenile rat barrel cortex. This modulation was apparent at room temperature when presynaptic NMDARs were activated by elevation of extracellular glutamate or application of exogenous NMDAR agonists. At near physiological temperatures, modulation of transmission by presynaptic NMDARs occurred naturally, without the need for external activation. Blockade of presynaptic NMDARs depressed unitary and extracellularly evoked EPSCs at L4 -L2/3 synapses, accompanied by increases in paired-pulse ratio and coefficient of variation, indicative of a decrease in presynaptic release probability. NMDAR agonists increased the frequency of miniature EPSCs in L2/3 neurons, without altering their amplitude or kinetics. Focal application of NMDAR antagonist revealed that the NMDARs that modulate L4 -L2/3 transmission are located in L2/3, not L4, consistent with localization on terminals or axons of L4 -L2/3 synapses, rather than on the somatodendritic compartment of presynaptic L4 neurons. In contrast, presynaptic NMDARs did not modulate L4 -L4 synapses, which originate from the same presynaptic neurons as L4 -L2/3 synapses, or cross-columnar L2/3-L2/3 horizontal projections, which synapse onto the same postsynaptic target neurons. Thus, presynaptic NMDARs selectively modulate L4 -L2/3 synapses, relative to other synapses made by the same neurons. Existence of these receptors may support specialized processing or plasticity by L4 -L2/3 synapses.