Dorsal hippocampus, retrosplenial cortex (RSC), and anterior thalamic nuclei (ATN) 19interact to mediate diverse cognitive functions, but the cellular basis for these interactions 20 is unclear. We hypothesized a long-range circuit converging in layer 1 (L1) of RSC, based 21 on the pathway anatomy of GABAergic CA1 retrosplenial-projecting (CA1-RP) neurons 22 and thalamo-restrosplenial projections from ATN. We find that CA1→RSC projections 23 stem from GABAergic neurons with a distinct morphology, electrophysiology, and 24 molecular profile, likely corresponding to recently described Ntng1-expressing 25 hippocampal interneurons. CA1-RP neurons monosynaptically inhibit L5 pyramidal 26 neurons, principal outputs of RSC, via potent GABAergic synapses onto apical tuft 27 dendrites in L1. These inhibitory inputs align precisely with L1-targeting thalamocortical 28 excitatory inputs from ATN, particularly the anteroventral nucleus, forming a convergent 29 circuit whereby CA1 inhibition can intercept ATN excitation to co-regulate RSC activity. 30 Excitatory axons from subiculum, in contrast, innervate proximal dendrites in deeper 31 layers. Short-term synaptic plasticity differs at each connection. Chemogenetically 32 abrogating inhibitory CA1→RSC or excitatory ATN→RSC connections oppositely affects 33 the encoding of contextual fear memory. Collectively, our findings identify multiple cellular 34 mechanisms underlying hippocampo-thalamo-retrosplenial interactions, establishing CA1 35 RSC-projecting neurons as a distinct class with long-range axons that target apical tuft 36 dendrites, and delineating an unusual cortical circuit in the RSC specialized for integrating 37 long-range inhibition and thalamocortical excitation. 38 39 the neocortically projecting GABAergic neurons mentioned above, however, the cellular 55 properties of the presynaptic neurons are obscure, and the targets of their axons in the RSC are 56 unknown. 57This GABAergic CA1→RSC projection lies within a larger network thought to support 58 the encoding and storage of hippocampally derived information in the RSC (reviewed in 9-11 ). 59Understanding how the GABAergic neurons in CA1 are connected to postsynaptic RSC neurons 60 could thus yield mechanistic insight into the cellular basis for behaviors mediated by 61 hippocampo-retrosplenial communication in this system. 62 4 Hippocampo-retrosplenial communication also involves excitatory circuits, but unlike 63 GABAergic CA1→RSC projections these appear to be mostly indirect, through polysynaptic 64 chains of connections 12 . Two major routes carrying hippocampus-related information to the 65 RSC are a corticocortical pathway via subiculum, and a subcortical pathway traversing the 66 anterior thalamic nuclei (ATN). Subiculum→RSC projections arise from burst-firing vGlut1-or 67 vGlut2-expressing subicular pyramidal neurons, which directly excite RSC pyramidal neurons 13 . 68They also drive feedforward (disynaptic) inhibition 13 , a local-circuit inhibitory mechanism 69 previously implicated in mnemonic func...

Hippocampus, granular retrosplenial cortex (RSCg), and anterior thalamic nuclei (ATN) interact to mediate diverse cognitive functions. To identify cellular mechanisms underlying hippocampo-thalamo-retrosplenial interactions, we investigated the potential circuit suggested by projections to RSCg layer 1 (L1) from GABAergic CA1 neurons and ATN. We find that CA1→RSCg projections stem from GABAergic neurons with a distinct morphology, electrophysiology, and molecular profile. Their long-range axons inhibit L5 pyramidal neurons in RSCg via potent synapses onto apical tuft dendrites in L1. These inhibitory inputs intercept L1-targeting thalamocortical excitatory inputs from ATN to co-regulate RSCg activity. Subicular axons, in contrast, excite proximal dendrites in deeper layers. Short-term plasticity differs at each connection. Chemogenetically abrogating CA1→RSCg or ATN→RSCg connections oppositely affects the encoding of contextual fear memory. Our findings establish retrosplenial-projecting CA1 neurons as a distinct class of long-range dendrite-targeting GABAergic neuron, and delineate an unusual cortical circuit specialized for integrating long-range inhibition and thalamocortical excitation.

BackgroundGenome-wide association studies have identified TRPM8 (transient receptor potential melastatin 8) as one of the susceptibility genes for common migraine. Here, we investigated the postnatal changes of TRPM8-expressing dural afferent fibers as well as the function of dural TRPM8 channels in mice.ResultsFirst, we quantified the density and the number of axonal branches of TRPM8-expressing fibers in the dura of mice expressing farnesylated enhanced green fluorescent protein (EGFPf) from one TRPM8 allele between postnatal day 2 (P2) to adulthood. The number of axonal branches on individual dural EGFP-positive fibers was decreased by 30% between P2 and P11. The density of dural EGFP-positive fibers was subsequently reduced by 50% between P16 and P21. Conversely, the density and the number of branches of axons expressing calcitonin gene-related peptide remained stable in postnatal mouse dura. The density of TRPM8-expressing fibers innervating the mouse cornea epithelium was significantly increased from P2 to adulthood. Next, we tested the function of dural TRPM8 channels in adult mice and found that TRPM8 agonist menthol effectively inhibited the nocifensive behavior evoked by dural application of inflammatory mediators.ConclusionsOur results indicate that the TRPM8-expressing dural afferent fibers undergo cell- and target tissue-specific axonal pruning during postnatal development. Activation of dural TRPM8 channels decreases meningeal irritation-evoked nocifensive behavior in adult mice. This provides a framework to further explore the role of postnatal changes of TRPM8-expressing dural afferents in the pathophysiology of pediatric and adult migraine.Electronic supplementary materialThe online version of this article (doi:10.1186/s12990-015-0043-0) contains supplementary material, which is available to authorized users.

Migraine and other primary headache disorders affect a large population and cause debilitating pain. Establishing animal models that display behavioral correlates of long-lasting and ongoing headache, the most common and disabling symptom of migraine, is vital for the elucidation of disease mechanisms as well as the identification of drug targets. We have developed a mouse model of headache, using dural application of capsaicin along with a mixture of inflammatory mediators (IScap) to simulate the induction of a headache episode. This elicited intermittent head-directed wiping and scratching as well as the phosphorylation of c-Jun N-terminal kinase in trigeminal ganglion neurons. Interestingly, dural application of IScap preferentially induced FOS protein expression in the excitatory but not inhibitory cervical/medullary dorsal horn neurons. The duration of IScap-induced behavior and the number of FOS-positive neurons correlated positively in individual mice; both were reduced to the control level by the pretreatment of anti-migraine drug sumatriptan. Dural application of CGRP(8–37), the calcitonin gene-related peptide (CGRP) receptor antagonist, also effectively blocked IScap-induced behavior, suggesting that the release of endogenous CGRP in dura is necessary for IScap-induced nociception. These data suggest that dural IScap-induced nocifensive behavior in mice may be mechanistically related to the ongoing headache in humans. In addition, dural application of IScap increased resting time in female mice.. Taken together, we present here the first detailed study using dural application of IScap in mice. This headache model can be applied to genetically modified mice to facilitate the research for the mechanisms and therapeutic targets for migraine headache.

N-Methyl D-Aspartate Receptors (NMDAR) are central mediators of glutamate actions underlying learning and memory processes including those required for extinction of fear and fear-related behaviors. Consistent with this view, in animal models, antagonists of NMDAR typically impair fear extinction, whereas partial agonists have facilitating effects. Promoting NMDAR function has thus been recognized as a promising strategy towards reduction of fear symptoms in patients suffering from anxiety disorders and post-traumatic disorder (PTSD). Nevertheless, application of these drugs in clinical trials, has proved of limited utility. Here we summarize recent advances in our knowledge of NMDAR pharmacology relevant for fear extinction, focusing on molecular, cellular, and circuit aspects of NMDAR function as they relate to fear extinction at the level of behavior and cognition. We also discuss how these advances from animal models might help to understand and overcome the limitations of existing approaches in human anxiety disorders and how novel, more specific, and personalized approaches might help advance future therapeutic strategies.