Hippocampal c-Jun-N-terminal kinases serve as negative regulators of associative learning.

In the adult mouse, signaling through c-Jun N-terminal kinases (JNKs) links exposure to acute stress to various physiological responses. Inflammatory cytokines, brain injury and ischemic insult, or exposure to psychological acute stressors induce activation of hippocampal JNKs. Here we report that exposure to acute stress caused activation of JNKs in the hippocampal CA1 and CA3 subfields, and impaired contextual fear conditioning. Conversely, intrahippocampal injection of JNKs inhibitors sp600125 (30 µm) or D-JNKI1 (8 µm) reduced activity of hippocampal JNKs and rescued stress-induced deficits in contextual fear. In addition, intrahippocampal administration of anisomycin (100 µg/µl), a potent JNKs activator, mimicked memory-impairing effects of stress on contextual fear. This anisomycin-induced amnesia was abolished after cotreatment with JNKs selective inhibitor sp600125 without affecting anisomycin's ability to effectively inhibit protein synthesis as measured by c-Fos immunoreactivity. We also demonstrated milder and transient activation of the JNKs pathway in the CA1 subfield of the hippocampus during contextual fear conditioning and an enhancement of contextual fear after pharmacological inhibition of JNKs under baseline conditions. Finally, using combined biochemical and transgenic approaches with mutant mice lacking different members of the JNK family (Jnk1, Jnk2, and Jnk3), we provided evidence that JNK2 and JNK3 are critically involved in stress-induced deficit of contextual fear, while JNK1 mainly regulates baseline learning in this behavioral task. Together, these results support the possibility that hippocampal JNKs serve as a critical molecular regulator in the formation of contextual fear.

Corticotropin-releasing factor receptors and urocortins, links between the brain and the heart.

Corticotropin-releasing factor (CRF), a 41 amino acid peptide, was discovered as a key signal in mediating neuroendocrine, autonomic, and behavioral responses to stress. It was revealed later that there exist additional CRF-like peptides, termed urocortins. The CRF receptor subtype 1 (CRF1 receptor) is predominant in the brain whereas subtype 2 (CRF2 receptor) is highly expressed in the brain and the heart. Both centrally and peripherally administered CRF and urocortins produce significant hemodynamic effects via activation of CRF receptors in the brain and the heart. CRF and urocortins are important neural and cardioactive hormones, and are potentially useful therapy for heart failure.

Suppression of the MEK/ERK signaling pathway reverses depression-like behaviors of CRF2-deficient mice.

The neuropeptide corticotropin-releasing factor (CRF) plays a critical role in the proper functioning of the stress response system through its actions on its receptors, CRF receptor 1 (CRF1) and CRF receptor 2 (CRF2), located at multiple anatomical sites. Clinical data indicate that stress response dysfunctions, such as excessive CRF activity and hyperstimulation of CRF1, are present in a range of stress-related disorders, including depression and anxiety disorders. Our previous work along with that of other laboratories has demonstrated that mice deficient in CRF2 (CRF2-/-) display increased anxiety and depression-like behaviors. In this study, we found CRF2-/- mice display increased hippocampal levels of activated (phosphorylated) mitogen-activated protein kinase (MAP kinase)/ERK kinase (MEK), extracellular signal-regulated kinases 1 and 2 (ERK1/2), and ribosomal protein S6 kinases 1 (RSK1). These changes can be explained by overactive hippocampal CRF1, in view of the finding that the application of the nonselective CRF receptor antagonist [Glu(11,16)] astressin ([Glu(11,16)]Ast) into the dorsal hippocampus of mutant mice returned the levels of the phosphorylated proteins to baseline. Moreover, inhibition of the hippocampal MEK/ERK pathway with the specific MEK inhibitor U0126, decreased depression-like behaviors in the forced swim test and tail suspension test of CRF2-/- mice. Similarly, treatment with [Glu(11,16)]Ast reversed depression phenotype of CRF2-/- mice without affecting the phenotype of wild-type littermates. Our results support an involvement of CRF receptors in the development of depression, such that elevated hippocampal CRF1 activity, in the absence of CRF2, produces a depression-dominated phenotype through the activation of the MEK/ERK pathway.

Cations affect the rate of gating charge recovery in wild-type and W434F Shaker channels through a variety of mechanisms.

In this study we examine the effects of ionic conditions on the gating charge movement in the fast inactivation-removed wild-type Shaker channel and its W434F mutant. Our results show that various ionic conditions influence the rate at which gating charge returns during repolarization following a depolarizing pulse. These effects are realized through different mechanisms, which include the regulation of channel closing by occupying the cavity, the modulation of transitions into inactivated states, and effects on transitions between closed states via a direct interaction with the channel's gating charges. In generating these effects the cations act from the different binding sites within the pore. Ionic conditions, in which conducting wild-type channels close at different rates, do not significantly affect the rate of charge recovery upon repolarization. In these conditions, channel closing is fast enough not to be rate-limiting in the charge recovery process. In the permanently P-inactivated mutant channel, however, channel closing becomes the rate-limiting step, presumably due to weakened ion-ion interactions inside the pore and a slower intrinsic rate of gate closure. Thus, variations in closing rate induced by different ions are reflected as variations in the rate of charge recovery. In 115 mM internal Tris(+) and external K(+), Cs(+), or Rb(+), low inward permeation of these ions can be observed through the mutant channel. In these instances, channel closing becomes slower than in Tris(+)(O)//Tris(+)(I) solutions showing resemblance to the wild-type channel, where higher inward ionic fluxes also retard channel closing. Our data indicate that cations regulate the transition into the inactivated states from the external lock-in site and possibly the deep site. The direct action of barium on charge movement is probably exerted from the deep site, but this effect is not very significant for monovalent cations.