Spatial, cellular and temporal basis of vasopressin potentiation of norepinephrine-induced cAMP formation

Cyclic AMP controls BDNF-induced TrkB phosphorylation and dendritic spine formation in mature hippocampal neurons

Synaptic actions of brain-derived neurotrophic factor (BDNF) are 'gated' by cyclic AMP (cAMP), but the underlying molecular mechanisms remain unclear. Here we report that cAMP regulates BDNF function in mature hippocampal neurons by modulating the signaling and trafficking of its receptor TrkB. cAMP gated the TrkB tyrosine kinase with three characteristic features: BDNF-induced TrkB phosphorylation was attenuated by inhibitors of cAMP signaling, it was potentiated by cAMP analogs, and activation of the cAMP pathway alone had no effect. In addition, cAMP facilitated trafficking of TrkB to dendritic spines, possibly by promoting its interaction with synaptic scaffolding protein PSD-95. Norepinephrinergic and dopaminergic agonists, which elevate intracellular cAMP concentration, also enhanced TrkB phosphorylation and its translocation to spines. cAMP gated long-term modulation by BDNF of spine density, but not the number of primary dendrites. These results reveal a specific role of cAMP in controlling BDNF actions in the brain, and provide new insights into the molecular mechanism underlying cAMP gating.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Effects of arginine-vasopressin (AVP) on long-term potentiation in intact anesthetized rats

We studied the effects of the neuropeptide arginine-vasopressin (AVP) on the long-term potentiation (LTP) paradigm in the dentate gyrus (DG) of urethane intact anesthetized rats. Intracerebroventricular injection of 1 microg of the hormone in 1 microl of physiological solution 3 min before tetanization, produced a significant increase in both components of the perforant path-evoked potentials (EP) in the DG. The effects were already evident 1 min after tetanization. Amplitude of the EPs increased continuously for the 2h of recording time, reaching values 100% above baseline, reference levels. In contrast, in previous in vitro studies, enhancement of LTP with AVP appeared only after 15 min of exposure of the hippocampal slice to the hormone, increased EPSPs were no higher than 50% from baseline, reached a plateau at 40 min decreasing slowly thereafter. Not only quantitative but also qualitative differences can be observed between in vitro and in vivo intact preparations in response to identical hormones. This study emphasizes the importance of hormone neurotransmitter interactions in determining electrophysiological characteristics of response to AVP.