Extensive work has shown that activation of the cAMP-dependent protein kinase A (PKA) is crucial for long-term depression (LTD) of synaptic transmission in the hippocampus, a phenomenon that is thought to be involved in memory formation. Here we studied the role of an alternative target of cAMP, the exchange protein factor directly activated by cyclic AMP (Epac). We show that pharmacological activation of Epac by the selective agonist 8-(4-chlorophenylthio)-2 -O-methyl-cAMP (8-pCPT) induces LTD in the CA1 region. Paired-pulse facilitation of synaptic responses remained unchanged after induction of this LTD, suggesting that it depended on postsynaptic mechanisms. The 8-pCPT-induced LTD was blocked by the Epac signalling inhibitor brefeldin-A (BFA), Rap-1 antagonist geranylgeranyltransferase inhibitor (GGTI) and p38 mitogen activated protein kinase (P38-MAPK) inhibitor SB203580. This indicated a direct involvement of Epac in this form of LTD. As for other forms of LTD, a mimetic peptide of the PSD-95/Disc-large/ZO-1 homology (PDZ) ligand motif of the AMPA receptor subunit GluR2 blocked the Epac-LTD, suggesting involvement of PDZ protein interaction. The Epac-LTD also depended on mobilization of intracellular Ca2+ , proteasome activity and mRNA translation, but not transcription, as it was inhibited by thapsigargin, lactacystin and anisomycin, but not actinomycin-D, respectively. Finally, we found that the pituitary adenylate cyclase activating polypeptide (PACAP) can induce an LTD that was mutually occluded by the Epac-LTD and blocked by BFA or SB203580, suggesting that the Epac-LTD could be mobilized by stimulation of PACAP receptors. Altogether these results provided evidence for a new form of hippocampal LTD.

The pattern of prolactin (PRL) secretion depends on the physiological state. Due to insufficient detection sensitivity of existing assays, the precise description of these patterns in mice is lacking. We described an ultrasensitive ELISA assay that can detect mouse PRL in small fractions of whole blood, allowing longitudinal studies of PRL secretion profiles in freely moving mice. Over a 24-hour period, males displayed no oscillation in PRL levels, whereas virgin and lactating females showed large pulses. Peaks of PRL secretion reached 30-40 ng/mL in lactating female mice and rarely exceeded 10 ng/mL in virgin females. These pulses of PRL in lactating females were associated with suckling. The return of pups after an experimental 12-hour weaning induced a pulse of PRL release, reaching 100 ng/mL. This approach also enabled us to assess the inhibitory tone from hypothalamic dopamine neurons on PRL secretion. We used a dopamine D2 receptor antagonist to relieve pituitary lactotrophs from the tuberoinfundibular dopaminergic inhibitory tone and demonstrate a D2-induced PRL rise that can be used to evaluate both the secretory capacity of lactotrophs and the magnitude of the inhibitory tone on pituitary PRL release. We demonstrate that, although lactotroph function is altered to enhance chronic PRL output, their secretory response to acute stimulus is not modified during lactation and that chronic hyperprolactinemia is linked to a lower inhibitory tone. The combination of a sensitive PRL ELISA and administration of D2 receptor antagonist provide a unique opportunity to investigate the function and plasticity of the lactotroph axis in freely moving mice.