Activation of Erk and JNK MAPK pathways by acute swim stress in rat brain regions.

BACKGROUND: The mitogen-activated protein kinases (MAPKs) have been shown to participate in a wide array of cellular functions. A role for some MAPKs (e.g., extracellular signal-regulated kinase, Erk1/2) has been documented in response to certain physiological stimuli, such as ischemia, visceral pain and electroconvulsive shock. We recently demonstrated that restraint stress activates the Erk MAPK pathway, but not c-Jun-N-terminal kinase/stress-activated protein kinase (JNK/SAPK) or p38MAPK, in several rat brain regions. In the present study, we investigated the effects of a different stressor, acute forced swim stress, on the phosphorylation (P) state of these MAPKs in the hippocampus, neocortex, prefrontal cortex, amygdala and striatum. In addition, effects on the phosphorylation state of the upstream activators of the MAPKs, their respective MAPK kinases (MAPKKs; P-MEK1/2, P-MKK4 and P-MKK3/6), were determined. Finally, because the Erk pathway can activate c-AMP response element (CRE) binding (CREB) protein, and swim stress has recently been reported to enhance CREB phosphorylation, changes in P-CREB were also examined. RESULTS: A single 15 min session of forced swimming increased P-Erk2 levels 2-3-fold in the neocortex, prefrontal cortex and striatum, but not in the hippocampus or amygdala. P-JNK levels (P-JNK1 and/or P-JNK2/3) were increased in all brain regions about 2-5-fold, whereas P-p38MAPK levels remained essentially unchanged. Surprisingly, levels of the phosphorylated MAPKKs, P-MEK1/2 and P-MKK4 (activators of the Erk and JNK pathways, respectively) were increased in all five brain regions, and much more dramatically (P-MEK1/2, 4.5 to > 100-fold; P-MKK4, 12 to approximately 300-fold). Consistent with the lack of forced swim on phosphorylation of p38MAPK, there appeared to be no change in levels of its activator, P-MKK3/6. P-CREB was increased in all but cortical (prefrontal, neocortex) areas. CONCLUSIONS: Swim stress specifically and markedly enhanced the phosphorylation of the MAPKKs P-MEK1/2 and P-MKK4 in all brain regions tested without apparent alteration in the phosphorylation of P-MKK3/6. Curiously, phosphorylation of their cognate substrates (Erk and JNK) was increased to a much more modest extent, and in some brain regions was not altered. Similarly, there was a region-specific discrepancy between Erk and CREB phosphorylation. Possible explanations for these findings and comparison with the effects of restraint stress will be discussed.

Region-specific effects of acute and repeated restraint stress on the phosphorylation of mitogen-activated protein kinases.

The mitogen-activated protein kinases (MAPKs) are a family of signal transduction mediators that regulate a host of cellular activities, including cell growth and proliferation, and differentiation and survival, via sequential phosphorylation and activation of a cassette of three protein kinases. MAPKs are also recruited when the brain undergoes synaptic plasticity and remodeling (e.g., during induction of long-term potentiation, learning and memory consolidation). The activities of some of these kinases are altered in response to various acute stimuli such as ischemic insult, visceral pain and electroconvulsive shock. In the present study we used immunoblotting techniques to examine the effects of acute and repeated restraint stress on the phosphorylation state of three MAPKs, the extracellular signal-regulated kinase Erk1/2, c-Jun-N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38 MAPK, in different brain regions. A single exposure to 30 min of restraint stress-elevated phospho-Erk1/2 (P-Erk1/2) levels in all three brain regions examined (hippocampus, medial prefrontal cortex and cingulate cortex), but did not alter the phosphorylation pattern of the other two MAPKs in any region. In marked contrast, exposure to restraint for 11 days (30 min/day) reduced the levels of all three MAPKs, but only in the prefrontal cortex. The results are compared to the reported effects of acute and chronic stress on other biochemical and functional measures.