TGF β1 plays an acute effect on neuronal excitability

The present study demonstrated TGFβ1 and its receptors are involved in the pancreatic hyperalgesia in rats with chronic pancreatitis (CP). It is well known that TGFβ1 is upregulated in the pancreas in rodents as well as patients with CP and plays a dominant role in the development of CP by contributing to local inflammation and promoting pancreatic fibrosis. A recent report shows that administration of TGFβ1 neutralizing antibody significantly attenuated the pancreatic hyperalgesia in rats with CP. In the present study, administration of SB431542, a potent and selective TGFβ receptor I antagonist, significantly attenuated abdominal hyperalgesia in CP rats. Together, these data suggest a role for TGFβ1 and its receptors in the development of pancreatic hyperalgesia in rats with CP. Of particular note is that SB431542 did not affect nociceptive behaviors in control rats in our study, while in the previous report, TGFβ1 neutralization in control rats resulted in pancreatic hyperalgesia. This discrepancy may be due to differences in the administration methods (i.t. vs i.p.) and in the focus on action time of dugs (0.5-8 hours after injection vs 1 week after injection), or more probably due to the fact that we used the TGFβ receptor I antagonist which blocks the TGFβ signaling including TGFβ1, TGFβ2 and TGFβ3, while TGFβ1 neutralizing antibody does not block the other two isoforms (TGFβ2 and TGFβ3) that can also bind to TGFβ receptors and further activate the downstream signaling.

Another important finding is that TGFβ1 plays an acute effect on neuronal excitability. To the best of our knowledge, this is the first time to report the acute effect of TGF β1 on neuron excitability. This is supported by the following observations. First, TGFβ1 application depolarized the membrane potential and caused firing activity of pancreas-specific DRG neurons. Second, TGFβ1 application also reduced the rheobase, hyperpolarized action potential threshold and increased the number of action potentials evoked by current injection of pancreas-specific DRG neurons. In the previous report [11] TGFβ1 sensitization of DRG neurons in vitro, occurring after incubation of TGFβ1 for 24 hours but not earlier (1 hour), was attributed to downregulation of Kv1.4 gene and decreased voltage-gated A-type K + currents (I A ), while TGFβ1 sensitization in our study, occurring after application of TGFβ1 for 3 minutes, was companied by decreased AP duration (data not shown), suggesting that the acute sensitized effect of TGFβ1 was due to some factors other than Kv1.4. Although these factors remain to be further investigated, one possibility is that two pore domain potassium channels (K 2 Ps) are the effectors in this pathway because inhibition of their activity leads to membrane depolarization similar to that observed in our study. This pathway mediates an immediate but mild depolarization that is not enough to trigger discharges.

In addition to enhance the neuronal excitability, TGFβ1 quickly increased [Ca 2+ ] i in pancreas-specific DRG neurons. Pretreatment of SB431542 largely inhibited or even completely blocked TGFβ1-induced [Ca 2+ ] i increase, indicating the involvement of TGFβ receptor I. Since TGFβ receptors (I and II) are serine-threonine kinase receptors that cannot act as ion channels downstream signaling and effectors are definitely necessary for TGFβ1-induced [Ca 2+ ] i increase. Recently studies indicate that TGFβ1 signaling sensitizes TRPV1 in primary sensory neurons. However, our result suggests that TGFβ1-induced [Ca 2+ ] i increase was independent of TRPV1. Of special note is that the proportion (16 of 55 neurons) of neurons displaying TGFβ1-induced firing is very similar to the proportion (32 of 114 neurons) of neurons showing [Ca 2+ ] i increase to TGFβ1, suggesting that TGFβ1-induced [Ca 2+ ] i increase might be due to the extracellular calcium influx through voltage-gated calcium channels (VGCCs). Nevertheless, our preliminary experiments show that neither pretreatment of CdCl 2 (a none selective VGCC antagonist) nor removal of the extracellular calcium inhibited TGFβ1-induced [Ca 2+ ] i increase (data not shown), suggesting that calcium release from intracellular calcium pools might be a main source of TGFβ1-induced [Ca 2+ ] i increase. Thus, another possibility mediating the acute effects of TGFβ1 is that calcium released from intracellular calcium pools as a second message opens one or more non-selective cation channels in direct or indirect ways, thus resulting in robust depolarization and discharges.

Although the detailed mechanisms have yet to be investigated, administration of TGFβ1 induced abdominal hyperalgesia dose- and time- dependently in health rats, confirming the involvement of acute effects of TGFβ1 in pancreatic nociception. Surprisingly, single administration of TGFβ1 at a high dose produced a persistent algogenic effect lasting for at least 48 hours. It is possible that excessive TGFβ1 may trigger some chronic effects. Our study does not exclude the importance of chronic effects of TGFβ1 in peripheral sensitization. Acute effects may be an initiating event and accumulation of acute effects probably contributes to the occurrence and development of chronic effects, and in return, chronic effects may amplify acute effects. Finally, both acute and chronic effects of TGFβ1 contribute to the peripheral sensitization in CP. The origin of TGFβ1 might be from mononuclear cells located in the fibrotic areas and ducts damaged by fibrosis in the pancreas of CP patients.

In conclusion, this study demonstrated for the first time that TGF β1 plays an acute effect on neuronal excitability, which might contribute to the pancreatic hyperalgesia in rats with chronic pancreatitis.

Read more as you like:http://www.cusabio.com/Recombinant-Protein/Recombinant-Natriuretic-peptides-A-for-Homo-sapiens-Human-11089619.html