## 1 Introduction  
Peripheral nerve injury can arise from systemic diseases or localized damage; however, unlike the central nervous system (CNS), the peripheral nervous system (PNS) displays a high regenerative capacity [1]. This is mainly driven by Schwann cells, the myelinating glia of the PNS, which display high plasticity following peripheral nerve injury. Upon nerve injury, Schwann cells are spontaneously reprogrammed to switch from a myelinating phenotype to a progenitor-like state that is able to promote nerve regeneration [2]. Part of this process includes the secretion of proteases such as plasminogen activators, whose proteolytic activities clear any residual cell debris and altered extracellular matrix, creatinga clear nerve conduit to allow axon regeneration [3, 4]. The two major plasminogen activators involved in this process are the tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA) [3].  
There is evidence that the neuropeptide pituitary adenylate cyclase-activating peptide (PACAP) is also involved in peripheral nerve regeneration [5, 6]. PACAP is widely distributed throughout the central and peripheral nervous systems and is associated with a myriad of biological functions including antiapoptotic, neuroprotective and antiinflammatory [7, 8, 9, 10]. The activity of PACAP is mediated by three G protein-coupled receptors (GPCRs) PAC1, VPAC1 and VPAC2. Whereas PACAP binds with a high affinity to these three receptors, it shows a much higher affinity (>100-fold) for the PAC1 receptor [11]. As such, the PACAP preferring receptor PAC1 is thought to mediate most of the antiapoptotic and neuroprotective effects of PACAP. Based on these findings, the PACAP/PAC1 axis has been investigated as a potential target for the treatment of neurological diseases, including Alzheimer's disease [12], stroke [13], headaches (using PAC1 antagonists) [14] and other neurological disorders (reviewed in [15, 16, 17, 18]).  
Emerging studies suggest that the PAC1 receptor might promote the regeneration and functional recovery following peripheral nerve injury, as some reports have shown that both PACAP and PAC1 are upregulated after nerve injury [19]. Moreover, it has been demonstrated that axon regeneration is strongly reduced in PACAP knockout animals [20]. In vitro, we have previously demonstrated that RT4 Schwann cell-like cultures express both PACAP and PAC1, and stimulation of these cells with PACAP promoted tPA and uPA expression and activity [21, 22]. Additionally, we demonstrated that BDNF mimicked PACAP-induced up-regulation of tPA expression and activity in these cell lines [22].  
Recently, it has been shown that doxycycline and minocycline act as positive allosteric modulators (PAMs) of the PAC1 receptor [23]. Doxycycline and minocycline are tetracyclines, broad spectrum antibiotics that share similar structures and functions [24]. Both tetracyclines have shown promising potential as therapeutic targets for neurological diseases due to their high lipophilic nature that allows them to readily cross the blood-brain barrier and other biological membranes [25]. Both doxycycline and minocycline have been shown to exert neuroprotection and anti-inflammatory activities in a range of neurological pathologies, similarly to PACAP [25, 26, 27, 28, 29]. Most importantly, it has been shown that the neuroprotective activity of doxycycline is enhanced by the activation of the PAC1 receptor [30].  
In the present study, we investigated the novel role of doxycycline and minocycline as potential PAMs of PAC1 receptor in Schwann cell lines. To achieve this goal, we tested if exogenous administration of either tetracycline would induce tPA and/or uPA expression and activity in a similar way as exogenous PACAP. In addition, we assessed if PAC1 receptor blockade prevented these effects in RT4 Schwann cell-like cultures.