Neuroprotective effects of growth hormone releasing hormone (GHRH) in traumatic optic neuropathy

Abstract

Purpose: We have previously shown that the hypothalamic secretagogue hormone peptide, growth hormone releasing hormone (GHRH), and its receptors (GHRH-R) are expressed in the retina and that a GHRH agonist prevents diabetes-induced retinal neurovascular injury. Here we have further explored GHRH neuroprotective properties by assessing the effects of the GHRH agonist (JI-34) in a mouse model of traumatic optic neuropathy (TON).

Methods: C57/Bl6J mice were subjected to unilateral optic nerve crush (ONC) injury performed in the left eye, with the contralateral eye serving as a control. A selected group of ONC mice were treated with 5mg/kg of the GHRH agonist JI-34, which received daily subcutaneous injections starting at day 1 post-injury. Both groups were subjected to spectral domain optical coherence tomography (SD-OCT) and scotopic threshold response of electroretinography (STR-ERG) to evaluate retinal structural and functional changes, respectively at 7d and 21d post ONC. Immunohistochemical and Western blot analyses were conducted to assess levels of NOGO-A, a marker for axonal regeneration. Retinal ganglion cell (RGC) counting was performed by immunohistochemical analysis of NeuN and Brnd3a (RGC markers) on flat mounts of extracted retinas. Specific detection and quantitation of apoptotic cells were done using The DeadEnd™ Fluorometric TUNEL System.

Results: GHRH agonist JI-34 prevented retinal ganglion cell loss following ONC at both 7 and 21 days post-injury. SD-OCT analysis showed a preservation of the nerve fiber layer thickness in JI-34 ONC mice. TUNEL assay showed a drastic reduction in the number of detectable apoptotic cells within the inner retina in ONC-mice given the GHRH agonist. Increased NOGO-A immunoreactivity was evident at sites proximal to the lesion and NOGO-A protein levels were increased in ONC injured eyes. GHRH-agonist treated ONC mice, however, displayed a significant decrease in immunopositive areas of NOGO-A as well as a global decrease in protein levels.

Conclusions: These results demonstrate GHRH neuroprotective effects in the injured retina and suggest the use of GHRH agonists as novel therapeutic agents for acute and chronic neurodegenerative retinal diseases.