Purpose: To evaluate the effect of intravitreal injection of N-methyl-D-aspartate (NMDA) on brain-derived neurotrophic factor (BDNF), pituitary adenylate cyclase-activating peptide-38 (PACAP-38), vasoactive intestinal peptide (VIP) and the VIP-associated glial protein activity-dependent neuroprotective protein (ADNP) in the rat retina. These elements have well-documented neuroprotective properties and may thus be integrated in endogenous neuroprotective mechanisms in the retina which break down in NMDA excitotoxicity. Methods: A volume of 2 μl of 100 nmol NMDA was intravitreally injected into one eye of rats, the untreated eye served as a control. Time-dependent effects of NMDA on VIP, PACAP-38 and BDNF were detected by radioimmunoassay and ELISA, and the effect on the expression of VIP, PACAP-38 and ADNP was evaluated by quantitative RT-PCR 20 days after NMDA injection. Topical flunarizine served to find out whether the effect of NMDA is counteracted. Results: Compared to PACAP-38, VIP levels significantly decreased on days 1, 7, 14, 28 and 56 after NMDA injection indicating that VIPergic cells are more vulnerable than PACAP-38-expressing cells. The expression of VIP and ADNP but not of PACAP-38 was found to be reduced, and application of topical flunarizine counteracted the decrease of VIP. BDNF levels significantly increased after days 1 and 3. Conclusion: The early upregulation of BDNF seems to act neuroprotectively and leads to a delay of ganglion cell loss. Although there is no direct evidence, the decrease of VIP and ADNP – the consequence of the presence of NMDA receptors on these peptide-expressing cells – might contribute to the breakdown of endogenous neuroprotective mechanisms given that the decrease of the VIP-related ADNP runs in parallel with the decrease of VIP. Activating and maintaining these mechanisms must be the primary aim in the therapy of diseases with retinal neuronal degeneration.

The chromogranin A-derived peptide catestatin has previously been shown to be a constituent of the sensory innervation of the eye and dental pulp. The antibody used in these studies not only recognizes authentic catestatin but also larger molecules containing the sequence of catestatin. Therefore, the authors now aimed to explore whether there are additional molecular forms present in the rat skin, rat trigeminal ganglion and rat dorsal root ganglia apart from free catestatin. Western blot analysis was performed in these tissues and and double immunofluorescence techniques in rat dorsal root ganglia. A very prominent band was found at approximately 50 kDa in the skin representing a middle-sized fragment containing the sequence of this peptide. Recently, the same result has been obtained in the dental pulp indicating that this fragment might represent a main molecular form in various other tissues of the body. It is likely of sensory nature since the band was also present of weaker intensity both in the trigeminal ganglion and dorsal root ganglia and since abundant cells expressed catestatin-like immunoreactivity in rat dorsal root ganglia. Another small sized fragment of weaker intensity was found in these tissues as well. No band was observed in the position of intact chromogranin A in the Western blot. Chromogranin A is proteolytically processed in sensory ganglia to at least two fragments containing the sequence of catestatin which are then anterogradely transported to the skin and the 50 kDa fragment obviously becomes aggregated there in nerve endings.