Microinjection of orexin neuropeptide into the brainstem raphe pallidus elevates intraocular pressure.

Abstract

Purpose : Loss of vision in glaucoma results from damage to retinal ganglion cells that transmit visual signaling from the retina to the brain. Currently, intraocular pressure (IOP) is the only modifiable risk factor for glaucomatous vision loss. In our search for CNS targets for novel glaucoma therapies, we present data that extends the role of the orexin neurotransmitter system in the regulation of IOP. We have previously demonstrated a direct efferent connection from the orexin-rich cell bodies within the dorsomedial hypothalamus and perifornical area to the nucleus raphe pallidus (RP) and we now demonstrate that orexin signaling within the RP can stimulate a significant and sustained increase in IOP.

Methods : In 24 male SD adult rats, the rostral RP was targeted following craniotomy and injected with 75 nL of either the synthetic peptide of orexin A (OxA, 20-40 pmoles), orexin B (OxB, 30-90 pmoles), or saline vehicle. IOP, heart rate (HR), mean arterial pressure (MAP), and temperature were monitored prior to, and for an hour following, injection. AUC values for each response value were calculated for the period 5-45 min post-injection and compared by one-way ANOVA. Injection sites were verified histologically.

Results : Orexin injections outside the RP (n=9) did not significantly raise IOP (p=0.74). OxA (n=12), and OxB (n=7) when injected directly in the RP increased IOP in the rats from a baseline of approximately 10.4 +/- 0.1 mm Hg to a peak mean of approximately 14.9 (+/- 0.7) mm Hg. The onset of IOP rise was delayed approximately 10 min post-injection and was sustained for approximately 25 min with the peak occurring approximately 26 min following RP injection. For IOP, the AUC values for the OxA and OxB responses to RP injection were significantly greater than responses to saline vehicle (n=5) (p=0.008, p=0.035, respectively). These IOP responses were linearly dose-dependent for OxA (R²=0.34, p=0.014) and OxB (R²=0.64, p=0.002). RP injection of OxB (R²=0.78, p=0.008) but not OxA (p=0.18) significantly increased HR AUC values in a dose-dependent manner.

Conclusions : The orexin neurotransmitters applied to the rostral RP nucleus in the brain stem increased IOP and stimulated efferent sympathetic pressor responses. This is the first demonstration of orexin-mediated IOP regulation through the RP.

This is a 2020 ARVO Annual Meeting abstract.