TRPV1 Tunes Optic Nerve Axon Excitability in Glaucoma

The transient receptor potential vanilloid member 1 (TRPV1) in the central nervous system may contribute to homeostatic plasticity by regulating intracellular Ca²⁺, which becomes unbalanced in age-related neurodegenerative diseases, including Alzheimer’s and Huntington’s. Glaucomatous optic neuropathy – the world’s leading cause of irreversible blindness – involves progressive degeneration of retinal ganglion cell (RGC) axons in the optic nerve through sensitivity to stress related to intraocular pressure (IOP). In models of glaucoma, genetic deletion of TRPV1 (Trpv1^–/–) accelerates RGC axonopathy in the optic projection, whereas TRPV1 activation modulates RGC membrane polarization. In continuation of these studies, here, we found that Trpv1^–/– increases the compound action potential (CAP) of optic nerves subjected to short-term elevations in IOP. This IOP-induced increase in CAP was not directly due to TRPV1 channels in the optic nerve, because the TRPV1-selective antagonist iodoresiniferatoxin had no effect on the CAP for wild-type optic nerve. Rather, the enhanced CAP in Trpv1^–/– optic nerve was associated with increased expression of the voltage-gated sodium channel subunit 1.6 (NaV1.6) in longer nodes of Ranvier within RGC axons, rendering Trpv1^–/– optic nerve relatively insensitive to NaV1.6 antagonism via 4,9-anhydrotetrodotoxin. These results indicate that with short-term elevations in IOP, Trpv1^–/– increases axon excitability through greater NaV1.6 localization within longer nodes. In neurodegenerative disease, native TRPV1 may tune NaV expression in neurons under stress to match excitability to available metabolic resources.

Progression and Pathology of Traumatic Optic Neuropathy From Repeated Primary Blast Exposure

Indirect traumatic optic neuropathy (ITON) is a condition that is often associated with traumatic brain injury and can result in significant vision loss due to degeneration of retinal ganglion cell (RGC) axons at the time of injury or within the ensuing weeks. We used a mouse model of eye-directed air-blast exposure to characterize the histopathology of blast-induced ITON. This injury caused a transient elevation of intraocular pressure with subsequent RGC death and axon degeneration that was similar throughout the length of the optic nerve (ON). Deficits in active anterograde axon transport to the superior colliculus accompanied axon degeneration and first appeared in peripheral representations of the retina. Glial area in the ON increased early after injury and involved a later period of additional expansion. The increase in area involved a transient change in astrocyte organization independent of axon degeneration. While levels of many cytokines and chemokines did not change, IL-1α and IL-1β increased in both the ON and retina. In contrast, glaucoma shows distal to proximal axon degeneration with astrocyte remodeling and increases in many cytokines and chemokines. Further, direct traumatic optic neuropathies have a clear site of injury with rapid, progressive axon degeneration and cell death. These data show that blast-induced ITON is a distinct neuropathology from other optic neuropathies.

Rapid Repeat Exposure to Subthreshold Trauma Causes Synergistic Axonal Damage and Functional Deficits in the Visual Pathway in a Mouse Model

We examined the effect of repeat exposure to a non-damaging insult on central nervous system axons using the optic projection as a model. The optic projection is attractive because its axons are spatially separated from the cell bodies, it is easily accessible, it is composed of long axons, and its function can be measured. We performed closed-system ocular neurotrauma in C57Bl/6 mice using bursts of 15 or 26-psi (pounds per square inch) overpressure air that caused no gross damage. We quantified the visual evoked potential (VEP) and total and degenerative axons in the optic nerve. Repeat exposure to a 15-psi air blast caused more axon damage and vision loss than a single exposure to a 26-psi air blast. However, an increased VEP latency was detected in both groups. Exposure to three 15-psi air blasts separated by 0.5 sec caused 15% axon degeneration at 2 weeks. In contrast, no axon degeneration above sham levels was detected when the interinjury interval was increased to 10 min. Exposure to 15-psi air blasts once a day for 6 consecutive days caused 3% axon degeneration. Therefore, repeat mild trauma within an interinjury interval of 1 min or less causes synergistic axon damage, whereas mild trauma repeated at a longer interinjury interval causes additive, cumulative damage. The synergistic damage may underlie the high incidence of traumatic brain injury and traumatic optic neuropathy in blast-injured service members given that explosive blasts are multiple injury events that occur in a very short time span. This study also supports the use of the VEP as a biomarker for traumatic optic neuropathy.

Reduced frequency prenatal visits in midwifery practice: attitudes and use

Recent research supports the use of reduced frequency prenatal visit schedules (RFVS) for women of low obstetric risk. However, for the RFVS to be widely adopted for use in practice, health care providers must implement and support its use. The purpose of this study was to explore midwives' attitudes toward and use of reduced frequency prenatal care visit schedules for the care of low-risk women. A descriptive, correlational study was conducted at the 1999 Annual Meeting of the American College of Nurse-Midwives with completed surveys received from 234 midwives. Seventy-two percent (n = 170) responded that they were familiar with the reduced frequency visit schedule. Of those, 71% agreed that they could give effective prenatal care by using reduced frequency scheduling, although few (17%) reported using it in practice. Significant differences were found between the midwives who were familiar versus those who were unfamiliar with the visit schedule in their perceptions for five central themes: 1) quality of care of the RFVS, 2) women's empowerment or self-care with the RFVS, 3) ability to manage practice, 4) patient satisfaction, and 5) barriers to the use of RFVS. Providers' responses to the use of RFVS have been mixed. Successful integration of this schedule into prenatal care services may require more than knowledge of its safety for low-risk women. Careful selection of women for whom the schedule is appropriate and a commitment from midwives to tailor prenatal care to the individual women's needs is indicated. Further research is also needed to evaluate the barriers that prevent midwives from using a reduced frequency visit schedule for the prenatal care of low-risk clients.

Evidence-based prenatal care visits: when less is more

Traditionally, low-risk pregnant women in the United States who participate in prenatal care have been scheduled for approximately 14-16 prenatal visits, which is the schedule recommended by the American College of Obstetricians and Gynecologists. In 1989, an expert panel convened by the United States Department of Health and Human Services proposed a reduced frequency prenatal visit schedule for low-risk, healthy women based on the timing of specific tests or events that occur in pregnancy. Available evidence shows no adverse effect on maternal or neonatal outcomes for low-risk pregnant women who follow a reduced visit schedule, making it a highly important consideration for pregnant women and their health care providers. Other important aspects of prenatal care, especially related to adequacy and content, will be explored in-depth in a future segment of this series on evidence-based prenatal care.