Effects of a Subanesthetic Ketamine Infusion on Inflammatory and Behavioral Outcomes after Closed Head Injury in Rats

Abstract

Traumatic brain injury (TBI) affects millions of people annually, and most cases are classified as mild TBI (mTBI). Ketamine is a potent trauma analgesic and anesthetic with anti-inflammatory properties. However, ketamine’s effects on post-mTBI outcomes are not well characterized. For the current study, we used the Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA), which replicates the biomechanics of a closed-head impact with resulting free head movement. Adult male Sprague–Dawley rats sustained a single-session, repeated-impacts CHIMERA injury. An hour after the injury, rats received an intravenous ketamine infusion (0, 10, or 20 mg/kg, 2 h period), during which locomotor activity was monitored. Catheter blood samples were collected at 1, 3, 5, and 24 h after the CHIMERA injury for plasma cytokine assays. Behavioral assays were conducted on post-injury days (PID) 1 to 4 and included rotarod, locomotor activity, acoustic startle reflex (ASR), and pre-pulse inhibition (PPI). Brain tissue samples were collected at PID 4 and processed for GFAP (astrocytes), Iba-1 (microglia), and silver staining (axonal injury). Ketamine dose-dependently altered locomotor activity during the infusion and reduced KC/GRO, TNF-α, and IL-1β levels after the infusion. CHIMERA produced a delayed deficit in rotarod performance (PID 3) and significant axonal damage in the optic tract (PID 4), without significant changes in other behavioral or histological measures. Notably, subanesthetic doses of intravenous ketamine infusion after mTBI did not produce adverse effects on behavioral outcomes in PID 1–4 or neuroinflammation on PID 4. A further study is warranted to thoroughly investigate beneficial effects of IV ketamine on mTBI given multi-modal properties of ketamine in traumatic injury and stress.

Effects of Mild Closed-Head Injury and Subanesthetic Ketamine Infusion on Microglia, Axonal Injury, and Synaptic Density in Sprague–Dawley Rats

“…Repetitive mTBI (rmTBI) using the CHIMERA paradigm over multiple injury days produced axonal injury in the OT of rodents [11,17,42,43]. Moreover, repetitive mTBI in a single session also produced axonal injury in the OT of rats [7]. These results suggest that mTBI produces white-matter damage such as axonal injury in the visual system of rodents.…”

“…The consequences of primary injury such as axonal damage are generally regarded as irreversible, but secondary injury such as neuroinflammatory processes may be a target for mTBI treatment [5]. Ketamine, a multimodal drug used in traumatically injured patients, has anti-inflammatory [7] and synaptogenic effects [8] and may even reverse synaptic deficits induced by TBI [9]. However, very few preclinical studies have examined the utility of intravenous (IV) ketamine infusion in ameliorating or preventing secondary injury after mTBI.…”

“…Ketamine, a non-competitive N-methyl-D-aspartate receptor (NMDAR) antagonist, is used as a multi-modal trauma analgesic because it does not produce respiratory depression or cardiovascular instability at sub-anesthetic doses [18][19][20]. The multi-modal properties of ketamine, including potential anti-inflammatory effects [7], may be beneficial for the treatment of TBI. Ketamine has been shown to be neuroprotective in preclinical models of post-traumatic stress disorder (PTSD), major depression, and TBI [9,[21][22][23][24].…”

“…However, no studies to date have examined synaptic density following a clinically relevant IV ketamine infusion after mTBI. We have established a clinically relevant paradigm of IV ketamine infusion in awake and freely moving rats [25,26] and combined this technique with CHIMERA to investigate the effects of mTBI and ketamine on inflammatory cytokines and behavioral outcomes [7]. Though IV ketamine infusion reduced certain inflammatory cytokines regardless of the injury condition, its effects on other injury outcomes, such as microglial morphology and synaptic density, are largely unknown.…”

“…The optic tract (OT), a continuation of the optic nerve, may be particularly vulnerable to TBI, as its multiplanar geometry in rodents [41] may introduce ample opportunities for impaction, shearing, and stretch injuries. Axonal injury in the OT is well established in rodent mTBI studies [7,11,16,17,42,43] and can be accompanied by impaired visual acuity and visual function [42,43]. Repetitive mTBI (rmTBI) using the CHIMERA paradigm over multiple injury days produced axonal injury in the OT of rodents [11,17,42,43].…”