Dose-response characteristics of intravenous ketamine on dissociative stereotypy, locomotion, sensorimotor gating, and nociception in male Sprague-Dawley rats

Highlights

• Intravenous (IV) ketamine is a valid administration route for rat behavior studies.

• Ketamine IV bolus induced hyper-locomotion, impaired PPI, and antinociception.

• Ketamine IV infusion induced hypo & hyper-locomotion and extended antinociception.

• A high-dose ketamine infusion reduced acoustic startle and impaired PPI.

• Plasma ketamine concentrations rapidly declined after bolus and infusion administration.

Abstract

Clinicians administer subanesthetic intravenous (IV) ketamine infusions for treatment of refractory depression, chronic pain, and post-traumatic stress disorder in humans. However, ketamine is administered via the subcutaneous (SC) or intraperitoneal (IP) routes to rodents in most pre-clinical research, which may limit translational application. The present study characterized the dose-response of a subanesthetic IV ketamine bolus (2 and 5 mg/kg) and 1-h infusion (5, 10, and 20 mg/kg/h) on dissociative stereotypy, locomotion, sensorimotor gating, and thermal nociception in male Sprague-Dawley rats. The secondary aim was to measure ketamine and norketamine plasma concentrations following IV ketamine bolus at 1, 20, and 50 min and at the conclusion of the 1-h infusion using liquid chromatography/mass spectrometry. The results showed that ketamine bolus and infusions produced dose-dependent dissociative stereotypy. Bolus (2 and 5 mg/kg) and 20 mg/kg/h infusion increased locomotor activity while 5 mg/kg/h infusion decreased locomotor activity. Both 10 and 20 mg/kg/h infusions reduced the acoustic startle reflex, while 5 mg/kg bolus and 20 mg/kg/h infusion impaired pre-pulse inhibition. Ketamine 5 mg/kg bolus and the 10 and 20 mg/kg/h infusions induced significant and prolonged antinociception to the hotplate test. Plasma concentrations of ketamine decreased quickly after bolus while norketamine levels increased from 1 to 20 min and plateaued from 20 to 50 min. The peak ketamine plasma concentrations [ng/ml] were similar between 5 mg/kg bolus [4100] vs. 20 mg/kg/h infusion [3900], and 2 mg/kg bolus [1700] vs. 10 mg/kg/h infusion [1500]. These results support the findings from previous ketamine injection studies and further validate the feasibility of administering subanesthetic doses of IV ketamine infusion to rats for neuropharmacological studies.

Introduction

Ketamine, a non-competitive antagonist at the glutamatergic N-methyl-d-aspartate (NMDA) receptor, is a potent analgesic and dissociative anesthetic with many applications in both rodent behavioral and human psychiatric research. Pre-clinical researchers utilize ketamine in various rodent models of psychosis, depression, addiction, and pain while clinicians administer ketamine for treatment of refractory depression, post-traumatic stress disorder (PTSD), and chronic pain. Ketamine is typically administered via subcutaneous (SC) or intraperitoneal (IP) injection to rodents, but these delivery routes may not translate well to human studies and treatments that primarily use continuous intravenous (IV) ketamine infusions. Previous research has focused on evaluating the effects of IP and SC ketamine on rodent behaviors, but little is known regarding the dose-response characteristics of subanesthetic doses of ketamine administered by the IV route.

Dose-dependent changes in locomotion and stereotypic dissociative-like behaviors in rodents are observed following subanesthetic ketamine injections. At higher doses, ketamine induces dose-dependent hyper-locomotion (Danysz et al., 1994, Hetzler and Swain Wautlet, 1985, Imre et al., 2006) through increased dopamine turnover in limbic-striatal regions (Jentsch et al., 1998, McCullough and Salamone, 1992, Steinpreis and Salamone, 1993). Interestingly, several studies have observed reduced locomotion following low-dose ketamine injections (Becker et al., 2003, Kotermanski et al., 2013, Trujillo et al., 2011). Phencyclidine (PCP) and ketamine not only impact locomotion, but also produce other characteristic changes in rodent behavior. Commonly described stereotypic behaviors include circling, side to side head motion, reduced rearing, ataxia, and reduced fine motor movements such as grooming (Danysz et al., 1994, Hetzler and Swain Wautlet, 1985, Kotermanski et al., 2013, Sturgeon et al., 1979). Similar to hyper-locomotion, dissociative stereotypy is mediated through dopaminergic pathways in striatal regions (Castellani and Adams, 1981b, Finnegan et al., 1976, Glick et al., 1976, Nabeshima et al., 1983).

In addition to stereotypy and locomotion changes, ketamine impacts sensorimotor gating in both humans and rodents (Abel et al., 2003, Geyer et al., 2001). Pre-pulse inhibition (PPI), a measure of sensorimotor gating and an indirect measure of information processing, occurs when a weak stimulus precedes a startle stimulus and reduces the acoustic startle reflex (ASR). Ketamine-induced disruption of PPI is well-described (Geyer et al., 2001) and is used to model schizophrenia-like behaviors in rodent psychosis models; however, the effects of ketamine on the ASR are inconsistent. Most investigators show ketamine to have either no effect or a non-significant reduction in ASR (de Bruin et al., 1999, Imre et al., 2006, Palenicek et al., 2011), while others report an increase in startle response (Cilia et al., 2007).

Ketamine, a potent analgesic in humans (Prommer, 2012), produces dose-dependent antinociception in rodents at high doses (Hoffmann et al., 2003, Smith et al., 1980). Ketamine mediates antinociception via central and peripheral mechanisms primarily through NMDA antagonism and partially through opioid receptor agonism (Gupta et al., 2011, Pacheco et al., 2014). Although researchers have described dose-dependent increases in hotplate and tail-flick latencies in rodents (Sarton et al., 2001, Smith et al., 1980), there are limited reports that describe the duration of antinociception following ketamine administration. The impact of ketamine on nociception over time provides useful information to researchers regarding dose-response characteristics and the duration required to determine optimum antinociceptive doses in rodents.

Overall, there is an abundance of literature characterizing rodent behaviors, information processing, and antinociception following ketamine injections, but there is little information describing the dose-response characteristics of subanesthetic IV ketamine and corresponding drug plasma concentrations in rodents. Therefore, our primary aim was to characterize the effects of subanesthetic IV ketamine administered as a bolus and continuous infusion on dissociative stereotypy, locomotor activity, ASR, PPI, and nociception in male Sprague-Dawley rats. Our secondary aim was to measure plasma concentrations of ketamine and norketamine, an active metabolite, following IV bolus at 1, 20, and 50 min and at the completion of a 1-h infusion to provide a comparison to previous rodent ketamine studies that used SC or IP injections. Based upon the previous literature, we hypothesized that administration of subanesthetic IV ketamine may dose-dependently increase dissociative stereotypy, locomotor activity, and antinociception while impairing PPI in rats.