A protocol for use of medetomidine anesthesia in rats for extended studies using task-induced BOLD contrast and resting-state functional connectivity

Abstract

The α2-adrenoreceptor agonist, medetomidine, which exhibits dose-dependent sedative effects and is gaining acceptance in small-animal functional magnetic resonance imaging (fMRI), has been studied. Rats were examined on the bench using the classic tail-pinch method with three infusion sequences: 100 μg/kg/hr, 300 μg/kg/hr, or 100 μg/kg/hr followed by 300 μg/kg/hr. Stepping the infusion rate from 100 to 300 μg/kg/hr after 2.5 hours resulted in a prolonged period of approximately level sedation that cannot be achieved by a constant infusion of either 100 or 300 μg/kg/hr. By stepping the infusion dosage, experiments as long as six hours are possible. Functional MRI experiments were carried out on rats using a frequency dependent electrical stimulation protocol—namely, forepaw stimulation at 3, 5, 7, and 10 Hz. Each rat was studied for a four-hour period, divided into two equal portions. During the first portion, rats were started at a 100 μg/kg/hr constant infusion. During the second portion, four secondary levels of infusion were used: 100, 150, 200, and 300 μg/kg/hr. The fMRI response to stimulation frequency was used as an indirect measure of modulation of neuronal activity through pharmacological manipulation. The frequency response to stimulus was attenuated at the lower secondary infusion dosages 100 or 150 μg/kg/hr but not at the higher secondary infusion dosages 200 or 300 μg/kg/hr. Parallel experiments with the animal at rest were carried out using both electroencephalogram (EEG) and functional connectivity MRI (fcMRI) methods with consistent results. In the secondary infusion period using 300 μg/kg/hr, resting-state functional connectivity is enhanced.

Rat brains also have a default mode network

“…It was recently introduced for longitudinal functional MRI and resting state fMRI studies in rodents (15,52). However, the sedative effect of medetomidine is reduced with constant infusion of medetomidine alone, and a stepwise dose increase has been suggested to maintain stable sedation (17). In pilot studies, we used a combination of low-dose isoflurane (0.25%) and dexmedetomidine (0.03 mg/kg i.v.…”

Dynamic Properties of Functional Connectivity in the Rodent

“…Five minutes after the bolus, isoflurane was discontinued, and 15 min later, a subcutaneous infusion of 0.05 mg/kg/h dexmedetomidine was initiated to maintain anesthesia for the duration of the experiment (Weber et al, 2006). Approximately 80 min after the initial dexmedetomidine bolus, the infusion dosage was increased to 0.15 mg/kg/h (3 · initial infusion rate) for maintaining anesthetic depth, in accordance with the protocol established in (Pawela et al, 2009). …”

Anesthesia and the Quantitative Evaluation of Neurovascular Coupling

“…However, in rats under urethane anesthesia (1.1 g/kg), electroencephalogram activity was shown to be stable for a prolonged time (8 to 12 hours) (Lincoln, 1969). To maintain steady electroencephalogram and fMRI responses with rats receiving medetomidine infusion (0.1 to 0.3 mg/kg per hour), the infusion rate needs to be adjusted over time due to potential pharmacokinetic changes in long-term experiments ( > 3 hours) (Pawela et al, 2009). Moreover, functional connectivity examined with blood oxygen level-dependent fMRI was well localized in rats anesthetized with either a-chloralose (27 mg/kg per hour) or medetomidine (0.1 mg/kg per hour) but less so under isoflurane (2%) (Williams et al, 2010).…”

“…The a2-adrenoreceptor was also known to significantly affect cardiovascular function and depress respiratory function (Sinclair, 2003). Animals can recover after the administration of a reversible a2-antagonist, making this agent also suitable for use in repeated longitudinal imaging experiments (Pawela et al, 2009;Weber et al, 2006). ( …”