Pavlovian conditioning is commonly used to investigate the mechanisms of fear learning. Because the Wistar-Kyoto (WKY) rat strain is particularly stress-sensitive, we investigated the effects of a psychological stressor on sleep in WKY compared to Wistar (WIS) rats. Male WKY and WIS rats were either fear-conditioned to tone cues or received electric foot shocks alone. In the fearconditioning procedure, animals were exposed to 10 tones (800 Hz, 90 dB, 5 sec), each coterminating with a foot shock (1.0 mA, 0.5 sec), at 30-sec intervals. In the shock stress procedure, animals received 10 foot shocks at 30-sec intervals, without tones. All subjects underwent a toneonly test both 24 hrs (Day 1) and again two weeks (Day 14) later. Rapid eye movement sleep (REMS) continuity was investigated by partitioning REMS episodes into single (inter-REMS episode interval > 3 min) and sequential (interval ≤ 3 min) episodes. In the fear-conditioned group, freezing increased from baseline in both strains, but the increase was maintained on Day 14 in WKY rats only. In fear-conditioned WKY rats, total REMS amount increased on Day 1, sequential REMS amount increased on Day 1 and Day 14, and single REMS amount decreased on Day 14. Alterations were due to changes in the number of sequential and single REMS episodes. Shock stress had no significant effect on REMS microarchitecture in either strain. The shift toward sequential REMS in fear-conditioned WKY rats may represent REMS fragmentation, and may provide a model for investigating the neurobiological mechanisms of sleep disturbances reported in posttraumatic stress disorder.

COVID-19 is a potentially fatal infection caused by the SARS-CoV-2 virus. The SARS-CoV-2 3CL protease (Mpro) is a viral enzyme essential for replication and is the target for nirmatrelvir. Paxlovid (nirmatrelvir co-administered with the pharmacokinetic enhancer ritonavir) showed efficacy in COVID-19 patients at high risk of progressing to hospitalization and/or death. Nonclinical safety studies with nirmatrelvir are essential in informing benefit-risk of Paxlovid and were conducted to support clinical development. In vivo safety pharmacology assessments included a nervous system/pulmonary study in rats and a cardiovascular study in telemetered monkeys. Potential toxicities were assessed in repeat dose studies of up to 1 month in rats and monkeys. Nirmatrelvir administration (1,000 mg/kg, p.o.) to male rats produced transient increases in locomotor activity and respiratory rate but did not affect behavioral endpoints in the functional observational battery. Cardiovascular effects in monkeys were limited to transient increases in blood pressure and decreases in heart rate, observed only at the highest dose tested (75 mg/kg per dose b.i.d; p.o.). Nirmatrelvir did not prolong QTc-interval or induce arrhythmias. There were no adverse findings in repeat dose toxicity studies up to 1 month in rats (up to 1,000 mg/kg daily, p.o.) or monkeys (up to 600 mg/kg daily, p.o.). Nonadverse, reversible clinical pathology findings without clinical or microscopic correlates included prolonged coagulation times at ≥60 mg/kg in rats and increases in transaminases at 600 mg/kg in monkeys. The safety pharmacology and nonclinical toxicity profiles of nirmatrelvir support clinical development and use of Paxlovid for treatment of COVID-19.

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The peripheral nervous system (PNS) is subject to a wide range of structural and functional insults including direct damage to axons, loss of myelin, and progressive deficits in saltatory conduction. Drugs that damage the PNS often result in neuropathies that impact the structure and function of targeted nerves. In most cases, both sensory and motor neurons are affected with damage initially evident in the distal extremities. Drug-induced neuropathies are potentially reversible following cessation of treatment, but early stages of neuropathy can be subclinical and asymptomatic making diagnosis difficult. Nerve biopsy is highly validated and provides definitive evidence of nerve injury and corresponding severity; however, it is limited in some respects and electrophysiological measures can complement histopathological assessments and provide a functional measure of potential toxicity. In a drug development setting, nerve conduction assessments are valuable to monitor nerve function longitudinally if nerve damage is suspected or confirmed, and importantly, can be used to monitor progression and/or recovery of a drug-induced neuropathy. This review will summarize the methodology used in nerve conduction assessments as well as discuss data interpretation and considerations for use in nonclinical species. Finally, the use of nerve conduction assessments in nonclinical drug development is discussed.

Recent investigations of rapid eye movement sleep (REMS) continuity have emphasized the importance of transitions both into and out of REMS. We have previously reported that, compared to Wistar rats (WIS), Wistar-Kyoto rats (WKY) responded to fear conditioning (FC) with more fragmented REMS. Gamma oscillations in the electroencephalogram (EEG) are synchronized throughout the brain in periods of focused attention, and such synchronization of cell assemblies in the brain may represent a temporal binding mechanism. Therefore, we examined the effects of FC on EEG gamma range activity (30–50 Hz) at REMS transitions in WKY compared to WIS. Relative power in the gamma range (measured as a percent of total power) at baseline and upon re-exposure to the fear-inducing conditioning stimulus was measured 35 s before REMS onset to 105 s after REMS onset (ARO) and 85 s before REMS termination (BRT) to 35 s after REMS termination. After baseline recording, rats received ten tones, each co-terminating with an electric foot shock. On Days 1 and 14 post-conditioning, rats were re-exposed to three tones. Fast-Fourier transforms created power spectral data in the gamma frequency domain. Relative power was extracted from an average of 4–5 REMS transitions. Relative gamma power was always higher in WIS. On Day 14, at 15 s and 25 s ARO, WKY had significant increases in relative gamma power from Baseline. WIS had a significant increase on Day 1 at 25 s ARO. Despite the increases in relative gamma power, WKY never achieved levels attained by WIS. Moreover, at 5 s BRT, only WKY had a significant decrease in relative gamma power from Baseline to Day 14. Gamma range activity may indicate neural activity underlying maintenance of REMS continuity. Low relative gamma power at REMS transitions may be associated with increased REMS fragmentation in WKY after FC.