Posture effects on spontaneous limb movements, alternated stepping, and the leg extension response in neonatal rats.

The development of postural control is considered an important factor for the expression of coordinated behavior such as locomotion. In the natural setting of the nest, newborn rat pups adapt their posture to perform behaviors of ecological relevance such as those related to suckling. The current study explores the role of posture in the expression of three behaviors in the newborn rat: spontaneous limb activity, locomotor-like stepping behavior, and the leg extension response (LER). One-day-old rat pups were tested in one of two postures--prone or supine--on each of these behavioral measures. Results showed that pups expressed more spontaneous activity while supine, more stepping while prone, and no differences in LER expression between the two postures. Together these findings show that posture affects the expression of newborn behavior patterns in different ways, and suggest that posture may act as a facilitator or a limiting factor in the expression of different behaviors during early development.

Sensorimotor training during expression of the leg extension response (LER) in 1-day-old rats.

In newborn rats, the leg extension response (LER) is a coordinated hyperextension of the hindlimbs that is shown in response to anogenital stimulation. Here we examined the influence of sensorimotor training on LER expression in postnatal day 1 rats. In Experiment 1, we examined if proprioceptive feedback facilitates LER expression. We did this by repeatedly stimulating the pup's anogenital region with a vibrotactile device, to experimentally evoke the LER, thus increasing LER-relevant hindlimb proprioceptive feedback during training. In trained subjects, the LER was evoked every 4 min for 15 trials, followed by a final LER test. Results indicated that proprioceptive feedback on its own did not alter later expression of the LER. In Experiment 2, we examined the effect of both proprioceptive and cutaneous feedback on LER expression, through the use of a range of motion (ROM) restriction during training. During ROM restriction, a Plexiglas plate was placed beneath the pup at 50% of limb length. After the 15th training trial, a final LER test occurred with no ROM restriction in place. Compared to controls, pups that experienced ROM restriction exhibited a significantly shorter LER duration, and smaller hip and ankle angles during the LER test (indicating greater limb flexion). Together these findings show that concurrent proprioceptive and cutaneous feedback, but not proprioceptive feedback alone, has persistent effects on expression of this newborn action pattern.

Prematurely delivered rats show improved motor coordination during sensory-evoked motor responses compared to age-matched controls.

The amount of postnatal experience for perinatal rats was manipulated by delivering pups one day early (postconception day 21; PC21) by cesarean delivery and comparing their motor behavior to age-matched controls on PC22 (the typical day of birth). On PC22, pups were tested on multiple measures of motor coordination: leg extension response (LER), facial wiping, contact righting, and fore- and hindlimb stepping. The LER and facial wiping provided measures of synchronous hind- and forelimb coordination, respectively, and were sensory-evoked. Contact righting also was sensory-evoked and provided a measure of axial coordination. Stepping provided a measure of alternated forelimb and hindlimb coordination and was induced with the serotonin receptor agonist quipazine. Pups that were delivered prematurely and spent an additional day in the postnatal environment showed more bilateral limb coordination during expression of the LER and facial wiping, as well as a more mature righting strategy, compared to controls. These findings suggest that experience around the time of birth shapes motor coordination and the expression of species-typical behavior in the developing rat.

Effectiveness of topical anesthetics on reducing tactile sensitivity in the paws of newborn rats.

The aim of this study was to evaluate the effectiveness of three local, topical anesthetics on touch response thresholds of the paws of 1-day-old rats. Touch response thresholds were measured using Semmes Weinstein monofilaments after treatment of the paws with EMLA (2.5% lidocaine and 2.5% prilocaine), alcaine (.5% proparacaine), triocaine (20% benzocaine, 6% lidocaine, and 4% tetracaine), or petroleum jelly (treatment control). Touch thresholds significantly increased after treatment with EMLA 18% of the time, and there was no evidence of a systemic effect. Touch thresholds were not significantly altered after treatment with alcaine, triocaine, or petroleum jelly. Therefore, EMLA appears to be a slightly effective topical anesthetic for reducing tactile sensitivity in newborn rats.

Sensory feedback modulates quipazine-induced stepping behavior in the newborn rat.

Research has shown that sensory feedback modulates locomotor behavior in intact as well as spinal adult animals. Here we examined if locomotor activity ("stepping") in newborn rats is influenced by cutaneous and proprioceptive feedback. One-day-old rats were treated with the serotonergic receptor agonist quipazine (3.0mg/kg) to induce air-stepping behavior or with saline (vehicle control). During stepping, a substrate/floor (elastic, stiff, or none) was placed beneath their limbs so that the feet could make plantar surface contact with a substrate. Pups treated with quipazine showed significantly more alternated fore- and hindlimb steps and plantar paw contact with the substrate, compared to pups treated with saline. Pups also made proportionately less contact with the stiff substrate versus the elastic substrate during stepping. Different types of movements made on the substrate (paw pushes, taps, swipes, and stances) were also characterized. These results indicate that sensory feedback modulates locomotor mechanisms and behavior in perinatal rats.