Sensory-motor function in the aging rat

Effect of Kv3 channel modulators on auditory temporal resolution in aged Fischer 344 rats

AUT00063 and AUT00202 are novel pharmaceutical modulators of the Kv3 subfamily of voltage-gated K⁺ channels. Kv3.1 channels, which control fast firing of many central auditory neurons, have been shown to decline with age and this may contribute to age-related deficits in central auditory processing. In the present study, the effects of the two novel compounds that specifically modulate Kv3 channels on auditory temporal processing were examined in aged (19-25-month-old) and young-adult (3-5 month-old) Fischer 344 rats (F344) using a behavioral gap-prepulse inhibition (gap-PPI) paradigm. The acoustic startle response (ASR) and its inhibition induced by a gap in noise were measured before and after drug administration. Hearing thresholds in tested rats were evaluated by the auditory brainstem response (ABR). Aged F344 rats had significantly higher ABR thresholds, lower amplitudes of ASR, and weaker gap-PPI compared with young-adult rats. No influence of AUT00063 and AUT00202 administration was observed on ABR hearing thresholds in rats of both age groups. AUT00063 and AUT00202 had suppressive effect on ASR of F344 rats that was more pronounced with AUT00063. The degree of suppression depended on the dose and age of the rats. Both compounds significantly improved the gap-PPI performance in gap detection tests in aged rats. These results indicate that AUT00063 and AUT00202 may influence intrinsic firing properties of neurons in the central auditory system of aged animals and have the potential to treat aged-related hearing disorders.

Comparison of operant escape and reflex tests of nociceptive sensitivity

Testing of reflexes such as flexion/withdrawal or licking/guarding is well established as the standard for evaluating nociceptive sensitivity and its modulation in preclinical investigations of laboratory animals. Concerns about this approach have been dismissed for practical reasons - reflex testing requires no training of the animals; it is simple to instrument; and responses are characterized by observers as latencies or thresholds for evocation. In order to evaluate this method, the present review summarizes a series of experiments in which reflex and operant escape responding are compared in normal animals and following surgical models of neuropathic pain or pharmacological intervention for pain. Particular attention is paid to relationships between reflex and escape responding and information on the pain sensitivity of normal human subjects or patients with pain. Numerous disparities between results for reflex and operant escape measures are described, but the results of operant testing are consistent with evidence from humans. Objective reasons are given for experimenters to choose between these and other methods of evaluating the nociceptive sensitivity of laboratory animals.

Age-dependent effects of modafinil on acoustic startle and prepulse inhibition in rats

Modafinil is a psychostimulant approved for treating excessive sleepiness in adults; off-label uses (e.g., treatment of cognitive impairment in schizophrenia, ADHD and age-related dementias) are currently being explored. The effects and mechanisms of action of modafinil have not been fully established. In the present study, the effects of modafinil were examined in young adult (7-month-old) and middle-aged (21-22-month-old) rats, using the acoustic startle response (ASR) and prepulse inhibition (PPI). In the control condition, middle-aged rats showed lower activity levels, significantly lower ASR amplitudes and significantly longer ASR latencies compared to young adult rats. The effects of modafinil differed by age: activity levels and ASR amplitudes were significantly increased in middle-aged rats, whereas activity levels were lower and ASR amplitude was significantly decreased in young adult rats. Modafinil did not significantly alter PPI or startle latencies relative to the control condition. Amphetamine, used as a positive control, significantly decreased ASR amplitude in young adult rats and significantly impaired PPI for both age groups. Amphetamine-induced PPI impairment was greater for young adult rats (34% reduction in ASR amplitude) than for middle-aged rats (24% reduction). The results offer new insights into the effects of modafinil and its mechanism of action, and are consistent with the idea that modafinil enhances vigilance and cognitive functioning in individuals with deficits but not in normal, healthy individuals.

Spatial learning and psychomotor performance of C57BL/6 mice: age sensitivity and reliability of individual differences

Two tests often used in aging research, the elevated path test and the Morris water maze test, were examined for their application to the study of brain aging in a large sample of C57BL/6JNia mice. Specifically, these studies assessed: (1) sensitivity to age and the degree of interrelatedness among different behavioral measures derived from these tests, (2) the effect of age on variation in the measurements, and (3) the reliability of individual differences in performance on the tests. Both tests detected age-related deficits in group performance that occurred independently of each other. However, analysis of data obtained on the Morris water maze test revealed three relatively independent components of cognitive performance. Performance in initial acquisition of spatial learning in the Morris maze was not highly correlated with performance during reversal learning (when mice were required to learn a new spatial location), whereas performance in both of those phases was independent of spatial performance assessed during a single probe trial administered at the end of acquisition training. Moreover, impaired performance during initial acquisition could be detected at an earlier age than impairments in reversal learning. There were modest but significant age-related increases in the variance of both elevated path test scores and in several measures of learning in the Morris maze test. Analysis of test scores of mice across repeated testing sessions confirmed reliability of the measurements obtained for cognitive and psychomotor function. Power calculations confirmed that there are sufficiently large age-related differences in elevated path test performance, relative to within age variability, to render this test useful for studies into the ability of an intervention to prevent or reverse age-related deficits in psychomotor performance. Power calculations indicated a need for larger sample sizes for detection of intervention effects on cognitive components of the Morris water maze test, at least when implemented at the ages tested in this study. Variability among old mice in both tests, including each of the various independent measures in the Morris maze, may be useful for elucidating the biological bases of different aspects of dysfunctional brain aging.

A diminished rate of "physiological decay" at noise offset contributes to age-related changes in temporal acuity in the CBA mouse model of presbycusis

The persistence of afferent activity at stimulus offset was examined in mice as a function of age (experiment 1) and noise level (experiment 2), using a procedure devised by von Bekesy to study the physiological decay of afferent activity. Noise offset was presented from 1 to 10 ms (the temporal gap) prior to an acoustic startle stimulus, with an abrupt edge or a linear ramp having the duration of the gap. Noise offset inhibited the startle reflex, this effect declining with age. For young (2-3 months old) and "young-old" mice (18-19 months old), the abrupt offset was always more inhibitory than the ramp, even for the one millisecond gap. In "old-old" mice (24-26 months old) abrupt and ramped offsets were not different until the gap exceeded 4 ms. The behavioral difference between abrupt and ramped decay times did not depend on noise level in young mice (4-5 months old), though overall inhibition increased with level. These data indicate that temporal acuity as measured by this gap detection method is limited in very old mice by their reduced ability to follow the abrupt change in noise level at the beginning of the gap.

The acoustic startle reflex and its modulation: effects of age and gender in humans

The acoustic startle reflex and its modulation by prepulse inhibition (PPI) and habituation are used in many studies in different fields of neuropsychiatric research. The aim of this study was to examine the effects of age and gender on PPI, startle magnitude, and habituation in healthy human volunteers. Twenty-seven male and 28 female participants of four different age groups (range: 20-60 years) were investigated in an acoustic startle paradigm using a startle stimulus of 115 dB and a prepulse of 86 dB (16 dB over the white noise background) with five different lead intervals (30, 60, 120, 240, and 2000 ms). Seventeen males and 16 female participants were tested three times at monthly intervals. Aged participants showed significantly lower startle magnitude and significantly more habituation than younger participants, but there was no effect of age on PPI or prepulse facilitation. Moreover, there were no effects of gender on startle magnitude, PPI, prepulse facilitation, or habituation measures. Healthy males and females exhibited stable startle magnitudes and PPI across sessions. The results demonstrated that PPI and startle are reliable measures of sensory information processing in both genders and that startle magnitude and habituation are age-dependent measures.

The relationship of age to prepulse inhibition and habituation of the acoustic startle response

Prepulse inhibition (PPI) of the startle response reflects an early stage of information processing that is abnormal in schizophrenia and certain other specific neuropsychiatric disorders that are distinguished by the inability to inhibit redundant or relatively irrelevant sensory, cognitive, or motor information. The goal of the present study was to characterize the effect of normal aging on PPI and habituation of the startle response and to examine the hypothesis that normal aging is characterized by a global decline in inhibitory function. Ninety-seven non-psychiatric controls (age range 18-88) were tested for startle eyeblink response using electromyogram (EMG) recording. Startle magnitude decreased and startle latency increased with aging. PPI demonstrated an inverted U-shaped function with age (greatest PPI at intermediate ages) while there was no significant effect of age on startle habituation. The results do not support the theory that aging is associated with a general decline in inhibitory function and contrast with previous studies that have compared only extreme age groups and have found no effects of age on PPI.

Mini-review: assessment of behavioural recovery following spinal cord injury in rats

Behavioural recovery is one of the primary goals of therapeutic intervention in animal models of disease. It is necessary, therefore, to have the means with which to quantify pertinent behavioural changes in experimental animals. Nevertheless, the number and diversity of behavioural measures which have been used to assess recovery after experimental interventions often makes it difficult to compare results between studies. The present review attempts to integrate and categorize the wide variety of behavioural assessments used to measure recovery in spinal-injured rats. These categories include endpoint measures, kinematic measures, kinetic measurements, and electrophysiological measurements. Within this categorization, we discuss the advantages and disadvantages of each type of measurement. Finally, we make some recommendations regarding the principles for a comprehensive behavioural analysis after experimental spinal cord injury in rats.

Changes in temporal acuity with age and with hearing impairment in the mouse: a study of the acoustic startle reflex and its inhibition by brief decrements in noise level

Temporal acuity for brief gaps in noise was studied in mice of different ages (1-36 months) from strains with differing susceptibility to age-related hearing loss, using reflex modification audiometry. Prepulse inhibition of the acoustic startle reflex (ASR) increased with gap depth (GD: 10-40 dB in 70 dB SPL noise) and lead time (LT: 1-15 ms). The increase in inhibition with LT followed an exponential function in which the two parameters, asymptotic inhibition (AINH) and the time constant (tau), were both affected by GD. AINH rapidly declined from 1 to 6 and then to 18 months of age in C57BL/6J mice with progressively severe hearing loss, but first increased with maturation and then gradually declined beyond 6-12 months of age in CBA/CaJ and CBA x C57BL Fl-hybrid mice, which show no apparent change in sensory function at these ages. In contrast, tau was unaffected by hearing loss or by age, this suggesting that age-related changes in this form of temporal acuity occur because of a reduction in the efficiency with which gaps are centrally processed, not from any reduced ability to follow their rapid shift in noise level.

Darkness facilitates the acoustic startle reflex in humans

The effects of darkness on startle reactivity and prepulse inhibition were investigated in two studies with 25 subjects participating in each study. Acoustic startle stimuli that were or were not preceded by an acoustic prepulse were delivered in alternating periods of complete darkness or light. In both studies, darkness significantly increased the magnitude of startle but did not affect prepulse inhibition (PPI). The PPI results suggest that darkness did not increase attention to the auditory modality, so that the startle facilitation in the dark probably did not result from an attentional process. The increased startle in the dark was significantly correlated with the intensity of subjects' fear of the dark as children based on retrospective rating scales. It is hypothesized that the startle facilitation in the dark results from a change in affect rather than from a change in attention.

Effects of stress and shock anticipation on prepulse inhibition of the startle reflex

The effects of shock anticipation and attention to external stimuli on prepulse inhibition (PPI) were compared. In the threat-of-shock experiment, acoustic startle stimuli were presented with and without prepulses when aversive shocks were or were not anticipated. In the control experiment, startle and prepulse stimuli were delivered during periods with attended or ignored external stimuli. In the threat-of-shock experiment, startle was potentiated (fear-potentiated startle) and PPI was increased by shock anticipation. A gradual reduction in the overall PPI throughout the experiment was also found. In the control experiment, only PPI was increased in the attend condition. The PPI level remained constant throughout the experiment. The increase in PPI in the threat and attend conditions may have resulted from an increase in the general level of alertness that facilitated the processing of the prepulse. The gradual decrease in PPI in the threat experiment was hypothesized to result from a progressive deficit in sensory functioning due to the stressful nature of repeated shock anticipation.

A signal detection theory analysis of gap detection in the rat

An innocuous sensory event (a prestimulus) briefly preceding a startle-eliciting stimulus (SES) reduces the amplitude of the elicited reflex. This study used signal detection theory (SDT) techniques to quantify the effects of gaps (pauses in otherwise continuous noise) on the rat's acoustic startle reflex. Sixteen rats were given four identical test sessions consisting of the randomized presentation of 150 trials of the SES alone and 150 trials of a gap-and-SES combination. Gap duration (1, 2, 4, and 8 msec) varied between sessions. Data analyses based on amplitude, difference scores, percentage scores, and SDT techniques identified similar patterns. The three longest gaps, but not the shortest, were reliably detected, and differences among these three were identified with percentage and SDT analyses. Analyses of amplitude changes over test sessions yielded different patterns for each measure. The results demonstrate that an SDT analysis is a sensitive index of prestimulus effects.

A longitudinal study of reaction time performance in Long-Evans rats

This study was undertaken to examine individual differences in the progression of age-related decline on a measure of sensorimotor function. Twenty-one adult rats were trained on a simple reaction time (RT) task and assessed every 6 weeks from 14.5 to 25 months of age. An overall slowing of RT latencies associated with chronological age was observed. However, some rats maintained a stable performance with age while others slowed considerably. Another measure within the RT task, accuracy of performance, appeared to represent a stable individual characteristic that was insensitive to the effects of age. However, no measure of performance on the RT task at 14.5 months of age predicted later impairment in RT latency. At the completion of longitudinal testing, spatial learning in the Morris water maze was assessed in these aged rats along with a young comparison group. The aged rats were impaired relative to young controls in locating the escape platform. Measures of cognitive function and sensorimotor function within the spatial task were, however, unrelated to slowing of RT latency.

Effects of chronic dietary restriction on sensory-motor function and susceptibility to stressor stimuli in the laboratory rat

Two sets of experiments describing the effects of chronic undernutrition on sensory-motor function and susceptibility to environmental stressors are described. In the first, Fischer 344 rats between 10 and 12 months of age were placed on an every-other-day feeding regimen. Behavioral tests designed to assess sensory function (auditory and visual thresholds), somato-motor competence (hang time from a horizontal wire, balance on a narrow beam, descent of a wire mesh pole), and sensory-motor integrity (auditory startle) were then conducted every 3 to 6 months. Chronic undernutrition significantly increased life span and increased somato-motor competence but did not affect sensory function or sensory-motor integrity. In the second set of experiments both acute and chronic dietary restriction impaired the ability of young adult Sprague-Dawley rats to effectively thermoregulate in response to a cold environment. Body temperature dropped more rapidly and recovered at a slower rate in the dietarily restricted animals. Fischer 344 rats maintained on a restricted diet for 16 months were similarly impaired. The latter findings suggest that techniques for extending life span based on laboratory animal models may have little relevance to either animals or humans living in their natural habitats where a wide variety of environmental stressors are encountered.

Influence of normobaric hypoxia on learning capacity of different aged rats

In this study it was tested to what extent subchronic normobaric hypoxia (10% oxygen by volume) influences the learning performance of rats of different ages (4, 20, 30 months). The animals were presented with two successive conditioning patterns (FR 10/DRL). After acquisition of the FR 10 under normoxic conditions in a Skinner box the rats needed to reestablish the learned behavior under normoxia and further under hypoxia. Following this, the rats had to learn the DRL in a test chamber under the conditions of reduced oxygen. Their performance was compared with that of control animals which had to accomplish the tasks under normoxia. No age differences were observed under normoxia in learning the FR 10. However, the reestablishment of FR 10 under hypoxic conditions was less well performed by old rats than younger ones. The young rats (4 months) had a high efficiency level on the DRL-schedule under normoxia which was not impaired by hypoxia. The old rats (30 months) had considerably lower performance level under normoxia which was not further reduced in individuals by hypoxia. The performance of the middle group (20 months) was, under normoxia, at an intermediate level, while oxygen reduction led to a pronounced reduction in efficiency.

Reflex modification by the human auditory startle blink by antecedent interruption of a visual stimulus

The effect of a preceding brief interruption in the illumination of a small numeral eight on the auditory startle blink was investigated in human subjects. In Exp. 1 (n = 9) trials were given in which a 20-msec. interruption of the visual stimulus preceded elicitation of the startle blink by a loud tone at intervals from 80 to 200 msec. Compared to trials on which the digit was continuously lit but the reflex was elicited, the blink was augmented at 80 msec. and depressed at longer intervals. Exp. 2 (n = 24) showed that the depressive effect peaked at about 200 msec. and did not depend upon requiring subjects to make judgements concerning the stimuli. In Exp. 3 (n = 17) dark intervals ranged from 1.25 msec. to 40 msec. and preceded the loud tone by 200 msec. Reflex effects paralleled subjects' reports: dark intervals of 10 msec. or greater reliably inhibited the reflex and subjects reported seeing the dark interval at least 50% of the time at intervals of 8 msec. or greater, suggesting that the reflex method may be useful as an alternative objective technique for measuring visual temporal resolution.

Sensory, motor and cognitive alterations in aged cats

These experiments were designed to assess some of the sensory, motor and cognitive alterations that occur in aged cats. Three groups of cats (1-3, 5-9 and 11-16 years of age) were tested in four behavioral tasks to assess age-dependent changes in locomotor activity, fine motor coordination, reactivity to auditory stimuli and spatial reversal learning. In tests of locomotor activity, 11-16 year old cats displayed altered patterns of habituation compared to 1-3 and 5-9 year cats. There were no decrements in fine motor coordination in the 11-16 year cats as measured by their ability to traverse planks of varying width or by their scores on a neurological examination. The 11-16 and 5-9 year cats both displayed increased reactivity to auditory stimuli. On tests of spatial reversal learning, 11-16 year cats displayed superior performance compared to 5-9 or 1-3 year animals, making fewer errors and requiring fewer trials to reach criterion. These findings indicate that a series of age-related behavioral changes occurs in the cat. Some of these may be related to morphological and neurophysiological alterations in neurons in the caudate nucleus.

Environmental conditions modulate degeneration and new dendrite growth in cerebellum of senescent rats

Spiny branchlets of cerebellar Purkinje neurons, thought to be sites of synaptic efficacy change during motor learning, degenerate during aging. To examine effects of behavioral experience on degeneration, Purkinje neurons were studied in aging rats housed for 4.5 months either under complex environment conditions promoting sensory-motor activity or in pairs in standard cages. Their data were compared with those of a baseline group of rats from standard cages sacrificed at the age of onset of differential housing for the older groups. Rats housed in the complex environment had more spiny branchlets than the other groups, indicating that new branches had formed. There was a net loss of summed total spiny branchlet material per Purkinje cell in both the laboratory cage and complex environment older groups, although the complex environment group had more spiny branchlet per cell than the laboratory cage group. Thus, dendrite loss in the aging cerebellum can be partly offset by appropriate experience. There was no net loss of Purkinje cell main branch dendrite with aging, as indicated by previous studies, and there was no effect of differential housing upon main branches in the older groups. However, changes in the pattern of branching in the main dendritic field suggested that this region undergoes reorganization with aging.

Increased novelty-induced grooming in aged rats: a preliminary observation

Age-related changes in grooming activity in female Fischer-344 rats were observed in a home cage and in a novel testing chamber for 50 min. Compared to the home cage condition with brief handling treatment, excessive grooming was found in the novel situation. The amount of novelty-induced grooming of aged rats (26-28 months) was about two times that of young animals (6-8 months). This increase in novelty-induced grooming of aged animals was attributable to an increase in the number of grooming bouts, prolongation of each grooming bout, and a slight increase in the duration of licking elements. However, there were no age differences in the percentage of face washing and the duration of face-washing elements. These results are discussed in terms of behavioral aging and age-related changes in peptidergic pathways in the brain.

Assessment of retention capacities in old rats

Young (3-6 month) and old (24-27 month) barrier reared Wistar rats were tested for their ability to retain an inhibitory (passive) avoidance, acquired immobility, and a conditioned taste aversion response as a function of time. Old rats exhibited accelerated forgetting of both the inhibitory avoidance and acquired immobility response in comparison to young rats. In contrast, old rats displayed good retention of the conditioned taste aversion response at all time intervals tested. It appears that the dynamic aspects of retention are altered in aged rats depending on the task, and in some instances may be expressed as accelerated forgetting.

Age related changes in blood-to-brain amino acid transport and incorporation into brain protein

Blood-to-brain amino acid transport consists of at least two components: 1. a fast rate or early process, commonly measured by the intra-carotid bolus injection method and attributed to transport across the capillary endothelium and entry into the astrocytes, and, 2. a slow rate or later component measured over 2 to 15 minutes probably associated with exit from the astrocytes and entry into the neurons. Incorporation into brain protein is temporally related to the second process. In the present study the slow and fast rate transport components and the incorporation into brain protein of tyrosine (Tyr) and Valine (Val) was measured in young adult and aged male C57BL/6 mice. The results indicate that the fast rate transport component is unaffected by age while the rates of the slow process and protein turnover show an exponential decline most marked between 3 and 8 months of age. Changes in the relative incorporation of Tyr and Val suggest that brain protein metabolism is altered qualitatively as well as quantitatively in aging, in these animals.