On nonlinear random effects models for repeated measurements

Mixed nonlinear models in ruminal in situ degradability trials

Abstract Classical methods of analysis of nonlinear models are widely used in studies of ruminal degradation kinetics. As this type of study involves repeated measurements in the same experimental unit, the use of mixed nonlinear models (MNLM) is proposed, in order to solve problems of heterogeneity of variances of the responses, correlation among repeated measurements and consequent lack of sphericity in the covariance matrix. The aims of this work are to present an evaluation of the applicability of MNLM in the estimation of parameters to describe the in situ ruminal degradation kinetics of the dry matter of Tifton 85 hay and to compare the results with those obtained from the usual analysis in two-phases. The steers used in the trial were fed diets composed of three different combinations of roughage and concentrate and two hays with different nutritional qualities. The proposed approach was proven as effective as the traditional one for estimating model parameters. However, it adequately models the correlation among the longitudinal data, which can affect the estimates obtained, the standard error associated with them and potentially change the results of the inferences. It is quite attractive when the research seeks to understand the behavior of the process of food degradation throughout the incubation times.

Comparison of Fetal Rabbit Brain Xenografts to Three Different Strains of Athymic Nude Rats: Electrophysiological and Immunohistochemical Studies of Intraocular Grafts

Interest in the use of neural tissue transplantation for the study of CNS development and maturation and the potential use of this technique for the treatment of certain degenerative CNS disorders has led to our use of transplantation of neural tissue across species lines. Prior to extensive transplantation studies using athymic rats as recipients, we wished to evaluate the currently available strains of athymic rat for their suitability as host animals for xenografts of neural tissue. Fetal cerebellar and cerebral cortex tissue from rabbit brain of gestational age 20-25 days was dissected and transplanted to the anterior chamber of the eye of Harlan Wisconsin, Fisher 344 Jnu, or NCI-Harlan athymic nude rat strains. The brain tissue grafts were allowed to mature for 3 mo during which time the size and vascularity of each graft was monitored through the cornea of anesthetized hosts. In each group all of the transplants survived and grew to varying extents in the anterior chamber of the eye. Following the growth study in vivo extracellular recording of single neuronal activity was performed. Spontaneous neural activity was found in most transplants in all three groups with no difference in the viability or discharge rates of neurons between the groups. Illumination of the ipsilateral eye increased the firing rate of neurons in all three groups, suggesting excitatory cholinergic innervation of the grafted neurons from the host parasympathetic iris ground plexus. Antibodies directed against neurofilament protein, glial fibrillary acidic protein, synapsin, and tyrosine hydroxylase were used to characterize the transplants immunocytochemically and revealed no differences between the grafts in the three groups of recipients. All transplants contained significant numbers of glial and neuronal elements with the distribution resembling that in adult brain tissue. Some of the transplants contained a sparse innervation of tyrosine hydroxylase–positive fibers from the sympathetic plexus of the host iris. Furthermore, synapsin-immunoreactivity suggested that synaptogenesis had taken place within the grafts. Histological examination of the grafts revealed that 67% of the grafts had been infiltrated, to varying extents, by lymphocytes which led to areas of cell lysis and necrosis. All host animals had populations of T-cell receptor positive cells, most of which also expressed the T-cell surface antigens CD4 and CD8. However, no transplants were overtly rejected over the 15 wk period of study. Our investigation demonstrates that all of the athymic strains used in this study are able to mount an immune response against grafted fetal tissue, despite the absence of rejection, and that none of these strains is superior to the others with respect to suitability as a host for the long-term study of fetal CNS xenografts in oculo.

Discriminant analysis for longitudinal data with multiple continuous responses and possibly missing data

Multiple outcomes are often used to properly characterize an effect of interest. This article discusses model-based statistical methods for the classification of units into one of two or more groups where, for each unit, repeated measurements over time are obtained on each outcome. We relate the observed outcomes using multivariate nonlinear mixed-effects models to describe evolutions in different groups. Due to its flexibility, the random-effects approach for the joint modeling of multiple outcomes can be used to estimate population parameters for a discriminant model that classifies units into distinct predefined groups or populations. Parameter estimation is done via the expectation-maximization algorithm with a linear approximation step. We conduct a simulation study that sheds light on the effect that the linear approximation has on classification results. We present an example using data from a study in 161 pregnant women in Santiago, Chile, where the main interest is to predict normal versus abnormal pregnancy outcomes.

Non-linear random effects model for multivariate responses with missing data

The use of random-effects models for the analysis of longitudinal data with missing responses has been discussed by several authors. In this paper, we extend the non-linear random-effects model for a single response to the case of multiple responses, allowing for arbitrary patterns of observed and missing data. Parameters for this model are estimated via the EM algorithm and by the first-order approximation available in SAS Proc NLMIXED. The set of equations for this estimation procedure is derived and these are appropriately modified to deal with missing data. The methodology is illustrated with an example using data coming from a study involving 161 pregnant women presenting to a private obstetrics clinic in Santiago, Chile.

Comparing linear and nonlinear mixed model approaches to cosinor analysis

The cosinor model, used for variables governed by circadian and other biological rhythms, is a nonlinear model in the amplitude and acrophase parameters that has a linear representation upon transformation. With linear cosinor analysis, amplitude and acrophase for each harmonic can be computed as nonlinear functions of the estimated linear regression coefficients. Here a flexible mixed model approach to cosinor analysis is considered, where the fixed effect parameters may enter nonlinearly as acrophase and amplitude for each harmonic or linearly after transformation to regression coefficients. In addition, the random effects may enter nonlinearly as subject-specific deviations from the acrophases and amplitudes or linearly as subject-specific deviations from the regression coefficients. It is also possible for the fixed effects to enter nonlinearly while the random effects enter linearly. Additionally, we evaluate whether including higher order linear harmonic terms as random effects, that is, Rao-Khatri 'covariance adjustment', improves precision. Applying the delta method to nonlinear functions of the parameters from linear mixed cosinor models to obtain approximate variances produces results that are often identical to results from nonlinear mixed models. Consequently, traditional linear cosinor analysis can often be used to estimate and compare the nonlinear parameters of interest, that is, amplitudes and acrophases, via the delta method. This is advantageous since the nonlinear mixed model may have convergence difficulties for more complex models. However, for some multiple-group analyses, the linear cosinor transformation should not be used and we clarify when the two methods are equivalent and when they differ.

Effects of hippocampal lesions on patterned motor learning in the rat

Motor skill learning in rats has been linked to cerebellar function as well as to cortical and striatal influences. The present study evaluated the contribution of the hippocampus to motor learning. Adult male rats received electrolytic lesions designed to selectively destroy the hippocampus; a sham-lesioned group of animals served as a control. The animals with hippocampal lesions acquired a patterned motor learning task as well as sham controls. In contrast, rats with hippocampal lesions were impaired in spatial, but not cued, learning in the Morris water maze. In addition, lesioned rats showed profound impairment in the novel object recognition memory task, when a 1-h delay was used between training and testing. Taken together, these results suggest that the hippocampus is not necessary during acquisition of the motor learning task.

Effects of branched-chain amino acids on placental amino acid transfer and insulin and glucagon release in the ovine fetus

OBJECTIVE

Competition for placental amino acid transporters can affect the fetal supply of amino acids. Specifically, the branched-chain amino acids-isoleucine, leucine, and valine-may inhibit the transfer of other amino acids. This study was undertaken to determine the effect of branched-chain amino acids on the umbilical uptake of amino acids.

STUDY DESIGN

Six late-gestation ewes were infused sequentially for 2 hours with 3 different mixtures of amino acids: (1) one that was comparable to commercial parenteral nutrition preparations, (2) the same solution without branched-chain amino acids, and (3) branched-chain amino acids alone. Maternal and fetal blood samples were collected simultaneously for the determination of uterine and umbilical uptake values of amino acids, and for concentrations of arterial insulin, glucagon, glucose, and lactate before (control) and during (experimental) infusion.

RESULTS

Umbilical uptake of branched-chain amino acids increased significantly when they were present in the infusates. The fetal uptake of several other amino acids could be increased by increasing their maternal concentrations. Inhibition of umbilical uptake by branched-chain amino acids could be shown for threonine and methionine. The infusion of branched-chain amino acids alone did not affect maternal and fetal insulin or glucagon concentrations.

CONCLUSIONS

In late-gestation sheep, an increase in maternal plasma concentration of branched-chain amino acids led to increased branched-chain amino acid umbilical uptake, but branched-chain amino acids can also inhibit the transport of some amino acids to the fetus. Changes in fetal plasma concentration and uptake of branched-chain amino acid appear to have no significant effect on fetal insulin or glucagon.

Motor learning in Ts65Dn mice, a model for Down syndrome

Ts65Dn mice are a genetic model for Down syndrome. Both individuals with Down syndrome and Ts65Dn mice have reduced cerebellar volumes and the cerebellum is involved in motor learning. Conflicting results have been reported on the motor learning abilities of Ts65Dn mice, which may be related to procedural differences between the motor learning tasks used in different laboratories and/or variability in phenotype because of the segregating background on which the mice are maintained. In this study, we examined learning in three types of motor tasks (peg running, accelerating rotorod, and rotating rod) which were initially easy for mice and gradually increased in difficulty. Ts65Dn mice learned the peg running task as well as controls, and learned the accelerating rotorod and rotating rod tasks as well as, and even better than, controls. These data indicate that Ts65Dn mice are not impaired in motor learning.

Linear discriminant models for unbalanced longitudinal data

This paper discusses statistical methods for the classification of observations into one of two or more groups based on longitudinal observations. Measurements on subjects in longitudinal medical studies are often collected at different times and on a different number of occasions. Classical multivariate methods for linear discriminant analysis are difficult to apply to repeated measurements due to the highly unbalanced structure observed in these data. Linear models for the analysis of longitudinal data proposed by Laird and Ware and non-linear models proposed by Lindstrom and Bates can be used to estimate population parameters for a discriminant model that classifies individuals into distinct predefined groups or populations. An example is presented using data from a study in 150 pregnant women in Santiago, Chile, in order to predict normal versus abnormal pregnancy outcomes.

Comparison of a live attenuated and an inactivated varicella vaccine to boost the varicella-specific immune response in seropositive people 55 years of age and older

Healthy, varicella-zoster virus (VZV)-seropositive subjects, aged 55-89 years (mean age 66 years), received either 4000 PFU of live, attenuated VZV vaccine (n=85) or an equal volume of this vaccine that was heat-inactivated (n=82). Both vaccines significantly boosted VZV antibody (enzyme immunoassay) and gamma-interferon production by peripheral blood mononuclear cells stimulated by VZV antigen. These responses returned to baseline by 12 months. Circulating mononuclear cells that proliferated in response to VZV antigen were significantly more numerous (responder cell frequency assay) after either vaccine, and persisted with a half-life of 17. 5-21.3 months. There were no differences in immune response to either vaccine in this older age cohort.

Antioxidant-rich diets improve cerebellar physiology and motor learning in aged rats

The free radical theory of aging predicts that reactive oxygen species are involved in the decline in function associated with aging. The present paper reports that diets supplemented with either spinach, strawberries or blueberries, nutritional sources of antioxidants, reverse age-induced declines in beta-adrenergic receptor function in cerebellar Purkinje neurons measured using electrophysiological techniques. In addition the spinach diet improved learning on a runway motor task, previously shown to be modulated by cerebellar norepinephrine. Motor learning is important for adaptation to changes in the environment and is thus critical for rehabilitation following stroke, spinal cord injury, and the onset of some neurodegenerative diseases. These data are the first to indicate that age-related deficits in motor learning and memory can be reversed with nutritional interventions.

Prediction of arterial partial pressure of oxygen with pulse oxygen saturation measurements

OBJECTIVES

We studied 22 neonates with gestational ages from 26 to 40 weeks to determine how accurately pulse oxygen saturation (SpO2) could predict arterial partial pressure of oxygen (PaO2) and how much time and effort would be required to achieve and maintain a desired SpO2 value.

STUDY DESIGN

SpO2 was maintained at 90%, 92%, 94%, 96%, and 98% by adjustment of FiO2. PaO2 was correlated with SpO2 by nonlinear mixed effects regression analysis.

RESULTS

PaO2(y) correlated positively with SpO2(x): y = (0.03). e0.08(x). Variation of PaO2 increased from a range of +/- 10 Torr at SpO2 = 90% to +/- 28 Torr at SpO2 = 98%, principally as a result of decreasing hemoglobin-oxygen affinity, with no effect of advancing gestational age, subject temperature, pH, PaCO2. Considerable time (18 +/- 14 SD minutes) and effort (5 +/- 3 SD FiO2 adjustments/min were required to achieve and maintain a desired SpO2.

CONCLUSIONS

PaO2 varied broadly with SpO2 in a clinically acceptable range of SpO2 (90% to 98%), and the variability increased at higher SpO2 values. Considerable time and effort were required to achieve and maintain desired SpO2 values.

The effects of aging on cerebellar beta-adrenergic receptor activation and motor learning in female F344 rats

Female Fischer 344 (F344) rats were tested for age-related differences in both cerebellar beta-adrenergic receptor activation and motor learning. Three and 20 month old rats were tested on a motor learning paradigm that required the rats to adjust their gait to navigate a runway of unevenly spaced pegs. Rates of acquisition were compared between age groups. After behavioral testing, cerebellar beta-adrenergic receptor function was assessed using extracellular recordings of Purkinje cells during iontophoresis of GABA and isoproterenol (ISO). Aged female rats showed deficits in motor learning and in beta-adrenergic receptor activation. In young rats, ISO modulated GABAergic inhibition of Purkinje cell firing in more cells compared with aged rats. Thus, aging produces deficits in both motor learning and in beta-adrenergic receptor activation in aged female F344 rats.

Acquisition of a runway motor learning task is impaired by a beta adrenergic antagonist in F344 rats

Performance of rats on a motor learning paradigm that has been demonstrated to be dependent upon cerebellar norepinephrine (NE) was studied in male F344 rats treated with an alpha 1 antagonist (prazosin), an alpha 2 antagonist (yohimbine) or a beta noradrenergic antagonist (propranolol). The ability of propranolol-treated rats (10 mg/kg i.p. 30 min prior to daily testing) to acquire proficiency on the novel motor task was impaired while prazosin-treated rats' (0.5 mg/kg i.p. 30 min prior to daily testing) and yohimbine-treated rats' (1 mg/kg i.p. 30 min prior to daily testing) rates of acquisition of the novel motor task were not different from controls. In an attempt to distinguish between alterations in motor coordination and motor learning, additional tests of psychomotor performance were assessed for all groups of rats. These examinations included a walking test on 2.5 and 5 cm rods and speed of running on the motor task. The data indicate that drug-treated rats show no difference from controls on the above parameters. Some differences, however, were observed between propranolol and controls in the time spent in the goal box. Overall, the data are consistent with our hypothesis that the beta noradrenergic receptor is involved in the ability to acquire novel motor tasks.

Mixed-effects Models in the Study of Individual Differences with Repeated Measures Data

Nonlinear mixed-effects models are used to describe each person's set of scores from a longitudinal design or repeated measures study by a function that includes an overall group effect plus an effect for the individual. The model is ideal for many kinds of behavioral data. Some characteristics of mixed models are reviewed in this article and illustrated by a series of examples.

Uteroplacental carbon substrate metabolism and O2 consumption after long-term hypoglycemia in pregnant sheep

Chronic maternal hypoglycemia leads to fetal growth restriction, but the effects on placental metabolism and its contribution to reduced fetal growth have not been determined. To study these factors, experiments were conducted in seven normal control (C) and six chronically insulin-infused hypoglycemic (H) pregnant sheep over the second half of gestation to measure effects of maternal and fetal hypoglycemia [maternal: 2.0 +/- 0.2 (H) vs. 3.6 +/- 0.1 mM (C); fetal: 0.7 +/- 0.1 (H) vs. 1.1 +/- 0.1 mM (C)] on placental metabolism of glucose, other carbon substrates, and O2. Placental and fetal weight were proportionately reduced in the H group [placental wt: 258 +/- 18 (H) vs. 376 +/- 18 g (C), -31%; fetal wt: 2,540 +/- 190 (H) vs. 3,558 +/- 230 g (C), -29%]. Glucose uptake by the uterus was reduced in the H group [0.10 +/- 0.01 (H) vs. 0.19 +/- 0.02 mmol/min (C)]; it was partitioned more to uteroplacental consumption (UPGC) and less to transfer to the fetus (PGT) [UPGC-to-PGT ratio: 20.0 +/- 2.1 (H) and 13.5 +/- 1.2 (C), P < 0.05 for both]. Metabolism of glucose by nonoxidative and oxidative pathways (as estimated by uteroplacental lactate production-to-UPGC ratio and normal rates of placental O2 consumption) was not significantly changed in the H group. Acetoacetate and beta-hydroxybutyrate net uptake and fructose production rates were significantly reduced in the H group, and there was no change in free fatty acid or amino acid uptake rates. With acute return of maternal and fetal glucose concentrations to normal, UPGC and PGT exceeded normal rates by approximately 25%. Thus chronic reduced glucose supply to the uteroplacenta produces a smaller placenta that uses glucose normally and in preference to transfer to the fetus, demonstrating placental metabolic autonomy at the expense of the fetus.

Effects of dietary restriction on motor learning and cerebellar noradrenergic dysfunction in aged F344 rats

Fisher 344 rats were fed either ad libitum or with a diet containing a 40% reduction of calories beginning at 4 months of age. At 14 months and 22 months male rats were tested for their ability to learn a complex motor skill. At both ages the diet restricted rats reached criterion of performing 10 successful crosses in 10 min at an earlier time than ad libitum fed controls. At 22 months of age the diet restricted rats showed improved acquisition of running times for the task. Male rats at 14 and 22 months and female rats at 24 months were examined electrophysiologically for the ability of isoproterenol to augment the action of GABA in the cerebellum when both substances were applied iontophoretically from an extracellular multibarreled glass electrode. In all 3 age and sex groups there was an improvement in the beta-adrenergic receptor modulation of GABA responses in the dietary restricted vs. ad libitum rats. However, no difference was observed between dietary restricted and ad libitum rats when the number and affinity of cerebellar beta-adrenergic receptors was assessed with 125I-iodopindolol binding. Overall, there was a significant improvement in cerebellar noradrenergic function in the dietary restricted rats and this was accompanied by an improvement in motor learning.

IGF-1 influences olfactory bulb maturation. Evidence from anti-IGF-1 antibody treatment of developing grafts in oculo

Recent studies have indicated that both insulin-like growth factor-1 (IGF-1) and IGF-1 receptor mRNA are abundant in developing and adult olfactory bulbs, and that IGF-1 receptor mRNA is abundant in the prenatal cerebral cortex. To examine the potential role of IGF-1 in development of a central nervous system region rich in IGF-1 and its receptor (the olfactory bulb), as compared to one in which IGF-1 is less abundant (the cerebral cortex), tissue pieces of these two central nervous system areas from E15-E17 rat fetuses were transplanted into the anterior chamber of the eye of adult host rats. The transplants were treated with either a total of 300 ng truncated IGF-1, two different IGF-1 polyclonal antisera, two different non-immune sera, a total of 15 micrograms IGF binding protein-1, or vehicle alone. Treatments were administered by preincubation just prior to grafting and by 5 microliters injections into the anterior chamber on days 5, 10 and 15 postgrafting. Olfactory bulb grafts treated with either of the two IGF-1 antisera grew significantly larger than grafts receiving any other treatment. No enhancement of graft size was seen in E16-E17 parietal cortex grafts after IGF-1 antibody treatment. Immunohistochemical studies revealed no difference between the treatments with regard to glial fibrillary acidic protein-, tyrosine hydroxylase- or neurofilament-immunoreactivity within the olfactory bulb grafts. Since, in the olfactory bulb the presumed reduction of endogenous IGF-1 achieved by antibody treatment caused enhanced growth, we suggest that the presence of appropriate endogenous levels of IGF-1 in this area induces maturation. This mechanism is not operative in all brain areas since it was not seen in cortex cerebri grafts. Thus, endogenous IGF-1 appears to influence brain development in a regionally specific manner.

Motor learning deficits in aged rats are correlated with loss of cerebellar noradrenergic function

We have demonstrated that aged rats show impairments in learning patterned motor movements. Similar behavioral impairment is observed in rats with noradrenergic lesions. Norepinephrine is known to act as a neuromodulator in the cerebellar cortex because it can augment the action of GABA and other neurotransmitters. This effect of NE to augment the signal to noise ratio of GABAergic inputs to cerebellar Purkinje neurons is a possible substrate for NE's effect on motor learning. Aged rats demonstrate deficits in the modulatory actions of NE to augment GABAergic inhibitions when both substances are locally applied onto cerebellar Purkinje neurons. In this report, we examined how motor learning and cerebellar noradrenergic function varied in individual young and 20-month-old Fischer 344 rats. There was a significant correlation between the loss of the neuromodulatory actions of norepinephrine (NE) in the cerebellar cortex and the rate of learning a novel motor task in individual rats. This report thus demonstrates for the first time a correlation between age-related impairments in motor plasticity and specific neurophysiological deficits in cerebellar Purkinje neurons in individual animals.

Increased responsiveness of hippocampal pyramidal neurons to nicotine in aged, learning-impaired rats

The effect of aging upon the responsiveness of hippocampal CA1 pyramidal neurons to nicotine was investigated using electrophysiological techniques in male Fischer 344 rats. Prior to electrophysiological recording, animals were behaviorally tested using the Morris water maze. All 3-6 and 18-21 month rats displayed rapid place learning in this task, while none of the 27-30 month animals learned within the 5-day test period. By contrast, rats of all age groups were able to learn a cue version of the water maze task. Following behavioral testing, the animals were anesthetized with sodium pentobarbital for acute recording. Nicotine was locally applied to electrophysiologically identified CA1 pyramidal neurons using pressure microejection from two-barreled glass microelectrodes. For each neuron, a dose of nicotine was found which elicited a 300-400% increase in basal firing rate. These data were used to construct cumulative dose response curves for populations of neurons tested in 3-6-, 18-21-, and 27-30-month-old animals. An age-related increase in the responsiveness of CA1 pyramidal neurons to locally applied nicotine was observed. The results of this study suggest that an increase in hippocampal CA1 pyramidal cell responsiveness to nicotine could be related to the impaired place learning ability seen with aging.

Electrophysiological interactions of ethanol with GABAergic mechanisms in the rat cerebellum in vivo

Biochemical studies indicate that ethanol (EtOH) will facilitate the activation of the GABAA/Cl- channel, and behavioral studies demonstrate that EtOH-induced sedative and incoordinating effects can be potentiated by GABA mimetics and blocked by GABA antagonists. It has been difficult, however, to demonstrate an EtOH-induced potentiation of the depressant electrophysiological effects of locally applied GABA in mammalian brain in vivo. Similarly, in this study, local EtOH applications only infrequently caused potentiations of the depressant effects of microiontophoretically applied GABA on cerebellar Purkinje neurons, and this interaction was modest when present. The predominant interaction of locally applied EtOH was an antagonism of GABA-induced depressions of neuronal activity. However, the GABAA receptor antagonist bicuculline reversibly and apparently competitively blocked the depressant effects of locally applied EtOH on single cerebellar Purkinje neurons. Our data suggest that EtOH potentiation of GABA responses alone is insufficient to account for EtOH-induced depressions of cerebellar Purkinje neurons. However, these data clearly imply that activation of a GABAA receptor is required for the expression of EtOH-induced depressions of neuronal activity in this brain area. It is less clear how lower, nondepressant doses of EtOH interact with GABA mechanisms. We hypothesize that either the GABAA receptor mechanism must be sensitized to the potentiative effects of EtOH through the influences of neuromodulatory and/or hormonal regulation, or that EtOH interacts directly with these regulatory processes.

Non-linear models for the analysis of longitudinal data

Given the importance of longitudinal studies in biomedical research, it is not surprising that considerable attention has been given to linear and generalized linear models for the analysis of longitudinal data. A great deal of attention has also been given to non-linear models for repeated measurements, particularly in the field of pharmacokinetics. In this article, a brief overview of non-linear models for the analysis of repeated measures is given. Particular emphasis is placed on mixed-effects non-linear models and on various estimation procedures proposed for such models. Several of these estimation procedures are compared via a simulation study. In addition, simulation is used to investigate the effects of model misspecification, particularly with respect to one's choice of random-effects. A relatively straightforward measure useful in selecting an appropriate set of random effects is investigated and found to perform reasonably well.

Impaired acquisition of novel locomotor tasks in aged and norepinephrine-depleted F344 rats

Performance of rats on a motor learning paradigm that has been demonstrated to be dependent upon cerebellar norepinephrine (NE) was studied in 20-month-old F344 rats. The behavioral task is identical to that described by Watson and McElligott: Rats are trained on a runway consisting of aluminum pegs arranged in a regular pattern. Rats receive a water reward at either end of the runway. Subsequent to training, rats are tested for running times on a runway with irregularly spaced rods. The ability of rats to improve their performance (decrease their running times) on this novel motor task is diminished in young rats that have received 6-hydroxydopamine lesions. Rats at 20 months of age are known to have deficits in cerebellar noradrenergic transmission; thus, the hypothesis to be tested was to determine if aged rats demonstrated performance deficits similar to young rats depleted of central stores of NE. The rate of acquisition of the task was determined by the decrease in running times with successive days of training. The ability of 20-month-old F344 rats to acquire proficiency on the novel motor task was impaired and the rate of acquisition of the novel motor task was not different from the young 6-hydroxydopamine-lesioned rats. In an attempt to distinguish between alterations in motor coordination and motor learning, additional tests of psychomotor performance were assessed for all groups of rats. These tests included a walking on 2.5- and 5-cm rods, speed of running on the motor task, and number and types of mistakes made on the motor learning task.(ABSTRACT TRUNCATED AT 250 WORDS)