The curious ability of polyethylene glycol fusion technologies to restore lost behaviors after nerve severance.

Traumatic injuries to PNS and CNS axons are not uncommon. Restoration of lost behaviors following severance of mammalian peripheral nerve axons (PNAs) relies on regeneration by slow outgrowths and is typically poor or nonexistent when after ablation or injuries close to the soma. Behavioral recovery after severing spinal tract axons (STAs) is poor because STAs do not naturally regenerate. Current techniques to enhance PNA and/or STA regeneration have had limited success and do not prevent the onset of Wallerian degeneration of severed distal segments. This Review describes the use of a recently developed polyethylene glycol (PEG) fusion technology combining concepts from biochemical engineering, cell biology, and clinical microsurgery. Within minutes after microsuturing carefully trimmed cut ends and applying a well-specified sequence of solutions, PEG-fused axons exhibit morphological continuity (assessed by intra-axonal dye diffusion) and electrophysiological continuity (assessed by conduction of action potentials) across the lesion site. Wallerian degeneration of PEG-fused PNAs is greatly reduced as measured by counts of sensory and/or motor axons and maintenance of axonal diameters and neuromuscular synapses. After PEG-fusion repair, cut-severed, crush-severed, or ablated PNAs or crush-severed STAs rapidly (within days to weeks), more completely, and permanently restore PNA- or STA-mediated behaviors compared with nontreated or conventionally treated animals. PEG-fusion success is enhanced or decreased by applying antioxidants or oxidants, trimming cut ends or stretching axons, and exposure to Ca(2+) -free or Ca(2+) -containing solutions, respectively. PEG-fusion technology employs surgical techniques and chemicals already used by clinicians and has the potential to produce a paradigm shift in the treatment of traumatic injuries to PNAs and STAs.

Polyethylene glycol-fused allografts produce rapid behavioral recovery after ablation of sciatic nerve segments.

Restoration of neuronal functions by outgrowths regenerating at ∼1 mm/day from the proximal stumps of severed peripheral nerves takes many weeks or months, if it occurs at all, especially after ablation of nerve segments. Distal segments of severed axons typically degenerate in 1-3 days. This study shows that Wallerian degeneration can be prevented or retarded, and lost behavioral function can be restored, following ablation of 0.5-1-cm segments of rat sciatic nerves in host animals. This is achieved by using 0.8-1.1-cm microsutured donor allografts treated with bioengineered solutions varying in ionic and polyethylene glycol (PEG) concentrations (modified PEG-fusion procedure), being careful not to stretch any portion of donor or host sciatic nerves. The data show that PEG fusion permanently restores axonal continuity within minutes, as initially assessed by action potential conduction and intracellular diffusion of dye. Behavioral functions mediated by the sciatic nerve are largely restored within 2-4 weeks, as measured by the sciatic functional index. Increased restoration of sciatic behavioral functions after ablating 0.5-1-cm segments is associated with greater numbers of viable myelinated axons within and distal to PEG-fused allografts. Many such viable myelinated axons are almost certainly spared from Wallerian degeneration by PEG fusion. PEG fusion of donor allografts may produce a paradigm shift in the treatment of peripheral nerve injuries.

Rapid, effective, and long-lasting behavioral recovery produced by microsutures, methylene blue, and polyethylene glycol after completely cutting rat sciatic nerves.

Behavioral function lost in mammals (including humans) after peripheral nerve severance is slowly (weeks to years) and often poorly restored by 1-2-mm/day, nonspecifically directed outgrowths from proximal axonal stumps. To survive, proximal stumps must quickly repair (seal) plasmalemmal damage. We report that, after complete cut- or crush-severance of rat sciatic nerves, morphological continuity, action potential conduction, and behavioral functions can be consistently (>98% of trials), rapidly (minutes to days), dramatically (70-85% recovery), and chronically restored and some Wallerian degeneration prevented. We assess axoplasmic and axolemmal continuity by intra-axonal dye diffusion and action potential conduction across the lesion site and amount of behavioral recovery by Sciatic Functional Index and Foot Fault tests. We apply well-specified sequences of solutions containing FDA-approved chemicals. First, severed axonal ends are opened and resealing is prevented by hypotonic Ca²âº-free saline containing antioxidants (especially methylene blue) that inhibit plasmalemmal sealing in sciatic nerves in vivo, ex vivo, and in rat B104 hippocampal cells in vitro. Second, a hypotonic solution of polyethylene glycol (PEG) is applied to open closely apposed (by microsutures, if cut) axonal ends to induce their membranes to flow rapidly into each other (PEG-fusion), consistent with data showing that PEG rapidly seals (PEG-seals) transected neurites of B104 cells, independently of any known endogenous sealing mechanism. Third, Ca²âº-containing isotonic saline is applied to induce sealing of any remaining plasmalemmal holes by Ca²âº-induced accumulation and fusion of vesicles. These and other data suggest that PEG-sealing is neuroprotective, and our PEG-fusion protocols that repair cut- and crush-severed rat nerves might rapidly translate to clinical procedures.

Sex differences in cognitive performance and alcohol consumption in High Alcohol-Drinking (HAD-1) rats.

Excessive alcohol (ethanol) consumption negatively impacts social, emotional, as well as cognitive function and well-being. Thus, identifying behavioral and/or biological predictors of excessive ethanol consumption is important for developing prevention and treatment strategies against alcohol use disorders (AUDs). Sex differences in alcohol consumption patterns are observed in humans, primates, and rodents. Selectively bred high alcohol-drinking rat lines, such as the "HAD-1" lines are recognized animal models of alcoholism. The present work examined sex differences in alcohol consumption, object recognition, and exploratory behavior in male and female HAD-1 rats. Naïve male and female HAD-1 rats were tested in an object recognition test (ORT) prior to a chronic 24 h intermittent ethanol access procedure for five weeks. Object recognition parameters measured included exploratory behavior, object investigation, and time spent near objects. During the initial training trial, rearing, active object investigation and amount of time spent in the object-containing section was significantly greater in female HAD-1 rats compared to their male counterparts. During the subsequent testing trial, time spent in the object-containing section was greater in female, compared to male, rats; but active object investigation and rearing did not statistically differ between females and males. In addition, female HAD-1 rats consumed significantly more ethanol than their male counterparts, replicating previous findings. Moreover, across all animals there was a significant positive correlation between exploratory behavior in ORT and ethanol consumption level. These results indicate there are significant sex differences in cognitive performance and alcohol consumption in HAD-1 rats, which suggests neurobiological differences as well.

Priming of rotational behavior by a dopamine receptor agonist in Hemiparkinsonian rats: movement-dependent induction.

Repetitive stimulation of dopamine receptors located in the basal ganglia may lead to the manifestation of sensitized, abnormal, motor responses in dopamine-denervated rats. In order to study the role of motor behavior execution on the expression of these altered motor responses, we evaluated how "priming", a phenomenon displaying neurochemical and behavioral features peculiar to a sensitized abnormal motor response in dopamine-denervated rats, depends on actual movement performance. To this end, unilaterally 6-hydroxydopamine-lesioned rats received apomorphine (0.2 mg/kg s.c.), being either allowed to move or immobilized (1 h) before, concomitantly to, or after its administration, respectively. Three days after apomorphine, the dopamine D(1) receptor agonist 1-Phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SKF 38393, 3 mg/kg s.c.) was administered to all animals. Rats that had performed rotational behavior following apomorphine administration displayed robust contraversive rotational behavior in response to SKF 38393, whereas rats that had been immobilized concomitantly to, but neither before nor after apomorphine, did not. To clarify whether stress, which may be increased by immobilization, mediated the results observed, additional rats received apomorphine paired with immobilization plus the corticosterone-synthesis inhibitor metyrapone (100 mg/kg i.p.), or apomorphine paired with a tail stressor, being not immobilized. Metyrapone did not affect the capacity of immobilization to prevent priming and tail stressor imposition did not affect priming magnitude, suggesting that stress has minimal or no effect on the results observed. This study demonstrates how movement performance following initial dopaminergic stimulation governs the occurrence of a sensitized, abnormal, motor response to a subsequent dopaminergic challenge in dopamine-denervated rats.

Compulsive, abnormal walking caused by anticholinergics in akinetic, 6-hydroxydopamine-treated rats.

In otherwise profoundly akinetic rats that had been severely depleted of brain catecholamines, anticholinergic drugs caused excessive walking. The effect did not appear until 10 days after surgery and then increased with time, suggesting that a phenomenon analogous to denervation supersensitivity may be involved. If the animals walked into corners, they were unable to turn around or back out. Their gait (extremely short steps) was reminiscent of that of patients with Parkinson's disease. The results are consistent with a mutually antagonistic interaction between cholinergic and dopaminergic brain systems and emphasize certain complexities in this interaction.

Sex-specific ultrasonic vocalization patterns and alcohol consumption in high alcohol-drinking (HAD-1) rats.

Ultrasonic vocalizations (USVs) have been established as an animal model of emotional status and are often utilized in drug abuse studies as motivational and emotional indices. Further USV functionality has been demonstrated in our recent work showing accurate identification of selectively-bred high versus low alcohol-consuming male rats ascertained exclusively from 22 to 28kHz and 50-55kHz FM USV acoustic parameters. With the hypothesis that alcohol-sensitive sex differences could be revealed through USV acoustic parameters, the present study examined USVs and alcohol consumption in male and female selectively bred high-alcohol drinking (HAD-1) rats. For the current study, we examined USV data collected during a 12-week experiment in male and female HAD-1 rats. Experimental phases included Baseline (2weeks), 4-h EtOH Access (4weeks), 24-h EtOH Access (4weeks) and Abstinence (2weeks). Findings showed that both male and female HAD-1 rats spontaneously emitted a large number of 22-28kHz and 50-55kHz FM USVs and that females drank significantly more alcohol compared to males over the entire course of the experiment. Analyses of USV acoustic characteristics (i.e. mean frequency, duration, bandwidth and power) revealed distinct sex-specific phenotypes in both 50-55kHz FM and 22-28kHz USV transmission that were modulated by ethanol exposure. Moreover, by using a linear combination of these acoustic characteristics, we were able to develop binomial logistic regression models able to discriminate between male and female HAD-1 rats with high accuracy. Together these results highlight unique emotional phenotypes in male and female HAD-1 rats that are differentially modulated by alcohol experience.

Neurological deficits and brain edema after intracerebral hemorrhage in Mongolian gerbils.

We examined the time course of neurological deficits in gerbils after an intracerebral hemorrhage (ICH) induced by autologous blood infusion and examined its correlation with the severity of perihematomal edema. Mongolian gerbils (n = 15) were subjected to stereotaxic autologous blood infusion (30 or 60 microL) into the left caudate nucleus. Corner-turn and forelimb-placing tests were performed before, and 1 and 3 days after ICH. Perihematomal water content was measured by tissue gravimetry. Gerbils developed neurological deficits and perihematomal edema at day 1 after ICH. Both neurological deficits and perihematomal edema were significantly greater in animals with 60 microL blood infusion compared to the 30 microL infusion group, and both neurological deficits and edema were also greater at 3 days compared to 1 day after ICH. The severity of neurological deficits paralleled the degree of perihematomal edema. We conclude that the Mongolian gerbil is a suitable model for studies on the behavioral effects of ICH.

Early pubertal female rats are more resistant than males to 6-hydroxydopamine neurotoxicity and behavioural deficits: a possible role for trophic factors.

PURPOSE: The infusion of 6-hydroxydopamine (6-OHDA) into the nigrostriatal pathway in rats is commonly used to produce an animal model of Parkinson's disease (PD). However, most studies use male adult animals only. The present study focused on possible gender differences in vulnerability to 6-OHDA during the early pubertal period when the effects exerted by gonadal steroid hormones are unpronounced. METHODS: Young Sprague-Dawley rats, 35 days of age, were given a low vs. a higher dose of 6-OHDA in the medial forebrain bundle (MFB). Control rats received equivalent saline infusions. At 14 days post-surgery the rats were evaluated for forelimb akinesia. RESULTS: For the higher dose of 6-OHDA the female rats were less impaired than males in making adjustment steps in response to a weight shift and in a vibrissae-evoked forelimb placing test. Tyrosine hydroxylase (TH) immunoreactivity was significantly higher for the female rats. CONCLUSION: Early gender differences in cell survival factors and/or other promoters of neuroplasticity may have contributed to the beneficial outcome in the females. For example, NGF was found to be higher in the female rats following administration of DA neurotoxin. It is unclear whether gonadal steroids are involved, and if so, whether female hormones are protective or whether male hormones are prodegenerative. Determining the mechanisms for the improved outcome in the young female rats may lead to potential treatment strategies in PD.

Behavioral recovery from unilateral photothrombotic infarcts of the forelimb sensorimotor cortex in rats: role of the contralateral cortex.

During sensorimotor recovery following stroke ipsi- and contralesional alterations in brain function have been characterized in patients as well as animal models of focal ischemia, but the contribution of these bilateral processes to the functional improvement is only poorly understood. Here we examined the role of the homotopic contralateral cortex for sensorimotor recovery after focal ischemic infarcts at different time periods after the insult. One group of animals received a unilateral single photothrombotic infarct in the forelimb sensorimotor cortex, while four additional groups received a second lesion in the contralateral homotopic cortex either immediately or 2 days, 7 days, or 10 days after the first infarct. The time course of functional recovery of the impaired forelimbs was assessed using different sensorimotor scores: forelimb-activity during exploratory behavior and frequency of forelimb-sliding in the glass cylinder as well as forelimb misplacement during grid walking. Focal infarcts in the forelimb sensorimotor cortex area significantly impaired the function of the contralateral forelimb in these different behavioral tests. The subsequent damage of the contralateral homotopic forelimb sensorimotor cortex only affected the forelimb opposite to the new lesion but did not reinstate the original deficit. The time course of sensorimotor recovery after bilateral sequential cortical infarcts did not significantly differ from animals with unilateral single lesions. These data indicate that following small ischemic cortical infarcts in the forelimb sensorimotor cortex the contralateral cortex homotopic to the lesion plays only a minor role for functional recovery.

Alterations in intracerebral hemorrhage-induced brain injury in the iron deficient rat.

BACKGROUND: Iron contributes to brain edema and cellular toxicity after intracerebral hemorrhage (ICH). Knowledge regarding ICH in the context of iron deficiency anemia (IDA), a common nutritional disorder, is limited. OBJECTIVE: To determine the effect of IDA on brain and behavioral outcome after ICH in rats. METHODS: Six-week-old male rats (n = 75) were randomized to non-IDA or IDA groups. After 1 month of iron sufficient or deficient diets, 100 microl autologous blood was infused into the right basal ganglia (BG). Brains were assessed for iron concentration, regional water content, BG transferrin, and transferrin receptor concentrations after ICH. Recovery of upper extremity sensorimotor function was assessed. Brain and behavioral variables were compared by diet group. Significance was set at p < 0.05. RESULTS: Whole brain iron was decreased and water content was increased for IDA rats in injured cortex and BG at day 3 (p < 0.05) compared with non-IDA rats. Transferrin and transferrin receptor content were increased in injured BG for IDA compared to non-IDA in the first week after ICH (p < 0.05). IDA rats had greater left vibrissae-stimulated forelimb-placing deficits and forelimb-use asymmetry than non-IDA after ICH (p < 0.05). CONCLUSIONS: Brain iron status may be an important determinant of injury severity and recovery after ICH.

Forced limb-use effects on the behavioral and neurochemical effects of 6-hydroxydopamine.

Rats with unilateral depletion of striatal dopamine (DA) show marked preferential use of the ipsilateral forelimb. Previous studies have shown that implementation of motor therapy after stroke improves functional outcome (Taub et al., 1999). Thus, we have examined the impact of forced use of the impaired forelimb during or soon after unilateral exposure to the DA neurotoxin 6-hydroxydopamine (6-OHDA). In one group of animals, the nonimpaired forelimb was immobilized using a cast, which forced exclusive use of the impaired limb for the first 7 d after infusion. The animals that received a cast displayed no detectable impairment or asymmetry of limb use, could use the contralateral (impaired) forelimb independently for vertical and lateral weight shifting, and showed no contralateral turning to apomorphine. The behavioral effects were maintained throughout the 60 d of observation. In addition to the behavioral sparing, these animals showed remarkable sparing of striatal DA, its metabolites, and the expression of the vesicular monoamine transporter, suggesting a decrease in the extent of DA neuron degeneration. Behavioral and neurochemical sparing appeared to be complete when the 7 d period of immobilization was initiated immediately after 6-OHDA infusion, only partial sparing was evident when immobilization was initiated 3 d postoperatively, and no sparing was detected when immobilization was initiated 7 d after 6-OHDA treatment. These results suggest that physical therapy may be beneficial in Parkinson's disease.

Transplants of fibroblasts genetically modified to express BDNF promote regeneration of adult rat rubrospinal axons and recovery of forelimb function.

Adult mammalian CNS neurons do not normally regenerate their severed axons. This failure has been attributed to scar tissue and inhibitory molecules at the injury site that block the regenerating axons, a lack of trophic support for the axotomized neurons, and intrinsic neuronal changes that follow axotomy, including cell atrophy and death. We studied whether transplants of fibroblasts genetically engineered to produce brain-derived neurotrophic factor (BDNF) would promote rubrospinal tract (RST) regeneration in adult rats. Primary fibroblasts were modified by retroviral-mediated transfer of a DNA construct encoding the human BDNF gene, an internal ribosomal entry site, and a fusion gene of lacZ and neomycin resistance genes. The modified fibroblasts produce biologically active BDNF in vitro. These cells were grafted into a partial cervical hemisection cavity that completely interrupted one RST. One and two months after lesion and transplantation, RST regeneration was demonstrated with retrograde and anterograde tracing techniques. Retrograde tracing with fluorogold showed that approximately 7% of RST neurons regenerated axons at least three to four segments caudal to the transplants. Anterograde tracing with biotinylated dextran amine revealed that the RST axons regenerated through and around the transplants, grew for long distances within white matter caudal to the transplant, and terminated in spinal cord gray matter regions that are the normal targets of RST axons. Transplants of unmodified primary fibroblasts or Gelfoam alone did not elicit regeneration. Behavioral tests demonstrated that recipients of BDNF-producing fibroblasts showed significant recovery of forelimb usage, which was abolished by a second lesion that transected the regenerated axons.

The role of thrombin in gliomas.

BACKGROUND: In a previous study we found that intracerebral infusion of argatroban, a specific thrombin inhibitor, reduces brain edema and neurologic deficits in a C6 glioma model. OBJECTIVES: To examine the role of thrombin in gliomas and whether systemic argatroban administration can reduce glioma mass and neurologic deficits and extend survival time in C6 and F98 gliomas. METHODS: The presence of thrombin in human glioblastoma samples and rat C6 glioma cells (in vitro and in vivo) was assessed using immunohistochemistry. The effect of thrombin on C6 cell proliferation in vitro was assessed using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide assay. The role of thrombin in vivo was assessed in rat C6 and F98 glioma cell models using argatroban, a thrombin inhibitor. The effects of argatroban on tumor mass, neurologic deficits and survival time were investigated. RESULTS: Thrombin immunoreactivity was found in cultured rat C6 glioma cells and human glioblastomas. Thrombin induced C6 cell proliferation in vitro. In C6 glioma, argatroban reduced glioma mass (P < 0.05) and neurologic deficits (P < 0.05) at day 9. In F98 glioma, argatroban prolonged survival time (P < 0.05). CONCLUSION: These results suggest that thrombin plays an important role in glioma growth. Thrombin may be a new therapeutic target for gliomas.

Systemic use of argatroban reduces tumor mass, attenuates neurological deficits and prolongs survival time in rat glioma models.

Our previous studies showed that intracerebral infusion of argatroban, a specific thrombin inhibitor, reduces brain edema and neurological deficits in a C6 glioma model. The present study investigated whether systemic argatroban administration can reduce glioma mass and neurological deficits and extend survival time in C6 and F98 gliomas. Rat C6 or F98 glioma cells were infused into the right caudate of adult male Fischer 344 rats. Osmotic minipump loaded with argatroban (0.3 mg/hour) or vehicle was implanted into abdomen immediately after glioma implantation. Tumor mass was determined at day 9. Over the period of the experiment, the animals underwent behavioral testing (forelimb placing and forelimb use asymmetry). In addition, survival time was tested in the F98 glioma model. In C6 glioma, argatroban reduced glioma mass (p < 0.05) and neurological deficits (p < 0.05) at day 9. In F98 glioma, agratroban prolonged the survival time (p < 0.05) and reduced the body weight loss (84 +/- 15 gram vs. 99 +/- 2 gram in the vehicle group, P < 0.05). In conclusion, systemic use of argatroban reduced tumor mass and neurological deficits, and prolonged survival time. These results suggest that thrombin plays a key role in glioma growth and thrombin inhibition with argatroban may be a novel treatment for gliomas.

Experience-associated structural events, subependymal cellular proliferative activity, and functional recovery after injury to the central nervous system.

Considerable structural plasticity is possible in the damaged neocortex and connected brain areas, and the potential for significant functional recovery remains even during the chronic phases of the recovery process. In this article, the authors review the literature on use-dependent morphologic events, focusing on the direct interaction of behavioral experience and structural changes associated with plasticity and degeneration. Experience-associated neural changes have the potential to either hinder or enhance functional recovery; therefore, issues concerning the nature, timing, and intensity of behavior-based intervention strategies are addressed.

CNS plasticity and assessment of forelimb sensorimotor outcome in unilateral rat models of stroke, cortical ablation, parkinsonism and spinal cord injury.

We have reviewed a battery of useful tests for evaluating sensorimotor function and plasticity acutely and chronically in unilateral rat models of central nervous system injury. These tests include forelimb use for weight shifting during vertical exploration in a cylindrical enclosure, an adhesive removal test of sensory function, and forelimb placing. These tests monitor recovery of sensorimotor function independent of the extent of test experience. Data are presented for four models, including permanent focal ischemia, focal injury to the forelimb area of sensorimotor cortex, dopaminergic neurodegeneration of the nigrostriatal system, and cervical spinal cord injury. The effect of the dendrite growth promoting factor, Osteogenic Protein-1 (OP-1) on outcome following permanent middle cerebral artery (MCA) occlusion was used as an example to illustrate how the tests can be applied preclinically. OP-1 showed a beneficial effect on limb use asymmetry in the cylinder test.

Focal brain injury, FGF-2 and the adverse effects of excessive motor demand on cortical and nigral degeneration: marked protection by delayed intermittent exposure to halothane.

The neuroprotective potential of halothane anesthesia was investigated following unilateral electrolytic lesions to the forelimb representation area of the sensorimotor cortex (FL-SMC). Previously, it was found that the FL-SMC lesion increases substantially in size when the intact forelimb is immobilized with a plaster of paris cast for the first 7 days postlesion, which forces extreme overuse of the impaired forelimb during a time when nonlethally damaged tissue is vulnerable to behavioral demand. Initially, the purpose of this study was to investigate whether intracisternal infusion of basic fibroblast growth factor (bFGF or FGF-2), a potent neurotrophic factor that has been shown to have neuroprotective and plasticity promoting properties in focal stroke and other injury models, could prevent this use-dependent exaggeration of injury. Although intracisternal bFGF (starting 24 h after surgery, twice per week) was not found to produce significant neuroprotective or behavioral effects, the brief exposure to halothane anesthesia (15-20 min) during bFGF or vehicle administration was found to prevent expansion of the lesion size, and to reduce delayed loss of neurons in the substantia nigra pars reticulata (SNr). The data have implications for investigations of the effects of neurotrophic factor in vivo, and other investigations requiring brief, intermittent halothane anesthesia.

Sensorimotor impairment and recovery of function in brain-damaged rats: reappearance of symptoms during old age.

Following unilateral lesions in the posterior-lateral hypothalamic area, rats displayed impaired orienting behavior to tactile stimuli placed on the contralateral side of the body, whereas sham-operated animals showed no sensorimotor impairment. Recovery from this impairment occurred during the first postoperative month. As the animals became senescent, there was a reinstatement of contralateral sensorimotor impairment in the brain-damaged animals. Although preliminary, these data may have important implications for the study of aging-dependent neurological and psychiatric disorders.

Aging and atropine effects on spatial navigation in the Morris water task.

A recent animal model that has been particularly useful in the neurobiology of aging has been the age-related decline of spatial information processing capacity in Sprague-Dawley rats measured in the place-learning water task developed by Morris (1981). In the first experiment of the present study, place behavior was examined in young (6 months), old (23-24 months), and very old (28 months) rats of another strain, Long-Evans. As an analogue of aging-related cholinergic dysfunction the effects of atropine sulfate (5-50 mg/kg), an anticholinergic drug that is known to disrupt behavior in this task, also was determined. Place navigation was not impaired in undrugged rats, even those in the oldest age group. Rats treated with atropine showed dose-dependent deficits. In a second experiment, young (4-5 months), old (18-20 months), and very old (28 months) Fischer-344 rats were examined. Place navigation was impaired in the old rats. The very old (28 months) rats could not swim well enough to be tested adequately. Although nonspatial deficits associated with aging may be found across most strains tested, there appear to be very large strain-related differences in spatial processing ability as a function of age.