A critical maturational period of reduced brain vulnerability to injury. A study of cerebral glucose metabolism in cats.

We have developed a feline cerebral hemispherectomy model as an analog to the surgical procedure used in pediatric intractable epilepsy. Previous work with this model has shown a remarkable plasticity associated with an early period of brain development, which we have defined using morphological, cerebral metabolic and behavioral methods. However, the important functional-metabolic bracketing of this period has not yet been performed. We have conducted the present study to answer questions raised by our previous findings using [14C] 2-deoxy-D-glucose autoradiography but only including animals lesioned at day 10 postnatally (P10) or in adulthood. The questions were; (a) is there any age better than P10 for an optimal metabolic outcome?, and (b) can we determine a cutoff point for the beneficial effects of the young age-at-lesion? Twenty-one adult cats were studied. Seven cats served as intact controls, five received a left hemineodecortication at P30, three at P60, three at P90 and three at P120, respectively. Histological analysis indicated that the extent of the lesion was similar between the age groups. Local glucose metabolic rates (LCMR(glc)) were measured in 50 structures bilaterally and used to calculate overall LCMR(glc) for seven grouped sites within the cerebral cortex, thalamus, basal ganglia, mesencephalic tegmentum (and tectum), limbic system and cerebellum. Results indicated a widespread bilateral depression of LCMR(glc) in all age-at-lesion groups. The depression in overall LCMR(glc) across all structures measured in each hemisphere was significant (P<0.05) for the P120 group relative to intacts for both ipsilateral (left) and contralateral (right) sides of the brain. The ipsilateral thalamus was the region most effected by the injury, with significant losses for all age-at-lesion groups. In addition, while there were widespread depressions for all lesion groups, these losses were significant for the P120 group in five groups of structures ipsilaterally (thalamus, basal ganglia, tectum, limbic system, cerebellum) and in three contralaterally (thalamus, tectum, cerebellum). In contrast, significant depressions for the earlier age-at-lesion groups (P30, P60, P90) were found only in the ipsilateral thalamus and bilaterally in the tectum. These results, together with our previous results for the P10 group, indicate a relative sparing of LCMR(glc) after hemineodecortication during the first 60 days of life, with gradually decreasing plasticity thereafter, such that there is some residual sparing at 90 days of age, and afterwards an almost complete loss of metabolic plasticity, with lesions at P120 producing a dismal outcome. These results complement earlier morphological and behavioral studies and support the concept of a 'Critical Maturational Period' of reduced vulnerability to developmental injury.

The growth of the feline brain from late fetal into adult life. I. A morphometric study of the neocortex and white matter.

We measured the growth of the neocortex (NCx) and telencephalic white matter (WM) in the brain of 64 cats allocated to the following 11 age-groups: fetal (E) 59 days (birth is at E63-65), postnatal (P) days 1, 7, 15, 30, 45, 60, 90, 120, 180, and adult. There were six subjects per group (except for E59, n=4). Using a projection microscope and cytochrome oxidase-stained coronal sections, a total of 4300 and 4325 sections at left and of 4282 and 4264 sections at right were drawn for the NCx and for the WM, respectively. With computer assistance, the drawings were digitized to calculate mean cross-sectional area and then the mean volume of each structure per age-group. The two structures grew heterochronously. In terms of percentage of the adult volume, for the left side (both side grew at a similar rate), the size of the NCx grew very fast from a 15.7% at E59 to an adult-range value of 93.7% at P30. In contrast, the WM grew slowly. Starting at a larger volume of 55%, the WM was only 72. 5% of the adult size at P30 reaching an adult-range value only by P180 (94.7%). After P30, both structures showed a small, albeit consistent, left versus right asymmetry with the right size been larger at all (but fetal) ages by a margin ranging between 0.4 and 4. 1%. In addition, after P30 the NCx tended to overgrow with all groups showing higher values relative to adult cats, and reaching significance at P60 (volume higher by 19.2%, P<0.01) and at P180 (higher by 14.5%, P<0.05). For the NCx there were no within group correlations between volume of the structures and the subjects' body weight, while a positive correlation was present for four of the WM postnatal groups. There were no correlations between the size of the structures and the sex of the cats. The data is discussed in the context of the extant human and animal literature and, in the ensuing paper, also within the context of growth of subcortical structures.

The growth of the feline brain from fetal into adult life. II. A morphometric study of subcortical nuclei.

As a continuation of the morphometric studies on the preceding paper, here we report on the rate of growth of the caudate nucleus (n.), thalamus, red n., and the substantia (s.) nigra using, with few exceptions, the same cohort of cats. The same previously used brains (n=64 cats) were allocated to the following age groups: fetal (E) 59 days, postnatal (P) days 1, 7, 15, 30, 45, 60, 90, 120, and 180. Sixteen additional cats, interspersed within the groups, were substituted for the red n. and s. nigra studies. There were six subjects per group (except for E59, n=4). Using a projection microscope and cytochrome oxidase-stained coronal sections, a combined (left plus right sides) total of 4693, 3822, 1636, and 1180 sections were drawn for the caudate, thalamus, s. nigra, and red n., respectively. With computer assistance, the drawings were digitized to calculate mean cross-sectional areas and then the mean volume of each structure per group. The growth time tables for the caudate n., thalamus and s. nigra were fairly synchronous. In terms of percentage of the adult volume, for the left side (both sides grew at a similar rate), the three structures grew at a fast pace between E59 and P30. Thus, at E59 their respective percentages relative to adult volume were 23.7, 29.8 and 22.6% and by P30 the percentages were within adult range (85.2, 115.1 and 87.5%, respectively). Starting at P30, for the thalamus and at P45 for the caudate n., there was a consistent tendency to an overgrow which ranged between 4.3 and 30.9% (at P180, P<0.5) for the caudate and between 0.3 and 15.1% for the thalamus. In addition, starting at P30, the right thalamus tended to be consistently larger than the left by a margin ranging between 0.5 and 11.2% (P120, P<0.05). The red n. grew at a different, slower pace. Starting from a fetal volume equivalent to an 18.6% of adult size, its volume was only a 61.0% of the adult value at P30 and came within range of adulthood size only by P60 (81. 3%). Neither the s. nigra nor the red n. showed any consistent tendency to overgrow or to asymmetry. These findings are discussed in the context of the literature. Furthermore, we discuss general conclusions and considerations pertaining to both papers as well as draw comparisons with the maturational time tables of other developmental landmarks in cats. Finally, in a comparison with growth of human brain structures, we point at the limitations and complexities involved in studying human material and, noting interspecies similarities, we propose that the present data from an advanced gyrencephalic mammal may form the bases for a model of structures maturation in humans.

Regional age-dependent effects of hemineodecortication upon contralateral neocortical thickness: comparison with other measures of cortical size.

This study investigated age-dependent changes in regional neocortical thickness after hemineodecortication in cats and compared the results to previously reported volumetric and cross-sectional data. Subjects sustained hemineodecortication on postnatal days (P) P10, P30, P60 or in adulthood. Neocortical thickness was quantified at 115 sites along 20 stereotaxic coronal anterior-posterior (AP) planes using defined sites of the main cerebral sulci for the measurements. The analysis established significantly lower thickness values for adult-lesioned as compared to (a) P30, P60 and control groups at AP +14, (b) P30 group at 7 planes along a range of AP +9 to AP +3, and (c) P10 and P60 groups at AP +6. Both the P10 and the P30 groups presented a significantly thicker neocortex than controls at select coronal planes clustering behind AP +10 (parietal and temporal cortices). When analyzed by sulcus, results once again reflected significant advantages for the early-lesioned cats with a significantly thicker cortex found at 4 of the 8 sulci examined. Again, significant advantages were also discovered for early-lesion subjects compared with control cats (splenial, cruciate sulci). Overall, the range of significant effects (from AP +14 to AP 0) and the direction of the means suggested that there was a significant, age-dependent (P10-P60), regional sparing of neocortical thickness with a peak effect occurring at P30. We concluded that: (a) there was a regional sparing/increase of neocortical thickness suggesting that discrete cortical areas are selectively involved in the resistance to structural atrophy following hemineodecortication in young cats (P10-P60) and (b) the global loss of neocortex volume found in our previous study was not apparent using the present thickness measurement. It is suggested that both of these measurements must be taken into account when assessing morphological effects upon the neocortex either in human pathology (i.e. hemispherectomy, intractable epilepsy) or in animal models.

Effects of a restricted unilateral neocortical lesion upon cerebral glucose and oxidative metabolisms in fetal and neonatal cats.

The present study was designed to measure cerebral glucose and oxidative metabolisms and to assess relationships with previously identified morphological changes in adult cats with a unilateral, restricted neocortical lesion sustained either during fetal life or neonatally. Local cerebral metabolic rates for glucose (LCMR(glc)) were measured using the [14C]2-deoxy-D-glucose (2 DG) autoradiography method and oxidative capacity was determined using cytochrome oxidase histochemistry (C.O.). Only glucose metabolism in the fetal-lesioned cats was affected substantially. There was a global decrease (31.0% relative to controls) of the LCMR(glc) for both cerebral hemispheres while focal decreases were seen mainly in thalamic and neostriatal nuclei (and reaching declines of over 50%). Cats with a neonatal lesion showed only a tendency to such declines (19.5% and 22.0% for the right and left hemispheres, respectively). C.O. values were not affected significantly either globally or locally in any of the age-at-lesion groups. In previous work using fetal animals with similar lesions, morphological evidence of subcortical neuropile degeneration was not observed; instead, a marked decrease in size of the ipsilateral remaining neocortex and a pronounced distortion of gyri and sulci patterns bilaterally were found. In this context, we propose that in the fetal-lesioned cats, there was a widespread lesion-induced decrease in corticofugal (and transcortical) synaptic inputs which was responsible for a decline in functional (synaptic) activities, and that this, in turn, caused a downturn in glucose utilization. In the neonatal cats minor degeneration, coupled with lack of reinnervation, would account for the tendency to 2 DG declines. These results indicate that the long-term metabolic response of the fetal brain to injury is also less adaptive than that of the neonatal brain. Since standard methods are available to measure cerebral metabolism in humans, our studies in animal models may help understanding the long term physiological consequences of developmental focal brain damage in patients as well as to predict the relationships between cerebral metabolism and the underlying long-term morphological effects of such lesions.

Dopaminergic and glutamatergic interactions in the expression of self-injurious behavior.

Self-injurious behavior occurring in persons with severe mental retardation is a clinically significant and poorly understood problem. Multiple neurotransmitter systems have been implicated in the pathogenesis of this behavior, particularly dopaminergic, opioidergic, and serotonergic systems. Pemoline, a central stimulant, administered systemically at high doses reliably produces self-biting behavior in the rat. The systemic bolus of pemoline produces sustained neostriatal levels of pemoline for over 24 h in a continuous infusion paradigm. Studies of the effect of cortical lesions on pemoline-mediated behaviors reveal that cortical damage, as is common in profound mental retardation, lowers the threshold for pemoline-induced self-biting behavior. Data from the corticostriatal slice suggests that sustained exposure to pemoline produces a shift in N-methyl-D-aspartate receptor-mediated responses rendering them more susceptible to dopaminergic enhancement. Thus, dopaminergic and glutamatergic interactions appear to play an important role in the development and expression of self-biting in the pemoline model.

A critical maturational period of reduced brain vulnerability to developmental injury. I. Behavioral studies in cats.

Groups of cats with resection of the neocortex of the left cerebral hemisphere at postnatal (P) ages (in days) 5-15 (P10), 30 (P30), 60 (P60), 90 (P90), 120 (P120), and in adulthood, were compared using a comprehensive battery of 16 neurobehavioral tests administered when they were at least 6 months post-lesion. For all behaviors, except 3 (including the paw contact placing reaction which never recovered), the performance was significantly better for the cats lesioned between P10 and P30 compared to cats lesioned at older ages. For 10 of the behaviors, the transition from age-at-lesion P30 to P60 was rather abrupt and characterized by a significant increment in impairments. However, cats with the resection at ages P90 and P120 still showed some behavioral advantage over the adult-lesioned animals. Overall, for most of the behaviors tested, there was a significant linear trend for an increase in the magnitude of impairments across the entire age-at-lesion range. We previously reported that cats with a unilateral frontal cortical lesion sustained during the late fetal life showed substantial behavioral impairments, while animals with a similar resection sustained early postnatally exhibited minimal abnormalities. These findings, together with the present results, indicate that the long-term behavioral outcome of neocortical injury is best when the lesion is sustained during a discrete period of the life of the cat. This period extends from about fetal age 55 days (the oldest lesion age in our fetal studies) to about P60, as shown in the present paper. For these reasons, we propose that there is a Critical Maturational Period (CMP) for optimal post injury brain and behavioral restoration. We hypothesize that this span of reduced vulnerability is linked to specific developmental morphological events which occur during the same time period. Since, as discussed, such ontogenetic events also occur in other mammal species (albeit at different chronological ages), we further propose that the timing of the CMP as delineated in cats, can be extrapolated to other higher mammals species including humans.

A critical period for reduced brain vulnerability to developmental injury. II. Volumetric study of the neocortex and thalamus in cats.

Groups of young adult cats with a left hemineodecortication at postnatal (P) ages (in days) 5-15 (P10), 30 (P30) 60 (P60), 90 (P90), 120 (P120) and in adulthood, were used to measure the volume of the thalamus, bilaterally, and of the remaining neocortex (right hemisphere). The same subjects were employed for the behavioral studies reported in the preceding paper. There was a bilateral, age-dependent, thalamic volume decrease. Ipsilateral to the resection, the thalamic shrinkage was the largest for the adult-lesioned cats (by 56.7%) and it was the smallest for the P30 group (43.4%), with a tendency towards a greater atrophy as the age at lesion increased. A similar pattern of atrophy was seen for the contralateral thalamus but the volume reduction was much less pronounced such that it was significant only for the four older age-at-lesion groups (ranging from 18.2% to 11.2% for the P120 and P90 groups respectively). Once again, the shrinkage was the smallest for the P30 group (5.3%). The remaining neocortex also shrunk in these animals, but the volume decrease was significant only for the adult-lesioned (17.8%) and the P120 group (15.4%), while the P30 group had practically no shrinkage (2.4%). The frontal cortex had no atrophy or it was minimal but the shrinkage gradually increased caudally such that all lesioned groups had some size reduction of the occipital cortex. The present results, together with the main conclusion of the preceding paper, indicate that there is a critical maturation period (CMP) of reduced forebrain vulnerability to neocortical injury which, in cats, tends to end between 30 to 60 days postnatally. The implications for developmental brain damage in other higher mammal species as well as the possible morphological ontogenetical underpinnings of this period are discussed.

Amphetamine and task-specific practice augment recovery of vibrissae-evoked forelimb placing after unilateral sensorimotor cortical injury in the rat.

This study investigated the relative contribution of amphetamine administration and task-specific practice during the period of drug action to recovery of forelimb-placing ability after unilateral electrolytic lesions of the sensorimotor cortex (SMC) in rats. Subjects were divided into groups receiving amphetamine plus postinjection forelimb-placing practice, amphetamine only, saline plus postinjection forelimb-placing practice, or saline only. The results revealed that groups of subjects receiving either amphetamine, postinjection practice, or a combination of these treatments exhibited the greatest enhancement of rates of vibrissae-evoked forelimb-placing recovery. These data suggest that these treatments can have an enduring beneficial effect on vibrissae-evoked forelimb-placing recovery without any immediate restorative effect on forelimb-placing ability. The recovery patterns and experimental evidence (see Feeney and Sutton, 1988; Chaouloff, 1989) suggest that the beneficial effect of the two therapies may be mediated by catecholamine release.

The effects of amphetamine on recovery of function after cortical damage in the rat depend on the behavioral requirements of the task.

The effects of amphetamine on the recovery of function following unilateral lesions of the rat somatic sensorimotor cortex (SMC) were examined. Rats with large SMC were tested on two measures of locomotor placing: the beam-walking test and the foot-fault test. Amphetamine produced an immediate and enduring facilitation of recovery on the beam-walking test. In contrast, the drug had no effect on the rats' ability to accurately place the forelimbs on the rungs of the elevated grid during locomotion on the foot-fault test. These data suggest that amphetamine may facilitate recovery when the requirements of the task produce a deficit in the initiation of locomotion but not when the animal is required to use somatosensory and proprioceptive cues to guide performance on the task. A second group of rats with smaller SMC lesions was evaluated with tactile-placing tests and the bilateral-tactile stimulation task. The forelimb placing reaction is elicited by unilateral tactile stimulation of the vibrissae or forelimb, whereas the ipsilateral asymmetry observed on the bilateral-tactile stimulation test has been interpreted as an impairment in processing stimuli presented on both sides of the body. On two measures of forelimb placing amphetamine produced a facilitation of recovery, but restoration of function was not observed during the period of drug intoxication. In contrast, amphetamine had no effect on recovery of function on the bilateral-tactile stimulation test. Taken together, these data suggest that the behavioral requirements of the task are an important factor in determining the facilitatory effects of amphetamine on recovery of function.