Embryonic pretreatment with bromodeoxyuridine blocks regeneration and functional recovery from perinatal medial frontal lesions in rats

Removal of the midline frontal cortex on postnatal day 10 is followed by apparent regeneration of the part of the lost cortex, correlated with substantial recovery of function in adulthood. The spontaneous regrowth of the medial frontal cortex after midline frontal lesions on postnatal day 10 was blocked by pretreatment with bromodeoxyuridine (BrdU) on embryonic days 11, 12, 13, 15, or 17. BrdU pretreatment on embryonic day 21 or postnatal day 10 did not block either functional recovery or cortical regrowth. These results demonstrate a teratological effect of BrdU and are consistent with the claim that functional recovery after midline frontal removal on postnatal day 10 is supported by the generation of new midline frontal tissue.     

Recovery from infant medial frontal cortical lesions in rats is reversed by cortical lesions in adulthood

Previous studies have shown that when the medial prefrontal cortex (mPFC) is removed at 7-10 days of age there is a spontaneous filling of the lesion cavity and a nearly complete restitution of behaviour. In the current study animals received mPFC lesions on postnatal day 10 and on day 160 the tissue occupying the mPFC region was again removed. Behavioural performance on the Morris water task was compared to animals with either only day 10 mPFC lesions or only day 160 mPFC lesions. Rats with the combined day 10 and day 160 lesions or day 160 lesions were severely impaired at the task whereas the rats with only day 10 lesions showed complete recovery. An analysis of dendritic arborization in pyramidal neurons adjacent to the lesion showed increased dendritic arborization in the basilar fields in both the P10 groups but this was not associated with functional recovery in the animals with the two mPFC lesions. It thus appears that the tissue that filled in the mPFC lesions on day 10 was functional.     

Very early treatment with fluoxetine and reboxetine causing long-lasting change of the serotonin but not the noradrenaline transporter in the frontal cortex of rats.

Interactions of the serotonergic and noradrenergic system at different sites of the brain may be important for efficacy and side effects of antidepressant drugs. Further, serotonin and noradrenaline play a critical role in the development of neurons during brain maturation. To gain further insight how brain maturation and the two monoaminergic systems are influenced by drug treatment during early postnatal development, this animal study investigated possible effects on the noradrenaline and serotonin transporter density of the frontal cortex very early in postnatal life. Rats were treated from postnatal day 2 to 5 either with fluoxetine (5 mg/kg per day s.c.) or with reboxetine (10 mg/kg per day s.c.). At day 90 the serotonin and noradrenaline transporter density in the frontal cortex was measured by ligand binding assay. Fluoxetine treatment led to a significant long-lasting increase of serotonin (not noradrenaline) transporter density (Bmax = 1231 +/- 34) in the frontal cortex (compared with saline-treated controls (Bmax = 1112 +/- 58)). Reboxetine treatment (surprisingly) led to an even more enhanced serotonin transporter density (Bmax = 1322 +/- 46), while noradrenaline transporter density seemed to be unaffected. There were no significant differences for KD values. The results support the idea that serotonin seems to play an important role during early brain development. Moreover, drug-related modulation of the noradrenergic system during brain maturation seems to cross-influence the serotonergic system.     

Early androgen treatment decreases cognitive function and catecholamine innervation in an animal model of ADHD

The spontaneously hypertensive rat (SHR) has been used as an animal model of attention deficit hyperactivity disorder (ADHD). The present study was designed to determine whether exposure to elevated androgen levels early in development demonstrated impairments in cognitive functioning, neuroendocrine control, and brain development parallel to those seen in ADHD children. The animals (SHR and Wistar (WKY) controls) were implanted with testosterone on postnatal day 10 and tested for behavior in a spatial cognition paradigm on postnatal day 45. Plasma samples were collected for determination of adrenocorticotrophin hormone (ACTH) and corticosterone levels as indicators of the basal tone of the pituitary-adrenal neuroendocrine axis. In addition, the density of tyrosine hydroxylase-immunoreactive fibers (an indicator of catecholamine innervation) in the frontal cortex was compared between animals. The current data show that early testosterone treatment in SHR animals resulted in additional deficits in spatial memory in the water maze, but was ineffective in altering the response of WKY animals. Furthermore, SHR rats had high basal ACTH and low corticosterone levels that may indicate a dysfunctional stress axis similar to other reports in humans with persistent ADHD. Finally, there was a further suppression of tyrosine hydroxylase-immunoreactivity in the frontal cortex of androgen-treated SHR rats. These results support the hypothesis that early androgen treatment may support the neurobiology of animals with genetic predisposition to hyperactivity, impulsivity and inattention in a manner consistent with the enhanced expression of ADHD-like behaviors.     

Ontogeny of dopaminergic function in the rat midbrain tegmentum, corpus striatum and frontal cortex

Ontogenic development of the dopaminergic system in rat brain was investigated. This was accomplished by monitoring changes in postsynaptic dopamine receptor formation and presynaptic dopamine content in the midbrain tegmentum, frontal cortex and corpus striatum from the 18th day of gestation through adulthood. The dopamine antagonist spiperone was used as the binding ligand to quantitate receptor number while dopamine content was measured chromatographically. [3H]Spiperone binding kinetics in adult animals revealed that the maximum number of receptor sites (Bmax) was 160, 900 and 597 fmol/mg protein in midbrain tegmentum, frontal cortex and corpus striatum, respectively, while the corresponding equilibrium constant (Kd) values were 0.15, 0.52 and 0.15 nM. During the course of development, the affinity for spiperone binding in corpus striatum and frontal cortex did not change significantly, while in midbrain tegmentum the binding affinity in younger animals was significantly lower. Results from competitive inhibition experiments using various serotonergic and dopaminergic antagonists suggested that at all ages dopamine D2-receptors were responsible for spiperone binding in corpus striatum and midbrain tegmentum. In frontal cortex, binding properties consistent with D2-receptors were observed in non-adult animals; by the time adulthood was reached, however, spiperone binding characteristics were altered and appeared to correspond to serotonin sites. The developmental patterns of the dopaminergic markers were different in all 3 tissues. Adult receptor levels were achieved very early in midbrain tegmentum, while increases in receptor number continued in corpus striatum and frontal cortex, at different rates, throughout the postnatal period. A marked increase in dopamine in corpus striatum occurred during the second and third postnatal weeks and the transmitter content remained relatively constant after this time. Transient fluctuations in endogenous dopamine during the postnatal period were observed in midbrain tegmentum and frontal cortex. A general feature of the ontogenic pattern in all tissues appeared to be increases in dopamine receptor preceding increases in dopamine synthesis. A hypothesis on the developmental regulation of dopamine neurons was derived.     

Lectins demarcate the barrel subfield in the somatosensory cortex of the early postnatal mouse

Plant lectins were used to examine the disposition of glycosylated molecules in vibratome sections through the barrel subfield of mouse somatosensory cortex at selected times during postnatal development. The peroxidase conjugates of peanut agglutinin (PNA, specific for N-acetylgalactosamine), concanavalin A (specific for mannose), and wheat germ agglutinin (specific for N-acetylglucosamine and N-acetylneuraminic acid) were used to study lectin binding in aldehyde-fixed tissue sections of cortex. Following peroxidase cytochemistry and light microscopy, it was found that all three lectins bound in the region of the barrel subfield as early as postnatal day 3 (day of birth = postnatal day 1). The lectins bound to the prospective sides and/or septae of individual barrels in preference to the prospective hollows. This lectin demarcation of the barrel field occurred prior to the detection of this region with cresyl violet staining and was still demonstrable on postnatal day 6, when the individual barrels became discernible with cresyl violet. This suggests that the lectin binding material is present before the barrel field becomes a fully formed and organized region. A decrease in lectin affinity for binding sites in these tissue sections occurs during postnatal development (Cooper and Steindler: Soc. Neurosci. (Abstr.) 10: 43a, '84) and this study demonstrates that lectins do not delineate the barrel field of more mature animals (2-3 months old), whereas barrels can be detected with cresyl violet at this time. A preliminary electron microscope analysis of the postnatal day 6 somatosensory cortex demonstrates that the lectin PNA binds to elements of the forming neuropil and also to Golgi apparatus intermediate saccules in neuronal cells. The prospective barrel field can be detected with lectins during a critical period in development in which alterations can occur in the barrel field in response to peripheral deprivation (Jeanmonod et al: Neuroscience 6:1503-35, '81) and therefore we suggest that the glycans visualized with lectin-peroxidase conjugates denote possible candidates for molecules involved in shaping barrel structure.     

Amphetamine with experience promotes recovery of locomotor function after unilateral frontal cortex injury in the cat

In cats, a single dose of D-amphetamine (AMP) given at 10 days after unilateral frontal cortex ablation produced an immediate and enduring (60 day) acceleration of beam-walking ability compared to saline control animals. Four doses of AMP at 4-day intervals promoted recovery faster than a single dose of AMP. Subjects with no beam-walking experience while under AMP intoxication were not different from saline controls after two doses of AMP. However, after 4 doses these cats recovered significantly faster than saline controls and were comparable to animals that received AMP and experience under the drug.     

Rehabilitation following early malnutrition in the rat: Body weight, brain size, and cerebral cortex development

Sprague-Dawley rats were malnourished by giving their mothers an 8% casein diet starting at day 10 of gestation, while controls were fed a 24% casein diet. Starting at postnatal day 20 (P20), rehabilitation of the malnourished animals was attempted by: (1) feeding both mother and young a 24% casein diet, (2) leaving the pups with their mothers until they were 40 days old, and (3) reducing the litter size from 8 to 4 pups. Observations were made on aldehyde-perfused tissue from animals 20, 40 and 70 days old. The somatosensory cortex from one hemisphere was embedded in Araldite, and that from the other side was processed fro Golgi staining. At 20 days of age the body weight of the malnourished animals was 21% that of the controls, but at 70 days it was no longer different. The anterior-posterior length, the width, and the height of the cerebral hemispheres were also significantly reduced at P20, but the differences had disappeared by P70. The thickness of area 3 of the cerebral cortex was measured in 1 micron sections. It was significantly reduced in the malnourished animals at P20, but at P40, following rehabilitation, the difference was no longer statistically significant. In tangential 1 micron sections the fraction of the volume of tissue occupied by neuropil was measured in layers II through IV. At P20 it was significantly reduced only in the upper half of layers II/III of the malnourished animals; at P40 this difference was no longer present. The mean volume of upper layer II/III cell bodies was estimated and found to be significantly reduced in the experimental animals at P20 but not at P40. In the Golgi preparations, pyramidal cells in upper layer II/III were studied. Their estimated volume, as well as the thickness of their basal dendrites, was significantly reduced in the 20 day malnourished animals, but not in the rehabilitated animals. These results show that animals severely malnourished until 20 days of age can reach normal body weight and attain cerebral hemispheres of normal size when proper nutrition is provided. The effects of malnutrition on the cerebral cortex of these animals are most apparent in upper layer II/III which, during the time of nutritional restriction, is the least developed of the cortical layers. However, when proper nutrition is provided, the cerebral cortex may attain normal morphology.     

Undernutrition and the developing cerebellar cortex in the rat.

Undernutrition of the newborn rats, produced during the first 3 weeks by increasing the litter size and restricting the mother's diet, resulted in reduction of the body and brain weights of the experimental animals. One group of undernourished animals showed especially severe reduction of body and cerebellar weights. These animals, on the 10th postnatal day, had an immature cerebellar cortex corresponding to that of the 7th day postnatal control animals. The external granular layer persisted in the cerebellar cortex of the underweight animals until the 23rd day, while it disappeared by 20th day in the control animals. Mitotic activity was evident until the 21st postnatal day in these animals while it stopped in the normal animal by 16th postnatal day. There was no marked difference in the fine structure of the various cell types in the control and undernourished animals. Midsagittal tracings of the cerebellar cortex showed a reduced surface area in the undernourished animals, while the thickness of the external granular layer and molecular layer did not show any significant difference when compared to that of the control animals, thus showing a reduction in total cell number, but not per unit area. The normal morphological appearance of the cerebellar cortex in the underfed animals of higher weight probably indicates that these animals are adequately nourished in spite of the reduction in weight when compared to the control animals, which probably are overfed.     

Distractibility following simultaneous bilateral lesions of the superior colliculus or medial frontal cortex in the rat

Hooded rats with bilateral lesions of the superior colliculus or medial frontal cortex were compared with controls for locomotor guidance in shuttling back and forth between goal-doors at two opposite ends of a large arena. Colliculectomized rats accomplished this with great accuracy. When flashing distractor lights were introduced midway down the runway, frontal corticals and controls were severely disrupted but colliculars continued to run normally. This result was obtained both when all training occurred postoperatively (Experiment 1) and when runway performance had been stabilized preoperatively (Experiment 2), thus after a long or short postoperative recovery interval. The results offer support for previous studies with rats which have demonstrated sensory 'neglect' but good locomotor guidance after collicular ablation. Frontal corticals differed from controls only in terms of their elevated rate of repeat door-pressing upon postoperative resumption of testing in Experiment 2. Despite the similarity between effects reported elsewhere of collicular and frontal lesions made unilaterally, bilateral deficits clearly demonstrable after collicular ablation were absent here after frontal lesions. The results imply that the functional responsibilities of superior colliculus and frontal cortex in the rat are separable; at least, they have different rates of functional recovery.     

FGF-2-induced cell proliferation stimulates anatomical, neurophysiological and functional recovery from neonatal motor cortex injury

Infant rats treated with basic fibroblast growth factor-2 (FGF-2) after postnatal day (P)10 motor cortical injury, show functional improvement in adulthood relative to those that do not receive FGF-2. In this study we used a combination of behavioural, immunohistochemical, electrophysiological, electron microscopic and teratological approaches to investigate possible mechanisms by which FGF-2 may influence functional recovery. We show that subcutaneous injections of FGF-2 following bilateral lesions to the motor cortex at P10 in the rat leads to filling of the lesion area with migrating neuroblasts and cycling cells. We assessed the functionality of this tissue in adulthood, and show that cells from the filled region spontaneously fire and form synapses. Behavioural analysis shows enhanced motor performance in the FGF-2-treated lesion rats in comparison to vehicle-treated lesion rats, and this improvement is reversed by removal of the tissue from the previously lesioned area or by blocking cortical regeneration by embryonic treatment with bromodeoxyuridine (BrdU). The results show that FGF-2 stimulates filling of the lesion cavity with cells after neonatal motor cortex lesions, that the new tissue has anatomical and physiological properties similar to control tissue, and that the filled region supports motor behaviour.     

TRKB signalling controls the expression of N-methyl-d-aspartate receptors in the visual cortex

NMDA receptors (NMDARs) are multimeric proteins, the biological and functional characteristics of which depend on differential subunit assembly during postnatal development. In the present paper, we investigated whether the expression of NMDAR subunits NR1, NR2A, NR2B is influenced by neurotrophins in rat visual cortex. We used a soluble form of the TrkB receptor engineered as an immunoadhesin (TrkB-IgG) in order to block TrkB ligands. TrkB-IgG was released through a cannula implanted in the occipital pole and connected to a mini-osmotic pump. TrkB-IgG was continuously released from postnatal day 20-21 (P20-21) to P36-37. In a different group of animals used as controls, osmotic pumps were filled with saline. Different antibodies were used to stain neurons expressing NR1, NR2A and NR2B. We counted the number of neurons stained for NR2A and NR2B subunits and expressed this as percentage with respect to the total number of cresyl-violet stained neurons in each cortical layer. In the visual cortex of TrkB-IgG-treated rats, the percentage of neurons expressing NR2A was significantly increased in all cortical layers. Concerning the NR2B subunit, the percentage of stained neurons was not significantly different between TrkB-IgG-treated and control rats. The staining level for both NR2A and NR2B, but not NR1, was reduced in all cortical layers in TrkB-IgG-treated animals. In agreement with this result, the endogenous levels of NR2A and NR2B subunits were reduced in TrkB-IgG-treated animals as shown by Western blotting. Thus, TrkB signalling controls the cellular expression of NMDAR subunits in visual cortical neurons during postnatal development.     

FGF-2 induces behavioral recovery after early adolescent injury to the motor cortex of rats

Motor cortex injuries in adulthood lead to poor performance in behavioral tasks sensitive to limb movements in the rat. We have shown previously that motor cortex injury on day 10 or day 55 allow significant spontaneous recovery but not injury in early adolescence (postnatal day 35 “P35”). Previous studies have indicated that injection of basic fibroblast growth factor (FGF-2) enhances behavioral recovery after neonatal cortical injury but such effect has not been studied following motor cortex lesions in early adolescence. The present study undertook to investigate the possibility of such behavioral recovery. Rats with unilateral motor cortex lesions were assigned to two groups in which they received FGF-2 or bovine serum albumin (BSA) and were tested in a number of behavioral tests (postural asymmetry, skilled reaching, sunflower seed manipulation, forepaw inhibition in swimming). Golgi-Cox analysis was used to examine the dendritic structure of pyramidal cells in the animals’ parietal (layer III) and forelimb (layer V) area of the cortex. The results indicated that rats injected with FGF-2 (but not BSA) showed significant behavioral recovery that was associated with increased dendritic length and spine density. The present study suggests a role for FGF-2 in the recovery of function following injury during early adolescence.     

Transplantation of hNT neurons into the ischemic cortex: cell survival and effect on sensorimotor behavior

Cell transplantation offers a potential new treatment for stroke. Animal studies using models that produce ischemic damage in both the striatum and the frontal cortex have shown beneficial effects when hNT cells (postmitotic immature neurons) were transplanted into the ischemic striatum. In this study, we investigated the effect of hNT cells in a model of stroke in which the striatum remains intact and damage is restricted to the cortex. hNT cells were transplanted into the ischemic cortex 1 week after stroke induced by distal middle cerebral artery occlusion (dMCAo). The cells exhibited robust survival at 4 weeks posttransplant even at the lesion border. hNT cells did not migrate, but they did extend long neurites into the surrounding parenchyma mainly through the white matter. Neurite extension was predominantly toward the lesion in ischemic animals but was bidirectional in uninjured animals. Extension of neurites through the cortex toward the lesion was also seen when there was some surviving cortical tissue between the graft and the infarct. Prolonged deficits were obtained in four tests of sensory-motor function. hNT-transplanted animals showed a significant improvement in functional recovery on one motor test, but there was no effect on the other three tests relative to control animals. Thus, despite clear evidence of graft survival and neurite extension, the functional benefit of hNT cells after ischemia is not guaranteed. Functional benefit could depend on other variables, such as infarct location, whether the cells mature, the behavioral tests employed, rehabilitation training, or as yet unidentified factors.     

Postnatal changes in the nitric oxide system of the rat cerebral cortex after hypoxia during delivery

The impact of hypoxia in utero during delivery was correlated with the immunocytochemistry, expression and activity of the neuronal (nNOS) and inducible (iNOS) isoforms of the nitric oxide synthase enzyme as well as with the reactivity and expression of nitrotyrosine as a marker of protein nitration during early postnatal development of the cortex. The expression of nNOS in both normal and hypoxic animals increased during the first few postnatal days, reaching a peak at day P5, but a higher expression was consistently found in hypoxic brain. This expression decreased progressively from P7 to P20, but was more prominent in the hypoxic group. Immunoreactivity for iNOS was also higher in the cortex of the hypoxic rats and was more evident between days P0 and P5, decreasing dramatically between P10 and P20 in both groups of rats. Two nitrated proteins of 52 and 38 kDa, were also identified. Nitration of the 52-kDa protein was more intense in the hypoxic animals than in the controls, increasing from P0 to P7 and then decreasing progressively to P20. The 38-kDa nitrated protein was seen only from P10 to P20, and its expression was more intense in control than in the hypoxic group. These results suggest that the NO system may be involved in neuronal maturation and cortical plasticity over postnatal development. Overproduction of NO in the brain of hypoxic animals may constitute an effort to re-establish normal blood flow and may also trigger a cascade of free-radical reactions, leading to modifications in the cortical plasticity.     

The effect of neonatal 6-hydroxydopamine treatment on synaptogenesis in the visual cortex of the rat

It has been proposed repeatedly that the noradrenergic (NE) system may exert an influence on cortical development. We have tested this proposition by examining synaptogenesis in the visual cortex of rats whose NE afferents were selectively lesioned by injections of the neurotoxin 6-hy-droxydopamine (6-OHDA). Control littermates were injected with equal volumes of vehicle. Montages of electron micrographs covering approximately 50 μm-wide strips of cortex were assembed from both groups of animals at 2,4,6,8,14, and 90 days of age. Symapses counts revealed a significantly higher density of synapses in the cortex of 6-OHDA-treated rats during the first week of postnatal life. The difference between the experimental and control rats was less apparent during the second postnatal week, and at day 90 the densities of synapses were similar for the two groups of animals. The enhanced density, which was the result of the increased number of Gray's type I synapses, was confined to the subplate region at day 2 but became more widespread in the cortex at subsequent stages of development. From these observations it would appear that the NE system exerts an inhibitory influence on synapse formation in the visual cortex in early postnatal life.     

Short- and long-term effects of low-dose prenatal X-irradiation in mouse cerebral cortex, with special reference to neuronal migration

To elucidate the short- and long-term effects of ionizing radiation on cell migration in the developing cerebral cortex, we labeled proliferating cells on day 14 of gestation of mice with bromodeoxyuridine (BrdU) followed by a single exposure to 0.1–1 Gy of X-rays. The brains of embryos on day 17 and offspring at 2, 3 and 8 weeks after birth were processed for BrdU immunohistochemistry to trace the movements of BrdU-labeled cells. The location of BrdU-labeled neurons in the cerebral cortex was quantitatively analyzed between irradiated animals and non-irradiated controls. We have demonstrated that the initial migration of BrdU-labeled cells from the matrix cell zone towards the cortical plate during embryonic periods was decelerated when exposed to X-rays of 0.25, 0.5 and 1 Gy on embryonic day 14, and that aberrantly placed neurons in the cerebral neocortex were noted in younger animals that were irradiated prenatally, whereas such derangement was less pronounced in mature animals. These observations suggest that some modification process might have occurred during the postnatal period. Received: 23 July 1996 / Revised, accepted: 18 October 1996     

Ontogeny of beta 1- and beta 2-adrenergic receptors in rat cerebellum and cerebral cortex

The development of beta 1- and beta 2-adrenergic receptors was studied in rat cerebral cortex and cerebellum. In the cerebral cortex, which contains mostly beta 1-adrenergic receptors, total beta-adrenergic receptor density increased sharply between postnatal days 10 and 21. The density of receptors remained fairly constant through 6 weeks of age and then subsequently declined. The proportion of beta 1 and beta 2 receptors was relatively constant throughout the development of the cerebral cortex. The development of the two receptor subtypes thus paralleled the development of total beta-adrenergic receptors in the cerebral cortex. The ontogeny of beta-adrenergic receptors in the cerebellum, which contains mainly beta 2 receptors, was strikingly different from that observed in the cortex. Total cerebellar beta receptor density exhibited a slow but steady increase from postnatal day 5 through day 42. The density of receptors then plateaued and remained constant until the animals were approximately 6 months of age. Unlike the results obtained in the cortex, the relative proportions of beta 1 and beta 2 receptors in the cerebellum changed markedly during development. Between postnatal days 8 and 13 approximately 18% of the receptors were of the beta 1 subtype. This proportion steadily decreased with age, and in 3- and 6-month-old animals only approximately 2% of the receptors were of the beta 1 subtype. The results demonstrate that the two subtypes of beta-adrenergic receptors can have different developmental patterns in the same brain area, and that a single receptor subtype can follow different developmental patterns in different brain regions. Possible correlations between the ontogeny of beta 1 and beta 2 receptors and various developmental events are discussed.     

Alterations of interneurons in the striatum and frontal cortex of mice during postnatal development

We investigated the postnatal alterations of neuronal nuclei (NeuN)-positive neurons, parvalbumin (PV)-positive interneurons, neuronal nitric oxide synthase (nNOS)-positive interneurons, and neurotrophic factors in the mouse striatum and frontal cortex using immunohistochemistry. NeuN, PV, nNOS, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) immunoreactivity were measured in 1-, 2-, 4- and 8-week-old mice. Total number of NeuN-positive neurons was unchanged in the mouse striatum and frontal cortex from 1 up to 8 weeks of age. In contrast, a significant decrease in the number of PV-positive interneurons was observed in the striatum and frontal cortex of 1-, 2- and 4-week-old mice. Furthermore, a significant increase of nNOS-positive interneurons was found in the striatum and frontal cortex of 1- and/or 2-week-old mice. NGF-positive neurons were unchanged in the mouse striatum from 1 up to 8 weeks of age. In the frontal cortex, a significant increase in the number of NGF-positive neurons was observed only in 1-week-old mice. In contrast, a significant increase in the number of NGF-positive glia 1 cells was found in the striatum and frontal cortex of 4-week-old mice. Our double-labeled immunostaining showed that nNOS immunoreactivity was not found in PV-immunopositive interneurons. Furthermore, BDNF immunoreactivity was observed in both nNOS-positive and PV-positive interneurons in the striatum of 1- or 2-week-old mice. These results show that the maturation of nNOS-immunopositive interneurons precedes the maturation of PV-immunopositive interneurons in the striatum and frontal cortex during postnatal development. Furthermore, our results demonstrate that the expression of BDNF may play some role in the maturation of interneurons in the striatum and frontal cortex during postnatal development. Moreover, our findings suggest that the expression of NGF in glia cells may play some role in the maturation of glial cells and PV-positive interneurons in the striatum and frontal cortex during postnatal development.