Effects of hypoxic-ischemic brain damage on dopaminergic markers in the neonatal rat: a regional autoradiographic analysis

Treadmill exercise ameliorates impairment of spatial learning ability through enhancing dopamine expression in hypoxic ischemia brain injury in neonatal rats

Substantia nigra and striatum are vulnerable to hypoxic ischemia brain injury. Physical exercise promotes cell survival and functional recovery after brain injury. However, the effects of treadmill exercise on nigro-striatal dopaminergic neuronal loss induced by hypoxic ischemia brain injury in neonatal stage are largely unknown. We determined the effects of treadmill exercise on survival of dopamine neurons in the substantia nigra and dopaminergic fibers in the striatum after hypoxic ischemia brain injury. On postnatal 7 day, left common carotid artery of the neonatal rats ligated for two hours and the neonatal rats were exposed to hypoxia conditions for one hour. The rat pups in the exercise groups were forced to run on a motorized treadmill for 30 min once a day for 12 weeks, starting 22 days after induction of hypoxic ischemia brain injury. Spatial learning ability in rat pups was determined by Morris water maze test after last treadmill exercise. The viability of dopamine neurons in the substantia nigra and dopamine fibers in the striatum were analyzed using immunohistochemistry. In this study, hypoxic ischemia injury caused loss of dopamine neurons in the substantia nigra and dopaminergic fibers in the striatum. Induction of hypoxic ischemia deteriorated spatial learning ability. Treadmill exercise ameliorated nigro-striatal dopaminergic neuronal loss, resulting in the improvement of spatial learning ability. The present study suggests the possibility that treadmill exercise in early adolescent period may provide a useful strategy for the recovery after neonatal hypoxic ischemia brain injury.

Vulnerability of the mesencephalic dopaminergic neurons of the human neonate to prolonged perinatal hypoxia: an immunohistochemical study of tyrosine hydroxylase expression in autopsy material

Experimental studies indicate that hypoxia to the fetus, a common occurrence in many birth complications in humans, results in long-term disturbances of the central dopaminergic (DA) systems that persist in adulthood. Because dysregulation of DA systems is involved in the pathophysiology of many neurological and psychiatric disorders, we investigated the effects of perinatal hypoxia on the mesencephalic DA neurons of the human neonate using immunohistochemistry. We studied the expression of tyrosine hydroxylase (TH), the first and rate-limiting enzyme in catecholamine synthesis, in substantia nigra, and ventral tegmental area of 18 neonates in relation to the age and severity/duration of hypoxic injury estimated by neuropathological criteria. In severe/abrupt perinatal hypoxia, intense TH staining was observed in substantia nigra, ventral tegmental area, and, surprisingly, in the nonpreganglionic Edinger-Westphal nucleus. In severe/prolonged hypoxia, there was a striking reduction or even absence of TH immunoreactivity in all the mesencephalic nuclei. These observations suggest that at early states of perinatal hypoxia, there is a massive increase in dopamine synthesis and release that is followed by feedback blockage of dopamine synthesis through inhibition of TH by the end product dopamine. Early dysregulation of DA neurotransmission could predispose infant survivors of severe perinatal hypoxia to dopamine-related neurological and/or cognitive deficits later in life.

Regional expression of dopamine D1 and D2 receptor proteins in the cerebral cortex of asphyxic newborn infants

Dopamine D(1) and D(2) receptor protein expression was examined by Western blotting in newborn infants dying from cerebral asphyxia between 31 and 42 weeks' gestation, and matched controls. Frontal, occipital, temporal, and motor cortex tissue samples were obtained at autopsy (median postmortem interval 35 hours) and frozen for storage at -80 degrees C. A total of 2 immunoreactive bands were detected with each primary antibody in infant brain, whereas a single band was present in adult human and rat tissue. Immunoreactivity varied between cortical areas for both receptors, but their regional patterns differed significantly. D(1) protein levels were higher in motor and temporal cortex than in frontal or occipital cortex. D(2) protein showed graded expression frontal > motor > occipital > temporal cortex. Asphyxia cases showed lower expression of the upper D(2) immunoreactive band, but no difference in regional pattern. Lower D(2) receptor expression may attenuate stress responses and underlie increased vulnerability to hypoxia at birth.

Dipyridamole promotes changes in calbindin-D28k and tyrosine hydroxylase expression in neonatal rats

BACKGROUND

Perinatal hypoxia alters the concentration of many neurochemicals in the brain, including adenosine, and promotes central nervous system (CNS) disorders in human infants such as periventricular leukomalacia or encephalopathy.

OBJECTIVE

Using the postnatal rat as a model of perinatal human development, we examined the effects of sustained increases in brain adenosine on CNS regions thought to be involved with both planning and execution of motor activity.

METHODS

To simulate hypoxia-induced changes in adenosine, Sprague-Dawley rats were injected twice daily from postnatal day (P) 3 to P14, with the adenosine uptake inhibitor dipyridamole (DIP) or the A(1) adenosine receptor agonist N(6)-cyclopentyladenosine (CPA). Vehicle-injected animals served as controls. Immunohistochemical and morphological analyses were then performed to examine the expression of calbindin D-28k (CB) and the thickness of the external granule cell layer (eGL) in the cerebellum. Additionally tyrosine hydroxylase (TH) expression in the caudate putamen and ventricular size were also examined.

RESULTS

In the cerebellum, both DIP and CPA reduced the number of CB-positive Purkinje cells as well as decreased the thickness of the eGL compared to vehicle. In the caudate putamen we found that DIP but not CPA decreased TH expression when compared to vehicle. Neither agent significantly altered ventricular size when compared to vehicle.

CONCLUSIONS

These observations suggest that elevations in brain adenosine, which can occur following hypoxia, leads to both neurochemical and cellular changes in regions of the brain which control the planning and execution of motor activity. Thus, therapeutic strategies that target brain regions most sensitive to adenosine may prevent or control at least some of the CNS damage observed following perinatal hypoxia.

Neonatal intermittent hypoxia impairs dopamine signaling and executive functioning

Mesotelencephalic dopamine (DA) pathways are exquisitely vulnerable to ischemic-anoxic insult. These insults are known to produce long-term derangements in DA signaling and have been hypothesized to contribute, at least in part, to pathologic behaviors such as cerebral palsy, schizophrenia, and attention deficit hyperactivity disorder (ADHD). Whether modest intermittent hypoxia, such as that encountered with repetitive apneas in premature infants, contributes to clinically significant impairments in DA signaling, and how these impairments manifest at a systems level, is unknown. To address these voids there is a need to develop animal models emulating features of a common disorder of prematurity, namely, apnea with hypoxia. Behavioral traits exhibited by such models include disturbed sleep-wake architecture, excessive locomotion, and impaired working memory persisting 1 to 2 months post-insult. Western-blot analysis of expression patterns of proteins involved in DA signaling (e.g., DA and vesicular monoamine transporters, tyrosine hydroxylase, and D1 receptors) are consistent with that which might be expected from hyper- or hypodopaminergic functioning in DA-responsive prefrontal cortex and striatal circuits, respectively. These novel observations suggest that intermittent hypoxia occurring during a period of critical brain development disrupts development of those mesotelencephalic pathways modulating the expression of sleep and wakefulness, locomotion, and executive functioning.

Cocaine-induced alterations in the density of monoaminergic receptors in the embryonic guinea pig cerebral wall

Quantitative receptor autoradiography was used to examine the effect of chronic cocaine exposure on the density of alpha1-, alpha2- and beta-adrenergic, 5-HT1A- and 5-HT2-serotonergic, and D1- and D2-dopaminergic receptors in the fetal guinea pig cerebral wall which contained forming motor area of the cerebral cortex. The pregnant guinea pig received two daily subcutaneous injections of 20 mg/kg cocaine beginning on the 20th day of pregnancy (E20). The control animals received injections of equivalent volume of saline. The receptor densities were examined between days 5-30 of the treatment, which corresponds to E25-E50. By the fifth day of treatment (E25), cocaine produced downregulation of all receptors studied throughout the entire depth of the fetal cerebral wall. More extended treatment, however, resulted in recovery of receptor levels. Finally, from days 20-30 of treatment (E40-E50) there was a significant upregulation of noradrenergic and dopaminergic receptor sites. These findings demonstrate that exposure to cocaine in utero can influence adrenergic, serotonergic, and dopaminergic receptors in the embryonic cerebral wall, which may lead to alteration in corticogenesis. Furthermore, the present study reveals that, in the course of chronic treatment, cocaine may completely reverse its receptor regulatory activity in the fetal brain.

Neuropathological characteristics and alteration of the dopamine D2 receptor in hypoxic-ischemic basal ganglia necrosis

The neuropathological characteristics and alteration of the dopamine D2 receptor (D2R) were investigated in 27 cases of hypoxic-ischemic basal ganglia necrosis (BGN) by means of neuropathological and immunohistochemical methods. Perinatal hypoxic-ischemic BGN manifested neuronal karyorrhexis as well as eosinophilia, karyorrhexis being more predominant in preterm infants and eosinophilia more predominant in full-term infants. Immunoreactivity to D2R was detected in the cytoplasm and dendrites of small and large neurons in the basal ganglia, and increased with neuronal maturation during the late gestational period in normal human basal ganglia. The number of D2R-positive neurons was smaller in all cases of acute BGN than that in controls, the areas of decreased D2R-positive neurons corresponding to the damaged regions observed on HE staining. Furthermore, neurons showed high expression of D2R in a few cases of remote BGN, suggesting some plasticity as to the recovery of D2R. Thus, the neuropathological characteristics of perinatal hypoxic-ischemic BGN may be related to neuronal maturation during different developmental stages in each region, and D2R development may play a role in the basal ganglia vulnerability to hypoxic-ischemia.

D1 and D2 dopamine receptors in perinatal and adult basal ganglia

There is reason to believe that dopamine is important in developmental programs of the basal ganglia, brain nuclei implicated in motor and cognitive processing. Dopamine exerts effects through dopamine receptors, which are predominantly of the D1 and D2 subtypes in the basal ganglia. Cocaine acts as a stimulant of dopamine receptors and may cause long-term abnormalities in children exposed in utero. Dopamine receptor (primarily D1) stimulation has been linked to gene regulation. Therefore, D1 and D2 receptor densities in perinatal and adult striatum and globus pallidus were examined using quantitative autoradiography. The most striking finding was that pallidal D1 receptor densities were 7-15 times greater in the perinatal cases than in the adult. Pallidal D2 receptor densities were similar at both ages. In both the adult and perinatal striatum, D2 receptor densities were greater in the putamen than in the caudate, and both D1 and D2 receptor densities were modestly enriched in caudate striosomes compared with the matrix. In both caudate and putamen, perinatal D1 receptor levels were within the adult range, whereas D2 receptor levels were only 50% of adult values. The development of D1 and D2 receptors appears to vary across the major subdivisions of the human basal ganglia. The facts that we found such extremely high levels of D1 receptors in the perinatal pallidum, and that D1 receptor activation influences gene regulation, suggest that the globus pallidus could be particularly susceptible to long-term changes with perinatal exposure to cocaine and other D1 receptor agonists or antagonists.

Evolution of dopamine receptors in the rat after neonatal hypoxia-ischemia: autoradiographic studies

The aim of this work was to follow the evolution of striatal dopamine D1 and D2 receptors after hypoxic-ischemic (H/I) insult in immature rats. SPET imaging of these receptors could be used as an index to assess brain dysfunctions after perinatal H/I without change in cerebral blood flow or neuronal loss. We submitted 1-week-old rat pups to unilateral ligation of the left carotid artery plus 2h exposure to 8% O2. After recovery periods of 1, 2 or 9 weeks, ex vivo and in vitro autoradiographic studies of dopamine receptors were performed on normal appearing brains using specific ligands usable in human SPET imaging. Striatal dopamine D2 receptors tended to decrease bilaterally after one week's recovery. The decrease then reached 40% at 3 weeks of age and at 10 weeks of age the level of receptors had returned to normal values. By contrast, no change in dopamine D1 receptors was seen, whatever the age studied. In conclusion, changes in dopamine D2 receptors could be a valuable index for SPET imaging to evaluate H/I brain damage in the absence of anatomical lesions.

Neuropathophysiology of movement disorders in cerebral palsy

Recent developments in understanding the pathophysiology of disordered motor control in cerebral palsy are reviewed. In spastic cerebral palsy, evidence for abnormal segmental as well as supraspinal control of motor neuron output exists. Impaired Ia inhibition of antagonist muscles has been suggested but recently contested. Evidence also supports the role of decreased presynaptic inhibition of Ia afferents and decreased nonreciprocal Ib inhibition. Furthermore, early cerebral injury results in reorganization of supraspinal (corticospinal) inputs to motor neuron pools. In extrapyramidal cerebral palsy, injury of basal ganglia or thalamus has been demonstrated. A scheme for understanding the neurochemical circuitry of the extrapyramidal system is discussed. Animal models and certain specific human diseases provide examples of how this circuitry may be disturbed, thereby resulting in an imbalance between the direct and indirect striatal output systems and in impaired motor control. Future studies employing postmortem neurochemical analysis, functional magnetic resonance imaging, and positron emission tomographic scanning may foster progress in this area.

Long-term effects of neonatal ischemic-hypoxic brain injury on sensorimotor and locomotor tasks in rats

Perinatal ischemia and/or hypoxia in humans are major risk factors for neurologic injury that often manifest as sensorimotor and locomotor deficits throughout development and into maturity. In these studies, we utilized an established model of neonatal ischemic-hypoxia that creates unilateral striatal, cortical, and hippocampal damage (Rice III, J.E., Vanucci, R.C. and Brierley, J.B., Ann. Neurol., 9 (1981) 131-141) to investigate sensorimotor and locomotor deficits in these animals during development and as adults. Sensorimotor deficits were examined by measuring the amount of time that the animals were able to remain on a rotating treadmill. Locomotor abnormalities were assessed by measuring apomorphine-induced rotational asymmetry. Following the neonatal ischemic-hypoxic episode, at 3-9 weeks of age, animals were not able to remain on the treadmill as long as their normal littermate controls. In addition, these animals demonstrated an abnormal, ipsiversive rotational asymmetry in response to systemic administration of apomorphine. When these animals reached adulthood, the degree of atrophy in specific regions of the damaged hemisphere was quantified using measurements of cross-sectional area. The mean cross-sectional area of the striatum was decreased by 29%, the sensorimotor cortex area by 26%, and the dorsal hippocampus cross-sectional area was approximately 6% of its normal size. These data suggest that this rodent model of neonatal ischemic-hypoxic brain injury results in cerebral atrophy and long-lasting sensorimotor and locomotor deficits. These particular behavioral tasks may be used in future studies to assess locomotor and sensorimotor deficits following neonatal ischemic-hypoxic brain injury.

The temporal evolution of striatal dopamine receptor binding and mRNA expression following hypoxia-ischemia in the neonatal rat

Neonatal hypoxic-ischemic (HI) brain injury in the rat alters dopamine receptors. To determine whether such changes are permanent, dopamine receptors and corresponding mRNA were examined at various time points after neonatal HI using receptor autoradiography and in situ hybridization. Rat pups underwent ligation of the left common carotid artery followed by hypoxic exposure (8.5% O2 for 3 h). Controls underwent sham surgery alone. Animals surviving for 2-80 days following HI were studied. Striatal D1 receptors (labeled by [3H]SCH23390) were reduced as early as 2 days following HI, remained depressed for 21 days, but recovered to control levels by young adulthood (3 months of age). D2 receptors (labeled by [125I] iodosulpride) did not decline until 10 days after HI, and remained uniformly depressed throughout the caudate-putamen thereafter. Changes in D1 receptor mRNA transcripts closely paralleled alterations in receptors: early reductions in D1 mRNA signal recovered by young adulthood. D2 mRNA exhibited a unique temporal profile with an early decrease (2 days following HI), and prompt, persistent recovery. Dopamine receptors and transcripts are differentially affected by HI injury early in development. Whereas D1 receptor expression recovers from neonatal HI injury, D2 receptors remain permanently affected despite the presence of normal levels of D2 receptor transcripts. A persistent, post-transcriptional effect of HI on D2 receptor expression is suggested.

Animal models of perinatal asphyxia: contributions, contradictions, clinical relevance

Animal models have contributed immensely to our understanding of hypoxic ischemic encephalopathy in the newborn. A number of animal models have been used, including both primate and subprimate species. Although the Rhesus monkey model has a dramatically similar pathological distribution of brain injury when compared with the human, other pathologic processes secondary to asphyxia may be more appropriately assessed in other species. The maxim that because primates are closer on the phylogenetic tree to humans than are subprimates all observations in the primate are applicable to the human is simply not true. Understanding of the neurochemical consequences of asphyxia in the past decade have arisen from experiments primarily in the neonatal rat. We have come to understand that not only is the hypoxic event of major significance, but that, once reperfused, reoxygenation causes further injury. Free-radical generation following reperfusion may be massive and may further contribute to cell membrane injury. These observations have lead to rational theoretic approaches to the treatment of hypoxic ischemic brain injury. On the other hand, previously used treatments such as osmotic agents and glucocorticoids would appear to be not only inefficacious but hazardous in the treatment of hypoxic ischemic brain injury. The role of nitric oxide (NO) in the pathogenesis of brain injury is yet uncertain, but there is little doubt that it plays a significant role. Although survival of the immature animal subjected to hypoxic environment is longer than in the mature animal, the central nervous system of the immature animal is more sensitive to glutamate and N-Methyl-D-aspartate (NMDA) receptor-mediated injury.

Effect of neonatal hypoxia-ischemia on nigro-striatal dopamine receptors and on striatal neuropeptide Y, dynorphin A and substance P concentrations in rats

Perinatal hypoxic-ischemic brain injury was induced in 7- to 8-day-old rats by ligating the left carotid artery with subsequent exposure to 9% oxygen atmosphere for 2.5 h. The animals were killed 7 days later and grouped according to the degree of brain injury sustained after hypoxia-ischemia. Total protein content measured in striatum ipsilateral to the ligation, and dissected from brains showing extensive damage, was reduced to 64% of contralateral tissue. The protein content was not altered in other groups including control animals exposed to air and in sham-operated animals exposed to hypoxic conditions. The concentration of (pg/mg protein) and total (pg/striatum) striatal dynorphin A-like immunoreactivity (DLI) from brains with extensive damage were increased to 481% and 285% of the contralateral side, respectively. Hypoxia-ischemia increased striatal neuropeptide Y-like immunoreactivity (NPYLI) concentration from brains with extensive damage to 157% of contralateral side, but when the results were expressed as total NPYLI content per striatum, NPYLI content in striatum with extensive damage remained unaltered. Substance P-like immunoreactivity (SPLI) concentration and total content per striatum from brains with extensive damage were reduced to 66% and 43% of the contralateral side, respectively. D1 and D2 receptor density in animals killed 10 days after injury was reduced by 24% and 22% of control, respectively, in striatum from brains with extensive damage. These results indicate complex changes in brain neuropeptides following neonatal hypoxia-ischemia. Damage in the substance P system could have functional effects on dopaminergic transmission while the increase in NPYLI and in DLI concentrations may respectively reflect the relative preservation from neuronal damage and possibly an increase in neuropeptide synthesis or decrease in release. The decrease in SPLI concentration and the increase DLI concentration induced by hypoxia-ischemia suggests that these peptides may be present in separate neurons.

Pharmacologic management of neonatal cerebral ischemia and hemorrhage: old and new directions

New developments in pharmacologic management of cerebral ischemia and hemorrhage are reviewed. A number of agents with diverse modes of action have now been shown to be neuroprotective in adult and neonatal animal models when administered either before or after a hypoxic-ischemic insult. As experience improves with these agents in hypoxic-ischemic injury and periventricular-intraventricular hemorrhage in human neonates, there is reason to be optimistic that effective neuroprotective strategies will soon be clinically available.