Birth insult interacts with stress at adulthood to alter dopaminergic function in animal models: possible implications for schizophrenia and other disorders.

Altered subcortical dopaminergic activity is thought to be involved in the pathophysiology of several disorders including schizophrenia, substance abuse and attention deficit hyperactivity disorder. Epidemiological studies have implicated perinatal insults, particularly obstetric complications involving fetal or neonatal hypoxia, as etiological risk factors for schizophrenia. This suggests the possibility that perinatal hypoxia might have lasting effects on dopaminergic function. In animal models, dopaminergic systems appears to be particularly vulnerable to a wide range of perinatal insults, resulting in persistent alterations in function of mesolimbic and mesostriatal pathways. This review summarizes recent work characterizing long-term changes in dopaminergic function and biochemistry in models of Caesarean section (C-section) birth and of C-section birth with added global anoxia in the rat and guinea pig. C-section birth and C-section with anoxia appear to be two distinct hypoxic birth insults, with somewhat differing patterns of lasting effects on dopamine systems. In addition, birth insult alters the manner in which dopaminergic function is regulated by stress at adulthood. The possible relevance of these finding to effects of human birth procedures is discussed.

Caesarean section birth produces long term changes in dopamine D1 receptors and in stress-induced regulation of D3 and D4 receptors in the rat brain.

Schizophrenia is associated with increased birth complications and altered mesocorticolimbic dopaminergic (DA) transmission, whereas stress also influences psychotic symptoms. Given this, the present study tested effects of two birth complications, Caesarean section (C-section) birth with or without acute global anoxia, on brain DA receptors in rats at adulthood. Effects of repeated stress at adulthood were also tested. Before stress, C-sectioned rats showed increased D1-like receptor binding in limbic areas, compared to vaginally born controls. There were no differences between birth groups in D2-like, D3, or D4-like receptor binding before stress. After stress, C-sectioned animals showed decreased D3 receptors in accumbens and increased D4-like receptors in dorsal striatum, accumbens, and olfactory tubercles, compared to vaginal birth. This occurred because stress upregulated D3 receptors only with vaginal birth and upregulated D4-like receptors only with C-section. Animals born by C-section + anoxia showed no change in DA receptors relative to vaginal birth, before or after stress. It is concluded that interactions between an individual's experience of stress at adulthood, together with other environmental events in their history, such as birth complications, can be important determinants of brain DA receptor levels.

The expression of mRNAs for the proteasome complex is developmentally regulated in the rat mesencephalon.

The proteasome is a large protease complex that recognizes, unfolds and degrades ubiquitinated proteins. Evidence is now accumulating that the ubiquitin-proteasome system may play an important role in neuronal apoptosis. However, little is known about the involvement of the proteasome in neuronal death in vivo, and there has been no prior analysis of the developmental expression of proteasome subunits in brain during periods of natural and inducible apoptotic death. We therefore studied the mRNA expression levels, using Northern analysis, of a subunit from each of the three key components of the proteasome in the rat mesencephalon from E21 through development and in adulthood. We measured mRNA expression for RC6 (a subunit of 20S), p112 (a subunit of 19S) and PA28-alpha (a subunit of 11S). The expression of PA28-alpha in rat mesencephalon was highest at the earliest times studied, and then decreased at PND 21, 28 and adult, in comparison to E21 (P<0.05) and PND 2, 4 and 7 (P<0.01). The expression of RC6 was lower in adult in comparison to PND 2, 4 and 21 (P<0.05) and PND 14 (P<0.01). There were no significant differences in the mRNA levels of p112 at various times studied. In situ hybridization at PND 7 indicated that all the subunits studied are particularly abundant in the SNpc. Thus, PA28-alpha and RC6 are developmentally regulated, and they may therefore play a role in developmental cell death or differentiation in neurons of the SN.

Expression of cyclin-dependent kinase 5 and its activator p35 in models of induced apoptotic death in neurons of the substantia nigra in vivo.

Cyclin-dependent kinase 5 is predominantly expressed in postmitotic neurons and plays a role in neurite elongation during development. It has also been postulated to play a role in apoptosis in a variety of cells, including neurons, but little is known about the generality and functional significance of cdk5 expression in neuronal apoptosis in living brain. We have therefore examined its expression and that of its known activators, p35, p39 and p67, in models of induced apoptosis in neurons of the substantia nigra. We find that cdk5 is expressed in apoptotic profiles following intrastriatal injection of 6-hydroxydopamine and axotomy. It is expressed exclusively in profiles which are in late morphologic stages of apoptosis. In these late stages, derivation of the profiles from neurons, and localization of expression to the nucleus, can be demonstrated by co-labeling with a neuron-specific nuclear marker, NeuN. In another model of induced apoptotic death in nigra, produced by developmental striatal lesion, kinase activity increases in parallel with cell death. While mRNAs for all three cdk5 activators are expressed in nigra during development, only p35 protein is expressed in apoptotic profiles. We conclude that cdk5/p35 expression is a general feature of apoptotic neuron death in substantia nigra neurons in vivo.

Transient birth hypoxia increases behavioral responses to repeated stress in the adult rat.

The aim of this study was to test if two birth complications, namely, transient global hypoxia during Caesarean section (C-section) birth or C-section birth per se, produce long-term changes in behavioral responses to repeated stress. Adult rats, that had been born vaginally, by C-section or by C-section with 10 min of global anoxia, were stressed for 8 days (15 min tail pinch daily) followed by challenge with the same stressor 2 weeks later. The main finding is that adult rats born by C-section + 10 min of anoxia showed enhanced locomotor activity on days 5 and 6 of the repeated stress period and at stress challenge 2 weeks later, compared to animals born vaginally or by C-section. It is concluded that transient global birth hypoxia can render the adult rat behaviorally hyper-responsive to repeated stress.

Birth insult increases amphetamine-induced behavioral responses in the adult rat.

We have previously reported that an apparently uncomplicated Caesarean section birth produces long-term alterations in steady-state levels of dopamine in the central nervous system of the rat. In addition, adult rats that had been born by Caesarean section, either with or without acute global anoxia, showed markedly greater dopamine release from the nucleus accumbens in response to repeated stress, in comparison to vaginally born controls. The aim of the present study was to test whether these birth complications also result in long-term changes in behavior mediated by dopamine systems. For this, we investigated effects of a low dose (0.5 mg/kg) of amphetamine on activity levels in three-month-old rats that had been born vaginally (control), by rapid Caesarean section, or by Caesarean section with 15 min of global anoxia. Amphetamine induced a significantly greater increase in locomotor activity in animals born by Caesarean section or by Caesarean section+ 15 min anoxia, in comparison to the drug's effects in vaginally born controls. Behavioral responses were further analysed from video recordings of the animals' behavior. In confirmation of automated activity counts, both animals born by Caesarean section and by Caesarean section + 15 min anoxia showed a significant increase in the duration and frequency of moving and a decrease in the duration and frequency of standing, in comparison to vaginally born controls. Animals delivered by Caesarean section showed a significant increase in the duration of sniffing and a decrease in the duration and frequency of grooming when compared to vaginally born controls. Animals delivered by Caesarean section + 15 min anoxia showed a significant increase in the duration and frequency of rearing, in comparison to controls. The pattern of behavioral changes observed indicates that, as adults, animals born by Caesarean section and by Caesarean section with added global anoxia both show heightened behavioral responses to amphetamine, in comparison to vaginally born animals. These findings highlight the sensitivity of dopamine pathways to variations in birth procedure and add experimental support to epidemiological evidence implicating birth complications in the pathophysiology of disorders involving central dopaminergic neurons, such as schizophrenia.

Long-term reciprocal changes in dopamine levels in prefrontal cortex versus nucleus accumbens in rats born by Caesarean section compared to vaginal birth.

Epidemiological evidence indicates a higher incidence of pregnancy and birth complications among individuals who later develop schizophrenia, a disorder linked to alterations in mesolimbic dopamine (DA) function. Two birth complications usually included in these epidemiological studies, and still frequently encountered in the general population, are birth by Caesarean section (C-section) and fetal asphyxia. To test the hypothesis that birth complications can produce long-lasting changes in DA systems, the present study examined the effects of Caesarean birth, with or without an added period of anoxia, on steady state monoamine levels and metabolism in various brain regions in a rat model. Pups born vaginally served as controls. At 2 months of age, in animals born by rapid C-section, steady state levels of DA were decreased by 53% in the prefrontal cortex and increased by 40% in both the nucleus accumbens and striatum, in comparison to the vaginally born group. DA turnover increased in the prefrontal cortex, decreased in the nucleus accumbens, and showed no significant change in the striatum, in the C-section group. Thus, birth by a Caesarean procedure produces long-term reciprocal changes in DA levels and metabolism in the nucleus accumbens and prefrontal cortex. This is consistent with the known inhibitory effect of increased prefrontal cortex DA activity on DA release in the nucleus accumbens. By contrast to birth by rapid C-section alone, young adult animals, that had been born by C-section with 15 min of added anoxia, showed no change in steady state DA levels in the prefrontal cortex, nucleus accumbens, or striatum and a significant decrease in DA turnover only in the nucleus accumbens, in comparison to the vaginally born group. Levels of norepinephrine, serotonin, and its metabolite, 5-hydroxyindole acetic acid, were unchanged in all groups, indicating relatively specific effects on DA systems. Although appearing robust at birth on gross observation, more subtle measurements revealed that rat pups born by C-section show altered respiratory rates and activity levels and increased levels of whole brain lactate, suggestive of low grade brain hypoxia, during the first 24 h of life, in comparison to vaginally born controls. Pups born by C-section with 15 min of added acute anoxia were pale, hypotonic, and inactive at birth and showed reduced respiration and high brain lactate levels. However, these alterations resolved by 1-5 h after birth and, with few exceptions, animals in the anoxic group remained normal with respect to these parameters during the remainder of the first 24 h of life. Immediately after birth, levels of plasma epinephrine, a hormone known to play a role in neonatal adaptation to extrauterine life and protection against hypoxia, were decreased in pups born by C-section but increased in pups born by C-section with 15 min added anoxia, in comparison to levels measured in vaginally born controls. These early developmental alterations could contribute to long-term alterations in dopaminergic parameters observed in rats born by C-section, with or without added anoxia. It is concluded that C-section birth is sufficient perturbation to produce long-lasting effects on DA levels and metabolism in the central nervous system of the rat. These findings highlight the sensitivity of DA pathways to variations in birth procedure and support the notion that birth complications might contribute to the pathophysiology of disorders involving central dopaminergic neurons, such as schizophrenia.