Perinatal asphyxia in the rat : a morphological and functional analysis of the dopamine systems and dopamine-mediated behaviours in a novel experimental model

Abstract

Asphyxia during labour in the human being is one of the important paediatric complications which can lead to various degree of injuries in the central nervous system. No efficient therapy is available at present. Thus, further research is necessary in order to understand the mechanisms causing asphyxia-induced lesioning of the CNS for development of rational therapies. The main purpose of this thesis was to develop a new animal model which has a high resemblance with the clinical situation resulting in an asphyctic state. The model is easy to perform, largely non-invasive to the pup, and highly reproducible. The model has been used to investigate the short- and long-term changes following different times (0-22 min) of perinatal asphyxia, with special reference to the meso-telencephalic dopamine systems and their behavioural functions.

The results show that there are short-term (80 min - 8 days) changes in several metabolic and cellular markers in brain tissue suggesting diffuse stimulatory and inhibitory responses to the progressive failure in energy reserves that follows prolonged perinatal asphyxia. During the acute phase (within the first postnatal 80 min), rats subjected to asphyctic periods longer than 16 min demonstrate a high mortality rate. Pups surviving this condition show a lactate accumulation, a decrease in pH, a decrease in the number of cells expressing Fos-IR and a decrease in glutamate and aspartate levels, indicating severe impairments in metabolism and neuronal activity.

The meso-striatal dopamine systems seem to develop earlier than the mesolimbic systems, and perinatal asphyxia appears not to change the gross development of dopamine systems. Furthermore, dopamine and its metabolites levels were acutely increased following 2 3 min and up to 19-20 min of asphyxia in the substantia nigra, ventral tegmental area, striatum and the accumbens nucleus, indicating that perinatal asphyxia acutely increases the release of dopamine from the meso-telencephalic dopamine systems.

In the 4-week-old rat, dopamine levels and turnover were decreased in striatum, the accumbens nucleus, the olfactory tubercle, the substantia nigra and ventral tegmental area following 19-20 min of asphyxia. However, the number of the dopamine cell bodies in the substantia nigra and ventral tegmental area were increased, suggesting that the apoptotic mechanism within dopamine cells do not develop properly. When asphyxia was extremely severe (>20 min), the number of dopamine cell bodies was decreased. The number of noradrenaline cell bodies in the locus ceruleus were decreased already after 15-16 min of perinatal asphyxia, while no further decrease was observed after more than 20 min of perinatal asphyxia.

An increase in dopamine D1 receptor affinity in the accumbens nucleus and the olfactory tubercle, in spite of a reduced number of D1 receptors, was observed in the 4-week-old animal subjected to 15-16 min of perinatal asphyxia. This finding may partly explain the increased locomotor activity observed in these animals, as well as the increase in the locomotor response to a low dose of d-amphetamine (1.5 mg/kg, s.c.). The animals subjected to 19-20 min of perinatal asphyxia showed decreased dopamine levels and utilisation in the meso-striatal/-limbic systems as well as a reduction in dopamine receptor function in the accumbens nucleus, the olfactory tubercle and striatum, which all may contribute to the demonstrated inhibition of locomotor activity in these animals.

A 4-week postnatal nicotine treatment counteracted the asphyxia-induced increase in DA cell body number in the mesencephalon, as well as the changes in motor activity, in particular those observed in the rats subjected to 19-20 min of perinatal asphyxia. The results indicate that this animal model may be used to characterise the behavioural impairment and the neurochemical mechanisms behind different neurodegenerative disorders in human. It may be particularly useful when studying conditions induced by mild asphyxia, such as attention deficit disorders in humans.