Autoradiographic changes in central benzodiazepine binding sites and their coupling to gamma-aminobutyric acid receptors after seizures in the developing rat.

Benzodiazepines are psychoactive substances classically used for their anticonvulsant properties in neonates as well as in adults. In a previous work, we have shown that seizures lead to an age-dependent upregulation of central benzodiazepine binding sites measured in isolated rat cerebral membranes. However, information concerning regional changes in the receptor density was lacking. In our present study, the effects of bicuculline-induced seizures were investigated by quantitative autoradiography of central benzodiazepine receptors in developing rats and in adults. Animals were killed 30 min after an intraperitoneal injection of either saline or a convulsive dose of bicuculline. Benzodiazepine binding sites in brain sections were labeled by [3H]flunitrazepam. Generalized seizures induced a widespread increase in benzodiazepine receptors, with a marked enhancement in structures that mediate seizure activity, such as substantia nigra, amygdala, septum, and hippocampus. The addition of exogenous gamma-aminobutyric acid to the incubation medium increased benzodiazepine binding by the same order of magnitude whether rats were given saline or bicuculline, suggesting that additional benzodiazepine sites are also functionally linked to gamma-aminobutyric acid receptors. The age-related postictal increase in benzodiazepine receptors might reflect a compensatory response for protection against recurrent seizures, especially in newborns.

Quantitative autoradiographic study of the postnatal development of benzodiazepine binding sites and their coupling to GABA receptors in the rat brain.

The postnatal development of benzodiazepine binding sites in the rat brain was studied by quantitative receptor autoradiography using [3H]flunitrazepam. The coupling of these sites to GABA receptors was assessed in 43 cerebral structures by examining the effects of in vitro addition of GABA on flunitrazepam specific binding. Benzodiazepine-specific binding was relatively high at birth and exhibited an heterogeneous distribution pattern, anatomically different from the adult one. Data showed a sequential development of benzodiazepine receptors in relation to the time course of maturation of cerebral structures. A proliferation peak which paralleled rapid brain growth was noticed. High levels of benzodiazepine sites were transiently observed in some areas, e.g. thalamus and hypothalamus, and might be related to maturational events. In every brain structure examined, benzodiazepine binding sites were linked to GABA receptors. However, enhancement of flunitrazepam specific binding by exogenous GABA differed according to the structures studied and decreased during development, suggesting some changes in the control of GABA/benzodiazepine regulation during postnatal maturation.

Quantitative autoradiographic study of the postnatal development of adenosine A1 receptors and their coupling to G proteins in the rat brain.

Adenosine is now considered as a major regulatory agent in the mammalian central nervous system. Its actions are mediated by specific receptors which are coupled with an adenylate cyclase system via a G protein. The postnatal development of adenosine A1 receptors was studied by quantitative autoradiography using [3H]N6-cyclohexyladenosine, a potent receptor agonist in 42 rat brain structures. The coupling of these sites to G proteins was examined by measuring the effects of in vitro addition of guanylyl-5'-imidodiphosphate, a stable analogue of guanosine triphosphate, on N6-cyclohexyladenosine binding. [3H]N6-Cyclohexyladenosine-specific binding was quite low at birth, around 10% of adult levels, and exhibited a rather homogeneous distribution pattern, except in thalamic nuclei. Data showed a sequential development of adenosine A1 receptors in relation to the time course of maturation of cerebral structures with a proliferation peak which paralleled rapid brain growth. The time period by which adult levels are reached differed according to the cerebral region studied. N6-Cyclohexyladenosine-specific binding sites appeared to be functionally linked to G proteins in all structures and at all postnatal stages. However, the potency of guanylyl-5'-imidodiphosphate to displace N6-cyclohexyladenosine binding was significantly lower before 5 days of age, suggesting functional changes during postnatal maturation in cerebral pathways modulated by adenosine.