Effects of neonatal exposure to clonidine on basal and activated central noradrenaline metabolism and in vivo overflow

The alpha2 adrenoreceptor agonist clonidine suppresses evoked and spontaneous seizures, whereas the alpha2 adrenoreceptor antagonist idazoxan promotes seizures in amygdala-kindled kittens

Microinfusion of alpha2 adrenoreceptor agonists and antagonists into amygdala has contrasting effects on evoked and spontaneous seizure susceptibility in amygdala-kindled kittens. Subjects were 14 preadolescent kittens between 3 and 4 months old at the beginning of kindling. The same protocol was followed except that half the kittens received microinfusions (1 mul) of the alpha2 agonist clonidine (CLON; 1.32 nmol), and half received the alpha2 antagonist idazoxan (IDA; 0.33 nmol). Infusions were made over 1 min through needles inserted into cannulae adjacent to stimulating electrodes in the kindled amygdala, and evoked seizures were tested 10-12 min later. The results were: (1) CLON elevated seizure thresholds obtained once at the beginning and end of kindling, but only when compared to sham control values (needle insertion only) in the same animals; IDA significantly reduced thresholds. (2) CLON retarded and IDA accelerated kindling rate, defined as the number of afterdischarges (ADs) required to achieve the first stage 6 seizure or generalized tonic-clonic convulsion (GTC). These effects were most pronounced on the emergence of seizure "generalization" stages (3-6) from "focal" seizure stages (1-2). (3) CLON prevented onset of spontaneous seizures, whereas IDA precipitated onset of spontaneous seizures in 100% of the animals before or during the 5-week post-kindling follow-up during which seizures were evoked once each work day. The study confirms previous findings in kindled rodents to show that CLON and IDA can have opposing effects on kindling development in kittens and is the first report to show contrasting effects on spontaneous epileptogenesis in kindled animals as well.

Clonidine increases caspase-3 mRNA level and DNA fragmentation in the developing rat brainstem

The densities of alpha2-adrenergic receptors, labeled by 3H-clonidine or 3H-RX821002, reach a peak in the rat brainstem during the first week of its life. This enables the agonist of alpha2-adrenergic receptor clonidine, which is used as a component of anaesthetic solution in infants and children, to have specific effects in this structure of the developing brain. Clonidine was injected into the fetal brain (5 microg in 5 microl of saline) or subcutaneously to the pups (1, 10 microg in 50 microl of saline) 3 days before investigation. Clonidine increased the level of apoptotic enzyme caspase-3 mRNA expression, as measured by RT-PCR and enhanced the DNA fragmentation, as determined by gel electrophoresis, in the brainstem of the 21-day-old fetuses and 8-day-old rats. In the cortex of 8-day-old rat, the alpha2-adrenergic receptors are at a much lower level than the brainstem. Clonidine treatment had no evident effects on caspase-3 mRNA level and DNA fragmentation in the cortex of an 8-day-old rat. The data suggest that clonidine facilitates cell death in the developing brainstem. This drug effect provides a potential mechanism whereby clonidine during early life could induce long-lasting alterations in brain neurochemistry, autonomic functions and behavior.

Genetic background, nature of event, and time of exposure to event direct the phenotypic expression of a particular genotype

The aim of this study was to investigate the differential long-term after-effects on the apomorphine-susceptibility in the apomorphine-susceptible (APO-SUS) and apomorphine-unsusceptible (APO-UNSUS) rats of either a saline injection (a mild stressor) or a clonidine-injection (a moderate stressor) applied around the onset of the stress-hypo-responsive-period (SHRP) on postnatal day (PND) 2, 4, 6 or 9. The present study shows that an injection of saline on PND 2 decreases the apomorphine-induced gnawing score in adult APO-SUS rats, but increases this score in adult APO-UNSUS rats; clonidine given on this day has no effect. An injection of saline on PND 4 decreases the apomorphine score in adult APO-SUS rats, but has no effect in adult APO-UNSUS rats. An injection of clonidine on PND 4 counteracts the long-term after-effects of a saline injection on this PND in adult APO-SUS rats, but has no effect in adult APO-UNSUS rats. Finally, an injection of clonidine on PND 9 has no effect on the apomorphine score in adult APO-SUS rats, but increases this score in adult APO-UNSUS rats; saline given on this day has no effect. It is concluded that the long-term after-effects of early postnatal stressors depend on the interrelationship between, rather than the separate impact of, the genetic background of the rat, the nature of the event, and time of exposure to the event, thereby implying that these factors direct the phenotypic expression of a particular genotype at adult age.

Early postnatal appearance of enhanced noradrenaline content in the brain of vasopressin-deficient Brattleboro rat; normal adrenoceptor densities and aberrant influences of vasopressin treatment

The course of postnatal development of noradrenaline (NA) and its unconjugated free metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG), as well as the influence on early chronic vasopressin treatment, were investigated in various brain regions of the hereditary vasopressin-deficient (homozygous di/di) Brattleboro rat. In addition, the densities of the adrenergic receptor subtypes were measured in adult brain. Brain NA levels of di/di pups appeared enhanced already at 7 days of age when compared with data of heterozygous (+/di) controls. This was also seen in areas not known to receive a vasopressinergic input, e.g. the frontal cortex. Levels of MHPG also differed between genotypes, but changes were slight and either a decrease or increase, depending on age and region tested. Saturation analyses of alpha 1-, alpha 2-, and beta-adrenoceptor binding on crude membrane preparations of some brain regions revealed no differences in adulthood. Chronic treatment with vasopressin between 6 and 13 days of age reduced the enhanced NA brain levels throughout the brain of the di/di Brattleboro pups. The known vasopressin-mediated enhancement of NA turnover in adult brain was also measurable in +/di pups of this neonatal period (MHPG/NA ratios), indicating the early maturation of the interaction of vasopressinergic and NAergic systems. However, the dose-response in the di/di Brattleboro rat was biphasic with a decrease at a low dose of vasopressin. Since changes were found throughout the brain, it was concluded that vasopressin deficiency had altered the maturation of NA neurons of the locus coeruleus which may be due to the absence of a presumed inhibitory control of vasopressin on synthesis and storage mechanisms at the perikaryal level.

Increased alpha 2-adrenoceptor mediated regulation of adult rat brain noradrenaline overflow after chronic neonatal exposure to propranolol; a microdialysis study

Direct and persistent effects of chronic neonatal administration of the beta-adrenoceptor antagonist propranolol on brain noradrenergic activity were investigated by measuring tissue concentrations of noradrenaline and its metabolites and in vivo overflow of noradrenaline during adulthood. Rat pups were chronically treated with propranolol from postnatal day 1 to day 10. Determination of monoamine metabolism after the last injection showed an increase in noradrenaline metabolism in frontal cortex, limbic system and hippocampus of propranolol-exposed rats, but 47 days after this last injection it was apparent that these effects were not long-lasting. Moreover, basal noradrenaline overflow in vivo in the hippocampus of 40-55 day-old propranolol-exposed rats did not differ from that in controls. However, the regulation of noradrenaline release seemed to have been altered, since a pharmacological challenge with the alpha 2-adrenoceptor antagonist idazoxan induced an enhanced increase in the in vivo noradrenaline overflow in propranolol-exposed rats compared to controls. It is suggested that the neonatal beta-blockade induced a supersensitivity of the presynaptic alpha 2-adrenoceptor. The precise mechanism underlying this effect has to be elucidated.