Adaptive changes of beta-adrenergic receptors after neonatal locus coeruleus lesion: regulation of serotoninergic unit activity

Norepinephrine, neurodevelopment and behavior

Neurotransmitters play critical roles in the developing nervous system. Among the neurotransmitters, norepinephrine (NE) is in particular postulated to be an important regulator of brain development. NE is expressed during early stages of development and is known to regulate both the development of noradrenergic neurons and the development of target areas. NE participates in the shaping and the wiring of the nervous system during the critical periods of development, and perturbations in this process can alter the brain's developmental trajectory, which in turn can cause long-lasting and even permanent changes in the brain function and behavior later in life. Here we will briefly review evidence for the role of noradrenergic system in neurodevelopmental processes and will discuss about the potential disruptors of noradrenergic system during development and their behavioral consequences.

Electrophysiological evidence for the tonic activation of 5-HT(1A) autoreceptors in the rat dorsal raphe nucleus

Serotonin (5-hydroxytryptamine, 5-HT) and norepinephine (NE) neurons have reciprocal connections. These may thus interfere with anticipated effects of selective pharmacological agents targeting these neurons. The main goal of the present study was to assess whether the somatodendritic 5-HT(1A) autoreceptor is tonically activated by endogenous 5-HT in anesthetised rats, using in vivo extracellular unitary recordings. In rats with their NE neurons lesioned using 6-hydroxydopamine (6-OHDA) and in controls administered the NE reuptake inhibitor desipramine to suppress NE neuronal firing, the alpha2-adrenoceptor agonist clonidine no longer inhibited 5-HT neuron firing, therefore indicating the important modulation of the firing activity of 5-HT neurons by NE neurons. In control rats, the administration of the potent and selective 5-HT(1A) receptor antagonist WAY 100,635 ((N-[2-[4(2-methoxyphenyl)-1-piperazinyl]ethy]-N-(2-pyridinyl)cyclohexanecarboxamide trihydroxychloride) (100 microg/kg, i.v.) did not modify the spontaneous firing activity of 5-HT neurons, but in NE-lesioned rats using either 6-OHDA or DSP-4, WAY 100,635 produced a mean firing increase of 80 and 69%, respectively. When desipramine and D-amphetamine were used in control rats to prevent alterations in the availability of NE in the dorsal raphe, again WAY 100,635 produced a significant disinhibition of the firing of 5-HT neurons (83 and 53%, respectively). These data support the notion that the NE system tonically activates the firing activity of 5-HT neurons. When the fluctuations of the function of NE neurons normally produced by WAY 100,635 were prevented, a tonic activation of 5-HT(1A) autoreceptors by endogenous 5-HT was unmasked.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

5-HT1A receptors in the median raphe nucleus and dorsal hippocampus may mediate anxiolytic and anxiogenic behaviours respectively

The behavioural response of rats in the high light unfamiliar condition of the social interaction test of anxiety was observed following direct administration of the 5-HT1A receptor agonist, (+/-)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT, 50, 100 or 200 ng) or antagonist tertatolol (3 micrograms) into the median raphe nucleus or dorsal hippocampus. In the median raphe nucleus, 8-OH-DPAT (200 ng) significantly increased social interaction without changing locomotor activity; lower doses were inactive. In the dorsal hippocampus, bilateral injection of 8-OH-DPAT (100 ng) significantly decreased social interaction, without effect on locomotor activity; both 50 and 100 ng significantly changed grooming. Tertatolol had no effect on social interaction following administration to the median raphe nucleus, but significantly increased locomotor activity. Bilateral injection of tertatolol into the dorsal hippocampus decreased social interaction and changed grooming. These effects are similar to those of 8-OH-DPAT suggesting tertatolol may have 5-HT1A receptor agonist properties. In conclusion, the findings of this study demonstrate that 5-HT1A somatodendritic autoreceptors and post-synaptic receptors mediate anxiolytic and anxiogenic effects, respectively, in the social interaction test.

Alpha 1-adrenoceptor antagonists differentially control serotonin release in the hippocampus and striatum: a microdialysis study

Using the in vivo microdialysis technique, we have studied the effect of the systemic administration of several alpha 1-adrenoceptor antagonists on the extracellular levels of serotonin (5-HT) in the rat hippocampus. Prazosin, and to a lesser extent, terazosin, decreased these levels by 50-65% for 0.03-0.4 mg/kg, i.v. and by 30-40% for 0.1-0.4 mg/kg, i.v., respectively. In contrast, alfuzosin, an alpha 1-adrenoceptor antagonist with poor brain penetration, did not significantly affect these levels even at the high dose of 0.4 mg/kg, i.v. When perfused into the hippocampus through the dialysis probe, prazosin (1-10 microM) induced a more limited (20-30%) and delayed decrease in 5-HT outflow. These results support the existence of a central noradrenergic facilitatory influence, mediated by alpha 1-adrenoceptors, on serotonergic neurons projecting to the hippocampus. In the striatum prazosin (0.4 mg/kg, i.v.) decreased 5-HT levels to a smaller extent (-35%) than in the hippocampus (-65%), suggesting the existence of differences in the degree of noradrenergic influence on median and dorsal raphé nuclei, which preferentially project to the hippocampus and striatum, respectively.

(-)Tertatolol is a potent antagonist at pre- and postsynaptic serotonin 5-HT1A receptors in the rat brain

The potential 5-HT1A antagonist properties of the beta-antagonist tertatolol were assessed using biochemical and electrophysiological assays in the rat. (+/-) Tertatolol bound with high affinity (Ki = 38 nM) to 5-HT1A sites labelled by [3H]8-OH-DPAT in hippocampal membranes. The (-)stereoisomer (Ki = 18 nM) was about 50-fold more potent than the (+)stereoisomer (Ki = 864 nM) to inhibit the specific binding of [3H]-8-OH-DPAT. As expected of a 5-HT1A antagonist, (-)tertatolol prevented in a concentration-dependent manner (Ki = 24 nM) the inhibitory effect of 8-OH-DPAT on forskolin-stimulated adenylate cyclase activity in rat hippocampal homogenates. Furthermore in vivo pretreatment with (-)tertatolol (5 mg/kg s.c.) significantly reduced the inhibitory influence of 8-OH-DPAT (0.3 mg/kg s.c.) on the accumulation of 5-hydroxytryptophan in various brain areas after the blockade of aromatic L-amino acid decarboxylase by NSD-1015 (100 mg/kg i.p.). In vitro (in brainstem slices; Ki approximately 50 nM) and in vivo (in chloral hydrate anaesthetized rats; ID50 approximately 0.40 mg/kg i.v.), (-)tertatolol prevented the inhibitory effects of the 5-HT1A receptor agonists 8-OH-DPAT, ipsapirone and lesopitron on the firing rate of serotoninergic neurones within the dorsal raphe nucleus. In about 25% of these neurones, the basal firing rate was significantly increased by (-)tertatolol (up to +47% in vitro, and +30% in vivo). These data indicate that (-)tertatolol is a potent competitive antagonist at both pre (in the dorsal raphe nucleus)-and post (in the hippocampus)-synaptic 5-HT1A receptors in the rat brain.

Flerobuterol, a beta-adrenoceptor agonist, enhances serotonergic neurotransmission: an electrophysiological study in the rat brain

The two beta-adrenoceptor agonists salbutamol and clenbuterol have been shown to be effective antidepressant drugs. Flerobuterol, a new beta-adrenoceptor agonist, exhibits antidepressant activity in animal models. Given the long-standing notion that the serotonergic (5-HT) system might be involved in the etiology and/or the therapeutics of affective disorders and that this class of adrenergic agents can alter factors regulating 5-HT transmission, the effects of acute and repeated administrations of flerobuterol on the 5-HT system were studied. Acute administration of flerobuterol (up to 2 mg/kg, IV) did not modify the firing rate of dorsal raphe 5-HT neurons. However, the sustained administration of flerobuterol for two days (0.5 mg/kg/day, SC. delivered by an osmotic minipump) produced a marked decrease of the firing rate of 5-HT neurons. The reversal of this effect of flerobuterol by the somatodendritic 5-HT autoreceptor antagonist spiperone suggests that this decrease in the firing activity of 5-HT neurons in rats treated for 2 days with flerobuterol resulted from an enhanced synaptic availability of 5-HT. This initial decrease in firing activity of 5-HT neurons was followed by a progressive recovery to normal after 14 days of treatment with flerobuterol. At this point in time, the effect of intravenous lysergic acid diethylamide on the firing of 5-HT neurons was attenuated, indicating that the somatodendritic 5-HT autoreceptors had desensitized. The effectiveness of the electrical stimulation of the ascending 5-HT pathway in suppressing the firing activity of dorsal hippocampus pyramidal neurons was markedly enhanced in rats treated with flerobuterol for 14 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental aspects of the locus coeruleus-noradrenaline system

The locus coeruleus-noradrenaline (LC-NA) system exhibits an early developmental pattern, so that its nerve terminals are present in target areas before formation of most synapses. Several properties of the source neurons in the LC change substantially during early postnatal periods: spontaneous activity patterns, responsiveness to sensory stimulation, and responsiveness to NA. The effect may be to confer enhanced responsiveness of LC neurons, and an enhanced release of NA in target areas, during early postnatal development. Developmental changes in density of adrenoceptors or adrenergic responsiveness in target areas have also been documented. The usual pattern is a progressive increase in adrenergic ligand binding, with some reduction during later phases of development. However, there are a number of examples of receptor subtypes and region-specific transient binding during the first few weeks of postnatal life, followed by reductions to very low levels. These observations may reflect developmentally transient adrenergic responsiveness in certain target areas. NA and the LC-NA system have been implicated in the control of morphological and functional properties of neurons in target areas, and in the control of developmentally important biochemical systems (ornithine decarboxylase). NA, as well as other neurotransmitters, may individually, or in cooperation, exert important trophic influences during a restricted developmental period.

Neonatal clonidine treatment results in long-lasting changes in noradrenaline sensitivity and kindling epileptogenesis

In the present experiment we tested the hypothesis that early interference with noradrenaline transmission can have permanent consequences for brain function in adulthood. Neonatal depletion of noradrenaline by daily subcutaneous injections of clonidine results in supersensitivity to noradrenaline in adult hippocampal CA1 cells as shown in our previous microiontophoretic study. These findings were confirmed and extended here with dose-response curves. Furthermore, we tested whether this form of neonatal interference with noradrenaline also permanently affects long-lasting plasticity as revealed in kindling epileptogenesis in adulthood. The initiation of the epileptic activity after the kindling stimulation was significantly delayed in the clonidine-treated group, and all measured parameters of seizure expression tended to be retarded in comparison with saline-treated control rats. This indicates that noradrenaline supersensitivity induced by neonatal clonidine treatment retards kindling development in adulthood.

The effect of regional differences in noradrenergic neuron growth patterns on juvenile kindling

Kindling can be altered by a variety of lesions designed to deplete norepinephrine (NE). However, the effect of the regional alteration in NE concentration on seizure susceptibility has not been studied. Two different concentrations of 6-hydroxydopamine (6-OHDA) were administered to one-day-old rat pups. At age 18 days, rat with significant rostral brain NE loss, due to high dose 6-OHDA, had a faster rate of electrical kindling in the entorhinal cortex than controls. In contrast rats receiving low dose 6-OHDA which resulted in comparable forebrain NE depletion but with a dramatic hindbrain noradrenergic overgrowth showed no enhancement of kindling. These results suggest that in the immature rat the proconvulsant effect of forebrain NE depletion can be overridden by an augmentation of hindbrain NE growth patterns.

Suppression of clinical weakness in experimental autoimmune encephalomyelitis associated with weight changes, and post-decapitation convulsions after intracisternal-ventricular administration of 6-hydroxydopamine

Selective depletion of central nervous system norepinephrine (NE) by the neurotoxin 6-hydroxydopamine (6-OHDA) in rats subsequently inoculated with myelin basic protein (MBP) and complete Freund's adjuvant (CFA) produced experimental autoimmune encephalomyelitis (EAE) without the usual expected degree of weakness. The preservation of strength occurred in spite of continued weight loss. Post-decapitation myoclonic convulsive kick latency and kick number, which are known to depend on spinal cord NE, agreed well with the degree of weakness through the clinical disease course. The only difference between EAE groups was that the stronger 6-OHDA pretreated EAE animals did not have an elevated pons-medulla NE compared to saline intracisternal-ventricular (i.c.v.) pretreated controls. We conclude that 6-OHDA can influence the clinical course of weakness by interfering with central noradrenergic activity independent of other features associated with disease in EAE. This effect of 6-OHDA may be exerted through alteration of the blood-spinal cord barrier function and/or central nervous system blood flow.

Autism: review of neurochemical investigation

The neurochemistry of autism, the most well-validated childhood neuropsychiatric disorder, has been studied extensively over the past three decades. Autism is of interest neurochemically because it represents a relatively homogeneous disorder with a triad of social, communicative, and intellectual developmental disturbance. Because a sufficient animal model has been lacking and relatively few diagnosed people with autism have died, most investigation has been of peripheral fluids and tissues. The most consistent finding has been that over 25% of autistic children and adolescents are hyperserotonemic. However, after 29 years of investigation, the mechanism of hyperserotonemia has not been determined. Hyperserotonemia has been found to be familial. Elevated plasma norepinephrine has also been a replicated finding. Cerebrospinal fluid (CSF) opiate activity has been found to be elevated in two studies. Plasma cyclic adenosine monophosphate (cAMP) has been found to be elevated in autistic children. A high rate of nonsuppression after dexamethasone and blunted or delayed growth hormone response to L-dopa have been found. Abnormal cell-mediated immunity has been replicated consistently in autism. Although several pharmacological trials have been conducted and shown promise in initial open trials, only "typical" antipsychotic drugs have shown replicable chronic ameliorating effects in double-blind trials. However, chronic neurotoxicity (tardive dyskinesia) has also been revealed. Findings of morphological changes in the cerebellum have been replicated. Findings in need of replication include diminished platelet function, increased baseline CSF homovanillic acid, decreased nerve cell adhesion molecule serum fragment, blunted prolactin response to fenfluramine, amelioration of symptoms by naltrexone and bromocriptine, reduced electroretinographic (ERG) b-wave amplitude, and morphological changes in the hippocampus, amygdala, and septal nuclei. In addition to refining and replicating past findings, future directions that may be fruitful include investigation of neurochemical aspects of platelet function, of interactions between monoaminergic systems, of phosphatidylinositides, and of pharmacological response to "atypical" antipsychotic agents and relatively selective serotonin receptor subtype agonists or antagonists.