Sex hormone-dependent desensitization of 5-HT1A autoreceptors in knockout mice deficient in the 5-HT transporter

The serotonin transporter (5-HTT) is the target of most antidepressant drugs, whose therapeutic action is related to their facilitatory influence on 5-HT neurotransmission. In this study, we investigated the functional adaptive properties of 5-HT1A autoreceptors, which regulate serotonergic neuronal firing, in knockout mice deficient in 5-HTT. Neurons of the dorsal raphe nucleus (DRN) were recorded extracellularly under chloral hydrate anaesthesia in male and female knockout 5-HTT mice and their wild-type counterparts. The inhibitory response of DRN neurons to intravenous injection of the 5-HT1A agonist 8-OH-DPAT was dramatically reduced in knockout 5-HTT compared with wild-type mice, especially in females. Changes in 8-OH-DPAT-induced hypothermia and autoradiographic labelling of 5-HT1A sites in the DRN confirmed a greater level of desensitization/down-regulation of 5-HT1A autoreceptors in female than in male knockout 5-HTT mice. After gonadectomy, the functional status of 5-HT1A autoreceptors was unchanged in wild-type mice, whereas in knockout 5-HTT, castrated males exhibited a down-regulation, and ovariectomized females an up-regulation of these receptors, as shown by electrophysiological recording and autoradiographic labelling in the DRN, as well as by changes in 8-OH-DPAT-induced hypothermia. Finally, in gonadectomized knockout 5-HTT mice, treatment with testosterone or estradiol restored the DRN neuronal firing sensitivity to 8-OH-DPAT back to sham control level in males or females, respectively. These data indicate that sexual hormones participate in the mechanisms responsible for the desensitization of 5-HT1A autoreceptors in knockout 5-HTT mice. The differential effects of testosterone and estradiol on 5-HT1A-mediated control of 5-HT neurotransmission might be related to the well-established gender differences in the vulnerability to depression.     

Differences in peripheral nerve degeneration/regeneration between wild-type and neuronal nitric oxide synthase knockout mice

Nitric oxide (NO), a unique biological messenger molecule, is synthesized by three isoforms of the enzyme NO synthase (NOS) and diffuses from the site of production across cellular membranes. A postulated role for NO in degeneration and regeneration of peripheral nerves has been explored in a sciatic nerve model comparing wild-type mice and mice lacking neuronal NOS after transection and microsurgical repair. In NOS knockout mice, regenerative delay was observed, preceded by a decelerated Wallerian degeneration (WD). In the regenerated nerve, pruning of uncontrolled sprouts was disturbed, leading to an enhanced number of axons, whereas remyelination seemed to be less affected. Delayed regeneration was associated with a delayed recovery of sensor and motor function. In such a context, possible NO targets are neurofilaments and myelin sheaths of the interrupted axon, filopodia of the growth cone, newly formed neuromuscular endplates, and Schwann cells in the distal nerve stump. The results presented suggest that 1) local release of NO following peripheral nerve injury is a crucial factor in degeneration/regeneration, 2) success of fiber regeneration in the peripheral nervous system depends on a regular WD, and 3) manipulation of NO supply may offer interesting therapeutic options for treatment of peripheral nerve lesions.     

Reflections on experimental and human pathology of aggression

On the basis of the already proposed distinction between "normal" and "pathological" aggression in laboratory animals, it is essayed an integration of the experimental findings derived from a specific animal model of aggression with the available clinical information on human violent behavior. The too disregarded importance of the role played by the inhibitory control of brain functions, appears instead reportedly essential in the regulation of emotions and behavior, and is of great relevance in explaining the behavioral changes that follow induced or spontaneous impairment of the serotonergic system of the brain. As a matter of fact, the numerous evidences indicate that genetic predisposition and induced or acquired defects of serotonergic inhibitory control greatly concur to precipitate anomalous strong aggression. Interestingly, the cluster of symptoms presented by laboratory rats in consequence of the serotonergic discontrol, has many unexpected similarities with several pathological conditions of man. This fact confers to laboratory experiments the value of a tool aimed at a better understanding of the biological mechanisms which underlie corresponding alterations of human conduct, with special reference to pathological aggression and violence. In this line, some specific nutrient defects and/or malabsorption conditions can be important in the facilitation or elicitation of mental illness including human aggression. In addition, the efficacy and neurochemical action of those substances capable to partially or completely block or prevent experimental aggression, will likely assume equal relevance in the management or prevention of human violent behavior.     

A novel role of prostaglandin E2 in neuropathic pain: blockade of microglial migration in the spinal cord

Neuropathic pain produced by damage to or dysfunction of the nervous system is a common and severely disabling state that affects millions of people worldwide. Recent evidence indicates that activated microglia are key cellular intermediaries in the pathogenesis of neuropathic pain and that ATP serves as the mediator. However, the in vivo mechanism underlying the retention of activated microglia in the injured region has not yet been completely elucidated. Prostaglandin E(2) (PGE(2)) is the principal proinflammatory prostanoid and plays versatile roles by acting via four PGE receptor subtypes, EP1-EP4. In the present study, we investigated the role of PGE(2) in spinal microglial activation in relation to neuropathic pain by using genetic and pharmacological methods. Mice deficient in microsomal prostaglandin E synthase-1 impaired the activation of microglia and the NMDA-nitric oxide (NO) cascade in spinal neurons in the dorsal horn and did not exhibit mechanical allodynia after peripheral nerve injury. The intrathecal injection of indomethacin, a nonsteroidal anti-inflammatory drug, ONO-8713, a selective EP1 antagonist, or 7-nitroindole, a neuronal NO synthase inhibitor, attenuated mechanical allodynia and the increase in activated microglia observed in the established neuropathic-pain state. We further demonstrated that ATP-induced microglial migration was blocked in vitro by PGE(2) via EP2 and by S-nitrosoglutathione, an NO donor. Taken together, the present study suggests that PGE(2) participated in the maintenance of neuropathic pain in vivo not only by activating spinal neurons, but also by retaining microglia in the central terminals of primary afferent fibers via EP2 subtype and via EP1-mediated NO production.     

Enkephalin contributes to the locomotor stimulating effects of 3,4-methylenedioxy-N-methylamphetamine

3,4-methylenedioxy-N-methylamphetamine (MDMA, 'Ecstasy') is a potent inhibitor of serotonin uptake, which induces both an increase in locomotion and a decrease in exploratory activity in rodents. Serotonin 5-HT1B receptors, located on the terminals of striatal efferent neurons, have been suggested to mediate these motor effects of MDMA. Striatal neurons projecting to the globus pallidus contain met-enkephalin, whilst those projecting to the substantia nigra contain substance P. We therefore analysed the levels of both peptides using radioimmunocytochemistry after MDMA administration (10 mg/kg, 3 h) in wild-type and 5-HT1B receptor knockout mice. Our results demonstrate that MDMA induces a decrease in pallidal met-enkephalin immunolabelling in wild-type, but not in 5-HT1B receptor knockout mice. Similar results were obtained following treatment with the 5-HT1A/1B agonist RU24969 (5 mg/kg, 3 h), suggesting that activation of 5-HT1B receptors leads to a reduction in met-enkephalin levels in the globus pallidus. In contrast, MDMA had no effect on the nigral substance P levels. We have previously shown that both MDMA and RU24969 fail to stimulate locomotor activity in 5-HT1B receptor knockout mice. Our present data indicate that the opioid antagonist naloxone suppressed the locomotor effects of MDMA. This study is the first to demonstrate that Enk contributes to MDMA-induced increases in locomotor activity. Such an effect may be related to the 5-HT control of pallidal met-enkephalin levels via the 5-HT1B receptors.     

Resident-intruder trait aggression is associated with differences in lysine vasopressin and serotonin receptor 1A (5-HT1A) mRNA expression in the brain of pre-pubertal female domestic pigs (Sus scrofa)

Aggressive behaviour exhibited by domestic pigs following encounters with unfamiliar individuals is a serious welfare and economical problem. Aggression resulting in skin lesions is similarly prevalent in prepubertal pigs of either sex. Little is known about the neural circuits and neuropeptides that control aggression in the pig. Because there is evidence for the involvement of the vasopressin and serotonergic systems in the regulation of aggressive behaviour in male mammals, we sought differences using quantitative in situ hybridisation of vasopressin and serotonin 1A receptor (5-HT1A) mRNA expression within specific brain regions of aggressive and nonaggressive prepubertal female pigs. The number of cells expressing vasopressin mRNA was significantly higher in aggressive pigs in the medial amygdala, lateral septum (LS) and showed a similar trend in the bed nucleus of the stria terminalis (BnST) but not the paraventricular nucleus (PVN) or supraoptic nucleus. The 5-HT1A receptor was widely expressed through the porcine brain and a significantly lower intensity (silver grain density) of 5-HT1A mRNA expression was observed in the BnST. In the medial amygdala and LS fewer cells expressed 5-HT1A mRNA in aggressive pigs but no differences were found in the PVN. In the absence of inbred strains or selection lines, these findings have shown that prior identification of phenotypic behavioural extremes in a population in advance of neural studies is a useful technique. Moreover, these findings support a central role for vasopressin and serotonin in the mediation of high trait aggression in prepubertal female pigs.     

Exaggerated aggression and decreased anxiety in mice deficient in brain serotonin

Serotonin is a major neurotransmitter in the central nervous system (CNS). Dysregulation of serotonin transmission in the CNS is reported to be related to different psychiatric disorders in humans including depression, impulsive aggression and anxiety disorders. The most frequently prescribed antidepressants and anxiolytics target the serotonergic system. However, these drugs are not effective in 20–30% of cases. The causes of this failure as well as the molecular mechanisms involved in the origin of psychological disorders are poorly understood. Biosynthesis of serotonin in the CNS is initiated by tryptophan hydroxylase 2 (TPH2). In this study, we used Tph2-deficient (Tph2^−/−) mice to evaluate the impact of serotonin depletion in the brain on mouse behavior. Tph2^−/− mice exhibited increased depression-like behavior in the forced swim test but not in the tail suspension test. In addition, they showed decreased anxiety-like behavior in three different paradigms: elevated plus maze, marble burying and novelty-suppressed feeding tests. These phenotypes were accompanied by strong aggressiveness observed in the resident–intruder paradigm. Despite carrying only one copy of the gene, heterozygous Tph2^+/− mice showed only 10% reduction in brain serotonin, which was not sufficient to modulate behavior in the tested paradigms. Our findings provide unequivocal evidence on the pivotal role of central serotonin in anxiety and aggression.     

Importance of NO/cGMP signalling via cGMP-dependent protein kinase II for controlling emotionality and neurobehavioural effects of alcohol

Cyclic GMP is a second messenger for nitric oxide (NO) that acts as a mediator for many different physiological functions. The cGMP-dependent protein kinases (cGKs) mediate cellular signalling induced by NO and cGMP. Here, we explored the localization of cGMP-dependent protein kinase type II (cGKII) in the mouse brain. In situ hybridization revealed high levels of cGKII mRNA in cerebral cortex, thalamic nuclei, hypothalamic nuclei, and in several basal forebrain regions including medial septum, striatum and amygdala. The close link to NO and the distribution pattern of cGKII suggested that this enzyme might be involved in emotional reactions and responses to drugs of abuse. Therefore, cGKII knockout animals (cGKII-/-) were compared with littermate controls in behavioural tests (i) for emotion-linked and (ii) for acute and chronic ethanol responses. Deletion of cGKII did not influence aggressive behaviour but led to enhanced anxiety-like behaviour. In terms of acute responses to ethanol, cGKII-/- mice were hyposensitive to hypnotic doses of ethanol as measured by the loss of righting reflex, without an alteration in their blood alcohol elimination. In a two-bottle free choice test, cGKII-/- mice showed elevated alcohol consumption. No taste differences to sweet solutions were observed compared to control animals. In summary, our data show that cGKII activity modulates anxiety-like behaviour and neurobehavioural effects of alcohol.