Antiepileptic drugs to treat pain in rheumatic conditions. Recommendations based on evidence-based review of the literature and expert opinion.

OBJECTIVES: Neuropathic pain is commonly encountered in rheumatology practice, often associated with nociceptive mechanisms. It is caused by nervous system lesions, and the usual treatments with analgesics and anti-inflammatory drugs are mostly ineffective. Antiepileptic drugs (AED) have proved effective in relieving neuropathic pain. AED are recently used by rheumatologists since the role of neuropathic pain in rheumatological conditions has only recently been documented. Nevertheless, the tendency seems to be reversed when these drugs are used inappropriately. The CEDR (Cercle d'Etude de la Douleur en Rhumatologie), a specific interest group of the French Society of Rheumatology that focuses on pain in rheumatology, undertook to develop recommendations for the use of AED in Rheumatology. METHODS: A list of questions concerning the prescription of AED in painful rheumatic conditions was validated by a working group of 7 experts from the CEDR. The list of questions was used to draw up the recommendations. A literature review was performed using electronic databases (Medline, Embase and Cochrane library between 1980 and 2007) without limitations on the type of publication: case reports, clinical trials, literature review and guidelines about therapeutic management of neuropathic pain. Selected studies were scored for quality. Based on the literature and clinical experience, recommendations were developed using the Delphi method. RESULTS: We identified 29 studies concerning the use of AED in painful rheumatic conditions and 16 studies were considered valid and scored for quality. These few studies, the guidelines published for neuropathic pain treatment and the clinical experience of each expert, were used to develop 11 recommendations for the use of AED in painful rheumatic conditions. CONCLUSION: These recommendations can be used as guidelines to help prescribers to use AED for the management of pain in rheumatic conditions until further scientific evidence becomes available.

Chemical transmission between dopaminergic neuron pairs.

Midbrain dopaminergic (DAergic) neurons play a major regulatory role in in goal-directed behavior and reinforcement learning. DAergic neuron activity, and therefore spatiotemporal properties of dopamine release, precisely encodes reward signals. Neuronal activity is shaped both by external afferences and local interactions (chemical and electrical transmissions). Numerous hints suggest the existence of chemical interactions between DAergic neurons, but direct evidence and characterization are still lacking. Here, we show, using dual patch-clamp recordings in rat brain slices, a widespread bidirectional chemical transmission between DAergic neuron pairs. Hyperpolarizing postsynaptic potentials were partially mediated by D2-like receptors, and entirely resulted from the inhibition of the hyperpolarization-activated depolarizing current (Ih). These results constitute the first evidence in paired recordings of a chemical transmission relying on conductance decrease in mammals. In addition, we show that chemical transmission and electrical synapses frequently coexist within the same neuron pair and dynamically interact to shape DAergic neuron activity.

Drugs of abuse specifically sensitize noradrenergic and serotonergic neurons via a non-dopaminergic mechanism.

A challenge in drug dependence is to delineate long-term neurochemical modifications induced by drugs of abuse. Repeated d-amphetamine was recently shown to disrupt a mutual regulatory link between noradrenergic and serotonergic neurons, thus inducing long-term increased responses to d-amphetamine and para-chloroamphetamine, respectively. We show here that such a sensitization of noradrenergic and serotonergic neurons also occurs following repeated treatment with cocaine, morphine, or alcohol, three compounds belonging to main groups of addictive substances. In all cases, this sensitization is prevented by alpha 1b-adrenergic and 5-HT2A receptors blockade, indicating the critical role of these receptors on long-term effects of drugs of abuse. However, repeated treatments with two non-addictive antidepressants, venlafaxine, and clorimipramine, which nevertheless inhibit noradrenergic and serotonergic reuptake, do not induce noradrenergic and serotonergic neurons sensitization. Similarly, this sensitization does not occur following repeated treatments with a specific inhibitor of dopamine (DA) reuptake, GBR12783. Moreover, we show that the effects of SCH23390, a D1 receptor antagonist known to inhibit development of d-amphetamine behavioral sensitization, are due to its 5-HT2C receptor agonist property. SCH23390 blocks amphetamine-induced release of norepinephrine and RS102221, a 5-HT2C antagonist, can reverse this inhibition as well as inhibition of noradrenergic sensitization and development of behavioral sensitization induced by repeated d-amphetamine. We propose that noradrenergic/serotonergic uncoupling is a common neurochemical consequence of repeated consumption of drugs of abuse, unrelated with DA release. Our data also suggest that compounds able to restore the link between noradrenergic and serotonergic modulatory systems could represent important therapeutic targets for investigation.

Effects of acute dopamine depletion on the electrophysiological properties of striatal neurons.

The striatum, the main input nucleus of basal ganglia, receives a massive innervation from the entire cerebral cortex and is in charge of the detection of behaviorally relevant signals. In turn, via its projections to the output nuclei of basal ganglia, the striatum contributes to the organization of appropriate compartmental responses. Substantia nigra pars compacta dopaminergic neurons project predominantly to the striatum and regulate striatal functions. Implications of dopaminergic receptors on the physiology of striatal neurons are now well documented. By contrast, the effects of acute dopamine depletion on striatal neurons remain poorly explored. Here, the alpha-methyl-para-tyrosine was used to deplete dopamine from rat brain slices. We analyzed the consequences of a alpha-methyl-para-tyrosine treatment on membrane properties of striatal neurons: the medium-sized spiny neurons and the interneurons (GABAergic, cholinergic and NO-synthase). After acute dopamine depletion, medium-sized spiny neurons became more excitable. GABAergic interneurons became less excitable whereas cholinergic cells displayed an increased excitability. NO-synthase-containing interneurons did not show noticeable changes in their excitability. Such membrane properties changes indicate that striatal circuits should undergo major alteration in cortico-basal ganglia information processing.

Electrical synapses in basal ganglia.

The basal ganglia (BG) provide a major integrative system of the forebrain involved in the organization of goal-directed behaviour. Pathological alteration of BG function leads to major motor and cognitive impairments such as observed in Parkinson's disease. Recent advances in BG research stress the role of neural oscillations and synchronization in the normal and pathological function of BG. As demonstrated in several brain structures, these patterns of neural activity can emerge from electrically coupled neuronal networks. This review aims at addressing the presence, functionality and putative role of electrical synapses in BG, with a particular emphasis on the striatum and the substantia nigra pars compacta (SNc), two main BG nuclei in which the existence and functional properties of neuronal coupling are best documented.

Impact of 6-hydroxydopamine lesions and cocaine exposure on mu-opioid receptor expression and regulation of cholinergic transmission in the limbic-prefrontal territory of the rat dorsal striatum.

Information processing within the striatum is regulated by local circuits involving dopamine, cholinergic interneurons and neuropeptides released by recurrent collaterals of striatal output neurons. In the limbic-prefrontal territory of the dorsal striatum, enkephalin inhibits the NMDA-evoked release of acetylcholine directly through micro-opioid receptors (MORs) located on cholinergic interneurons and indirectly through MORs of output neurons of striosomes. In this territory, we investigated the consequence of changes in dopamine transmission, bilateral 6-hydroxydopamine-induced degeneration of striatal dopaminergic innervation or cocaine (acute and chronic) exposure on (i) MOR expression in both cholinergic interneurons and output neurons of striosomes, and (ii) the direct and indirect enkephalin-MOR regulations of the NMDA-evoked release of acetylcholine. Expression of MORs in cholinergic interneurons was preserved after 6-hydroxydopamine and down-regulated after cocaine treatments. Accordingly, the direct enkephalin-MOR control of acetylcholine release was preserved after 6-hydroxydopamine treatment and lost after cocaine exposure. Expression of MORs in output neurons of striosomes was down-regulated in the 6-hydroxydopamine situation and either preserved or up-regulated after acute or chronic cocaine exposure, respectively. Accordingly, the indirect enkephalin-MOR control of acetylcholine release disappeared in the 6-hydroxydopamine situation but surprisingly, despite preservation of MORs in striosomes, disappeared after cocaine treatment. Showing that MORs of striosomes are still functional in this situation, the MOR agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin inhibited the NMDA-evoked release of acetylcholine after cocaine exposure. Therefore, alteration in the regulation of cholinergic transmission by the enkephalin-MOR system might play a major role in the motivational and cognitive disorders associated with dopamine dysfunctions in fronto-cortico-basal ganglia circuits.

Connexin mRNA expression in single dopaminergic neurons of substantia nigra pars compacta.

Dopaminergic neurons of the substantia nigra pars compacta play a major role in goal-directed behavior and reinforcement learning. The study of their local interactions has revealed that they are connected by electrical synapses. Connexins, the molecular substrate of electrical synapses, constitute a multigenic family of 20 proteins in rodents. The permeability and regulation properties of electrical synapses depend on their connexin composition. Therefore, the knowledge of the molecular composition of electrical synapses is fundamental to the understanding of their specific functions. We have investigated the connexin mRNA expression pattern of dopaminergic neurons by single-cell RT-PCR analysis, during two periods in which dopaminergic neurons are electrically coupled in vitro (P7-P10 and P17-P21). Our results show that dopaminergic neurons express mRNAs of various connexins (Cx26, Cx30, Cx31.1, Cx32, Cx36 and Cx43) in a developmentally regulated manner. Furthermore, we have observed that dopaminergic neurons display different connexin expression patterns, and that multiple connexins can be expressed in a single dopaminergic neuron. These observations underline the importance of electrical coupling in the development of dopaminergic neurons and raise the question of the existence of functionally distinct electrically coupled networks in the substantia nigra pars compacta.

Irreversible blockade of monoamine oxidases reveals the critical role of 5-HT transmission in locomotor response induced by nicotine in mice.

Although nicotine is generally considered as the main compound responsible for addictive properties of tobacco, some experimental data indicate that nicotine does not exhibit all the characteristics of other substances of misuse such as psychostimulants and opiates. For example, nicotine generally fails to induce locomotor response in mice and self-administration of nicotine is difficult to obtain in rats. We have shown recently that a pretreatment with mixed irreversible monoamine oxidase inhibitors (MAOIs), such as tranylcypromine, triggers a locomotor response to nicotine in mice and induces a robust self-administration of nicotine in rats. We show here that when mice were pretreated with enhancers of extracellular levels of noradrenaline, dopamine or serotonin (D-amphetamine, GBR12783 or para-chloro-amphetamine, respectively) and injected with nicotine (1 mg/kg), only those animals pretreated with para-chloro-amphetamine exhibited a specific locomotor response to nicotine. These data indicate a critical role of serotonin in nicotine-induced locomotor activity in mice. This was further confirmed in microdialysis experiments showing that nicotine induces an increase in extracellular serotonin levels in the ventral striatum in mice pretreated with tranylcypromine. This effect of nicotine on extracellular serotonin levels was absent in mice lacking the beta2-subunit of the nicotinic acetylcholine receptor. Our data suggest that mixed irreversible MAOIs contained in tobacco facilitate the effects of nicotine on serotonin release, thus allowing the locomotor and rewarding effects of nicotine.

Calpain product of WT-CRMP2 reduces the amount of surface NR2B NMDA receptor subunit.

The brain is particularly vulnerable to ischaemia; however, neurons can become tolerant to ischaemic insult. This tolerance has been shown to involve activation of NMDA receptors, but its mechanisms have not yet been fully elucidated. Using a preconditioning protocol, we show that neurons surviving to a transient NMDA exposure become resistant to the glutamatergic agonist. Using a proteomic approach, we found that alterations of the protein pattern of NMDA-resistant neurons are restricted mainly to the five collapsin response mediator proteins (CRMPs). A sustained increase in calpain activity following NMDA treatment is responsible for the production of cleaved CRMPs. Finally, we provide evidence for the involvement of the cleaved form of WT-CRMP2 in the down-regulation of NR2B. Our data suggests that, beside their role in neuronal morphogenesis, CRMPs may contribute to neuronal plasticity.

Behavioral sensitization to amphetamine results from an uncoupling between noradrenergic and serotonergic neurons.

In rodents, drugs of abuse induce locomotor hyperactivity, and repeating injections enhances this response. This effect, called behavioral sensitization, persists many months after the last administration, thus mimicking long-term sensitivity to drugs observed in human addicts. We show here that, in naïve animals, noradrenergic and serotonergic systems, besides their behavioral activating effects, inhibit each other by means of the stimulation of alpha1b-adrenergic and 5-HT(2A) receptors and that this mutual inhibition vanishes with repeated injections of d-amphetamine; this uncoupling may be responsible for behavioral sensitization and for an increased reactivity of dopaminergic neurons. First, after repeated d-amphetamine injections, a d-amphetamine challenge induces a dramatic increase in cortical extracellular norepinephrine (NE) levels. This increased cortical NE release still occurs after 1 month of withdrawal but is diminished or blocked if sensitization is performed in the presence of prazosin, SR46349B, or both alpha1-adrenergic and 5-HT(2A) receptor antagonists, respectively. A strong correlation between increases in cortical extracellular NE levels and the expression of behavioral sensitization was found. Second, repeated d-amphetamine injections induce an increased reactivity of serotonergic neurons measured by cortical extracellular serotonin (5-HT) levels after the administration of a 5-HT releaser, p-chloroamphetamine. Third, knockout mice for alpha1b-adrenergic (alpha1b-AR KO) or 5-HT(2A) (5-HT(2A)-R KO) receptor, respectively, exhibit a behavioral and biochemical hyperreactivity to the acute injection of p-chloroamphetamine (alpha1b-AR KO; 5-HT levels) and d-amphetamine (5-HT(2A)-R KO; NE levels). Uncoupling between noradrenergic and serotonergic neurons may occur not only in addiction but also during chronic stressful situations, thus facilitating the onset of mental illness.

Bidirectional activity-dependent plasticity at corticostriatal synapses.

Corticostriatal projections originate from the entire cerebral cortex and provide the major source of glutamatergic inputs to the basal ganglia. Despite the importance of corticostriatal connections in sensorimotor learning and cognitive functions, plasticity forms at these synapses remain strongly debated. Using a corticostriatal slice preserving the connections between the somatosensory cortex and the target striatal cells, we report the induction of both non-Hebbian and Hebbian forms of long-term potentiation (LTP) and long-term depression (LTD) on striatal output neurons (SONs). LTP and LTD can be induced selectively by different stimulation patterns (high-frequency trains vs low-frequency pulses) and were evoked with similar efficiency in non-Hebbian and Hebbian modes. Combination of LTP-LTD and LTD-LTP sequences revealed that bidirectional plasticity occurs at the same SONs and provides efficient homeostatic mechanisms leading to a resetting of corticostriatal synapses avoiding synaptic saturation. The effect of temporal relationship between cortical stimulation and SON activity was assessed using spike-timing-dependent plasticity (STDP) protocols. An LTP was observed when an action potential was triggered in the striatal neuron before the cortical stimulus, and, conversely, an LTD was induced when the striatal neuron discharge was triggered after the cortical stimulation. Such STDP was reversed when compared with those described so far in other mammalian brain structures. This mechanism may be essential for the role of the striatum in learning of motor sequences in which sensory and motor events are associated in a precise time sequence.

A pro- and an anti-inflammatory cytokine are synthesised in distinct brain macrophage cells during innate activation.

Brain macrophages are known to exert dual and opposing functions on neuronal survival, which can be either beneficial or detrimental. The rationale of our study is that this duality could arise from an exclusive secretion of either pro- or anti-inflammatory cytokine by distinct cell subsets, cytokines that could respectively mediate neurotoxic or neurotrophic effects. Innate immune response was induced in macrophage cultures prepared from embryonic-day-16 to postnatal-day-8 mouse brains. By immunofluorescent detection of intracellular cytokines, we have assessed the occurrence of TNFalpha or IL10 synthesis at single cell level and observed distinct secretory patterns that include cells producing exclusively TNFalpha or IL10, cells producing both cytokines and non-producer cells. These secretory patterns are differentially regulated by MAP-kinase inhibitors. Altogether, these results demonstrate that synthesis of either a pro- or an anti-inflammatory cytokine can segregate distinct brain macrophages and suggests a functional cell-subset-specialisation.

Neuroleptic-induced catalepsy: electrophysiological mechanisms of functional recovery induced by high-frequency stimulation of the subthalamic nucleus.

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) remarkably alleviates motor disorders in parkinsonian patients. The mechanisms by which STN HFS exerts its beneficial effects were investigated in anesthetized rats, using a model of acute interruption of dopaminergic transmission. Combined systemic injections of SCH-23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5,-tetrahydro-1H-3-benzazepine] and raclopride, antagonists of the D1 and D2 classes of dopaminergic receptors, respectively, were performed, and the parameters of STN HFS that reversed the neuroleptic-induced catalepsy were determined in freely moving animals. The effects of neuroleptics and the impact of STN HFS applied at parameters alleviating neuroleptic-induced catalepsy were analyzed in the substantia nigra pars reticulata (SNR), a major basal ganglia output structure, by recording the neuronal firing pattern and the responses evoked by cortical stimulation. Neuroleptic injection altered the tonic and regular mode of discharge of SNR neurons, most of them becoming irregular with bursts of spikes and pauses. The inhibitory component of the cortically evoked response, which is attributable to the activation of the direct striatonigral circuit, was decreased, whereas the late excitatory response resulting from the indirect striato-pallido-subthalamo-nigral circuit was reinforced. During STN HFS, the spontaneous firing of SNR cells was either increased or decreased with a global enhancement of the firing rate in the overall population of SNR cells recorded. However, in all of the cases, SNR firing pattern was regularized, and the bias between the trans-striatal and trans-subthalamic circuits was reversed. By these effects, STN HFS restores the functional properties of the circuits by which basal ganglia contribute to motor activity.

Functional mu opioid receptors are expressed in cholinergic interneurons of the rat dorsal striatum: territorial specificity and diurnal variation.

Striatal cholinergic interneurons play a crucial role in the control of movement as well as in motivational and learning aspects of behaviour. Neuropeptides regulate striatal cholinergic transmission and particularly activation of mu opioid receptor (MOR) inhibits acetylcholine (ACh) release in the dorsal striatum. In the present study we investigated whether this cholinergic transmission could be modulated by an enkephalin/MOR direct process. We show that mRNA and protein of MORs are expressed by cholinergic interneurons in the limbic/prefrontal territory but not by those in the sensorimotor territory of the dorsal striatum. These MORs are functional because potassium-evoked release of ACh from striatal synaptosomes was dose-dependently reduced by a selective MOR agonist, this effect being suppressed by a MOR antagonist. The MOR regulation of cholinergic interneurons presented a diurnal variation. (i) The percentage of cholinergic interneurons containing MORs that was 32% at the beginning of the light period (morning) increased to 80% in the afternoon. (ii) The MOR-mediated inhibition of synaptosomal ACh release was higher in the afternoon than in the morning. (iii) While preproenkephalin mRNA levels remained stable, enkephalin tissue content was the lowest (-32%) in the afternoon when the spontaneous (+35%) and the N-methyl-d-aspartate-evoked (+140%) releases of enkephalin (from microsuperfused slices) were the highest. Therefore, by acting on MORs present on cholinergic interneurons, endogenously released enkephalin reduces ACh release. This direct enkephalin/MOR regulation of cholinergic transmission that operates only in the limbic/prefrontal territory of the dorsal striatum might contribute to information processing in fronto-cortico-basal ganglia circuits.

Albumin stimulates monocyte chemotactic protein-1 expression in rat embryonic mixed brain cells.

Albumin, a blood protein absent from the adult brain in physiological situations, can be brought into contact with brain cells during development or, in adult, following breakdown of the blood-brain barrier occurring as a result of local inflammation. In the present study, we show that ovalbumin and albumin induce the release of monocyte chemotactic protein 1 (MCP-1/CCL2) from rat embryonic mixed brain cells. A short-term exposure to ovalbumin during the cell dissociation procedure is sufficient to generate MCP-1 mRNA. A comparable effect is observed when the cells are incubated for 4 hr with ovalbumin or rat albumin, while MCP-1 messengers are barely detectable following bovine albumin exposure. The amount of MCP-1 protein measured in 4 hr-supernatants of albumin-treated cells followed the same albumin-inducing pattern as that of MCP-1 mRNA, while all albumins tested induced MCP-1 protein after a 17 hr-incubation period. The albumin-induced MCP-1 production is significantly inhibited in calphostin C-treated cells, suggesting the implication of a protein kinase C-dependent signaling pathway. This MCP-1-inducing activity is maintained after a lipid extraction procedure but abolished by proteinase K or trypsin treatments of albumin. The MCP-1 secretion following albumin contact with nervous cells could thus interfere, by chemotactic gradient formation, with the brain infiltration program of blood-derived cells during development or brain injury.

Differential expression of CRMP1, CRMP2A, CRMP2B, and CRMP5 in axons or dendrites of distinct neurons in the mouse brain.

CRMP1, CRMP2, and CRMP5 have been identified as cytosolic proteins relaying semaphorin 3A signalling, one of the molecular cues conducting axon and dendrite growth and guidance. They are highly expressed during brain ontogenesis, but, because of their lower levels in the adult, their distribution in the mature brain is poorly documented. By using specific antibodies, we investigated the cellular distribution of these CRMPs in different adult brain structures and in neural cell cultures with a special focus on the splice variants CRMP2A and CRMP2B. In brain sections of adult mouse, CRMP1, CRMP2B, and CRMP5 were located predominantly in dendrites of specific neuronal populations, such as cortical pyramidal neurons, hippocampal CA1 pyramidal cells, or Purkinje cerebellar cells. On the contrary, CRMP2A was specifically associated with axons of the corpus callosum, bundles of the striatum, and mossy fibers of the hippocampus. In cultures of cortical neurons, CRMP1, CRMP2A, CRMP2B, and CRMP5 were equally distributed throughout cell bodies, axons, or dendrites of neurons, whereas CRMP2A and CRMP5 were completely absent from Purkinje cerebellar cells in 12-day-old animals. By comparison, oligodendrocytes exclusively express CRMP2B and CRMP5 in cell bodies and processes both in situ in the adult brain and in primary cultures. Overall, our results demonstrate specific subcellular localizations of CRMP1, CRMP2A, CRMP2B, and CRMP5 depending on cell types, neuronal compartment, and developmental stage. This study suggests that, beyond their signalling function in axon outgrowth and guidance, CRMPs also play a role in mature neurons both in axons and in dendrites.

Electrical synapses between dopaminergic neurons of the substantia nigra pars compacta.

Spatiotemporal properties of dopamine release play a major role both in striatal and nigral physiology because dopamine is released from nerve terminals and dendrites of nigrostriatal dopaminergic (DA) neurons. Pioneering work revealed gap junctional communication (assessed by dye-coupling experiments) between DA cells in the substantia nigra pars compacta (SNc). However, direct evidence of functional electrical synapses between DA neurons is still lacking. In this study, gap junctional communication between DA neurons was investigated in rat brain slices. Tracer coupling was observed in postnatal day 5 (P5) to P10 and P15-P25 rats. Dual whole-cell patch-clamp recordings revealed that 96% of DA neurons were coupled by electrical synapses in P7-P10 rats, and 20% were coupled in P17-P21 rats. These electrical synapses were mainly symmetrical and displayed strong low-pass filtering properties. When spontaneous firing activity was monitored, no significant synchronization was observed. Nevertheless, an efficient modulation of the spontaneous firing frequency of the postsynaptic cell occurred during injection of hyperpolarizing and depolarizing currents in the coupled presynaptic cell. Together, these observations demonstrate the existence of a fast communication between SNc DA neurons through electrical synapses.

5-HT2A and alpha1b-adrenergic receptors entirely mediate dopamine release, locomotor response and behavioural sensitization to opiates and psychostimulants.

Addictive properties of drugs of misuse are generally considered to be mediated by an increased release of dopamine (DA) in the ventral striatum. However, recent experiments indicated an implication of alpha1b-adrenergic receptors in behavioural responses to psychostimulants and opiates. We show now that DA release induced in the ventral striatum by morphine (20 mg/kg) is completely blocked by prazosin (1 mg/kg), an alpha1-adrenergic antagonist. However, morphine-induced increases in DA release in the ventral striatum were found to be similar in mice deleted for the alpha1b-adrenergic receptor (alpha1b-AR KO) and in wild-type (WT) mice, suggesting the presence of a compensatory mechanism. This acute morphine-evoked DA release was completely blocked in alpha1b-AR KO mice by SR46349B (1 mg/kg), a 5-HT2A antagonist. SR46349B also completely blocked, in alpha1b-AR KO mice, the locomotor response and the development of behavioural sensitization to morphine (20 mg/kg) and D-amphetamine (2 mg/kg). Accordingly, the concomitant blockade of 5-HT2A and alpha1b-adrenergic receptors in WT mice entirely blocked acute locomotor responses but also the development of behavioural sensitization to morphine, D-amphetamine or cocaine (10 mg/kg). We observed, nevertheless, that inhibitory effects of each antagonist on locomotor responses to morphine or D-amphetamine were more than additive (160%) in naïve WT mice but not in those sensitized to either drug. Because of these latter data and the possible compensation by 5-HT2A receptors for the genetic deletion of alpha1b-adrenergic receptors, we postulate the existence of a functional link between these receptors, which vanishes during the development of behavioural sensitization.