Multiple sclerosis (MS) is a complex chronic disease with an unknown etiology. It is considered an inflammatory demyelinating and neurodegenerative disorder of the central nervous system (CNS) characterized, in most cases, by an unpredictable onset of relapse and remission phases. The disease generally starts in subjects under 40; it has a higher incidence in women and is described as a multifactorial disorder due to the interaction between genetic and environmental risk factors. Unfortunately, there is currently no definitive cure for MS. Still, therapies can modify the disease's natural history, reducing the relapse rate and slowing the progression of the disease or managing symptoms. The limited access to human CNS tissue slows down. It limits the progression of research on MS. This limit has been partially overcome over the years by developing various experimental models to study this disease. Animal models of autoimmune demyelination, such as experimental autoimmune encephalomyelitis (EAE) and viral and toxin or transgenic MS models, represent the most significant part of MS research approaches. These models have now been complemented by ex vivo studies, using organotypic brain slice cultures and in vitro, through induced Pluripotent Stem cells (iPSCs). We will discuss which clinical features of the disorders might be reproduced and investigated in vivo, ex vivo, and in vitro in models commonly used in MS research to understand the processes behind the neuropathological events occurring in the CNS of MS patients. The primary purpose of this review is to give the reader a global view of the main paradigms used in MS research, spacing from the classical animal models to transgenic mice and 2D and 3D cultures.

Nociceptin/orphanin FQ (N/OFQ) has been reported to inhibit dopamine (DA) release in basal ganglia mainly by acting on NOP receptors in substantia nigra and ventral tegmental area. We investigated whether N/OFQ could affect DA transmission by acting at either DA nerve endings or DA‐targeted post‐synaptic neurons. In synaptosomes of rat nucleus accumbens and striatum N/OFQ inhibited DA synthesis and tyrosine hydroxylase (TH) phosphorylation at Ser40 via NOP receptors coupled to inhibition of the cAMP/protein kinase A pathway. Immunofluorescence studies showed that N/OFQ preferentially inhibited phospho‐Ser40‐TH in nucleus accumbens shell and that in this subregion NOP receptors partly colocalized with either TH or DA D₁ receptor positive structures. In accumbens and striatum N/OFQ inhibited DA D₁ receptor‐stimulated cAMP formation, but failed to affect either adenosine A_2A or DA D₂ receptor regulation of cAMP. In accumbens slices, N/OFQ inhibited DA D₁‐induced phosphorylation of NMDA and α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate glutamate receptors, whereas in primary cultures of accumbal cells, which were found to coexpress NOP and DA D₁ receptors, N/OFQ curtailed DA D₁ receptor‐induced cAMP‐response element‐binding protein phosphorylation. Thus, in accumbens and striatum N/OFQ exerts an inhibitory constraint on DA transmission by acting on either pre‐synaptic NOP receptors inhibiting TH phosphorylation and DA synthesis or post‐synaptic NOP receptors selectively down‐regulating DA D₁ receptor signaling.

Tricyclic antidepressants (TCAs) have been reported to interact with the opioid system, but their pharmacological activity at opioid receptors has not yet been elucidated. In the present study, we investigated the actions of amoxapine, amitriptyline, nortriptyline, desipramine, and imipramine at distinct cloned and native opioid receptors. In Chinese hamster ovary (CHO) cells expressing δ-opioid receptors (CHO/DOR), TCAs displaced [3H]naltrindole binding and stimulated guanosine 5′-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding at micromolar concentrations with amoxapine displaying the highest potency and efficacy. Amoxapine and amitriptyline inhibited cyclic AMP formation and induced the phosphorylation of signaling molecules along the extracellular signal-regulated kinase 1/2 (ERK1/2) and phosphatidylinositol-3 kinase pathways. Amoxapine also activated δ-opioid receptors in rat dorsal striatum and nucleus accumbens and human frontal cortex. In CHO cells expressing κ-opioid receptors (CHO/KOR), TCAs, but not amoxapine, exhibited higher receptor affinity and more potent stimulation of [³⁵S]GTPγS binding than in CHO/DOR and effectively inhibited cyclic AMP accumulation. Amitriptyline regulated ERK1/2 phosphorylation and activity in CHO/KOR and C6 glioma cells endogenously expressing κ-opioid receptors, and this effect was attenuated by the κ-opioid antagonist nor-binaltorphimine. In rat nucleus accumbens, amitriptyline slightly inhibited adenylyl cyclase activity and counteracted the inhibitory effect of the full κ agonist trans-(−)-3,4dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide (U50,488). At the cloned μ-opioid receptor, TCAs showed low affinity and no significant agonist activity. These results show that TCAs differentially regulate opioid receptors with a preferential agonist activity on either δ or κ subtypes and suggest that this property may contribute to their therapeutic and/or side effects.