Since many years, the ob/ob mouse is conceived as a suitable experimental model for studying neuropeptide substrates and metabolic pathways implicated in human obesity and type 2 diabetes : these rodents bear a nonsense mutation in the coding region of the leptin gene causing the lack of secretion of a functional peptide. An important aspect should be considered when using this mutant model: human obesity is rarely due to a single gene mutation  and obese humans prevalently present high circulating levels of leptin  together desensitized, leptin-resistant pathways; by contrast, ob/ob mice are defective in leptin production. Differences between these two conditions have not been still elucidated at the biochemical level. The ob/ob syndrome can be reversed by exogenous administration of leptin or by leptin gene transfection , proving that leptin receptors and networks are functional. On the other side, the study of ob/ob mice permits to dissect leptin-dependent neuroendocrine loops, involved in appetite control and energy balance dysfunctions. Indeed, as already mentioned, leptin pathways, co-modulation or connection with corticotropin releasing factor (CRF), NPY, α-MSH and neurotransmitter systems have not been fully characterized . Specifically, energy expenditure balance and feeding behavior are regulated by redundant pathways: monoamines and leptin are both able to modulate food intake at the hypothalamic levels, but it is not precisely known if and how these signal molecules interact . Some authors also consider leptin a signal evolved to prevent starvation rather than food plenty . Beside alterations of glucose metabolism, another important feature of the ob/ob syndrome is the increased blood cholesterol in mutant animals. Cholesterol is the precursor of steroidogenesis, being a main, high-affinity ligand for the benzodiazepine site-translocator protein TSPO [7, 9]. Reduced cholesterol levels have been evidenced inside macrophages of ob/ob mice, along with a diminished capacity in inflammatory response , supporting metabolic and hormonal cross-talks between immune response, inflammation and body weight signals [20, 21]. Interestingly, platelet TSPO and SERT densities have been found altered in fibromyalgia [12, 13], panic disorders [11, 22] and suicide attempters [23, 24]. This prompted us to preliminary assess the equilibrium-binding parameters of SERT and TSPO proteins, either in brain or high expression peripheral tissues, circulating platelets and kidneys, of ob/ob mice: to our knowledge, this is the first study that simultaneously evaluates SERT and TSPO expression in distinct anatomical district of a rodent genetic model of obesity.
Prior to SERT and TSPO analyses, blood chemical parameters were determined in ob/ob and WT animals to monitor cell metabolism: higher total cholesterol and glucose concentrations were observed in mutant vs. WT animals, according to data provided by the commercial source. Blood levels of the γ-GT enzyme were low in ob/ob and WT mice, indicating the absence of hepatic alterations. Moreover, ob/ob mice presented similar circulating triglyceride or calcium levels to those measured in WT animals. Concerning binding results, no difference of SERT density or affinity was reported in brain and platelets of ob/ob and control mice. Also, [3H]-paroxetine autoradiographic sections showed no appreciable binding differences between animals. This finding could signify that leptin-dependent pathways are altered in ob/ob mice without affecting 5-HT transmission. On the other hand, the fact that ob/ob mice show no changes in SERT density or affinity (present results) but a reduced SERT mRNA  is intriguing. Since leptin has been found able to decrease SERT binding sites in the rat brain , the ob/ob mouse could maintain the capacity at counteracting the decrease of SERT transcripts at the protein level, in the absence of SERT (or SERT-related) gene mutations. In fact, SERT underlies posttranslational regulation, trafficking or protein inactivation causing a differential distribution within cell compartments and/or discrepancies between mRNA and protein expression, as reported during megakaryoblastic differentiation . Leptin-lacking mice could present an altered 5-HT responsiveness together a modified SERT reserve/function (uptake), without significant differences in binding sites. A deeper biochemical analysis of SERT should be therefore carried out, including flow cytofluorimetry, immunoprecipitation, gene expression, proteomic-functional (uptake) studies, in the context of 5-HT (or other monoamine) levels in blood and tissues of these animals. Concerning TSPO binding results, this protein was found increased in the ob/ob mouse brain while being similar in kidneys of mutant and WT animals. The [3H]PK11195 autoradiography of hippocampal-hypothalamic sections has revealed an up-regulation of TSPO density in two brain regions of ob/ob mice, the dentate gyrus of hippocampus and choroids plexus, indicating that TSPO number variations in ob/ob mouse brain are region-dependent. These results also underline that leptin chronic deficiency affects brain protein patterns.
The interpretation of our TSPO finding is difficult. In fact, to date, the precise role of TSPO within CNS is not understood. A brain region-specific regulatory mechanism in response to hypercholesterolemia and hyperglycemia could be active in these animals. Brain TSPO is mainly localized on glial cells and can be modulated by protein kinase C signaling  and the cAMP-protein kinase A pathway , activated by G-protein coupled receptors, including, therefore, metabotropic 5-HT receptor subtypes. In hippocampus and choroids plexus there could be an unbalance of regulatory signaling cascades, resulting in the enhancement of TSPO number. This could depend upon many factors, such as different receptor sub-type localization and activation: hippocampus and choroids plexus are brain regions at high expression of 5-HT1 and 5-HT2C receptors, respectively, and insulin inhibits choroids plexus 5-HT2C receptors . Additional difficulties in interpreting our result come from the observation that increased TSPO has been associated with either tissue/neuronal damage or repair. Some authors have reported protective effects of TSPO agonists in experimental diabetic neuropathy , suggesting, hypothetically, reparative actions of brain TSPO in such a disease.
The same as for SERT, a deeper study of TSPO gene and protein expression together the investigation of its function and drug activities in ob/ob animals by means of different methodologies, including leptin treatment, is essential to confirm present results as well as to understand leptin-dependent neuronal trophism, metabolism and transmission. Another variable to consider is the different age of mutant animals: in our study, 4 month old mice were examined, when all symptoms of the "leptin-lacking syndrome" are present , but this does not exclude the diverse SERT and TSPO expression at other development or aging stages.