Expression and function of striatal nAChRs differ in the flinders sensitive (FSL) and resistant (FRL) rat lines

Major Depression: One Brain, One Disease, One Set of Intertwined Processes

Major depressive disorder (MDD) is a heterogeneous disease affecting one out of five individuals and is the leading cause of disability worldwide. Presently, MDD is considered a multifactorial disease with various causes such as genetic susceptibility, stress, and other pathological processes. Multiple studies allowed the formulation of several theories attempting to describe the development of MDD. However, none of these hypotheses are comprehensive because none of them can explain all cases, mechanisms, and symptoms of MDD. Nevertheless, all of these theories share some common pathways, which lead us to believe that these hypotheses depict several pieces of the same big puzzle. Therefore, in this review, we provide a brief description of these theories and their strengths and weaknesses in an attempt to highlight the common mechanisms and relationships of all major theories of depression and combine them together to present the current overall picture. The analysis of all hypotheses suggests that there is interdependence between all the brain structures and various substances involved in the pathogenesis of MDD, which could be not entirely universal, but can affect all of the brain regions, to one degree or another, depending on the triggering factor, which, in turn, could explain the different subtypes of MDD.

The reinforcement threshold and elasticity of demand for nicotine in an adolescent rat model of depression

BACKGROUND

The Food and Drug Administration (FDA) is considering setting a nicotine standard for tobacco products to reduce their addictiveness. Such a standard should account for the apparent greater vulnerability to nicotine addiction in some subpopulations, such as adolescents with depression. The present study examined whether the reinforcement threshold and elasticity of demand (i.e., reinforcing efficacy) for nicotine in a genetic inbred rat model of depression (Flinders Sensitive Line [FSL]) differs from an outbred control strain.

METHODS

Acquisition of nicotine self-administration (NSA) across a wide range of nicotine doses was measured in both FSL and Sprague-Dawley (SD) control adolescent rats. At the highest dose, elasticity of demand was also measured. Nicotine pharmacokinetics was examined to determine whether it might modulate NSA, as it does smoking in humans.

RESULTS

FSL rats acquired self-administration quicker and showed more inelastic demand (greater reinforcing efficacy) than SDs at the highest unit dose. However, there was no strain difference in the reinforcement threshold of nicotine. FSL rats exhibited faster nicotine clearance, larger volume of distribution, and lower plasma and brain nicotine concentrations. However, these differences were not consistently related to strain differences in NSA measures.

CONCLUSION

These findings are consistent with studies showing greater dependence and reinforcing efficacy of cigarettes in smokers with depression and those with relatively fast nicotine metabolism. However, these findings also suggest that a nicotine standard to reduce initiation of tobacco use should be similarly effective in both the general adolescent population and those with depression.

A computerized exposure system for animal models to optimize nicotine delivery into the brain through inhalation of electronic cigarette vapors or cigarette smoke

Pre-clinical studies investigated the effects of chronic exposure to nicotine on lungs, kidneys and brains using animal models. Most of these studies delivered nicotine into the circulatory and central nervous systems (CNS) through intraperitoneal injection or oral consumption methods. Few studies used inhalation machine system for nicotine delivery into brains in rodents to mimic human exposure to cigarettes. However, finding a more accurate and clinically relevant method of nicotine delivery is critical. A computerized inhalation machine has been designed (SciReq) and is currently employed in several institutions. The computerized machine delivers electronic (e)-cigarette vapor as well as tobacco smoke to rodents using marketed e-cigarette devices or tobacco cigarettes. This provides evidence about clinical effects of nicotine delivery by traditional methods (combustible cigarettes) and new methodologies (e-cigarettes) in physiological systems. Potential neurobiological mechanisms for the development of nicotine dependence have been determined recently in mice exposed to e-cigarette vapors in our laboratory using SciReq system. In this review article, the discussion focuses on the efficiency and practical applicability of using this computerized inhalation exposure system in inducing significant changes in brain protein expression and function as compared to other nicotine delivery methods. The SciReq inhalation system utilized in our laboratory and others is a method of nicotine delivery to the CNS, which has physiological relevance and mimics human inhalant exposures. Translation of the effects of inhaled nicotine on the CNS into clinical settings could provide important health considerations.

Effects of chronic inhalation of electronic cigarettes containing nicotine on glial glutamate transporters and α-7 nicotinic acetylcholine receptor in female CD-1 mice

Alteration in glutamate neurotransmission has been found to mediate the development of drug dependence, including nicotine. We and others, through using western blotting, have reported that exposure to drugs of abuse reduced the expression of glutamate transporter-1 (GLT-1) as well as cystine/glutamate antiporter (xCT), which consequently increased extracellular glutamate concentrations in the mesocorticolimbic area. However, our previous studies did not reveal any changes in glutamate/aspartate transporter (GLAST) following exposure to drugs of abuse. In the present study, for the first time, we investigated the effect of chronic exposure to electronic (e)-cigarette vapor containing nicotine, for one hour daily for six months, on GLT-1, xCT, and GLAST expression in frontal cortex (FC), striatum (STR), and hippocampus (HIP) in outbred female CD1 mice. In this study, we also investigated the expression of alpha-7 nicotinic acetylcholine receptor (α-7 nAChR), a major pre-synaptic nicotinic receptor in the glutamatergic neurons, which regulates glutamate release. We found that inhalation of e-cigarette vapor for six months increased α-7 nAChR expression in both FC and STR, but not in the HIP. In addition, chronic e-cigarette exposure reduced GLT-1 expression only in STR. Moreover, e-cigarette vapor inhalation induced downregulation of xCT in both the STR and HIP. We did not find any significant changes in GLAST expression in any brain region. Finally, using liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques, we detected high concentrations of nicotine and cotinine, a major metabolite of nicotine, in the FC tissues of e-cigarette exposed mice. These data provide novel evidence about the effects of chronic nicotine inhalation on the expression of key glial glutamate transporters as well as α-7 nAChR. Our work may suggest that nicotine exposure via chronic inhalation of e-cigarette vapor may be mediated in part by alterations in the glutamatergic system.

A neurochemical basis for an epigenetic vision of psychiatric disorders (1994-2009)

In 1996, Dr. Costa was invited by Prof. Boris Astrachan, Chairman of the Department of Psychiatry at the University of Illinois at Chicago, to direct the research of the "Psychiatric Institute, Department of Psychiatry, School of Medicine, at the University of Illinois at Chicago." He was asked to develop a seminal research program on psychiatric disorders. Viewed in retrospect, Dr. Costa met and surpassed the challenge, as was usual for him. To elucidate the molecular mechanisms whereby nurture (epigenetic factors) and nature (genetic factors) interact to cause major psychiatric disorders was at the center of Dr. Costa's mission for the last 15 years of his research at the Psychiatric Institute. The challenge for Dr. Costa and his colleagues (Auta, Caruncho, Davis, Grayson, Guidotti, Impagnatiello, Kiedrowski, Larson, Manev, Pappas, Pesold, Pinna, Sharma, Smalheiser, Sugaya, Tueting, Veldic [1-111]) had always been to find new ways to prevent and treat psychiatric disorders with pharmacological agents that failed to have major unwanted side effects. In this list, we have quoted the first authors of the papers pertaining to the field of research highlighted in the title. As you know, Dr. Costa was an eclectic scientist and in his 15 years of studies at UIC, he touched many other aspects of neuroscience research that are not discussed in this overview.

Nicotine receptors and depression: revisiting and revising the cholinergic hypothesis

There is a well-established connection between smoking and depression. Depressed individuals are over-represented among smokers, and ex-smokers often experience increased depressive symptoms immediately after stopping smoking. Nicotine in tobacco binds, activates and desensitizes nicotinic acetylcholine receptors (nAChRs), but it is not known whether activation or desensitization is more important for the effects of nicotine on depressive symptoms. Here we review, based on clinical and preclinical studies of nicotinic drugs, the hypothesis that blockade (rather than activation) of neuronal nAChRs might be important for the effects of nicotinic agents on depressive symptoms. The endogenous neurotransmitter for nAChRs is acetylcholine, and the effects of nicotine on depression-like behaviors support the idea that dysregulation of the cholinergic system might contribute to the etiology of major depressive disorder. Thus, pharmacological agents that limit acetylcholine signaling through neuronal nAChRs might be promising for the development of novel antidepressant medications.

Both acute and chronic buspirone treatments have different effects on regional 5-HT synthesis in Flinders Sensitive Line rats (a rat model of depression) than in control rats

The main objective of this investigation was to evaluate the effects of buspirone, a 5-HT(1A) agonist with some partial agonist properties and also an antidepressant, on regional 5-HT synthesis in Flinders Sensitive Line (FSL) rats ("depressed"), and to compare the effects to the Flinders Resistant Line (FRL) control rats (not "depressed"). In addition results were compared to those previously reported in normal Sprague-Dawley (SPD) rats (normal control). Serotonin synthesis in both FSL and FRL rats was measured following acute and chronic treatments with buspirone. Both of these strains were derived from the SPD rats. No direct comparison was done between the FSL saline and FRL saline groups, or the FSL buspirone and FRL buspirone groups, because the objective of the studies was to evaluate effects of buspirone in these two strains. The results show that acute treatment with buspirone elevates 5-HT synthesis throughout the brain in the FRL rats. In the FSL rats, there were reductions in some brain regions (e.g., dorsal and median raphe, amygdala, anterior olfactory nucleus, substantia nigra reticulate), while in other regions, there were increases in the synthesis observed (e.g., frontal, parietal, visual and somatosensory cortices, ventral hippocampus). In 20 out of the 30 brain regions investigated in the FSL rats, there was no significant change in the synthesis following acute buspirone treatment. During the chronic treatment, buspirone produced a significant reduction of 5-HT synthesis in 15 out of 30 brain regions in the FRL rats. In the FSL rats, buspirone produced a significant elevation of the synthesis in 10 out of 30 brain regions. In both the FSL and FRL rats, buspirone produced rather different effects than those reported previously for SPD (normal) rats. The acute effect in the FSL rats was somewhat similar to the effect reported previously for the SPD rats, while in the FRL rats, the acute buspirone treatment produced an effect observed previously in treatments with 5-HT(1A) antagonists suggesting an action of buspirone as partial agonist in FRL rats. The data suggest that with respect to 5-HT synthesis, FRL rats differ from SPD rats (a natural control; normal rats) and, as such, indicate that when the effects related to the serotonergic system (e.g., influence of serotonergic drugs) are studied in the FSL rats and compared to those in the FRL rats, any conclusions drawn may not reflect differences relative to a normal rat.

Chronic nicotine exposure has dissociable behavioural effects on control and beta2-/- mice

Nicotine exerts beneficial effects on various neurological and psychiatric pathologies, yet its effects on cognitive performance remain unclear. Mice lacking the beta2 subunit of the nicotinic receptor (beta2-/-) show characteristic deficits in executive functions and are suggested as reliable animal models for some specific endophenotypes of human pathologies, notably ADHD. We use beta2-/- and their controls to investigate the consequences of chronic nicotine exposure on cognitive behaviour. We show that in control mice, this treatment elicits somewhat slight effects, particularly affecting nocturnal activity and self-grooming. By contrast, in beta2-/- mice, chronic nicotine treatment had restorative effects on exploratory behaviour in the open-field and affected rearing, but did not modify motor functions. We confirmed that beta2-/- mice exhibit impaired exploratory and social behaviour, and further demonstrated their nocturnal hyperactivity. These data support the proposal that beta2-/- mice represent a relevant model for cognitive disorders in humans and that nicotine administered chronically at low dose may relieve some of these.

Genetics of nicotinic acetylcholine receptors: Relevance to nicotine addiction

Human twin studies have suggested that there is a substantial genetic component underlying nicotine dependence, ongoing smoking and ability to quit. Similarly, animal studies have identified a number of genes and gene products that are critical for behaviors related to nicotine addiction. Classical genetic approaches, gene association studies and genetic engineering techniques have been used to identify the gene products involved in nicotine dependence. One class of genes involved in nicotine-related behavior is the family of nicotinic acetylcholine receptors (nAChRs). These receptors are the primary targets for nicotine in the brain. Genetic engineering studies in mice have identified a number of subunits that are critical for the ability of nicotine to activate the reward system in the brain, consisting of the dopaminergic cell bodies in the ventral tegmental area and their terminals in the nucleus accumbens and other portions of the mesolimbic system. In this review we will discuss the various lines of evidence suggesting that nAChRs may be involved in smoking behavior, and will review the human and animal studies that have been performed to date examining the genetic basis for nicotine dependence and smoking.

Blunted response to cocaine in the Flinders hypercholinergic animal model of depression

The Flinders sensitive line (FSL) rat is a proposed genetic hypercholinergic animal model of human depression. Considering the strong comorbidity between depression and cocaine dependence we investigated the well-documented behavioral and molecular effects of cocaine in the FSL and their control Flinders resistant line (FRL) rats. First, we found no difference between the two lines to establish cocaine self-administration; both lines reached stable responding within 10 days of training at a fixed ratio-1 schedule of reinforcement (1.5 mg/kg/injection). However, the FSL rats exhibited reduced cocaine intake at a dose of 0.09 mg/kg/injection in a within-session dose-response curve (0.02, 0.09, 0.38, 1.5 mg/kg/injection). Second, we examined the effects of repeated cocaine administration on locomotor activity, dopamine overflow and striatal prodynorphin mRNA expression. We found the FSL rats to be low responders to novelty and to exhibit less locomotor activation after repeated cocaine administration (30 mg/kg, i.p., daily injections for 10 days) than their controls. Microdialysis sampling from the nucleus accumbens shell revealed no significant difference in the dopamine overflow between the rat lines, neither during baseline nor after cocaine stimulation. Postmortem analyses of striatal prodynorphin mRNA expression (using in situ hybridization histochemistry) revealed a differentiated response to the cocaine exposure. In contrast to control FRL rats, the FSL rats showed no typical cocaine-evoked elevation of prodynorphin mRNA levels in rostral subregions of the striatum whereas both strains expressed increased prodynorphin mRNA levels in the caudal striatum after cocaine administration. In conclusion, the FSL animal model of depression demonstrates marked blunting of the locomotor and dynorphin neuroadaptative responses to cocaine in accordance with its enhanced cholinergic sensitivity.

Self-administration of 5-iodo-A-85380, a beta2-selective nicotinic receptor ligand, by operantly trained rats

It is widely accepted that nicotine is the active ingredient of tobacco smoke that promotes tobacco dependence. Nicotine interacts with several subtypes of nicotinic acetylcholine receptors (nAChRs). In brain, it primarily targets nAChRs that contain beta2 and alpha4 subunits in combination and those composed of solely alpha7 subunits. The present study tested whether operantly trained rats would self-administer a ligand active at beta2-containing (i.e. not alpha7) nAChRs. Male Sprague-Dawley rats were trained to lever press for i.v. cocaine self-administration. After 2 weeks of cocaine washout, rats were given operant access to 5-iodo-A-85380 (5IA), a beta2-selective nAChR ligand, in daily 1 h sessions. The rats rapidly developed a stable level of 5IA self-administration behavior (unit dose = 5 nmol/kg/infusion). This finding suggests that interaction with beta2-containing nAChRs, without direct involvement of alpha7 receptors, can produce reinforcement and thereby can support self-administration behavior.

Muscarinic and nicotinic presynaptic modulation of EPSCs in the nucleus accumbens during postnatal development

We have studied the modulatory effects of cholinergic agonists on excitatory postsynaptic currents (EPSCs) in nucleus accumbens (nAcb) neurons during postnatal development. Recordings were obtained in slices from postnatal day 1 (P1) to P27 rats using the whole cell patch-clamp technique. EPSCs were evoked by local electrical stimulation, and all experiments were conducted in the presence of bicuculline methchloride in the bathing medium and with QX-314 in the recording pipette. Under these conditions, postsynaptic currents consisted of glutamatergic EPSCs typically consisting of two components mediated by AMPA/kainate (KA) and N-methyl-D-aspartate (NMDA) receptors. The addition of acetylcholine (ACh) or carbachol (CCh) to the superfusing medium resulted in a decrease of 30-60% of both AMPA/KA- and NMDA-mediated EPSCs. In contrast, ACh produced an increase ( approximately 35%) in both AMPA/KA and NMDA receptor-mediated EPSCs when administered in the presence of the muscarinic antagonist atropine. These excitatory effects were mimicked by the nicotinic receptor agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) and blocked by the nicotinic receptor antagonist mecamylamine, showing the presence of a cholinergic modulation mediated by nicotinic receptors in the nAcb. The antagonistic effects of atropine were mimicked by pirenzepine, suggesting that the muscarinic depression of the EPSCs was mediated by M(1)/M(4) receptors. In addition, the inhibitory effects of ACh on NMDA but not on AMPA/KA receptor-mediated EPSC significantly increased during the first two postnatal weeks. We found that, under our experimental conditions, cholinergic agonists produced no changes on membrane holding currents, on the decay time of the AMPA/KA EPSC, or on responses evoked by exogenous application of glutamate in the presence of tetrodotoxin, but they produced significant changes in paired pulse ratio, suggesting that their action was mediated by presynaptic mechanisms. In contrast, CCh produced consistent changes in the membrane and firing properties of medium spiny (MS) neurons when QX-314 was omitted from the recording pipette solution, suggesting that this substance actually blocked postsynaptic cholinergic modulation. Together, these results suggest that ACh can decrease or increase glutamatergic neurotransmission in the nAcb by, respectively, acting on muscarinic and nicotinic receptors located on excitatory terminals. The cholinergic modulation of AMPA/KA and NMDA receptor-mediated neurotransmission in the nAcb during postnatal development could play an important role in activity-dependent developmental processes in refining the excitatory drive on MS neurons by gating specific inputs.