Gabriella Gobbi

Deletion of the background potassium channel TREK-1 results in a depression-resistant phenotype

Depression is a devastating illness with a lifetime prevalence of up to 20%. The neurotransmitter serotonin or 5-hydroxytryptamine (5-HT) is involved in the pathophysiology of depression and in the effects of antidepressant treatments. However, molecular alterations that underlie the pathology or treatment of depression are still poorly understood. The TREK-1 protein is a background K+ channel regulated by various neurotransmitters including 5-HT. In mice, the deletion of its gene (Kcnk2, also called TREK-1) led to animals with an increased efficacy of 5-HT neurotransmission and a resistance to depression in five different models and a substantially reduced elevation of corticosterone levels under stress. TREK-1–deficient (Kcnk2−/−) mice showed behavior similar to that of naive animals treated with classical antidepressants such as fluoxetine. Our results indicate that alterations in the functioning, regulation or both of the TREK-1 channel may alter mood, and that this particular K+ channel may be a potential target for new antidepressants.

Cannabinoids Elicit Antidepressant-Like Behavior and Activate Serotonergic Neurons through the Medial Prefrontal Cortex

Preclinical and clinical studies show that cannabis modulates mood and possesses antidepressant-like properties, mediated by the agonistic activity of cannabinoids on central CB1 receptors (CB1Rs). The action of CB1R agonists on the serotonin (5-HT) system, the major transmitter system involved in mood control and implicated in the mechanism of action of antidepressants, remains however poorly understood. In this study, we demonstrated that, at low doses, the CB1R agonist WIN55,212-2 [R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate] exerts potent antidepressant-like properties in the rat forced-swim test (FST). This effect is CB1R dependent because it was blocked by the CB1R antagonist rimonabant and is 5-HT mediated because it was abolished by pretreatment with the 5-HT-depleting agent parachlorophenylalanine. Then, using in vivo electrophysiology, we showed that low doses of WIN55,212-2 dose dependently enhanced dorsal raphe nucleus 5-HT neuronal activity through a CB1R-dependent mechanism. Conversely, high doses of WIN55,212-2 were ineffective in the FST and decreased 5-HT neuronal activity through a CB1R-independent mechanism. The CB1R agonist-induced enhancement of 5-HT neuronal activity was abolished by total or medial prefrontocortical, but not by lateral prefrontocortical, transection. Furthermore, 5-HT neuronal activity was enhanced by the local microinjection of WIN55,212-2 into the ventromedial prefrontal cortex (mPFCv) but not by the local microinjection of WIN55,212-2 into the lateral prefrontal cortex. Similarly, the microinjection of WIN55,212-2 into the mPFCv produced a CB1R-dependent antidepressant-like effect in the FST. These results demonstrate that CB1R agonists possess antidepressant-like properties and modulate 5-HT neuronal activity via the mPFCv.

Chronic exposure to cannabinoids during adolescence but not during adulthood impairs emotional behaviour and monoaminergic neurotransmission

The pathophysiological neural mechanism underlying the depressogenic and anxiogenic effects of chronic adolescent cannabinoid use may be linked to perturbations in monoaminergic neurotransmission. We tested this hypothesis by administering the CB(1) receptor agonist WIN55,212-2, once daily for 20 days to adolescent and adult rats, subsequently subjecting them to tests for emotional reactivity paralleled by the in vivo extracellular recordings of serotonergic and noradrenergic neurons. Chronic adolescent exposure but not adult exposure to low (0.2 mg/kg) and high (1.0 mg/kg) doses led to depression-like behaviour in the forced swim and sucrose preference test, while the high dose also induced anxiety-like consequences in the novelty-suppressed feeding test. Electrophysiological recordings revealed both doses to have attenuated serotonergic activity, while the high dose also led to a hyperactivity of noradrenergic neurons only after adolescent exposure. These suggest that long-term exposure to cannabinoids during adolescence induces anxiety-like and depression-like behaviours in adulthood and that this may be instigated by serotonergic hypoactivity and noradrenergic hyperactivity.

Mirtazapine and paroxetine in major depression: A comparison of monotherapy versus their combination from treatment initiation

This double-blind study compared initial combination therapy against monotherapy using two antidepressant drugs with complementary mechanisms of action on the serotonin (5-HT) and norepinephrine (NE) systems. Sixty one adult patients with a DSM-IV diagnosis of unipolar depression were randomized to receive mirtazapine (30 mg/day), paroxetine (20 mg/day), or the combination of both drugs for 6 weeks. Response at week 4 was defined as a 30% reduction in the Montgomery-Asberg Depression Rating Scale (MADRS), and at week 6 as a 50% reduction in the MADRS. Remission was defined as a reduction in the MADRS score to 10 points or less. After 4 weeks, non-responders in the monotherapy groups had their medication dose increased by 50%. After 6 weeks, non-responders on monotherapy had the second trial drug added to their current regimen. Non-responders on combination therapy had the dosage of both drugs increased by 50%. There was a significantly greater decrease in MADRS scores in the combination group compared to the monotherapy groups at days 28, 35 and 42, with a 10 point difference separating the combination from the monotherapies at day 42. Remission rates at week 6 were 19% on mirtazapine, 26% on paroxetine, and 43% on the combination. Fifteen patients in the mirtazapine arm and 10 in the paroxetine arm who did not respond had the other drug added to their current regimen, and 5 on the combination had an increase in dose of both drugs secondary to non-response. Of these 30 patients, approximately 50% went on to achieve remission in the subsequent 2 weeks. These results indicate that the combined use of two antidepressants was well tolerated and produced a greater improvement than monotherapy.

Spadin, a Sortilin-Derived Peptide, Targeting Rodent TREK-1 Channels: A New Concept in the Antidepressant Drug Design

We found that spadin, a natural peptide derived from sortilin, blocks the mouse TREK-1 channel and might be an efficient and fast-acting antidepressant.

Decline in serotonergic firing activity and desensitization of 5-HT1A autoreceptors after chronic unpredictable stress

Chronic stressful life events are risk factors for contracting depression, the pathophysiology of which is strongly associated with impairments in serotonergic (5-HT) neurotransmission. Indeed, in rodents, exposure to chronic unpredictable stress (CUS) produces depressive-like behaviours such as behavioural despair and anhedonia. To date, there have not been many studies that especially explore in vivo changes in 5-HT neurotransmission associated with CUS in the rat. Therefore, using in vivo electrophysiology, we evaluated whether CUS that induces anhedonia-like behaviours concurrently impairs midbrain raphe 5-HT neuronal activity. Unlike unstressed and acutely stressed rats, CUS produced progressive reductions in sucrose intake and preference (anhedonia-like). These were associated with a decrease in the spontaneous firing activity (35.4%) as well as in the number of spontaneously active 5-HT neurons, and a desensitization of somatodendritic 5-HT1A autoreceptors in the dorsal raphe. These results suggest that CUS dramatically decreases 5-HT neural activity and 5-HT1A autoreceptor sensitivity, and may represent endophenotypic features of depressive-like states.

Genetic Deletion of Fatty Acid Amide Hydrolase Alters Emotional Behavior and Serotonergic Transmission in the Dorsal Raphe, Prefrontal Cortex, and Hippocampus

Pharmacological blockade of the anandamide-degrading enzyme, fatty acid amide hydrolase (FAAH), produces CB1 receptor (CB1R)-mediated analgesic, anxiolytic-like and antidepressant-like effects in murids. Using behavioral and electrophysiological approaches, we have characterized the emotional phenotype and serotonergic (5-HT) activity of mice lacking the FAAH gene in comparison to their wild type counterparts, and their response to a challenge of the CB1R antagonist, rimonabant. FAAH null-mutant (FAAH−/−) mice exhibited reduced immobility in the forced swim and tail suspension tests, predictive of antidepressant activity, which was attenuated by rimonabant. FAAH−/− mice showed an increase in the duration of open arm visits in the elevated plus maze, and a decrease in thigmotaxis and an increase in exploratory rearing displayed in the open field, indicating anxiolytic-like effects that were reversed by rimonabant. Rimonabant also prolonged the initiation of feeding in the novelty-suppressed feeding test. Electrophysiological recordings revealed a marked 34.68% increase in dorsal raphe 5-HT neural firing that was reversed by rimonabant in a subset of neurons exhibiting high firing rates (33.15% mean decrease). The response of the prefrontocortical pyramidal cells to the 5-HT2A/2C agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((±)-DOI) revealed desensitized 5-HT2A/2C receptors, likely linked to the observed anxiolytic-like behaviors. The hippocampal pyramidal response to the 5-HT1A antagonist, WAY-100635, indicates enhanced tonus on the hippocampal 5-HT1A heteroreceptors, a hallmark of antidepressant-like action. Together, these results suggest that FAAH genetic deletion enhances anxiolytic-like and antidepressant-like effects, paralleled by altered 5-HT transmission and postsynaptic 5-HT1A and 5-HT2A/2C receptor function.

Endocannabinoids in the Treatment of Mood Disorders: Evidence from Animal Models

Among all mental disorders, major depression has the highest rate of prevalence and incidence of morbidity. Currently available antidepressant therapies have limited efficacies; consequently, research on new drugs for the treatment of mood disorders has become increasingly critical. Recent preclinical evidences that cannabinoid agonists and endocannabinoid enhancers, such as the fatty acid amide hydrolase (FAAH) inhibitors, can impact mood regulation have opened a new line of research in antidepressant drug discovery. However, the neurobiological mechanisms linking the endocannabinoid system with the pathophysiology of mood disorders and antidepressant action remain unclarified. In this review, we have presented an update on preclinical data indicating the antidepressant potential of cannabinoid agonists and endocannabinoid enhancers in comparison to standard antidepressants. Data obtained from CB(1) knockout (CB(1)-/-) and FAAH knockout (FAAH-/-) mice have also been examined within this context. We have illustrated how the various classes of antidepressants exert their therapeutic action. In particular, all antidepressants increase the neurotransmission of serotonin after long-term treatment, enhance the tonic activity of hippocampal 5-HT(1A) receptors, promote neurogenesis, and modulate (decrease or increase) the firing activity of noradrenergic neurons. Interestingly, cannabinoid agonists and endocannabinoid enhancers increase serotonin and noradrenergic neuronal firing activity, increase serotonin release in the hippocampus, as well as promote neurogenesis. Since cannabinoid-derived drugs potentiate monoaminergic neurotransmission and hippocampal neurogenesis through distinct pathways compared to classical antidepressants, they may represent an alternative drug class in the pharmacotherapy of mood and other neuropsychiatric disorders.

Effect of neurokinin-1 receptor antagonists on serotoninergic, noradrenergic and hippocampal neurons: Comparison with antidepressant drugs

Neurokinin-1 (NK1) receptor antagonists have been reported to possess antidepressant and anxiolytic properties in controlled trials. Since antidepressant and anxiolytic drugs act mainly by enhancing serotonin (5-HT) and norepinephrine (NE) neurotransmission in forebrain areas, the main focus of the present review is to critically examine the electrophysiological effects of NK1 receptor antagonists on serotoninergic and noradrenergic neurons, and then hippocampal neurons. It is concluded that NK1 antagonists increase the firing and burst activity of 5-HT neurons, increase burst activity of NE neurons, and modulate postsynaptic transmission at the hippocampus level. Further research is needed in order to develop more selective ligands for the human NK1 receptor and to gain better knowledge of required brain penetration and optimal pharmacodynamic conditions for their use in patients.

Cannabinoids and emotionality: A neuroanatomical perspective

The endocannabinoid system has recently emerged as a promising therapeutic target for the treatment of stress-related emotional disorders. A growing literature base has collectively demonstrated that facilitation of endocannabinoid signaling promotes antidepressant- and anxiolytic-like responses in preclinical animal models, while disruption of this system profoundly affects emotion, cognition, and neuroendocrine functioning. Although these findings are encouraging, the role of endocannabinoid signaling within discrete corticolimbic brain structures is considerably complex. Consequently, researchers have recently shifted focus to examining the effects of local cannabinoid manipulations on emotion from a neuroanatomical standpoint. This review provides an overview of the site-specific effects of cannabinergic compounds in preclinical tests of emotionality, as well as the alterations in endocannabinoid signaling observed in animal models of depression. Broadly speaking, these studies indicate that CB(1) receptors in the medial prefrontal cortex and ventral hippocampus appear to be responsible for the antidepressant- and anxiolytic-like phenotype elicited by systemic CB(1) receptor agonists, which parallels biochemical studies showing that endocannabinoids are downregulated in these two regions following exposure to chronic stress. Conversely, CB(1) receptor activation within distinct amygdalar nuclei yields opposing effects on emotional behavior, such that local stimulation of CB(1) receptors in the basolateral amygdala and central amygdala promoting anxiogenesis and anxiolysis, respectively. Moreover, a series of elegant studies has revealed that cannabinoid transmission in the basolateral amygdala strongly modulates the acquisition and processing of associative fear memory via interactions with the medial prefrontal cortex. Given the crucial role of this corticolimbic network in regulating emotional behavior, it is palpable that alterations in endocannabinoid signaling within any of these structures could have profound implications for the pathophysiological development of affective illnesses. Accordingly, local pharmacological augmentation of endocannabinoid signaling within discrete corticolimbic subregions may serve as a promising therapeutic strategy for the treatment of these debilitating disorders.