Endocannabinoids blunt the augmentation of synaptic transmission by serotonin 2A receptors in the nucleus tractus solitarii (nTS)

Cannabinoids, Inner Ear, Hearing, and Tinnitus: A Neuroimmunological Perspective

Cannabis has been used for centuries for recreational and therapeutic purposes. Whereas, the recreative uses are based on the psychotropic effect of some of its compounds, its therapeutic effects range over a wide spectrum of actions, most of which target the brain or the immune system. Several studies have found cannabinoid receptors in the auditory system, both at peripheral and central levels, thus raising the interest in cannabinoid signaling in hearing, and especially in tinnitus, which is affected also by anxiety, memory, and attention circuits where cannabinoid effects are well described. Available studies on animal models of tinnitus suggest that cannabinoids are not likely to be helpful in tinnitus treatment and could even be harmful. However, the pharmacology of cannabinoids is very complex, and most studies focused on neural CB1R-based responses. Cannabinoid effects on the immune system (where CB2Rs predominate) are increasingly recognized as essential in understanding nervous system pathological responses, and data on immune cannabinoid targets have emerged in the auditory system as well. In addition, nonclassical cannabinoid targets (such as TRP channels) appear to play an important role in the auditory system as well. This review will focus on neuroimmunological mechanisms for cannabinoid effects and their possible use as protective and therapeutic agents in the ear and auditory system, especially in tinnitus.

The effect of cannabidiol on the pharmacokinetics of carbamazepine in rats

Carbamazepine (CBZ) is mainly metabolized by CYP3A4 into carbamazepine-10,11-epoxide (CBZE). Cannabidiol (CBD) is a potent inhibitor of the CYP3A family. The aim of this study is to determine the effect of acute and chronic administration of CBD on the pharmacokinetics of CBZ and CBZE. Male SD rats were assigned into four acute and four chronic groups: control (CBZ only), positive control (ketoconazole), low-dose cannabidiol (l-CBD), and high-dose cannabidiol (h-CBD). Acute CBD groups were administered a single dose of CBD, while chronic CBD groups were given multiple doses of CBD for 14 days (q.d.) before CBZ administration. Plasma samples had been collected and analyzed for CBZ and CBZE, then their noncompartmental pharmacokinetic parameters before and after CBD administration were determined. The co-administration of a single l-CBD has significantly increased CBZ's [Formula: see text] by 53.1%. Furthermore, CBZE kinetics showed a significant decrease in C_max by 31.8%. Acute h-CBD caused similar effects on CBZ's [Formula: see text] with 40.4% significant decrease in CBZE's C_max, when compared to the control. Chronic h-CBD caused a significant decrease in CBZ's C_max and [Formula: see text] by 75.3% and 65.7%, respectively. Besides, [Formula: see text] and C_max of CBZE significantly decreased by 75.3% and 78.3%, respectively. These results demonstrated that the pharmacokinetics of CBZ and CBZE had been significantly affected by CBD. When CBD has been administered as a single dose, the effect is believed to be mainly caused by the inhibition of CBZ metabolism through CYP3A. The effect of chronic administration of CBD probably includes kinetic pathways other than the inhibition of CYP3A-dependent pathways. Graphical abstract.

Serotonin and Adenosine G-protein Coupled Receptor Signaling for Ventilatory Acclimatization to Sustained Hypoxia

Different patterns of hypoxia evoke different forms of plasticity in the neural control of ventilation. For example, acute intermittent hypoxia produces long term facilitation (LTF) of ventilation, while chronic sustained hypoxia (CH) causes ventilatory acclimatization to hypoxia (VAH). In both LTF and VAH, ventilation in normoxia is greater than normal after the hypoxic stimulus is removed and the acute hypoxic ventilatory response can increase. However, the mechanisms of LTF and VAH are thought to be different based on previous results showing serotonin 5HT₂ receptors, which are G protein coupled receptors (GPCR) that activate G_Q signaling, contribute to LTF but not VAH. Newer results show that a different GPCR, namely adenosine A_2A receptors and the G_S signaling pathway, cause LTF with more severe intermittent hypoxia, i.e., PaO₂ = 25–30 Torr for G_S versus 35–45 Torr for LTF with the G_Q signaling pathway. We hypothesized adenosine A_2A receptors and G_S signaling are involved in establishing VAH with longer term moderate CH and tested this in adult male rats by measuring ventilatory responses to O₂ and CO₂ with barometric pressure plethysmography after administering MSX-3 or ketanserin (A_2A and 5HT₂ antagonists, respectively, both 1 mg/Kg i.p.) during CH for 7 days. Blocking G_S or G_Q signals throughout CH exposure, significantly decreased VAH. After VAH was established, G_Q blockade did not affect ventilation while G_S blockade increased VAH. Similar to LTF, data support roles for both G_Q and G_S pathways in the development of VAH but after VAH has been established, the G_S pathway inhibits VAH.

Activation of 5-hyrdoxytryptamine 7 receptors within the rat nucleus tractus solitarii modulates synaptic properties

Serotonin (5-HT) is a potent neuromodulator with multiple receptor types within the cardiorespiratory system, including the nucleus tractus solitarii (nTS)--the central termination site of visceral afferent fibers. The 5-HT7 receptor facilitates cardiorespiratory reflexes through its action in the brainstem and likely in the nTS. However, the mechanism and site of action for these effects is not clear. In this study, we examined the expression and function of 5-HT7 receptors in the nTS of Sprague-Dawley rats. 5-HT7 receptor mRNA and protein were identified across the rostrocaudal extent of the nTS. To determine 5-HT7 receptor function, we examined nTS synaptic properties following 5-HT7 receptor activation in monosynaptic nTS neurons in the in vitro brainstem slice preparation. Application of 5-HT7 receptor agonists altered tractus solitarii evoked and spontaneous excitatory postsynaptic currents which were attenuated with a selective 5-HT7 receptor antagonist. 5-HT7 receptor-mediated changes in excitatory postsynaptic currents were also altered by block of 5-HT1A and GABAA receptors. Interestingly, 5-HT7 receptor activation also reduced the amplitude but not frequency of GABAA-mediated inhibitory currents. Together these results indicate a complex role for 5-HT7 receptors in the nTS that mediate its diverse effects on cardiorespiratory parameters.

The involvement of medial septum 5-HT1 and 5-HT2 receptors on ACPA-induced memory consolidation deficit: possible role of TRPC3, TRPC6 and TRPV2

The present study evaluates the roles of serotonergic receptors of the medial septum on amnesia induced by arachidonylcyclopropylamide (ACPA; as selective cannabinoid CB1 receptor agonist) in adult male Wistar rats. Cannulae were implanted in the medial septum of the brain of the rats. The animals were trained in a passive avoidance learning apparatus, and were tested 24 hours after training for step-through latency. Results indicated that post-training medial septum administration of CP94253 (5-HT1B/1D receptor agonist) and cinancerine (as 5-HT2 receptor antagonist) reduced the step-through latency showing an amnesic response, while GR127935 (5-HT1B/1D receptor antagonist) and αm5htm (as 5-HT2A/2B/2D receptor agonist) did not alter memory consolidation by themselves. On continuing the test, the results showed that CP94253 increased and GR127935 did not alter ACPA (0.02 µg/rat)-induced memory impairment, respectively. Other data indicated that αm5htm induced a modulatory effect, while cinancerine restored ACPA-induced amnesia. Using SKF-96365 (inhibitor of transient receptor potential TRPC3/6 and TRPV2 channels) demonstrated that TRPC3, TRPC3 and TRPV2 channels have a significant role, according to our results.

Molecular Targets of Cannabidiol in Neurological Disorders

Cannabis has a long history of anecdotal medicinal use and limited licensed medicinal use. Until recently, alleged clinical effects from anecdotal reports and the use of licensed cannabinoid medicines are most likely mediated by tetrahydrocannabinol by virtue of: 1) this cannabinoid being present in the most significant quantities in these preparations; and b) the proportion:potency relationship between tetrahydrocannabinol and other plant cannabinoids derived from cannabis. However, there has recently been considerable interest in the therapeutic potential for the plant cannabinoid, cannabidiol (CBD), in neurological disorders but the current evidence suggests that CBD does not directly interact with the endocannabinoid system except in vitro at supraphysiological concentrations. Thus, as further evidence for CBD's beneficial effects in neurological disease emerges, there remains an urgent need to establish the molecular targets through which it exerts its therapeutic effects. Here, we conducted a systematic search of the extant literature for original articles describing the molecular pharmacology of CBD. We critically appraised the results for the validity of the molecular targets proposed. Thereafter, we considered whether the molecular targets of CBD identified hold therapeutic potential in relevant neurological diseases. The molecular targets identified include numerous classical ion channels, receptors, transporters, and enzymes. Some CBD effects at these targets in in vitro assays only manifest at high concentrations, which may be difficult to achieve in vivo, particularly given CBD's relatively poor bioavailability. Moreover, several targets were asserted through experimental designs that demonstrate only correlation with a given target rather than a causal proof. When the molecular targets of CBD that were physiologically plausible were considered for their potential for exploitation in neurological therapeutics, the results were variable. In some cases, the targets identified had little or no established link to the diseases considered. In others, molecular targets of CBD were entirely consistent with those already actively exploited in relevant, clinically used, neurological treatments. Finally, CBD was found to act upon a number of targets that are linked to neurological therapeutics but that its actions were not consistent withmodulation of such targets that would derive a therapeutically beneficial outcome. Overall, we find that while >65 discrete molecular targets have been reported in the literature for CBD, a relatively limited number represent plausible targets for the drug's action in neurological disorders when judged by the criteria we set. We conclude that CBD is very unlikely to exert effects in neurological diseases through modulation of the endocannabinoid system. Moreover, a number of other molecular targets of CBD reported in the literature are unlikely to be of relevance owing to effects only being observed at supraphysiological concentrations. Of interest and after excluding unlikely and implausible targets, the remaining molecular targets of CBD with plausible evidence for involvement in therapeutic effects in neurological disorders (e.g., voltage-dependent anion channel 1, G protein-coupled receptor 55, CaV3.x, etc.) are associated with either the regulation of, or responses to changes in, intracellular calcium levels. While no causal proof yet exists for CBD's effects at these targets, they represent the most probable for such investigations and should be prioritized in further studies of CBD's therapeutic mechanism of action.

(In)activity-related neuroplasticity in brainstem control of sympathetic outflow: unraveling underlying molecular, cellular, and anatomical mechanisms

More people die as a result of physical inactivity than any other preventable risk factor including smoking, high cholesterol, and obesity. Cardiovascular disease, the number one cause of death in the United States, tops the list of inactivity-related diseases. Nevertheless, the vast majority of Americans continue to make lifestyle choices that are creating a rapidly growing burden of epidemic size and impact on the United States healthcare system. It is imperative that we improve our understanding of the mechanisms by which physical inactivity increases the incidence of cardiovascular disease and how exercise can prevent or rescue the inactivity phenotype. The current review summarizes research on changes in the brain that contribute to inactivity-related cardiovascular disease. Specifically, we focus on changes in the rostral ventrolateral medulla (RVLM), a critical brain region for basal and reflex control of sympathetic activity. The RVLM is implicated in elevated sympathetic outflow associated with several cardiovascular diseases including hypertension and heart failure. We hypothesize that changes in the RVLM contribute to chronic cardiovascular disease related to physical inactivity. Data obtained from our translational rodent models of chronic, voluntary exercise and inactivity suggest that functional, anatomical, and molecular neuroplasticity enhances glutamatergic neurotransmission in the RVLM of sedentary animals. Collectively, the evidence presented here suggests that changes in the RVLM resulting from sedentary conditions are deleterious and contribute to cardiovascular diseases that have an increased prevalence in sedentary individuals. The mechanisms by which these changes occur over time and their impact are important areas for future study.