Reworking:
In a nutshell, chronic stress changes the plasticity of the receptors that regulate mood, causing depression. This process appears to be triggered by the downregulation of CB1 receptors in certain areas of the brain closely associated with the adrenal system. This downregulation may be caused by the increase in stress hormones floating in the internal sea of the brain.
It's CB1 receptors we want to activate for the mood regulation, and I believe CBD may fascilitate this by working on the receptors in subtle ways. It's been shown that injury to the brain causes an uptick in the number of CB2 receptors, but this is for purposes of reducing inflammatory response, as far as I can understand.
My experience has been that a balanced ratio, or one slightly tilted in favor of CBD, works effectively in the treatment of depression, at least with my patients. Every individual body will have an individual experience. This is why we start low and work up slowly.
To be annotated:
Chronic Stress Impairs α1-Adrenoceptor-Induced Endocannabinoid-Dependent Synaptic Plasticity in the Dorsal Raphe Nucleus
Samir Haj-Dahmane and Roh-Yu Shen
Journal of Neuroscience 29 October 2014, 34 (44) 14560-14570; DOI:
Chronic Stress Impairs α1-Adrenoceptor-Induced Endocannabinoid-Dependent Synaptic Plasticity in the Dorsal Raphe Nucleus
Serotonin (5-hydroxytryptamine [5-HT]) neurons in the dorsal raphe nucleus (DRn) (Dahlström and Fuxe, 1964) provide major serotonergic projections to brain areas controlling the behavioral and neuroendocrine responses to stress (Petrov et al., 1994). By modulating the stress-associated neuronal circuits, DRn 5-HT neurons control stress homeostasis and mood (Joëls and Baram, 2009).
Indeed, animal studies have shown that the behavioral responses to various stressors are mediated, at least in part, by the activation of 5-HT system. For instance, exposure to uncontrollable stressors (e.g., tail shock) activates DRn 5-HT neurons and enhances 5-HT transmission (Amat et al., 1998; Maswood et al., 1998; Grahn et al., 1999).
Activation of DRn 5-HT neurons also regulates uncontrollable stress-induced learned helplessness (Grahn et al., 1999), characterized by a set of behaviors, including reduced escape to aversive stimuli, increased fear conditioning, and anxiety (Maier et al., 1994, 1995). Conversely, inhibition of DRn 5-HT neurons reduces the behavioral responses to uncontrollable stressors (Maier et al., 1994, 1995), indicating that DRn 5-HT neurons play a key role in modulating the behavioral responses to uncontrollable stress (Maier and Watkins, 2005).
Furthermore, results from clinical studies have established that stress-induced dysregulation of the 5-HT system is a major contributing factor for the development of mood disorders, such as depression and anxiety (Southwick et al., 2005; Lupien et al., 2009).
The DRn receives a major noradrenergic input from the locus ceruleus (Baraban and Aghajanian, 1981), which activates DRn 5-HT neurons (Baraban and Aghajanian, 1981) and regulates arousal and stress homeostasis (Morilak et al., 2005; Stone et al., 2007).
Previous studies have shown that exposure to various stressors increases noradrenaline release in the DRn (Tanaka et al., 1983; Shimizu et al., 1994) and induces anxiety-like behaviors (Chiba et al., 2012; Kim et al., 2012), at least in part, via the activation of α1-ARs located on DRn 5-HT neurons (Stone et al., 2007).
α1-AR signaling in the DRn also regulates fear conditioning, as blockade of these receptors prevents conditioned fear and impairs escape performance (Grahn et al., 2002). Furthermore, disruption of DRn α1-AR signaling alters the behavioral effects of antidepressant and antianxiety drugs (O'Leary et al., 2007; Doze et al., 2009). Collectively, these studies indicate that α1-AR-mediated control of DRn 5-HT neurons plays an important role in the regulation of stress homoeostasis and that the alteration of α1-AR signaling in the DRn might contribute to stress-related mood disorders.
Remarkably, despite the crucial role of α1-AR signaling in the DRn in controlling the behavioral responses to stress, the effects of chronic stress on α1-AR-mediated control of the excitability of 5-HT neurons and synaptic transmission in the DRn remain unknown. In this study, we show that exposure to chronic restraint stress (CRS) impairs α1-AR LTD of glutamate synapses in the DRn but has no effects on α1-AR-induced membrane depolarization/inward current in DRn 5-HT neurons. The CRS-induced impairment of α1-AR LTD is mediated by a downregulation of eCB signaling. Such results unravel a novel cellular mechanism by which chronic stress could induce long-lasting changes in the function of the 5-HT system.
Discussion
The present study shows that activation of α1-ARs elicits LTD of glutamate synapses onto DRn 5-HT neurons. This form of synaptic plasticity is initiated by the activation of postsynaptic α1-ARs but expressed presynaptically by a decrease in glutamate release.
The α1-AR LTD is signaled by retrograde eCB messengers acting on presynaptic CB1 receptors. More importantly, we report that exposure to CRS profoundly reduces of the magnitude of α1-AR LTD.
The CRS-induced impairment of α1-AR LTD is essentially mediated by a downregulation of presynaptic CB1 receptors. As such, our study reveals a novel cellular mechanism by which noradrenaline controls the function of DRn 5-HT. It also provides the first direct evidence that chronic stress reduces eCB signaling at glutamate synapses of DRn 5-HT neurons, which could have important functional implications for stress-induced maladaptation of the 5-HT system.
Previous studies have examined the regulation of glutamate synapses by α1-ARs in various brain areas (Scanziani et al., 1993; Scheiderer et al., 2004; Choi et al., 2005; McElligott and Winder, 2008; Marzo et al., 2010; McElligot et al., 2010). Generally, these studies have reported that activation of α1-ARs elicits a transient inhibition of glutamatergic transmission.
However, in some brain areas, such as the cerebral cortex (e.g., visual cortex and prefrontal cortex), hippocampus, and the bed nucleus of striata terminalis, activation of α1-ARs induces LTD of glutamate synapses (Scheiderer et al., 2004; Choi et al., 2005; McElligott and Winder, 2008; Marzo et al., 2010; McElligot et al., 2010).
Depending on the brain area studied, the α1-AR LTD seems to be mediated by different cellular mechanisms. In the hippocampus and cerebral cortex, the α1-AR LTD is mediated by a postsynaptic mechanism that involves α1-AR-induced activation of the extracellular signal regulating kinase (ERK1/2) pathways (Scheiderer et al., 2008; Marzo et al., 2010).
Activation of ERK 1/2 induces LTD by reducing the function and/or number of AMPARs. At glutamate synapses of the bed nucleus of striata terminalis, the α1-AR LTD is mediated by a switch of the subunit composition of AMPARs from GluA2-lacking, which exhibit higher unitary conductance and calcium permeability (Kamboj et al., 1995; Dingledine et al., 1999), to GluA2 containing AMPARs (McElligott et al., 2010).
In the DRn, the present study shows that the α1-AR LTD is initiated by the activation of postsynaptic α1-ARs but mediated by a decrease in glutamate release induced by retrograde eCB messengers. This cellular mechanism is supported by multiple lines of evidence. First, inhibition of postsynaptic α1-AR signaling with G-protein inhibitors abolishes the LTD.
Second, α1-AR LTD is associated with a persistent decrease in glutamate release as indicated by the increase in PPR and CV. Finally, blockade of presynaptic CB1 receptors or inhibition of 2-AG synthesis abolishes the α1-AR LTD.
The conclusion that the α1-AR LTD of glutamate synapses onto DRn 5-HT neurons is mediated by 2-AG acting at presynaptic CB1 receptors is consistent with numerous studies showing that activation of Gq/11-coupled receptors, such as Group I metabotropic glutamate receptors (mGluR1/5), M1/M5 muscarinic receptors, and orexin receptors, increase the synthesis/release of 2-AG in various brain areas (Maejima et al., 2001; Kim et al., 2002; Ohno-Shosaku et al., 2003), including the DRn (Haj-Dahmane and Shen, 2005).
Generally, Gq/11 coupled receptor-driven 2-AG synthesis and release mediate transient inhibition of excitatory and inhibitory synaptic transmission (Maejima et al., 2001; Kim et al., 2002; Haj-Dahmane and Shen, 2005). However, growing evidence indicates that this mode of 2-AG synthesis/release also mediates the presynaptic form of LTD at glutamate and GABA synapses and, hence, plays an ubiquitous role in regulating synaptic plasticity in the brain (Castillo et al., 2012).
Results from previous studies have reported that chronic exposure to various stressors increases the expression in the DRn of various synaptic proteins, such as synaptosomal-associated protein 25 and synaptic vesicle glycoprotein 2B (Abumaria et al., 2006, 2007), suggesting that chronic stress can induce a long-lasting alteration of synaptic function and plasticity in the DRn.
Consistent with this idea, we report that CRS impairs the α1-AR LTD of glutamate synapses onto DRn 5-HT neurons. The alteration of α1-AR-mediated synaptic plasticity in the DRn may represent an important cellular mechanism by which chronic stress can induce a long-lasting alteration of the 5-HT system.
The effects of restraint stress on the α1-AR-mediated control of synaptic transmission have also been examined in several other brain areas. In the amygdala, exposure to acute restraint stress combined with tail shock blocks the α1-AR-mediated facilitation of GABA-ergic transmission (Braga et al., 2004). The mechanisms underlying this effect remain unknown.
Here, we find that acute exposure to restraint stress has no effect on the ability of α1-ARs to control the function of glutamate synapses. In contrast, exposure to CRS profoundly impairs the α1-AR LTD of glutamate synapses in the DRn by blocking the induction and maintenance of the LTD. Such finding is in agreement with a previous study showing that exposure to CRS, but not to acute restraint stress, also impairs the LTD of glutamate synapses induced by α1-ARs (McElligott et al., 2010) in the basal nucleus of striata terminalis.
Collectively, these studies suggest that the impairment of the α1-AR-mediated control of the strength and plasticity of glutamate synapses may represent a common response to chronic stress exposure. Importantly, such alterations of synaptic plasticity may mediate the maladaptive behavioral responses to chronic stress, including depression and anxiety.
An interesting finding of the present study is that exposure to CRS has no significant effect on the amplitude of the α1-AR-induced inward current but reduces the effect of presynaptic CB1 receptors on glutamate release.
These results strongly indicate that the CRS-induced impairment of the α1-AR LTD is not mediated by a downregulation of α1-ARs but by a profound reduction of presynaptic CB1 receptor function. However, it remains possible that CRS could also reduce eCB synthesis/release, which may contribute to the impairment of the α1-AR LTD.
The conclusion that CRS reduces the function of presynaptic CB1 receptors is consistent with previous studies showing that chronic exposure to various stressors, including CRS, downregulates CB1 receptors and impairs eCB-mediated control of glutamatergic (Rossi et al., 2008; Wang et al., 2010; Reich et al., 2013) and GABAergic synaptic transmission in other brain areas (Wamsteeker et al., 2010; Hu et al., 2011).
Importantly, in the nucleus accumbens, exposure to chronic stress has also been shown to block the eCB-mediated LTD induced by mGluR1 at glutamate synapses onto medium spiny neurons (Wang et al., 2010). As in the DRn, the blockade of the mGluR1 LTD is mainly attributed to a reduction of presynaptic CB1 receptor function (Wang et al., 2010).
Together, the results of these studies indicate that reduced retrograde eCB signaling (e.g., downregulation of CB1 receptors) may represent a common mechanism by which chronic stress impairs Gq/11-coupled receptor-mediated control of synaptic function and plasticity in the brain.
Although exposure to chronic stress has been shown to impair the function of presynaptic CB1 receptors in various brain areas (Hill et al., 2005; Wang et al., 2010), the precise molecular mechanisms underlying this effect remain unknown. It is well established that exposure to chronic stress increases the circulating levels of corticosterone and noradrenaline (Krugers et al., 2012).
Because both of these stress mediators stimulate eCB synthesis and release in the DRn (Wang et al., 2012; present study) and other brain areas (Di et al., 2003), it is tempting to speculate that the high circulating levels of noradrenaline and corticosterone during daily stress lead to chronic increase in eCB release and activation of CB1 receptors, which could induce downregulation of these receptors.
Consistent with this idea, results from previous studies have shown that chronic exposure to stress enhances the release of 2-AG in various brain regions (Patel and Hillard, 2008; Patel et al., 2009). More importantly, chronic activation of CB1 receptors with eCBs or exogenous cannabinoids has been shown to reduce the function of presynaptic CB1 receptors (Sim et al., 1996; Breivogel et al., 1999).
Thus, it is possible that the CRS-induced functional downregulation of presynaptic CB1 receptors reported in this study could be attributed to an agonist-induced downregulation. However, future studies are required to further test this notion and determine the precise cellular mechanisms underlying the downregulation of presynaptic CB1 receptors.
Extensive work has established that noradrenergic inputs from the locus ceruleus provide a major excitatory drive to the DRn, which is mediated by α1-ARs. Activation of these receptors increases the excitability of DRn 5-HT neurons by inducing membrane depolarization (Aghajanian, 1985; Pan et al., 1994) and reducing the amplitude of after hyperpolarizing potential (Pan et al., 1994).
In addition to these excitatory effects, the present study shows that activation of postsynaptic α1-ARs enhances 2-AG release, which in turn reduces the strength of glutamate synapses onto DRn 5-HT neurons (Fig. 8). Combined, these studies indicate that the noradrenergic modulation of 5-HT neurons is more complex than initially thought and that α1-AR signaling in the DRn exerts a bidirectional control on the excitability of 5-HT neurons.
The bidirectional control exerted by α1-AR could play an important role in maintaining the activity of 5-HT neurons within desirable range and prevent excessive excitation of DRn 5-HT neurons, especially during heightened arousal (e.g., stress), which is characterized by increased noradrenergic tone (Krugers et al., 2012).
As such, the reduction of eCB signaling and the impairment of α1-AR LTD induced by chronic stress may lead to an abnormal increase in the excitability of DRn 5-HT neurons and persistent alteration of central 5-HT transmission.
Furthermore, the impairment of eCB signaling in the DRn could mediate, at least in part, some of the behavioral consequences of chronic stress exposure, such as depression-like behaviors. It is noteworthy that pharmacological manipulation that increases eCB signaling has been shown to block chronic stress-induced depression-like behaviors (Zhong et al., 2014).
A model of α1-ARs mediated regulation of glutamate synapses onto DRn 5-HT neurons. Activation of α1-ARs elicits an increase in the synthesis/release of the eCB messenger 2-AG. The release of 2-AG reduces the strength of glutamate synapses by the activation of presynaptic CB1 receptors. Exposure to CRS impairs α1-AR-mediated depression of glutamate synapses by reducing the function of presynaptic CB1 receptors.
I’m having too much fun to feel any less than the thrill of learning. 
