**4. Role of 5‐HT and NE in the BLA in alcohol addiction**

There is significant evidence that supports the role of NE and 5‐HT in drug dependence and alcohol addiction [87, 155–157]. Moreover, the BLA which is highly implicated in depend‐ ence to alcohol‐seeking [17, 123, 131, 132] is densely innervated by these neurotransmitters [58, 71, 158, 159]. Since chronic alcohol exposure causes neuroadaptations that affect the signal‐ ing and receptor subtypes of these neurotransmitters, dysregulation of NE and 5‐HT trans‐ mission in the BLA may lead to a constellation of aversive outcomes including altered consolidation of alcohol‐related memories, anxiety disorders, and eventually higher rates of relapse [132, 138].

NE plays a vital role in facilitating the function of the BLA in fear memory consolidation [70]. It has been shown that intra‐BLA infusions of *β*‐AR agonists enhance retention of inhibitory avoidance [160], while *β*‐AR antagonists block fear memory enhancement [69]. Also, *α*1‐AR activation in the BLA enhances fear memory consolidation through an interaction with *β*‐ ARs [161]. This evidence suggests that noradrenergic receptors strongly contribute to BLA function. It is possible that alteration in NE activity in the BLA may lead to altered memory consolidation and stress‐coping mechanism that may enhance alcohol‐seeking and relapse [162]. Indeed, antagonism of *α*1‐ARs reduced dependence‐induced increase in alcohol consumption in rats [88]. Furthermore, recent evidence supports the role of *β*‐ARs in alcohol‐ induced enhancement of GABA synapses in the BLA, suggesting a possible noradrenergic mechanism mediating the anxiolytic effects of alcohol [58] (**Figure 1**). This was further evidenced by intra‐BLA infusions of a *β*3‐AR agonist that enhanced inhibitory GABA signaling on BLA pyramidal cells to reduce anxiety‐like and alcohol‐seeking behavior [163]. Furthermore, the neuroadaptive changes associated with chronic alcohol consumption including desensitization of *β*‐ARs in the BLA have been shown to modulate its activity [164] (**Figure 1**).

In contrast to excitatory dopaminergic/glutamatergic signaling in the BLA that increases its activity, serotonergic transmission in the BLA is inhibitory [165]. The serotonergic innerva‐ tions on principal glutamate cells in the BLA decrease the overall excitatory activity of these cells [166] through 5‐HT1A receptors [167] and modulate BLA output (**Figure 2**). This is supported by a recent study where depletion of serotonin in the BLA increased glutamate receptor density and fear‐potentiated startle in mice, indicating that serotonergic inhibition regulates excitatory signaling in the BLA to modulate affective behaviors like anxiety [68]. Contribution of Noradrenaline, Serotonin, and the Basolateral Amygdala to Alcohol Addiction: Implications for Novel Pharmacotherapies for AUDs http://dx.doi.org/10.5772/62843 123

Chronic alcohol‐induced neuroadaptations change 5‐HT receptor expression and function in the brain [168] that alters the regulatory control of serotonin over BLA principal cells. Loss of inhibition on BLA principal neurons increases BLA output, increasing anxiety [169, 170] and other symptoms of withdrawal. In support of this, chronic alcohol or withdrawal stress increases the expression of 5‐HT1A autoreceptors in the raphe nucleus [168] which causes a reduction in 5‐HT levels in the BLA. This increases BLA activity, which contributes to anxiety‐ like behaviors following withdrawal from chronic alcohol (**Figure 2**). Furthermore, 5‐HT2A/2C receptors have been suggested to potentiate inhibitory GABAergic tone on principal BLA glutamatergic cells to decrease excitability [67]. Chronic alcohol causes adaptive changes that lower the expression levels of these receptors, reducing inhibition over BLA principal neurons [67]. This augments BLA output and increases the possibility of anxiety‐induced relapse following a period of chronic alcohol exposure [141] (**Figure 2**). In addition to this,

Long‐term exposure to alcohol simulates chronic stress‐like conditions [130] that have a profound effect on fear memory consolidation [151]. Alcohol withdrawal‐induced stress has been shown to increase conditioned fear [152] and impair extinction of fear memory [153]. A recent study also showed that repeated alcohol exposures enhance retrieval of previously consolidated fear memories and augments activity in BLA and other brain regions involved

There is significant evidence that supports the role of NE and 5‐HT in drug dependence and alcohol addiction [87, 155–157]. Moreover, the BLA which is highly implicated in depend‐ ence to alcohol‐seeking [17, 123, 131, 132] is densely innervated by these neurotransmitters [58, 71, 158, 159]. Since chronic alcohol exposure causes neuroadaptations that affect the signal‐ ing and receptor subtypes of these neurotransmitters, dysregulation of NE and 5‐HT trans‐ mission in the BLA may lead to a constellation of aversive outcomes including altered consolidation of alcohol‐related memories, anxiety disorders, and eventually higher rates of

NE plays a vital role in facilitating the function of the BLA in fear memory consolidation [70]. It has been shown that intra‐BLA infusions of *β*‐AR agonists enhance retention of inhibitory avoidance [160], while *β*‐AR antagonists block fear memory enhancement [69]. Also, *α*1‐AR activation in the BLA enhances fear memory consolidation through an interaction with *β*‐ ARs [161]. This evidence suggests that noradrenergic receptors strongly contribute to BLA function. It is possible that alteration in NE activity in the BLA may lead to altered memory consolidation and stress‐coping mechanism that may enhance alcohol‐seeking and relapse [162]. Indeed, antagonism of *α*1‐ARs reduced dependence‐induced increase in alcohol consumption in rats [88]. Furthermore, recent evidence supports the role of *β*‐ARs in alcohol‐ induced enhancement of GABA synapses in the BLA, suggesting a possible noradrenergic mechanism mediating the anxiolytic effects of alcohol [58] (**Figure 1**). This was further evidenced by intra‐BLA infusions of a *β*3‐AR agonist that enhanced inhibitory GABA signaling on BLA pyramidal cells to reduce anxiety‐like and alcohol‐seeking behavior [163]. Furthermore, the neuroadaptive changes associated with chronic alcohol consumption including desensitization of *β*‐ARs in the BLA have been shown to modulate its activity [164]

In contrast to excitatory dopaminergic/glutamatergic signaling in the BLA that increases its activity, serotonergic transmission in the BLA is inhibitory [165]. The serotonergic innerva‐ tions on principal glutamate cells in the BLA decrease the overall excitatory activity of these cells [166] through 5‐HT1A receptors [167] and modulate BLA output (**Figure 2**). This is supported by a recent study where depletion of serotonin in the BLA increased glutamate receptor density and fear‐potentiated startle in mice, indicating that serotonergic inhibition regulates excitatory signaling in the BLA to modulate affective behaviors like anxiety [68].

**4. Role of 5‐HT and NE in the BLA in alcohol addiction**

in fear memory retrieval [154].

122 Recent Advances in Drug Addiction Research and Clinical Applications

relapse [132, 138].

(**Figure 1**).

**Figure 1. Changes in NE signaling and BLA output following acute and chronic alcohol exposure or withdrawal**. Acute alcohol decreases NE signaling in the BLA, which is regulated by a feedback loop through presynaptic *α*2‐adre‐ nergic autoreceptors expressed on NE fibers in the LC. Decreased BLA‐NE levels decrease the excitation of BLA princi‐ pal cells through postsynaptic *α*1‐ARs. Acute alcohol further enhances the inhibition of BLA principal cells by NE‐ mediated enhancement of GABA synapses through *β*‐ARs expressed on GABAergic LPSCs. The net result of this inhibition is decreased BLA principal neuron excitability and BLA activity which has been suggested to reduce anxiety and may explain the anxiolytic effect of acute alcohol. Chronic alcohol/withdrawal increases NE levels that enhance *α*1‐ AR mediated excitation of BLA principal cells. Chronic ethanol has been shown to desensitize *β*‐ARs in the brain which leads to a reduction in NE's effects on GABA‐LPSCs causing a decrease in the inhibitory tone over BLA princi‐ pal cells, increasing excitability. This increases the net excitability of BLA principal cells and increases BLA activity causing anxiety during withdrawal and may contribute to alcohol‐seeking and relapse.

chronic alcohol‐induced neuroadaptations in other receptor subtypes like the GABA‐A receptors facilitate the anxiolytic effects of alcohol [171]. Increasing the activity of 5‐HT on GABA‐A receptors on BLA principal cells may contribute in reducing withdrawal‐induced anxiety and alcohol‐seeking.

**Figure 2. Changes in 5‐HT signaling and BLA output following acute and chronic alcohol exposure or withdrawal**. Acute alcohol increases 5‐HT release in the BLA which is regulated by a feedback loop through 5‐HT1A autoreceptors expressed on 5‐HT neurons in the DRN. Increased BLA‐5‐HT levels enhance the inhibition of BLA activity through postsynaptic 5‐HT1A receptors expressed on principal neurons. Increased 5‐HT signaling also activates 5‐HT2A/2C recep‐ tors expressed on GABAergic interneurons in the BLA that further increase the inhibition on BLA principal cells through increased GABAergic tone. The net result of this inhibition is decreased BLA principal neuron excitability and BLA activity, which has been shown to reduce anxiety and may explain the anxiolytic effect of acute alcohol. Chronic alcohol/withdrawal increases the expression of 5‐HT1A autoreceptors in the DRN which decreases 5‐HT levels in the BLA. This reduces 5‐HT1A‐mediated inhibition on BLA principal cells. Chronic alcohol‐induced withdrawal downregu‐ lates the expression of 5‐HT2A/2C receptors on GABAergic interneurons to further decrease the inhibitory GABA tone on BLA principal cells, increasing excitability. Chronic alcohol also upregulates GABA receptors on principal cells. This results in a net increase in BLA activity causing anxiety that may contribute to alcohol‐seeking and relapse.

Furthermore, cross‐modulation of synaptic transmission in the BLA by 5‐HT1A/1B receptors and *β*‐ARs dictates BLA output [159] that may affect behavioral outcomes like stress, anxiety, and drug dependence. In support of this, we have shown that pindolol, a drug having dual pharmacological activity on 5‐HT1A/1B receptors and *β*1/*β*<sup>2</sup> ARs, decreases alcohol consump‐ tion in mice following long‐term alcohol exposure. Our electrophysiological experiments also indicate that the BLA may mediate the effects of pindolol on alcohol consumption [172].
