Elsevier

Brain Research

Volume 1770, 1 November 2021, 147628
Brain Research

Increased inhibitory activity in the basolateral amygdala and decreased anxiety during estrus: A potential role for ASIC1a channels

https://doi.org/10.1016/j.brainres.2021.147628Get rights and content

Highlights

  • Rhythmic spontaneous IPSCs in the basolateral amygdala (BLA) increase in estrus.

  • Anxiety-like behavior decreases in estrus.

  • Lower temperature enhances sIPSCs; the effect is blocked by an ASIC1a antagonist.

  • Rhythmic sIPSCs are the strongest in early estrus, when body temperature drops.

  • Increased BLA sIPSCs in estrus may underlie lower anxiety; potential role for ASIC1a.

Abstract

The amygdala is central to emotional behavior, and the excitability level of the basolateral nucleus of the amygdala (BLA) is associated with the level of anxiety. The excitability of neuronal networks is significantly controlled by GABAergic inhibition. Here, we investigated whether GABAergic inhibition in the BLA is altered during the rat estrous cycle. In rat amygdala slices, most principal BLA neurons display spontaneous IPSCs (sIPSCs) in the form of “bursts” of inhibitory currents, occurring rhythmically at a frequency of about 0.5 Hz. The percentage of BLA neurons displaying sIPSC bursts, along with the inhibitory charge transferred by sIPSCs and the frequency of sIPSC bursts, were significantly increased during the estrus phase; increased inhibition was accompanied by reduced anxiety in the open field, the light–dark box, and the acoustic startle response tests. sIPSC bursts were blocked by ibuprofen, an antagonist of acid-sensing-1a channels (ASIC1a), whose activity is known to increase by decreasing temperature. A transient reduction in the temperature of the slice medium, strengthened the sIPSCs bursts; this effect was blocked in the presence of ibuprofen. Further analysis of the sIPSC bursts during estrus showed significantly stronger rhythmic inhibitory activity in early estrus, when body temperature drops, compared with late estrus. To the extent that these results may relate to humans, it is suggested that “a calmer amygdala” due to increased inhibitory activity may underlie the positive affect in women around ovulation time. ASIC1a may contribute to increased inhibition, with their activity facilitated by the body-temperature drop preceding ovulation.

Introduction

In a significant percentage of women, the menstrual cycle is associated with a cyclicity in mood, affecting emotional processing (Hoyer et al., 2013, Sundström Poromaa and Gingnell, 2014, Lorenz et al., 2017, Yamazaki and Tamura, 2017). Most well-known of the negative emotional changes related to the menstrual cycle are the premenstrual syndrome and its more severe form, the premenstrual dysphoric disorder, which many women experience in the later part of the luteal phase (Sanders et al., 1983, Bäckström et al., 1983, Pearlstein and Steiner, 2008, Sundström Poromaa and Gingnell, 2014). In contrast, an increased positive affect has been observed in the pre-/periovulatory phase (Rossi and Rossi, 1977, Bäckström et al., 1983, Sanders et al., 1983, Ocampo Rebollar et al., 2017), which may influence reproductive behavior (Hedricks et al., 1987, Bullivant et al., 2004, Pillsworth et al., 2004, Gangestad and Thornhill, 2008, Roney and Simmons, 2013) and fertility (Lynch et al., 2014, Rooney and Domar, 2018). The biological mechanisms underlying the emotional changes during the menstrual cycle are not well-understood. Changes in progesterone levels play a central role in the negative affect associated with the luteal phase (Sundström Poromaa and Gingnell, 2014), while increased levels of estradiol and/or luteinizing hormone, which have anxiolytic and mood-elevating effects (Meller et al., 2001, Walf and Frye, 2006), could be involved in the positive affect pre-ovulation; however, where these hormones act in the brain and by what mechanisms they affect emotional behavior is not clear. Central to emotional functions is the limbic system (Morgane et al., 2005, Rajmohan and Mohandas, 2007), with the amygdala playing a pivotal role (Phelps and LeDoux, 2005, Pessoa, 2010, Bonnet et al., 2015, Weymar and Schwabe, 2016). Structural and functional changes have been observed in the amygdala during the menstrual cycle, such as an increase in gray matter volume associated with an enhanced negative affect, and an increased reactivity to negative stimuli, during the premenstrual/luteal phase when compared with the preovulatory phase (Ossewaarde et al., 2013, Gingnell et al., 2014).

A change in the reactivity of a neuronal network implies changes in neuronal excitability, which, in turn, is significantly controlled by the GABAergic inhibitory system. It is unknown, however, whether GABAergic inhibition in the amygdala changes during the menstrual cycle. Studies in animals can provide an insight into the answer of this question. Thus, in the present study, we investigated whether the level of basal GABAergic inhibition in the rat amygdala changes during the estrous cycle, and if such changes are reflected in emotional behavior, as determined by testing the level of anxiety. We studied spontaneous GABAergic activity in the basolateral nucleus of the amygdala (BLA), a region that plays a central role in processing the emotional components of sensory information (Olucha-Bordonau et al., 2015, Phelps and LeDoux, 2005), modulates cognitive functions via extensive interconnections with the prefrontal cortex (Grace and Rosenkranz, 2002, McIntyre et al., 2003) and the hippocampus (Pitkänen et al., 2000, Phelps, 2004), and its excitability level is closely related to anxiety (Sajdyk and Shekhar, 1997, Shekhar et al., 2003, Zhou et al., 2010, Prager et al., 2014b). We found that basal inhibitory activity in the BLA is enhanced during the estrus/periovulatory phase of the cycle, and this is associated with reduced anxiety.

There are a number of mechanisms involved in the regulation of GABAergic synaptic transmission in the BLA (Rainnie, 1999, Aroniadou-Anderjaska et al., 2007, Chung and Moore, 2009, Ohshiro et al., 2011, Popescu and Paré, 2011, Prager et al., 2016, Aroniadou-Anderjaska et al., 2018), which could potentially influence it during the estrous cycle. One of them involves the calcium-permeable, acid-sensing ion channels-subtype 1a (ASIC1a; Pidoplichko and Dani, 2006, Pidoplichko et al., 2014), which display the paradoxical characteristic of increasing their activity when temperature is reduced (Askwith et al., 2001, Pidoplichko et al., 2014). In the BLA of male rats, ASIC1a channels are active in the basal state, and facilitation or suppression of their activity increases or decreases spontaneous GABAergic inhibition, respectively, with a corresponding decrease or increase in anxiety-like behavior (Pidoplichko et al., 2014). In the present study, we also investigated the regulation of GABAergic inhibition by ASIC1a in the BLA of female rats, and considered a potential mechanism by which these channels could affect inhibitory activity in the BLA during the estrous cycle.

Section snippets

Inhibitory activity in the BLA during the phases of the estrous cycle

Basal, spontaneous inhibitory activity in the BLA has the form of large-amplitude “bursts” of GABAA receptor-mediated IPSCs (“sIPSC bursts”), occurring at an average frequency of 0.5–1 Hz, over a background of irregular, low amplitude IPSCs (Rainnie, 1999, Chung and Moore, 2009, Ohshiro et al., 2011, Popescu and Paré, 2011, Aroniadou-Anderjaska et al., 2018). For the most part, this information has been obtained from studies in male rats. Here, we first examined if this oscillatory inhibitory

Discussion

In the present study, we found that during the estrus phase of the rat estrous cycle, spontaneous inhibitory activity (sIPSCs) increases significantly in the BLA, and this is accompanied by a reduction in anxiety-like behavior. We also found that NMDARs and ASIC1a channels play a pivotal, facilitating role in the regulation of sIPSCs in the BLA of female rats, as we have previously found in male rats (Pidoplichko et al., 2014, Aroniadou-Anderjaska et al., 2018). Lowering temperature increases

Animals

Female, Sprague-Dawley rats (Taconic Farms, Derwood, MD), 3–5-months old, were housed in an environmentally controlled room (23-24C, 44% humidity, 12-h light/12-h dark cycle [350–400 lx], lights on at 6:00 pm), with food and water supplied ad libitum. To standardize experimental conditions, cages were cleaned frequently, ensuring clean air for at least 18 h before an experiment, as the buildup of ammonia in the air the rats breath could affect the results from the present experiments (ammonium

CRediT authorship contribution statement

Volodymyr I. Pidoplichko: Investigation, Software, Formal analysis. Vassiliki Aroniadou-Anderjaska: Conceptualization, Methodology, Writing – original draft, Writing – review and editing, Supervision. Taiza H. Figueiredo: Investigation, Software, Formal analysis. Camilla Wilbraham: Investigation. Maria F.M. Braga: Conceptualization, Methodology, Resources, Writing – review and editing, Project administration.

Funding

Supported by the CounterACT Program, National Institutes of Health, Office of the Director and the National Institute of Neurologic Disorders and Stroke [Grant Number 5U01NS102135-03].

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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