The neurons expressing calcium-binding proteins in the amygdala of the guinea pig: precisely designed interface for sex hormones

The amygdala is composed of a set of nuclei and cortical regions within the temporal lobe which are thought to be critically important for emotional behaviour and emotional learning (LeDoux 2003; Sah et al. 2003; Pape and Pare 2010), mediation of pheromonal and reproductive functions (Baum 2009; Kevetter and Winans 1981; Martel and Baum 2009; Morris et al. 2008; Segovia et al. 2006), drug addiction mechanisms (Buffalari and See 2010) and stress response (LeDoux 1994). Although the general mechanisms of all these functions are similar in both sexes, there are also sex-specific details which make these functions sexually dimorphic (Hamann 2005; Fagergren and Hurd 1999; Gruene et al. 2015; Rollins and King 2000; Swaab et al. 2003). For example, women on average retain stronger and more vivid memories of emotional events than men (Seidlitz and Diener 1998; Canli et al. 2002). In addition, the fact that emotional memories tend to be stronger in women may be linked to the greater prevalence of depression and some types of anxiety disorders in women (Bekker and van Mens-Verhulst 2007; Davidson et al. 2002). Indeed, there are many other psychological disorders involving emotional function which occur with substantially different rates in men versus women. For example, anorexia is another disorder associated with amygdala dysfunction, which is observed more often in women than in men (Swaab et al. 2003). In contrast, schizophrenia, autism and drug addiction, also associated with abnormalities in the functioning of the amygdala, are quoted more frequently in men than in women (Swaab et al. 2003). It is worth mentioning that various aspects of amygdala anatomy are sexually dimorphic (Morris et al. 2008; Równiak 2013; Równiak et al. 2015; Segovia et al. 2006).

Circulating levels of sex hormones in the bloodstream constitute additional factors which may influence amygdala physiology and may drive amygdala responses in a sex-specific manner by acting through receptors expressed in specific amygdala nuclei. Indeed, the amygdala has a high density of oestrogen and androgen receptors, as well as high aromatase expression and activity (Shughrue et al. 1997; Lephart 1996). Moreover, these receptors and aromatase were found in both the glutamatergic principal neurons (Kiss et al. 2013) and γ-aminobutyric acid (GABA)ergic interneurons (Blurton-Jones and Tuszynski 2002), suggesting that excitation and inhibition in the amygdala may be substantially modulated by sex hormones. There is a growing body of evidence that oestrogens and androgens enhance glutamatergic neurotransmission and down-regulate GABAergic neurotransmission (Barth et al. 2015; Frye et al. 2008), although opposite effects have also been reported (Tan et al. 2012; Zhou et al. 2005). For example, Murphy et al. (1998) reported that oestradiol down-regulated brain-derived neurotrophic factor (BDNF) in cultured hippocampal cells and this effect was associated with an increase in excitatory tone leading to a twofold increase in dendritic spine density and a decrease in inhibition, as evidenced by decrease in glutamate decarboxylase (GAD) and GABA level. Moreover, as both glutamatergic and GABAergic cell types express GABA receptors, an oestradiol-induced decrease in GABAergic transmission to glutamatergic cells could effectively increase excitatory drive and thus favour the formation of new dendritic spines (Murphy et al. 1998; McDonald and Mascagni 2004). Further studies have confirmed that oestradiol enhances the magnitude of long-term potentiation in the hippocampus through facilitation of the N-methyl-d-aspartate receptors (NMDA) (Foy 2001; Córdoba Montoya and Carrer 1997; Smith et al. 2009). On the other hand, oestrogen has an ability to alter the frequency of GABAergic miniature inhibitory postsynaptic currents (Zhou et al. 2005). Moreover, other experiments have shown that ovariectomy and subsequent oestrogen replacement can alter mRNA levels of GABA receptor subunits, GABA transporter, and GAD, the rate-limiting enzyme in GABA synthesis (Nakamura et al. 2004; Herbison et al. 1995; Herbison and Fénelon 1995).

The generation of responses by the amygdala is determined largely by the balance of excitatory and inhibitory inputs to its principal projecting neurons. The activity of these neurons is tightly controlled by GABAergic interneurons, especially these expressing calbindin (CB), parvalbumin (PV) and calretinin (CR) (Sorvari et al. 1996; Muller et al. 2007; Woodruff and Sah 2007a). It is worth mentioning that each of these subpopulations, apart from GABA, also uses different neuropeptides. For example, CB-containing neurons often co-express somatostatin (SOM) and/or neuropeptide Y, whereas CR-positive cells often co-express cholecystokinin and/or vasoactive intestinal peptide (McDonald and Mascagni 2002; Mascagni and McDonald 2003). In contrast, PV neurons are devoid of any of these neuropeptides (Mascagni et al. 2009). Moreover, it should be kept in mind that some of the CB and CR neurons are not GABAergic interneurons but glutamatergic projecting neurons (Moryś et al. 1999; Kemppainen and Pitkänen 2000; McDonald et al. 2012). If the activity of various subpopulations of GABAergic neurons was strongly influenced by circulating steroid hormones via their receptors, inhibitory control in the amygdala could easily be driven in a sex-specific manner. Such modulation may have a huge impact on the amygdala’s overall processing, and this way promotes sexually dimorphic responses. Although the amygdala has a large population of GABAergic neurons co-expressing CB, PV or CR (Pitkänen and Amaral 1994) and a large population of cells bearing oestrogen alfa (ERα), oestrogen beta (ERβ) and androgen (AR) receptors (Shughrue et al. 1997; Warembourg and Leroy 2004; Wood and Newman 1999), little is known about relationships among all these populations. There are only two studies on the relationship between neurons expressing calcium-binding proteins and steroid receptors in the amygdala, and only one of them is quantitative (Blurton-Jones and Tuszynski 2002; Perez et al. 2004). However, in both of these studies the amygdala was just one of many brain centres studied. The quantitative study of Blurton-Jones and Tuszynski (2002) describes the co-expression pattern of PV interneurons with ERβ-bearing cells in the selected nuclei of rat amygdala, where the counts were limited to the lateral and basomedial nuclei of female subjects. Perez et al. (2004) reported a lack of association between CB or PV-expressing neurons and ERα in monkey forebrain, which was also studied in female subjects only. Thus, the present study investigates for the first time the co-localization pattern of CB, PV or CR with ERα, ERβ or AR in the amygdala of the guinea pig. Female and male subjects were taken into account to examine possible sex differences as well. To make the text easier to read, the neurons expressing CB, PV, CR, SOM, GABA, ERα, ERβ and AR will be uniformly described as CB+, PV+, CR+, SOM+, GABA+, ERα+, ERβ+ and AR+ neurons.

To the best of our knowledge, the present study describes for the first time a systematic analysis of relationships between populations of CB+, PV+ and CR+ neurons, which form in the amygdala main subsets of GABA+ system and neurons bearing steroid receptors such as ERα or ERβ or AR. The results show that in the amygdala of male and female guinea pig, subpopulations of the CB+ and PV+ neurons co-express ERβ, while many of the CR+ neurons co-express ERα. In addition, in the medial nucleus and some other neighbouring regions there are also subpopulations of CB+ and CR+ neurons which co-express AR. The localization of ERα, ERβ or AR within subsets of GABAergic interneurons across diverse amygdaloid regions suggests that steroid hormones may exert a significant influence over local neuronal activity by directly modulating inhibitory tone. The control of inhibitory tone may be a mechanism whereby oestrogen and androgen modulate amygdalar processing in a sex-specific manner. Moreover, as the numbers of CB+, PV+ and CR+ neurons (Równiak et al. 2015) and the numbers of such cells co-expressing steroid receptors (present results) are sexually dimorphic in various nuclei of the guinea pig amygdala, sensitivity to sex hormones and their impact on many regions may substantially differ in male and female subjects. Thus, GABA+ neurons co-expressing CB+, PV+ or CR+ provide a precisely designed inhibitory interface which is steroid-sensitive and sexually dimorphic. This interface could be the natural basis for sex-specific processing in the amygdala.

The present results demonstrate that in the amygdala of the guinea pig, subpopulations of CB+, PV+ and CR+ neurons are oestrogen or androgen-sensitive as many of these cells co-express ERα, ERβ or AR. The patterns of co-expression are similar in both sexes. However, these patterns are quite different in the “cortex-like” nuclei (the cortical and basolateral amygdala) and “striatum-pallidum-like” nuclei (the central and medial amygdala). In the “cortex-like” regions, CB+, PV+ and CR+ neurons use mostly oestrogen receptors, and the pattern of co-expression is similar to relationships observed in the cerebral cortex (Blurton-Jones and Tuszynski 2002; Kritzer 2002). In the “striatum-pallidum-like” nuclei PV+ neurons are not present, while CB+ and CR+ cells are much more influenced by androgens. However, there are no data for comparisons in the available literature. As the amygdala is composed of “cortex-like” and “striatum-pallidum-like” nuclei (Swanson and Petrovich 1998), one important fact should be kept in mind. In the “cortex-like” regions, most projection neurons (pyramidal and semi-pyramidal cells) use glutamate as a fast, excitatory neurotransmitter (McDonald 1996), whereas, in contrast, many projection cells of the “striatum-pallidum-like” nuclei use GABA as a fast, inhibitory neurotransmitter (Swanson and Petrovich 1998; Saha et al. 2000; Seo et al. 2016). In the “cortex-like” regions, GAD is expressed in interneurons (McDonald 1985; Pitkänen and Amaral 1994), whereas in the “striatum-pallidum-like” nuclei, it is often expressed in projection neurons (Swanson and Petrovich 1998; Saha et al. 2000; Seo et al. 2016). The finding that substantial subpopulations of PV+ neurons and small subsets of CB+ neurons in the amygdala of the guinea pig co-express ERβ, while these cells almost never co-express ERα coincide well with some previous studies on the “cortex-like” amygdala (Blurton-Jones and Tuszynski 2002; Perez et al. 2004) and cerebral cortex (Kritzer 2002) of the rat. For example, extensive co-localization of PV+ neurons with ERβ was reported in this species in the lateral and basomedial nuclei of the amygdala (Blurton-Jones and Tuszynski 2002) and sensorimotor and association areas of the cerebral cortex (Blurton-Jones and Tuszynski 2002; Kritzer 2002). The co-expression of CB+ neurons with ERβ in the sensorimotor and association cortices was rather limited. However, in the piriform and enthorinal cortices, such co-expression was much more extensive (Kritzer 2002). On the other hand, there is no or very weak co-expression between CB+ or PV+ neurons with ERα in the amygdala (present results; Perez et al. 2004) and various areas of the cerebral cortex (Perez et al. 2004; Kritzer 2002). The present results also demonstrate that substantial populations of CR+ neurons in the amygdala of the guinea pig co-express ERα, while these cells were almost without exception immunonegative for ERβ. These findings are in agreement with observations in the juvenile rat cerebral cortex, which revealed substantial overlap between CR+ and ERα+ immunoreactivity but virtually no co-localization of CR+ and ERβ+ immunolabelling (Hayashi et al. 2001; Kritzer 2005). The cells double-labelled for CR and ERα were also described in the adult rat hippocampus (Nakamura and McEwen 2005). Finally, the present results demonstrate that in the medial nucleus and in some other neighbouring regions, there are also subpopulations of CB+ and CR+ neurons which co-express AR. Data concerning relationships between CB+ neurons and AR in the amygdala are not as yet available. In the cerebral cortex AR is mostly present in the pyramidal neurons, and in the most cortical areas virtually none of the non-pyramidal CB+ or PV+ neurons were found to be immunoreactive for AR (Kritzer 2004). Only in the proisocortical areas of the piriform and entorhinal cortices, many of the non-pyramidal CB+ or PV+ cells were simultaneously AR+ (Kritzer 2004). Data concerning relationships between CR+ neurons and AR in the amygdala or cerebral cortex are not yet available.

The present results demonstrate that there is strict cellular separation of ERβ in CB+ and/or PV+ neurons and ERα in CR+ cells. In contrast, AR is expressed in non-overlapping subpopulations of CB+ and CR+ cells, because CR and CB were not co-localized in amygdala neurons of the guinea pig. Such separaration of ERα, ERβ or AR within subsets of non-overlapping subpopulations of GABA+ interneurons does not seem to be accidental, and this suggests that these steroid-sensitive subsets form a kind of precisely designed interface for sex hormones’ actions. For example, there is no or very little overlap in adult life between immunoreactivity for ERβ and ERα in any major region of the cortex, hippocampus or amygdala (Kritzer 2002; Perez et al. 2004), although there are brain regions where such co-existence exists (Gréco et al. 2001). There is also no or very little overlap between CR+ cells either with CB+ or PV+ neurons in the cortex, hippocampus or amygdala (present results, and Miettinen et al. 1992; Mascagni et al. 2009). However, in all these areas there is extensive co-localization of CB and PV in the same neurons (present results, and Mascagni et al. 2009). Moreover, there are intriguing parallels in the development of ERβ+ and PV+ neurons on the one hand and ERα+ and CR+ cells on the other, suggesting that the cellular separation observed in adult life is established during brain development (Kritzer 2005). Specifically, in the postnatal rat cerebrum, like ERβ+ cells, PV+ neurons are identified initially in low numbers around PND 7 (7th day of postnatal life), and from that time steadily increase in number and density to reach mature patterns of labelling by the end of the third postnatal week (Alcántara et al. 1993; Solbach and Celio 1991; Sánchez et al. 1992; Hof et al. 1999). In contrast, like ERα+ cells, CR+ neurons peak in density early, at about PND 7, transiently include large numbers of pyramidal and non-pyramidal cells, and sharply decrease from peak densities in early postnatal life to much lower, near-adult values by PND 15–21 (Fonseca et al. 1995; Schierle et al. 1997; Hof et al. 1999). Finally, double-labelling studies in the rat and monkey indicated that in the adult mammalian amygdala, there are at least three non-overlapping subpopulations of GABA+ interneurons: (1) PV+ neurons (most of which also contain CB); (2) SOM+ neurons (most of which also contain CB and/or neuropeptide Y); and (3) CR+ neurons (most of which also contain cholecystokinin and/or vasoactive intestinal polypeptide) (McDonald and Mascagni 2001, 2002; Mascagni and McDonald 2003; Mascagni et al. 2009). Interestingly, the present results indicate that the first population uses ERβ to respond to sex hormones. The second one is endowed with AR. The third subpopulation co-expresses usually ERα, but some of these neurons also utilize AR. It is worth mentioning that the present work was done in the guinea pig, which could be a good model system for studying brain sexual dimorphism, as well as neuroendocrine and developmental processes because of the many similarities to primates and humans (Wallen and Baum 2002; Bartesaghi and Severi 2004, Kaiser and Sachser 2005). As the results in the guinea pig are quite similar to available data obtained from the rat (Blurton-Jones and Tuszynski 2002) and monkey (Perez et al. 2004), it seems that in the amygdala the co-expression patterns of neurons containing calcium-binding proteins and steroid receptors may be similar across various mammalian species.

The present results seem to be very interesting in the light of synaptic studies which indicated that PV+, CB+ and CR+ neurons perform quite different roles in the inhibitory mechanism of the amygdala (Muller et al. 2007; Miles et al. 1996; Woodruff and Sah 2007a). It should be added, however, that available data are limited to the “cortex-like” amygdala only. For example, PV+ neurons are considered to be a critical component of the inhibitory circuitry in the basolateral and cortical amygdala, where they form characteristic basket-like plexuses around the unstained somata and cartridges, which represent axo-axonic contacts on initial segments of axons (McDonald and Mascagni 2001; Muller et al. 2005; Woodruff and Sah 2007b). Thus, as in the cortex, many PV+ interneurons in both the rodent and primate “cortex-like” amygdala appear to be basket or chandelier cells that provide a strong perisomatic inhibition of local pyramidal neurons (Pitkänen and Amaral 1993; Sorvari et al. 1995, 1996; Woodruff and Sah 2007a, b). Such inhibition is considered to control the output, most notably the synchrony of action potentials of large principal cell populations (Cobb et al. 1995; Miles et al. 1996). Taking into account that many PV+ neurons in the amygdala co-express ERβ (present results, and Blurton-Jones and Tuszynski 2002), and that the numbers of these cells in various amygdaloid nuclei are sexually dimorphic (Równiak et al. 2015; Stefanova et al. 1997), the mechanism described above may be strongly modulated by circulating steroid hormones and sex-specific. CB+ neurons perform a dual role in the intrinsic inhibitory mechanism of the amygdala. In rodents (but not primates) some of them appear to be basket cells, which provide a strong perisomatic inhibition of local pyramidal neurons, and such cells co-express PV but do not contain somatostatin and/or neuropeptide Y (Pitkänen and Amaral 1993; Sorvari et al. 1995, 1996; Woodruff and Sah 2007a, b). The other CB+ neurons are considered as dendrite-targeting interneurons, providing a robust innervation of dendrites in the rat and primate amygdala (Mascagni et al. 2009). Such neurons contain somatostatin and/or neuropeptide Y (but not PV) (McDonald and Mascagni 2002; Muller et al. 2007), and are responsible for the control of the efficacy and plasticity of inputs from specific sources that terminate in the same dendritic domain (Cobb et al. 1995; Miles et al. 1996). Interestingly, it seems probable that the double-labelled CB+/ERβ+ neurons observed in the present study are those CB+ cells which contain PV. On the other hand, CB+/AR+ neurons are most probably those CB+ cells which co-express somatostatin. Thus, both subpopulations of CB+ neurons and both mechanisms described above may be modulated by circulating steroid hormones and sex-specific. There are no detailed synaptic studies on the amygdala concerned with CR+ neurons. However, such studies in the neocortex revealed that CR+ neurons are mostly dendrite-targeting, and their buttons typically innervate dendritic shafts, and less often dendritic spines or somata (Meskenaite 1997). Moreover, at least in some cortical areas and layers, CR+ neurons innervate preferentially other GABA+ neurons (Melchitzky and Lewis 2008), and by inhibiting other classes of inhibitory interneurons they may exert a significant disinhibitory effect on the pyramidal neurons (Meskenaite 1997; Gonchar and Burkhalter 1999). Analogous relationships were also described in the hippocampus, where CR+ interneurons were found to be specialized to control other interneuronal types (Gulyás et al. 1996). Taking into account that many CR+ neurons in the amygdala co-express ERα or AR (present results), the mechanisms described above may be strongly modulated by both oestrogen or testosterone, and thus sex-specific.

The present results and some other studies (Krȩżel et al. 2001; Tan et al. 2012; Blurton-Jones and Tuszynski 2002) provide evidence that there is a relationship between steroid hormone receptors and inhibitory tone in the amygdala. Thus, the question arises what may be the functional significance of such a relationship. There are several lines of evidence that modulation of inhibitory tone by oestrogen and/or androgen via steroid receptors on GABAergic cells may be important for proper amygdala physiology, and any alterations in such modulation may lead to various psychiatric disorders, which differentially affect men and women (prevalence, severity) (Krȩżel et al. 2001; Kinrys and Wygant 2005). For example, studies in ERβ knock-out mice revealed that mutant females displayed increased anxiety and/or depression-like behaviour associated with a reduced threshold for the induction of synaptic plasticity in the basolateral amygdala (Krȩżel et al. 2001). A reduced threshold in this structure could result in exaggerated responses to normally innocuous stimuli or environments, a characteristic feature of fear and anxiety disorders. Whether a reduction of GABA responses in the basolateral amygdala is the mechanism by which anxiety is increased in ERβ mutant mice remains to be tested, but some evidence indicates that pharmacological manipulation of GABA type A receptors in wild type mice can mimic some of the effects of ERβ null mutation (Krȩżel et al. 2001). Moreover, selective ERβ agonists typically exert potent anxiolytic activity (Lund et al. 2005; Oyola et al. 2012). Such anxiolytic effects are mediated by the cortex and hippocampus, and most probably also by the basolateral amygdala, by elevation of GABAergic inhibitory signalling and reduction of glutamatergic excitatory drive (Tan et al. 2012). On the other hand, male ERα knock-out mice display reduced aggression (Ogawa et al. 1997, 1998), while selective ERα agonists typically elevate anxiety and/or aggression (Nomura et al. 2002). All these data from ERβ and ERα knock-out mice are in accordance with recent physiological studies (Adhikari et al. 2015; Hong et al. 2014) and the present description of ERα, ERβ and/or AR enrichment in several amygdala nuclei. For example, the basomedial nucleus is the region in the amygdala, which is directly engaged in anxiolysis, however, the medial and cortical nuclei cannot be excluded as well (Adhikari et al. 2015). Interestingly, in all these nuclei, ERβ is very abundant according to present results. On the other hand, in the medial nucleus, which is especially rich in ERα and other steroid receptors, there are many aggression-promoting GABAergic neurons (Hong et al. 2014).

The results of the present study provide evidence that in the amygdala of the guinea pig subpopulations of CB+ and PV+ neurons co-express ERβ, while many of CR+ neurons co-express ERα. In addition, in the selected nuclei, there are also subpopulations of CB+ and CR+ neurons which co-express AR. The localization of ERα, ERβ or AR within subsets of GABAergic interneurons across diverse amygdaloid regions suggests that steroid hormones may exert a significant influence over local neuronal activity by directly modulating inhibitory tone. The control of inhibitory tone may be one of the mechanisms whereby oestrogen and androgen could modulate amygdala processing in a sex-specific manner. Another mechanism may be thorough steroid-sensitive projection neurons, which are most probably located in the medial and central nuclei.