Although this technology is relatively new in brain research, its origins can be traced back to the early the 1970s when Oesterhelt and Stoeckenius (1971) showed that bacteria of the genus Halobacterium contain red-colored light-sensitive proteins (opsins) in their cellular membrane. Subsequent studies found that similar proteins are present in the membranes of a series of other microorganisms; in each case, these seemed to be involved in light “recognition,” because they elicited the trafficking of ions across the cell membrane when illuminated (Harz and Hegemann 1991; Matsuno-Yagi and Mukohata 1977; Nagel et al. 2002). The technique was introduced in neuroscience by Boyden et al. (2005) who inserted the gene for the naturally occurring algal protein Channelrhodopsin-2, a rapidly gated light-sensitive cation channel into the genome of mouse neurons, which allowed a reliable, millisecond-time scale control of neuronal spiking by blue light. It was also discovered that halorhodopsin, another microbial opsin, is a light-sensitive chloride channel (Schobert and Lanyi 1982); its introduction into neurons by viral vectors and its stimulation with yellow light silence the neurons reversibly also on a millisecond time scale (Han and Boyden 2007). Importantly, the properties of neurons do not appear to be changed by the mere expression of opsins.

The technique shows advantages over electrical stimulation even if employed alone, because it allows both stimulation and silencing; in addition, passing fibers are not affected. Its combination with advanced gene engineering techniques makes the optogenetic technology one of the most powerful tools of neuroscientific research developed so far. Opsin expression can be made selective to neuron types; moreover, the stimulation of axons can dissect the roles of projections.

The optogenetic technology was introduced into behavioral research rather early (Adamantidis et al. 2007; Aravanis et al. 2007). Aggression research benefited so far from it by two research papers and five reviews. Lin et al. (2011) showed that the optogenetic stimulation of hypothalamic neurons induces attacks on both inanimate objects and social partners in mice, while Challis et al. (2013) showed that GABAergic neurons in the raphe mediate the acquisition of avoidance after social defeat. The reviews acclaimed the potentials of the methodology and proposed new directions for aggression research to exploit these potentials (Anderson 2012; Chamero et al. 2012; Nieh et al. 2013; Sternson 2013; Tourino et al. 2013). Thus, the optogenetic technology had no major impact on this field so far. Its potentials, however, are indeed tremendous. We showed above that understanding the mechanisms of aggression control often requires neuron-level analysis of functions, which can be achieved by the optogenetic technology only. Our own experience shows that the introduction of the technology needs expertise in various fields and is not easy; one can confidently state, however, that it constitutes the future in aggression research just like in other fields.

The phrase covers processes by which the function of the genome is altered after birth by chromatin remodeling. One of the basic epigenetic mechanisms, DNA methylation, was discovered rather early, but its roles and consequences remained unknown by that time (Gold et al. 1966). In an initial wave of discoveries, epigenetics was associated with viral infections and carcinogenesis (Chu et al. 1977; Rubinstein 1976). However, it became gradually recognized that the phenomenon has more profound roles; it constitutes a basic mechanism that governs vertebrate gene function and cell differentiation (Cooper 1983; Riggs 1975; Razin and Riggs 1980). Nowadays it is well established that epigenetic mechanisms including DNA methylation, as well as histone phosphorylation, acetylation, and methylation, control the transcription machinery, determine cell differentiation, and maintain cell phenotypes over the life cycle (Blomen and Boonstra 2011).

Based on the roles of glucocorticoids in epigenetics (Hofmann et al. 1989), it was proposed that early exposure to stress “predetermines” the development of diseases later in life (including psychopathologies) by affecting epigenetic phenomena (Meaney and Szyf 2005; Seckl and Meaney 2006). Afterwards it was discovered that the phenomenon is more dynamic than previously thought and can be activated and can alter gene expression over the lifespan. For example, chromatin remodeling by histone phosphorylation and acetylation was shown to have a role in the encoding of memories of psychologically stressful events (Reul and Chandramohan 2007). The studies reviewed in these articles in fact revealed a new mechanism by which environmental events affect the functioning of the organism. Nowadays it is widely accepted that major events that affect the individual at any time point of its lifetime elicit lasting changes in gene expression profiles, which is highly consequential for brain function and behavior.

Epigenetic changes—the basic mechanisms which “translate” environmental events into alterations in gene expression—appear highly relevant for understanding aggression-related psychopathologies which are in fact developmental disorders. Moreover, the manipulation of epigenetic phenomena may be important from the point of view of therapy. While this idea did not yet penetrate aggression research, the manipulation of epigenetic processes with therapeutic purposes is an emerging research area in various diseases and psychopathologies (Castren et al. 2012; Gnyszka et al. 2013; Warnault et al. 2013; Zimmermann et al. 2012).

A PubMed search identified 11 studies that addressed the role of epigenetic phenomena in aggressive behavior¹; 5 were reviews and 6 were experimental studies. The reviews usually interpreted findings obtained by other techniques through the prism of epigenetics and prized the importance and relevance of the phenomenon for aggression research (Cushing and Kramer 2005; Lesch et al. 2012; Tremblay 2008; Veenema 2009, 2012). Interestingly, three of the six experimental studies were done in humans despite the fact that obtaining brain samples is rather problematic in living people. In these reports, peripheral tissues were perceived as markers of central functions. It was found that childhood physical aggression is associated with epigenetic changes in serotonin synthesis (Wang et al. 2012a, b); antisocial behavior in women is linked to childhood sexual abuse by the methylation of the promoter of the serotonin transporter gene (Beach et al. 2011), while BDNF epigenetics was associated with therapeutic responsiveness in borderline personality disorder (Perroud et al. 2013). The main findings of animal studies are as follows: MAO_A epigenetics plays a role in the pubertal stress model of abnormal aggression (Márquez et al. 2013); DNA methyltransferase-1 expression in the amygdala and hippocampus differentiates high novelty-responding and low novelty-responding rats, the former selection line showing high aggressiveness (Simmons et al. 2012); testosterone treatment in the egg affects adult aggressiveness in house sparrows presumably by epigenetic mechanisms (Partecke and Schwabl 2008). Clearly, epigenetic processes are understudied in aggression research. Nevertheless, the general roles played by the former and the type of information the latter needs make the expansion of the field not only likely but also necessary.

Studies on the pharmacology of aggression tacitly or openly assume that the brain control of this behavior is realized on two levels: the behavior per se is controlled by neural circuits, which, however, are under neurochemical influences that impact the circuits as a whole. “Neural mechanistic” approaches operate with concepts like “prefrontal cortex,” “amygdala,” “hypothalamus,” and “periaqueductal gray,” while neurochemical approaches deal with “serotonin,” “noradrenaline,” “dopamine,” and “vasopressin.” The two approaches seem to run on “roads” that occasionally cross each other but are quite independent overall. While neurochemical approaches negate the role of neurocircuits by no means, there are a large number of studies where, e.g., “serotonin” is perceived as a regulatory mechanism of its own right and where reference to local effects or neuroanatomical entities is minimal. In this section we propose that the amalgamation of the neuroanatomical and neurochemical approaches would markedly improve our understanding of aggressive behavior per se and of its abnormal forms. We will illustrate this by examples.

The three examples presented above demonstrate that neurochemical and anatomical mechanisms of aggression control cannot be separated: the overall effects of pharmacological treatments are realized by multiple, often opposite, effects at particular brain levels. The neurochemical aspects of anatomical connections received some attention in the past, especially in the studies published by the group of Siegel (amply discussed above). Even in this case, however, the neurochemical mediators of the main anatomical connections were investigated only. The neurochemical identity of the various inputs (and their subcellular localization) that a particular group of neurons received was never studied. Overall effects (seen, e.g., after systemic or oral treatments with psychoactive drugs) are important from a therapeutic perspective; as such, “global” pharmacology will always be an issue. Nevertheless, the understanding of brain mechanisms requires studies on local effects and on the interactions of these.

The quest for “aggression genes” was prompted by the strong heritability of aggressiveness (van Oortmerssen et al. 1992; Miles and Carey 1997). The search was successful, probably more successful than expected. The number of genes associated with aggressiveness was surprisingly large irrespective to the technology employed and the subjects studied (36 genes identified by the transgenic technology till 2003, Maxson and Canastar (2003); 335 genes identified by sequence analysis in tame and aggressive foxes, Kukekova et al. (2011); 262 genes associated with seasonal variation in aggression in song sparrows, Mukai et al. (2009); the majority of the marker genes studied in feral rats, de Boer et al. (2003) etc.). A detailed analysis of the genetic underpinnings of aggression is not within the scope of this book. We draw attention on one emerging line of research only, namely, pharmacogenetics.

This phrase refers to genetic differences that affect individual responses to drugs, e.g., genetic differences in receptor structure and ligand metabolism (Klotz 2007). The field is not particularly new; the first paper on the phenomenon was published a long time ago (Evans and Clarke 1961). Although the primary focus was on drugs so far, pharmacogenetic differences are unlikely to affect drug responses alone, because endogenous ligands and therapeutic agents affect similar mechanisms. As such, pharmacogenetic approaches address the very essence of neurochemical communication.

The PubMed search with the terms pharmacogenetics and aggression identified seven relevant papers, out of which four were research studies, while three were reviews (Nesher et al. 2013; Singh et al. 2012; Guillot et al. 1999; Miner et al. 1993; Patel and Barzman 2013; Takahashi et al. 2012; Veenstra-VanderWeele et al. 2000). While such limited evidence seems insufficient to address the issue systematically, the available evidence suggests that (1) abnormal forms of aggression are associated with genetic variability that forecasts significant variations in drug responses and (2) genetic variability in receptor characteristics brings about changes in aggressive behavior. We mention that there are a series of studies which do not bear the flag of pharmacogenetics but are still important in this respect. For example, genetic polymorphisms in the encoding of neurotransmitter receptors affect not only the functioning of aggression-controlling brain mechanisms but also their responses to drug treatments (for recent examples of such studies, see Beitchman et al. 2012; Benis and Hobgood 2011; Butovskaya et al. 2012; Vaillancourt et al. 2012; Zai et al. 2012).

One of the main conclusions deriving from the studies reviewed in Chap. 4 is that the interpretation of findings on aggression control is quite often hampered by the lack of detail. Missing pieces in the multidimensional puzzle of aggression control may be supplied by the advanced technologies presented above. The identification of particular groups of neurons that play roles in aggression control and the mapping of their anatomical connections can be done with unprecedented precision by the optogenetic technology and functional pharmacology; the ultimate mechanisms of abnormal aggression and their relationship with environmental factors may be resolved by studies on epigenetic phenomena, while pharmacogenetic studies may reveal how drug effects and neurochemical connections are altered in abnormal aggression. Their combined application will elevate understanding to a qualitatively new level.