Ivo H. Machatschke

The evolution of violence in men: the function of central cholesterol and serotonin.

Numerous studies point to central serotonin as an important modulator of maladaptive behaviors. In men, for instance, low concentrations of this neurotransmitter are related to hostile aggression. A key player in serotonin metabolism seems to be central cholesterol. It plays a fundamental role in maintaining the soundness of neuron membranes, especially in the exocytosis transport of serotonin vesicles into the synaptic cleft. In this review, we attempt an evolutionary approach to the neurobiological basis of human male violence. Hominid evolution was shaped by periods of starvation but also by energy demands of an increasingly complex brain. A lack of food resources reduces uptake of glucose and results in a decreased energy-supply for autonomous brain cholesterol synthesis. Consequently, concentrations of neuromembrane cholesterol decrease, which lead to a failure of the presynaptic re-uptake mechanism of serotonin and ultimately to low central serotonin. We propose that starvation might have affected the larger male brains earlier than those of females. Furthermore, this neurophysiological process diminished the threshold for hostile aggression, which in effect represented a prerequisite for being a successful hunter or scavenger. In a Darwinian sense, the odds to acquire reliable energetic resources made those males to attractive spouses in terms of paternal care and mate support. To underpin these mechanisms, a hypothetical four-stage model of synaptic membrane destabilization effected by a prolonged shortage of high-energy, cholesterol-containing food is illustrated.

Non-invasive measurement of adrenocortical and gonadal activity in male and female guinea pigs (Cavia aperea f. porcellus).

Taking blood samples is a common method in biomedical and biological research using guinea pigs. However, most blood sampling techniques are complicated and highly invasive and may therefore not be appropriate for certain research topics concerning stress and reproduction. Thus, a non-invasive method to measure steroid hormones is critically needed. The aim of this study was the biological validation of corresponding enzyme immunoassays for the measurement of fecal cortisol, progesterone, estrogen, and testosterone metabolites in guinea pigs. We examined the effect of subcutaneous injections of ACTH or saline on fecal cortisol metabolites to investigate the suitability of fecal samples to monitor adrenocortical activity. Furthermore, we investigated whether fecal sex steroid metabolites accurately reflected endocrine changes observed in plasma samples during female estrous cycles and male puberty, respectively. In addition, we compared fecal testosterone metabolites of intact males, castrated males, and females to investigate the reliability of fecal samples in discriminating gonadal status of males. Concentrations of fecal cortisol metabolites were significantly increased following ACTH challenge, indicating that adrenocortical activity can be monitored via fecal samples. Secondly, in females, plasma and fecal gonadal steroids were significantly correlated in most subjects. The assay for testosterone metabolites, on the other hand, could not clearly discriminate between test groups. From these findings we conclude that fecal samples can be used for the non-invasive assessment of adrenocortical and female reproductive status in guinea pigs. Testosterone metabolism seems to be more complex and further investigations are needed to establish a more suitable assay.

Social stimuli cause changes of plasma oxytocin and behavior in guinea pigs

The neuropeptide oxytocin (OXT) is a key factor in the initiation and regulation of sociosexual behavior. The present study analyzes the effects of cohabitation and social challenge on plasma OXT concentration rates in guinea pig pairs in relation to male sociosexual behavior. The cohabitation phase lasted 3 days. On day 4, the pair was socially challenged by introducing an unfamiliar male. Displayed male sexual behavior varied significantly during cohabitation, with peaks on day 1. Sociopositive behavior, i.e., side-by-side contact, was increased on days 3 and 4. Cohabitation per se led to elevated plasma OXT concentrations only in males. In contrast, both sexes reacted with increased plasma OXT concentrations to the social challenge (day 4). At that time, male OXT was significantly correlated with sexual behavior and female OXT with sociosexual behavior received from the partner. Additionally, pairs were synchronized in their OXT release during days 3 and 4. We conclude that cohabitation causes sexually dimorphic plasma OXT concentration patterns in guinea pigs. Secondly, the conformity of OXT release in both sexes may represent an endocrine marker for long-term cohabitation, which is reflected behaviorally by increased spatial proximity.

Impact of social environment on female chimpanzee reproductive cycles

The socio-sexual environment of a female is known to affect ovarian function. Increased male contact can enhance menstrual cycle regularity. Conversely, social deprivation constitutes a form of stress that often alters cyclicity and the secretion of reproductive hormones. The present study was carried out on captive female chimpanzees to examine possible interactions among housing conditions, menstrual cycle length, morphological changes in secondary sexual character expression and endocrine release patterns related to follicular and luteal function. Animals were housed over a period of 2 years either with a male conspecific or singly. Blood samples were collected over three cycles, and anogenital swelling changes registered to define menstrual cycle phases. Fecal sampling techniques were used to monitor cortisol as a measure of stress-load. Male presence seemed to affect female cyclicity. Females housed with a male had shorter and more regular cycles than singly housed females. Prolactin, gonadotropins and estradiol levels were generally higher in paired females during specific cycle phases. Group variation was not always significant. No differences were found in progesterone. Sexually cohabited females tended to have lower fecal cortisol metabolites immediately before and after maximum tumescence. We suggest that the close behavioral, physical and olfactory contact with a male conspecific can act as a sort of zeitgeber to modulate ovarian function by stabilizing the female cycle and, perhaps, enhancing folliculogenesis and ovulation.

Spatial learning and memory differs between single and cohabitated guinea pigs.

In socially-living animals, social enrichment enhances spatial learning and memory while separation from conspecifics can severely impair these abilities. In the present work, guinea pigs were kept in isolation or cohabitated in heterosexual pairs and then subjected to a labyrinth task. Latency-time to bait, error-rate, amount of movement and pre- and post-experimental cortisol (CORT) were registered. During a 5d-acquisition phase, single animals (N=19) showed a more efficient encoding of spatial information, with significantly decreased latency-time and error-rate over the time course. In contrast, cohabitated animals (N=19) did not show a significant improvement. Three days after acquisition, memory was tested in a retention test, under the same conditions. With regard to behavioral performance, there was no significant difference between cohabitated and single animals. Pre-experimental CORT was significantly higher in cohabitated animals when compared to single ones. Post-experimentally, CORT increased significantly in singles but not in cohabitated animals when compared to pre-experimental values. Thus, both groups did not differ from each other at that point. Social condition seemed to be an important modulator, in that learning and memory were more impaired in paired animals than in single ones. The failure of cohabitated animals to encode spatial memory more quickly may have been caused by a more chronically up-regulated HPA-axis. The post-experimental CORT increase of singles may be due to more efficient handling of short-term stress exposure.

Non-lactating versus lactating females: a comparison of sex steroids, sexual coloration, and sexual behavior in Japanese macaques

Female Japanese macaques are seasonal breeders distinguished by their red-colored hindquarters, face, and nipple skin areas. Intensity of coloration seems to be associated with sexual attractiveness, behavior, and fluctuating sex steroids. Our aim was to investigate whether the color intensity of these regions differed between lactating (LA) and non-lactating (NLA) females during sexually inactive (SI) and active (SA) phases. Coloration scores of 19 adult females were classified using color tables. Estrogen and progesterone metabolites were determined in fecal samples. Weekly comparison between both groups revealed significantly increased coloration of the hindquarters area from week 13 (SI) until the end of the observation period, and for the nipple skin throughout the SI and SA periods. Face coloration differed marginally. Hormonally, NLA females showed significantly increased excretion rates of sex steroids at the end of the SI phase and throughout the whole SA period. Logistic regression analyses between elevated fecal steroids and nipple coloration disclosed a significant relationship for NLA females during the SI period. This connection persisted and included hindquarter coloration during the SA period. NLA females showed increased intromission with ejaculation, but no difference was found for intromission without ejaculation. In conclusion, results demonstrate increased endocrine excretion rates for NLA females during the whole observation period, paralleled by an enhanced, fertility-signaling sexual attractiveness.

Reply to: Low cholesterol, impulsivity and economy: The influence of statin drugs to behavior

In their reply Goldstein et al. (2009) raise some important aspects regarding cholesterol-lowering statin drugs and their possible association with behavioral changes, which may have ultimately resulted in the current financial crisis. Golomb et al. (2004a) describe in theirwork significant irritability in association with the use of cholesterol-lowering drugs. In six case studies (four men, two women), patients suffered maladaptive behavioral side effects after administration of statin compounds. Men reported gratuitous violent thoughts, for example the desire to kill someone, or became angry and explosive against family members. Interestingly,womenshoweda lower risk for violence as a sideeffect but complained about short temper and the tendency to become increasingly confrontational. Behaviorally, these case reports do fit the assumption of a sexual dimorphism in central serotonergic-mediated behavior related to the cholesterol metabolism. It is worth to mention that the patients in this study were treated with various statin drugs. Surprisingly, side effects were reported regardless of the blood-brain barrier (BBB) crossing capability of the different compounds used. The women patients were administered either simvastatin (which can cross the BBB) or atorvastatin (which cannot cross the BBB), whereas themen received atorvastatin, or lovostatin (crosses the BBB as well) or several other cholesterol-lowering statins, which can or cannot cross the BBB (Caballero and Nahata, 2004). Another study enrolled thousand patients that were being treated with cholesterol-lowering drugs and investigated for adverse or favorable side effects on noncardiac outcomes such as mood, cognition, or serotonin biochemistry (Golomb et al., 2004b). No clear picture emerged, and the authors concluded that the issue of statin-mediated effects on behavior remains highly unresolved. This is in itself alarming as a high number of people in theWesternworld get treatedwith statin drugs to reduce their risk for cardiovascular diseases. A recent study revealed quite limited potential effects of statin administration in association with psychiatric events (Tuccori et al., 2008). More than 35000 reports were analyzed and the most reported psychiatric events in combination with statin were insomnia, somnolence, agitation, confusion, andhallucination.However, significant results were only found for insomnia. The hypothesis suggested by Goldstein et al. (2009) that treatment with statins impairs CNS serotoninmetabolismandmayhave eventually resulted in the risky business policies (impulsive action) that have led to the current global economic disaster is interesting and worthwhile to discuss. We agree that further studies on this topic are needed. So far, work in this field of research has been limited, but results are at least partially disturbing. Comparative studies on the capability of different statin compounds to cross the BBB and others which cannot would be a first step to investigate potential interfering with the central autonomous cholesterol metabolism. In this sense, a testable neuro-physiological model could be worked out using the four step synaptic paradigm by Wallner and Machatschke (2009).

Asthma: Side effects of holistic treatment on behavior, central serotonin, and cholesterol

In her comment to our recent hypothesis on the role of central serotonin and cholesterol in relation to the evolution of violence in men (Wallner and Machatschke, 2009), Pretorius (2009) suggests that aggressive behavior shown in asthma patients could result from neurophysiological mechanisms similar to those proposed by us. Specifically, holistic treatment with corticosteroids (CS) and a concomitant dietary fat restriction would cause a reduction of central serotonin (5HT) and synaptic membrane cholesterol. Medicationwith CS is common in asthma disease as wheezing and exacerbations are significantly reduced in affected patients (CastroRodriguez and Rodrigo, 2009). Maladaptive behavioral side effects have, however, been reported. For instance, children affected with acute exacerbation that were treated with oral CS show several adverse behaviors. Those who were administered higher concentrations displayed significantly increased anxiety and aggression compared to groups subjected to a lower concentration regimen (Kayani and Shannon, 2002). Another study compared children with persistent asthma with groups having intermittent or no symptoms. Those with persistent symptoms scored significantly higher in negative social peer skills (including “hurts others, fights with other children, and bothers other children”) (Halterman et al., 2006). Potential behavioral differences between boys and girls were not considered then, but the authors reported that boys were overrepresented among the persistent symptoms group. Unfortunately, no information about medical treatment was available to the authors at that time; however, a follow-up study on the same cohort of children indicated that around 50% of subjects with persistent symptoms had been treated with CS (Halterman, pers com). Finally, a work carried out on teenaged students (average age 15 years) found significantly more violent behavior in asthmatic boys compared to asthmatic girls (Annesi-Maesano et al., 2001). Again, no information on medical treatment was available to the authors. Returning to the neurobiological model suggested by Pretorius, further studies on functional aspects of CS and their role in the physiology of the stress response could be helpful. Obviously, mutual influence exists between stress mechanisms and the serotonergic system. An upregulated 5HT metabolism can have stimulatory effects on the hypothalamic part of the HPA-axis (Calogero, 1995). In rats, icv administered corticotrophin-releasing factor (CRF) resulted in significantly elevated concentrations of the 5HT metabolite 5-hydroxyindoleacetic acid (5HIAA) in several brain areas. Behaviorally, such a treatment caused dose-dependent displays of anxiety (Song et al., 1995). Furthermore, repeated exposure to various stressors has been shown to increased 5HT turnover or metabolism in the prefrontal cortex, the nucleus accumbens, the lateral hypothalamus and the amygdala (cited from Leonard, 2005). For example, repeated enforced swimming tests caused a decrease of 5HT and 5HIAA concentrations in the hypothalamus and amygdala of rats 24 h after the last test (Shishkina et al., 2008). From these results the authors concluded that the reduced tissue concentration could derive from increased secretion patterns of 5HT during the swimming test in these brain regions and that the later decrease corresponds to an exhausted serotonergic capability. Leonard (2005) pointed out that under chronic stress presynaptic 5HT1A receptors become desensitized in the dorsal raphe region. This is an exciting issue since receptors of this type are involved in anti-aggressive behavior (Simon et al., 1998). Wallner and Machatschke (2009) put forward in their hypothesis that prolonged deficiency of high-energy food triggers a gradual reduction of cholesterol in synaptic membranes due to loss of glucose as an energy source for the maintenance of autonomous cholesterol production. This leads to membrane destabilization, a process which conceivably is associated with dietary fat restriction in asthma therapy. The effects centrally acting CS obviously have could aggravate the synaptic impairment of serotonergic neurons. First, CS would initially increase 5HT concentrations in the synaptic cleft, a phenomenon caused by a decreased re-uptake of these molecules into the presynaptic part due to membrane destabilization. Secondly, CS would cause a down-regulation of 5HT1A receptors. These two processes are the precondition for the four-stage synaptic model described by us (Wallner and Machatschke, 2009) and may equally be responsible for the increased aggressive behavior found in chronic asthma patients who receive a holistic medical treatment characterized by both CS medication and dietary fat intake reduction. Finally, the indication that asthmatic boys seem to be more aggressive fits perfectly into the sexual dimorphic hypothesis about the evolutionary origin of violent behavior in humans (Wallner and Machatschke, 2009).

Fight or flight? Effects of vaginal oestrus on cortisol, testosterone, and behaviour in guinea pig female-female interaction

It is accepted that social stress in relation to confrontation and competition can elicit behavioural and hormonal changes in social mammals. These effects have, however, been less frequently studied among female-female interactions. In the present study female-female confrontation experiments were carried out to monitor socio-positive and agonistic behaviour by controlling for the oestrus cycles of 12 individuals. Additionally, plasma cortisol (CORT) and testosterone (T) levels were determined before and after the experiments. During non-oestrus conditions a significant increase in CORT levels from pre- to post confrontation was registered and females spent more time to sit side by side. During vaginal oestrus the confrontation experiments revealed avoiding of a conspecific female by showing increased flight behaviour. However, during that period no changes in CORT levels were found. But, a non-significant increase in T was measured from pre- to post confrontation in both cycle phases, while no differences in the display of aggressive behaviours were found. These findings indicate considerable influences of different oestrus cycle phases on social stress-induced CORT secretion and the modulation of socio-positive and agonistic behaviour in female guinea pigs.