Differences in serotonin serum concentration between aggressive English cocker spaniels and aggressive dogs of other breeds

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1 Journal of Veterinary Behavior (2013) 8, RESEARCH Differences in serotonin serum concentration between aggressive English cocker spaniels and aggressive dogs of other breeds Marta Amat, Susana Le Brech, Tomàs Camps, Carlos Torrente, Valentina M. Mariotti, José L. Ruiz, Xavier Manteca School of Veterinary Science, Universitat Autònoma de Barcelona, Bellaterra, Cerdanyola del Vallès, Barcelona, Spain. KEYWORDS: aggression; behavior; dog; English cocker spaniel; serotonin Abstract Aggression is one of the most common behavioral problems in dogs and may have important negative effects on public health, human animal bond, and animal welfare. There is ample evidence showing a negative correlation between serum serotonin concentration and aggressive behavior in a variety of species, including the domestic dogs. This negative correlation is particularly pronounced in dogs that show impulsive aggression. Data obtained in some previous studies suggest that the English cocker spaniel (ECS) is more likely to show impulsive aggression than other breeds. Therefore, the aim of this study was to analyze possible differences in serum serotonin levels between aggressive ECS and aggressive dogs of other breeds. Nineteen ECSs presented for aggression at the Animal Behavior Service (School of Veterinary Science, Barcelona, Spain) were evaluated and compared with 20 aggressive dogs of other breeds attended at the same center. Serum serotonin levels were measured using an enzyme-linked immunosorbent assay method. Statistical analysis was done using the SPSS 15.0 for Windows. Aggressive ECSs had significantly (P, 0.001) lower levels of serum serotonin than aggressive dogs of other breeds ( and ng/ml, respectively). Variances were not significantly different between ECSs and other breeds (standard deviation ng/ml vs ng/ml, P. 0.05). This finding may explain why ECSs are more likely to show impulsive aggression than other breeds, and suggests that the ECS could be a good model to study the neurophysiologic mechanisms underlying impulsive aggression. Ó 2013 Elsevier Inc. All rights reserved. Introduction Dog aggressive behavior has important negative effects on the welfare of dogs and on public health (Goldstein, 1992; Sacks et al., 1996; Palacio et al., 2005; De Keuster et al., Address for reprint requests and correspondence: Marta Amat, DVM, MSc, PhD, Dip. ECVBM-CA, School of Veterinary Science, Universitat Autònoma de Barcelona, Bellaterra, Cerdanyola del Vallès 08193, Barcelona, Spain; Tel: ; Fax: Marta.amat@uab.es 2006). For instance, in the United States, the treatment of the physical and psychological effects of dog bites costs between 30 and 100 million US dollars each year (Berzon et al., 1972; Matter and Arbeitsgemeinschaft, 1998; Weiss et al., 1998; Borud and Friedman, 2000). In fact, bite accidents are one of the main threats that domestic dogs pose to human health (Overall, 1997; De Keuster et al., 2006). In contrast, behavior problems are one of the main reasons for euthanasia and relinquishment of healthy dogs to animal shelters (Reisner et al., 1994; Edney, 1998; Salman et al., 1998). In addition, because /$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. doi: /j.jveb

2 20 Journal of Veterinary Behavior, Vol 8, No 1, January/February 2013 stress causes an activation of the adrenocortical stress response, most aggression problems have a negative effect on the welfare of dogs (Moberg, 2000; Kruk et al., 2004; Stafford, 2007). Aggressive behavior can be divided into affective (offensive and defensive aggression) and nonaffective aggression (predatory aggression). Affective aggression is associated with a marked autonomic activation and involves neurotransmitters such as serotonin, dopamine, noradrenalin, acetylcholine, and g-amino butyric acid (Dodman and Shuster, 1994), whereas nonaffective aggression in controlled by acetylcholine mainly and does not involve autonomic activation (Beaver, 2006). In addition, the posture of the dog, the brain areas that control the aggressive response, and the function of aggression are different between the 2 types of aggression (Moyer, 1968). There is strong evidence that breeds differ in their behavior (Takeuchi and Houpt, 2004; Scott and Fuller, 1965; Hart, 1995; Svartberg, 2006; Duffy et al., 2008). For example, Scott and Fuller (1965) found significant differences in all behavior traits under study between 5 breeds that were raised in the same environment. Lately, Hart and Hart compared the behavior of 56 breeds based on expert opinion and concluded that breeds differ in all behavior traits, although the breed effect was more pronounced for some traits than for others (Hart and Hart, 1985). With respect to aggressive behavior, there are many studies that suggest that some breeds may be more prone to show aggression problems than others (Overall, 1997; Sacks et al., 2000; Guy et al., 2001; Fatjó et al., 2007). For instance, English cocker spaniels (ECSs) are presented to the Animal Behavior Service of the School of Veterinary Science in Barcelona (Spain) owing to aggressive behavior more frequently than any other breed and more frequently than expected according to their relative abundance in the population (Fatjó et al., 2007). Other authors have obtained similar results (Lund et al., 1996; Overall, 1997; Bradshaw and Goodwin, 1998; Svartberg, 2006; Takeuchi and Mori, 2006). However, a large individual variation exists within breeds, and preventive programs should be based on individuals rather than breeds themselves (Hart and Hart, 1985). There are several studies showing an association between low levels of serotonin and aggressiveness in different species (Brown et al., 1979; Da Prada et al., 1988; Mehlman et al., 1995; Virkkunen et al., 1995; Reisner et al., 1996; Berman and Coccaro, 1998 (revision); Peremans et al., 2003; Miczek and Fish, 2006; Cakiroglu et al., 2007; Rosado et al., 2010). For instance, in rodents, the reduction of brain serotonin activity increases some forms of aggression (Vergnes et al., 1986), whereas pharmacological manipulations that increase serotonin activity reduce aggression (Nelson and Chiavegatto, 2001). Nevertheless, other authors observed a positive correlation between cerebrospinal fluid levels of 5-hydroxyindoleacetic acid (5-HIAA, the main serotonin metabolite) and aggression (Van der Vegt et al., 2003). In nonhuman primates, some studies indicate a negative correlation between cerebrospinal fluid 5-HIAA levels and aggression (Higley et al., 1992; Westergaard et al., 2003). A similar link between serotonin levels and aggression has been reported in dogs. For example, in one study, dogs with dominance-type aggression had lower levels of the main metabolite of serotonin in the central nervous system than nonaggressive dogs (Reisner et al., 1996). Similarly, a group of dogs with owner-directed aggression had lower levels of serum serotonin than a control group without aggression problems (Cakiroglu et al., 2007; Rosado et al., 2010). In addition, treatment with drugs that increase the serotonin levels reduces aggressive behavior (Coccaro et al., 1990; Fava et al., 1996; Ferris et al., 1999; León et al., 2006), and diets with low levels of tryptophan, which is the precursor of serotonin, cause an increase in aggressive behavior (Young, 1991). The aim of this study was to ascertain whether the differences in aggressive behavior between ECSs and other breeds could be related to lower levels of serotonin. Material and methods Nineteen ECSs attended for aggression (toward family members, unfamiliar people, and other dogs) at the Animal Behavior Service of the School of Veterinary Science in Barcelona (Spain) were compared with 20 aggressive dogs of other breeds presented at the same center. All dogs were considered healthy based on physical examination, serum biochemistry, Thyroxine (T4) and Thyroid-stimulating hormone (TSH) evaluation, and complete blood count. Dogs included in the study were not receiving pharmacological treatment. A 5-mL blood sample was collected from the jugular vein of each dog into anticoagulant-free tubes. Samples were centrifuged at 4,500! g for 15 minutes. Serum was frozen and stored at 280 C until its analysis. Serotonin ELISA kit (DLD Diagnostika GMBH, Hamburg, Germany) was used to measure serotonin levels in serum (Chauveau et al., 1991). A c 2 test was used to assess the association between experimental groups and serotonin levels. A value of P, 0.05 was considered significant for all analyses. Data were analyzed using statistical software for Windows (SPSS version 15.0, SPSS Inc., Chicago, IL, December 18, 2006). Results In the ECS group, there were 7 male (85.71 % of which were intact) and 12 female dogs (50% of which were intact). The average age was 9.5 years. In the non-ecs group, there were 13 male (84.61 % of which were intact) and 7 female dogs (all were intact). In this group, we included 11 purebred and 9 crossbred dogs.

3 Amat et al Differences in serum serotonin concentration 21 Table Serum serotonin concentration in the treatment group and in the control group Dog Group Breed Sex Neutered status Serotonin (ng/ml) 1 Treatment ECS M Intact Treatment ECS M Intact Treatment ECS F Neutered Treatment ECS F Neutered Treatment ECS F Intact Treatment ECS F Neutered Treatment ECS F Intact Treatment ECS F Neutered Treatment ECS F Intact Treatment ECS F Intact Treatment ECS M Neutered Treatment ECS M Intact Treatment ECS F Intact Treatment ECS F Intact Treatment ECS M Intact Treatment ECS F Neutered Treatment ECS F Neutered Treatment ECS M Intact Treatment ECS M Intact Control Labrador F Intact Control Crossbred M Intact Control Jack Russell terrier M Intact Control Crossbred F Intact Control Crossbred M Neutered Control Boxer F Intact Control Crossbred F Intact Control Labrador M Intact Control Bouvier M Intact Control Crossbred M Neutered Control Beagle F Intact 1, Control Argentine mastiff M Intact 1, Control Crossbred F Intact 1, Control Crossbred M Intact 1, Control Bull terrier M Intact Control Golden retriever M Intact 1, Control Pit bull terrier M Intact 1, Control Crossbred M Intact Control Catalan sheepdog M Intact Control Crossbred F Intact M, male; F, female; ECS, English cocker spaniel. Breeds included in this group are detailed in the Table. The average age of the non-ecs dogs was 5 years. Regarding the type, all were diagnosed with affective aggression. The serum serotonin levels are given in the Table. The mean serum serotonin level in the ECSs was (standard error ) ng/ml. The mean serum serotonin level in the non-ecs dogs was (standard error ) ng/ml. Thus, aggressive ECSs had significantly lower levels of serum serotonin than aggressive dogs of other breeds (P, 0.05). The standard deviations in the ECS group and in the non-ecs group were and ng/ml, respectively, and variances were not significantly different between both groups (P. 0.05). Discussion Dog behavior is a complex trait influenced by genetic, epigenetic, and environmental factors (Brain, 2000; Nelson, 2006), and all these may be partly responsible for the differences found in our study. It has been suggested that aggressive behavior in humans could be related to polymorphisms in genes involved in the transport, synthesis, release, or metabolism of serotonin (Courtet et al., 2001; Sukonick et al., 2001). For instance, a polymorphism in one of the most important enzymes in the metabolism of serotonin (monoamine oxidase A) is associated with abnormal impulsive behavior,

4 22 Journal of Veterinary Behavior, Vol 8, No 1, January/February 2013 including aggression (Brunner et al., 1993). In laboratory animals, the absence of 5-hydroxytryptamine receptors 1B (5-HT 1B receptors), one of the main receptors involved in aggression behavior, is linked to impulsiveness (Bouwknecht et al., 2001). In dogs, recent studies have identified polymorphisms that could explain behavioral differences between breeds (Niimi et al., 1999; Takeuchi et al., 2005), although thus far, the polymorphisms studied are not related to serotonin, but to other neurotransmitters such as dopamine (Niimi et al., 1999). However, the possibility exists that putative differences in serotonin levels between breeds, as shown in our study, are caused by genetic factors. Epigenetic factors may also be involved in neurophysiological differences within a given species. For instance, several studies in laboratory rodents have found that the tactile stimulation provided by the mother during the first few days after birth caused by maternal licking causes an increase in glucocorticoid receptor expression in the hippocampus (Liu et al., 1997; Meaney, 2001; Weaver et al., 2004; Holmes et al., 2005) and an increase in serotonin levels in the central nervous system owing to epigenetic effects (Meaney et al., 2000; Matthews et al., 2001). Therefore, different mothering styles within a given species could cause differences in serotonin levels in the offspring. Although further research is needed to see whether dog breeds, in general, and ECSs, in particular, differ in their maternal behavior, breed differences in the expression of maternal behavior has been observed in other species (Dwyer and Lawrence, 1998). Breed differences could also be due to the fact that dogs of the same breed share a similar environment, including owner personality, husbandry, or training. In fact, some authors have observed some resemblance in the owner personality of aggressive ECSs (Peachey, 1993; Podberscek and Serpell, 1997). Additionally, some handling aspects might have an important influence on dog behavior. For example, dog training methods based on punishment increase aggression problems (Roll and Unshelm, 1997; Hiby et al., 2004; Blackwell et al., 2007; Herron et al., 2009). Also, owners of smaller dogs are more inconsistent than owners of larger dogs in the sense that they reward some behaviors in some cases but not always (Arhant et al., 2010). Because the stress response is influenced by the predictability and controllability of events (Weiss, 1970; Beerda et al., 1998; Schalke et al., 2007) and there is a clear link between stress and aggression (Kruk et al., 2004), inconsistent handling could be related to increased aggressive behavior (Arhant et al., 2008). Therefore, it would be helpful to investigate whether the owners of ECSs are more inconsistent and use punishment training methods more frequently than the owners of other breeds. Possible differences between ECSs and other breeds in the amount of physical exercise and their diet could also explain differences in serotonin levels, although studies on this particular issue are lacking. For example, some studies in laboratory rodents and humans suggest that physical exercise increases serotonin levels (Chaouloff, 1997) and that low levels of exercise correlate with aggressive behavior (Jagoe and Serpell, 1996; Tsatsoulis and Fountoulakis, 2006); however, other studies have failed to find such an effect (Acworth et al., 1986; Hoffmann et al., 1994). Preliminary findings suggest that aggressive dogs exercise less than nonaggressive dogs (Mariotti et al., 2009). Lowprotein and high-carbohydrate diets increase the amount of tryptophan that enters the brain and, consequently, the synthesis of serotonin (Benton, 2002). Further studies evaluating the possible differences in physical exercise and diet between breeds could be useful. ECSs are obtained more frequently from pet shops that dogs of other breeds (Amat et al., 2009), perhaps owing to the popularity of this breed (Serpell and Jagoe, 1995). Dogs obtained from pet shops have more behavioral problems than those obtained from other sources, probably owing to the early weaning and a stressful environment in the pet shop (Serpell and Jagoe, 1995). Regarding the neutering status, all female dogs included in the non-ecs group were intact. Nevertheless, although female sex hormones may have an effect on some types of affective aggression (Kim et al., 2006), there is no report on any link between female sex hormones and serotonin level or impulsivity. Finally, an effect of age cannot be ruled out because in our study, the average ages of the dogs in the ECS group and the non-ecs group were different. In humans, the levels of serotonin increase during development in children and decline before puberty (Chugani et al., 2001), and there is an inverse correlation between serotonin levels and age (Seifert et al., 2004). Therefore, it would be useful to analyze whether the serotonin levels of ECSs change during their development. Low levels of serotonin are correlated with impulsive aggression (Soubrié, 1986; Virkkunen and Linnoila, 1993; Mehlman et al., 1994; Higley et al., 1996; Kavoussi et al., 1997; Peremans et al., 2003). Studies in human and nonhuman primates show that low levels of serotonin are correlated with impulsive aggression rather than with overall aggression (Coccaro, 1989; Virkkunen and Linnoila, 1993; Higley, 2003). The term impulsiveness has been defined as a reduction or complete lack of warning signals previous to an attack (Peremans et al., 2003) or as the tendency to respond prematurely without adequate foresight (Robbins and Crockett, 2010). Studies in laboratory rats indicate that decreasing serotonergic activity intensifies impulsiveness, whereas increasing serotonergic activity enhances the preferences for delayed rewards (Manuck et al., 2003). A previous study found that ECSs show impulsiveness more frequently than other purebred dogs (Amat et al., 2009), and impulsive aggression is one of the main risk factors for euthanasia of aggressive dogs (Reisner et al., 1994). Violent impulsive humans have lower levels of 5-HIAA in cerebrospinal fluid than violent individuals who had committed a crime

5 Amat et al Differences in serum serotonin concentration 23 with premeditation (Virkkunen et al., 1995). In dogs, impulsive individuals have lower levels of 5-HIAA in cerebrospinal fluid than nonimpulsive ones (Reisner et al., 1996; Quadros et al., 2010), and there is a lower activity of serotonin receptors in some areas of brain in impulsive dogs compared with nonimpulsive ones (Peremans et al., 2003; Vermeire et al., 2011). Conclusions According to these results, lower levels of serotonin could influence aggressive behavior. However, further research is needed to find out the precise role of serotonin on aggressive behavior in this particular breed. References Acworth, I., Nicholass, J., Morgan, B., Newsholme, E.A., Effect of sustained exercise on concentrations of plasma aromatic and branched-chain amino acids and brain amines. Biochem. Biophys. Res. Commun. 137, Amat, M., Manteca, X., Mariotti, V., Ruiz de la Torre, J.L., Fatjó, J., Aggressive behavior in the English Cocker Spaniel. J. Vet. Behav.: Clin. Appl. Res. 4, Arhant, C., Troxler, J., Mittmann, A., Are problem behaviours in urban pet dogs related to inappropriate use of positive punishment? In: Boyle, L., O Connel, N., Hanlon, A. (Eds.), Proceedings of the 42nd Congress of the ISAE Applied Ethology Addressing Future Challenges in Animal Agriculture. Wageningen Academic Publishers, Wageningen, p Arhant, C., Bubna-Littitz, H., Bartels, A., Futschik, A., Troxler, J., Behaviour of smaller and larger dogs: effects of training methods, inconsistency of owner behaviour and level of engagement in activities with the dog. Appl. Anim. Behav. Sci. 123, Beaver, B.V., Canine Behavior Insights and Answers, 2nd Ed. Saunders Elsevier, St. Louis, MO, pp Beerda, B., Schilder, M.B.H., van Hooff, J.A.R.A.M., de Vries, H.W., Mol, J.A., Behavioural, saliva cortisol and heart rate responses to different types of stimuli in dogs. Appl. Anim. Behav. Sci. 58, Benton, B., Carbohydrate ingestion, blood glucose and mood [review]. Neurosci. Biobehav. Rev. 26, Berman, M.E., Coccaro, E.F., Neurobiologic correlates of violence: relevance to criminal responsibility. Behav. Sci. Law 16, Berzon, D.R., Farber, R.E., Gordon, J., Kelley, E.B., Animal bites in a large city-a report on Baltimore, Maryland. Am. J. Public Health 62, Blackwell, E.J., Twells, C., Seawright, A., Casey, R.A., The relationship between training methods and the occurrence of behaviour problems in a population of domestic dogs. Proceedings of the 6th International Veterinary Behaviour Meeting. Fondazione Iniziative Zooprofilattiche e Zootecniche, Brescia, Italy, pp Borud, L.J., Friedman, D.W., Dog bites in New York City. Plast. Reconstr. Surg. 106, Bouwknecht, J.A., Hijzen, T.H., van der Gugten, J., Maes, R.A., Hen, R., Olivier, B., Absence of 5-HT(1B) receptors is associated with impaired impulse control in male 5-HT(1B) knockout mice. Biol. Psych. 49, Bradshaw, J.W.S., Goodwin, D., Determination of behavioral traits of pure-bred dogs using factor analysis and cluster analysis; a comparison of studies in the USA and UK. Res. Vet. Sci. 66, Brain, P.F., The Biology of Aggression. In: Biological Psychiatry. JAI Press Inc., Stamford, pp Brown, G.L., Goodwin, F.K., Goyer, P.F., Major, L.F., Aggression in humans correlates with cerebrospinal fluid amine metabolites. Psychiatry Res. 1, Brunner, H.G., Nelen, M., Breakefield, X.O., Ropers, H.H., van Oost, B.A., Abnormal behavior associated with a point mutation in the structural gene for monoamine oxidase A. Science 262, Cakiroglu, D., Meral, Y., Sancak, A.A., Cifti, G., Relationship between the serum concentrations of serotonin and lipids and aggression in dogs. Vet. Rec. 161, Chaouloff, F., Effects of acute physical exercise on central serotonergic systems. Med. Sci. Sports Exerc. 29, Chauveau, J., Fert, V., Morel, A.M., Delaage, M.A., Rapid and specific enzyme immunoassay of serotonin. Clin. Chem. 37, Chugani, D.C., Muzik, O., Behen, M., Rothermel, R., Janisse, J.J., Lee, J., Chugani, H.T., Developmental changes in brain serotonin synthesis capacity in autistic and nonautistic children. Ann. Neurol. 45, Coccaro, E.F., Astill, J.L., Herbert, J.L., Schut, A.G., Fluoxetine treatment of impulsive aggression in DSM-III-R personality disorder patients. J. Clin. Psychopharmacol. 10, Coccaro, E.F., Central serotonin and impulsive aggression. Br. J. Psychiatry 8, Courtet, P., Baud, P., Abbar, M., Boulenger, J.P., Castelnau, D., Mouthon, D., Malafosse, A., Buresi, C., Association between violent suicidal behavior and the low activity allele of the serotonin transporter gene. Mol. Psychiatry 6, Da Prada, M., Cesura, A.M., Launay, J.M., Richards, J.G., Platelets as a model for neurones? Experientia 44, De Keuster, T., Lamoureux, L., Kahn, A., Epidemiology of dog bites: a Belgian experience of canine behaviour and public health concerns. Vet. J. 172, Dodman, N.H., Shuster, L., Pharmacologic approaches to managing behavior problems in small animals. Vet. Med. 89, 960. Duffy, D.L., Hsu, H., Serpell, J., Breed differences in canine aggression. Appl. Anim. Behav. Sci. 114, Dwyer, C.M., Lawrence, A.B., Variability in the expression of maternal behaviour in primiparous sheep. Appl. Anim. Behav. Sci. 58, Edney, A.T.B., Reasons for euthanasia of dogs and cats. Vet. Rec. 143, 114. Fatjó, J., Amat, M., Mariotti, V.M., De la Torre, J.L., Manteca, X., Analysis of 1040 cases of canine aggression in a referral practice in Spain. J. Vet. Behav.: Clin. Appl. Res. 2, Fava, M., Alpert, J., Nierenberga, A., Ghaemi, N., O Sullivan, R., Tedlow, J., Worthington, J., Rosenbaum, J.F., Fluoxetine treatment of anger attacks: a replication study. Ann. Clin. Psychiatry 8, Ferris, C.F., Stolberg, T., Delville, Y., Serotonin regulation of aggressive behavior in male golden hamsters (Mesocricetus auratus). Behav. Neurosci. 113, Goldstein, E.J., Bite wounds and infection. Clin. Infect. Dis. 14, Guy, N.C., Luescher, U.A., Dohoo, S.E., Spangler, E., Miller, J.B., Dohoo, I.R., Bate, L.A., Risk factors for dog bites to owners in a general veterinary case load. Appl. Anim. Behav. Sci. 74, Hart, B.L., Hart, L.A., Selecting pet dogs on the basis of cluster analysis of breed behavior profiles and gender. J. Am. Vet. Med. Assoc. 186, Hart, B.L., Analysing breed and gender differences in behaviour. In: Serpell, J.N. (Ed.), The Domestic Dog. Its Evolution, Behavior and Interactions with People. Cambridge University Press, Cambridge, pp Herron, M.E., Shofer, F.S., Reisner, I.R., Survey of the use and outcome of confrontational and non-confrontational training methods in client owned dogs showing undesired behaviours. Appl. Anim. Behav. Sci. 117,

6 24 Journal of Veterinary Behavior, Vol 8, No 1, January/February 2013 Hiby, E.F., Rooney, N.J., Bradshaw, J.W.S., Dog training methodsd their use, effectiveness and interaction with behaviour and welfare. Anim. Welf. 13, Higley, J.D., Mehlman, P.T., Taub, D.M., Higley, S.B., Vickers, J.H., Suomi, S.J., Linnoila, M., Cerebrospinal fluid monoamine and adrenal correlates of aggression in free-ranging rhesus monkeys. Arch. Gen. Psychiatry 49, Higley, J.D., Mehlman, P.T., Poland, R.E., Taub, D.M., Vickers, J., Suomi, S.J., Linnoila, M., CSF testosterone and 5-HIAA correlate with different types of aggressive behaviors. Biol. Psychiatry 40, Higley, J.D., Aggression. In: Maestripieri, D. (Ed.), Primate Psychology. Harvard University Press, Cambridge, MA, pp Hoffmann, P., Elam, M., Thoren, P., Hjorth, S., Effects of longlasting voluntary running on cerebral levels of dopamine, serotonin and their metabolites in the spontaneously hypertensive rat. Life Sci. 54, Holmes, A., Guisquet, A.M., Vogel, E., Millstein, R.A., Leman, S., Belzung, C., Early life genetic, epigenetic and environmental factors shaping emotionality in rodents. Neurosci. Biobehav. Rev. 29, Jagoe, J.A., Serpell, J.A., Owner characteristics and interactions and the prevalence of canine behaviour problems. Appl. Anim. Behav. Sci. 47, Kavoussi, R., Armstead, P., Coccaro, E., The neurobiology of impulsive aggression. Psychiatr. Clin. North Am. 20, Kim, H.H., Yeon, S.C., Houpt, K.A., Lee, H.C., Chang, H.H., Lee, H.J., Effects of ovariohysterectomy on reactivity in German Shepherd dogs. Vet. J. 172, Kruk, M.R., Meelis, W., Halász, J., Haller, J., Fast positive feedback between the adrenocortical stress response and a brain mechanism involved in aggressive behaviour. Behav. Neurosci 118, León, M.A., Palacio, J.L., García-Belenguer, S.L., Agresividad Canina: Epidemiología en la Comunidad Valenciana e Implicación de los Sistemas Serotoninérgicos [Tesis doctoral]. Valencia, Spain, Departamento de Medicina y Cirugía Animal, Universidad Cardenal Herrera CEU. Liu, D., Diorio, J., Tannenbaum, B., Caldji, C., Francis, D., Freedman, A., Sharma, S., Pearson, D., Plotsky, P.M., Meaney, M.J., Maternal care, hippocampal glucocorticoid receptors, and hypothalamic pituitary adrenal responses to stress. Science 277, Lund, J.D., Agger, J.F., Vestergaard, K.S., Reported behaviour problems in pet dogs in Denmark: age distribution and influence of breed and gender. Prev. Vet. Med. 28, Manuck, S.B., Flory, J.D., Muldoon, M.F., Ferrell, R.E., A neurobiology of intertemporal choice. In: Loewenstein, D., Read, D., Baumeister, R. (Eds.), Time and Decision: Economic and Psychological Perspective on Intertemporal Choice. Sage, New York, pp Mariotti, V.M., Amat, M., Ruíz De la Torre, J.L., Baucells, M.D., Fatjó, J., Manteca, X., Management and environmental influences on owner-directed aggression in dogs. In: ACVB/AVSAB Scientific Meeting; July 10, 2009; Seattle, WA. Matthews, K., Dalley, J.W., Matthews, C., Tsai, T.H., Robbins, T.W., Periodic maternal separation of neonatal rats produces region- and gender-specific effects on biogenic amine content in postmortem adult brain. Synapse 40, Matter, H.C., Arbeitsgemeinschaft, S., The epidemiology of bite and scratch injuries by vertebrate animals in Switzerland. Eur. J. Epidemiol. 14, Meaney, M.J., Diorio, J., Francis, D., Weaver, S., Yau, J., Chapman, K., Seckl, J.R., Postnatal handling increases the expression of camp inducible transcription factors in the rat hippocampus: the effects of thyroid hormones and serotonin. J. Neurosci. 20, Meaney, M.J., Maternal care, gene expression, and the transmission of individual differences in stress reactivity across generations. Annu. Rev. Neurosci. 24, Mehlman, P.T., Higley, J.D., Faucher, I., Lilly, A.A., Taub, D.M., Vickers, J., Low CSF 5-HIAA concentrations and severe aggression and impair impulsive control in nonhuman primates. Am. J. Psychiatry 151, Mehlman, P.T., Higley, J.D., Faucher, I., Lilly, A.A., Taub, D.M., Vickers, J., Suomi, S.J., Linnoila, M., Correlation of CSF 5-HIAA concentration with sociality and the timing of emigration in free- ranging primates. Am. J. Psychiatry 152, Miczek, K.A., Fish, E.W., Monoamines, GABA, glutamate, and aggression. In: Nelson, R.J. (Ed.), Biology of Aggression. Oxford University Press, Inc., UK, pp Moberg, G.P., Biological response to stress: implications for animal welfare. In: Moberg, G.P., Mench, J.A. (Eds.), The Biology of Animal Stress. Basic Principles and Implications for Animal Welfare. CABI Publishing, Oxfordshire, UK, pp Moyer, K.E., Kinds of aggression and their physiological basis. Commun. Behav. Biol. 2(Part A), Nelson, R.J., Chiavegatto, S., Molecular basis of aggression. Trends Neurosci. 24, Nelson, R.J., Biology of Aggression. Oxford University Press, Inc, UK. Niimi, Y., Inoue-Murayama, M., Murayama, Y., Ito, S., Iwasaki, T., Allelic variation of the D4 dopamine receptor polymorphic region in two dog breeds, Golden retriever and Shiba. J. Vet. Med. Sci. 61, Overall, K.L., Clinical Behavioral Medicine for Small Animals. Mosby, St. Louis, MO, pp Palacio, J., León, M., García-Belenguer, S., Aspectos epidemiológicos de las mordeduras caninas. Gac. Sanit. 19, Peachey, E., Problems with people. In: Fisher, J. (Ed.), The Behavior of Dogs and Cats. Stanley Paul, London, pp Peremans, K., Audenaert, K., Coopman, F., Blanckaert, P., Jacobs, F., Otte, A., Verschooten, F., van Bree, H., van Heeringen, K., Mertens, J., Slegers, G., Dierckx, R., Estimates of regional cerebral blood flow and 5-HT2A receptor density in impulsive, aggressive dogs with 99mTc-ECD and 123I-5-I-R Eur. J. Nucl. Med. Mol. Imaging 30, Podberscek, A.L., Serpell, J.A., Aggressive behavior in English cocker spaniels and the personality of their owners. Vet. Rec. 141, Quadros, I.M., Takahashi, A., Miczek, K.A., Serotonin and aggression. In: Müller, C.P., Jacobs, B.L. (Eds.), Handbook of Behavioral Neuroscience. Elsevier, London, pp Reisner, I., Erb, H.N., Houpt, K.A., Risk factors fort behaviorrelated euthanasia among dominant-aggressive dogs: 110 cases ( ). J. Am. Vet. Med. Assoc. 205, Reisner, I.R., Mann, J.J., Stanley, M., Huang, Y., Houpt, K.A., Comparison of cerebrospinal fluid monoamine levels in dominantaggressive and non-aggressive dogs. Brain Res. 714, Robbins, T.W., Crockett, M.J., Role of central serotonin in impulsivity and compulsivity: comparative studies in experimental animals and humans. In: Müller, C., Jacobs, B. (Eds.), Handbook of Behavioral Neuroscience. Elsevier, Netherlands, pp Roll, A., Unshelm, J., Aggressive conflicts amongst dogs and factors affecting them. Appl. Anim. Behav. Sci. 52, Rosado, B., García-Belenguer, S., León, M., Chacón, G., Villegas, A., Palacio, J., Blood concentrations of serotonin, cortisol and dehydroepiandrosterone in aggressive dogs. Appl. Anim. Behav. Sci. 123, Sacks, J.J., Kresnow, M., Houston, B., Dog bites: how big a problem? Inj. Prev. 2, Sacks, J.J., Sinclair, L., Gilchrist, J., Golab, G.C., Lockwood, R., Breed of dogs involved in fatal human attacks in The United States between 1979 and J. Am. Vet. Assoc. 217, Salman, M.D., New Jr., J.G., Scarlett, J.M., Kass, P.H., Ruch-Gallie, R., Hetts, S., Human and animal factors related to the relinquishment of dogs and cats to 12 selected animal shelters in the United States. J. App. Anim. Welf. Sci. 1,

7 Amat et al Differences in serum serotonin concentration 25 Seifert, W.E., Fox, J.L., Butler, I.J., Age effect on dopamine and serotonin metabolite levels in cerebrospinal fluid. Ann. Neurol. 8, Serpell, J., Jagoe, J.A., Early experience and the development of behaviour. In: The Domestic Dog, its Evolution, Behaviour and Interaction with People. Cambridge University Press, Cambridge, pp Scott, J.P., Fuller, J.L., The critical period. In: Genetics and the Social Behavior of the Dog. The Classic Study. The University of Chicago Press, Chicago and London, pp Schalke, E., Stichnoth, J., Ott, S., Jones-Baade, R., Clinical signs caused by the use of electric training collars on dogs in everyday life situations. Appl. Anim. Behav. Sci. 105, Soubrié, P., Reconciling the role of central serotonin neurons in human and animal behavior. Behav. Brain Sci. 9, Stafford, K.J., The Welfare of Dogs. Springer, The Netherlands, pp Sukonick, D.L., Pollock, B.G., Sweet, R.A., Mulsant, B.H., Rosen, J., Klunk, W.E., Kastango, K.B., DeKosky, S.T., Ferrell, R.E., The 5-HTTPR*S/*L polymorphism and aggressive behavior in Alzheimer disease. Arch. Neurol. 58, Svartberg, K., Breed-typical behavior in dogsdfistorical remnants or recent constructs? Appl. Anim. Behav. Sci. 96, Takeuchi, Y., Houpt, K.A., Behavior genetics. Clin. Tech. Small Anim. Pract. 19, Takeuchi, Y., Hashizume, C., Chon, E.M., Momozawa, Y., Masuda, K., Kikusui, T., Mori, Y., Canine tyrosine hydroxylase (TH) gene and dopamine beta -hydroxylase (DBH) gene: their sequences, genetic polymorphisms, and diversities among five different dog breeds. J. Vet. Med. Sci. 67, Takeuchi, Y., Mori, Y., A comparison of the behavioral profiles of purebred dogs in Japan to profiles of those in the United States and the United Kingdom. J. Vet. Med. Sci. 68, Tsatsoulis, A., Fountoulakis, S., The protective role of exercise on stress system dysregulation and comorbidities. Ann. NY Acad. Sci. 1083, Van der Vegt, B.J., Lieuwes, N., Cremers, T.I.F.H., de Boer, S.F., Koolhaas, J.M., Cerebrospinal fluid monoamine and metabolite concentrations and aggression in rats. Horm. Behav 44, Vergnes, M., Depaulis, A., Boehrer, A., Parachlorophenylalanineinduced serotonin depletion increases offensive but not defensive aggression in male rats. Physiol. Behav 36, Vermeire, S., Audenaert, K., De Meester, R., Vandermeulen, E., Waelbers, T., De Spiegeleer, B., Eersels, J., Peremans, K., Neuro-imaging the serotonin 2A receptor as a valid biomarker for canine behavioural disorders. Res. Vet. Sci. 91, Virkkunen, M., Linnoila, M., Brain serotonin, type II alcoholism and impulsive violence. J. Stud. Alcohol 11, Virkkunen, M., Goldman, D., Nielsen, D.A., Linnoila, M., Low brain serotonin turnover rate (low CSF 5-HIAA) and impulsive violence. J. Psychiatry Neurosci. 20, Weaver, I.C., Cervoni, N., Champagne, F.A., D Alesio, A.C., Sharma, S., Seckl, J.R., Dymov, S., Szyf, M., Meaney, M.J., Epigenetic programming by maternal behaviour. Nat. Neurosci. 7, Weiss, J.M., Somatic effects of predictable and unpredictable shock. Psychosom. Med. 32, Weiss, H.B., Friedman, D.I., Coben, J.H., Incidence of dog bite injuries treated in emergency departments. JAMA 279, Westergaard, G.C., Suomi, S.J., Chavanne, T.J., Houser, L., Hurley, A., Cleveland, A., Snoy, P.J., Higley, J.D., Physiological correlates of aggression and impulsivity in free-ranging female primates. Neuropsychopharmacology 28, Young, S.N., Some effects of dietary components (amino acids, carbohydrate, folic acid) on brain serotonin synthesis, mood and behaviour. Can. J. Physiol. Pharmacol 69,