ANXIETY RELATED BEHAVIOURAL DISORDERS.pdf

Anxiety related behavioural disorders and

neurotransmitters in dogs

Jacopo Riva a,*, Gianpietro Bondiolotti b, Manuela Michelazzi a,Marina Verga a, Corrado Carenzi a

a Department of Animal Science, Faculty of Veterinary Medicine, University of Milan, via Celoria 10, 20133 Milan, Italyb Department of Pharmacology, Chemotherapy and Toxicology, Faculty of Medicine and Surgery,

University of Milan, via Vanvitelli 32, 20133 Milan, Italy

Accepted 30 January 2008

Available online 28 March 2008

Abstract

The aim of this research has been to verify possible correlations among some neurotransmitters and

anxiety related behavioural disorders in the domestic dog.

Twenty dogs affected by anxiety related behavioural disorders and 13 control dogs were studied. The

dogs’ behaviour has been analysed through a detailed history case. Blood samples have been analysed by

HPLC method to evaluate the plasma levels of dopamine (DA), norepinephrine (NE), serotonin (5-HT), 5-

hydroxyindolacetic acid (5-HIAA), 3,4-diidroxyfenilacetic acid (DOPAC), L-DOPA and platelets levels of

NE, DA and 5-HT in the two groups.

Plasma levels of DA and 5-HT are significantly higher in the anxious dogs with respect to the control

ones (DA: ng/mg prot. 0.074 � 0.071 vs. 0.030 � 0.001, [P < 0.01] and 5HT: ng/mg prot. 24.95 � 36.64

vs. 5.94 � 3.20, [P < 0.01]). The plasma levels of the other neurotransmitters are similar in the two groups.

Platelet levels of NE (ng/mg prot. 0.133 � 0.047 vs. 0.124 � 0.061) and DA (ng/mg prot. 0.0552 � 0.018

vs. 0.074 � 0.039) are also similar in the two groups. A trend to higher 5-HT platelet levels (ng/mg prot.

509 � 100 vs. 425 � 117) in the control dogs compared to the anxious ones has been found.

# 2008 Elsevier B.V. All rights reserved.

Keywords: Anxiety; Dogs; Aggression; Serotonin; Neurotransmitters; Dopamine

1. Introduction

Correlations among behavioural problems and neurotransmitters, especially plasma and

platelets concentrations of serotonin (5-HT), dopamine (DA) and norepinephrine (NE), have

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Applied Animal Behaviour Science 114 (2008) 168–181

* Corresponding author. Tel.: +39 02 50318028; fax: +39 02 50318030.

E-mail address: [email protected] (J. Riva).

0168-1591/$ – see front matter # 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.applanim.2008.01.020

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been found in some species, such as rats, rabbits, humans and dogs (Higley et al., 1992, 1996a;

Gray, 1982; Rogeness et al., 1992; Reisner et al., 1996).

5-HTand catecholamines are involved in the regulation of several behavioural systems that play

an important role in the interaction of the organism with its external environment. DA appears to be

involved in the expression of active behavioural patterns, including aggression, sexual behaviour

(Haney et al., 1990; Rogeness et al., 1984) and locomotion (Kiley-Worthington, 1983). On the

contrary behavioural inhibition of affective aggression seems to be regulated by 5-HT (Spoont,

1992). Moreover, in defensive aggression the threshold for agonistic behaviour is lowered by the

release of catecholamines (Higley et al., 1992). Dynamic changes in central 5-HT levels are thought

to be involved in the establishment and maintenance of dominant attitudes and behaviour (Raleigh

et al., 1980, 1985). These interactions have been found to be a common denominator in a very broad

body of literature (Mann, 1995). For example the correlation between 5-HT and aggressive

behaviour has been experimentally obtained inducing shortages of cerebral 5-HT by lesions of the

serotoninergic cells of rafe in laboratory rats: animals were becoming irritable, reacting in an

exaggerated way to light stimulations (Miczek and Donat, 1989). Other studies highlighted this

correlation: mice that respond to isolation by increased inter-male fighting show reduced 5-HT

function (Valzelli and Garattini, 1968). Similar conclusions were reached by researches on

dominance and aggressiveness in non-human primates where low serotonin levels have been

indicated (Higley et al., 1996b). The results are often contradictory; actually some authors have

shown that increased 5HT was correlated to increased aggression (Brunner et al., 1993; Birmaher

et al., 1990). In dogs, a research on CFS (Cerebro-Spinal-Fluid) monoamine metabolites showed

that CSF 5-HIAA concentration (5-hydroxyindolacetic acid is a metabolite of 5-HTand an indirect

measure of central 5-HT concentration) was lower in dogs diagnosed with dominance related

aggression than in the control ones (Reisner et al., 1996) furthermore a research on nuclear imaging

technology showed that 5HT-2a receptors was significantly higher in the cortical regions in dogs

diagnosed with aggressive behaviour, compared to normal dogs (Peremans et al., 2006).

Moreover, dopaminergic and serotoninergic systems play an important role in the

neurotrasmission system involved in stress response (Puglisi-Allegra and Cabib, 1990; Le

Moal and Simon, 1991).

The knowledge of the involvement of NE in aggressive behaviour has been possible thanks to

the availability of brain tissues of animals which had just fought (Welch and Welch, 1965;

Hendley et al., 1973). An increase in NE has been discovered in the brain of aggressive mice

which had been previously held in a state of isolation (Modigh, 1973); increases in noradrenergic

turnover in the brain of cats or rats were shown in association to biting (Salama and Goldberg,

1973; Reis and Fuxe, 1969).

The aim of this research was to find out the possible differences in some neurotransmitters

(DA, NE, 5-HT, 5-HIAA, L-Dopa and DOPAC) in dogs suffering from anxiety related

behavioural disorders compared to control dogs, not suffering from any behavioural disorder.

2. Materials and methods

2.1. Subjects

The research has been carried out on 20 dogs suffering from anxiety related behavioural disorders and 13

control dogs not suffering from behaviour problems. The 20 problem dogs had been presented at the

University of Milan, Faculty of Veterinary Medicine, Behaviour Clinic and diagnosed as suffering from

J. Riva et al. / Applied Animal Behaviour Science 114 (2008) 168–181 169

anxiety related disorders after taking case. The behaviours of anxious dogs have been described by their

owners during the behaviour visit. The analysis carried out by the therapist has been done through the

description of the owner, the direct observation in the clinic and the results of the questionnaire. In this way

the therapist has correctly carried out different diagnoses, in order to separate the dogs in the two groups

which have been analysed in this study. The control group did not show any disorder problem. The diagnosis

of anxiety related behavioural disorders was done according to the literature models (Overall, 1997; Beaver,

1983). The behavioural problems included: generalised anxiety, separation anxiety, phobias, compulsive

behaviours and anxiety related aggression. The 13 control dogs were randomly chosen and diagnosed as not

suffering from any behavioural problem according to the same history case.

The dogs age range was 8 month to 9 years of age. Breeds represented in anxious dogs included 3

Labrador Retrievers, 2 Doberman Pinschers, 2 German Shepherds, 3 Giant Schnauzers and one of each of

the following breeds: Brittany Spaniel, Akita-inu (Federation Cynologique Internationale standard), English

Cocker Spaniel, Pitbull, English bulldog and Weimaraner. Four mixed-breed dogs were also included.

Breeds represented in the control group included 2 Boxers, 2 Rottweilers and one of each of the following

breeds: Mastiff, Pitbull, German Shepherd, Labrador Retriever. Five mixed-bred dogs were also included.

Sex represented in anxious dogs included 10 intact males, 6 neutered males, 3 intact females and 1 neutered

female. Sex represented in controls dogs included 6 intact males, 3 neutered males, 4 neutered females and

no intact female.

No dog was taking any drug.

2.2. History case recording

A case history has been completed for each subject, recording the following information on the dog and

on the living environment:

(A) individual features: age of adoption, place of origin (rescue kennel, pure breed kennel, private

breeders);

(B) environmental features: kind of family (single, couple, a numerous family, children), house physical

characteristics, number of walks and length of time, active level when walked, walked on leash or not,

sleeping site, food at disposal, presence of other animals;

(C) behavioural features: escaping from physical handling, submission postures toward the owner, not

accepting to be alone, digging holes, destroying objects, showing aggressiveness signals and/or biting

the owners or strangers or other dogs, not allowing the food to be taken away, asking for physical

contact, emotional induced soiling, supine position, coming back in response to recalls, accepting food

or objects or bone to be taken away, continuously following the owner through the house, reacting to

blaming, house-soiling, chasing other animals, excessively barking, pica (which means the ingestion of

non-nutritive, non-food items, such as fabric, plastics, sticks and rocks) (Overall, 1997), coprophagy,

compulsive behaviours, phobia of crackers (phobia is defined as a profound and quickly developed fear

reaction that does not extinguish with gradual exposure to the object or firework or thunderstorm, or

with exposure over time) (Overall, 1997), even if other authors described systematic desensitization

phobia treatment (Mills, 2002).

2.3. Statistical analysis of behaviours

Answers to the single questions in the case history have been scored and analysed through descriptive

statistical analysis and Chi Square tests. The seldom appearing variables have not been considered in the

analysis. The significance was accepted when p < 0.05.

In order to evaluate the interdependence among the different behaviours a principal component analysis

(PCA with Varimax rotation), has been carried out on the most often exhibited behaviours of the dogs in the

two groups (SPSS, 2003). PCAwas used in order to point out the relationships among the different variables.

PCA was considered a suitable method to treat this data set, as it presents a number of advantages. First of

J. Riva et al. / Applied Animal Behaviour Science 114 (2008) 168–181170

all, it is a statistical method which condenses into a few latent variables the information contained in many

original variables. Secondly, it offers the possibility of using discrete variables as well as continuous

variables, as it is independent from data distribution; furthermore, the use of PCA with binary variables

allows for qualitative considerations (Jolliffe, 1986; Jackson, 1991; Mattiello et al., 1997).

2.4. Blood analyses

Plasma and platelet concentration levels of DA, NE, 5-HT, 5-HIAA, DOPAC and L-Dopa have been

evaluated in the two groups, according to the methodology described below.

Five ml of blood have been taken with addition of 10% in volume of EDTA (disodiumethylene-

diaminotetraacetate) already diluted to 1%. The blood has been centrifuged to 120 � g (15 min at

20 8C); afterwards the plasma has been centrifuged to 2700 � g (15 min at 4 8C) (Da Prada and Picotti,

1979; Bondiolotti et al., 1987). The platelet pellet was washed with saline and analysed with the HPLC

method.

Monoamines and their derivatives were detected and measured using liquid chromatography with

electrochemical detection. For the analysis of the catecholamines, platelet pellets were homogenized with

0.3N perchloric acid and plasma deproteinized (1/1 v/v) with 0.6N perchloric acid containing dihydrox-

ybenzylamine as the internal standard. Catecholamines were extracted onto alumina before the injection.

For the analysis of 5-HTand 5-HIAA, a volume of 100 ml of plasma was deproteinized with an equal volume

of 0.6N perchloric acid containing 3-metoxytyramine as internal standard, 50 ml of supernantant was then

injected in the chromatograph. Evaluation of chromatographed products was accomplished by coulometric

detection.

HPLC separation was performed on a reverse-phase analytical column as described elsewhere (Lollis

et al., 1979; Alleva et al., 1998). The protein content of platelets were assayed using a microassay (Lowry

et al., 1951) with bovine serum albumin as a standard.

Platelet levels of DA, NE and 5-HT could be evaluated only on 5 anxious and in 5 control dogs, due to

problems of haemolysis and of difficult separation between plasma and platelets.

2.5. Statistical analysis of blood variables

Plasma and platelet levels have been statistically analysed according to the data distribution, whether

normal or not. Actually plasma DA and 5-HT have been analysed through Mann-Whitney U-test while L-

DOPA, DOPAC, and 5-HIAA have been analysed through Student T-test.

3. Results

3.1. Individual environmental and behavioural data

3.1.1. Individual features

3.1.1.1. Age of adoption. 77% of control dogs were adopted in the correct period between 60

and 90 days of age (Overall, 1997), while in the anxious group, late adoptions were more frequent

(P < 0.05). In fact 40% of these latter dogs had been adopted after 3 months of age.

3.1.1.2. Place of origin. A statistically significant difference (P < 0.05) has been found

according to the dog’s origin: in fact 35% of anxious subjects had come from a kennel or had been

found by chance, being abandoned, vs. 0% in control. This result agrees with the literature

(Beerda et al., 1999), showing that abandoned or kenneled dogs show behavioural problems more

frequently.


3.1.2. Environmental features

3.1.2.1. Kind of family. More problem dogs than the control ones (50% vs. 10%; P < 0.05) live

with a couple as against living in a family with children. In anxiety related behavioural disorders

the family dynamics may play a relevant role, which sometimes might interfere also with the

behaviour treatment plans (Dodman et al., 1996; Dehasse, 1997). Moreover sometimes a dog

may be treated by owners like a human baby.

3.1.2.2. Sleeping site. A statistically significant difference (P < 0.005) has been found as far as

the dog sleeping place is concerned. In fact 45% of anxious dogs sleep on the bed/sofa, 45% in a

dog basket and 10% in other places in the house compared with all the control group dogs

sleeping in the dog basket. This result is important because the dog basket must represent a

relaxing and protective place for the dog.

3.1.2.3. Food at disposal. Dogs in the control group have no food always at disposal; on the

contrary 35% of the anxious dogs have continuously food at disposal (P < 0.05). The food

distribution through a series of regulars and rhythmical acts allows to obtain a clear hierarchy

between the dog and its owner. Actually dogs that are pushed in a rhythm by their owner’s habits,

may get stressed when this rhythm is broken and their expectations frustrated (Horwitz, 2002).

Beagles living in individual kennels with dry food freely available tend to eat three times a day, at

dawn, at dusk and whenever fresh food is given (Rashotte et al., 1984).

3.1.3. Behaviour

Statistically significant differences have been found on the following dogs’ behaviours.

3.1.3.1. Escaping from physical handling. More anxious dogs (60%) do not accept to be

handled by the owner compared to the control ones (P < 0.05), where only one subject refused to

be handled. This behaviour could be due to wrong rank in the social group hierarchy (Rowell,

1974), leading to ambiguous and unclear dominance-submission relationship between the dog

and the owner, and consequently raising anxiety in the dog. In fact in a dog pack usually members

which most frequently display aggression are the middle rank, rather than the high rank ones

(Lockwood, 1979).

3.1.3.2. Submission posture towards the owner. 90% of control dogs do not show this

behaviour; on the contrary, in the anxious group 77% of the subjects show it (P < 0.001). Visual

canine communication is conventionally described in terms of the signals performed by the wolf

during dominance/submission interactions within the pack (Goodwin et al., 1997). Submission

posture may be displayed in response to a threatening challenge inducing fear and avoidance.

Not accepting to be alone, digging holes, destruction, excessively barking.

High statistically significant differences have been found between anxious and control dogs on

all these anxiety related behaviours (not accepting to be alone 55% in anxious dogs vs. 0% in control

dogs; digging holes 65% in anxious dogs vs. 0% in control dogs; destruction 75% in anxious dogs

vs. 10% in control dogs; excessively barking 65% in anxious dogs vs. 0% in control dogs)

(P < 0.001). These are the most common distress behaviours seen in dogs showing separation

anxiety (McCrave, 1991; Lund and Jorgensen, 1997; Simpson, 2000), although sometimes

destructiveness may be an element of play or exploratory behaviour in young active animals

(Simpson, 2000). Also vocalisation is common and may be due to outside stimuli, social facilitation

with other dogs, territorial displays or play (Horwitz, 1998). However both destructiveness and


barking become problem behaviours when performed excessively, as shown in anxiety suffering

dogs, mainly in the ones suffering from separation anxiety (Overall, 1997; Houpt et al., 1996).

3.1.3.3. Showing aggressiveness – biting owners or strangers. Only the anxious dogs display

aggressive behaviours (85%) towards the owner or towards strangers (P < 0.01). According to

Reisner (2002) anxiety ‘may play an important part in the genesis of aggression’, leading to

dangerous attacks also towards human beings. Moreover, aggressive behaviour may appear as a

consequence of affective emotional arousal (Reis, 1974).

3.1.3.4. Phobia of crackers – pica – compulsive behaviours. Statistically significant differences

have been found between anxious and control dogs also on these anxiety related behaviours

(phobia of cracker 60% in anxious dogs vs. 0% in control dogs; pica 45% in anxious dogs vs. 0%

in control dogs; compulsive behaviours 45% in anxious dogs vs. 0% in control dogs) (P < 0.05

for fear of crackers; P < 0.01 for pica and compulsive behaviours).

Noise phobias may be among the most commonly recognized and exhibited phobic responses

in dogs, though few substantive data exist. As with many fearful, phobic and anxious responses,

noise phobias are characterized by the exhibition of non-specific signs when the dog is presented

with the stimulus or is anticipating the stimulus (Overall, 1997, Overall et al., 2001). Pica may be

exhibited on particular chosen objects in a compulsive way, thus showing a form of compulsive

disorder especially when the dog disregards other activities in favour of ‘‘stones chewing’’ or

similar activities (Overall, 1997).

The results of the Principal Component Analysis (PCA) performed on the behaviours are

shown in Table 1 and Fig. 1, which include the loadings of the variables explaining the 49.513%

of the total variance on the 1st and 2nd component.

PCA revealed two underlying components. The first component (PC1) shows positive

loadings for the following behaviours: escaping from physical handling, destructiveness, digging

holes and excessively barking and a negative loading for not accepting to be alone (Table 1). The

second component (PC2) shows positive loadings for the following variables: fear of crackers,

pica, compulsive behaviours and a negative loading for the submission posture. Thus the 1st

component identifies the generalised anxiety and separation anxiety problems, while the 2nd

component identifies the anxiety related phobias and compulsive disorders. Each dog has been

plotted on the 2 principal components in Fig. 2. The control dogs are grouped opposite to the


Table 1

Component variance and cumulative variance explained by the 1st and 2nd component

Variables Components

EH: escaping from physical handling 0.653 0.299

SP: submission posture �0.281 S0.686

NA: not accepting to be alone S0.797 �0.068

SA: show aggressiveness �0.292 0.033

DT: destruction 0.657 0.396

DH: digging holes 0.552 0.294

EB: excessively barking 0.776 0.162

FC: phobia of cracker 0.282 0.587

PC: pica 0.096 0.790

CB: compulsive behaviours �0.036 0.802

Variance (%) 26.666 22.847

Cumulative (%) 26.666 49.513

anxious dogs, particularly for factor 1, thus showing that the two groups are well divided

according to the variables included into the PCA analysis.

3.2. Neurotransmitters

3.2.1. Results in plasma

The plasma concentration values of NE, DA and 5-HT found in this research agree with the

levels shown in the literature by some authors in dogs: in Table 2 the reference average

concentration values of these neurotransmitters in some species, including the dog, are shown, as

well as in Table 3 the average levels of all the neurotransmitters found in this study. In plasma no

reference on dogs have been found as far as 5-HIAA, DOPAC and L-Dopa are concerned.

Statistically significant differences (P < 0.01) have been found between anxious and control

dogs on DA and 5-HT; the anxious dogs showing higher DA and 5-HT plasmatic values

compared to the control dogs (Table 3). However, the levels of the other studied neurotransmitters

do not show any significant difference between the two groups.

3.3. Results in platelets

The platelet levels of 5-HT found in this research also agree with the levels shown in literature

by some authors in dogs (Clagett et al., 1981, 1987; Mezzano et al., 1991). No reference value has


Fig. 1. Projection of the loadings of the behavioural variables considered for the first and second principal component.

See Table 1 for the legend.


Fig. 2. Plots of scores (anxious and controls dogs) on the first and second principal components. Some subjects in the

control group are overlapping.

Table 2

Plasma concentration values (�standard deviation) in different species of norepinephrine (NE), dopamine (DA) and

serotonin (5-HT) according to the literature

NE (ng/ml) DA (ng/ml) References 5-HT (ng/ml) References

Human 0.198 � 0.01 0.064 � 0.01 Da Prada and Picotti (1979) 4.7 � 0.6 Benedict et al. (1986)

Rabbit 0.392 � 0.3 0.216 � 0.02 Da Prada and Picotti (1979) 6.69 � 1.70 Da Prada and Picotti (1979)

Dog 0.451 � 0.046 0.044 � 0.004 Roche et al. (2002) 171 � 14.6 Da Prada and Picotti (1979)

Mice 8.75 � 0.15 Yamamoto et al. (1986)

Table 3

Blood variables mean plasma values (�standard deviation) found in this study

NE (ng/ml) L-DOPA (ng/ml) DOPAC (ng/ml) DA (ng/ml) 5-HT (ng/ml) 5-HIAA (ng/ml)

Controls 0.256 � 0.087a 1.630 � 0.587a 0.610 � 0.192a 0.030 � 0.001a 5.94 � 3.20a 5.59 � 1.59a

Anxious 0.229 � 0.078a 2.041 � 0.890a 0.767 � 0.309a 0.074 � 0.071b 24.95 � 36.64b 6.68 � 4.13b

Note: a, b: different superscripts within the same column indicate statistically significant differences (P < 0.01).

been found in the literature on DA and NE platelet values in dogs: in Table 4 the reference levels

of DA-NE in some species, and 5-HT in the dog are shown, for comparison with the average

concentration values of platelet DA, NE and 5-HT found in this study (Table 5). The platelet DA

and NE values are similar in the anxious compared to the control dogs, while a trend to higher

values of 5-HT has been found in control dogs.

Peripheral measures of 5-HTactivity may reflect central 5-HT functioning despite the fact that

the major portion of 5-HT in the periphery is derived from the gut (Coccaro et al., 1997).

4. Discussion

5-HT seem to inhibit affective aggression, while DA seems to enhance it (Spoont, 1992;

Dodman and Shuster, 1998). Increased DA and indices of DA synthesis and turnover in the whole

brain of laboratory mice that had just fought have been shown (Modigh, 1973), especially in

nucleus accumbens, striatum, frontal cortex and hypothalamus (Haney et al., 1990; Tizabi et al.,

1979; Hutchins et al., 1975). Moreover, plasma DA levels increase together with the sympathetic

activation (Van Loon, 1983). In children showing high affective aggressiveness and

impulsiveness a reduction of the serotonergic function in the brain and lower concentrations

of 5-HIAA in the cerebrospinal fluid (CSF) have been found (Tuinier et al., 1995). Serotoninergic

mechanisms have been shown to operate in the acquisition and maintenance of dominance in

primates. Engaging in and winning dominance related struggles in some monkeys, generate high

blood levels of serotonin (Raleigh et al., 1991a). In our research, DA and 5-HT higher plasma

levels in anxious dogs compared to the control ones suggest that also in dogs a relationship exists

between these two neurotransmitters and more frequent exhibition of anxiety related behaviours.

However a question remains to be solved, that is the possibility that plasma levels of the

evaluated neurotransmitters may reflect what is happening at the brain level. According to some

authors, this is true (Da Prada et al., 1988; Coccaro et al., 1997). Yan et al. (1993) found a

statistically significant relationship between serotoninergic measures (including 5-HIAA) in CSF


Table 4

Platelet concentration values (�standard deviation) of norepinephrine (NA), dopamine (DA) and serotonin (5-HT) in

different species according to the literature

NE (ng/mg) DA (ng/mg) References 5-HT (ng/mg) References

Human 0.241 � 0.057 0.012 � 0.001 Picotti et al. (1984) 139 � 61.6 Maurer-Spurej et al. (2004)

227 � 55 Sagud et al. (2007)

Mice 0.181 � 0.043 0.041 � 0.009 Picotti et al. (1984) 211 Da Prada and Picotti (1979)

Guinea pig 0.263 � 0.023 0.035 � 0.009 Picotti et al. (1984)

Rabbit 0.698 � 0.13 0.186 � 0.127 Picotti et al. (1984) 9680 Da Prada and Picotti (1979)

Dog 956 � 126 Clagett et al. (1981)

796 � 103 Clagett et al. (1987)

626 � 196 Mezzano et al. (1991)

Table 5

Platelets amine concentration mean values (�standard deviation) found in this study

NE (ng/mg) DA (ng/mg) 5-HT (ng/mg)

Controls 0.133 � 0.047 0.0552 � 0.018 509 � 100

Anxious 0.124 � 0.061 0.074 � 0.039 425 � 117

and blood in sub-human primates, indicating that measurements in blood may be used as a

peripheral indicator of central 5-HT functioning (Stahl et al., 1982). No indications are up to date

at disposal on dogs in the literature.

Blood concentrations of catecholamines in many species such as humans beings, cats, mice

and rabbits (Da Prada and Picotti, 1979; Picotti et al., 1984) are higher in the platelets compared

to in plasma. Some factors affect the distribution of 5-HT and DA in platelets and plasma

(Pletscher, 1968). In vitro findings on rat’s platelets show that 5-HT and DA compete for the same

active transport mechanism (carrier) but 5-HT has a higher affinity than DA for this transport

process (Gordon and Olverman, 1978). In vivo this competition is likely to occur, since it has

been shown both in humans and in rabbits that the concentrations measured in plasma were

higher for 5-HT than for DA (Da Prada and Picotti, 1979). Moreover, the respective neuronal and

platelet proteins have been shown to be encoded, both for 5-HT and for DA, by the same genes

(Cook et al., 1994; Ramamoorthy et al., 1993). These factors may explain why different

concentrations of DA and 5-HT are found between platelets and plasma.

As far as the relationships between blood and platelet neurotransmitters concentrations are

concerned, our study suggests that also in dogs anxiety related behavioural disorders may be

enhanced by high DA and 5-HT levels in plasma, and inhibited by low 5-HT concentrations in

platelets. According to Dodman and Shuster (1998) the threshold for aggression may be raised also

by dopaminergic antagonists, being aggressive behaviour increased by the release of

catecholamines (Eichelman, 1987). The important role of catecholamines in the flight or fight

stress response, where the limbic system is involved (Henry and Stephens, 1977) is well known.

Moreover, DA may be involved also in compulsive behavioural disorders. The treatment with

dopamine antagonists has reduced some compulsive behaviours in cats and humans (Cools and Van

Rossum, 1970; Stein and Hollander, 1992) as well the Tourette-like self-injurious behaviour in

horses (Dodman and Shuster, 1998) and dopamine agonist-induced stereotypic grooming and self-

mutilation in rats (Hartgraves and Randall, 1986). High blood levels of 5-HT have been revealed in

relation with affective aggressive behaviour for example in vervet monkeys, showing the

involvement of serotonin dynamics in this stress response (Raleigh et al., 1980, 1985, 1991a,b).

The involvement of the 5-HT with aggressive behaviour has been experimentally proven,

inducing shortages of brain 5-HT (Garattini et al., 1967; Salama and Goldberg, 1973). A few

researches on aggressive behaviour together with reduced brain 5-HT concentration have

highlighted this correlation in mice after isolation, revealing an increase of fights between males

in response to the treatment (Valzelli and Bernasconi, 1979). Reisner et al. (1996) studied the

relationship between cerebrospinal fluid (CSF) monoamine metabolite levels in dominant-

aggressive and non-aggressive dogs. Post-mortem CSF 5-HIAA levels were measured by HPLC:

concentrations of 5-HIAA (202.0 pmol/ml) were lower in the aggressive group than in the control

one (298.0 pmol/ml). This study suggests that reduced serotonergic function is associated with

aggressive behaviour and impaired impulse control in dogs and that serotonin may modulate

aggressive behaviour in mammals.

As far as the relationship between platelet 5-HT and disturbed behaviours is concerned, 5-HT

platelet content was reported to be lower in human adult depressed patients who attempted

suicide than in depressed patients who did not (Mann et al., 1992a,b). Platelet serotonin content

was positively correlated also with a lifetime history of aggression and current hostility in

humans (Pliszka et al., 1988). According to Dodman and Shuster (1998), anxiety related

aggressive behaviour may be decreased enhancing central serotoninergic activity. We also have

found a trend to lower platelet 5-HT levels in anxious dogs compared to the control ones, thus

suggesting too an involvement of serotoninergic function in anxiety related behaviours in dogs.


5. Conclusion

This study shows that 5-HT plasma and DA are higher in the anxious dogs than in the control

ones, while NE plasma levels are similar in the two groups. Another important result of this study

is related to the platelet concentrations of 5-HT, showing a trend to lower concentration levels in

anxious dogs compared to the control ones.

This result could demonstrate that also in dogs the serotoninergic and dopaminergic systems

play an important role in determining affective reactions such as the exhibition of anxiety related

behaviour problems. This hypothesis raised by the present research needs to be confirmed on a

larger sample of dogs possibly comparing the platelet 5-HT levels to the 5-HT levels at the CNS

level.

Acknowledgments

We wish to thank Dr. Michela Minero for the support in data statistical analysis, Dr.

Alessandra Trotti and Prof. Elena Zenoni for the English language final check.

Tanks to The European Social Fund, Ministero del Lavoro e delle Politiche Sociali, Regione

Lombardia and Ingenio to partially funding this work.

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