Veterinary professionals regularly have to treat dogs with behavioural problems associated with anxiety and stress. Within this, the monoamine neurotransmitters – including serotonin and dopamine – play a pivotal role in raising mood and in the regulation of arousal states. Indeed, much focus has been placed on raising serotonin levels to reduce anxiety and arousal in dogs. However, serotonin is involved in the reward mechanism of the brain as well as the release of dopamine, which reduces stress and is key to the feeling of well-being, motivation and happiness, so the complete reward cascade must be considered when managing behaviour.
Neurophysiological basis of learning and behaviour
Learning involves the strengthening of the connections between those neural circuits that detect a particular stimulus and the neural circuits that produce a particular response. An inability to experience reinforcing events, either positive or negative, will lead to reduced learning capabilities and associated behavioural problems. A combination of genetic and environmental factors can affect an animal’s ability to cope with novel situations and learning tasks. The secretion of hormones such as corticosteroids through dysregulation of the hypothalamic–pituitary–adrenal axis in response to prolonged stress can reduce brain levels of serotonin, explaining some abnormal behaviour patterns sometimes seen in dogs.
An inability to experience reinforcing events, either positive or negative, will lead to reduced learning capabilities and associated behavioural problems
If the neurophysiological basis of learning processes is impaired, for example by a lack of availability of dietary precursors to key neurotransmitters or their availability and function in emotional systems, then the ability and capacity for learning new and alternative behaviours will be compromised.
When the brain’s reinforcement mechanisms are impaired, then the ability to experience reinforcing events will be reduced and learning affected accordingly. In fact, if an individual has a biochemical inability to derive reward from ordinary, everyday activities, behavioural problems such as addictive, compulsive or impulse control disorders may result.
The role of dopamine and serotonin
Low levels of serotonin have been implicated in increased pain sensitivity, indicating a link with the touch sensitivity seen in a number of diet-related canine behaviour problems
Dopamine is the primary neurotransmitter of reward in the limbic system, but at least three other neurotransmitters are known to be involved: serotonin, the enkephalins and gamma-aminobutyric acid. ln a normal individual, these neurotransmitters work together in a cascade of excitation or inhibition leading to a feeling of well-being, which is the ultimate reward. A disruption of these intracellular interactions results in anger, anxiety and other “negative feelings”, or in a craving for substances or activities which alleviate these negative emotions.
Low levels of serotonin have been implicated in increased pain sensitivity, indicating a link with the touch sensitivity seen in a number of diet-related canine behaviour problems. Serotonin, gamma-aminobutyric acid, glutamate, dopamine and opioid systems have been shown to be involved in mediating positive reinforcement systems. A deficiency in the reward cascade causing lowered levels of serotonin can lead to negative states, such as depressed mood, dysphoria, irritability and impulsive behaviour via a reduction of the behavioural inhibition system during learning.
The reward cascade
The reward cascade (Figure 1) begins with the excitatory activity of serotonin-releasing neurons in the hypothalamus. This then causes the release of the opioid peptide met-enkephalin in the ventral tegmental area, which inhibits the activity of neurons that release the inhibitory neurotransmitter gamma-aminobutyric acid. The disinhibition of dopamine-containing neurons in the ventral tegmental area allows them to release dopamine in the nucleus accumbens and in certain parts of the hippocampus, completing the cascade and mediating reinforcement effects.
Irrelevant behaviour patterns, displacement activities or coping strategies such as scratching or barking may be exhibited in response to the frustration of intermittent reinforcement. Inadequate or ineffective reinforcement may elicit adjunctive behaviours such as drinking or redirected aggression or a hyperactivity state caused by the arousal of the noradrenergic pathways.
Thus, the success of behaviour modification must follow a holistic approach to ensure that all relevant neurotransmitters involved in the reward cascade are available in sufficient levels for new learning to take place.