MOOD

The "mood triangle" often used to depict how three major neurotransmitters interact in determining mood. While applied most frequently to humans, pharmacological studies and veterinary practice indicates that some of the same principles apply to many, perhaps most, mammals, although the exact behavioral effects can vary dramatically (Overall 2000). These same neurotransmitters are present in the brains of other animals, such as birds, but their role in regulating behavior is less well understood in non-mammals.

Serotonin is the most commonly manipulated neurotransmitter in pharmacological interventions in animals. Selective Serotonin Reuptake Inhibitors (SSRI's), which are commonly used as antidepressants in humans, are effective in moderating aggression in dogs. Clomimaprine, an older drug which also affects serotonin-based behavioral responses is commonly used in dogs to treat anxiety. In humans, clomipramine has been used to treat depression and obsessive compulsive disorder, although it (and other tricyclics) have largely been replaced by SSRI's in the treatment of human depression. There is thought that sometimesaggression in dogs may be a response to anxiety; if this is true, this would explain the efficicacy of serotonin-related treatments on aggression (eg, Overall and Dunham 2002). In mice, the expression of "anxiety" is higher in strains which have been genetically selected to be non-aggressive (Nyberg et al. 2003). While this anxiety/aggression relationship is the opposite of that suggested for dogs, the existence of a correlation between these two factors does imply a similarity in how these behaviors are controlled in the two species.

Dopamine is most commonly thought of in the context of Parkinson's disease in humans. Parkinson's, a progressive loss of muscular control due to death of the cells in the brain which produce dopamine, can be counteracted to a certain extent by treatment with compounds that supplement brain dopamine, such as L-dopa. Because of its link to Parkinson's, physicians treating humans are justifiably wary of prescribing drugs which lower dopamine levels, even though dopamine is known to also modulate mood and behavior. An exception is made for patients with severe pychoses (hallucinations, loss of contact with reality, such as hearing voices); antipsychotics such as clozapine bind the dopamine receptor, effectively reducing dopamine concentrations. In dogs, selegiline hydrochloride, a drug developed as a Parkinson's treatment for humans, may enhance training (Mills and Ledger 2001).

GABA is most strongly associated with wakefullness/sleepiness. Hypnotic sleep aids such as Ambien activate GABA receptors, imitating the effect of increased GABA concentrations. Low GABA may also be associated with depression. In animals, low GABA levels are, generally, associated with aggression, and treatments which increase GABA (or bind GABA receptors, mimicking the effect of GABA) lower aggression (Miczek et al. 2003).

Miczek KA, Fish EW, De Bold JF 2003 Neurosteroids, GABA(A) receptors, and escalated aggressive behavior HORMONES AND BEHAVIOR 44 (3): 242-257
Mills D, Ledger R 2001 The effects of oral selegiline hydrochloride on learning and training in the dog: A psychobiological interpretation PROGRESS IN NEURO-PSYCHOPHARMACOLOGY & BIOLOGICAL PSYCHIATRY25 (8): 1597-1613
Nyberg, J. M. O.Vekovischeva, N. K. Sandnabba 2003 Anxiety Profiles of Mice Selectively Bred for Intermale Aggression Behavior Genetics 33:503-511
Overall KL 2000 Natural animal models of human psychiatric conditions: Assessment of mechanism and validity PROGRESS IN NEURO-PSYCHOPHARMACOLOGY & BIOLOGICAL PSYCHIATRY 24 (5): 727-776
Overall KL, Dunham AE 2002 Clinical features and outcome in dogs and cats with obsessive-compulsive disorder: 126 cases (1989-2000). JOURNAL OF THE AMERICAN VETERINARY MEDICAL ASSOCIATION. 221 (10): 1445-1452