Altered mesoaccumbens and nigro-striatal dopamine physiology is associated with stereotypy development in a non-rodent species

Stress-induced changes in mesoaccumbens dopamine neurophysiology have been associated with the development of stereotypic behaviour in in-bred strains of laboratory rodents. This experiment evaluated whether similar changes are associated with environmentally-induced stereotypic behaviour in a higher-vertebrate species, the horse. D1- and D2-like dopamine receptor densities (Bmax) and dissociation constants (Kd) were measured in control (n = 9) and stereotypy (n = 9) horses in the nucleus accumbens, caudate nucleus, putamen, substantia nigra and ventral tegmentum brain regions. Results revealed that stereotypy horses had significantly higher (P < 0.05) dopamine D1 and D2 receptor densities (Bmax) in the nucleus accumbens compared to non-stereotypy controls. D1 receptor densities (Bmax) and D2 receptor affinity (Kd) were also significantly lower in the caudate nucleus brain region of stereotypy horses (P < 0.05). No other significant results were observed. These results demonstrate that stereotypy horses have increased activity within the mesoaccumbens dopamine pathway and, thus, that the development of environmentally-induced stereotypy may be associated with changes in motivational systems within the animal.