Research reportLocomotor response to novelty correlates with differences in number and morphology of hypothalamic tyrosine hydroxylase positive cells in rats

- Number and morphology of hypothalamic TH+ cells differed with locomotor activity.
- The number of TH+ cells positively correlated with higher locomotor activity.
- HR rats had higher TH+ number cells particularly in ARC and Pe.
- Morphology of A11-A15 cells was more diverse in HRs (in comparison to MRs and LRs).

Individual differences in the intensity of locomotor response to a new environment (exploratory reaction) are generally used as a model to study individual vulnerability to stress and drug addiction. In the present work we studied the number, distribution and morphology of the hypothalamic cells expressing tyrosine hydroxylase (TH+ cells) (immunohistochemical and immunofluorescent staining) in male Wistar rats divided based on high (HR), midline (MR) or low (LR) locomotor activity in response to novelty. Morphology and total number of TH+ cells were analyzed for A11-A15 dopaminergic groups. We found correlation between the total number of hypothalamic TH+ cells in the whole A11-A15 area and the locomotor activity. The differences were most pronounced in some of the hypothalamic nuclei, i.e. in the rostro-caudal extension of the A11, A12 and A14 structures, where the HR rats had a significantly higher number of TH+ cells in comparison to the MR and LR rats.Morphology analysis of TH+ cells showed HR/MR/LR differences in single cell area and perimeter and, to a lesser extent, in the other morphometric parameters such as length of the major and minor axes, or circularity factor.The results suggest that the behavioral traits which characterize the HR animals and are correlated with increased susceptibility to stress and propensity to develop drug addictions can be determined by the number, distribution, activity and perhaps the morphology of the cells in the dopaminergic systems.