Distribution of tyrosine hydroxylase positive cells in different subregions of the VTA in morphine-dependent rats

Dopamine neurons originating in the VTA are fundamental in the pathophysiology of opiate dependence. In particular, abnormal structural plasticity linked to the long-lasting consequences of opiate addiction has been observed and previous work has shown that DA-containing neurons 'shrink' after morphine-withdrawal (Spiga et al., 2003) but the number of TH-positive cells has not been investigated. Importantly, it has been suggested that the VTA displays a functional regional heterogeneity (Margolis et al., 2010; Moore 2009; Satoshi 2007). We then investigated the possible anatomical reflections of these findings in morphine withdrawn rats. Male Sprague-Dawley albino rats were used. Morphine hydrochloride was administered twice daily for 14 days. Rats were divided in four groups: controls (CTRL), chronically treated with saline; chronically treated with morphine (CH-M), spontaneous morphine withdrawal (S-W); naloxone-induced morphine withdrawal (NLX-W). Coronal Slices were stained whit tyrosine hydroxylase (TH) immunofluorescence for confocal microscopy. We considered the entire VTA (from -4.8mm to 6.8mm from bregma) and counted TH positive cells at 300 µm intervals in coronal sections, rostro-caudally. The distribution of stained neurons, in CH-M rats in each VTA sub-regions, shows no statistical differences vs CTRL group. On the contrary S-W and NLX-W groups show a reduction of marked neurons of ~50% in anterior and an increase by more than 100% in posterior VTA. These opposite and selective changes, observed in the two sub regions of the VTA, might reflect different degrees of activation and/or activity of DAaergic neurons. Therefore, the supposed decreased activity in anterior VTA is consistent with previous reports that support the hypodopaminergic hypothesis in the N. Accumbens, the major target of DA afferents, while the increase in number of TH-positive cells in posterior VTA suggests that another brain area, the prefrontal cortex, may be involved, with an increase of DA, in behavioral (cognitive) abnormalities of opiate dependence