Ipsilateral alterations in tryptophan hydroxylase activity in rat brain after hypothalamic 5,7-di-hydroxytryptamine lesion

Abstract

The in vivo relationship between the amounts of tryptophan hydroxylase (TPH) protein and its intrinsic synthetic activity, measured by quantifying the amounts of α-[ 3H]methyl-5-hydroxytryptamine (α-[ 3H]M5-HT), is reported in cell body and terminal areas of intact and disturbed serotonergic neurons following a unilateral 5,7-dihydroxytryptamine (5,7-DHT) lesion of the dorsolateral hypothalamus. Five days after the lesion, the relationships between TPH and its synthetic product 5-HT were evaluated on adjacent brain sections in serotonergic cells bodies of the dorsal raphe nucleus (DRN) and nerve fibres of the medial forebrain bundle (MFB). On the side contralateral to the lesion, TPH and α-[ 3H]M5-HT levels in the intact hemi-DRN exhibited a caudo-rostral distribution and were positively and significantly correlated ( P ⪯ 0.001); the calculated TPH-specific activity was 0.76 nCi of α-[ 3H]M5-HT formed per U TPH. In the MFB, quantitative measurements of TPH and α-[ 3H]M5-HT showed no correlation between enzyme and product and no specific activity for TPH could be determined. On the side ipsilateral to the lesion, the density of TPH-immunoreactive fibers was drastically decreased in the dorsolateral hypothalamus where a significant reduction in TPH content (45.5% of control side, P < 0.001) was found. In the overall ipsilateral hemi-DRN, TPH and α-[ 3H]M5-HT levels, their correlation as well as TPH-specific activity were unaltered by the lesion but a significant increase in α-[ 3H]M5-HT and TPH contents was observed in the lateral wings of the DRN. The lesion also induced a significant increase in α-[ 3H]M5-HT and TPH levels (136% and 93.8%, P < 0.001, respectively) in the ipsilateral MFB, which resulted in a positive and significant correlation between these two markers and yielded a TPH-specific activity of 1.0 nCi of α-[ 3H]M5-HT formed per U TPH. TPH topological area was also significantly increased in the lateral aspect of the ipsilateral MFB 5 days post lesion. These results show that 5-HT synthesis in the intact DRN is proportional to and dependent on TPH activity while in the MFB, 5-HT accumulation appears unrelated to TPH content which is most likely in an inactive enzymatic form. Moreover, the data show that a local disruption of serotonergic terminals in the dorsolateral hypothalamus does not affect 5-HT synthesis in the overall ipsilateral DRN neurons but results in local activation of TPH within the serotonergic projection neurons and the ipsilateral MFB, as evidenced by active de novo synthesis of 5-HT. Altogether the results point to circumscribed activation of compensatory mechanisms in 5-HT synthesis after selective destruction of serotonergic terminals.