The Effect of Mesencephalic Lesions on Tyramine and Dopamine in the Caudate Nucleus of the Rat

Abstract

Abstract: The dopamine (DA)‐containing nerve terminals in the caudate nucleus arise from cell bodies located in the substantia nigra (pars compacta), and it is possible that p‐tyramine‐ and m‐tyramine‐containing neurons may also exist in this nucleus. We have studied the effects of unilateral electrolytic lesions of the pars compacta in rat on levels of DA, p‐tyramine, m‐tyramine, and homovanillic acid in the caudate nucleus after various survival times. At 12 and 24 h following lesioning the ipsilateral level of p‐tyramine was significantly reduced compared with the contralateral side, whereas the concentrations of m‐tyramine, DA, and homovanillic acid were significantly increased. Thus, in the short term, the lesion results in an increase in DA turnover, which is accompanied by an increase in m‐tyramine levels and a decrease in p‐tyramine levels. Similar changes occur following pharmacological treatments (chlorpromazine, d‐amphetamine, l‐DOPA) that increase DA turnover. At survival times of 2, 11, and 25 days, the ipsilateral concentrations of m‐tyramine, DA, and homovanillic acid were reduced along with p‐tyramine. These longer‐term alterations in amine levels are most likely a consequence of degeneration of nigro‐striatal axons. Placement of a lesion 1 mm dorsal to the usual position centering on the pars compacta produced different biochemical changes from those seen after the pars compacta lesion. One day following this lesion the concentration of p‐tyramine was slightly reduced; DA was unaffected, but the concentration of m‐tyramine was profoundly increased, even more so than after the pars compacta lesion. This could indicate the existence of specific m‐tyramine‐containing cell bodies located dorsal to the substantia nigra. The results suggest that p‐ and m‐tyramine in the caudate nucleus originate from neurons in or close to the substantia nigra. The results in the short term following the lesion support the observation that there is an inverse relationship between p‐tyramine concentration and DA turnover in the caudate nucleus.