Thalamic control of dopaminergic functions in the caudate-putamen of the rat--II. Studies using ibotenic acid injection of the parafascicular-intralaminar nuclei.
We have investigated the role of the parafascicular-intralaminar thalamus in the regulation of dopaminergic function in the caudate-putamen by making unilateral injections of the excitotoxin, ibotenic acid, into the thalamus of the halothane-anaesthetized rat. Dopamine utilization was measured at 4 h, 18 h, and 7 days after operation in microdissected tissue from caudate-putamen, substantia nigra, and nucleus accumbens. High performance liquid chromatography with electrochemical detection was used to simultaneously determine dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid. Dopamine utilization was recorded as a ratio of metabolite to its parent amine. At 4 h following injection large bilateral increases in dopamine utilization were recorded in both medial and lateral sectors of caudate-putamen. The percentage increases found for homovanillic acid-based ratios were larger than those found for 3,4-dihydroxyphenylacetic acid-based ratios. These results probably reflect increased dopamine release resulting from the acute effects of ibotenic acid. These changes were independent of dopamine utilization ratios recorded in the substantia nigra, which showed no change either ipsilateral or contralateral to the injection. In contrast to these findings, at 7 days following injection, dopamine utilization ratios were reduced both ipsilateral and contralateral to the injection, although only the ipsilateral reductions were significant. Again, no change was found for this survival time in the substantia nigra. At 18 h survival an intermediate pattern between the 4 h and 7 day result was found. In the nucleus accumbens, ibotenic acid injection produced similar results to those found in caudate-putamen, i.e. a bilateral increase in dopamine utilization at early time intervals and a unilateral ipsilateral decrease at long intervals following injection. These results show that dopamine release in the caudate-putamen is sensitive to experimentally induced changes in neural activity and lesions of its thalamic input. Since the effect of presumed stimulation is markedly greater than lesion, it would appear that, under the conditions employed in these experiments, the thalamus is relatively silent; a suggestion consistent with other evidence. Furthermore, since the changes found occurred in the absence of changes in utilization ratios in the substantia nigra, the mechanisms whereby thalamus regulates dopamine release may be exerted via a local circuit and/or a presynaptic mechanism in the region of dopamine terminals. The anatomical routes responsible for these effects are discussed.(ABSTRACT TRUNCATED AT 400 WORDS)