Thalamic control of subcortical dopamine function in the rat and the effects of lesions applied to the medial prefrontal cortex

Abstract

Dopamine (DA) utilisation has been assessed in medial and lateral segments of the caudate-putamen complex (CP M and CP L, respectively) in response to unilateral manipulations aimed at the thalamic mediodorsal nucleus, lateral division (MD L). The ratios of 3,4-dihydroxyphenylacetic acid (DOPAC):DA and 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid, HVA):DA are used as indices of DA utilisation and, in the case of HVA:DA, may also reflect DA release. Neither electrical stimulation nor ibotenate (IBO) treatment followed by long recovery periods (2 days or 1 week) had any significant effect on DA utilisation in CP M or CP L. Cell-specific activation of neurones produced by short-term (1 h recovery) infusions of IBO aimed unilaterally at MD L (right side) resulted in bilateral increases of DA utilisation in both CP sectors. These changes tended to be slightly more marked in the hemisphere ipsilateral to the side of IBO infusion. Unilateral infusions of IBO were then aimed at MD L of either (1) the left or right hemisphere of animals which had already received a 1-week-old unilateral (right side) prefrontal cortex (FCx) lesion or (2) the right hemisphere of animals which had previously received a 1 week-old bilateral FCx lesion. The pattern of changes, when expressed relative to the ‘sham-operated’ animals which received the FCx lesion alone, were similar to those described above following intra-MD L infusions of IBO into animals with an intact cortex. The FCx lesions themselves were shown to have no significant effect on DA utilisation in any CP sector. In view of the known neuroanatomical connections, it is likely that the effects observed in CP are not due to activation of MD L neurones themselves but are more likely the result of activation of neurones in the intralaminar nuclei which border MD L. Nevertheless, these findings support the concept that activation of thalamic nuclei will enhance DA function in a variety of forebrain areas in the rat.