Chemical Anatomy of the Basal Ganglia in Primates

The connections of the dopaminergic system with the striatum in rats and primates: an analysis with respect to the functional and compartmental organization of the striatum

This Commentary compares the connections of the dopaminergic system with the striatum in rats and primates with respect to two levels of striatal organization: a tripartite functional (motor, associative and limbic) subdivision and a compartmental (patch/striosome-matrix) subdivision. The topography of other basal ganglia projections to the dopaminergic system with respect to their tripartite functional subdivision is also reviewed. This examination indicates that, in rats and primates, the following observations can be made. (1) The limbic striatum reciprocates its dopaminergic input and in addition innervates most of the dopaminergic neurons projecting to the associative and motor striatum, whereas the motor and associative striatum reciprocate only part of their dopaminergic input. Therefore, the connections of the three striatal subregions with the dopaminergic system are asymmetrical, but the direction of asymmetry differs between the limbic versus the motor and associative striatum. (2) The limbic striatum provides the main striatal input to dopamine cell bodies and proximal dendrites, with some contribution from a subset of neurons in the associative and motor striatum (patch neurons in rats; an unspecified group of neurons in primates), while striatal input to the ventrally extending dopamine dendrites arises mainly from a subset of neurons in the associative and motor striatum (matrix neurons in rats; an unspecified group of neurons in primates). (3) Projections from functionally corresponding subdivisions of the striatum, pallidum and subthalamic nucleus to the dopaminergic system overlap, but the specific targets (dopamine cells, dopamine dendrites, GABA cells) of these projections differ. Major differences include the following. (1) In rats, neurons projecting to the motor and associative striatum reside in distinct regions, while in primates they are arranged in interdigitating clusters. (2) In rats, the terminal fields of projections arising from the motor and associative striatum are largely segregated, while in primates they are not. (3) In rats, patch- and matrix-projecting dopamine cells are organized in spatially, morphologically, histochemically and hodologically distinct ventral and dorsal tiers, while in primates there is no (bi)division of the dopaminergic system that results in two areas which have all the characteristics of the two tiers in rats. Based on the anatomical data and known dopamine cell physiology, we forward an hypothesis regarding the influence of the basal ganglia on dopamine cell activity which captures at least part of the complex interplay taking place within the substantia nigra between projections arising from the different basal ganglia nuclei. Finally, we incorporate the striatal connections with the dopaminergic system into an open-interconnected scheme of basal ganglia-thalamocortical circuitry.

Ontogeny of the striatal neurons expressing neuropeptide genes in the human fetus and neonate

The distribution patterns of neurons expressing mRNAs for four neuropeptides in the human striatum were studied during ontogeny by the use of in situ hybridization. The results of our study demonstrate that somatostatin, enkephalin, dynorphin, and substance P mRNAs are present in striatal neuronal populations from week 12 of fetal life. Each neuronal population undergoes a specific differentiation. Neurons containing somatostatin mRNA are scattered throughout the caudate-putamen up until birth. Neurons containing enkephalin, dynorphin, or substance P mRNAs evolve throughout fetal life in relation to caudate-putamen and patch-matrix compartmentalization. Neurons containing enkephalin mRNA (distinct from those containing substance P or dynorphin mRNAs) are present in the matrix from week 12 of fetal life. These neurons are preferentially distributed in the matrix and, at birth, display higher enkephalin mRNA content in the matrix than in the patches. Dynorphin mRNA is found in the caudate and putamen, preferentially in the patch neurons; nevertheless, a low level of dynorphin mRNA is also present in neurons of the caudate matrix. Substance P mRNA is initially restricted to caudate neurons. At birth, both substance P and dynorphin mRNAs are expressed at high levels in the patches. These results demonstrate that each neuropeptide gene is expressed during human fetal life in neurons with a specific topology and pace of development in relation to caudate-putamen and patch-matrix differentiation. These results also contribute evidence that neurochemical evolution of the striatal neuronal populations is not complete at birth in humans.

Golgi study of the mouse striatum: age-related dendritic changes in different neuronal populations

The Van der Loos modification of the Golgi-Cox method and morphometric analyses were used to study the neuronal types in the striatum of adult (3, 6, and 10 months) and aged (20, 25, and 30 months) C57BL/6N mice. In adult mice six types of striatal neurons were distinguished primarily on the basis of the morphology of their cell body and dendrites. Each of these types was compared with morphologically similar neurons from previous Golgi classifications in other species and discussed within the framework of recent immunocytochemical work. With similar methods the age-related changes occurring on the dendrites of three of the six striatal types were also analyzed. In the medium-sized neuron with spine-laden dendrites, various dendritic tree shapes and sizes were distinguished in all age groups studied. Qualitative observations as well as measurements of total dendritic length per cell suggested that the dendrites in this type may both grow and regress throughout the life span, although signs of dendritic atrophy and regression were observed only in the aged groups. In the other two types of neuron, one a medium aspiny cell with thin varicose dendrites and the other a large spiny neuron with many dendrites, measurements of total dendritic lengths revealed sustained growth of the tree well into advanced age, followed by moderate regression in the oldest groups. The present findings also indicate that the dendrites of each type of striatal neuron follow unique temporal patterns of growth and regression during the life span of the mouse.

The response of striatal neuropeptide Y and cholinergic neurons to excitatory amino acid agonists

The effect of excitatory amino acid antagonists on antagonist on neuropeptide Y (NPY) and cholinergic neurons in the striatum of the rat was studied by means of NPY immunocytochemistry, DFP histochemistry for acetylcholinesterase (AChE), and biochemical determinations of choline acetyltransferase (ChAT). Intrastriatal infusion of drugs revealed that striatal neurons containing NPY are more sensitive than cholinergic neurons to the neurotoxic actions of kainic acid (KA), quinolinic acid (QA) and L-glutamic acid (GA); all 3 compounds produced a marked loss of NPY neurons, but only a moderate decrease in the number of AChE neurons or ChAT activity. Co-injection experiments showed that the neurotoxicity of QA and GA, but not that of KA, can be antagonized by the specific N-methyl-D-aspartate (NMDA) receptor antagonist 3-((+/-)-2-(carboxypiperazine-4-yl))-propyl-1-phosphonic acid (CPP). Destruction of the glutamatergic corticostriatal projection by cerebral decortication protected striatal NPY and cholinergic neurons against KA neurotoxicity. These results indicate that striatal NPY and cholinergic neurons receive prominent cortical amino acid afferents, and that the neurotoxic effect of QA and GA on these neurons is mediated through NMDA receptors.