The Journal of Neuroscience, June 1992, 72(6): 2079-2103 Compartmental Distribution of Ventral Striatal Neurons Projecting to the Mesencephalon in the Rat Henk W. Berendse,a Henk J. Groenewegen, and Anthony H. M. Lohman Department of Anatomy and Embryology, Vrije Universiteit, 1081 BT Amsterdam, The Netherlands The ventral striatum is characterized by an intricate neu- turn. The other component can, by way of the pars reticulata rochemical compartmentation that is reflected in the distri- of the substantia nigra, participate in nigrothalamic and ni- bution of most of its afferent fiber systems. In the present grotectal output pathways of the basal ganglia. study, the compartmental relationships of ventral striatal neurons projecting to the mesencephalon were studied by In spite of their long-standing exclusive associationwith motor combining tract tracing with the immunohistochemical lo- functions, the basal ganglia are now considered as a group of calization of leu-enkephalin. Injections of the retrograde interconnected nuclei that are involved in the processing of tracer cholera toxin subunit B were placed at various sites information related to many aspectsof behavior, ranging from in the ventral mesencephalon. The anterograde tracer fha- motor acts to complex emotional and motivational influences. seolus vu/garis leucoagglutinin was injected in single com- The underlying anatomical substratehas only recently been elu- partments in the rostrolateral part of the nucleus accumbens. cidated by demonstrating a parallel organizational principle The projections from the ventral striatum to the dopaminer- within basal ganglia connections (Heimer and Wilson, 1975; gic cell groups in the ventral mesencephalon and those to Alexander et al., 1986, 1990; Nauta, 1986). Information from the substantia nigra pars reticulata originate from distinct functionally diverse parts of the cerebral cortex is channeled subpopulations of ventral striatal neurons that respect neu- through different parts of the basal ganglia and thalamus back rochemically defined compartmental boundaries. In the to the cortex, thus forming a number of parallel cortico-striato- “shell” of the nucleus accumbens, neurons that project to pallido-thalamo-cortical circuits that may each be involved in the dopaminergic cell groups are located outside areas of different aspects of behavior. The importance of this organi- high cell density and weak enkephalin immunoreactivity zational principle is underscoredby the parallel arrangement of (ENK-IR). Rostrolaterally in the “core” of the nucleus ac- the projections from the midline and intralaminar thalamic nu- cumbens, neurons inside large areas of strong ENK-IR sur- clei and the amygdala to both the striatum and the prefrontal rounding the anterior commissure project to the dorsomedial cortex (Berendseand Groenewegen, 1990, 1991; Groenewegen part of the substantia nigra pars reticulata, whereas neurons et al., 1990b). outside these areas innervate the ventral tegmental area An important issue in basal ganglia research is whether and and/or the medial part of the substantia nigra pars compacta. where integration takes place in this systemof segregatedcircuits By contrast, more caudally in the dorsal part of the nucleus in order to produce coherent behavior. Local interactions may accumbens and in the ventral part of the caudate-putamen, occur at the level of the striatum through overlapping inputs. the relationships are reversed: neurons in- or outside small Widespread integration may take place through the projections patches of strong ENK-IR project respectively to the pars from the ventral striatum to the dopaminergic cell groups in the compacta or the pars reticulata of the substantia nigra. Since mesencephalon(Nauta et al., 1978; Somogyi et al., 1981; Nauta the thalamic and cortical afferents of the ventral striatum are and Domesick, 1984) which seemto defy the parallel arrange- compartmentally ordered as well, the present results imply ment of basal ganglia connections. By way of projections to the that through the ventral striatal compartments information substantia nigra pars compacta, the ventral tegmental area, and from disparate combinations of cortical and thalamic sources the retrorubral field, containing the A9, A 10, and A8 dopamin- may be conveyed to distinct mesencephalic targets. The ergic cell groups, respectively (Dahlstriim and Fuxe, 1964; component of the ventral striatomesencephalic system Bjiirklund and Lindvall, 1984) the ventral striatum appearsto reaching the dopaminergic cell groups Al 0, A9, and A8 may be in a position to influence the dopaminergic input to virtually modulate the dopaminergic input to virtually the entire stria- the entire striatum (Fallon and Moore, 1978; Bjiirklund and Lindvall, 1984; Gerfen et al., 1987) and, consequently, the in- formation processingin each of the parallel cortico-striato-pal- Received June 7, 1991; revised Dec. 30, 1991; accepted Jan. 6, 1992. lidofugal pathways. Whether the ventral striatal projections to We thank Dr. Pieter Voorn for helpful discussions, Mrs. Yvonne Galis-de Graaf for technical assistance, Mr. Dirk de Jong for his photographical skills, and Ms. the dorsomedial part of the substantia nigra pars reticulata Loes van Zanten for secretarial support. This work was supported by NW0 Pro- (Swansonand Cowan, 1975; Conrad and Pfaff, 1976; Nauta et gram Grant 900-550-093. Correspondence should be addressed to Henk J. Groenewegen, M.D., Ph.D., al., 1978; Troiano and Siegel, 1978) originate from the same Department of Anatomy and Embryology, Vrije Universiteit, Van der Boechorst- neurons remains to be established. straat 7, 1081 BT Amsterdam, The Netherlands. Superimposedupon the topographical organization of its con- = Present address: Neurology Unit, University of Rochester, Monroe Com- munity Hospital, 435 East Henrietta Road, Rochester, NY 14620. nections, the striatum is characterized by a heterogeneousdis- Copyright 0 1992 Society for Neuroscience 0270-6474/92/122079-25$05.00/O tribution of many neurotransmitters,neuromodulators, and their 2080 Berendse et al. * Ventral Striatomesencephalic Projections receptors. In the dorsal striatum, two neurochemically distinct more detailed features of the neurochemical compartmentation compartments, the patch or striosomal compartment and the of the ventral striatum (Voom et al., 1989). matrix compartment, have been identified (Graybiel and Rags- In order to label the ventral striatal neurons projecting to the dale, 1978; Graybiel et al., 1981; Gerfen, 1984; Gerfen et al., different mesencephalic targets, retrograde tracer injections were 1985; for reviews, see Graybiel, 1984,199O). Virtually all striatal made in the different parts of the dorsomedial substantia nigra, afferent and efferent systems observe the boundaries of these in the ventral tegmental area, and in the retrorubral field. Ad- neurochemical compartments (e.g., Herkenham and Pert, 198 1; ditional injections were placed in the peribrachial region to iden- Ragsdale and Graybiel, 198 1, 1990; Gerfen, 1984, 1985, 1989; tify the neuronal population projecting to more caudal parts of Donoghue and Herkenham, 1986; Jimtnez-Castellanos and the midbrain. The opioid peptide enkephalin (ENK), visualized Graybiel, 1987, 1989; Gimknez-Amaya and Graybiel, 1990; for immunohistochemically, was used as a marker for the ventral reviews, see Graybiel, 1984, 1990). The differential connections striatal compartments (Berendse et al., 1988; Voorn et al., 1989; of the patch/striosome and matrix compartments with the sub- Berendse and Groenewegen, 1990). A preliminary set of exper- stantia nigra and associated dopaminergic cell groups constitute iments, using fluorogold and diamidine yellow as retrograde the most striking example of the compartmental ordering. The tracers, provided us with the first indications of differential com- efferents of the striatal patches or striosomes preferentially reach partmental origins of the ventral striatomesencephalic projec- the pars compacta of the substantia nigra, whereas the matrix tions (Groenewegen et al., 1989). The retrograde tracer cholera projects mainly to the pars reticulata (Gerfen, 1984, 1985; Ger- toxin subunit B (CTb) proved to be more sensitive and produced fen et al., 1985; JimCnez-Castellanos and Graybiel, 1989). The a more extensive filling of the dendritic trees of the rather small projection from the patch or striosomal compartment to the striatal projection neurons. Therefore, the present account is pars compacta is reciprocated by a projection from particular based on CTb experiments only. The analysis of a limited num- subgroups of dopaminergic neurons, which in the rat reside in ber of PHA-L injections in the nucleus accumbens and the ven- the ventral tier of the pars compacta or in the ventrolateral part tral part of the caudate-putamen was necessary to supplement of the pars reticulata, and in the cat in the densocellular portion the data obtained by means of retrograde tracing. of the pars compacta (Gerfen et al., 1985, 1987; Jimknez-Cas- tellanos and Graybiel, 1987; see also Feigenbaum-Langer and Materials and Methods Graybiel, 1989). Neurons in the dorsal strata of the pars com- In the present study, 32 female Wistar rats (Harlan/CPB, Zeist, The pacta, in the ventral tegmental area, and in the retrorubral area Netherlands) weighing 180-220 gm were used. Following anesthesia with an intramuscular injection of a mixture (4:3) of ketamine and 2- project to the striatal matrix compartment. The striatal