The nigrostriatal pathway in the rat: A single-axon study of the relationship between dorsal and ventral tier nigral neurons and the striosome/matrix striatal compartments

Axons from dorsal/ventral tiers of substantia nigra pars compacta (SNc), ventral tegmental area (VTA), and retrorubral field (RRF) were traced after injecting their cell body with biotinylated dextran amine. Fifty-three single axons were reconstructed from serial sagittal sections with a camera lucida, and mu-opiate receptor immunostaining served to differentiate the striosome/matrix striatal compartments. Most dorsal tier SNc axons terminate within the matrix of the dorsal striatum, but their patterns of arborization vary markedly; some axons innervate one specific matriceal area, whereas others arborize in multiple discontinuous loci. Some dorsal tier SNc axons also project to both striosomes and matrix. Other dorsal tier SNc axons, as well as VTA axons, innervate the ventral striatum and send collaterals to striosomes lying ventrally in the dorsal striatum or to the ventral sector of the subcallosal streak (SS). Ventral tier SNc axons arborize principally in striosomes, but some ramify in both compartments or in striosomes and the SS. Ventral tier neurons that form deep clusters in substantia nigra pars reticulata innervate principally the matrix and the SS. The amygdala and ventral pallidum receive secondary collaterals from striatal axons of dorsal/ventral tier neurons or RRF neurons. The subthalamic nucleus receives collaterals from striatal axons of SNc clustered neurons, whereas the globus pallidus gets collaterals from striatal axons of dorsal/ventral tier SNc neurons. These findings reveal that the nigrostriatal pathway is composed of several neuronal subsystems, each endowed with a widely distributed axonal arborization that allows them to exert a multifaceted influence on striatal and/or extrastriatal structures.

Regulation of the nigrostriatal pathway by metabotropic glutamate receptors during development

Dopamine neurons in the substantia nigra heavily innervate the striatum, making it the nucleus with the highest levels of dopamine in the adult brain. The present study analyzes the time course and the density of striatal innervation by nigral dopamine neurons and characterizes the role of the neurotransmitter glutamate during the development of the nigrostriatal pathway. For this purpose, organotypic cultures containing the cortex, the striatum, and the substantia nigra (triple cultures) were prepared from rat brains at postnatal day (PND) 0-2 and were cultured for up to 60 d in vitro (DIV). Dopamine fibers and neurons were labeled using tyrosine hydroxylase (TH) immunohistochemistry. Striatal TH-ir fiber density was quantitatively analyzed using confocal laser scanning microscopy (CLSM). In long-term triple cultures (44 +/- 3 DIV), the striatal dopamine fiber density was high and was weakly correlated with the number of nigral dopamine neurons. The high striatal dopamine fiber density mainly resulted from an enhanced ingrowth and ramification of dopamine fibers from nigral neurons during 8-17 DIV. The metabotropic glutamate receptor (mGluR) antagonist L(+)-2-amino-3-phosphonopropionic acid (L-AP-3) selectively inhibited this dopaminergic innervation of the striatum, whereas ionotropic GluR antagonists had no effect. The L-AP-3-mediated inhibition was prevented by the mGluR agonist 1S, 3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). The inhibition of the striatal dopaminergic innervation by L-AP-3 was further confirmed by anterograde tracing of the nigrostriatal projection with Phaseolus vulgaris leucoagglutinin. These results indicate that glutamate, by acting on group I mGluRs, plays an important "trophic" role for the development of the nigrostriatal dopamine pathway.

Up and down states in striatal medium spiny neurons simultaneously recorded with spontaneous activity in fast-spiking interneurons studied in cortex-striatum-substantia nigra organotypic cultures

In vivo intracellular spontaneous activity in striatal medium spiny (MS) projection neurons is characterized by "up" and "down" states. How this type of activity relates to the neuronal activity of striatal fast-spiking (FS) interneurons was examined in the presence of nigral and cortical inputs using cortex-striatum-substantia nigra organotypic cultures grown for 45 +/- 4 d. The nigrostriatal projection was confirmed by tyrosine hydroxylase immunoreactivity. Corticostriatal (CS) projection neurons, striatal MS neurons, and FS neurons were intracellularly recorded and morphologically and electrophysiologically characterized. Intracellular spontaneous activity in the cultures consisted of intermittent depolarized periods of 0.5-1 sec duration. Spontaneous depolarizations in MS neurons were restricted to a narrow membrane potential range (up state) during which they occasionally fired single spikes. These up states were completely blocked by the glutamate antagonist CNQX. In FS interneurons, depolarized periods were characterized by large membrane potential fluctuations that occupied a wide range between rest and spike threshold. Also, FS interneurons spontaneously fired at much higher rates than did MS neurons. Simultaneous intracellular recordings established that during spontaneous depolarizations MS neurons and FS interneurons displayed correlated subthreshold neuronal activity in the low frequency range. These results indicate that (1) the CS projection neurons, striatal MS neurons, and FS interneurons grown in cortex-striatum-substantia nigra organotypic cultures show morphological and electrophysiological characteristics similar to those seen in vivo; (2) striatal MS neurons but not FS interneurons show an up state; (3) striatal MS neurons and FS interneurons receive common, presumably cortical inputs in the low frequency range. Our results support the view that the cortex provides a feedforward inhibition of MS neuron activity during the up state via FS interneurons.