No abstract

The present study describes the thalamic innervation coming from the rat parafascicular nucleus (PF) onto striatal and subthalamic efferent neurons projecting either to the globus pallidus (GP) or to the substantia nigra pars reticulata (SNr) by using a protocol for multiple neuroanatomical tracing. Both striatofugal neurons targeting the ipsilateral SNr (direct pathway) as well as striatal efferent neurons projecting to the ipsilateral GP (indirect pathway) were located within the terminal fields of the thalamostriatal afferents. In the subthalamic nucleus (STN), both neurons projecting to ipsilateral GP as well as neurons projecting to ipsilateral SNr also appear to receive thalamic afferents. Although the projections linking the caudal intralaminar nuclei with the ipsilateral striatum and STN are far more prominent, we also noticed that thalamic axons could gain access to the contralateral STN. Furthermore, a small number of STN neurons were seen to project to both the contralateral GP and PF nuclei. These ipsi-and contralateral projections enable the caudal intralaminar nuclei to modulate the activity of both the direct and the indirect pathway. J. Comp. Neurol. 483:143-153, 2005.

The present study analyses the anatomical arrangement of the projections linking the Wistar rat parafascicular thalamic nucleus (PF) and basal ganglia structures, such as the striatum and the subthalamic nucleus (STN), by using neuroanatomical tract-tracing techniques. Both the thalamostriatal and the striato-entopeduncular projections were topographically organized, and several areas of overlap between identi®ed circuits were noticed, sustaining the existence of up to three separated channels within the Nauta±Mehler loop. Thalamic afferents arising from dorsolateral PF territories are in register with striatofugal neurons located in dorsolateral striatal areas, which in turn project to dorsolateral regions of the entopeduncular nucleus (ENT). Medial ENT regions are innervated by striatal neurons located within medial striatal territories, these neurons being the target for thalamic afferents coming from medial PF areas. Finally, afferents from neurons located in ventrolateral PF areas approached striatal neurons in ventral and lateral striatal territories, which in turn project towards ventral and lateral ENT regions. Efferent STN neurons projecting to ENT were found to be the apparent postsynaptic target for thalamo-subthalamic axons. The thalamo-subthalamic projection was also topographically organized. Medial, central and lateral STN territories are innervated by thalamic neurons located within medial, ventrolateral and dorsolateral PF areas, respectively. Thus, each individual PF subregion projects in a segregated fashion to speci®c parts of the striato-entopeduncular and subthalamo-entopeduncular systems. These circuits enabled the caudal intralaminar nuclei to modulate basal ganglia output.

This work is focused on the study of neuronal circuits arising from the rodent caudal intralaminar nuclei and their presumed role on basal ganglia function. Emphasis was placed on the analysis of the architecture of thalamostriatal and thalamo-subthalamic projections in albino rats. Our major interest was to elucidate whether thalamic inputs were related to projection neurons or local circuit neurons within targeted structures (striatum and subthalamic nucleus). Projections coming from the parafascicular nucleus (PF) to the striatum displayed a patchy organization throughout the matrix compartment. These patches are composed by dense terminal axonal arborizations, often containing striatal neurons projecting to the entopeduncular nucleus (ENT) (medial globus pallidus in primates) and neurons projecting to the substantia nigra reticulata (SNR). The thalamostriatal projections under scrutiny were also seen to be in register with all the major classes of striatal interneurons (nitrergic neurons, neurons containing the calcium binding protein parvalbumin (PV) and cholinergic interneurons). Subthalamic neurons projecting to the ENT are the presumed postsynaptic target for fibers coming from the sensorimotor part (dorsolateral) of the PF. In summary, glutamatergic axons arising from the PF might exert a dual control of the striatal output, either by directly exciting striatal projection neurons or indirectly by means of a previous synaptic contact onto an striatal interneuron which in turn modulates the activity of projection neurons. Furthermore, thalamic inputs can also gain access to basal ganglia output nuclei via subthalamo-pallidal projecting neurons, neurons receiving glutamatergic thalamo-subthalamic projections. Thus, activation of either circuit has an opposite physiological effect on the basal ganglia output nucleus. Taken together, these data suggest that the PF may influence neuronal activity in the direct and indirect circuits and could be considered as an additional component of the basal ganglia motor loops.

Experimental neuroanatomical tracing methods lie at the basis of the study of the nervous system. When the scientific question is relatively straightforward, it may be sufficient to derive satisfactory answers from experiments in which a single neuroanatomical tracing method is applied. In various scientific paradigms however, for instance when the degree of convergence of two different projections on a particular cortical area or subcortical nucleus is the subject of study, the application of single tracing methods can be either insufficient or uneconomical to solve the questions asked. In cases where chains of projections are the subjects of study, the simultaneous application of two tracing methods or even more may be compulsory. The present contribution focuses on combinations of several neuroanatomical tract-tracing strategies, enabling in the end the simultaneous, unambiguous and permanent detection of three transported markers according to a three-color paradigm. A number of combinations of three tracers or of two tracers plus the immunocytochemical detection of a neuroactive substance can be conceived; we describe several of these combinations implemented by us using the present multitracer protocol.