Effects of thalamic lesion on the bilateral regulation of serotoninergic transmission in rat basal ganglia

The primate centromedian-parafascicular complex: anatomical organization with a note on neuromodulation

In addition to the cerebral cortex, the striatum receives excitatory input from the thalamus. The centromedian (centre median, CM) and parafascicular (Pf) nuclei are an important source of thalamostriatal projections. Anterograde tract-tracing indicates the CM-Pf complex provides dense afferents to the matrix compartment of the striatum. Whereas CM projects to the entire sensorimotor territory of the striatum, the Pf provides complementary input to the entire associative sector. The Pf also provides lighter input to the nucleus accumbens. Both CM and Pf provide light to moderately dense inputs to other components of the basal ganglia in a largely complementary manner, covering motor or associative-limbic territories of the subthalamic nucleus, globus pallidus and ventral midbrain. In turn, the CM and Pf receive mainly segregated input from parallel motor and associative-limbic circuits of the basal ganglia. The CM and Pf may therefore be considered important participants in parallel processing of motor and associative-limbic information in the basal ganglia. Connections of the CM and Pf with other thalamic nuclei suggest they also participate in integrative functions within the thalamus. In addition, inputs from the brainstem reticular core, reciprocal connections with the cerebral cortex and reticular thalamic nucleus suggest a role in state-dependant information processing. Consideration of the differential connections of the CM and Pf, and better understanding of their role in pathophysiology, may eventually lead to development of an important new target for relief of a variety of neurological and psychiatric disorders.

The thalamostriatal systems: anatomical and functional organization in normal and parkinsonian states

Although we have gained significant knowledge in the anatomy and microcircuitry of the thalamostriatal system over the last decades, the exact function(s) of these complex networks remain(s) poorly understood. It is now clear that the thalamostriatal system is not a unique entity, but consists of multiple neural systems that originate from a wide variety of thalamic nuclei and terminate in functionally segregated striatal territories. The primary source of thalamostriatal projections is the caudal intralaminar nuclear group which, in primates, comprises the centromedian and parafascicular nuclei (CM/Pf). These two nuclei provide massive, functionally organized glutamatergic inputs to the whole striatal complex. There are several anatomical and physiological features that distinguish this system from other thalamostriatal projections. Although all glutamatergic thalamostriatal neurons express vGluT2 and release glutamate as neurotransmitter, CM/Pf neurons target preferentially the dendritic shafts of striatal projection neurons, whereas all other thalamic inputs are almost exclusively confined to the head of dendritic spines. This anatomic arrangement suggests that transmission of input from sources other than CM/Pf to the striatal neurons is likely regulated by dopaminergic afferents in the same manner as cortical inputs, while the CM/Pf axo-dendritic synapses do not display any particular relationships with dopaminergic terminals. A better understanding of the role of these systems in the functional circuitry of the basal ganglia relies on future research of the physiology and pathophysiology of these networks in normal and pathological basal ganglia conditions. Although much remains to be known about the role of these systems, recent electrophysiological studies from awake monkeys have provided convincing evidence that the CM/Pf-striatal system is the entrance for attention-related stimuli to the basal ganglia circuits. However, the processing and transmission of this information likely involves intrinsic GABAergic and cholinergic striatal networks, thereby setting the stage for complex physiological responses of striatal output neurons to CM/Pf activation. Finally, another exciting development that will surely generate significant interest towards the thalamostriatal systems in years to come is the possibility that CM/Pf may be a potential surgical target for movement disorders, most particularly Tourette syndrome and Parkinson's disease. Although the available clinical evidence is encouraging, these procedures remain empirical at this stage because of the limited understanding of the thalamostriatal systems.

Effects of intralaminar thalamic nuclei lesion on glutamic acid decarboxylase (GAD65 and GAD67) and cytochrome oxidase subunit I mRNA expression in the basal ganglia of the rat

This study investigated the influence of thalamic inputs on neuronal metabolic activity in the rat basal ganglia. By means of in situ hybridization histochemistry, we examined the consequences of ibotenate-induced unilateral lesion of intralaminar thalamic nuclei on mRNA expression of cytochrome oxidase subunit-I (CoI) in the striatum and the subthalamic nucleus (STN) and of the two isoforms of glutamate decarboxylase (GAD65 and GAD67) in the striatum, globus pallidus (GP), entopeduncular nucleus (EP) and substantia nigra pars reticulata (SNr). In the striatum, GAD67 mRNA expression decreased selectively in the rostral part of the structure at 5 and 12 days postlesion (approximately -30%), whereas, GAD65 mRNA levels was downregulated only in the caudal striatum at 12 days (-29%). In both the striatum and STN, CoI mRNA expression decreased ipsilaterally at 5 and bilaterally at 12 days. In GP, GAD67 and GAD65 mRNA expression decreased ipsilaterally at 5 (-20% and -26%) and 12 days (-23% and -36%). In EP, selective bilateral decreases in GAD67 mRNA expression were found at 5 and 12 days (-50% and -40%). Conversely, in SNr, only GAD65 mRNA expression was reduced bilaterally at both time points. These data show that the thalamus exerts a widespread excitatory influence on the basal ganglia network that cannot be accounted for solely by its known direct connections. Given the recent data showing that intralaminar thalamic nuclei are a major nondopaminergic site of neurodegeneration in Parkinson's disease, these results may have a critical bearing on understanding the cellular basis of basal ganglia dysfunction in parkinsonism.

Differential effects of corticostriatal and thalamostriatal deafferentation on expression of the glutamate transporter GLT1 in the rat striatum

This study compared the effects of the disruption of the two main presumably glutamatergic striatal inputs, the corticostriatal and thalamostriatal pathways, on GLT1 expression in the rat striatum, using in situ hybridization and immunohistochemistry. Unilateral ibotenate-induced thalamic lesion produced no significant changes in striatal GLT1 mRNA labeling and immunostaining as assessed at 5 and 12 days postlesion. In contrast, significant increases in both parameters were measured after bilateral cortical lesion by superficial thermocoagulation. GLT1 mRNA levels increased predominantly in the dorsolateral part of the striatum; there, the increases were significant at 5 (+84%), 12 (+101%), and 21 (+45%) but not at 35 days postlesion. GLT1 immunostaining increased significantly and homogeneously by 17-26% at 12 and 21 days postlesion. The increase in GLT1 expression at 12 days postlesion was further confirmed by western blot analysis; in contrast, a 36% decrease in glutamate uptake activity was measured at the same time point. These data indicate that striatal GLT1 expression depends on corticostriatal but not thalamostriatal innervation. Comparison of our results with previous data showing that cortical lesion by aspiration downregulates striatal GLT1 expression further suggests that differential changes in GLT1 expression, and thus presumably in glial cell function, may occur in the target striatum depending on the way the cortical neurons degenerate.

Thalamo-striatal deafferentation affects preproenkephalin but not preprotachykinin gene expression in the rat striatum

This study examined the effects of thalamo-striatal deafferentation on preprotachykinin and preproenkephalin mRNA expression in the rat neostriatum, using quantitative in situ hybridization histochemistry. Unilateral ibotenate-induced intralaminar thalamic lesion produced a significant decrease in preproenkephalin mRNA levels (-27%) restricted to the ipsilateral striatum at 5 days post-lesion. At 12 days post-lesion, significant decreases in striatal preproenkephalin mRNA expression were found on both brain sides. This post-lesional response was more pronounced in the ipsilateral (-32%) than contralateral (-18%) striatum. All these changes were homogeneously distributed between the dorsolateral and ventromedial parts of the striatum. In parallel, no significant change in preprotachykinin mRNA expression was found at either 5 or 12 days after thalamic lesion, when considering the striatum as a whole. However, at 5 days post-lesion, the regional analysis revealed a slight decrease (-17%) in preprotachykinin mRNA expression, confined to the dorsolateral part of the ipsilateral striatum. These results show that thalamic lesion preferentially affects preproenkephalin vs. preprotachykinin gene expression in the striatum, suggesting, at the first site, a predominant influence of thalamo-striatal inputs on the enkephalin-containing striato-pallidal pathway. However, given that the thalamo-striatal projection is strictly ipsilateral, the bilateralization of the down-regulation of preproenkephalin mRNA expression at 12 days post-lesion suggests an involvement of interhemispheric adaptive mechanisms via cortical networks.

Evaluation of the function of microdialysis probes permanently implanted into the rat CNS and coupled to an on-line HPLC system of analysis

The aim of the microdialysis technique is to reflect as closely as possible the status and fluctuations of substances contained in the extracellular space. Most often, microdialysis is performed with repetitively implanted probes. We have recently devised an experimental set-up which allows microdialysis to be performed in the spinal cord of unrestrained rats through chronically permanently implanted probes. In the present study, we have compared the in vitro recovery of a non-biogenic amine (DHBA) and of 5-HT, and the in vivo recovery of the former. Thus, we could extrapolate the in vivo recovery of endogenous 5-HT released. Moreover, we have found that the recovery does not vary irrespective of whether the animal is at rest or performing sustained physical exercise, and that it also remains stable with time, from 8 to 36 days after permanent implantation of the probe. We conclude that this simple method can be applied to standard experiments of microdialysis, and thus allow one to measure the actual rate of recovery for a given probe. Moreover, it permits control of the stability of dialysis parameters with time for long-term permanently implanted probes.

Study of 5-HT release with a chronically implanted microdialysis probe in the ventral horn of the spinal cord of unrestrained rats during exercise on a treadmill

The ventral horn of the spinal cord is profusely innervated by serotonin (5-hydroxytryptamine, 5-HT) which presumably modulates locomotor activity through motoneurons. However, direct evidence of correlation between 5-HT release and activation of motoneurons is still lacking. In order to appreciate the functional characteristics of this innervation, we have used microdialysis to monitor the release of 5-HT in the spinal cord of rats spontaneously running on a treadmill. For this purpose, we developed an original surgical procedure adapted for the chronic implantation of a microdialysis probe in the lumbar spinal cord. The probe was kept in place for 40 days, and microdialysis experiments were carried out at days 8, 16, 20 and 32. 5-HT was detected with HPLC coupled to electrochemistry. This technique demonstrated that release of 5-HT is not increased during exercise. However, a significant decrease was measured during postexercise rest. 5-HT could still be detected 32 days after probe implantation. Detailed histological and immunocytochemical analysis based on glial fibrillary acidic protein and 5-HT immunocytochemistry showed minimal gliosis and the presence of serotonergic varicose fibers, respectively, at the probe contact. It thus appears that microdialysis can be performed through a probe implanted chronically in the spinal cord of unrestrained rat during an endurance running exercise.

Effect of thalamic parafascicularis nucleus stimulation in regulation of serotoninergic transmission in the cat caudate nucleus: involvement of autoreceptors in the dorsalis raphe nucleus

The mechanisms involved in parafascicularis nucleus control on serotoninergic neurons projecting into the caudate nucleus were investigated in "encéphale-isole" cats. The effects of unilateral stimulation of the parafascicularis nucleus on the release of newly synthesized [3H]serotonin were simultaneously determined in the ipsilateral caudate nucleus and the dorsalis raphe nucleus using push-pull cannulae. The actions of various pharmacological treatments performed either in the caudate nucleus or in the dorsalis raphe nucleus were also examined. The electrical or chemical stimulation of the parafascicularis nucleus induced a decrease in striatal [3H]serotonin release and an increase in [3H]serotonin release in the dorsalis raphe nucleus. The blockade of cholinergic (mecamylamine) and glutamatergic (PK 26124) transmissions at the striatal level did not modify the thalamic stimulation-induced effect on serotonin release in the caudate nucleus or in the dorsalis raphe nucleus. However, a decrease induced by parafascicularis nucleus stimulation in serotonin release in the caudate nucleus could not be observed when the autoreceptors present on serotoninergic nerve cell bodies localized in the dorsalis raphe nucleus were blocked by a methiothepin perfusion within the nucleus. These results indicate that the parafascicularis nucleus controls striatal serotonin transmission by inducing changes in the nerve activity of serotoninergic neurons in the dorsalis raphe nucleus via somatodendritic serotonin release and autoreceptors.