Dopaminergic and GABAergic modulation of glutamate release from rat subthalamic nucleus efferents to the substantia nigra

Abstract

The regulation of the glutamatergic projection from the subthalamic nucleus (STN) to the substantia nigra (SN) was investigated using dual-probe microdialysis in the awake behaving rat. Reverse dialysis of the cholinergic receptor agonist carbachol (1 mM) into the STN caused an increase in the extracellular concentrations of glutamate and dopamine in the SN. The increase in glutamate was transient and returned toward basal values despite the continued perfusions of the STN with carbachol. Carbachol-stimulated glutamate release was prolonged by perfusion of the selective D₂ dopamine receptor antagonist raclopride (100 μM) into the SN and was attenuated by the perfusion of the selective D₂-like receptor agonist quinpirole (10 μM). In contrast, perfusion of the D₁ dopamine receptor antagonist SCH-23390 (100 μM) did not alter the carbachol-stimulated glutamate release even though it increased basal glutamate concentrations. Perfusion of the GABA_A receptor antagonist bicuculline (10 μM) into the SN prolonged the carbachol-stimulated glutamate release in similar fashion as raclopride. The present findings suggest that somatodendritically released dopamine in the SN regulates glutamate release from subthalamic axon terminals by differentially activating dopamine D₂ and D₁ receptors. Activation of D₂ heteroreceptors, located on STN axon terminals, provides a negative feedback control on stimulated subthalamic glutamate release, while D₁ receptor activation preferentially regulates basal glutamate concentrations. The findings of the present study also indicate that GABA exerts an inhibitory control on glutamate release in the SN through GABA_A receptors.

Introduction

The subthalamic nucleus (STN) is the primary glutamatergic nucleus in the basal ganglia (Smith and Parent, 1988) and provides the main excitatory efferent projections to the substantia nigra pars reticulata (SNr) and the medial globus pallidus (Kita and Kitai, 1987). The STN plays an important role in movement control, and dysregulation of STN activity has been associated with the pathophysiology of Parkinson's disease (Hamada and DeLong, 1992, Rodriguez et al., 1998). The activity of STN neurons is regulated primarily by an inhibitory GABAergic projection from the globus pallidus (Shink et al., 1996) and by excitatory glutamatergic projections from multiple cortical areas (Fujimoto and Kita, 1993, Maurice et al., 1998). Moreover, the STN neuronal activity is influenced by a cholinergic projection from the pedunculopontine nucleus (PPN) (Lavoie and Parent, 1994, Mouroux et al., 1995) and by a direct dopaminergic projection from the substantia nigra pars compacta (SNc) (Hassani et al., 1997). Cholinergic afferents are believed to exert an excitatory effect on STN neuronal activity and are mediated via cholinergic muscarinic M₃ receptors (Flores et al., 1996). However, there is no consensus on the nature of the dopaminergic modulation of glutamatergic projections from the STN to the SN (Johnson et al., 1994).

The glutamatergic projection from the STN to the SN, referred to as the subthalamonigral pathway, plays a pivotal role in the firing activity and bursting behavior of SNc DA cells (Smith and Grace, 1992) and SNr GABA neurons (Maurice et al., 2003). Glutamate, presumably from subthalamic afferents, stimulates SNc DA cells through an activation of ionotropic (Overton and Clark, 1991, Chergui et al., 1994) and metabotropic (Meltzer et al., 1997) glutamate receptors. The majority of these receptors are located on the long dendritic processes of DA cells, which enter into and branch within the SNr (Chatha et al., 2000). The GABAergic neurons of the SNr also express ionotropic glutamate receptors (Tse and Yung, 2000). It has been demonstrated that stimulation of the STN increases DA cell firing (Benazzouz et al., 2000a) and results in somatodendritic DA release (Rosales et al., 1994, Mintz et al., 1986) as well as GABA release in the SN (Windels et al., 2005, Rosales et al., 1997).

The functional role of somatodendritically released DA in the SN is not as well characterized as DA released from nigrostriatal terminals. Previous studies have shown that DA released in the SN functions in the feedback regulation of DA cell activity via D₂ autoreceptor activation (Pucak and Grace, 1994) and also in regulating the activity of GABAergic neurons in the SNr through its actions on D₁ receptors (Abercrombie and DeBoer, 1997, Waszczak, 1990). In the present study, we investigated the possibility that somatodendritically released DA may play the additional role in regulating glutamate release from subthalamic terminals in the SN. This dopaminergic regulation could be mediated via an activation of D₂ heteroreceptors located on asymmetric terminals in the SN that presumably are glutamatergic (Pickel et al., 2002) and/or through the activation of D₁ heteroreceptors on glutamate terminals (Rosales et al., 1997).

To investigate the regulation of the glutamatergic projection from the STN to the SN, dual-probe microdialysis was used in the awake behaving rat. The cholinergic receptor agonist carbachol was reverse dialyzed into the STN, and extracellular glutamate and dopamine concentrations were simultaneously monitored in the SN. Reverse dialysis of selective dopaminergic receptor agonists and antagonists into the SN during carbachol stimulation of the STN was used to assess the differential contributions of D₁ and D₂ receptor subtypes on glutamate release in the SN. Furthermore, the contribution of GABA on glutamate release in the SN was assessed by perfusing the selective GABA_A antagonist bicuculline into the SN during carbachol stimulation of the STN. It was posited that the administration of the cholinergic receptor agonist carbachol into the STN will stimulate glutamate and, consequently, dopamine release in the SN. It was further hypothesized that the increase in extracellular dopamine within the SN negatively modulates the stimulated release of glutamate from the subthalamic terminals via the activation of D₂ heteroreceptors.