A direct projection from superior colliculus to substantia nigra for detecting salient visual events.

Midbrain dopaminergic neurons respond to unexpected and biologically salient events, but little is known about the sensory systems underlying this response. Here we describe, in the rat, a direct projection from a primary visual structure, the midbrain superior colliculus (SC), to the substantia nigra pars compacta (SNc) where direct synaptic contacts are made with both dopaminergic and non-dopaminergic neurons. Complementary electrophysiological data reveal that short-latency visual responses in the SNc are abolished by ipsilateral lesions of the SC and increased by local collicular stimulation. These results show that the tectonigral projection is ideally located to relay short-latency visual information to dopamine-containing regions of the ventral midbrain. We conclude that it is within this afferent sensory circuitry that the critical perceptual discriminations that identify stimuli as both unpredicted and biologically salient are made.

Localization of GABA receptors in the basal ganglia.

The majority of neurons in the basal ganglia utilize GABA as their principal neurotransmitter and, as a consequence, most basal ganglia neurons receive extensive GABAergic inputs derived from multiple sources. In order to understand the diverse roles of GABA in the basal ganglia it is necessary to define the precise localization of GABA receptors in relation to known neuron subtypes and known afferents. In this chapter, we summarize data on the ultrastructural localization of ionotropic GABA(A) receptors and metabotropic GABA(B) receptors in the basal ganglia. In each of the regions of the basal ganglia that have been studied, GABA(A) receptor subunits are located primarily at symmetrical synapses formed by GABAergic boutons, where they display a several-hundred-fold enrichment over extrasynaptic sites. In contrast, GABA(B) receptors are widely distributed at synaptic and extrasynaptic sites on both presynaptic and postsynaptic membranes. Presynaptic GABA(B) receptors are localized on striatopallidal, striatonigral and pallidonigral afferent terminals, as well as glutamatergic terminals derived from the cortex, thalamus and subthalamic nucleus. It is concluded that fast GABA transmission mediated by GABA(A) receptors in the basal ganglia occurs primarily at synapses whereas GABA transmission mediated by GABA(B) receptors is more complex, involving receptors located at presynaptic, postsynaptic and extrasynaptic sites.

Subcellular localization of GABAB receptor subunits in rat globus pallidus.

The inhibitory amino acid gamma-aminobutyric acid (GABA) is the major neurotransmitter in the globus pallidus. Although electrophysiological studies have indicated that functional GABA(B) receptors exist in rat globus pallidus, the subcellular localization of GABA(B) receptor subunits and their spatial relationship to glutamatergic and GABAergic synapses are unknown. Here, we use pre-embedding immunogold labeling to study the subcellular localization of GABA(B) receptor subunits, GABA(B1) and GABA(B2), in globus pallidus neurons and identified populations of afferent terminals. Immunolabeling for GABA(B1) and GABA(B2) was observed throughout the globus pallidus, with GABA(B1) more strongly expressed in perikarya and GABA(B2) mainly expressed in the neuropil. Electron microscopic analysis revealed that the majority of GABA(B1) labeling was localized within the cytoplasm, whereas most of GABA(B2) labeling was associated with the plasma membrane. At the subcellular level, both the GABA(B1) and GABA(B2) immunogold labeling was localized at pre- and postsynaptic sites. At asymmetric, putative excitatory, synapses, GABA(B1) and GABA(B2) immunogold labeling was found at perisynaptic sites of both pre- and postsynaptic specializations. Double immunolabeling, using the vesicular glutamate transporter 2 (VGLUT2), revealed the glutamatergic nature of most immunogold-labeled asymmetric synapses. At symmetric, putative GABAergic, synapses, including those formed by anterogradely labeled striatopallidal terminals, GABA(B1) and GABA(B2) immunogold labeling was found in the main body of both pre- and postsynaptic specializations. These results demonstrate the existence of presynaptic GABA(B) auto- and heteroreceptors and postsynaptic GABA(B) receptors, which may be involved in modulating synaptic transmission in the globus pallidus.

Functional presynaptic HCN channels in the rat globus pallidus.

Hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels are expressed postsynaptically in the rodent globus pallidus (GP), where they play several important roles in controlling GP neuronal activity. To further elucidate the role of HCN channels in the GP, immunocytochemical and electrophysiological approaches were used to test the hypothesis that HCN channels are also expressed presynaptically on the local axon collaterals of GP neurons. At the electron microscopic level, immunoperoxidase labelling for HCN1 and HCN2 was localized in GP somata and dendritic processes, myelinated and unmyelinated axons, and axon terminals. One population of labelled terminals formed symmetric synapses with somata and proximal dendrites and were immunoreactive for parvalbumin, consistent with the axon collaterals of GABAergic GP projection neurons. In addition, labelling for HCN2 and, to a lesser degree, HCN1 was observed in axon terminals that formed asymmetric synapses and were immunoreactive for the vesicular glutamate transporter 2. Immunogold labelling demonstrated that HCN1 and HCN2 were located predominantly at extrasynaptic sites along the plasma membrane of both types of terminal. To determine the function of presynaptic HCN channels in the GP, we performed whole-cell recordings from GP neurons in vitro. Bath application of the HCN channel blocker ZD7288 resulted in an increase in the frequency of mIPSCs but had no effect on their amplitude, implying that HCN channels tonically regulate the release of GABA. Their presence, and predicted role in modulating transmitter release, represents a hitherto unidentified mechanism whereby HCN channels influence the activity of GP neurons.

The subcellular localization of GABA(B) receptor subunits in the rat substantia nigra.

The inhibitory effects of GABA within the substantia nigra (SN) are mediated in part by metabotropic GABA(B) receptors. To better understand the mechanisms underlying these effects, we have examined the subcellular localization of the GABA(B) receptor subunits, GABA(B1) and GABA(B2), in SN neurons and afferents using pre-embedding immunocytochemistry combined with anterograde or retrograde labelling. In both the SN pars compacta (SNc) and pars reticulata (SNr), GABA(B1) and GABA(B2) showed overlapping, but distinct, patterns of immunolabelling. GABA(B1) was more strongly expressed by putative dopaminergic neurons in the SNc than by SNr projection neurons, whereas GABA(B2) was mainly expressed in the neuropil of both regions. Immunogold labelling for GABA(B1) and GABA(B2) was localized in presynaptic and postsynaptic elements throughout the SN. The majority of labelling was intracellular or was associated with extrasynaptic sites on the plasma membrane. In addition, labelling for both subunits was found on the presynaptic and postsynaptic membranes at symmetric, putative GABAergic synapses, including those formed by anterogradely labelled striatonigral and pallidonigral terminals. Labelling was also observed on the presynaptic membrane and at the edge of the postsynaptic density at asymmetric, putative excitatory synapses. Double immunolabelling, using the vesicular glutamate transporter 2, revealed the glutamatergic nature of many of the immunogold-labelled asymmetric synapses. The widespread distribution of GABA(B) subunits in the SNc and SNr suggests that GABA(B)-mediated effects in these regions are likely to be more complex than previously described, involving presynaptic autoreceptors and heteroreceptors, and postsynaptic receptors on different populations of SN neurons.