Delta-opioid receptor gene expression in the mouse forebrain: localization in cholinergic neurons of the striatum

Synaptic connections between spiny neurons of the direct and indirect pathways in the neostriatum of the rat: evidence from dopamine receptor and neuropeptide immunostaining

The flow of cortical information through the basal ganglia occurs through the so-called 'direct pathway' and 'indirect pathways'. The object of the present work was to attempt to determine whether spiny neurons in the neostriatum that give rise to the direct pathway (i.e. the striatonigral/entopeduncular pathway) and those giving rise to the indirect pathways (i.e. striatopallidal pathway) are synaptically interconnected. The approach was to carry out double immunocytochemistry at the electron microscopic level using antibodies against peptides or dopamine receptor subtypes that are selectively associated with the neurons that give rise to the direct (substance P or D1 receptors) and indirect pathways (enkephalin or D2 receptors). Sections of perfuse-fixed rat neostriatum were immunostained to reveal both substance P immunoreactivity and D2 receptor immunoreactivity or enkephalin and D1 receptor immunoreactivity, respectively. Double peroxidase methods were employed using different chromogens that were distinguishable at both the light and electron microscopic levels. In the electron microscope substance P-immunoreactive terminals were seen in synaptic contact with dendritic structures that displayed immunoreactivity for D2 receptor. Similarly, enkephalin-immunoreactive terminals were seen in synaptic contact with D1-immunoreactive dendritic structures. Thus, axon collaterals of neurons giving rise to the direct pathway form synaptic contacts with neurons that give rise to the indirect pathway and axon collaterals of neurons giving rise to the indirect pathway form synaptic contact with neurons that give rise to the direct pathway. These results indicate that the two pathways of information flow through the basal ganglia are synaptically linked at the level of the neostriatum.

KEYWORDS

spiny neurons, direct pathway,indirect pathways, rat neostriatum

Striatal interneurones: chemical, physiological and morphological characterization

The neostriatum is the largest component of the basal ganglia, and the main recipient of afferents to the basal ganglia from the cerebral cortex and thalamus. Studies of the cellular organization of the neostriatum have focused upon the spiny projection neurones, which represent the vast majority of neurones, but the identity and functions of interneurones in this structure have remained enigmatic despite decades of study. Recently, the discovery of cytochemical markers that are specific for each of the major classes of striatal interneurones, and the combination of this with intracellular recording and staining, has revealed the identities of interneurones and some of their functional characteristics in a way that could not have been imagined by the classical morphologists. These methods also suggest some possible modes of action of interneurones in the neostriatal circuitry.

Molecular biology of the opioid receptors: structures, functions and distributions

Opiates like morphine and endogenous opioid peptides exert their pharmacological and physiological effects through binding to their endogenous receptors, opioid receptors. The opioid receptors are classified into at least three types, mu-, delta- and kappa-types. Recently, cDNAs of the opioid receptors have been cloned and have greatly advanced our understanding of their structure, function and expression. This review focuses on the recent advances in the studies on opioid receptors using the cloned cDNAs. We describe the molecular cloning of the opioid receptor gene family and studies of the structure-function relationships, modes of coupling to second messenger systems, pharmacological effects of antisense oligonucleotide and anatomical distributions of opioid receptors.

D1 and D2 dopamine receptor gene expression in the rat striatum: sensitive cRNA probes demonstrate prominent segregation of D1 and D2 mRNAs in distinct neuronal populations of the dorsal and ventral striatum

The postsynaptic effects of dopamine in the striatum are mediated mainly by receptors encoded by D1, D2, and D3 dopamine receptor genes. The D1 and D2 genes are the most widely expressed in the caudate-putamen, the accumbens nucleus, and the olfactory tubercle. Several anatomical studies, including studies using in situ hybridization with oligonucleotide and cDNA probes, have suggested that D1 and D2 receptors are segregated into distinct efferent neuronal populations of the striatum: D1 in substance P striatonigral neurons and D2 in enkephalin striatopallidal neurons. In contrast, on the basis of several in vivo and in vitro studies, other authors have suggested the existence of an extensive colocalization of D1 and D2 in the same striatal neurons. Our study was undertaken in order to analyze in detail the expression of the D1 and D2 receptor genes in the efferent striatal populations, with special reference to the various striatal areas, and to yield insights into the question about D1 and D2 mRNA localization in the striatum. We have, therefore, used highly sensitive digoxigenin- and 35S-labeled cRNA probes to address this question. The present results demonstrate that the D1 and D2 receptor mRNAs are segregated, respectively, in substance P and enkephalin neurons in the caudate-putamen and accumbens nucleus (shell and core) and in the olfactory tubercle (for their largest part). A very small percentage of neurons may coexpress both genes. These results confirm that the D1 and D2 receptor genes are expressed in distinct populations of striatal efferent neurons in the normal adult rat.

Molecular pharmacology of the opioid receptors

Opioid receptors are the primary sites of actions of opiates and endogenous opioid peptides, which have a wide variety of pharmacological and physiological effects. The opioid receptors are classified into at least three subtypes, mu, delta, and kappa, and their cDNAs have been cloned. In this review, we describe the molecular cloning of opioid receptor gene family and studies of the structure-function relationships, modes of coupling to second messenger systems, pharmacological effects of antisense oligonucleotides, and anatomical distribution of opioid receptor mRNAs.