Marjorie A. Ariano

Are glial cells targets of the central noradrenergic system? A review of the evidence

It has been suggested by a number of investigators that glial cells as well as neurons are targets of the central noradrenergic system. This important hypothesis, however, has not been presented previously in a systematic and unified manner. The present review was therefore undertaken to accomplish this. The evidence supporting noradrenergic action on glia consists primarily of findings that beta-adrenoceptors, norepinephrine (NE)-stimulated cyclic AMP (cAMP) responses and glycogen are localized preferentially in glial cells and that beta-receptor density and glycogen hydrolysis are under the control of neuronally released NE. While there is some disagreement as to the extent to which beta-receptors are preferentially localized in glia, there is a consensus that most glycogen in the forebrain is localized in this cellular compartment. The presumed function of the noradrenergic action on glia appears to be the release of glucose for production of energy, the synthesis of neurotrophic factors such as nerve growth factor, and the release of substances which may affect local neurotransmission including taurine, cAMP and its metabolites. These glial responses may be intimately related to the electrophysiological actions of NE on neurons.

Cellular localization of cyclic nucleotide changes in rat superior cervical ganglion

Summary1.Cyclic nucleotides were localized by immunohistochemistry using light microscopy in the rat superior cervical ganglion. Cyclic GMP was found in the cytoplasm of postganglionic neurons and their processes. Cyclic AMP immunoreactivity was intense in satellite cells only. Ganglion sections incubated with antisera to cyclic AMP occasionally showed a faint fluorescence in postganglionic neurons.2.Cyclic AMP staining intensity was greatly augmented within satellite cells following preganglionic stimulation or isoproterenol incubation. Stimulation of the postganglionic nerve did not alter cyclic AMP immunofluorescence.3.Cyclic GMP immunofluorescence in the postganglionic neurons was increased by stimulation of the preganglionic nerve or following exposure to sodium azide. Stimulation of the postganglionic nerve increased cyclic GMP immunofluorescence in a few instances; the increases were small and were consistent with radioimmunoassay data.4.There was no difference between experimental and control ganglia in the localization of cyclic nucleotide by cell type. Some treatments were selective for a cyclic nucleotide, but there was no apparent selectivity for cell type.5.Qualitative changes in immunohistochemical localization of cyclic AMP and cyclic GMP reflect the quantitative alterations in these compounds assessed by radioimmunoassay after these same experimental procedures.

Striatal D1 dopamine receptor distribution following chemical lesion of the nigrostriatal pathway

The morphochemical and biochemical distribution of the adenylate cyclase-linked dopamine receptor, or D1 subpopulation, has been examined in the rat striatum following a chemical lesion of the dopaminergic nigrostriatal pathway. The cellular pattern of D1 dopaminergic binding was assessed using in vitro autoradiographic localization of [3H]SCH 23390 or [125I]SCH 23982, selective D1 receptor antagonists, 1 week following unilateral infusion of the neurotoxin 6-hydroxydopamine (6-OHDA) into the substantia nigra. The specific association of the D1 binding sites with cyclic AMP-immunoreactive striatal neurons was abolished after lesion of the dopaminergic nigral afferents. The morphochemical disruption of the caudate D1 dopamine binding sites in relation to cyclic AMP-positive elements, a large proportion of which are striatonigral neurons, probably contributes to the dysfunctions in this subpopulation of dopamine receptor after depletion of the catecholamine neurotransmitter.

Cyclic nucleotide metabolism in the sympathetic ganglion

Summary1.The synaptic processes controlling cyclic nucleotide metabolism in the rat superior cervical ganglion were examined through electrical stimulation of preganglionic and postganglionic nerves.2.Both cyclic AMP and cyclic GMP were measured in each ganglion. Cyclic AMP was increased twofold by preganglionic but not postganglionic stimulation for 30 sec.3.Carbachol also increased cyclic AMP twofold. These elevations were calcium dependent and atropine sensitive, consistent with a reliance on muscarinic transmission. However, atropine did not inhibit the elevation of cyclic AMP following preganglionic stimulation for 60 sec.4.A much larger increase in cyclic AMP could be induced by stimulation ofβ-adrenergic receptors, but this mechanism did not appear to be invoked by nervous activity.5.Cyclic GMP levels were increased sixfold by preganglionic stimulation, twofold by postganglionic stimulation, and twofold by carbachol. All of these effects on cyclic GMP levels were calcium sensitive.6.Atropine didnot inhibit the effect of 30-sec preganglionic stimulation on cyclic GMP. However, atropine blocked the cyclic GMP increases induced by carbachol or by postganglionic stimulation and partially inhibited the increase following 60-sec preganglionic stimulation.Thus, synaptic activity alters the levels of both cyclic AMP and cyclic GMP in the rat ganglion, but more than one process is involved and these processes appear to be considerably different.

Peptide coincidence in rat superior cervical ganglion.

The immunofluorescent localization of substance P (SP), somatostatin and methionine enkephalin has been determined in the rat superior cervical ganglion of the sympathetic nervous system. Immunoreactivity was confined to the norepinephrine-containing post-ganglionic neurons and their processes. Nearly 20% of the postganglionic somata demonstrate dual coincidence in staining for SP-somatostatin, somatostatin-enkephalin, or SP-enkephalin. Almost 10% of the positively stained neurons show a coincident staining of all 3 peptide compounds within their somata. This suggests that multiple neurotransmitter chemicals may be employed by the postganglionic cells of the superior cervical ganglion.

Comparison of dopamine binding sites in the rat superior cervical ganglion and caudate nucleus

A comparison of morphological and biochemical characteristics of the D1-type dopamine receptor has been assessed in two experimental tissues, the superior cervical ganglion (SCG) and caudate nucleus of the rat. Correlation of the distribution of this dopaminergic binding site using in vitro autoradiographic localization of [3H]SCH 23390, a selective D1-binding antagonist, demonstrated no specific association of this receptor subtype with cyclic AMP immunoreactive structures in the superior cervical ganglion. In contrast, the caudate nucleus demonstrated specific D1-binding sites associated with cyclic AMP immunoreactive elements. Biochemical analyses of the D1-dopamine binding sites showed only 20% of the amount of radioligand bound in SCG homogenates as compared to the quantity bound in homogenates of the caudate nucleus. The non-cyclase linked dopaminergic receptor, assessed using D2-type radioligand binding, was much less prevalent than the binding of the D1-subtype in either experimental tissue. Only a small amount of [3H]sulpiride binding, indicative of the D2-receptor subtype, could be measured in the SCG as compared to the caudate nucleus. This work has demonstrated differences in the amount, and the cellular association of dopamine binding sites in peripheral versus central nervous system areas with dopamine sensitive adenylate cyclase mechanisms.

Striatal muscarinic receptors are associated with substance P and somatostatin containing neurons

The cellular localization of muscarinic acetylcholine binding sites (mAChr) in relation to immunohistochemically characterized cell populations within the rat caudate nucleus has been determined using in vitro autoradiography of the reversible antagonist ligand, quinuclidinyl benzilate [( 3H]QNB). The pattern of autoradiographic silver grain deposition in the striatum was contrasted with the localization of two peptide-containing neuronal populations in the striatum. Substance P-immunoreactive somata demonstrated prevalent association of mAChr binding sites, as did somatostatin-immunoreactive cells. Substantially more striatal muscarinic binding sites were aggregated over the somatostatin interneuron population of the caudate nucleus than were associated with the substance P somata in concurrently performed experiments. This data further substantiates the heterogeneity in organization of the caudate nucleus, and the results are discussed in relation to the processing of information within this basal ganglia region.

Rat striatal cyclic nucleotide-reactive cells and acetylcholinesterase reactive interneurons are separate populations

The concurrent localization of cyclic nucleotide immunofluorescent cells and acetylcholinesterase-containing neurons in the rat caudate-putamen complex has been examined. Cyclic AMP and cyclic GMP stained elements do not exhibit coincident localization with the enzymatically detectable hydrolysis of acetylcholine. These data add further support for the preferential association of the cyclic nucleotides with striatal efferent projection systems, while the large cholinergic somata are part of the interneuron population of the rat caudate-putamen complex.

Neurotransmitter receptor autoradiography in immunohistochemically identified neurons

A method for the simultaneous visualization of neurotransmitter receptor binding sites and immunohistochemically characterized cells is described using in vitro sections of rat striatum. The striatum provides a rich neurochemical environment in which to examine muscarinic acetylcholine receptor relations to peptide-reactive somata, or to assess the cellular locale of the dopamine receptor linked to adenylate cyclase activation. Different modifications of the procedure are employed to determine the localization pattern of 3H-radioligands with slow or rapid receptor dissociation times. The present technique is compatible for use with any combination of immunohistochemical antibody/radioligand probe that can be studied on fresh-frozen, slide-mounted tissue sections.

Striatal D1 dopamine receptor morphochemistry following continuous or intermittent L-dopa replacement therapy.

Striatal dopamine deafferentation has previously been found to diminish D1 dopamine receptor clustering in association with striatal cyclic AMP-immunoreactive neurons. The administration of the dopamine precursor levodopa (L-DOPA) to animals with unilaterally placed 6-hydroxydopamine nigrostriatal tract lesions now appears to partially restore D1 dopamine receptor morphochemical organization in the deafferented striatum. Differences in the mode of levodopa delivery produced dissimilar D1 recovery patterns. The prodrug, L-DOPA methyl ester, was administered in combination with the peripheral aromatic amino acid decarboxylase inhibitor, benserazide, to achieve consistent plasma levels of the dopamine precursor. Continuous levodopa infusion (100 mg/kg/day, ip) led to a slight dorsomedial reassociation of D1 receptor binding sites with the postsynaptic cyclic AMP transduction system on the deafferented side. In contrast, intermittent levodopa therapy (50 mg/kg, ip, bid) produced a noticeable down regulation of the dopamine receptor system and also contributed to some region-specific recovery of the morphochemical pattern of D1 receptor binding site reaggregation with the postsynaptic cyclic AMP second messenger transduction system. These results suggest that exogenous levodopa replacement therapy desensitizes striatal D1 dopamine receptors. This was substantiated using image analysis of densitometric histograms. The down regulation of D1 receptors is dependent on the levodopa treatment regimen employed. Our findings provide a potential morphological basis for the behavioral desensitization shown previously in response to chronic, intermittent levodopa administration.