Selective modulation of heteromeric ASIC proton-gated channels by neuropeptide FF

Proton-gated channels of the acid-sensing ion channel (ASIC) family are candidates for mediating the fast ionotropic transduction of extracellular acidification in neurons. ASIC subunits can assemble in homomeric and heteromeric channels with specific biophysical and pharmacological properties. Using heterologous expression of ASIC subunits in Xenopus oocytes, we show here that the biphasic response of heteromeric rat and human ASIC2A+3 subtypes to low pH is selectively modulated by the neuropeptide FF (NPFF) and by the related peptide FMRFamide. We recorded both a dramatic potentiation (up to 275%) of the amplitude of acid-gated human ASIC2A+3 maximal currents and a change of desensitization kinetics in the presence of NPFF (EC(50)=2 microM) leading to a slowly inactivating phenotype. These modulatory effects were not observed with the corresponding homomeric human ASIC2A or ASIC3 receptor subtypes. Moreover, the sensitivity of ASIC2A+3 receptors to extracellular protons was increased in the presence of NPFF (DeltapH(50)=+0.5). Our data therefore suggest that the direct sensitization of heteromeric proton-gated channels by endogenous neuropeptides might play a role in the neuronal response to noxious acidosis in sensory and central pathways.

International union of pharmacology. XXIV. Current status of the nomenclature and properties of P2X receptors and their subunits

ATP acts as a humoral mediator to control cell function extracellularly. The receptors that mediate the actions of ATP belong to two classes, the metabotropic P2Y receptors and the transmitter-gated, ion channel P2X receptors. This review describes the structure, distribution, function, and ligand recognition characteristics of P2X receptors, which comprise seven distinct subunits that can function as both homo- and hetero- polymers. The pharmacology of P2X receptors is complicated by marked differences between species orthologues. The current nomenclature is based largely on recombinant receptor studies and detailed knowledge of endogenous P2X receptors in native tissues is limited because of lack of good selective agonists and antagonists for each receptor type.

Regional and subunit-specific downregulation of acid-sensing ion channels in the pilocarpine model of epilepsy

Acid-sensing ion channels (ASICs) constitute a recently discovered family of excitatory cation channels, structurally related to the superfamily of degenerin/epithelial sodium channels. ASIC1b and ASIC3 are highly expressed in primary sensory neurons and are thought to play a role in pain transmission related to acidosis. ASIC1a, ASIC2a, and ASIC2b are also distributed in the central nervous system where their function remains unclear. We investigated here the regulation of their expression during status epilepticus (SE), a condition in which neuronal overexcitation leads to acidosis. In animals treated with pilocarpine (380 mg/kg) to induce SE, we observed a marked decrease of ASIC2b mRNA levels in all hippocampal areas and of ASIC1a mRNA levels in the CA1-2 fields. These changes were also observed after protective treatment from neuronal cell death with diazepam (10 mg/kg) and pentobarbital (30 mg/kg). These findings suggest a key role of channels containing ASIC1a and ASIC2b subunits in both normal and pathological activity of hippocampus.

Unique functional properties of a sensory neuronal P2X ATP-gated channel from zebrafish

We report here the structural and functional characterization of an ionotropic P2X ATP receptor from the lower vertebrate zebrafish (Danio rerio). The full-length cDNA encodes a 410-amino acid-long channel subunit zP2X(3), which shares only 54% identity with closest mammalian P2X subunits. When expressed in Xenopus oocytes in homomeric form, ATP-gated zP2X(3) channels evoked a unique nonselective cationic current with faster rise time, faster kinetics of desensitization, and slower recovery than any other known P2X channel. Interestingly, the order of agonist potency for this P2X receptor was found similar to that of distantly related P2X(7) receptors, with benzoylbenzoyl ATP (EC(50) = 5 microM) >> ATP (EC(50) = 350 microM) = ADP > alpha,beta-methylene ATP (EC(50) = 480 microM). zP2X(3) receptors are highly sensitive to blockade by the antagonist trinitrophenyl ATP (IC(50) < 5 nM) but are weakly sensitive to the noncompetitive antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid. zP2X(3) subunit mRNA is exclusively expressed at high levels in trigeminal neurons and Rohon-Beard cells during embryonic development, suggesting that neuronal P2X receptors mediating fast ATP responses were selected early in the vertebrate phylogeny to play an important role in sensory pathways.

Mammalian ASIC2a and ASIC3 subunits co-assemble into heteromeric proton-gated channels sensitive to Gd3+

Proton receptors of the acid-sensing ion channel (ASIC) family are expressed in sensory neurons and thus could play a critical role in the detection of noxious acidosis. To investigate the subunit composition of native ASICs in peripheral and central neurons, we co-injected human as well as rodent ASIC2a and ASIC3 subunits in Xenopus oocytes. The amplitudes of acid-induced biphasic responses mediated by co-expressed ASIC2a and ASIC3 subunits were much larger (as much as 20-fold) than the currents mediated by the respective homomers, clearly indicating functional association. The reversal potential of the ASIC2a+3 current (>/=+20 mV) reflected a cationic current mainly selective for sodium. The sensitivity to pH or amiloride of single versus co-expressed ASIC subunits was not significantly different; however, gadolinium ions inhibited ASIC3 and ASIC2a+3 responses with much higher potency (IC(50) approximately 40 microm) than the ASIC2a response (IC(50) >/=1 mm). Biochemical interaction between ASIC2a and ASIC3 subunits was demonstrated by co-purification from transfected human embryonic kidney (HEK293) cells and Xenopus oocytes. Our in situ hybridization data showed that rat ASIC2a and ASIC3 transcripts are co-localized centrally, whereas reverse transcription-polymerase chain reaction data led us to detect co-expression of human ASIC2a and ASIC3 subunits in trigeminal sensory ganglia, brain, and testis where they might co-assemble into a novel subtype of proton-gated channels sensitive to gadolinium.

A protein kinase C site highly conserved in P2X subunits controls the desensitization kinetics of P2X(2) ATP-gated channels

P2X receptors are nonselective cation channels gated by extracellular ATP. Recombinant mammalian P2X subunits assemble in homomeric ionotropic ATP receptors that differ by their agonist sensitivity and desensitization rate in heterologous expression systems. Using site-directed mutagenesis and voltage clamp recording in Xenopus oocytes, we identified the highly conserved protein kinase C site TX(K/R) located in the intracellular N terminus of P2X subunits as a critical determinant of kinetics in slowly desensitizing (time constant, >1 min) rat P2X(2) receptors. Mutant receptors P2X(2)T18A, T18N, and K20T devoid of this consensus site exhibited quickly desensitizing properties (time constant, <1 s). In contrast with wild-type receptors, mutant P2X(2) receptors with truncated C terminus exhibited variable cell-specific kinetics with quickly desensitizing currents converted to slowly desensitizing currents by phorbol ester-mediated stimulation of protein kinase C. Phosphorylation of Thr(18) was demonstrated directly by immunodetection using specific monoclonal antibodies directed against the phosphothreonine-proline motif. Our data indicate that both phosphorylation of the conserved threonine residue in the N-terminal domain by protein kinase C and interaction between the two cytoplasmic domains of P2X(2) subunits are necessary for the full expression of slowly desensitizing ATP-gated channels.

Functional and biochemical evidence for heteromeric ATP-gated channels composed of P2X1 and P2X5 subunits

The mammalian P2X receptor gene family encodes two-transmembrane domain nonselective cation channels gated by extracellular ATP. Anatomical localization data obtained by in situ hybridization and immunocytochemistry have shown that neuronal P2X subunits are expressed in specific but overlapping distribution patterns. Therefore, the native ionotropic ATP receptors diversity most likely arises from interactions between different P2X subunits that generate hetero-multimers phenotypically distinct from homomeric channels. Rat P2X1 and P2X5 mRNAs are localized within common subsets of peripheral and central sensory neurons as well as spinal motoneurons. The present study demonstrates a functional association between P2X1 and P2X5 subunits giving rise to hybrid ATP-gated channels endowed with the pharmacology of P2X1 and the kinetics of P2X5. When expressed in Xenopus oocytes, hetero-oligomeric P2X1+5 ATP receptors were characterized by slowly desensitizing currents highly sensitive to the agonist alpha,beta-methylene ATP (EC50 = 1.1 microM) and to the antagonist trinitrophenyl ATP (IC50 = 64 nM), observed with neither P2X1 nor P2X5 alone. Direct physical evidence for P2X1+5 co-assembly was provided by reciprocal subunit-specific co-purifications between epitope-tagged P2X1 and P2X5 subunits transfected in HEK-293A cells.

Molecular cloning and regional distribution of a human proton receptor subunit with biphasic functional properties

Small changes of extracellular pH activate depolarizing inward currents in most nociceptive neurons. It has been recently proposed that acid sensitivity of sensory as well as central neurons is mediated by a family of proton-gated cation channels structurally related to Caenorhabditis elegans degenerins and mammalian epithelial sodium channels. We describe here the molecular cloning of a novel human proton receptor, hASIC3, a 531-amino acid-long subunit homologous to rat DRASIC. Expression of homomeric hASIC3 channels in Xenopus oocytes generated biphasic inward currents elicited at pH <5, providing the first functional evidence of a human proton-gated ion channel. Contrary to the DRASIC current phenotype, the fast desensitizing early component and the slow sustained late component differed both by their cationic selectivity and by their response to the antagonist amiloride, but not by their pH sensitivity (pH50 = 3.66 vs. 3.82). Using RT-PCR and mRNA blot hybridization, we detected hASIC3 mRNA in sensory ganglia, brain, and many internal tissues including lung and testis, so hASIC3 gene expression was not restricted to peripheral sensory neurons. These functional and anatomical data strongly suggest that hASIC3 plays a major role in persistent proton-induced currents occurring in physiological and pathological conditions of pH changes, likely through a tissue-specific heteropolymerization with other members of the proton-gated channel family.

Central P2X4 and P2X6 channel subunits coassemble into a novel heteromeric ATP receptor

Ionotropic ATP receptors are widely expressed in mammalian CNS. Despite extensive functional characterization of neuronal homomeric P2X receptors in heterologous expression systems, the subunit composition of native central P2X ATP-gated channels remains to be elucidated. P2X4 and P2X6 are major central subunits with highly overlapping mRNA distribution at both regional and cellular levels. When expressed alone in Xenopus oocytes, P2X6 subunits do not assemble into surface receptors responsive to ATP applications. On the other hand, P2X4 subunits assemble into bona fide ATP-gated channels, slowly desensitizing and weakly sensitive to the partial agonist alpha,beta-methylene ATP and to noncompetitive antagonists suramin and pyridoxal-5-phosphate-6-azophenyl-2',4'-disulfonic acid. We demonstrate here that the coexpression of P2X4 and P2X6 subunits in Xenopus oocytes leads to the generation of a novel pharmacological phenotype of ionotropic ATP receptors. Heteromeric P2X4+6 receptors are activated by low-micromolar alpha, beta-methylene ATP (EC50 = 12 microM) and are blocked by suramin and by Reactive Blue 2, which has the property, at low concentrations, to potentiate homomeric P2X4 receptors. The assembly of P2X4 with P2X6 subunits results from subunit-dependent interactions, as shown by their specific copurification from HEK-293 cells transiently transfected with various epitope-tagged P2X channel subunits. Our data strongly suggest that the numerous cases of neuronal colocalizations of P2X4 and P2X6 subunits observed in mammalian CNS reflect the native expression of heteromeric P2X4+6 channels with unique functional properties.

Sensory presynaptic and widespread somatodendritic immunolocalization of central ionotropic P2X ATP receptors

Recent evidence suggests that extracellular ATP plays a neurotransmitter role in the central nervous system. Its fast ionotropic effects are exerted through a family of P2X ATP-gated channels expressed in brain and spinal cord. To determine the physiological significance of central ATP receptors, we have investigated the localization of a major neuronal P2X receptor at the cellular and subcellular levels using affinity-purified antibodies directed against the C-terminal domain of P2X4 subunit. Subunit-specific anti-P2X4 antibodies detected a single band of 57,000 +/- 3000 mol. wt in transfected HEK-293 cells and in homogenates from adult rat brain. The strongest expression of central P2X receptors was observed in the olfactory bulb, lateral septum, cerebellum and spinal cord. P2X4 immunoreactivity was also evident in widespread areas including the cerebral cortex, hippocampus, thalamus and brainstem. In all regions examined, P2X receptors were associated with perikarya and dendrites where they were concentrated at the level of afferent synaptic junctions, confirming a direct involvement of postsynaptic ATP-gated channels in fast excitatory purinergic transmission. Moreover, P2X4-containing purinoceptors were localized in axon terminals in the olfactory bulb and in the substantia gelatinosa of nucleus caudalis of the medulla and dorsal horn of the spinal cord, demonstrating an important selective presynaptic role of ATP in the modulation of neurotransmitter release in central sensory systems.

Primary structure and expression of a naturally truncated human P2X ATP receptor subunit from brain and immune system

A novel member of the ionotropic ATP receptor gene family has been identified in human brain. This 422 amino acid long P2X receptor subunit has 62% sequence identity with rat P2X5. Several characteristic motifs of ATP-gated channels are present in its primary structure, but this P2X5-related subunit displays a single transmembrane domain. Heterologous expression of chimeric subunits containing the C-terminal domain of rat P2X5 leads to the formation of desensitizing functional ATP-gated channels in Xenopus oocytes. The developmentally regulated mRNA, found in two splicing variant forms, is expressed at high levels in brain and immune system.

Identification in the rat neurotensin receptor of amino-acid residues critical for the binding of neurotensin

In order to identify charged amino-acid residues of the cloned rat brain neurotensin (NT) receptor (NTR) that are critical for NT binding, we performed site-directed mutagenesis on the cDNA encoding this protein, followed by transient expression into mammalian COS-7 cells and in Xenopus laevis oocytes. Point substitutions of charged residues in the N-terminal part and in the 2nd and 3rd extracellular loop of the receptor either did not affect (125)I-Tyr3-NT binding or resulted in a decrease in binding affinity by a factor of 2-3. Mutations of amino acids Asp113 in the second transmembrane domain (TM) and of Arg149 or Asp150 in TM III yielded receptors that bound NT as efficiently as the native receptor. By contrast, replacement of the Asp139 residue in the 1st extracellular loop, or of Arg143 or Arg327-Arg328 residues at the top of TM III and in TM VI, respectively, completely abolished ligand binding. Confocal and EM immunocytochemical studies of the expression of these affected receptors, tagged with the C-terminal sequence of the vesicular stomatitis virus glycoprotein (VSV-G), indicated that this loss of binding was not due to altered receptor expression or to their improper insertion into the plasma membrane. When these mutated forms of neurotensin receptor were expressed into Xenopus oocytes, Asp139-Gly- and Arg143-Gly-modified receptors remained functional in spite of a lowered response to NT whereas the Arg327-Arg328 mutant form was totally insensitive to NT at concentrations up to 10 microM. In the case of the Arg327-Arg328 mutation, the observed insensibility to NT could be the result of a drastic conformational alteration of this mutant protein. By contrast, it would appear that Asp139 and Arg143 residues located in the first extracellular loop of the receptor may be directly involved in the interaction of the receptor with neurotensin.

Differential expression of sumatriptan-sensitive 5-hydroxytryptamine receptors in human trigeminal ganglia and cerebral blood vessels

The efficacy of sumatriptan in migraine relief has been attributed to its interaction with 5-hydroxytryptamine1D (5-HT1D) receptors in cerebral blood vessels and/or on nerve endings of the trigeminovascular system in the dura mater. Using the high sensitivity of polymerase chain reaction (PCR) amplification, we investigated the expression of the sumatriptan-sensitive 5-HT receptors, namely, the 5-HT1D alpha, 5-HT1D beta, and 5-HT1F subtypes in human trigeminal ganglia (10 experiments) and cerebral blood vessels (seven experiments) obtained postmortem. Messages for the 5-HT1D alpha and 5-HT1D beta receptors were expressed in all except one of the 10 trigeminal ganglia studied. Expression of the 5-HT1F receptor was detected by gel electrophoresis of the PCR products in six ganglia and by Southern blot hybridization in two additional cases. In human brain vessels, message for the 5-HT1D beta receptor was present in all samples, whereas specific PCR products corresponding to the 5-HT1D alpha receptor could hardly be detected in only two preparations. PCR products indicative of the 5-HT1F receptor message were detected by gel electrophoresis in three brain vessel preparations and confirmed in the other four by Southern blot hybridization. Restriction mapping and sequence analysis of all PCR products identified the expected human 5-HT receptor DNA sequences. The data confirm that the 5-HT1D beta receptor is the dominant species in human cerebral blood vessels and further show that this receptor and the 5-HT1F are expressed in both neural and vascular tissues. In contrast, the data point to a preferential expression of 5-HT1D alpha receptors in neural versus vascular tissues and strongly reemphasize the need for selective 5-HT1D alpha agonists in the identification of the target tissue(s) for antimigraine drugs. Moreover, the data stress the importance to better understand the role of 5-HT1F receptors in cerebrovascular functions and dural inflammation and further raise interest regarding their possible involvement in migraine therapy.

Immunological characterization and transmembrane topology of 5-hydroxytryptamine3 receptors by functional epitope tagging

5-Hydroxytryptamine3 (5-HT3) receptors are the only known monoamine receptors mediating fast excitatory responses in mammalian neurons. Their primary structure as well as their electrophysiological and pharmacological properties show a phylogenetic relation to nicotinic acetylcholine, GABAA, and glycine receptors. As a prototypical member of this gene superfamily, we investigated the membrane topology of functional homomeric 5-HT3 receptors by using epitope tagging of the channel subunits expressed in heterologous systems. Visualization of 5-HT3 receptors in transfected COS-7 cells, either in western blot (molecular mass 61.2 +/- 0.8 kDa) or in situ, was performed with previously characterized antibodies recognizing artificial epitopes as well as with anti-fusion protein antibodies directed against a wild-type receptor intracellular domain. The extracellular location of the distal C-terminal tagged domain demonstrates the presence of a fourth transmembrane domain in 5-HT3 serotonin-gated channels. In this region, the significant homology between members of this class of neurotransmitter-gated channels suggests strongly that they have a common transmembrane organization basically different from glutamate-gated and ATP-gated channels.

A novel neuronal P2x ATP receptor ion channel with widespread distribution in the brain

There is strong evidence that ATP acts as an excitatory neurotransmitter in the periphery, yet little is known about fast central ATP-mediated transmission. We report here the molecular cloning of a novel neuronal ionotropic ATP receptor of the P2x subtype (P2x3) isolated from rat brain. This central P2x channel subunit has significant amino acid homology with two recently cloned ATP-gated channels from rat smooth muscle (47%) and pheochromocytoma PC12 cells (37%). P2x3 receptor contains the characteristic 10 conserved cysteines of ATP-gated channels, a putative extracellular region homologous to the Walker type A motif found in various nucleotide-binding proteins, and two potential sites for phosphorylation by protein kinase C. Homomeric receptor P2x3 channels expressed in Xenopus oocytes produce rapid cation-selective purinergic currents that are potentiated by zinc ions and reversibly blocked by the P2x antagonists suramin, Reactive Blue 2, and pyridoxalphosphate-6-axophenyl-2U,4U-disulfonic acid. P2x3-receptor subunit mRNA is found in the Purkinje cells and the granule cells of the cerebellum as well as in CA3 pyramidal cells of the hippocampus that are innervated by zinc-rich axon terminals of mossy fibers. Our results suggest that fast excitatory synaptic transmission mediated by zinc-sensitive ATP-gated channels is widespread in mammalian brain.

Molecular cloning, functional properties, and distribution of rat brain alpha 7: a nicotinic cation channel highly permeable to calcium

A full-length clone coding for the rat alpha 7 nicotinic receptor subunit was isolated from an adult brain cDNA library and expressed in Xenopus oocytes. A significant proportion of the current through alpha 7-channels is carried by Ca2+. This Ca2+ influx then activates a Ca(2+)-dependent Cl- conductance, which is blocked by the chloride channel blockers niflumic and fluflenamic acid. Increasing the external NaCl concentration caused the reversal potentials for the alpha 7-channels and the Ca(2+)-dependent Cl- channels to be shifted in opposite directions. Under these conditions, agonist application activates a biphasic current with an initial inward current through alpha 7-channels followed by a niflumic acid- and fluflenamic acid-blockable outward current through Ca(2+)-dependent Cl- channels. A relative measure of the Ca2+ permeability was made by measuring the shift in the reversal potential caused by adding 10 mM Ca2+ to the external solution. Measurements made in the absence of Cl-, to avoid artifactual current through Ca(2+)-activated Cl- channels, indicate that alpha 7-homooligomeric channels have a greater relative Ca2+ permeability than the other nicotinic ACh receptors. Furthermore, alpha 7-channels have an even greater relative Ca2+ permeability than the NMDA subtype of glutamate receptors. High levels of alpha 7-transcripts were localized by in situ hybridization in the olfactory areas, the hippocampus, the hypothalamus, the amygdala, and the cerebral cortex. These results imply that alpha 7-containing receptors may play a role in activating calcium-dependent mechanisms in specific neuronal populations of the adult rat limbic system.

Ultrastructural features of the serotonin innervation in adult rat hippocampus: an immunocytochemical description in single and serial thin sections

This study was aimed at characterizing the fine-structural features of the normal serotonin (5-HT) innervation in adult rat hippocampus, by means of electron microscopic immunocytochemistry with a polyclonal antiserum against 5-HT-glutaraldehyde-protein conjugate (donated by Michel Geffard, Bordeaux). Two hippocampal sectors were examined, at mid-level along the septo-temporal axis: CA3-a of Ammon's horn and crest of the dentate gyrus (DG-c). A large number of axonal varicosities (terminals) were sampled in single ultrathin sections, to achieve a statistically significant comparison of their size and of their relative frequency of synaptic specialization, junctional targets and juxtaposed elements, between the oriens and the radiatum layer of CA3-a, and the molecular and the polymorph layer of DG-c. In both CA3-a layers, the microenvironment of the immunostained terminals was also compared to that of a population of unlabeled varicosities randomly selected from the same micrographs. Moreover, 57 varicosities from the oriens and the radiatum layer of CA3-a were visualized in a long series of thin sections, allowing for their examination from end to end in 43 instances. As measured in single sections, hippocampal 5-HT varicosities were of comparable diameter (0.57 microns on the average) in the two anatomical sectors and four neuropil layers examined. As extrapolated stereologically to whole varicosities, the proportion making a synaptic membrane specialization (synaptic incidence) ranged from 18 to 33% (average of 24%), without statistically significant differences between the two sectors and four layers. The synaptic incidence determined directly from serial sections of CA3-a (18%) was nearly identical to that extrapolated from single sections (18.1% in the oriens and 19.5% in the radiatum layer). In both CA3-a and DG-c, the 5-HT varicosities showing a junctional complex were slightly larger than their non-junctional counterparts. In CA3-a, only dendritic shafts were targeted by synaptic 5-HT varicosities, whereas in DG-c there were also a few axo-spinous synapses. The microenvironment of CA3-a 5-HT varicosities differed markedly from that of randomly selected unlabeled varicosities, due to its much lower frequency of synaptic targets and higher frequency of juxtaposed axonal varicosities, at least in the radiatum layer. In all four layers examined, other axonal varicosities were indeed the most frequently encountered neuronal element in the immediate vicinity of immunostained 5-HT varicosities. Neurites and dendritic shafts were also common, but dendritic spines (4%) were relatively infrequent.(ABSTRACT TRUNCATED AT 400 WORDS)

Nicotine receptors in the mammalian brain

Nicotine is a drug of abuse that presumably exerts its psychoactive effect through its interactions with nicotine binding sites in the central nervous system. Among its potential sites of action are the neuronal nicotinic acetylcholine receptors and the neuronal alpha-bungarotoxin binding sites. In this review we focus on the neuronal nicotinic acetylcholine receptors, their diversity, distribution, and functions as nicotine receptors or as mediators of synaptic transmission in the mammalian brain. We find that the complexity characteristic of the gene family encoding the subunits of these receptors is reflected both in the pattern of expression of the genes and in the pharmacological diversity of the expressed receptors.

Noradrenaline axon terminals in adult rat neocortex: an immunocytochemical analysis in serial thin sections

Peroxidase-antiperoxidase electron microscope immunocytochemistry with an antiserum against noradrenaline-glutaraldehyde-protein conjugate was used to identify cortical noradrenaline terminals (axonal varicosities) from the upper layers of the frontal, parietal and occipital cortex in adult rat. A large number of immunostained varicosities were examined in serial thin sections, and compared with a control population of randomly chosen unlabeled terminals from the same sections. Both groups of varicosities were measured and scrutinized for the presence of a junctional complex indicative of synaptic specialization. Cellular elements juxtaposed to the membrane of both types of varicosities were also identified and counted. Noradrenaline varicosities in all three cortical regions averaged 0.65 microns in diameter. In contrast to their unlabeled counterparts, these profiles rarely showed a membrane differentiation characteristic of a synaptic contact (junctional complex). The rare junctional complexes formed by cortical noradrenaline varicosities were invariably symmetrical and almost always found on dendritic shafts. The microenvironment of noradrenaline varicosities also differed, exhibiting a greater number of apposed axonal varicosities and a smaller number of dendritic spines than that of the random population. The proportion of noradrenaline varicosities making a synaptic contact (synaptic incidence) was determined by plotting the incidence of visible junctions as a function of the number of thin sections available for examination. As extrapolated for whole varicosities after linear transformation (double reciprocal plot), this proportion was 17% or 26% depending on the stringency of the criteria used in identifying the junctional complex. The same analysis provided a figure of 98% for the control population. The present study largely confirmed our initial radioautographic characterization of the cortical noradrenaline innervation as a mostly non-junctional system, and also indicated that these varicosities are set in a particular microenvironment. These new data further support the eventuality of a diffuse release of cortical noradrenaline in the extracellular space, compatible with both its neuromodulatory role and multiplicity of actions on diverse cellular targets in the cerebral cortex. The functions assigned to the coeruleocortical noradrenaline system must therefore be viewed as the product of a widespread and ubiquitously distributed neuronal organization characterized by loose intercellular relationships. This system might be capable of selectivity and specificity of action, however, owing to the distribution of its receptors, and in view of intrinsically or extrinsically driven control mechanisms triggered by the release of its own or other transmitters and which may also involve target-initiated feedback mechanisms.

Ultrastructural relationships of serotonin axon terminals in the cerebral cortex of the adult rat

PAP immunocytochemistry with an antiserum against serotonin (5-HT)-glutaraldehyde-protein conjugate (kindly donated by M. Geffard) was used to analyze the ultrastructural relationships of 5-HT axon terminals (varicosities) in the frontal (Fr1-Fr2), parietal (Par1), and occipital (Oc1M-Oc2) cortex of adult rats. One hundred-forty-five immunostained varicosities from Fr1-Fr2 (54 from layers I-II; 91 from layer VI) and 97 each from the upper layers (I-II) of Par1 and OcM1-Oc2 were examined in groups of serial thin sections (mean number of sections in series: 3.2 to 7.3). These terminals were of comparable shape and size in the 4 cortical sectors examined, and averaged 0.66 +/- 0.2 microns in mean diameter. The proportion of varicosities engaged in synaptic contact was evaluated by linear transformation of the relationship between the frequency of observed synaptic junctions and the number of thin sections available for examination. Reliability of the sampling was evidenced by a high coefficient of correlation (r greater than 0.95) in each cortical sector. The synaptic incidence extrapolated for whole varicosities ranged from 28% (layer VI of Fr1-Fr2) to 46% (Par1), without statistically significant differences between the 4 sectors examined. The interregional mean could thus be evaluated at 38%. The synaptic 5-HT terminals always made asymmetrical junctions, which were exclusively found on dendritic spines and shafts, and appeared more frequent on spines than shafts in the deep frontal and the upper occipital cortex. In all 4 sectors, dendritic shafts and spines and other axonal varicosities were frequently encountered in the immediate microenvironment of the immunostained varicosities. It is concluded that the cortical 5-HT innervation is predominantly nonjunctional throughout the neocortex of the adult rat, which reinforces earlier views of a highly divergent afferent system with particular functional properties and perhaps capable of widespread, global and/or sustained influences in this part of the brain.

Ultrastructural features of dopamine axon terminals in the anteromedial and the suprarhinal cortex of adult rat

The ultrastructural features and synaptic relationships of dopamine (DA) axon terminals were examined in the prefrontal cortex of adult rat after immunocytochemical staining with a highly specific polyclonal antiserum directed against DA-glutaraldehyde-lysyl-protein conjugate (donated by M. Geffard). Single and serial ultrathin sections were obtained from the deep layers of the anteromedial and the suprarhinal DA fields. The DA axon terminals from both regions averaged 0.7 micron in diameter, contained a mixed population of small, round and clear synaptic vesicles associated with a few larger dense-cored or fully immunostained vesicles, and frequently exhibited synaptic contacts which were exclusively made on dendritic shafts and spines. These synapses were mostly of the symmetrical type (80%) and were more often seen on dendritic shafts than spines, particularly in the suprarhinal (89%) compared with the anteromedial cortex (62%). As estimated either by stereological extrapolation from single sections or by direct observation in serial sections, the synaptic incidence of these DA varicosities was significantly greater in the anteromedial than suprarhinal DA field. In the longest series of thin sections, a junctional complex could be observed on 93% of the DA varicosities from the anteromedial cortex but only on 56% in the suprarhinal cortex. Such an inter-regional disparity in the relational characteristics of the DA input will need to be taken into account in elucidating the role and properties of this monoamine in cerebral cortex.

Distribution and morphological characteristics of dopamine-immunoreactive neurons in the midbrain of the squirrel monkey (Saimiri sciureus)

The distribution and morphological characteristics of dopamine (DA) neurons in the midbrain of the squirrel monkey (Saimiri sciureus) were investigated by peroxidase-antiperoxidase (PAP) immunohistochemistry with a highly specific antiserum raised against DA-glutaraldehyde-lysyl-protein conjugate (donated by M. Geffard). Four contiguous areas contained DA-immunostained nerve cell bodies: (1) the substantia nigra, pars compacta (SNc), (2) the ventral tegmental area (VTA), (3) the retrorubral area (RRA), and (4) the periaqueductal gray (PAG). The SNc composed the vast majority of DA-immunostained neurons. Most of these neurons were relatively large (mean diameters: 35 x 15 micron) and varied in shape from fusiform to polygonal, but a few smaller (16 x 10.5 micron) globular cells were dispersed among them. The caudal two-thirds of the SNc was particularly rich in DA somata. Rostrally, these DA cells formed several distinct columns impinging deeply upon the underlying pars reticulata. Large oval sectors mostly devoid of immunoreactivity were delineated by these trabeculae. The long dendritic processes of DA neurons in the SNc were generally oriented in prominent dorsoventral bundles the ventralmost portion of which arborized diffusely along the dorsal surface of the cerebral peduncle. In the VTA, the DA neurons were regrouped in a triangular zone located dorsal to the interpeduncular nucleus, medial to the substantia nigra and ventral to the oculomotor nucleus. These DA cells were of medium size (19 x 10.5 micron), globular or fusiform, and usually showed one or two thick primary dendrites oriented dorsoventrally. The DA cells in the RRA lay in continuity with the most caudal DA-containing elements of the substantia nigra but could be distinguished by their smaller size (26 x 12 micron), shorter and more profusely branched dendrites, and darker immunostaining. These DA neurons were characteristically scattered among and medial to the fibers of the medial lemniscus, and a few could be observed as far caudally as the pedunculopontine nucleus. In the PAG, DA-immunostained neurons were seen in the rostral half of the mesencephalic central gray and predominated in its ventral half. These cells were of medium size (22.5 x 10 micron) and some of them were found in proximity to the ventricular lining. At caudal levels, the DA-positive cells in the PAG did not intermingle with dorsal raphe neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of GABA-immunoreactive neurons in the thalamus of the squirrel monkey (Saimiri sciureus)

A light microscopic study of the cellular localization of GABA in the thalamus of the squirrel monkey (Saimiri sciureus) was undertaken by means of the indirect peroxidase-antiperoxidase method using a highly purified antiserum directed against GABA-glutaraldehyde-lysyl-protein conjugate. GABA-immunoreactive cell bodies and axon terminals were visualized in all thalamic nuclei in the squirrel monkey but their relative density varied from one nucleus to the other. At the level of the anterior nuclear group, GABA-positive cells and terminals abounded in the anterodorsal nucleus but were much less numerous in the anteromedial and anteroventral nuclei. In the nuclei of the ventral group, GABA-immunoreactive cells were found to be smaller and less numerous than nonimmunoreactive neurons. In the ventral anterior nucleus, GABA-positive neuronal profiles formed typical clusters, whereas they were more uniformly distributed in the posterior nuclei of the ventral group. In the intralaminar nuclei, GABA-immunoreactive cells and terminals abounded in the dorsal portion of the paracentral and centrolateral nuclei, whereas more caudally, GABA-positive terminals pervaded the entire parafascicular nucleus. In the mediodorsal nucleus, GABA-positive cell bodies and axon terminals formed typical clusters of various sizes scattered within the lateral parvocellular portion of the nucleus, while GABA-immunoreactive neuronal profiles were less numerous and more uniformly distributed in the medial portion of this structure. In the nuclei of the posterior group, GABA-immunoreactive neuronal profiles were uniformly distributed except in the pulvinar where they abounded in the inferior and oral parts but were scarce in the medial part. In the dorsal lateral geniculate nucleus, the magnocellular layers received the most massive GABA-positive innervation and contained the largest number of GABA-immunoreactive cell bodies. In the ventral lateral geniculate nucleus, GABA-positive cells occurred only ventrolaterally while GABA-immunoreactive terminals pervaded the entire structure. In the medial geniculate nucleus, GABA-immunoreactive cell bodies and terminals abounded particularly within the ventromedial third of the structure. In the habenula, a few GABA-immunoreactive cell bodies and numerous GABA-positive terminals were scattered throughout the lateral habenular nucleus, whereas only a few GABA-immunoreactive terminals surrounded the closely packed unreactive cells in the medial habenular nucleus. In contrast to other thalamic nuclei all neurons in the reticular nucleus displayed GABA immunoreactivity.(ABSTRACT TRUNCATED AT 400 WORDS)