Foreground contextual fear memory consolidation requires two independent phases of hippocampal ERK/CREB activation

Fear conditioning is a popular model for investigating physiological and cellular mechanisms of memory formation. In this paradigm, a footshock is either systematically associated to a tone (paired conditioning) or is pseudorandomly distributed (unpaired conditioning). In the former procedure, the tone/shock association is acquired, whereas in the latter procedure, the context/shock association will prevail. Animals with chronically implanted recording electrodes show enhanced amplitude of the extracellularly recorded field EPSP in CA1 pyramidal cells for up to 24 h after unpaired, but not paired, fear conditioning. This is paralleled by a differential activation of the ERK/CREB pathway in CA1, which is monophasic in paired conditioning (0-15 min post-conditioning), but biphasic (0-1 h and 9-12 h post-conditioning) in unpaired conditioning as revealed by immunocytochemistry and Western blotting. Intrahippocampal injection of the MEK inhibitor U0126 prior to each phase prevents the activation of both ERK1/2 and CREB after unpaired conditioning. Block of any activation phase leads to memory impairment. We finally reveal that the biphasic activation of ERK/CREB activity is independently regulated, yet both phases are critically required for the consolidation of long-term memories following unpaired fear conditioning. These data provide compelling evidence that CA1 serves different forms of memory by expressing differential cellular mechanisms that are dependent on the training regime.

Effects of age and spatial learning on adenylyl cyclase mRNA expression in the mouse hippocampus

Adenylyl cyclase (AC) subtypes have been implicated in memory processes and synaptic plasticity. In the present study, the effects of aging and learning on Ca2+/calmodulin-stimulable AC1, Ca2+-insensitive AC2 and Ca2+/calcineurin-inhibited AC9 mRNA level were compared in the dorsal hippocampus of young-adult and aged C57BL/6 mice using in situ hybridization. Both AC1 and AC9 mRNA expression were downregulated in aged hippocampus, whereas AC2 mRNA remained unchanged, suggesting differential sensitivities to the aging process. We next examined AC mRNA expression in the hippocampus after spatial learning in the Morris water maze. Acquisition of the spatial task was associated with an increase of AC1 and AC9 mRNA levels in both young-adult and aged groups, suggesting that Ca2+-sensitive ACs are oppositely regulated by aging and learning. However, aged-trained mice had reduced AC1 and AC9, but greater AC2, mRNA levels relative to young-trained mice and age-related learning impairments were correlated with reduced AC1 expression in area CA1. We suggest that reduced levels of hippocampal AC1 mRNA may greatly contribute to age-related defects in spatial memory.

Resistance to extinction is associated with impaired immediate early gene induction in medial prefrontal cortex and amygdala

Extinction of classical fear conditioning is thought to involve activity-dependent potentiation of synaptic transmission in the medial prefrontal cortex (mPFC), resulting in the inhibition of amygdala-dependent fear responses. While many studies have addressed the mechanisms underlying extinction learning, it is unclear what determines whether extinction memory is consolidated or whether spontaneous recovery of the fear response occurs. Here we show, using a combined electrophysiological and immunocytochemical approach, that spontaneous recovery of conditioned fear in mice is associated with a prolonged expression of long-term depression of synaptic transmission in the mPFC and the failure of induction of the immediate-early genesc-Fos and zif268 in the mPFC and the basolateral nucleus of the amygdala. This suggests that coordinated activity-dependent changes in gene expression in the mPFC and the amygdala may underlie the formation of long-term fear extinction memory.

Chronic ethanol consumption restores the age-related decrease in neurogranin mRNA level in the hippocampus of mice

Neurogranin (Ng) is a Ca(2+)-sensitive calmodulin-binding neuron-specific protein that has been implicated in the regulation of numerous post-synaptic signalling pathways. Here, we investigate the effects of 5 months low level ethanol consumption (approximately 20% of total calories intake) on Ng mRNA expression in the brain of adult (approximately 7-8 months) and aged (approximately 21-22 months) mice using in situ hybridization histochemistry. Results showed that ageing was accompanied by a decrease in amounts of mRNA coding for Ng, especially in the hippocampus (approximately 25% of adults) known to play a critical role in higher cognitive functions. Chronic ethanol consumption restored this decline up to pre-senescent (adult) levels without altering Ng mRNA levels in adult mice. On the basis of recent data indicating a central role for Ng in the regulation of hippocampal synaptic plasticity and spatial learning, our results suggest that moderate ethanol consumption might have a beneficial influence on cognitive deterioration during senescence. Such a possibility is in fact congruent with recent follow-up studies conducted in elderly people.

Selective age-related changes in the PKC-sensitive, calmodulin-binding protein, neurogranin, in the mouse brain

Brain ageing is associated with a dysregulation of intracellular calcium (Ca(2+)) homeostasis which leads to deficits in Ca(2+)-dependent signalling pathways and altered neuronal functions. Given the crucial role of neurogranin/RC3 (Ng) in the post-synaptic regulation of Ca(2+) and calmodulin levels, age-dependent changes in the levels of Ng mRNA and protein expression were analysed in 3, 12, 24 and 31-month-old mouse brains. Ageing produced significant decreases in Ng mRNA expression in the dorsal hippocampal subfields, retrosplenial and primary motor cortices, whereas no reliable changes were seen in any other cortical regions examined. Western blot indicated that Ng protein expression was also down-regulated in the ageing mouse brain. Analysis of Ng immunoreactivity in both hippocampal CA1 and retrosplenial areas indicated that Ng protein in aged mice decreased predominantly in the dendritic segments of pyramidal neurones. These data suggest that age-related changes of post-synaptic Ng in selected brain areas, and particularly in hippocampus, may contribute to altered Ca(2+)/calmodulin-signalling pathways and to region-specific impairments of synaptic plasticity and cognitive decline.

Differential regulation of Ca(2+)-calmodulin stimulated and Ca(2+)-insensitive adenylyl cyclase messenger RNA in intact and denervated mouse hippocampus

The Ca(2+)-calmodulin stimulated AC1 and Ca(2+)-insensitive AC2 are major isoforms of adenylyl cyclase, playing an important role in synaptic plasticity in the mammalian brain. We studied the pattern of expression of AC1 and AC2 genes in the hippocampus of C57BL/6 mice. We found that there were differences in their patterns of distribution in the dentate gyrus. AC1 messenger RNA was detected both in the dentate granule cell bodies and the corresponding molecular field whereas AC2 messenger RNA was preferentially distributed in the dentate granule cell layer, suggesting that AC1 and AC2 messenger RNA are differentially regulated in the dentate gyrus. In order to examine the regulation of AC1 and AC2 expression in response to synaptic deafferentation and reinnervation, the distribution patterns of the two AC messenger RNA in the hippocampal fields and the parietal cortex were analysed 2, 5, 9 and 30 days following an unilateral entorhinal cortex lesion. Interestingly, we found significantly reduced levels of AC1 hybridization signal following the lesion whereas the level of AC2 messenger RNA remained unaffected in all lesioned groups. The changes in AC1 messenger RNA were transient, with a maximal reduction at five days postlesion, and were restricted to the granule cell bodies and stratum moleculare of the deafferented dentate gyrus. No significant change in AC1 messenger RNA levels was detected in other hippocampal fields nor for any other postlesion times studied. These findings suggest that, at least in the dentate gyrus, messenger RNA for AC1 and AC2 might be differentially compartmentalized in cell bodies and dendritic fields. The activity-dependent regulation of AC1 messenger RNA levels by afferent synapses may provide an elegant mechanism for achieving a selective local regulation of AC1 protein, close to its site of action.

The role of Ca2+/calmodulin-stimulable adenylyl cyclases as molecular coincidence detectors in memory formation

Evidence from systems as diverse as mollusks, insects and mammals has revealed that adenylyl cyclase, cyclic adenosine 3',5'-monophosphate (cAMP) cascade, cAMP-dependent protein kinases and their substrates are required for the cellular events underlying the short-term and long-term forms of memory. In Aplysia and Drosophila models, the coincident activation of independent paths converge to produce a synergistic activation of Ca2+/calmodulin-stimulable adenylyl cyclase, thereby enhancing the cAMP level that appears as the primary mediator of downstream events that strengthen enduring memory. In mammals, in which long-term memories require hippocampal function, our understanding of the role of adenylyl cyclases is still fragmentary. Of the differently regulated isoforms present in the hippocampus, the susceptibility of type 1 and type 8 to stimulation by the complex Ca2+/calmodulin and their expression in the hippocampus suggest a role for these two isoforms as a molecular coincidence device for hippocampus-related memory function. Here, we review the key features of Ca2+/calmodulin stimulable adenylyl cyclases, as well as the involvement of cAMP-regulated signaling pathway in the processes of learning and memory.

Ca2+-sensitive adenylyl cyclases, key integrators of cellular signalling

The concept of second messenger signalling originated from the discovery of the role of cyclic AMP, although it is now known that cytosolic calcium [Ca2+]i mediates numerous signalling pathways and plays an equally vital role in many cellular events. In the last few years there has been a great deal of interest in the substantial molecular and functional diversity of mammalian adenylyl cyclases (ACs). Although AC was viewed as a generic activity, which was either stimulated or inhibited by stimulatory or inhibitory receptors, respectively, acting via alpha-subunits of trimeric GTP-regulatory proteins, the recent cloning of nine full-length isoforms, which significantly differ in their regulatory properties and tissue distributions, has revealed an unexpected level of complex regulation. In fact, each AC may integrate convergent inputs from many distinct signal-generating pathways. The nine isoforms can be divided into four distinct families, which reflect their distinct patterns of regulation by betagamma subunits of G-proteins, protein kinase C (PKC) and Ca2+. The mechanisms of regulation are often highly synergistic or conditional, suggesting a function of ACs as coincident detectors. Since all nine isoforms can be regulated either directly or indirectly by Ca2+ or PKC, a complex range of responses is possible. The Ca2+ concentration that stimulates the major ACs in brain has been found to inhibit AC activity in a number of peripheral tissues and cell lines. The purpose of this article is to review many of the important aspects about the distinct regulatory properties and cellular distribution of Ca2+-regulated ACs. Indeed, the notion that Ca2+ and cAMP are "synarchic" messengers acting in concert to regulate cellular activity was formally proposed some time ago. Here, we will focus on acute interactions between Ca2+ and cAMP and attempt to understand how AC activities can be regulated by discrete, physiological [Ca2+]i rises in intact cells. All Ca2+-regulated isoforms have characteristic distribution patterns in the brain. Also discussed are emerging insights on the temporal and spatial regulation of Ca2+- and cAMP-regulated pathways which may enable cell stimuli to elicit specific responses.

Dependence of the Ca2+-inhibitable adenylyl cyclase of C6-2B glioma cells on capacitative Ca2+ entry

The ability of adenylyl cyclases to be regulated by physiological transitions in Ca2+ provides a key point for integration of cytosolic Ca2+ concentration ([Ca2+]i) and cAMP signaling. Ca2+-sensitive adenylyl cyclases, whether endogenously or heterologously expressed, require Ca2+ entry for their regulation, rather than Ca2+ release from intracellular stores (Chiono, M., Mahey, R., Tate, G., and Cooper, D. M. F. (1995) J. Biol. Chem. 270, 1149-1155; Fagan, K., Mahey, R., and Cooper, D. M. F. (1996) J. Biol. Chem. 271, 12438-12444). The present study compared the regulation by capacitative Ca2+ entry versus ionophore-mediated Ca2+ entry of an endogenously expressed Ca2+-inhibitable adenylyl cyclase in C6-2B cells. Even in the face of a dramatic [Ca2+]i rise generated by ionophore, Ca2+ entry via capacitative Ca2+ entry channels was solely responsible for the regulation of the adenylyl cyclase. Selective efficacy of BAPTA over equal concentrations of EGTA in blunting the regulation of the cyclase by capacitative Ca2+ entry defined the intimacy between the adenylyl cyclase and the capacitative Ca2+ entry sites. This association could not be impaired by disruption of the cytoskeleton by a variety of strategies. These results not only establish an intimate spatial relationship between an endogenously expressed Ca2+-inhibitable adenylyl cyclase with capacitative Ca2+ entry sites but also provide a physiological role for capacitative Ca2+ entry other than store refilling.

Immunological assessment of the distribution of type VII adenylyl cyclase in brain

The localization of the nine identified isoforms of adenylyl cyclase in brain has been largely based on determination of patterns of mRNA expression. A polyclonal antibody has now been developed that specifically recognizes Type VII adenylyl cyclase. This antibody was used for immunocytochemical analysis of the distribution of Type VII adenylyl cyclase in rat brain. Labeling of Type VII adenylyl cyclase was observed in several areas, including cerebellum, caudate-putamen, nucleus accumbens, hippocampus and cerebral cortex. In some of these areas, the staining of the adenylyl cyclase protein suggested the possibility of presynaptic localization. For example, in situ hybridization showed Type VII adenylyl cyclase mRNA concentrated in cerebellar granule neurons. The cerebellar granule cell layer, however, showed little immunostaining, while punctate immunostaining was observed in the molecular layer. These results suggested that protein synthesized in the granule neurons may be targeted to the neuron terminals. Punctate staining in the caudate-putamen, globus pallidus and nucleus accumbens also suggested the possibility of axonal and/or dendritic localization of Type VII adenylyl cyclase in these regions. Labeling of the soma of cerebellar Purkinje cells, cortical pyramidal and non-pyramidal cells and interneurons in the cerebellum and hippocampus was also observed. Type VII adenylyl cyclase, like the other adenylyl cyclase isoforms, has distinct regulatory characteristics, including sensitivity to stimulation by Gsalpha and G protein betagamma subunits, modulation by protein kinase C, and high sensitivity to stimulation by ethanol. These characteristics, and the discrete localization of this enzyme, may contribute to its ability to provide signal integration and/or control of neurotransmitter release in particular neurons or brain areas.

Ca(2+)-inhibitable adenylyl cyclase and pulmonary microvascular permeability

Intracellular mechanisms responsible for endothelial cell disruption are unknown, although either elevated cytosolic Ca2+ ([Ca2+]i) or decreased adenosine 3',5'-cyclic monophosphate (cAMP) promotes permeability. Recent identification that Ca(2+)-inhibitable adenylyl cyclase establishes an inverse relationship between [Ca2+]i and cAMP in macrovascular endothelial cells provided a possible mechanism of development of permeability. However, these data utilized an in vitro model; lacking was evidence supporting 1) expression of Ca(2+)-inhibitable adenylyl cyclase in pulmonary microvascular endothelium and 2) Ca2+ inhibition of adenylyl cyclase and cAMP content as a paradigm for inflammatory mediator-induced permeability in the intact circulation. We therefore addressed these issues in microvascular endothelial cells derived from rat lung and in an isolated perfused rat lung preparation. Results demonstrate expression of a Ca(2+)-inhibitable adenylyl cyclase in microvascular endothelial cells. Furthermore, data suggest that Ca2+ inhibition of adenylyl cyclase is necessary for development of microvascular permeability in the intact circulation. We conclude Ca2+ inhibition of cAMP represents a critical step in genesis of microvascular permeability in the intact pulmonary circulation.

Regulation of growth hormone secretion by amino acid neurotransmitters in the goldfish (I): Inhibition by N-methyl-D, L-aspartic acid

High levels of the amino acid neurotransmitter glutamate were found in the goldfish hypothalamus and pituitary using high performance liquid chromatography with fluorometric detection. A specific polyclonal antibody to glutamate was generated in the rabbit for immunocytochemistry. Localization studies demonstrated that glutamatergic neurons of undetermined origin innervate the particular part of the goldfish adenohypophysis where somatotrophs and gonadotrophs are located. Intraperitoneal and brain third ventricle injection of the glutamate agonist N-methyl-D,L-aspartic acid (NMA) inhibited GH release in vivo. The gonadal steroid estradiol plays an important role in regulating GH secretion by stimulating basal serum GH levels and enhancing the inhibitory effects of NMA on GH secretion. Taken together, these results demonstrate that glutamate is an important regulator of GH secretion in goldfish.

Adenylate cyclases: critical foci in neuronal signaling

Current findings show that adenylate cyclases comprise a heterogeneous multigene family, members of which are variously regulated by the alpha and beta gamma subunits of G proteins, by Ca2+ and by protein kinases. In the CNS, individual isoforms of adenylate cyclase are expressed discretely in select regions of the brain. At the subcellular level, adenylate cyclases can be concentrated into dendritic spines, thereby increasing their susceptibility to multiple regulatory influences. Altogether, such findings greatly expand knowledge of the potential role of this archetypical signaling system in the modulation of neuronal function.

Immunohistochemical localization of adenylyl cyclase in rat brain indicates a highly selective concentration at synapses

Only three isoforms of adenylyl cyclase (EC 4.6.1.1) mRNAs (AC1, -2, and -5) are expressed at high levels in rat brain. AC1 occurs predominantly in hippocampus and cerebellum, AC5 is restricted to the basal ganglia, whereas AC2 is more widely expressed, but at much lower levels. The distribution and abundance of adenylyl cyclase protein were examined by immunohistochemistry with an antiserum that recognizes a peptide sequence shared by all known mammalian adenylyl cyclase isoforms. The immunoreactivity in striatum and hippocampus could be readily interpreted within the context of previous in situ hybridization studies. However, extending the information that could be gathered by comparisons with in situ hybridization analysis, it was apparent that staining was confined to the neuropil--corresponding to immunoreactive dendrites and axon terminals. Electron microscopy indicated a remarkably selective subcellular distribution of adenylyl cyclase protein. In the CA1 area of the hippocampus, the densest immunoreactivity was seen in postsynaptic densities in dendritic spine heads. Labeled presynaptic axon terminals were also observed, indicating the participation of adenylyl cyclase in the regulation of neurotransmitter release. The selective concentration of adenylyl cyclases at synaptic sites provides morphological data for understanding the pre- and postsynaptic roles of adenylyl cyclase in discrete neuronal circuits in rat brain. The apparent clustering of adenylyl cyclases, coupled with other data that suggest higher-order associations of regulatory elements including G proteins, N-methyl-D-aspartate receptors, and cAMP-dependent protein kinases, suggests not only that the primary structural information has been encoded to render the cAMP system responsive to the Ca(2+)-signaling system but also that higher-order strictures are in place to ensure that Ca2+ signals are economically delivered and propagated.

The characterization of a novel human adenylyl cyclase which is present in brain and other tissues

We characterized a human cDNA clone which encodes a novel adenylyl cyclase. Data from Southern and Northern blot analysis, and analysis of sequence similarity with a recently cloned mouse adenylyl cyclase (10), indicated that the human adenylyl cyclase was a species variant of type VII adenylyl cyclase. The sequence of the novel human adenylyl cyclase indicated it was a member of the type II adenylyl cyclase family, and we compared the regulatory characteristics of the novel human enzyme with those of type II adenylyl cyclase. The human type VII and rat type II adenylyl cyclases, expressed in human embryonic kidney 293 cells, were activated by prostaglandin E1 (PGE1), but only type VII was activated by isoproterenol. The stimulation of type VII adenylyl cyclase by PGE1 and isoproterenol was attenuated by pretreatment of the cells with staurosporine. Phorbol 12,13-dibutyrate synergistically enhanced the stimulation of both type VII and type II enzyme activity by PGE1 and by the constitutively active Gs mutant Gs (Q227L). The human type VII adenylyl cyclase activity was unresponsive to capacitatively induced changes in intracellular Ca2+. The functional characteristics of human type VII adenylyl cyclase resemble those of the rat type II enzyme, but the enzymes may respond differently to in vivo phosphorylation conditions. While the mRNA for adenylyl cyclase type II was found in several brain areas, the message for type VII adenylyl cyclase was localized primarily to the cerebellar granule cell layer.

Adenylyl cyclases and the interaction between calcium and cAMP signalling

Adenylyl cyclase is the prototypical second messenger generator. Nearly all of the eight cloned adenylyl cyclases are regulated by one or other arm of the phospholipase C pathway. Functional and ultrastructural investigations have shown that adenylyl cyclases are intimately associated with sites of calcium ion entry into the cell. Oscillations in cellular cyclic AMP levels are predicted to arise because of feedback inhibition of adenylyl cyclase by Ca2+. Such findings inextricably intertwine cellular signalling by cAMP and internal Ca2+ and extend the known regulatory modes available to cAMP.

Adenylyl cyclase mRNA expression does not reflect the predominant Ca2+/calmodulin-stimulated activity in the hypothalamus

Only three (Types I, II, V) of the six currently-described subtypes of adenylyl cyclase are prominently expressed in the rat brain. These species are differently sensitive to Ca2+, beta gamma subunits of G-proteins and protein kinase C. A knowledge of the susceptibility of the cAMP-signalling system in particular brain regions to these diverse modes of regulation can shed light on the mechanism of action of the neurotransmitters that modify neuronal activity in such regions. Cyclic AMP is extensively involved in the physiological functions of the hypothalamus. We have used in situ hybridization histochemistry with synthetic oligonucleotides to examine the expression in the rat hypothalamus of the three major brain subtypes of adenylyl cyclase-Ca2+/calmodulin-stimulable (Type I), Ca(2+)-insensitive (Type II) and Ca(2+)-inhibitable (Type V). The hypothalamus expresses high levels only of Type II mRNA, particularly in the supraoptic and paraventricular nuclei. Curiously, the strong expression of the Ca(2+)-insensitive Type II mRNA and the lack of expression of the major brain specific Type I mRNA does not correlate with the adenylyl cyclase activity, which is largely Ca2+/calmodulin stimulable in plasma membranes prepared from the hypothalamus.

Type VIII adenylyl cyclase. A Ca2+/calmodulin-stimulated enzyme expressed in discrete regions of rat brain

A cDNA that encodes type VIII adenylyl cyclase has been isolated from two rat brain libraries. The open reading frame encodes a 1248-amino acid protein predicted to have two sets of six transmembrane spans and two putative nucleotide binding domains as is characteristic of other mammalian adenylyl cyclases. Two type VIII messages are detected in rat brain with estimated sizes of 5.5 and 4.4 kilobases. In situ hybridization indicates that the type VIII messages are most abundantly expressed in the granule cells of the dentate gyrus, the pyramidal cells of hippocampal fields CA1-CA3, the entorhinal cortex, and the piriform cortex. Hybridization is also detected in the neocortex, the amygdaloid complex, and regions of the thalamus and hypothalamus. Stable expression of the type VIII cDNA in human embryonal kidney cells leads to the appearance of a novel 165-kDa glycoprotein in the membrane fraction. Stimulation of these cells with agents that increase intracellular Ca2+ results in up to 43-fold increases in cAMP accumulation over that of control cells transfected with the expression vector. Addition of isoproterenol alone does not lead to type VIII-specific effects in intact cells. Adenylyl cyclase activity in membranes prepared from type VIII-transformed cells is stimulated up to 40-fold by the addition of Ca2+/calmodulin (EC50 = 53 nM calmodulin). The addition of activated recombinant alpha subunit of Gs synergistically increases the Ca2+/calmodulin-stimulated activity. A possible role for type VIII adenylyl cyclase in long-term potentiation is discussed.

Selective expression of one Ca(2+)-inhibitable adenylyl cyclase in dopaminergically innervated rat brain regions

Type I adenylyl cyclase, which can be stimulated by elevated cellular levels of Ca2+, has been proposed to provide a positive coincidence signal detection system, which can integrate signals arising via Gs- and Ca(2+)-mediated pathways. The occurrence of this adenylyl cyclase in brain regions implicated with associative learning in invertebrates and with the mammalian model of plasticity--hippocampal long-term potentiation, supports the notion that the ability of this species of adenylyl cyclase to detect two signals simultaneously may play a role in this neuronal function. In the present study, two recently cloned, closely-related adenylyl cyclases (Types V and VI), are shown to be inhibited by physiological elevation in [Ca2+]i. As a first step towards probing the neuronal significance of Ca(2+)-inhibitable adenylyl cyclases, their distribution was evaluated by in situ hybridization analysis of the rat brain. Strikingly distinct patterns of gene expression were found, ranging from a highly selective distribution of Type V mRNA within the striatum, nucleus accumbens and olfactory tubercle, to a weak and ubiquitous distribution of Type VI mRNA. Type V AC mRNA is expressed exclusively in medium-sized striatal neurons, which also express D1-dopaminergic (Gs-linked) and M1-muscarinic cholinergic (Ca(2+)-linked) receptors. Thus the adenylyl cyclase is primed for simultaneous detection of opposing regulatory influences. The utility of this novel mode of signal detection to dopaminergic function remains to be established.

Discrete expression of Ca2+/calmodulin-sensitive and Ca(2+)-insensitive adenylyl cyclases in the rat brain

Physiological studies and inferences from invertebrate models implicate Ca2+/calmodulin-sensitive adenylyl cyclase with memory processes. However, Ca2+/calmodulin-insensitive adenylyl cyclase also occurs in brain, and its neuronal functions are less clear. Two oligonucleotide probes, based on rat cDNAs for Types I and II adenylyl cyclase, which appear to correspond to these functional forms, were used to conduct in situ hybridization analysis of the relative abundance and localization of these two species in the rat brain as a first step in evaluating their neuronal role. Quite discrete patterns of expression were encountered; in some areas, both species were co-expressed, but in others, little overlap was observed. The differential expression of the two mRNAs suggests that discrete roles may be fulfilled by the two adenylyl cyclases in neural tissues.

Preparation and characterization of a specific antibody for the immunohistochemical detection of L-dopa in paraformaldehyde-fixed rodent brains

A rat polyclonal antiserum has been obtained after coupling of L-3,4-dihydroxyphenylalanine (L-DOPA) to larger proteins using a low concentration of glutaraldehyde. The antiserum was tested for its affinity and specificity using an enzyme-linked-immunosorbent-assay (ELISA). From competition experiments, the most immunoreactive compound was found to be the non-reduced L-DOPA conjugate. Our specific L-DOPA antiserum enables us to visualize L-DOPA molecule on brain of guinea pigs and rats. We examined the immunohistochemical distribution of the polyclonal L-DOPA antiserum after the fixation of brains with a mixture of paraformaldehyde and picric acid. The presence of L-DOPA-immunoreactive (IR) neurons and fibers was described in the posterior, dorsal and periventricular hypothalamic areas and in the arcuate nucleus. Finally, the distribution of L-DOPA-IR cells was compared to that of tyrosine hydroxylase (TH)-IR cells, by means of a double staining procedure. The presence of two populations of TH-IR cells (TH-positive/L-DOPA-negative and TH-positive/L-DOPA-positive cells) was described in the dorsal part of the hypothalamus.

Simultaneous monitoring of cytosolic free calcium and exocytosis at the single cell level

Abstract Quinacrine, a fluorescent basic molecule, accumulates in secretory granules of pituitary cells, as was revealed by its colocalization with immunoreactive prolactin. Thus quinacrine fluorescence may be used to monitor secretory activity at the single cell level. Rat pituitary cells in primary culture were loaded with quinacrine and stimulated with physiological secretagogues, such as thyrotrophin-releasing hormone or bradykinin, which induced a multiphasic lowering of fluorescence, corresponding to the loss of quinacrine contained in exocytosed granules. Quinacrine was further used in combination with the fluorescent calcium probe fura-2, in order to monitor simultaneously exocytosis and variations in the cytosolic free calcium concentration, [Ca(2+)](i). With an appropriate selection of the excitation wavelengths, in dual excitation microfluorimetry experiments, it was possible to distinguish between fluorescence changes due to altered [Ca(2+)](i) versus quinacrine exocytosis. Transient elevations of [Ca(2+)](i) were provoked in individual pituitary cells by enhancing calcium influx through voltage gated channels. In part of the cells an initial increase in [Ca(2+)](i) coincided with stimulated quinacrine release. The approach was also applied to cells of the neuroblastoma line NCB20, where stimulation with bradykinin caused a transient rise in [Ca(2+)](i), concomitantly with enhanced exocytosis. No increase in exocytosis was ever detected without an elevation of [Ca(2+)](i), suggesting that in both cellular systems, an increase in [Ca(2+)](i), is absolutely necessary, but not sufficient to induce secretion.

Quantitative image analysis with densitometry for immunohistochemistry and autoradiography of receptor binding sites--methodological considerations

Major technical progress in the development of computer-based image analysis has made possible the entry of autoradiography and immunohistochemistry into a new era where quantification by densitometry has become easily accessible. Autoradiography could become quantitative and displayed adequate reproducibility with the help of emulsion-coated films and the use of scales of standards of known radioactivity exposed and analyzed in parallel to the tissue sections. Immunohistochemistry after revelation by a color-based enzymatic technique can also become quantitative, providing that standardization of the crucial steps of the procedure and calibration through a parallel treatment of a scale of antigen standards can be ensured. Such an approach is described here in the rat with reference to tyrosine hydroxylase (TH), the main synthesizing enzyme for catecholamines, and with dopamine (DA) itself, a catecholaminergic neurotransmitter. The different parts of the procedure, which can influence the results, such as the fixation of the animals by perfusion and the evaluation of the fluctuations via the calibration curve, are discussed in detail. Biological validation of the proposed procedure is described by reference to experiments already well documented biochemically, such as the induction effect of reserpine on TH in the rat locus coeruleus and the depleting effect of alpha-methyltyrosine (AMPT), a well-known blocker of TH activity, on rat striatal DA content. Finally the importance of restricting the measurements to the (pseudo)linear portion of the calibration curve is illustrated by the autoradiographic identification of the differential intrastriatal repartition of the dopaminergic D1 and D2 receptor sites, particularly the dual patch-matrix compartments.

The metabolism of exogenous L-dopa in the brain: an immunohistochemical study of its conversion to dopamine in non-catecholaminergic cells of the rat brain

The characterization and localization of non-catecholaminergic cells producing dopamine after L-Dopa load have been investigated in the normal rat brain by a direct immunohistochemical labelling of amines using specific antibodies. The detection of dopamine-containing non-catecholaminergic cells has been achieved in rats given a commonly used mixture of L-Dopa plus peripheral decarboxylase inhibitor, and compared to controls. Results indicate that serotoninergic neurons tend toward a switch of their metabolism into dopamine production after L-Dopa load in a dose-dependent manner. In addition small non-aminergic cells, identified as aromatic amino-acid decarboxylase-containing cells, were observed to produce dopamine after exogenous L-Dopa load. Possible implications of such results concerning the mode of action of L-Dopa in the brain are discussed.