Mismatches between neurotransmitter and receptor localizations in brain: observations and implications

The TIP39-PTH2 receptor system: unique peptidergic cell groups in the brainstem and their interactions with central regulatory mechanisms

Tuberoinfundibular peptide of 39 residues (TIP39) is the recently purified endogenous ligand of the previously orphan G-protein coupled parathyroid hormone 2 receptor (PTH2R). The TIP39-PTH2R system is a unique neuropeptide-receptor system whose localization and functions in the central nervous system are different from any other neuropeptides. TIP39 is expressed in two brain regions, the subparafascicular area in the posterior thalamus, and the medial paralemniscal nucleus in the lateral pons. Subparafascicular TIP39 neurons seem to divide into a medial and a lateral cell population in the periventricular gray of the thalamus, and in the posterior intralaminar complex of the thalamus, respectively. Periventricular thalamic TIP39 neurons project mostly to limbic brain regions, the posterior intralaminar thalamic TIP39 neurons to neuroendocrine brain areas, and the medial paralemniscal TIP39 neurons to auditory and other brainstem regions, and the spinal cord. The widely distributed axon terminals of TIP39 neurons have a similar distribution as the PTH2R-containing neurons, and their fibers, providing the anatomical basis of a neuromodulatory action of TIP39. Initial functional studies implicated the TIP39-PTH2R system in nociceptive information processing in the spinal cord, in the regulation of different hypophysiotropic neurons in the hypothalamus, and in the modulation of affective behaviors. Recently developed novel experimental tools including mice with targeted mutations of the TIP39-PTH2R system and specific antagonists of the PTH2R will further facilitate the identification of the specific roles of TIP39 and the PTH2R.

Neuropeptide signaling near and far: how localized and timed is the action of neuropeptides in brain circuits?

Neuropeptide signaling is functionally very diverse and one and the same neuropeptide may act as a circulating neurohormone, as a locally released neuromodulator or even as a cotransmitter of classical fast-acting neurotransmitters. Thus, neuropeptides are produced by a huge variety of neuron types in different parts of the nervous system. Within the central nervous system (CNS) there are numerous types of peptidergic interneurons, some with strictly localized and patterned branching morphologies, others with widespread and diffuse arborizations. From morphology alone it is often difficult to predict the sphere of influence of a peptidergic interneuron, especially since it has been shown that neuropeptides can diffuse over tens of micrometers within neuropils, and that peptides probably are released exclusively in perisynaptic (or non-synaptic) regions. This review addresses some questions related to peptidergic signaling in the insect CNS. How diverse are the spatial relations between peptidergic neurons and their target neurons and what determines the sphere of functional influence? At one extreme there is volume transmission and at the other targeted cotransmission at synapses. Also temporal aspects of peptidergic signaling are of interest: how transient are peptidergic messages? Factors important for these spatial and temporal aspects of peptidergic signaling are proximity between release sites and cognate receptors, distribution of peptidase activity that can terminate peptide action and colocalization of other neuroactive compounds in the presynaptic peptidergic neuron (and corresponding receptors in target neurons). Other factors such as expression of different channel types, receptor inactivation mechanisms and second messenger systems probably also contribute to the diversity in temporal properties of peptide signaling.

The morphology and biochemistry of nanostructures provide evidence for synthesis and signaling functions in human cerebrospinal fluid

Background

Cerebrospinal fluid (CSF) contacts many brain regions and may mediate humoral signaling distinct from synaptic neurotransmission. However, synthesis and transport mechanisms for such signaling are not defined. The purpose of this study was to investigate whether human CSF contains discrete structures that may enable the regulation of humoral transmission.

Methods

Lumbar CSF was collected prospectively from 17 participants: with no neurological or psychiatric disease, with Alzheimer's disease, multiple sclerosis, or migraine; and ventricular CSF from two cognitively healthy participants with long-standing shunts for congenital hydrocephalus. Cell-free CSF was subjected to ultracentrifugation to yield supernatants and pellets that were examined by transmission electron microscopy, shotgun protein sequencing, electrophoresis, western blotting, lipid analysis, enzymatic activity assay, and immuno-electron microscopy.

Results

Over 3,600 CSF proteins were identified from repeated shotgun sequencing of cell-free CSF from two individuals with Alzheimer's disease: 25% of these proteins are normally present in membranes. Abundant nanometer-scaled structures were observed in ultracentrifuged pellets of CSF from all 16 participants examined. The most common structures included synaptic vesicle and exosome components in 30-200 nm spheres and irregular blobs. Much less abundant nanostructures were present that derived from cellular debris. Nanostructure fractions had a unique composition compared to CSF supernatant, richer in omega-3 and phosphoinositide lipids, active prostanoid enzymes, and fibronectin.

Conclusion

Unique morphology and biochemistry features of abundant and discrete membrane-bound CSF nanostructures are described. Prostaglandin H synthase activity, essential for prostanoid production and previously unknown in CSF, is localized to nanospheres. Considering CSF bulk flow and its circulatory dynamics, we propose that these nanostructures provide signaling mechanisms via volume transmission within the nervous system that are for slower, more diffuse, and of longer duration than synaptic transmission.

The central nucleus of the amygdala and corticotropin-releasing factor: insights into contextual fear memory

The central nucleus of the amygdala (CeA) has been traditionally viewed in fear conditioning to serve as an output neural center that transfers conditioned information formed in the basolateral amygdala to brain structures that generate emotional responses. Recent studies suggest that the CeA may also be involved in fear memory consolidation. In addition, corticotropin-releasing factor systems were shown to facilitate memory consolidation in the amygdala, which contains a high density of CRF immunoreactive cell bodies and fibers in the lateral part of the CeA (CeAl). However, the involvement of CeA CRF in contextual fear conditioning remains poorly understood. Therefore, we first conducted a series of studies using fiber-sparing lesion and reversible inactivation methods to assess the general role of the CeA in contextual fear. We then used identical training and testing procedures to compare and evaluate the specific function of CeA CRF using CRF antisense oligonucleotides (CRF ASO). Rats microinjected with ibotenic acid, muscimol, or a CRF ASO into the CeA before contextual fear conditioning showed typical levels of freezing during acquisition training but exhibited significant reductions in contextual freezing in a retention test 48 h later. Furthermore, CeA inactivation induced by either muscimol or CRF ASO administration immediately before retention testing did not impair freezing, suggesting that the previously observed retention deficits were caused by inhibition of consolidation rather than fear expression. Collectively, our results suggest CeA involvement in the consolidation of contextual fear memory and specifically implicate CeA CRF as an important mediator.

Neural distribution of nonapeptide binding sites in two species of songbird

Vasotocin (VT) and its mammalian homologue, vasopressin (VP), modulate many social behaviors in a variety of vertebrate species. In songbirds, the effects of centrally administered VT vary according to species, which may reflect species-specific distributions of VT binding sites. Different radioligands used to map receptors in previous autoradiographical studies have revealed nonoverlapping distributions of VT binding, suggesting a heterogeneous population of more than one type of VT receptor. For two model songbird species, the white-throated sparrow (Zonotrichia albicollis) and zebra finch (Taeniopygia guttata), we labeled putative VT receptors with two radioligands, [(125)I]ornithine vasotocin analog ([(125)I]OVTA) and [(125)I]linear VP antagonist ([(125)I]HO-LVA). Competitive binding assays in the lateral septum showed that both ligands were effectively displaced by both VT and a related nonapeptide, mesotocin (MT), showing that these radioligands, which were developed to label mammalian nonapeptide receptors, label at least one population of related receptors in songbirds. [(125)I]OVTA labeled receptors throughout the telencephalon, diencephalon, midbrain, and brainstem, with a similar distribution in both species. In contrast, the binding of [(125)I]HO-LVA was restricted to the septal area, dorsal arcopallium, and optic tectum in sparrow and was essentially undetectable in zebra finch. Because the avian brain is likely to express multiple types of VT receptors, we hypothesize that the binding patterns of these radioligands represent a heterogeneous receptor population.

Neuronal and glial localization of alpha(2A)-adrenoceptors in the adult zebrafish (Danio rerio) brain

The alpha(2A)-adrenoceptor (AR) subtype, a G protein-coupled receptor located both pre- and postsynaptically, mediates adrenaline/noradrenaline functions. The present study aimed to determine the alpha(2A)-AR distribution in the adult zebrafish (Danio rerio) brain by means of immunocytochemistry. Detailed mapping showed labeling of alpha(2A)-ARs, in neuropil, neuronal somata and fibers, glial processes, and blood vessels. A high density of alpha(2A)-AR immunoreactivity was found in the ventral telencephalic area, preoptic, pretectal, hypothalamic areas, torus semicircularis, oculomotor nucleus (NIII), locus coreruleus (LC), medial raphe, medial octavolateralis nucleus (MON), magnocellular octaval nucleus (MaON), reticular formation (SRF, IMRF, IRF), rhombencephalic nerves and roots (DV, V, VII, VIII, X), and cerebellar Purkinje cell layer. Moderate levels of alpha(2A)-ARs were observed in the medial and central zone nuclei of dorsal telencephalic area, in the periventricular gray zone of optic tectum, in the dorsomedial part of optic tectum layers, and in the molecular and granular layers of all cerebellum subdivisions. Glial processes were found to express alpha(2A)-ARs in rhombencephalon, intermingled with neuronal fibers. Medium-sized neurons were labeled in telencephalic, diencephalic, and mesencephlic areas, whereas densely labeled large neurons were found in rhombencephalon, locus coeruleus, reticular formation, oculomotor area, medial octavolateralis and magnocellular octaval nuclei, and Purkinje cell somata. The functional role of alpha(2A)-ARs on neurons and glial processes is not known at present; however, their strong relation to the ventricular system, somatosensory nuclei, and nerves supports a possible regulatory role of alpha(2A)-ARs in autonomic functions, nerve output, and sensory integration in adult zebrafish brain.

Truncation of human dopamine transporter by protease calpain

It has been shown recently that the N-terminal domain of the dopamine transporter (DAT) plays a role in several transporter functions. Here we provide evidence for a possible cellular mechanism of how the N-terminus of dopamine transporter might be removed in vivo. We isolated a recombinant N-terminal protein region of human dopamine transporter and cleaved it with calpain protease. Peptide fragment analysis revealed the existence of two calpain cleavage sites at positions Thr43/Ser44 and Leu71/Ser72 of the DATN-terminus. We show that calpain activation in rat striatal synaptosomes leads to a rapid decrease of dopamine transporter N-terminal epitopes corresponding to the protein sequences removed by a calpain cleavage at Thr43/Ser44 and that the process is totally blocked by a calpain inhibitor. Calpain truncation of the DATN-terminus abolishes its interaction with the receptor of activated protein kinase C, RACK1 and removes protein sequences previously implicated in amphetamine-induced dopamine release, PKC-dependent endocytosis and the interaction of DAT with the dopamine D2 receptor. The above suggests that cleavage of DAT by calpain may significantly modify dopamine homeostasis under pathological or physiological conditions.

Listening to neuropeptides by microdialysis: echoes and new sounds?

Neuropeptides represent the largest class of neuromessengers in the central nervous system. They are involved in the regulation of growth processes, reproduction, social behavior, emotion/motivation and cognition. Particularly in subcortical structures, neuropeptides act as neuromodulators, which reach their target sites via diffusion through the extracellular space. This route of information transfer together with the ability of neurons to release neuropeptides from their whole membrane surface predisposes neuropeptides for microdialysis experiments. This review outlines the special characteristics of neuropeptide signaling in relation to other classes of neuromessengers. It further provides a survey of the application of the microdialysis technique for monitoring neuropeptide release patterns in laboratory rodents exemplarily for the two neuropeptides arginine vasopressin and oxytocin, discusses pros and cons of such experiments and outlines perspectives for future neuroendocrine studies in rats and mice.

Nonsynaptic Chemical Transmission Through Nicotinic Acetylcholine Receptors

This review attempts to organize the different aspects of nicotinic transmission in the context of nonsynaptic interactions. Nicotinic acetylcholine receptors (nAChRs) dominantly operate in the nonsynaptic mode in the central nervous system despite their ligand-gated ion-channel nature, which would otherwise be better suited for fast synaptic transmission. This fast form of nonsynaptic transmission, most likely unique to nAChRs, represents a new avenue in the communication platforms of the brain. Cholinergic messages received by nAChRs, arriving at a later phase following synaptic activation, can interfere with dendritic signal integration. Nicotinic transmission plays a role in both neural plasticity and cellular learning processes, as well as in long-term changes in basic activity through fast activation, desensitization of receptors, and fluctuations of the steady-state levels of ACh. ACh release can contribute to plastic changes via activation of nAChRs in neurons and therefore plays a role in learning and memory in different brain regions. Assuming that nAChRs in human subjects are ready to receive long-lasting messages from the extracellular space because of their predominantly nonsynaptic distribution, they offer an ideal target for drug therapy at low, nontoxic drug levels.

Prolactin-releasing peptide

Prolactin-releasing peptide (PrRP) was initially isolated from the bovine hypothalamus as an activating component that stimulated arachidonic acid release from cells stably expressing the orphan G protein-coupled receptor hGR3 (Hinuma et al. 1998) [also known as GPR10 (Marchese et al. 1995), or UHR-1 for the rat orthologue (Welch et al. 1995)]. Initially touted as a prolactin-releasing factor (therefore aptly named prolactin-releasing peptide), the perspective on the function of this peptide in the organism has been greatly expanded. Over 120 papers have been published on this subject since its initial discovery in 1998. Herein I review the state of knowledge of the PrRP system, its putative function in the organism, and implications for therapy.

Theory of active antidepressants: A nonsynaptic approach to the treatment of depression

Although depression is one of the major neuropsychiatric disorders, the success rate of medication for any drug is about 60%, which means that approximately 40% of the patients does not respond to the initial treatment. The major aim of this review is to provide a possible explanation for the relative inefficacy of currently used antidepressants and to propose a novel mechanism of action, which might improve the success rate of clinical treatment. According to the monoamine theory the most important neurochemical process in depression is the impairment of monoaminergic neurotransmission and the concomitant decrease of extracellular concentration of noradrenaline and/or serotonin. Since the vast majority of monoaminergic varicosities makes no synaptic contact but is able to release transmitters directly into the extrasynaptic space, the monoaminergic neurotransmission is predominantly nonsynaptic in nature. Depression can be regarded, therefore, as a disease, which is developed (at least in part) on the basis of the impairment of nonsynaptic interactions and the effective treatment has to improve this non-conventional communication in the nervous system. The currently used antidepressants (reuptake inhibitors, negative feedback inhibitors, monoamino oxidase inhibitors) can increase the monoamine levels in the extracellular space only if the monoaminergic cells are electrically active and without an action potential-induced vesicular exocytosis these compounds are ineffective. It is proposed that a selective and moderate induction of the carrier-mediated release of NA and 5-HT might be a better therapeutic approach to the treatment of depression, since this new class of antidepressants, the so-called 'active antidepressants' have a mechanism of action, which is independent from the electrical activity of monoaminergic cells, therefore the extrasynaptic concentration of monoamines and thereby the nonsynaptic communication can be enhanced more efficiently.

Role of nonsynaptic communication in regulating the immune response

The discovery of nonsynaptic communication in the 1960s and 1970s was an important milestone in investigating the function of the nervous system, and it revolutionized our view about information transmission between neurons. In addition, nonsynaptic communication has a practical importance not only within the nervous system, but in the communication between the peripheral nervous system and other organ systems. Nonsynaptic communication takes place in different immune organs, which are innervated by sympathetic nerve terminals. In addition, the function of microglia, one of the immunocompetent cell types of the brain, can also be affected by neurotransmitters released from axon varicosities. The various functions of immune cells are modulated by released neurotransmitters without any direct synaptic contact between nerve endings and targeted immune cells requiring only functional neurotransmitter receptors on immune cells. Here, we briefly overview the role of the various receptor subtypes mediating nonsynaptic modulation of the function of immunocompetent cells both in the periphery and in the central nervous system.

Extrasynaptic transmission and the diffusion parameters of the extracellular space

Extrasynaptic volume transmission, mediated by the diffusion of neuroactive substances in the extracellular space (ECS), plays an important role in short- and long-distance communication between nerve cells. The ability of a substance to reach extrasynaptic high-affinity receptors via diffusion depends on the ECS diffusion parameters, ECS volume fraction alpha (alpha=ECS volume/total tissue volume) and tortuosity lambda (lambda2=free/apparent diffusion coefficient), which reflects the presence of diffusion barriers represented by, e.g., fine astrocytic processes or extracellular matrix molecules. These barriers channel the migration of molecules in the ECS, so that diffusion may be facilitated in a certain direction, i.e. anisotropic. The diffusion parameters alpha and lambda differ in various brain regions, and diffusion in the CNS is therefore inhomogeneous. Changes in diffusion parameters have been found in many physiological and pathological states, such as development and aging, neuronal activity, lactation, ischemia, brain injury, degenerative diseases, tumor growth and others, in which cell swelling, glial remodeling and extracellular matrix changes are key factors influencing diffusion. Changes in ECS volume, tortuosity and anisotropy significantly affect the accumulation and diffusion of neuroactive substances and thus extrasynaptic transmission, neuron-glia communication, mediator "spillover" and synaptic crosstalk as well as, cell migration. The various changes occurring during pathological states can be important for diagnosis, drug delivery and treatment.

Receptors and neural effects of oxytocin in the rodent hypothalamus and preoptic region

Vasopressin and oxytocin are produced in and secreted from not only hypothalamo-hypophysial neurons which shed their products into the circulation to act as hormones or releasing factors, but also from neurons whose axons form tracts which remain within the central nervous system. Using tritiated or radioiodinated ligands, binding sites for vasopressin and for oxytocin have been detected by in vitro autoradiography. In the rat hypothalamus binding sites for vasopressin are present in the suprachiasmatic, sigmoid and arcuate nuclei, and oxytocin receptors in the area of the ventromedial nucleus. Electrophysiological evidence obtained using single cell recordings in slices suggests that oxytocin-binding sites present in the ventromedial hypothalamus and in the bed nucleus of the stria terminalis mostly represent functional, neuronal receptors. The expression of these receptors (but not of the vasopressin receptors) depends on gonadal steroid hormones, as does that of uterine and mammary gland oxytocin receptors. Modifications of the hormonal status associated with, for example, puberty and lactation cause 'up-regulation' of central and peripheral oxytocin receptors. The central administration of oxytocin facilitates (and the administration of oxytocin agonists inhibits) maternal behaviour and the milk ejection reflex, therefore the hormonal and neural actions of oxytocin appear to be complementary in ensuring the birth and development of the offspring.

From the Golgi–Cajal mapping to the transmitter-based characterization of the neuronal networks leading to two modes of brain communication: Wiring and volume transmission

After Golgi-Cajal mapped neural circuits, the discovery and mapping of the central monoamine neurons opened up for a new understanding of interneuronal communication by indicating that another form of communication exists. For instance, it was found that dopamine may be released as a prolactin inhibitory factor from the median eminence, indicating an alternative mode of dopamine communication in the brain. Subsequently, the analysis of the locus coeruleus noradrenaline neurons demonstrated a novel type of lower brainstem neuron that monosynaptically and globally innervated the entire CNS. Furthermore, the ascending raphe serotonin neuron systems were found to globally innervate the forebrain with few synapses, and where deficits in serotonergic function appeared to play a major role in depression. We propose that serotonin reuptake inhibitors may produce antidepressant effects through increasing serotonergic neurotrophism in serotonin nerve cells and their targets by transactivation of receptor tyrosine kinases (RTK), involving direct or indirect receptor/RTK interactions. Early chemical neuroanatomical work on the monoamine neurons, involving primitive nervous systems and analysis of peptide neurons, indicated the existence of alternative modes of communication apart from synaptic transmission. In 1986, Agnati and Fuxe introduced the theory of two main types of intercellular communication in the brain: wiring and volume transmission (WT and VT). Synchronization of phasic activity in the monoamine cell clusters through electrotonic coupling and synaptic transmission (WT) enables optimal VT of monoamines in the target regions. Experimental work suggests an integration of WT and VT signals via receptor-receptor interactions, and a new theory of receptor-connexin interactions in electrical and mixed synapses is introduced. Consequently, a new model of brain function must be built, in which communication includes both WT and VT and receptor-receptor interactions in the integration of signals. This will lead to the unified execution of information handling and trophism for optimal brain function and survival.

Distribution of tuberoinfundibular peptide of 39 residues and its receptor, parathyroid hormone 2 receptor, in the mouse brain

Tuberoinfundibular peptide of 39 residues (TIP39) was identified as a potent parathyroid hormone 2 receptor (PTH2R) agonist. Existing anatomical data also support the suggestion that TIP39 is the PTH2R's endogenous ligand, but a comprehensive comparison of TIP39 and PTH2R distributions has not been performed. In the present study, we compared the distributions of TIP39 and PTH2R on adjacent mouse brain sections. In addition, we determined the locations of PTH2R-expressing cell bodies by in situ hybridization histochemistry and by labeling beta-galactosidase driven by the PTH2R promoter in knockin mice. An excellent correlation was found between the distributions of TIP39-containing fibers and PTH2R-containing cell bodies and fibers throughout the brain. TIP39 and the PTH2R are abundant in medial prefrontal, insular, and ectorhinal cortices, the lateral septal nucleus, the bed nucleus of the stria terminalis, the fundus striati, the amygdala, the ventral subiculum, the hypothalamus, midline and intralaminar thalamic nuclei, the medial geniculate body, the periaqueductal gray, the ventral tegmental area, the superior and inferior colliculi, the parabrachial nuclei, the locus coeruleus, subcoeruleus and periolivary areas, and the nucleus of the solitary tract. Furthermore, even the subregional distribution of TIP39- and PTH2R-immunoreactive fibers in these regions showed remarkable similarities, providing anatomical evidence that TIP39 may act on the PTH2R. Based on these observations and on previous pharmacological data, we propose that TIP39 is an endogenous ligand of the PTH2R and that they form a neuromodulator system, which is optimally positioned to regulate limbic, endocrine, and auditory brain functions. Published 2007 Wiley-Liss, Inc.

Immunohistochemical distribution of glucose-dependent insulinotropic polypeptide in the adult rat brain

We have previously demonstrated that glucose-dependent insulinotropic polypeptide (GIP; gastric inhibitory polypeptide) is present in the adult rat hippocampus. This finding leads to the conclusion that all members of the secretin-glucagon family of gastrointestinal regulatory polypeptides can be found in the brain. To investigate the localization of GIP-producing cells, we used immunohistochemistry on sections of the adult rat brain. High levels of GIP immunoreactivity were observed in the olfactory bulb, hippocampus, and Purkinje cells in the cerebellum. Moreover, a moderate but distinct GIP immunoreactivity was observed in the cerebral cortex, amygdala, substantia nigra, and lateral septal nucleus as well as in several nuclei in the thalamus, hypothalamus, and brainstem. GIP immunoreactivity was frequently found to colocalize with the neuronal marker NeuN but never with the glial marker glial fibrillary acidic protein. Thus, GIP appears to be mainly neuronal to its distribution. This widespread distribution of GIP-immunoreactive cells suggests the involvement of GIP in various neuronal functions and suggests that GIP may act as a neurotransmitter or neuromodulator. This is the first characterization of the anatomical distribution of GIP-immunoreactive cells in the rat brain providing an anatomical framework for future investigations regarding the functions of GIP in the central nervous system.

Cellular and subcellular distributions of delta opioid receptor activation sites in the ventral oral pontine tegmentum of the cat

The ventral division of the reticular oral pontine nucleus (vRPO) is a pontine tegmentum region critically involved in REM sleep generation. Previous reports of morphine microinjections in the cat pontine tegmentum have shown that opioid receptor activation in this region modulates REM sleep. Even though opiate administration has marked effects on sleep-wake cycle architecture, the distribution of opioid receptors in vRPO has only been partially described. Using an antiserum directed against delta opioid receptor (DOR), to which morphine binds, in the present study, we use (1) light microscopy to determine DOR cellular distribution in the rostral pontine tegmentum and (2) electron microscopy to determine DOR subcellular distribution in the cat vRPO. In the dorsal pons, DOR immunoreactivity was evenly distributed throughout the neuropil of the reticular formation and was particularly intense in the parabrachial nuclei and locus coeruleus; the ventral and central areas of the RPO and locus coeruleus complex were especially rich in DOR-labeled somata. Within the vRPO, DOR was localized mainly in the cytoplasm and on plasma membranes of medium to large dendrites (47.8% of DOR-labeled profiles), which received both symmetric and asymmetric synaptic contacts mainly from non-labeled (82% of total inputs) axon terminals. Less frequently, DOR was distributed presynaptically in axon terminals (19% of DOR-labeled profiles). Our results suggest that DOR activation in vRPO regulates REM sleep occurrence by modulating postsynaptic responses to both excitatory and inhibitory afferents. DOR activation in vRPO could have, however, an additional role in direct modulation of neurotransmitter release from axon terminals.

Relaxin-3 in GABA projection neurons of nucleus incertus suggests widespread influence on forebrain circuits via G-protein-coupled receptor-135 in the rat

Relaxin-3 (RLX3) is a newly identified member of the relaxin/insulin peptide family that is highly conserved across a range of species from fish to mammals and is highly expressed in rat, mouse and human brain. Extensive pharmacological studies have demonstrated that RLX3 is a high affinity, selective ligand for G-protein-coupled receptor-135 (GPCR135, now classified as relaxin family peptide-3 receptor; RXFP3). In ongoing studies to understand the physiological functions of RLX3, the distribution of RLX3-containing neuronal elements in rat brain was determined by immunohistochemistry, using an affinity-purified polyclonal antiserum raised against a conserved segment of the RLX3 C-peptide (AS-R3(85-101)). Consistent with the distribution of RLX3 mRNA, neurons containing RLX3-like immunoreactivity (LI) were observed in the pontine nucleus incertus and the majority of these cells, which are known to express corticotropin-releasing factor receptor-1, were shown to express glutamic acid decarboxylase-65-immunoreactivity, suggesting a GABA phenotype. Nerve fibers and terminals containing RLX3-LI were observed adjacent to cells in the nucleus incertus and in various forebrain regions known to receive afferents from the nucleus incertus, including cortex, septum, hippocampus, thalamus, hypothalamus and midbrain. Regions that contained highest densities of RLX3-positive fibers included the medial septum, lateral preoptic area, lateral hypothalamus/medial forebrain bundle and ventral hippocampus; and additional fibers were observed in olfactory bulb and olfactory and frontal/cingulate cortices, bed nucleus of the stria terminalis, dorsal endopiriform, intergeniculate, and supramammillary nuclei, and the periaqueductal gray and dorsal raphe. The RLX3-positive network overlapped the regional distribution of GPCR135 mRNA and specific binding sites for an [125I]-GPCR135-selective, chimeric peptide. These anatomical findings further support the proposition that RLX3 is the endogenous ligand for GPCR135 in rat brain and provide evidence for broad modulatory activity of RLX3 in behavioral activation relating to autonomic and neuroendocrine control of metabolism and reproduction and higher-order processes such as stress and cognition.

The vasopressin 1b receptor is prominent in the hippocampal area CA2 where it is unaffected by restraint stress or adrenalectomy

The vasopressin 1b receptor (Avpr1b) is one of two principal receptors mediating the behavioral effects of vasopressin (Avp) in the brain. Avpr1b has recently been shown to strongly influence social forms of aggression in mice and hamsters. This receptor appears to play a role in social recognition and motivation as well as in regulating the hypothalamic-pituitary-adrenal axis. Most of these studies have been performed in knockout mice, a species in which the localization of the Avpr1b has not been described, thus precluding correlations with the behaviors. We performed in situ hybridization histochemistry (ISHH) with specific probes and found especially prominent expression within the CA2 pyramidal neurons of the hippocampus, with much lower expression in the hypothalamic paraventricular nucleus and amygdala. Reverse transcriptase-polymerase chain reaction (RT-PCR) confirmed expression in those as well other areas in which the ISHH was not sensitive enough to detect labeled cells (e.g. piriform cortex, septum, caudate-putamen and lower brainstem areas). Mouse Avpr1b transcript levels were not altered in the CA2 field by restraint stress or adrenalectomy. Finally, ISHH and RT-PCR showed expression of the Avpr1b gene in the rat and human hippocampi as well. We suggest that the CA2 field may form or retrieve associations (memories) between olfactory cues and social encounters.

The mysterious trace amines: protean neuromodulators of synaptic transmission in mammalian brain

The trace amines are a structurally related group of amines and their isomers synthesized in mammalian brain and peripheral nervous tissues. They are closely associated metabolically with the dopamine, noradrenaline and serotonin neurotransmitter systems in mammalian brain. Like dopamine, noradrenaline and serotonin the trace amines have been implicated in a vast array of human disorders of affect and cognition. The trace amines are unique as they are present in trace concentrations, exhibit high rates of metabolism and are distributed heterogeneously in mammalian brain. While some are synthesized in their parent amine neurotransmitter systems, there is also evidence to suggest other trace amines may comprise their own independent neurotransmitter systems. A substantial body of evidence suggests that the trace amines may play very significant roles in the coordination of biogenic amine-based synaptic physiology. At high concentrations, they have well-characterized presynaptic "amphetamine-like" effects on catecholamine and indolamine release, reuptake and biosynthesis; at lower concentrations, they possess postsynaptic modulatory effects that potentiate the activity of other neurotransmitters, particularly dopamine and serotonin. The trace amines also possess electrophysiological effects that are in opposition to these neurotransmitters, indicating to some researchers the existence of receptors specific for the trace amines. While binding sites or receptors for a few of the trace amines have been advanced, the absence of cloned receptor protein has impeded significant development of their detailed mechanistic roles in the coordination of catecholamine and indolamine synaptic physiology. The recent discovery and characterization of a family of mammalian G protein-coupled receptors responsive to trace amines such as beta-phenylethylamine, tyramine, and octopamine, including socially ingested psychotropic drugs such as amphetamine, 3,4-methylenedioxymethamphetamine, N,N-dimethyltryptamine, and lysergic acid diethylamide, have revitalized the field of scientific studies investigating trace amine synaptic physiology, and its association with major human disorders of affect and cognition.

The brain as a system of nested but partially overlapping networks. Heuristic relevance of the model for brain physiology and pathology

A new model of the brain organization is proposed. The model is based on the assumption that a global molecular network enmeshes the entire central nervous system. Thus, brain extra-cellular and intra-cellular molecular networks are proposed to communicate at the level of special plasma membrane regions (e.g., the lipid rafts) where horizontal molecular networks can represent input/output regions allowing the cell to have informational exchanges with the extracellular environment. Furthermore, some "pervasive signals" such as field potentials, pressure waves and thermal gradients that affect large parts of the brain cellular and molecular networks are discussed. Finally, at least two learning paradigms are analyzed taking into account the possible role of Volume Transmission: the so-called model of "temporal difference learning" and the "Turing B-unorganised machine". The relevance of this new view of brain organization for a deeper understanding of some neurophysiological and neuropathological aspects of its function is briefly discussed.

The role of substance P in stress and anxiety responses

Substance P (SP) is one of the most abundant peptides in the central nervous system and has been implicated in a variety of physiological and pathophysiological processes including stress regulation, as well as affective and anxiety-related behaviour. Consistent with these functions, SP and its preferred neurokinin 1 (NK1) receptor has been found within brain areas known to be involved in the regulation of stress and anxiety responses. Aversive and stressful stimuli have been shown repeatedly to change SP brain tissue content, as well as NK1 receptor binding. More recently it has been demonstrated that emotional stressors increase SP efflux in specific limbic structures such as amygdala and septum and that the magnitude of this effect depends on the severity of the stressor. Depending on the brain area, an increase in intracerebral SP concentration (mimicked by SP microinjection) produces mainly anxiogenic-like responses in various behavioural tasks. Based on findings that SP transmission is stimulated under stressful or anxiety-provoking situations it was hypothesised that blockade of NK1 receptors may attenuate stress responses and exert anxiolytic-like effects. Preclinical and clinical studies have found evidence in favour of such an assumption. The status of this research is reviewed here.

Dendritic peptide release and peptide-dependent behaviours

Neuropeptides that are released from dendrites, such as oxytocin and vasopressin, function as autocrine or paracrine signals at their site of origin, but can also act at distant brain targets to evoke long-lasting changes in behaviour. Oxytocin, for instance, has profound effects on social bonding that are exerted at sites that richly express oxytocin receptors, but which are innervated by few, if any, oxytocin-containing projections. How can a prolonged, diffuse signal have coherent behavioural consequences? The recently demonstrated ability of neuropeptides to prime vesicle stores for activity-dependent release could lead to a temporary functional reorganization of neuronal networks harbouring specific peptide receptors, providing a substrate for long-lasting effects.

The distribution and neurochemistry of the parathyroid hormone 2 receptor in the rat hypothalamus

This study reports the distribution of parathyroid hormone 2 receptor (PTH2R)-immunoreactive fibers in the hypothalamus using fluorescent amplification immunocytochemistry. The pattern of immunolabeling is strikingly similar to that of tuberoinfundibular peptide of 39 residues (TIP39), a peptide recently purified from bovine hypothalamus and proposed to be a ligand of the PTH2R based on pharmacological data. To investigate the anatomical basis of suggestions that TIP39 affects the secretion of several hypophysiotropic hormones we performed double-labeling studies and found that only somatostatin fibers contain PTH2R in the median eminence, which suggests that somatostatin release could be directly regulated via the PTH2R. However, several hypothalamic nuclei projecting to the median eminence contain a high density of both TIP39 and PTH2R fibers and terminals. We report here, that the PTH2R terminals also contain vesicular glutamate transporter-2, and suggest that TIP39 terminals are ideally positioned to modulate glutamatergic influences on hypophysiotropic neurons.

Effects of cochlear ablation on muscarinic acetylcholine receptor binding in the rat cochlear nucleus

Cholinergic synapses in the cochlear nucleus (CN) have been reported to modulate spontaneous activity via muscarinic acetylcholine receptors. In this study, muscarinic receptor binding was measured as specific binding of 1-[N-methyl-(3)H]scopolamine in CN regions of control rats and 7 days, 1 month, and 2 months after unilateral cochlear ablation. In control rats, the strongest binding was found in granular regions, followed in order by fusiform soma, molecular, and deep layers of the dorsal cochlear nucleus (DCN), with much lower binding in the anteroventral CN (AVCN) and posteroventral CN (PVCN). After unilateral cochlear ablation, binding in the AVCN, PVCN, and their associated granular regions on the lesion side became progressively greater than on the control side through 2 months after lesion. A significant asymmetry, with binding higher on the lesion side, was also found in the DCN fusiform soma layer at 7 days, and there and in the DCN deep layer at 1 and 2 months after lesion. There was also evidence of increased binding on the control side in most CN regions. By contrast, binding in the ipsilateral facial nucleus decreased, compared with the control side, by 7 days after the lesion and showed some recovery toward symmetry by 2 months after lesion, and there was no evidence for contralateral changes. These muscarinic receptor binding changes reflect receptor plasticity after loss of auditory nerve innervation. Such plasticity may underlie some of the central auditory functional changes that occur following peripheral lesions, such as tinnitus and hyperacusis.

The distribution of dopamine D1 receptor and μ-opioid receptor 1 receptor immunoreactivities in the amygdala and interstitial nucleus of the posterior limb of the anterior commissure: Relationships to tyrosine hydroxylase and opioid peptide terminal systems

Mismatches between dopamine innervation and dopamine D1 receptor (D1) distribution have previously been demonstrated in the intercalated cell masses of the rat amygdala. Here the distribution of enkephalin and beta-endorphin immunoreactive (IR) nerve terminals with respect to their mu-opioid receptors is examined in the intercalated cell masses, along with a further immunohistochemical analysis of the dopamine/D1 mismatches. A similar analysis is also made within the extended amygdala. A spatial mismatch in distribution patterns was found between the mu-opioid receptor-1 immunoreactivity and enkephalin IR in the main intercalated island of the amygdala. Discrete cell patches of dopamine D1 receptor and mu-opioid receptor-1 IR were also identified in a distinct region of the extended amygdala, the interstitial nucleus of the posterior limb of the anterior commissure, medial division (IPACM), which displayed sparse tyrosine hydroxylase or enkephalin/beta-endorphin IR nerve terminals. Furthermore, distinct regions of the main intercalated island that showed dopamine/D1 receptor matches (the rostral and rostrolateral parts) were associated with strong dopamine and cyclic AMP regulated phosphoprotein, 32 kDa-IR in several D1 IR neuronal cell bodies and dendrites, whereas this was not the case for the dopamine/D1 mismatch areas (the rostromedial and caudal parts) of the main intercalated island. The lack of correlation between the terminal/receptor distribution patterns suggests a role for volume transmission for mu-opioid receptor- and dopamine D1 receptor-mediated transmission in distinct regions of the amygdala and extended amygdala. This may have implications for amygdaloid function, where slow long lasting responses may develop as a result of volume transmission operating in opioid peptide and dopaminergic communication.

Methodological considerations in rat brain BOLD contrast pharmacological MRI

RATIONALE AND OBJECTIVES

Blood oxygen level dependent (BOLD) contrast pharmacological magnetic resonance imaging (phMRI) is an increasingly popular technique that allows the non-invasive investigation of spatial and temporal changes in rat brain function in response to pharmacological stimulation in vivo. Rat brain BOLD contrast phMRI is, at present, established in few neuropharmacological laboratories, and various issues associated with the technique require attention. The present review is primarily aimed at psychopharmacologists with no previous experience of phMRI, who are interested in the practical aspects that phMRI studies entail.

RESULTS AND DISCUSSION

Experimental and analytical considerations, including anaesthesia, physiological monitoring, drug dose and delivery, scanning protocols, statistical approaches and the interpretation of phMRI data, are discussed.

EMERGING CONCEPTS OF BRAIN FUNCTION

For over 40 years, since I first obtained evidence for nonsynaptic diffusion neurotransmission (most scientists call it Volume Transmission), I have been convinced that we scientists were ignoring organizational dynamics other than the mechanistic synaptic organization of the brain. For many years it was an uneasy feeling, since I was aware there are so many avenues to explore in brain function. I have wondered how much we scientists have ignored, in our quest to understand how the brain really works, due to our efforts to "be scientific". In addition to the difficulty of understanding how the brain functions, how could we even begin to explore the human experience? In this paper I will first discuss some emerging concepts of brain function. I will then comment on the development of concepts that have been a part of my own research experience.

Ontogeny of respiratory sensitivity and tolerance to the mu-opioid agonist fentanyl in rat

Whereas developmental changes in analgesic sensitivity and tolerance to the mu-opioid agonist fentanyl have been reported, knowledge of respiratory responses to that drug is lacking. Using 7- and 14-day-old (P7, P14) and adult conscious rats, we first established, using whole body plethysmography, the fentanyl dose that decreased minute ventilation by 50% (ED50) at each age. ED50 increased with postnatal age (40, 60 and 120 microg/kg sc, respectively), indicating a high sensitivity to fentanyl in the youngest rats that decreased with maturation. In separate rat groups of the 3 ages, we injected each ED50 dose, once a day, for several consecutive days, until tolerance was established. Tolerance was defined as a reduction in respiratory depression from 50% to 75% of baseline. All age groups reached tolerance in minute ventilation, respiratory frequency, tidal volume and instantaneous flow (equivalent to respiratory drive). The P14 rat pups attained tolerance more rapidly (at 2.6 days) than did either the younger (5.1 days) or the adult rats (4.4 days). These results indicate that respiratory sensitivity and tolerance to fentanyl in rat vary in a distinct manner during maturation.

The N-methyl D-aspartate receptor glycine site and D-serine metabolism: an evolutionary perspective

The N-methyl D-aspartate (NMDA) type of glutamate receptor requires two distinct agonists to operate. Glycine is assumed to be the endogenous ligand for the NMDA receptor glycine site, but this notion has been challenged by the discovery of high levels of endogenous d-serine in the mammalian forebrain. I have outlined an evolutionary framework for the appearance of a glycine site in animals and the metabolic events leading to high levels of D-serine in brain. Sequence alignments of the glycine-binding regions, along with the scant experimental data available, suggest that the properties of invertebrate NMDA receptor glycine sites are probably different from those in vertebrates. The synthesis of D-serine in brain is due to a pyridoxal-5'-phosphate (B(6))-requiring serine racemase in glia. Although it remains unknown when serine racemase first evolved, data concerning the evolution of B(6) enzymes, along with the known occurrences of serine racemases in animals, point to D-serine synthesis arising around the divergence time of arthropods. D-Serine catabolism occurs via the ancient peroxisomal enzyme d-amino acid oxidase (DAO), whose ontogenetic expression in the hindbrain of mammals is delayed until the postnatal period and absent from the forebrain. The phylogeny of D-serine metabolism has relevance to our understanding of brain ontogeny, schizophrenia and neurotransmitter dynamics.

Nonsynaptic communication in the central nervous system

Classical synaptic functions are important and suitable to relatively fast and discretely localized processes, but the nonclassical receptorial functions may be providing revolutionary possibilities for dealing at the cellular level with many of the more interesting and seemingly intractable features of neural and cerebral activities. Although different forms of nonsynaptic communication (volume transmission) often appear in different studies, their importance to modulate and mediate various functions is still not completely recognized. To establish the existence and the importance of nonsynaptic communication in the nervous system, here we cite pieces of evidence for each step of the interneuronal communication in the nonsynaptic context including the release into the extracellular space (ECS) and the extrasynaptic receptors and transporters that mediate nonsynaptic functions. We are now faced with a multiplicity of chemical communication. The fact that transmitters can even be released from nonsynaptic varicosities without being coupled to frequency-coded neuronal activity and they are able to diffuse over large distances indicates that there is a complementary mechanism of interneuronal communication to classical synaptic transmission. Nonconventional mediators that are also important part of the nonsynaptic world will also be overviewed.

Plasticity of somatostatin and somatostatin sst2A receptors in the rat dentate gyrus during kindling epileptogenesis

Increasing evidence suggests that somatostatin may control neuronal excitability during epileptogenesis. In the hippocampus, sst2A receptors are likely to mediate somatostatin inhibitory actions but little is known about their status in kindled tissues. In the present study, sst2A receptor and somatostatin immunoreactivity were examined by confocal microscopy in the hippocampus during and after kindling acquisition. In control rats, somatostatin-positive axon terminals were mainly found in the stratum lacunosum moleculare of CA1 area and in the outer molecular layer of the dentate gyrus. sst2A receptor immunoreactivity was diffusely distributed in the strata radiatum and oriens of CA1 and in the stratum moleculare of the dentate gyrus. Immunogold electron microscopy revealed that sst2A receptors were predominantly localized postsynaptically, at the plasma membrane of dendritic shafts and spines of principal neurons. During kindling epileptogenesis, qualitative and semiquantitative analysis revealed a progressive decrease of sst2A immunoreactivity in the outer molecular layer, which was spatially associated with an increase in somatostatin immunoreactivity. No obvious changes in sst2A receptor immunoreactivity were observed in other hippocampal subfields. These results suggest that the decrease of sst2A receptor immunoreactivity in the outer molecular layer reflects receptor down-regulation in distal dendrites of granule cells in response to chronic somatostatin release. Because the sst2A receptor appears to mediate anticonvulsant and antiepileptogenic effects of somatostatin, this may represent a pivotal mechanism contributing to epileptogenesis.

A 25 year adventure in the field of tachykinins

Several aspects of our 25 year adventure in the field of tachykinins will be successively described. They concern: substance P (SP) synthesis and release in the basal ganglia, the identification and pharmacological characterization of central tachykinin NK(1), NK(2) and NK(3) binding sites and their topographical distribution, the description of some new biological tests for corresponding receptors, the identification of tachykinin NK(1) receptor subtypes or conformers sensitive to all endogenous tachykinins (substance P, neurokinin A (NKA), neurokinin B (NKB), neuropeptide gamma (NP gamma) and neuropeptide K (NPK)) and finally, the functional involvement of these receptors and their subtypes in tachykinin-induced regulations of dopamine and acetylcholine release in the striatum.

Galanin in the brain: chemoarchitectonics and brain cartography--a historical review

We present a review of galanin in the brain from a historical perspective of the development of "chemoarchitectonics" and "brain cartography" accomplished in the Histopharmacology Section at the National Institutes of Health. It was the mapping of potential brain neuroregulators that served as a springboard of ideas from which behavioral studies emanate. The integration of the known localization of neurotransmitter/neuromodulatory nerves ("chemoarchitectonic maps") and receptor binding sites with biochemical data derived from brain micropunches coupled with behavioral analysis at the level of discrete brain allows one to define the anatomical circuits which support behavioral changes and which ultimately will improve our understanding of mental disorders.

Opioid binding in DYT1 primary torsion dystonia: An11C-diprenorphine PET study

The opioid transmitters enkephalin and dynorphin are known to regulate pallidal output and consequently cortical excitability. Indeed, abnormal basal ganglia opioid transmission has been reported in several involuntary movement disorders, including levodopa-induced dyskinesias in Parkinson's disease (PD), tardive dyskinesias/dystonia, Huntington's disease, and Tourette's syndrome. Moreover, a previous 11C-diprenorphine PET study investigating levodopa-induced dyskinesias found reduced opioid receptor availability in PD with but not without dyskinesias. We wished to investigate if a similar alteration in basal ganglia opioid binding was present in DYT1 primary torsion dystonia (PTD). Regional cerebral 11C-diprenorphine binding was investigated in 7 manifesting carriers of the DYT1 gene and 15 age-matched normal controls using a region-of-interest (ROI) approach and statistical parametric mapping (SPM). No difference in regional mean 11C-diprenorphine binding was found between DYT1-PTD and controls, and no correlation between the severity of dystonia and opioid binding was seen. We conclude that aberrant opioid transmission is unlikely to be present in DYT1-PTD and altered opioid transmission is not a common mechanism underlying all disorders of involuntary movement.

Weighing the Role of Hypothalamic Feeding Neurotransmitters

Growing health problems related to obesity have focused considerable attention on a number of neurotransmitters, particularly hypothalamic neuropeptides, involved in regulating energy homeostasis and food intake. As the fast-acting transmitters GABA and glutamate underlie the majority of fast synaptic activity in the hypothalamus, understanding neuropeptide modulation of amino acid transmitter actions may be key to a full appreciation of how the brain controls caloric balances.

Can the brain be protected through exercise? Lessons from an animal model of parkinsonism☆

Evidence suggests that following injury the brain has the capacity for self-repair and that this can be promoted through a variety of experiences including motor activity. In their article, Döbrössy and Dunnett have provided further evidence that this is the case in an animal model in which an excitotoxin is applied to the neostriatum. Under standard conditions, such a toxin would cause considerable damage to the GABAergic cells of this region and produce behavioral deficits. This model has been used to explore certain aspects of Huntington's disease, which also involves the loss of these neurons. However, Döbrössy and Dunnett show that the damage can be reduced by prior motor training. We have been exploring the neuroprotective effects of motor exercise in a different model, one involving 6-hydroxydopamine, which normally destroys dopamine neurons. Our results indicate that forced exercise can reduce the vulnerability of dopamine neurons to 6-hydroxydopamine. The results further suggest that this protection is due in part to an increase in the availability of the trophic factor GDNF, which can in turn stimulate certain signaling cascades, including one that activates ERK. Our results, together with those of Döbrössy and Dunnett and others, raise the possibility that exercise will protect against a variety of neurodegenerative conditions.

Diencephalic neurons producing melanin-concentrating hormone are influenced by local and multiple extra-hypothalamic tachykininergic projections through the neurokinin 3 receptor

As melanin-concentrating hormone (MCH) neurons express the neurokinin 3 receptor (NK3) in the rat diencephalon, their innervation by tachykininergic fibers, the origin of this innervation and the effect of a NK3 agonist on MCH mRNA expression were researched. The obtained results show that the tachykininergic system develops complex relationships with MCH neurons. Overall, MCH cell bodies appeared targeted by both NKB- and SP-inputs. These afferents have multiple hypothalamic and extra-hypothalamic origins, but a local (intra-lateral hypothalamic area) origin from small interneurons was suspected as well. MCH cell bodies do not express NK1, but around 2.7% of the MCH neurons contained SP after colchicine injection. Senktide, a NK3 agonist, produced an increase of the MCH mRNA expression in cultured hypothalamic slices. This effect was reversed by two NK3 antagonists. Tachykinins enhance MCH mRNA expression, and, thus, may modulate the effect of MCH in functions such as feeding and reproductive behaviors in which this peptide has been experimentally involved.

Urocortin in the central nervous system of a primate (Cebus apella): sequencing, immunohistochemical, and hybridization histochemical characterization

The urocortin (UCN)-like immunoreactivity and UCN mRNA distribution in various regions of the nonprimate mammalian brain have been reported. However, the Edinger-Westphal nucleus (EW) appears to be the only brain site where UCN expression is conserved across species. Although UCN peptides are present throughout vertebrate phylogeny, the functional roles of both UCN and EW remain poorly understood. Therefore, a study focused on UCN system organization in the primate brain is warranted. By using immunohistochemistry (single and double labeling) and in situ hybridization, we have characterized the organization of UCN-expressing cells and fibers in the central nervous system and pituitary of the capuchin monkey (Cebus apella). In addition, the sequence of the prepro-UCN was determined to establish the level of structural conservation relative to the human sequence. To understand the relationship of acetylcholine cells in the EW, a colocalization study comparing choline acetyltransferase (ChAT) and UCN was also performed. The cloned monkey prepro-UCN is 95% identical to the human preprohormone across the matched sequences. By using an antiserum raised against rat UCN and a probe generated from human cDNA, we found that the EW is the dominant site for UCN expression, although UCN mRNA is also expressed in spinal cord lamina IX. Labeled axons and terminals were distributed diffusely throughout many brain regions and along the length of the spinal cord. Of particular interest were UCN-immunoreactive inputs to the medial preoptic area, the paraventricular nucleus of the hypothalamus, the oral part of the spinal trigeminal nucleus, the flocculus of the cerebellum, and the spinal cord laminae VII and X. We found no UCN hybridization signal in the pituitary. In addition, we observed no colocalization between ChAT and UCN in EW neurons. Our results support the hypothesis that the UCN system might participate in the control of autonomic, endocrine, and sensorimotor functions in primates.

Activation of beta1-adrenoceptors excites striatal cholinergic interneurons through a cAMP-dependent, protein kinase-independent pathway

The role of noradrenergic neurotransmission was analyzed in striatal cholinergic interneurons. Conventional intracellular and whole-cell patch-clamp recordings were made of cholinergic interneurons in rat brain slice preparations. Bath-applied noradrenaline (NA) (1-300 microm) dose-dependently induced both an increase in the spontaneous firing activity and a membrane depolarization of the recorded cells. In voltage-clamped neurons, an inward current was induced by NA. This effect was not prevented by alpha-adrenoceptor antagonists, whereas it was mimicked by the beta-adrenoceptor agonist isoproterenol and blocked by the beta1 antagonists propranolol and betaxolol. Interestingly, forskolin, activator of adenylate cyclase, mimicked and occluded the membrane depolarization obtained at saturating doses of both dopamine and NA. Accordingly, SQ22,536, a selective adenylate cyclase inhibitor, reduced the response to NA. Analysis of the reversal potential of the NA-induced current did not provide homogeneous results, indicating the involvement of multiple membrane conductances. Because cAMP is known to modulate Ih, the effects of ZD7288, a selective inhibitor of Ih current, were examined on the NA-induced membrane depolarization/inward current. ZD7288 mostly reduced the response to NA. However, both KT-5720 and H-89, selective protein kinase A (PKA) blockers, failed to prevent the excitatory action of NA. Likewise, calphostin C, antagonist of PKC, genistein, inhibitor of tyrosine kinase, and 8-Bromo-cGMP, blocker of PKG, did not affect the response to NA. Finally, double-labeling experiments combining beta1-adrenoceptor and choline acetyltransferase immunocytochemistry by means of confocal microscopy revealed a strong beta1-adrenoceptor labeling on cholinergic interneurons. We conclude that NA depolarizes striatal cholinergic interneurons via beta1-adrenoceptor activation, through a cAMP-dependent but PKA-independent mechanism.

Distribution of ORL-1 receptor binding and receptor-activated G-proteins in rat forebrain and their experimental localization in anterior cingulate cortex

Opioid receptor-like (ORL-1) receptors and ORL-1-activated G-proteins are found in high levels in the forebrain, particularly cingulate cortex, an area involved in processing of nociceptive stimuli. [(3)H]nociceptin/orphanin FQ (N/OFQ) and N/OFQ-stimulated [(35)S]GTPgammaS autoradiography in rat brain were used to localize ORL-1 receptors and activated G-proteins, respectively. N/OFQ binding and activated G-proteins were highest in anterior cingulate, agranular insula, piriform, perirhinal and entorhinal cortices; midline and intralaminar thalamic nuclei; and subnuclei of the amygdala and hippocampus. In anterior cingulate area 24, [(3)H]N/OFQ and N/OFQ-stimulated [(35)S]GTPgammaS binding were highest in layers V and VI. The cellular localization of ORL-1 receptors and activated G-proteins in area 24 was examined using two strategies: ibotenic acid injection into the cortex or undercut lesions to remove afferent axons, followed by autoradiography. Ibotenic acid lesions that destroyed neurons in the anterior cingulate cortex decreased [(3)H]N/OFQ binding by 75-80% and reduced N/OFQ-stimulated [(35)S]GTPgammaS binding to basal levels seen in the absence of agonist. Deafferentation lesions increased [(3)H]N/OFQ binding by 40-50%, with no significant change in N/OFQ-stimulated [(35)S]GTPgammaS binding. These data demonstrate that ORL-1 receptors in layer V of anterior cingulate cortex are located on somatodendritic elements and that deafferentation increases ORL-1 receptor binding.

Theoretical basis for brain plasticity after a TBI

Evidence has been accumulating that the brain can reorganize extensively after damage and that reorganization can be obtained even many years after the trauma with appropriate late rehabilitation. An understanding of the brain plasticity mechanisms should lead to more effective rehabilitation and neuropharmacology. In this communication, several emerging concepts with supporting experimental evidence have been presented. These include non-synaptic diffusion neurotransmission, extracellular space volume fraction, neurotransmitters, regeneration and neurogenesis and multiplexing.

Expression and distribution of tuberoinfundibular peptide of 39 residues in the rat central nervous system

Tuberoinfundibular peptide of 39 residues (TIP39) has been recently purified and identified as a selective ligand for the parathyroid hormone 2 receptor. As a next step toward understanding its functions, we report the expression and distribution of TIP39 in the rat central nervous system. In situ hybridization histochemistry and immunocytochemistry revealed TIP39-containing cell bodies in three distinct areas. The major one comprises the subparafascicular area posterior through the intralaminar nucleus of the thalamus; a second is the medial paralemniscal nucleus at the pontomesencephalic junction; and a third is in the dorsal and dorsolateral hypothalamic areas, which contained a few, scattered cell bodies. We found, in contrast to the highly restricted localization of TIP39-containing cell bodies, a much more widespread localization of TIP39-containing fibers. The highest density of fibers was observed in limbic areas such as the septum, the amygdala, and the bed nucleus of the stria terminalis; in areas involved in endocrine regulation, such as the hypothalamic dorsomedial, paraventricular, periventricular, and arcuate nuclei; in auditory areas, such as the ectorhinal and temporal cortices, inferior colliculus, medial geniculate body, and some of the nuclei of the superior olivary complex; and in the dorsolateral funiculus of the spinal cord. The localization of TIP39-containing nuclei and fibers provides an anatomical basis for previously demonstrated endocrine and nociceptive effects of TIP39 and suggests additional functions for TIP39, one apparent candidate being the regulation of auditory information processing.

Ultrastructural localization of enkephalin and mu-opioid receptors in the rat ventral tegmental area

Enkephalins are endogenous ligands for opioid receptors whose activation potently modulates the output of mesocorticolimbic dopaminergic neurons within the ventral tegmental area. Many of the reinforcing effects of enkephalins in the mesocorticolimbic system are mediated by mu-opioid receptors. To determine the sites for Leu(5)-enkephalin activation of mu-opioid receptors in the ventral tegmental area, we examined the dual electron microscopic immunocytochemical localization of their respective antigens in this region of rat brain. Enkephalin immunoperoxidase reaction product and mu-opioid receptor immunogold-silver labeling showed similar cellular and subcellular distribution in both the paranigral and parabrachial subdivisions of the ventral tegmental area. Enkephalin immunoreactivity was mainly localized in small unmyelinated axons (50.4%) and in axon terminals (40.4%). The majority of these terminals formed symmetric, inhibitory-type synapses, many of which were on dendrites expressing plasmalemmal mu-opioid receptors. Appositional contacts were also often seen between axons or terminals that were differentially labeled for the two antigens. In addition, some of the enkephalin-labeled terminals and a few somatodendritic profiles showed a plasmalemmal or vesicular localization of mu-opioid receptors. Our results indicate that dendritic targets of inhibitory terminals, as well as nearby axon terminals, are potential sites for enkephalin activation of mu-opioid receptors throughout the ventral tegmental area. Moreover, co-localization of enkephalin and mu-opioid receptors in selective neuronal profiles may indicate an autoregulatory role for these receptors or their internalization along with the bound ligand in this brain region.