Preprodynorphin-, preproenkephalin-, preprotachykinin A- and preprotachykinin B-immunoreactive neurons in the accumbens nucleus and olfactory tubercle: double-immunofluorescence analysis

Endogenous opioids in the olfactory tubercle and their roles in olfaction and quality of life

Olfactory dysfunctions decrease daily quality of life (QOL) in part by reducing the pleasure of eating. Olfaction plays an essential role in flavor sensation and palatability. The decreased QOL due to olfactory dysfunction is speculated to result from abnormal neural activities in the olfactory and limbic areas of the brain, as well as peripheral odorant receptor dysfunctions. However, the specific underlying neurobiological mechanisms remain unclear. As the olfactory tubercle (OT) is one of the brain's regions with high expression of endogenous opioids, we hypothesize that the mechanism underlying the decrease in QOL due to olfactory dysfunction involves the reduction of neural activity in the OT and subsequent endogenous opioid release in specialized subregions. In this review, we provide an overview and recent updates on the OT, the endogenous opioid system, and the pleasure systems in the brain and then discuss our hypothesis. To facilitate the effective treatment of olfactory dysfunctions and decreased QOL, elucidation of the neurobiological mechanisms underlying the pleasure of eating through flavor sensation is crucial.

Cellular Profiles of Prodynorphin and Preproenkephalin mRNA-Expressing Neurons in the Anterior Olfactory Tubercle of Mice

The olfactory tubercle (OT) is a striatal region that receives olfactory inputs. mRNAs of prodynorphin (Pdyn) and preproenkephalin (Penk), precursors of dynorphins and enkephalins, respectively, are strongly expressed in the striatum. Both produce opioid peptides with various physiological effects such as pain relief and euphoria. Recent studies have revealed that OT has anatomical and cytoarchitectonic domains that play different roles in odor-induced motivated behavior. Neuronal subtypes of the OT can be distinguished by their expression of the dopamine receptors D1 (Drd1) and D2 (Drd2). Here, we addressed whether and which type of opioid peptide precursors the D1- and D2-expressing neurons in the OT express. We used multiple fluorescence in situ hybridization for mRNAs of the opioid precursors and dopamine receptors to characterize mouse OT neurons. Pdyn was mainly expressed by Drd1-expressing cells in the dense cell layer (DCL) of the OT, whereas Penk was expressed primarily by Drd2-expressing cells in the DCL. We also confirmed the presence of a larger population of Pdyn-Penk-Drd1 co-expressing cells in the DCL of the anteromedial OT compared with the anterolateral OT. These observations will help understand whether and how dynorphins and enkephalins in the OT are involved in diverse odor-induced motivated behaviors.

Neurochemical organization of the ventral striatum's olfactory tubercle

The ventral striatum is a collection of brain structures, including the nucleus accumbens, ventral pallidum and the olfactory tubercle (OT). While much attention has been devoted to the nucleus accumbens, a comprehensive understanding of the ventral striatum and its contributions to neurological diseases requires an appreciation for the complex neurochemical makeup of the ventral striatum's other components. This review summarizes the rich neurochemical composition of the OT, including the neurotransmitters, neuromodulators and hormones present. We also address the receptors and transporters involved in each system as well as their putative functional roles. Finally, we end with briefly reviewing select literature regarding neurochemical changes in the OT in the context of neurological disorders, specifically neurodegenerative disorders. By overviewing the vast literature on the neurochemical composition of the OT, this review will serve to aid future research into the neurobiology of the ventral striatum.

Quantitative Analyses of the Projection of Individual Neurons from the Midline Thalamic Nuclei to the Striosome and Matrix Compartments of the Rat Striatum

A fundamental organizing principle of the striatum is the striosome/matrix system that is defined by inputs/outputs and neurochemical markers. The thalamostriatal projection is highly heterogeneous originating in many subnuclei of the thalamus including the midline (ML) and intralaminar (IL) nuclei. We examined the dendritic morphology and axonal trajectory of 15 ML and 11 IL neurons by single-neuron labeling with viral vectors in combination with mu-opioid receptor immunostaining in rat brains. Dendritic and axonal morphology defined ML neurons as type II cells consisting of at least two subclasses according to the presence or absence of striatal axon collaterals. In the striatum, ML neurons preferentially innervated striosomes, whereas parafascicular neurons preferentially innervated the matrix. Almost all single thalamostriatal neurons favoring striosome or matrix compartments also innervated the cerebral cortical areas that supplied cortical input to the same striatal compartment. We thus revealed that thalamostriatal projections are highly organized 1) by the similarity in morphological characteristics and 2) their preference for the striatal compartments and cortical areas. These findings demonstrate that the functional properties of striatal compartments are influenced by both their cortical and thalamic afferents presumably with a different time latency and support selective dynamics for the striosome and matrix compartments.

Preprotachykinin A is expressed by a distinct population of excitatory neurons in the mouse superficial spinal dorsal horn including cells that respond to noxious and pruritic stimuli

Expression of the substance P precursor preprotachykinin A defines a distinct population of superficial dorsal horn excitatory neurons, many of which respond to noxious or pruritic stimuli.

The superficial dorsal horn, which is the main target for nociceptive and pruritoceptive primary afferents, contains a high density of excitatory interneurons. Our understanding of their roles in somatosensory processing has been restricted by the difficulty of distinguishing functional populations among these cells. We recently defined 3 nonoverlapping populations among the excitatory neurons, based on the expression of neurotensin, neurokinin B, and gastrin-releasing peptide. Here we identify and characterise another population: neurons that express the tachykinin peptide substance P. We show with immunocytochemistry that its precursor protein (preprotachykinin A, PPTA) can be detected in ∼14% of lamina I-II neurons, and these are concentrated in the outer part of lamina II. Over 80% of the PPTA-positive cells lack the transcription factor Pax2 (which determines an inhibitory phenotype), and these account for ∼15% of the excitatory neurons in this region. They are different from the neurotensin, neurokinin B, or gastrin-releasing peptide neurons, although many of them contain somatostatin, which is widely expressed among superficial dorsal horn excitatory interneurons. We show that many of these cells respond to noxious thermal and mechanical stimuli and to intradermal injection of pruritogens. Finally, we demonstrate that these cells can also be identified in a knock-in Cre mouse line (Tac1^Cre), although our findings suggest that there is an additional population of neurons that transiently express PPTA. This population of substance P–expressing excitatory neurons is likely to play an important role in the transmission of signals that are perceived as pain and itch.

Arborization patterns of amygdalopetal axons from the rat ventral pallidum

We previously analyzed the arborization patterns of rat ventral pallidal (VP) axons that coursed caudally to innervate the thalamus and brainstem (Tripathi et al. in Brain Struct Funct 218:1133-1157, 2013). Here, we have reconstructed 16 previously undetected axons from the same tracer deposits that follow a more lateral trajectory. Virtually all 16 axons emanating from the different VP compartments collateralized in the extended amygdala system (EAS) and amygdaloid complex. The most frequent targets of axons from the lateral and medial (VPm) VP compartments were the rostral sublenticular extended amygdala, the extended amygdala (EA), the central nucleus of the amygdala and the posterior part of the basolateral amygdaloid nucleus. In contrast, axons from the rostral extension of the VP preferentially innervated the anterior amygdaloid area, the magnocellular preoptic nucleus, and the anterior part of the basomedial amygdaloid nucleus. We additionally found and reconstructed a single corticopetal axon arising from the VPm. The new results show that both direct and indirect projections from the basolateral complex and EAS to the ventral striatopallidal system are reciprocated by VP projections, and suggest that the systems can be activated simultaneously. The results additionally suggest that the amygdaloid complex and cortex are innervated separately from the VP. Finally, the combination of new and previous data indicate that approximately 84 % of VP axons (88/105) participate in basal ganglia circuits, 15 % (16/105) target the amygdaloid complex, and less than 1 % innervate the cortex.

Effects of footshocks on anxiety-like behavior and mRNA levels of precursor peptides for corticotropin releasing factor and opioids in the forebrain of the rat

Corticotropin releasing factor (CRF) and dynorphin are neuropeptides that are associated with the negative emotional states. Experimental evidence indicates that dynorphin neurons located in the nucleus accumbens and CRF neurons in the bed nucleus of the stria terminalis (BST) and the central nucleus of the amygdala (CeA) mediate anxiety-like behaviors immediately after the stressful experience (24-48h). The present study was done to evaluate if changes in the levels of the mRNA for these peptides in the striatum, BST, and CeA were associated with the long-lasting avoidance of novelty, a measure of an anxiety-like state, in a subset of rats exposed to unpredictable and moderately intense footshocks (5×2s of 1.5mA). Shocked rats with enhanced fear to a novel tone 24h after the footshocks (high responders; HR) displayed long-lasting avoidance in the elevated T-maze whereas shocked rats with low levels of acute fear (low responders; LR) had low levels of avoidance similar to nonshocked rats. An increase in the level of proCRF mRNA was detected in the CeA of the HR compared to LR and nonshocked rats but not in other areas of the brain sampled. In contrast, prodynorphin and proenkephalin mRNA levels in the striatum, BST and CeA were not different between HR, LR and nonshocked rats. This study provides evidence that CRF neurons in the CeA may play a role in the anxiety-like state produced in a subset of rats exposed to footshocks.

Mapping of endogenous morphine-like compounds in the adult mouse brain: Evidence of their localization in astrocytes and GABAergic cells

Endogenous morphine, morphine-6-glucuronide, and codeine, which are structurally identical to vegetal alkaloids, can be synthesized by mammalian cells from dopamine. However, the role of brain endogenous morphine and its derivative compounds is a matter of debate, and knowledge about its distribution is lacking. In this study, by using a validated antibody, we describe a precise mapping of endogenous morphine-like compounds (morphine and/or its glucuronides and/or codeine) in the mouse brain. First, a mass spectrometry approach confirmed the presence of morphine and codeine in mouse brain, but also, of morphine-6-glucuronide and morphine-3-glucuronide representing two metabolites of morphine. Second, light microscopy allowed us to observe immunopositive cell somas and cytoplasmic processes throughout the mouse brain. Morphine-like immunoreactivity was present in various structures including the hippocampus, olfactory bulb, band of Broca, basal ganglia, and cerebellum. Third, by using confocal microscopy and immunofluroscence co-localization, we characterized cell types containing endogenous opiates. Interestingly, we observed that morphine-like immunoreactivity throughout the encephalon is mainly present in γ-aminobutyric acid (GABA)ergic neurons. Astrocytes were also labeled throughout the entire brain, in the cell body, in the cytoplasmic processes, and in astrocytic feet surrounding blood vessels. Finally, ultrastructural localization of morphine-like immunoreactivity was determined by electron microscopy and showed the presence of morphine-like label in presynaptic terminals in the cerebellum and postsynaptic terminals in the rest of the mouse brain. In conclusion, the presence of endogenous morphine-like compounds in brain regions not usually involved in pain modulation opens the exciting opportunity to extend the role and function of endogenous alkaloids far beyond their analgesic functions.

The endocannabinoid 2-arachidonoylglycerol mediates D1 and D2 receptor cooperative enhancement of rat nucleus accumbens core neuron firing

Many motivated and addiction-related behaviors are sustained by activity of both dopamine D1- and D2-type receptors (D1Rs and D2Rs) as well as CB1 receptors (CB1Rs) in the nucleus accumbens (NAc). Here, we use in vitro whole-cell patch-clamp electrophysiology to describe an endocannabinoid (eCB)-dopamine receptor interaction in adult rat NAc core neurons. D1R and D2R agonists in combination enhanced firing, with no effect of a D1R or D2R agonist alone. This D1R+D2R-mediated firing increase required CB1Rs, since it was prevented by the CB1R antagonists AM251 and Rimonabant. The D1R+D2R firing increase also required phospholipase C (PLC), the major synthesis pathway for the eCB 2-arachidonoylglycerol (2-AG) and one of several pathways for anandamide. Further, inhibition of 2-AG hydrolysis with the monoglyceride lipase (MGL) inhibitor JZL184 allowed subthreshold levels of D1R+D2R receptor agonists to enhance firing, while inhibition of anandamide hydrolysis with the fatty acid amide hydrolase (FAAH) inhibitors URB597 or AM3506 did not. Filling the postsynaptic neuron with 2-AG enabled subthreshold D1R+D2R agonists to increase firing, and the 2AG+D1R+D2R increase in firing was prevented by a CB1R antagonist. Also, the metabotropic glutamate receptor 5 (mGluR5) blocker MPEP prevented the ability of JZL184 to promote subthreshold D1R+D2R enhancement of firing, while the 2-AG+D1R+D2R increase in firing was not prevented by the mGluR5 blocker, suggesting that mGluR5s acted upstream of 2-AG production. Thus, our results taken together are consistent with the hypothesis that NAc core eCBs mediate dopamine receptor (DAR) enhancement of firing, perhaps providing a cellular mechanism underlying the central role of NAc core D1Rs, D2Rs, CB1Rs, and mGluR5s during many drug-seeking behaviors.

The PtdIns(3,4)P(2) phosphatase INPP4A is a suppressor of excitotoxic neuronal death

Phosphorylated derivatives of phosphatidylinositol, collectively referred to as phosphoinositides, occur in the cytoplasmic leaflet of cellular membranes and regulate activities such as vesicle transport, cytoskeletal reorganization and signal transduction. Recent studies have indicated an important role for phosphoinositide metabolism in the aetiology of diseases such as cancer, diabetes, myopathy and inflammation. Although the biological functions of the phosphatases that regulate phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) have been well characterized, little is known about the functions of the phosphatases regulating the closely related molecule phosphatidylinositol-3,4-bisphosphate (PtdIns(3,4)P(2)). Here we show that inositol polyphosphate phosphatase 4A (INPP4A), a PtdIns(3,4)P(2) phosphatase, is a suppressor of glutamate excitotoxicity in the central nervous system. Targeted disruption of the Inpp4a gene in mice leads to neurodegeneration in the striatum, the input nucleus of the basal ganglia that has a central role in motor and cognitive behaviours. Notably, Inpp4a(-/-) mice show severe involuntary movement disorders. In vitro, Inpp4a gene silencing via short hairpin RNA renders cultured primary striatal neurons vulnerable to cell death mediated by N-methyl-d-aspartate-type glutamate receptors (NMDARs). Mechanistically, INPP4A is found at the postsynaptic density and regulates synaptic NMDAR localization and NMDAR-mediated excitatory postsynaptic current. Thus, INPP4A protects neurons from excitotoxic cell death and thereby maintains the functional integrity of the brain. Our study demonstrates that PtdIns(3,4)P(2), PtdIns(3,4,5)P(3) and the phosphatases acting on them can have distinct regulatory roles, and provides insight into the unique aspects and physiological significance of PtdIns(3,4)P(2) metabolism. INPP4A represents, to our knowledge, the first signalling protein with a function in neurons to suppress excitotoxic cell death. The discovery of a direct link between PtdIns(3,4)P(2) metabolism and the regulation of neurodegeneration and involuntary movements may aid the development of new approaches for the treatment of neurodegenerative disorders.

Role of mu- and delta-opioid receptors in the nucleus accumbens in cocaine-seeking behavior

Earlier studies suggest that opioid receptors in the ventral tegmental area, but not the nucleus accumbens (NAc), play a role in relapse to drug-seeking behavior. However, environmental stimuli that elicit relapse also release the endogenous opioid beta-endorphin in the NAc. Using a within-session extinction/reinstatement paradigm in rats that self-administer cocaine, we found that NAc infusions of the mu-opioid receptor (MOR) agonist DAMGO moderately reinstated responding on the cocaine-paired lever at low doses (1.0-3.0 ng/side), whereas the delta-opioid receptor (DOR) agonist DPDPE induced greater responding at higher doses (300-3000 ng/side) that also enhanced inactive lever responding. Using doses of either agonist that induced responding on only the cocaine-paired lever, we found that DAMGO-induced responding was blocked selectively by pretreatment with the MOR antagonist, CTAP, whereas DPDPE-induced responding was selectively blocked by the DOR antagonist, naltrindole. Cocaine-primed reinstatement was blocked by intra-NAc CTAP but not naltrindole, indicating a role for endogenous MOR-acting peptides in cocaine-induced reinstatement of cocaine-seeking behavior. In this regard, intra-NAc infusions of beta-endorphin (100-1000 ng/side) induced marked cocaine-seeking behavior, an effect blocked by intra-NAc pretreatment with the MOR but not DOR antagonist. Conversely, cocaine seeking elicited by the enkephalinase inhibitor thiorphan (1-10 microg/side) was blocked by naltrindole but not CTAP. MOR stimulation in more dorsal caudate-putamen sites was ineffective, whereas DPDPE infusions induced cocaine seeking. Together, these findings establish distinct roles for MOR and DOR in cocaine relapse and suggest that NAc MOR could be an important therapeutic target to neutralize the effects of endogenous beta-endorphin release on cocaine relapse.

An association of prodynorphin polymorphisms and opioid dependence in females in a Chinese population

Prodynorphin (PDYN) binds to kappa-opioid receptors and is known to regulate dopaminergic tone, making this system important for the reinforcing and rewarding properties of drugs of abuse such as opioids. The binding of dynorphins to kappa-opioid receptors also produces aversive states that may affect the development of opioid dependence. Recent animal results have shown that PDYNknockout mice show decreased ethanol consumption; however, this finding was restricted to female mice. We were interested to analyse a possible gender specificity of dynorphin effects in humans and to this end three single-nucleotide polymorphisms (SNPs) in PDYN were genotyped in a Chinese population of 484 opioid dependents and 374 controls. An interaction between sex and genotype was found in female opioid dependents. Chi-squared tests for association revealed that the genotype distributions of SNPs rs1997794 (P = 0.019) and rs1022563 (P = 0.006) in the promoter and 3' region of PDYN, respectively, were found to be associated with opioid dependence. Therefore, SNPs in PDYN are significantly associated with the risk of developing opioid dependence; however, this effect may only be seen in females. These data suggest that PDYN polymorphisms should be studied in additional female opioid-dependent populations with an emphasis on the promoter and 3' regions of the gene.

Expression of D1 but not D2 dopamine receptors in striatal neurons producing neurokinin B in rats

Neostriatal projection neurons are known to be largely divided into two groups, striatoentopeduncular/striatonigral and striatopallidal neurons, which mainly express D1 and D2 dopamine receptors, respectively. Recently, a small population of neostriatal neurons have been reported to produce neurokinin B (NKB), and send their axons mainly to the basal forebrain regions. To reveal which type of dopamine receptors were expressed by these NKB-producing neurons, we examined rat striatal neurons by combining immunofluorescence labeling for preprotachykinin B (PPTB), the precursor of NKB, and fluorescence in situ hybridization labeling for dopamine receptors. Fluorescent signals for D1 receptor mRNA were detected in 85-89% of PPTB-immunopositive neurons in the neostriatum, accumbens nucleus and lateral stripe of the striatum, whereas almost no signal for D2 receptor was observed in PPTB-positive striatal neurons. To further reveal intracellular signaling downstream of D1 receptor in PPTB-producing neurons, we used a double immunofluorescence labeling method to study the localization of some substrates for protein kinase A (PKA), which was known to be activated by D1 receptor. Although only 3-7% of PPTB-immunopositive striatal neurons displayed immunoreactivity for dopamine- and cAMP-regulated phosphoprotein of 32 kDa, a well-known PKA substrate expressed in the two major groups of neostriatal projection neurons, 60-64% of PPTB-positive striatal neurons exhibited immunoreactivity for striatal-enriched tyrosine phosphatase. These results suggest that NKB-producing neostriatal neurons are similar to striatoentopeduncular/striatonigral neurons in the usage of dopamine receptor subtypes, but different from the two major groups of neostriatal projection neurons in terms of the downstream signaling of dopamine receptors.

Coexpression of prodynorphin and corticotrophin-releasing hormone in the rat central amygdala: Evidence of two distinct endogenous opioid systems in the lateral division

The lateral subdivision of the central nucleus of the amygdala (CeA) comprises two groups of gamma-aminobutyric acid (GABA) neurons that express corticotrophin-releasing hormone (CRH) and enkephalin. Regulation of the expression and release of these neuropeptides by glucocorticoids and other factors has been suggested to have a regulatory function on the diverse somatic, autonomic, and neuroendocrine responses that are coordinated by the CeA. Because another opioid peptide, dynorphin, has been reported to be also expressed by neurons in the lateral CeA, this study examined the neuronal expression of this kappa-opioid (KOP) receptor-preferring ligand by using immunohistochemistry for the precursor peptide prodynorphin. Prodynorphin neurons in the extended amygdala were observed mostly in the medial and central regions of the lateral CeA and the oval of the bed nucleus of the stria terminalis (BST). About one-third of the prodynorphin neurons in the CeA coexpressed CRH, whereas no coexpression with CRH was detected in the BST. Prodynorphin was not expressed by calbindin neurons in the medial part of the lateral CeA, and indirect evidence suggested that it was not expressed by enkephalin neurons. Coexpression of prodynorphin in extrahypothalamic CRH neurons in the CeA could provide an anatomical basis for regulation of the stress responses and other CRH-related functions by the brain dynorphin/KOP receptor system.

Dopamine D1 receptors have subcellular distributions conducive to interactions with prodynorphin in the rat nucleus accumbens shell

Activation of dopamine (DA) D1 receptors (D1Rs) in the nucleus accumbens (Acb) markedly affects the levels of prodynorphin, the precursor of aversion-associated dynorphin peptides. The location of prodynorphin, specifically as related to the dopaminergic inputs and D1Rs in the Acb, is fundamental for establishing the physiologically relevant sites. To determine these sites, we examined the electron microscopic dual-immunolabeling of prodynorphin and D1R or tyrosine hydroxylase (TH), a marker of catecholamine terminals in the rat Acb shell. This subregion is targeted by mesolimbic dopaminergic inputs affecting reward-aversion responses and locomotor activity. Prodynorphin was prominently localized to large (100-200 nm) granular aggregates in somatodendritic and axonal profiles, some of which expressed dynorphin A/B. In somata and dendrites, prodynorphin was often found in punctate clusters in the cytoplasm. Of the total prodynorphin-labeled dendrites, approximately 63% expressed D1Rs, which were largely located on the plasma membranes. In comparison with dendrites, many more axon terminals contained prodynorphin, although only 15% of these terminals contained D1R-labeling. Prodynorphin terminals formed symmetric synapses with D1R-labeled or unlabeled dendrites, and also apposed TH-containing axon terminals. Our results provide ultrastructural evidence that in the Acb shell, the prodynorphin is available for cleavage to physiologically active peptides in both dendrites and terminals of neurons that express D1Rs. They also indicate that dynorphin peptides have distributions that would enable their participation in modulation of DA release or D1R-mediated postsynaptic responses in Acb shell neurons.

Single-cell RT-PCR, in situ hybridization histochemical, and immunohistochemical studies of substance P and enkephalin co-occurrence in striatal projection neurons in rats

Single-cell RT-PCR studies in 3-4-week-old rats have raised the possibility that as many as 20% of striatal projection neurons may be a unique type that contains both substance P (SP) and enkephalin (ENK). We used single-cell RT-PCR, retrograde labeling, in situ hybridization histochemistry, and immunolabeling to characterize the abundance of this cell type, its projection target(s), and any developmental changes in its frequency. We found by RT-PCR that 11% of neurons containing either SP or ENK contained both in 4-week-old rats, while in 4-month-old rats SP/ENK colocalization was only 3%. SP-only neurons tended to co-contain dynorphin and ENK-only neurons neurotensin, while SP/ENK neurons tended to contain dynorphin. Single-cell RT-PCR showed SP/ENK co-occurrence in 4-week-old rats to be no more common among striatal neurons retrogradely labeled from the substantia nigra than among those retrogradely labeled from globus pallidus. Double-label in situ hybridization showed SP/ENK perikarya to be scattered throughout striatum, making up 8% of neurons containing either SP or ENK at 4 weeks, but only 4% at 4 months. Immunolabeling showed that presumptive striatal terminals in globus pallidus externus, globus pallidus internus and substantia nigra pars reticulata that colocalized SP and ENK were scarce. Terminals colocalizing SP and ENK were, however, abundant in the substantia nigra pars compacta. Thus, SP-only and ENK-only neurons make up the vast majority of striatal projection neurons in rats, the frequency of SP/ENK colocalizing striatal neurons is low in adult rats (3-4%), and SP/ENK colocalizing neurons primarily project to SNc but do not appear to be confined to striosomes.

Third pathway in the cortico-basal ganglia loop: Neurokinin B-producing striatal neurons modulate cortical activity via striato-innominato-cortical projection

In the cortico-basal ganglia loop, striatal regions serve as 'entrances' to the basal ganglia, receiving massive inputs from the cerebral cortex and sending 'direct' and 'indirect' pathways to the output nuclei of the basal ganglia. However, we have recently identified a new striatofugal subgroup which produces neurokinin B (NKB). Although NKB-producing neurons constitute a minority of striatal neurons, this subgroup is distinguished by the unique distribution and chemical characteristics. NKB-producing striatal neurons are distributed in association with mu-opioid receptor localization, and rarely express DARPP32, which is produced by the major striatofugal neurons and coupled with dopaminergic signaling. Further interestingly NKB-producing striatal neurons send axons to basal forebrain regions, but not to the main target regions of striatal outflow, pallidal or mesencephalic regions. In the basal forebrain, some GABAergic inhibitory neurons express NK3 receptor, selective receptor for NKB, and directly send axons to the cerebral cortex. The NK3-expressing neurons show different electrical properties from cholinergic basal forebrain neurons, and display facilitatory responses to stimulation of NK3 receptor. These findings strongly suggest that NKB-producing striatal neurons and NK3-expressing basal forebrain neurons constitute a third pathway which bypasses the common output nuclei of the basal ganglia, and more directly control or modulate cortical activity.

Colocalization of CART with substance P but not enkephalin in the rat nucleus accumbens

CART peptide is a novel neurotransmitter that, due to its distribution in the brain and its modulation of dopamine systems, may be involved in aspects of reward and drug abuse. In the nucleus accumbens (NAcc), CART peptide immunoreactivity (IR) is colocalized with substance P-IR in neurons. Approximately 86% of CART-IR cells colocalize with substance P, while only 19% of substance P-IR neurons contain CART. CART peptide does not colocalize with enkephalin-IR in this region. The substance P-CART colocalization exists in a rostro-caudal gradient with more colocalization in rostral regions. The presence of CART in substance P NAcc neurons suggests that CART neurons may be a subset of the basal ganglia direct pathway or that CART neurons are involved in limbic projections of the NAcc, such as to the ventral pallidum.

The distribution of m4 muscarinic acetylcholine receptors in the islands of Calleja and striatum of rats and cynomolgus monkeys

The distribution of m4 muscarinic acetylcholine receptors, and their relation to a number other markers, was examined using immunocytochemical techniques. Staining in the dorsal striatum tended to be more pronounced in the striosomal than the matrix compartment of both rats and cynomolgus monkeys. Within the ventral striatum, immunoreactivity was more pronounced within the olfactory tubercle and the shell region of the nucleus accumbens than in the nucleus accumbens core and was especially marked within the lateral striatal stripe. Modest staining was also seen in the external plexiform layer of the olfactory bulb. By far, the most intense staining in the forebrain of both rats and cynomolgus monkeys was found in islands of Calleja, where it appeared to be a selective marker for the core or hilus regions of the islands, or an analogous region found adjacent to them. The core regions of different islands appear to be continuous with each other so as to form a complex three-dimensional structure, which is largely encased by layers of granule cells. The neuronal elements in the islands of Calleja, which express m4 receptors, remain to be identified, but it is unlikely that cholinergic neurons are a major locus of these receptors. Although there are certain similarities between the islands of Calleja and other components of the striatal complex, the current studies emphasize the extent to which the islands are unique in terms of their architecture and chemical anatomy.

Neurokinin B-producing projection neurons in the lateral stripe of the striatum and cell clusters of the accumbens nucleus in the rat

Neurons producing preprotachykinin B (PPTB), the precursor of neurokinin B, constitute 5% of neurons in the dorsal striatum and project to the substantia innominata (SI) selectively. In the ventral striatum, PPTB-producing neurons are collected mainly in the lateral stripe of the striatum (LSS) and cell clusters of the accumbens nucleus (Acb). In the present study, we first examined the distribution of PPTB-immunoreactive neurons in rat ventral striatum and found that a large part of the PPTB-immunoreactive cell clusters was continuous to the LSS, but a smaller part was not. Thus, we divided the PPTB-immunoreactive cell clusters into the LSS-associated and non-LSS-associated ones. We next investigated the projection targets of the PPTB-producing ventral striatal neurons by combining immunofluorescence labeling and retrograde tracing. After injection of Fluoro-Gold into the basal component of the SI (SIb) and medial part of the interstitial nucleus of posterior limb of the anterior commissure, many PPTB-immunoreactive neurons were retrogradely labeled in the LSS-associated cell clusters and LSS, respectively. When the injection site included the ventral part of the sublenticular component of the SI(SIsl), retrogradely labeled neurons showed PPTB-immunoreactivity frequently in non-LSS-associated cell clusters. Furthermore, these PPTB-immunoreactive projections were confirmed by the double-fluorescence method after anterograde tracer injection into the ventral striatum containing the cell clusters. Since the dorsalmost part of the SIsl is known to receive strong inputs from PPTB-producing dorsal striatal neurons, the present results indicate that PPTB-producing ventral striatal neurons project to basal forebrain target regions in parallel with dorsal striatal neurons without significant convergence.

Compartment-specific localization of cannabinoid 1 (CB1) and mu-opioid receptors in rat nucleus accumbens

Interactions between cannabinoid and opioid systems have been implicated in reward and drug seeking behaviors involving neuronal circuitry in the nucleus accumbens (Acb) shell and core. To determine the relevant sites, we examined the electron microscopic localization of cannabinoid type-1 (CB1) receptors and mu-opioid receptors in each Acb compartment in rat brain. CB1 receptor immunogold labeling was seen on the plasma membrane and within the cytoplasm of neuronal and glial profiles throughout the Acb. These neuronal profiles included somata and dendrites as well as axon terminals, many of which formed excitatory-type, asymmetric synapses with notable perforations that are often associated with synaptic plasticity. The number of CB1-labeled terminals within the neuropil of the Acb shell was significantly greater than in the core. Mu-opioid receptors were also detected in axonal and dendritic profiles. These dendrites were most prevalent in the Acb shell, where mu-receptors also were located in 21% of the dendritic profiles and 3% of the axon terminals containing CB1 receptors. More of the CB1-labeled terminals contacted dendrites expressing mu-opioid receptors in the shell (19%) compared with the core (13%). Conversely, of the synaptic mu-labeled terminals, 20% in the shell and 10% in the core contacted dendrites containing CB1 receptors. These findings provide ultrastructural evidence that cannabinoid-opioid interactions are mediated by activation of CB1 and mu-opioid receptors within the same or synaptically linked neurons in the Acb shell and core. They also suggest a particularly important role for presynaptic CB1 receptors in the reward circuit of the Acb shell.

Revised nomenclature for avian telencephalon and some related brainstem nuclei

The standard nomenclature that has been used for many telencephalic and related brainstem structures in birds is based on flawed assumptions of homology to mammals. In particular, the outdated terminology implies that most of the avian telencephalon is a hypertrophied basal ganglia, when it is now clear that most of the avian telencephalon is neurochemically, hodologically, and functionally comparable to the mammalian neocortex, claustrum, and pallial amygdala (all of which derive from the pallial sector of the developing telencephalon). Recognizing that this promotes misunderstanding of the functional organization of avian brains and their evolutionary relationship to mammalian brains, avian brain specialists began discussions to rectify this problem, culminating in the Avian Brain Nomenclature Forum held at Duke University in July 2002, which approved a new terminology for avian telencephalon and some allied brainstem cell groups. Details of this new terminology are presented here, as is a rationale for each name change and evidence for any homologies implied by the new names. Revisions for the brainstem focused on vocal control, catecholaminergic, cholinergic, and basal ganglia-related nuclei. For example, the Forum recognized that the hypoglossal nucleus had been incorrectly identified as the nucleus intermedius in the Karten and Hodos (1967) pigeon brain atlas, and what was identified as the hypoglossal nucleus in that atlas should instead be called the supraspinal nucleus. The locus ceruleus of this and other avian atlases was noted to consist of a caudal noradrenergic part homologous to the mammalian locus coeruleus and a rostral region corresponding to the mammalian A8 dopaminergic cell group. The midbrain dopaminergic cell group in birds known as the nucleus tegmenti pedunculopontinus pars compacta was recognized as homologous to the mammalian substantia nigra pars compacta and was renamed accordingly; a group of gamma-aminobutyric acid (GABA)ergic neurons at the lateral edge of this region was identified as homologous to the mammalian substantia nigra pars reticulata and was also renamed accordingly. A field of cholinergic neurons in the rostral avian hindbrain was named the nucleus pedunculopontinus tegmenti, whereas the anterior nucleus of the ansa lenticularis in the avian diencephalon was renamed the subthalamic nucleus, both for their evident mammalian homologues. For the basal (i.e., subpallial) telencephalon, the actual parts of the basal ganglia were given names reflecting their now evident homologues. For example, the lobus parolfactorius and paleostriatum augmentatum were acknowledged to make up the dorsal subdivision of the striatal part of the basal ganglia and were renamed as the medial and lateral striatum. The paleostriatum primitivum was recognized as homologous to the mammalian globus pallidus and renamed as such. Additionally, the rostroventral part of what was called the lobus parolfactorius was acknowledged as comparable to the mammalian nucleus accumbens, which, together with the olfactory tubercle, was noted to be part of the ventral striatum in birds. A ventral pallidum, a basal cholinergic cell group, and medial and lateral bed nuclei of the stria terminalis were also recognized. The dorsal (i.e., pallial) telencephalic regions that had been erroneously named to reflect presumed homology to striatal parts of mammalian basal ganglia were renamed as part of the pallium, using prefixes that retain most established abbreviations, to maintain continuity with the outdated nomenclature. We concluded, however, that one-to-one (i.e., discrete) homologies with mammals are still uncertain for most of the telencephalic pallium in birds and thus the new pallial terminology is largely devoid of assumptions of one-to-one homologies with mammals. The sectors of the hyperstriatum composing the Wulst (i.e., the hyperstriatum accessorium intermedium, and dorsale), the hyperstriatum ventrale, the neostriatum, and the archistriatum have been renamed (respectively) the hyperpallium (hypertrophied pallium), the mesopallium (middle pallium), the nidopallium (nest pallium), and the arcopallium (arched pallium). The posterior part of the archistriatum has been renamed the posterior pallial amygdala, the nucleus taeniae recognized as part of the avian amygdala, and a region inferior to the posterior paleostriatum primitivum included as a subpallial part of the avian amygdala. The names of some of the laminae and fiber tracts were also changed to reflect current understanding of the location of pallial and subpallial sectors of the avian telencephalon. Notably, the lamina medularis dorsalis has been renamed the pallial-subpallial lamina. We urge all to use this new terminology, because we believe it will promote better communication among neuroscientists. Further information is available at http://avianbrain.org

Efferent connections of the nucleus of the lateral olfactory tract in the rat

The efferent connections of the nucleus of the lateral olfactory tract (LOT) were examined in the rat with the Phaseolus vulgaris leucoagglutinin (PHA-L) technique. Our observations reveal that layers II and III of LOT have largely segregated outputs. Layer II projects chiefly ipsilaterally to the olfactory bulb and anterior olfactory nucleus, bilaterally to the anterior piriform cortex, dwarf cell cap regions of the olfactory tubercle and lateral shell of the accumbens, and contralaterally to the lateral part of the interstitial nucleus of the posterior limb of the anterior commissure. Layer III sends strong bilateral projections to the rostral basolateral amygdaloid complex, which are topographically organized, and provides bilateral inputs to the core of the accumbens, caudate-putamen, and agranular insular cortex (dorsal and posterior divisions). Layer II projects also to itself and to layers I and II of the contralateral LOT, whereas layer III projects to itself, to ipsilateral layer II, and to contralateral layer III of LOT. In double retrograde labeling experiments using Fluorogold and cholera toxin subunit b tracers, LOT neurons from layers II and III were found to provide collateral projections to homonymous structures on both sides of the brain. Unlike other parts of the olfactory amygdala, LOT neither projects directly to the extended amygdala nor to the hypothalamus. Thus, LOT seemingly influences nonpheromonal olfactory-guided behaviors, especially feeding, by acting on the olfactory bulb and on ventral striatal and basolateral amygdaloid districts that are tightly linked to lateral prefrontal cortical operations.

Chemical organization of projection neurons in the rat accumbens nucleus and olfactory tubercle

Projection neurons in the ventral striatum, the accumbens nucleus and olfactory tubercle, were examined by combining the retrograde tracing method and immunocytochemistry with antibodies against C-terminals of the preprodynorphin (PPD), preproenkephalin (PPE), preprotachykinin A (PPTA) and preprotachykinin B (PPTB). When the retrograde tracer was injected into the ventral pallidum, about 60% and 40% of retrogradely labeled neurons in the accumbens nucleus were immunoreactive for PPD and PPE, respectively. In contrast, all accumbens nucleus neurons projecting to the ventral mesencephalic regions including the substantia nigra and ventral tegmental area were immunopositive for PPD but not for PPE. Although no olfactory tubercle neurons projected fibers to the mesencephalic regions, 60% and 40% of olfactory tubercle neurons projecting to the ventrolateral portion of the ventral pallidum were immunoreactive for PPD and PPE, respectively, as were the accumbens nucleus neurons. About 70% of accumbens nucleus and olfactory tubercle neurons projecting to the ventral pallidum and all accumbens nucleus neurons projecting to the ventral mesencephalic regions showed PPTA immunoreactivity. A small population (2-12%) of accumbens neurons projecting to the ventral pallidum and mesencephalic regions displayed immunoreactivity for PPTB. Compared with the dorsal striatopallidal projection neurons that were reported to mostly express PPE, it was characteristic of the ventral striatum that only the smaller population (about 40%) of ventral striatopallidal projection neurons expressed PPE. This suggests that the ventral striatopallidal projection system is less specialized than the dorsal striatopallidal system in terms of peptide production, or that the ventral pallidum should be compared with a combined region of the globus pallidus and entopeduncular nucleus in the dorsal system.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Temporal upregulation of prodynorphin mRNA in the primate striatum after cocaine self-administration

Several human and rat studies suggest that the striatal dynorphin system is important for neuroadaptation following cocaine exposure. In the current study, prodynorphin (PDYN) mRNA expression was examined in monkeys at initial and chronic phases of cocaine self-administration. Adult Rhesus monkeys were trained to self-administer food (banana flavoured pellets) or cocaine (0.03 or 0.3 mg/kg per injection) on a fixed interval 3-min schedule for 5 or 100 sessions. Each session ended after 30 reinforcers were delivered. The PDYN mRNA expression was analysed in the precommissural striatum using in situ hybridization histochemistry. We found a specific activation of PDYN mRNA expression in the limbic-innervated patch/striosome compartment of the dorsal caudate and dorsal putamen during the initial (i.e. 5 day) phase of the high dose cocaine self-administration. After 100 days of the high dose exposure, the patch/striosome compartment remained activated, but an increase in PDYN mRNA levels was also evident in the sensorimotor-connected matrix compartment of the caudate. Neither self-administration phase resulted in significant changes in the corresponding striatal regions of the low dose cocaine-exposed primates. Moreover, cocaine self-administration failed to alter the PDYN mRNA expression in high- or low-expressing PDYN cell populations in the nucleus accumbens during any condition studied. These results demonstrate the vulnerability of the dorsal striatum (in particular the caudate) to neuroadaptations following long-term high dose cocaine self-administration. In addition, the temporal nature of the changes in PDYN gene expression within the striatal compartments could reflect a change in drug responsivity that occurs during the transition to drug dependence.