Enkephalin systems in diencephalon and brainstem of the rat

Organization of enkephalinergic neuronal system in the central nervous system of the gecko Hemidactylus frenatus

Enkephalins are endogenous opioid pentapeptides that play a role in neurotransmission and pain modulation in vertebrates. However, the distribution pattern of enkephalinergic neurons in the brains of reptiles has been understudied. This study reports the organization of the methionine-enkephalin (M-ENK) and leucine-enkephalin (L-ENK) neuronal systems in the central nervous system of the gecko Hemidactylus frenatus using an immunofluorescence labeling method. Although M-ENK and L-ENK-immunoreactive (ir) fibers extended throughout the pallial and subpallial subdivisions, including the olfactory bulbs, M-ENK and L-ENK-ir cells were found only in the dorsal septal nucleus. Enkephalinergic perikarya and fibers were highly concentrated in the periventricular and lateral preoptic areas, as well as in the anterior and lateral subdivisions of the hypothalamus, while enkephalinergic innervation was observed in the hypothalamic periventricular nucleus, infundibular recess nucleus and median eminence. The dense accumulation of enkephalinergic content was noticed in the pars distalis of the hypophysis. In the thalamus, the nucleus rotundus and the dorsolateral, medial, and medial posterior thalamic nuclei contained M-ENK and L-ENK-ir fibers, whereas clusters of M-ENK and L-ENK-ir neurons were observed in the pretectum, mesencephalon, and rhombencephalon. The enkephalinergic fibers were also seen in the area X around the central canal, as well as the dorsal and ventral horns. The widespread distribution of enkephalin-containing neurons within the central nervous system implies that enkephalins regulate a variety of functions in the gecko, including sensory, behavioral, hypophysiotropic, and neuroendocrine functions.

Neurochemistry of the mammillary body

The mammillary body (MB) is a component of the extended hippocampal system and many studies have shown that its functions are vital for mnemonic processes. Together with other subcortical structures, such as the anterior thalamic nuclei and tegmental nuclei of Gudden, the MB plays a crucial role in the processing of spatial and working memory, as well as navigation in rats. The aim of this paper is to review the distribution of various substances in the MB of the rat, with a description of their possible physiological roles. The following groups of substances are reviewed: (1) classical neurotransmitters (glutamate and other excitatory transmitters, gamma-aminobutyric acid, acetylcholine, serotonin, and dopamine), (2) neuropeptides (enkephalins, substance P, cocaine- and amphetamine-regulated transcript, neurotensin, neuropeptide Y, somatostatin, orexins, and galanin), and (3) other substances (calcium-binding proteins and calcium sensor proteins). This detailed description of the chemical parcellation may facilitate a better understanding of the MB functions and its complex relations with other structures of the extended hippocampal system.

The role of enkephalinergic systems in substance use disorders

Enkephalin, an endogenous opioid peptide, is highly expressed in the reward pathway and may modulate neurotransmission to regulate reward-related behaviors, such as drug-taking and drug-seeking behaviors. Drugs of abuse also directly increase enkephalin in this pathway, yet it is unknown whether or not changes in the enkephalinergic system after drug administration mediate any specific behaviors. The use of animal models of substance use disorders (SUDs) concurrently with pharmacological, genetic, and molecular tools has allowed researchers to directly investigate the role of enkephalin in promoting these behaviors. In this review, we explore neurochemical mechanisms by which enkephalin levels and enkephalin-mediated signaling are altered by drug administration and interrogate the contribution of enkephalin systems to SUDs. Studies manipulating the receptors that enkephalin targets (e.g., mu and delta opioid receptors mainly) implicate the endogenous opioid peptide in drug-induced neuroadaptations and reward-related behaviors; however, further studies will need to confirm the role of enkephalin directly. Overall, these findings suggest that the enkephalinergic system is involved in multiple aspects of SUDs, such as the primary reinforcing properties of drugs, conditioned reinforcing effects, and sensitization. The idea of dopaminergic-opioidergic interactions in these behaviors remains relatively novel and warrants further research. Continuing work to elucidate the role of enkephalin in mediating neurotransmission in reward circuitry driving behaviors related to SUDs remains crucial.

The role of endogenous opioid neuropeptides in neurostimulation-driven analgesia

Due to the prevalence of chronic pain worldwide, there is an urgent need to improve pain management strategies. While opioid drugs have long been used to treat chronic pain, their use is severely limited by adverse effects and abuse liability. Neurostimulation techniques have emerged as a promising option for chronic pain that is refractory to other treatments. While different neurostimulation strategies have been applied to many neural structures implicated in pain processing, there is variability in efficacy between patients, underscoring the need to optimize neurostimulation techniques for use in pain management. This optimization requires a deeper understanding of the mechanisms underlying neurostimulation-induced pain relief. Here, we discuss the most commonly used neurostimulation techniques for treating chronic pain. We present evidence that neurostimulation-induced analgesia is in part driven by the release of endogenous opioids and that this endogenous opioid release is a common endpoint between different methods of neurostimulation. Finally, we introduce technological and clinical innovations that are being explored to optimize neurostimulation techniques for the treatment of pain, including multidisciplinary efforts between neuroscience research and clinical treatment that may refine the efficacy of neurostimulation based on its underlying mechanisms.

Endogenous opioid peptides in the descending pain modulatory circuit

The opioid epidemic has led to a serious examination of the use of opioids for the treatment of pain. Opioid drugs are effective due to the expression of opioid receptors throughout the body. These receptors respond to endogenous opioid peptides that are expressed as polypeptide hormones that are processed by proteolytic cleavage. Endogenous opioids are expressed throughout the peripheral and central nervous system and regulate many different neuronal circuits and functions. One of the key functions of endogenous opioid peptides is to modulate our responses to pain. This review will focus on the descending pain modulatory circuit which consists of the ventrolateral periaqueductal gray (PAG) projections to the rostral ventromedial medulla (RVM). RVM projections modulate incoming nociceptive afferents at the level of the spinal cord. Stimulation within either the PAG or RVM results in analgesia and this circuit has been studied in detail in terms of the actions of exogenous opioids, such as morphine and fentanyl. Further emphasis on understanding the complex regulation of endogenous opioids will help to make rational decisions with regard to the use of opioids for pain. We also include a discussion of the actions of endogenous opioids in the amygdala, an upstream brain structure that has reciprocal connections to the PAG that contribute to the brain's response to pain.

Regulation of Hypothalamo-Pituitary-Adrenocortical Responses to Stressors by the Nucleus of the Solitary Tract/Dorsal Vagal Complex

Hindbrain neurons in the nucleus of the solitary tract (NTS) are critical for regulation of hypothalamo-pituitary-adrenocortical (HPA) responses to stress. It is well known that noradrenergic (as well as adrenergic) neurons in the NTS send direct projections to hypophysiotropic corticotropin-releasing hormone (CRH) neurons and control activation of HPA axis responses to acute systemic (but not psychogenic) stressors. Norepinephrine (NE) signaling via alpha1 receptors is primarily excitatory, working either directly on CRH neurons or through presynaptic activation of glutamate release. However, there is also evidence for NE inhibition of CRH neurons (possibly via beta receptors), an effect that may occur at higher levels of stimulation, suggesting that NE effects on the HPA axis may be context-dependent. Lesions of ascending NE inputs to the paraventricular nucleus attenuate stress-induced ACTH but not corticosterone release after chronic stress, indicating reduction in central HPA drive and increased adrenal sensitivity. Non-catecholaminergic NTS glucagon-like peptide 1/glutamate neurons play a broader role in stress regulation, being important in HPA activation to both systemic and psychogenic stressors as well as HPA axis sensitization under conditions of chronic stress. Overall, the data highlight the importance of the NTS as a key regulatory node for coordination of acute and chronic stress.

Chronic nicotine-induced changes in gene expression of delta and kappa-opioid receptors and their endogenous ligands in the mesocorticolimbic system of the rat

Delta and kappa opioid receptors (DOR and KOR, respectively) and their endogenous ligands, proenkephalin (PENK) and prodynorphin (PDYN)-derived opioid peptides are proposed as important mediators of nicotine reward. This study investigated the regulatory effect of chronic nicotine treatment on the gene expression of DOR, KOR, PENK and PDYN in the mesocorticolimbic system. Three groups of rats were injected subcutaneously with nicotine at doses of 0.2, 0.4, or 0.6 mg/kg/day for 6 days. Rats were decapitated 1 hr after the last dose on day six, as this timing coincides with increased dopamine release in the mesocorticolimbic system. mRNA levels in the ventral tegmental area (VTA), lateral hypothalamic area (LHA), amygdala (AMG), dorsal striatum (DST), nucleus accumbens, and medial prefrontal cortex were measured by quantitative real-time PCR. Our results showed that nicotine upregulated DOR mRNA in the VTA at all of the doses employed, in the AMG at the 0.4 and 0.6 mg/kg doses, and in the DST at the 0.4 mg/kg dose. Conversely, PDYN mRNA was reduced in the LHA with 0.6 mg/kg nicotine and in the AMG with 0.4 mg/kg nicotine. KOR mRNA was also decreased in the DST with 0.6 mg/kg nicotine. Nicotine did not regulate PENK mRNA in any brain region studied.

The relationship between naloxone-induced cortisol and delta opioid receptor availability in mesolimbic structures is disrupted in alcohol-dependent subjects

Hypothalamic-pituitary-adrenal (HPA) axis responses following naloxone administration have been assumed to provide a measure of opioid receptor activity. Employing positron emission tomography (PET) using the mu opioid receptor (MOR) selective ligand [(11)C] carfentanil (CFN), we demonstrated that cortisol responses to naloxone administration were negatively correlated with MOR availability. In this study, we examined whether naloxone-induced cortisol and adrenocorticotropin (ACTH) responses in 15 healthy control and 20 recently detoxified alcohol-dependent subjects correlated with delta opioid receptor (DOR) availability in 15 brain regions using the DOR-selective ligand [(11)C] methyl-naltrindole (MeNTL) and PET imaging. The day after the scan, cortisol responses to cumulative doses of naloxone were determined. Peak cortisol and ACTH levels and area under the cortisol and ACTH curve did not differ by group. There were negative relationships between cortisol area under curve to naloxone and [(11)C] MeNTL-binding potential (BP(ND)) in the ventral striatum, anterior cingulate, fusiform cortices, temporal cortex, putamen and a trend in the hypothalamus of healthy control subjects. However, in alcohol-dependent subjects, cortisol responses did not correlate with [(11)C]MeNTL BP(ND) in any brain region. Plasma ACTH levels did not correlate with [(11)C]MeNTL BP(ND) in either group. The study demonstrates that naloxone provides information about individual differences in DOR availability in several mesolimbic structures. The data also show that the HPA axis is intimately connected with mesolimbic stress pathways through opioidergic neurotransmission in healthy subjects but this relationship is disrupted during early abstinence in alcohol-dependent subjects.

Effects of electroacupuncture of different frequencies on the release profile of endogenous opioid peptides in the central nerve system of goats

To investigate the release profile of met-enkephalin, β-endorphin, and dynorphin-A in ruminants' CNS, goats were stimulated by electroacupuncture of 0, 2, 40, 60, 80, or 100 Hz for 30 min. The pain threshold was measured using potassium iontophoresis. The peptide levels were determined with SABC immunohistochemisty. The results showed that 60 Hz increased pain threshold by 91%; its increasing rate was higher (P < 0.01) than any other frequency did. 2 Hz and 100 Hz increased met-enkephalin immunoactivities (P < 0.05) in nucleus accumbens, septal area, caudate nucleus, amygdala, paraventricular nucleus of hypothalamus, periaqueductal gray, dorsal raphe nucleus, and locus ceruleus. The two frequencies elicited β-endorphin release (P < 0.05) in nucleus accumbens, septal area, supraoptic nucleus, ventromedial nucleus of hypothalamus, periaqueductal gray, dorsal raphe nucleus, locus ceruleus, solitary nucleus and amygdala. 60 Hz increased (P < 0.05) met-enkephalin or β-endorphin immunoactivities in the nuclei and areas mentioned above, and habenular nucleus, substantia nigra, parabrachial nucleus, and nucleus raphe magnus. High frequencies increased dynorphin-A release (P < 0.05) in spinal cord dorsal horn and most analgesia-related nuclei. It suggested that 60 Hz induced the simultaneous release of the three peptides in extensive analgesia-related nuclei and areas of the CNS, which may be contributive to optimal analgesic effects and species variation.

C1 catecholamine neurons form local circuit synaptic connections within the rostroventrolateral medulla of rat

C1 catecholamine neurons reside within the rostroventrolateral medulla (RVLM), an area that plays an integral role in blood pressure regulation through reticulospinal projections to sympathetic preganglionic neurons in the thoracic spinal cord. In a previous investigation we mapped the efferent projections of C1 neurons, documenting supraspinal projections to cell groups in the preautonomic network that contribute to the control of cardiovascular function. Light microscopic study also revealed putative local circuit connections within RVLM. In this investigation we tested the hypothesis that RVLM C1 neurons elaborate a local circuit synaptic network that permits communication between C1 neurons giving rise to supraspinal and reticulospinal projections. A replication defective lentivirus vector that expresses enhanced green fluorescent protein (EGFP) under the control of a synthetic dopamine beta hydroxylase (DβH) promoter was used to label C1 neurons and their processes. Confocal fluorescence microscopy demonstrated thin varicose axons immunopositive for EGFP and tyrosine hydroxylase that formed close appositions to C1 somata and dendrites throughout the rostrocaudal extent of the C1 area. Dual-labeled electron microscopic analysis revealed axosomatic, axodendritic and axospinous synaptic contacts with C1 and non-C1 neurons with a distribution recapitulating that observed in the light microscopic analysis. Labeled boutons were large, contained light axoplasm, lucent spherical vesicles, and formed asymmetric synaptic contacts. Collectively these data demonstrate that C1 neurons form a synaptic network within the C1 area that may function to coordinate activity among projection-specific subpopulations of neurons. The data also suggest that the boundaries of RVLM should be defined on the basis of function criteria rather than the C1 phenotype of neurons.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Neuroanatomical characterization of endogenous opioids in the bed nucleus of the stria terminalis

Numerous neuroanatomical data indicate that the bed nucleus of the stria terminalis (BST) provides an interface between cortical and amygdaloid neurons, and effector neurons modulating motor, autonomic and neuroendocrine responses. Distinct divisions of the BST may be involved in stress response, homeostatic regulation, nociception, and motivated behaviors. Endogenous opioid peptides and receptors are expressed in the BST, but their exact distribution is poorly characterized. The present study used in situ hybridization in order to characterize the endogenous opioid system of the BST, focusing on both enkephalin and dynorphin neuropeptides, as well as their respective receptors (mu, delta, and kappa opioid receptors). We report that preprodynorphin mRNA is observed in distinct nuclei of the BST, namely the fusiform, oval and anterior lateral nuclei. In contrast, there is a widespread expression of preproenkephalin mRNA in both anterior and posterior divisions of the BST. Similarly, mu and kappa opioid receptors are broadly expressed in the BST, whereas delta opioid receptor mRNA was observed only in the principal nucleus. For further characterization of enkephalin-expressing neurons of the BST, we performed a double fluorescent in situ hybridization in order to reveal the coexpression of enkephalin peptides and markers of GABAergic and glutamatergic neurons. Although most neurons of the BST are GABAergic, there is also a modest population of glutamatergic cells expressing vesicular glutamate transporter 2 (VGLUT2) in specific nuclei of the BST. Finally, we identified a previously unreported population of enkephalinergic neurons expressing VGLUT2, which is principally located in the posterior BST.

Neurochemistry of identified motoneurons of the tensor tympani muscle in rat middle ear

The objective of the present study was to identify efferent and afferent transmitters of motoneurons of the tensor tympani muscle (MoTTM) to gain more insight into the neuronal regulation of the muscle. To identify MoTTM, we injected the fluorescent neuronal tracer Fluoro-Gold (FG) into the muscle after preparation of the middle ear in adult rats. Upon terminal uptake and retrograde neuronal transport, we observed FG in neurons located lateral and ventrolateral to the motor trigeminal nucleus ipsilateral to the injection site. Immunohistochemical studies of these motoneurons showed that apparently all contained choline acetyltransferase, demonstrating their motoneuronal character. Different portions of these cell bodies were immunoreactive to bombesin (33%), cholecystokinin (37%), endorphin (100%), leu-enkephalin (25%) or neuronal nitric oxide synthase (32%). MoTTM containing calcitonin gene-related peptide, tyrosine hydroxylase, substance P, neuropeptide Y or serotonin were not found. While calcitonin gene-related peptide was not detected in the region under study, nerve fibers immunoreactive to tyrosine hydroxylase, substance P, neuropeptide Y or serotonin were observed in close spatial relationship to MoTTM, suggesting that these neurons are under aminergic and neuropeptidergic influence. Our results demonstrating the neurochemistry of motoneuron input and output of the rat tensor tympany muscle may prove useful also for the general understanding of motoneuron function and regulation.

Mu (mu) opioid receptor regulation of ethanol-induced dopamine response in the ventral striatum: evidence of genotype specific sexual dimorphic epistasis

BACKGROUND

Ethanol stimulates the dopaminergic mesoaccumbal pathway, which is thought to play a role in ethanol reinforcement. Mu (mu)-opioid (MOP) receptors modulate accumbal dopamine activity, but it is not clear whether MOP receptors are involved in the mechanism of ethanol-stimulated accumbal dopamine release.

METHODS

We investigated the role that MOP receptors play in ethanol (2.0 g/kg)-stimulated accumbal dopamine release by using MOP receptor knockout mice (C57BL/6J-129SvEv and congenic C57BL/6J genotypes) along with blockade of MOP receptors with a mu1 selective antagonist (naloxonazine).

RESULTS

Both gene deletion and pharmacological antagonism of the MOP receptor decreased ethanol-stimulated accumbal dopamine release compared with controls with female mice showing a larger effect in the C57BL/6J-129SvEv genotype. However, both male and female mice showed reduced ethanol-stimulated dopamine release in the congenic MOP receptor knockout mice (C57BL/6J). No differences in the time course of dialysate ethanol concentration were found in any of the experiments.

CONCLUSIONS

The data demonstrate the existence of a novel interaction between genotype and sex in the regulation of ethanol-stimulated mesolimbic dopamine release by the MOP receptor. This implies that a more complete understanding of the epistatic influences on the MOP receptor and mesolimbic dopamine function may provide more effective pharmacotherapeutic interventions in the treatment of alcoholism.

The involvement of spinal bovine adrenal medulla 22-like peptide, the proenkephalin derivative, in modulation of nociceptive processing

Bovine adrenal medulla 22 (BAM22), one of the cleavage products of proenkephalin A, possesses high affinity for opioid receptors and sensory neuron-specific receptor (SNSR). The present study was designed to examine the expression of BAM22 in the spinal cord and dorsal root ganglion (DRG) of naive rats as well as in a model of inflammation. BAM22-like immunoreactivity (BAM22-IR) was expressed in fibers in the spinal cord, with high density seen in lamina I in naïve rats. The expression of BAM22-IR in the superficial laminae was greatly reduced following dorsal rhizotomy. BAM22-IR was also located in 19% of DRG cells, mainly in the small- and medium-sized subpopulations. Following injection of complete Freund's adjuvant (CFA) in the hindpaw, the expression of BAM22-IR in the superficial laminae of the spinal cord and small-sized DRG neurons on the ipsilateral side was markedly increased. Double labeling showed that the Fos-positive nucleus was surrounded by BAM22-IR cytoplasm in the spinal dorsal horn neurons or closely associated with BAM22-IR fibers in the superficial laminae. Furthermore, CFA-induced mechanical allodynia in the inflamed paw was potentiated by intrathecal administration of anti-BAM22 antibody. Together, these results demonstrate for the first time that BAM22-like peptide is mainly located in the superficial laminae of the spinal cord and mostly originates from nociceptive DRG neurons. BAM22 could thus act as a ligand for presynaptic opioid receptors and SNSR. Our study also provides evidence suggesting that BAM22 plays a role in the modulation of nociceptive processing at the spinal level under normal and inflammatory conditions.

Dynorphin-containing axons directly innervate noradrenergic neurons in the rat nucleus locus coeruleus

Stress causes increased dynorphin (DYN) expression in limbic brain regions and antagonism of kappa-opioid receptors may offer therapeutic potential for the treatment of depression. A potential site of DYN action relevant to stress and related neuropsychiatric disorders is the locus coeruleus (LC), the primary source of forebrain norepinephrine. Therefore, using immunofluorescence and immunoelectron microscopic analyses, we characterized the cellular substrates for interactions between DYN and tyrosine hydroxylase (TH), a catecholamine synthesizing enzyme in single sections through the rat LC. Light microscopic analysis of DYN immunoreactivity indicated that DYN fibers are distributed within the core and pericoerulear subregions of the LC. Using electron microscopy, immunoperoxidase labeling for DYN was primarily found in axon terminals, although in some cases was diffusely localized to somatodendritic processes. When DYN-containing axons formed synaptic contacts, they typically (89%) exhibited an asymmetric morphology. Almost a third (28%) of the postsynaptic targets of DYN-containing axons contained immunogold labeling for TH. These findings reveal some diversity as to the localization of DYN in the LC within axons that contact both TH and non-TH containing dendrites. However, the present data provide the first ultrastructural evidence that DYN-containing axon terminals directly innervate catecholaminergic LC dendrites. Moreover, DYN axon terminals targeting catecholaminergic LC dendrites via asymmetric synapses are consistent with localization within excitatory type afferents to the LC. Therefore, direct modulation of catacholaminergic LC neurons maybe an important site of action for DYN relevant to stress and stress-related disorders.

Bovine adrenal medulla 22 reverses antinociceptive morphine tolerance in the rat

Acute intrathecal (i.t.) bovine adrenal medulla 22 (BAM22, 10 nmol), an endogenous opioid peptide, induced equipotent thermal antinociception in naïve and morphine-tolerant rats while chronic BAM22 resulted in hyperalgesia and decrease in the effectiveness of antinociception. In rats made tolerant to morphine, prior administration of BAM22 (10 nmol, i.t.) significantly resumed antinociceptive response of morphine. The present study demonstrated that BAM22 was able to modulate maintenance of morphine tolerance.

Endogenous opioids and attenuated hypothalamic-pituitary-adrenal axis responses to immune challenge in pregnant rats

In late pregnant rats, the hypothalamic-pituitary-adrenal (HPA) axis is hyporesponsive to psychogenic stressors. Here, we investigated attenuated HPA responses to an immune challenge and a role for endogenous opioids. ACTH and corticosterone were assayed in blood samples from virgin and 21 d pregnant rats before and after endotoxin [lipopolysaccharide (LPS); 1 microg/kg, i.v.], interleukin-1beta (IL-1beta; 500 ng/kg, i.v.), or vehicle. In virgins, plasma ACTH concentrations increased 1 h after LPS and 15 min after IL-1beta, as did corticosterone, with no responses in pregnant rats. In situ hybridization revealed increased corticotrophin releasing hormone (CRH) mRNA expression in the dorsomedial parvocellular paraventricular nucleus (pPVN) and increased anterior pituitary pro-opiomelanocortin mRNA expression 4 h after IL-1beta in virgins; these responses were absent in pregnant rats. In contrast, immunocytochemistry showed that Fos expression was similarly increased in the nucleus tractus solitarius (NTS) A2 region in virgin and pregnant rats 90 min and 4 h after IL-1beta. Naloxone pretreatment (5 mg/kg, i.v.) restored ACTH and pPVN CRH mRNA responses after IL-1beta in pregnant rats but reduced the CRH mRNA response in virgins without affecting ACTH. Proenkephalin-A and mu-opioid receptor mRNA expression in the NTS was significantly increased in the pregnant rats, indicating upregulated brainstem opioid mechanisms. IL-1beta increased noradrenaline release in the PVN of virgin, but not pregnant, rats. However, naloxone infused directly into the PVN increased noradrenaline release after IL-1beta in pregnant rats. Thus, the HPA axis responses to immune signals are suppressed in pregnancy at the level of pPVN CRH neurons through an opioid mechanism, possibly acting by preterminal autoinhibition of NTS projections to the pPVN.

Distribution of methionine and leucine enkephalin neurons within the social behavior circuitry of the male Syrian hamster brain

Enkephalin plays a role in the social behaviors of many species, but no corresponding role for this peptide has been investigated in the male Syrian hamster, a species in which brain nuclei controlling social behaviors have been identified. Previous studies have shown the distribution of dynorphin and beta-endorphin throughout social behavior circuits within the male hamster brain. To date, the only studies of enkephalin in the hamster brain address the distribution of this peptide in the olfactory bulb and hippocampus. The present study provides a complete map of enkephalinergic neurons within the forebrain and midbrain of the male Syrian hamster and addresses the question of whether enkephalin immunoreactive (Enk-ir) cells are found within brain regions relevant to male hamster social behaviors. Following immunocytochemistry for either methionine enkephalin (met-enkephalin) or leucine enkephalin (leu-enkephalin), we observed enkephalin localization consistent with data that have previously been reported in the rat, with notable exceptions including lateral septum, ventromedial nucleus of the hypothalamus and cingulate gyrus. Additionally, met- and leu-enkephalin localization patterns largely overlap. Consistent with the post-translational processing of preproenkephalin, met-enkephalin was more abundant than leu-enkephalin both within individual cells (darker staining), and within given brain nuclei (more met-enkephalin immunoreactive cells). Two exceptions were the posterointermediate bed nucleus of the stria terminalis, containing more neurons heavily labeled for leu-enkephalin, and the main olfactory bulb, where only met-enkephalin was observed. Of most interest for this study was the observation of Enk-ir cells and terminals in areas implicated in both sexual and agonistic behaviors in this species.

Enkephalinergic inhibition of raphe pallidus inputs to rat hypoglossal motoneurones in vitro

Hypoglossal motoneurones play a major role in maintaining the patency of the upper airways and in determining airways resistance. These neurones receive inputs from many different regions of the neuroaxis including the caudal raphe nuclei. Whilst we have previously shown that glutamate is utilised in projections from one of these caudal raphe nuclei, the raphe pallidus, to hypoglossal motoneurones, these raphe pallidus-hypoglossal projections also contain multiple co-localised neuropeptides, including a population that are immunopositive for enkephalin. The role of enkephalin in the control of hypoglossal motoneurones is unknown. Therefore the aim of these studies was to determine whether enkephalins modulate caudal raphe glutamatergic inputs to hypoglossal motoneurones. Whole cell recordings were made from rat hypoglossal motoneurones in vitro, with glutamate-mediated excitatory postsynaptic currents (EPSCs) evoked in these neurones following electrical stimulation within the raphe pallidus. Superfusion of enkephalin significantly decreased the amplitude of these raphe pallidus evoked EPSCs (56.1+/-29% of control, P<0.001), an action that was mirrored by the tau-opioid receptor agonist, [D-Ala, N-Me-Phe, Gly-ol]-enkephalin acetate (DAMGO;53.8+/-26%, P<0.01), but not by the delta-opioid receptor agonist, [D-Pen]-enkephalin (DPDPE). Enkephalin also increased the amplitude ratio (1.57+/-0.36 vs. 1.14+/-0.27, P<0.01) of pairs of evoked EPSCs (paired pulse ratio), decreased the frequency (P<0.0001) but not the amplitude of miniature EPSCs, whilst having no effect on the inward current evoked by glutamate applied directly to the postsynaptic cell (97.8+/-2.2% of control, P=n.s.). Likewise, DAMGO also increased the paired pulse ratio (1.62+/-0.35 vs. 1.31+/-0.14, P<0.05) and decreased the frequency of miniature EPSCs (P<0.0001). Together, these data suggest that enkephalin acts at tau-opioid receptors located on the presynaptic terminals of raphe pallidus inputs to hypoglossal motoneurones to significantly decrease glutamate release from these projections.

Effects of intrathecal BAM22 on noxious stimulus-evoked c-fos expression in the rat spinal dorsal horn

The effects of bovine adrenal medulla 22 (BAM22), a cleaved product of proenkephalin A, were investigated on the noxious stimulus-evoked expressions of spinal c-fos-like immunoreactivity (FLI). Heat (51 degrees C) applied to the tail evoked FLI predominantly in laminae I-II of the sacral spinal cord. Intrathecal (i.t.) BAM22 at a dose of 7 nmol decreased the expressions of the heat-evoked FLI by 68%, 64% and 56% in laminae I-II, III-IV and V-VI, respectively, and the decrease pattern was comparable to that induced by i.t. morphine (10 mug). Naloxone (1 mg/kg, i.p.) significantly enhanced the heat-evoked FLI in laminae III-VI, prevented the morphine-induced inhibition, and decreased the potencies of BAM22 in laminae I-II and V-VI by 23-40%. Higher dose of naloxone (10 mg/kg, i.p.) also partially reduced the BAM22-induced suppression. Following intraplantar injection of formalin (2.5%), FLI neurons were preferentially distributed not only in laminae I-II but also in laminae III-IV and V-VI of segments L4-L5. Pretreatment with BAM22 (7 nmol, i.t.) reduced the formalin-evoked FLI neurons by 72%, 61% and 58%, in laminae I-II, III-IV and V-VI, respectively. Naloxone (1 mg/kg. i.p.) enhanced the formalin-evoked expressions of FLI in laminae III-VI and decreased the potencies of BAM22 by 22-38% in laminae I-II and V-VI. The present study provided evidence at a cellular level showing that opioid and non-opioid effects of BAM22 on nociceptive processing in acute and persistent pain models were associated with modulation of noxious stimulus-evoked activity of the spinal dorsal horn neurons.

Opiate-like effects of sugar on gene expression in reward areas of the rat brain

Drugs abused by humans are thought to activate areas in the ventral striatum of the brain that engage the organism in important adaptive behaviors, such as eating. In support of this, we report here that striatal regions of sugar-dependent rats show alterations in dopamine and opioid mRNA levels similar to morphine-dependent rats. Specifically, after a chronic schedule of intermittent bingeing on a sucrose solution, mRNA levels for the D2 dopamine receptor, and the preproenkephalin and preprotachykinin genes were decreased in dopamine-receptive regions of the forebrain, while D3 dopamine receptor mRNA was increased. While morphine affects gene expression across the entire dopamine-receptive striatum, significant differences were detected in the effects of sugar on the nucleus accumbens and adjacent caudate-putamen. The effects of sugar on mRNA levels were of greater magnitude in the nucleus accumbens than in the caudate-putamen. These areas also showed clear differences in the interactions among the genes, especially between D3R and the other genes. This was revealed by a novel multivariate analysis method that identified cooperative interactions among genes, specifically in the nucleus accumbens but not the caudate-putamen. Finally, a role for these cooperative interactions in a load-sharing response to perturbations caused by sugar was supported by the finding of a different pattern of correlations between the genes in the two striatal regions. These findings support a major role for the nucleus accumbens in mediating the effects of naturally rewarding substances and extend an animal model for studying the common substrates of drug addiction and eating disorders.

Ultrastructural localization of Leu5-enkephalin immunoreactivity in mesocortical neurons and their input terminals in rat ventral tegmental area

Enkephalin (ENK) immunoreactivity is widely distributed in the ventral tegmental area (VTA), where endogenous ENK and dynorphin opioid peptides are known to have opposing actions in reward, stress, cognition, and fear-related behaviors. Many neurons in the VTA give rise to mesocortical projections terminating in the medial prefrontal cortex (mPFC), and these projections have been implicated to varying extents in all these functions. To determine whether there is a synaptic basis for ENK and/or dynorphin modulation of cortically projecting neurons within the VTA, we combined retrograde tract-tracing from the mPFC with dual immunocytochemical-labeling electron microscopy in the rat VTA. The retrograde tracer Fluorogold (FG) was microinjected into mPFC. At optimal survival periods, sections through the VTA were processed for immunolabeling of anti-FG and a Leu(5)-ENK antibody recognizing both ENK and dynorphin peptides. Over 26% of the retrogradely labeled neuronal somatodendritic profiles (n = 177) were contacted by ENK-immunoreactive axonal profiles including small axons and axon terminals. The axon terminals varied in their subcellular distribution of ENK immunoreactivity and also differed in forming either inhibitory-type (symmetric) or excitatory-type (asymmetric) synapses. Many of the axonal profiles also were apposed to FG-labeled somata or dendrites without forming recognizable synapses. Approximately one-third of the mesocortical neuronal perikarya also showed sparsely distributed somatodendritic ENK-immunoreactivity. Our results provide ultrastructural evidence that ENK and possibly dynorphin in the rat VTA have distributions consistent with involvement in diverse physiological actions affecting the output of mesocortical neurons, some of which also contain one or both peptides.

Cellular interactions between axon terminals containing endogenous opioid peptides or corticotropin-releasing factor in the rat locus coeruleus and surrounding dorsal pontine tegmentum

Recent evidence suggests that certain stressors release both endogenous opioids and corticotropin-releasing factor (CRF) to modulate activity of the locus coeruleus (LC)-norepinephrine (NE) system. In ultrastructural studies, axon terminals containing methionine(5)-enkephalin (ENK) or CRF have been shown to target LC dendrites. These findings suggested the hypothesis that both neuropeptides may coexist in common axon terminals that are positioned to have an impact on the LC. This possibility was examined by using immunofluorescence and immunoelectron microscopic analysis of the rat LC and neighboring dorsal pontine tegmentum. Ultrastructural analysis indicated that CRF- and ENK-containing axon terminals were abundant in similar portions of the neuropil and that approximately 16% of the axon terminals containing ENK were also immunoreactive for CRF. Dually labeled terminals were more frequently encountered in the "core" of the LC vs. its extranuclear dendritic zone, which included the medial parabrachial nucleus (mPB). Triple labeling for ENK, CRF, and tyrosine hydroxylase (TH) showed convergence of opioid and CRF axon terminals with noradrenergic dendrites as well as evidence for inputs to TH-labeled dendrites from dually labeled opioid/CRF axon terminals. One potential source of ENK and CRF in the dorsal pons is the central nucleus of the amygdala (CNA). To determine the relative contribution of ENK and CRF terminals from the CNA, the CNA was electrolytically lesioned. Light-level densitometry revealed robust decreases in CRF immunoreactivity in the LC and mPB on the side ipsilateral to the lesion but little or no change in ENK immunoreactivity, confirming previous studies of the mPB. Degenerating terminals from the CNA in lesioned rats were found to be in direct contact with TH-labeled dendrites. Together, these data indicate that ENK and CRF may be colocalized to a subset of individual axon terminals in the LC "core." The finding that the CNA provides, to dendrites in the area examined, a robust CRF innervation, but little or no opioid innervation, suggests that ENK and CRF axon terminals impacting LC neurons originate from distinct sources and that terminals that colocalize ENK and CRF are not from the CNA.

Up-regulation of methionine-enkephalin-like immunoreactivity by 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment in the forebrain of the Long-Evans rat

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is considered to be one of the most toxic environmental contaminants, named dioxin. Exposure to TCDD induces a plethora of intoxication symptoms, including anorexia and hypothermia, in several mammals and human. Enkephalin, an endogenous pentapeptide, is an important neuroregulator of autonomic functions, such as food intake and body temperature. In this study, we investigated the effects of TCDD gastric administration on methionine-enkephalin (MEK) immunoreactivity in the brain of the Long-Evans rat, the species strain considered to be the most TCDD-susceptible, using immunohistochemical staining. A single dose of TCDD (dissolved in olive oil, 50 microg/kg) or olive oil alone was administrated to the rats by gavage. Compared with the vehicle-treated rat, a marked increase in the density of MEK immunoreactive cell bodies, fibers and terminals was found 2 weeks after TCDD treatment in the forebrain of the TCDD-treated rat, i.e. the central amygdaloid nucleus, field CA3 of the hippocampus, paraventricular hypothalamic nucleus, medial preoptic nucleus, interstitial nucleus of the posterior limb of the anterior commissure, lateral globus pallidus, ventral pallidum and lateral division of the bed nucleus of the stria terminalis. These results demonstrated for the first time a site-specific increased enkephalinergic activity in certain brain regions of the Long-Evans rat. It is suggested that the increased MEK immunoreactivity may act as a compensatory adaptation for the pathophysiological alterations caused by TCDD exposure.

Endomorphin-1 and -2 immunoreactive cells in the hypothalamus are labeled by fluoro-gold injections to the ventral tegmental area

Endomorphin-1 and -2 (EM1, EM2) are endogenous opioids with high affinity and selectivity for the mu-opioid receptor. Cells expressing EM-like immunoreactivity (EM-LI) are present in the hypothalamus, and fibers containing EM-LI project to many brain regions, including the ventral tegmental area (VTA). The VTA is one of the most sensitive brain regions for the rewarding and locomotor effects of opioids. It contains mu-opioid receptors, which are thought to mediate gamma-aminobutyric acid-dependent disinhibition of dopamine transmission to the nucleus accumbens. We investigated whether hypothalamic EM-LI cells project to the VTA, where they could play a natural role in this circuitry. The retrograde tracer Fluoro-Gold (FG) was microinjected into the anterior or posterior VTA in rats. Nine days later, colchicine was injected, and 24 hours later, the animals were perfused and processed for fluorescence immunocytochemistry. Numerous FG-labeled cells were detected in the hypothalamus. Both EM1-LI and EM2-LI cells were present in the periventricular nucleus, between the dorsomedial and ventromedial hypothalamus and between the ventromedial and arcuate nuclei. Subpopulations of EM1-LI and EM2-LI cells were labeled by FG. Injections of FG to the anterior and posterior VTA were both effective in producing double-labeled cells, and an anterior-posterior topographical organization between the VTA and hypothalamus was observed. The results support the idea that some endomorphin-containing neurons in the hypothalamus project to the VTA, where they may modulate reward and locomotor circuitry.

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.

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

Monoaminergic and peptidergic axonal projections to the vagal motor cell column of a teleost, the filefish Stephanolepis cirrhifer

In an immunohistochemical study, the vagal motor nucleus of a teleost, the filefish Stephanolepis cirrhifer, could be divided into a rostral part and a caudal part, and the former into a dorsolateral group and a ventromedial group. The dorsolateral group consisted of neurons immunoreactive for calcitonin gene-related peptide, whereas the ventrolateral-caudal group was negative for calcitonin gene-related peptide. The latter group was retrogradely labeled after dextran amine injection to the visceral ramus of the vagus nerve, suggesting that it is a general visceral efferent column, made up of parasympathetic preganglionic neurons, whereas the dorsolateral rostral group is a special visceral efferent column. In the general visceral efferent column, a dense concentration of nerve fibers immunoreactive for serotonin, tyrosine hydroxylase, cholecystokinin-8, and substance P, and a small number of fibers immunoreactive for neuropeptide Y was observed. Perikarya in contact with varicose terminals immunoreactive for these substances were frequently seen. In contrast, in the special visceral efferent column, only a moderate concentration of neuropeptide Y-immunoreactive nerve fibers and a sparse distribution of fibers immunoreactive for tyrosine hydroxylase were observed. Perikarya in contact with varicose terminals immunoreactive for these substances were rare. These results suggest that the vagal parasympathetic preganglionic neurons might receive multiple inputs of monoaminergic and peptidergic fibers involved in the regulation of the visceral organs. On the other hand, monoaminergic and peptidergic afferent fibers might be of much less significance in the activity of the special visceral efferent component of the vagus nerve.

Distribution of alpha-neoendorphin immunoreactivity in the diencephalon and the brainstem of the dog

Alpha-neoendorphin (alpha-NE) is an opiate decapeptide derived from the prodynorphin protein. Its anatomical distribution in the brain of mammals other than the rat, particularly in carnivores, is less well known than for other opiate peptides. In the present work, we have charted the distribution of alpha-NE immunoreactive fibers and perikarya in the diencephalon and the brainstem of the dog. The highest densities of labeled fibers were found in the substantia nigra and in patches within the nucleus of the solitary tract. Moderate densities appeared in the arcuate nucleus (Ar), median eminence, entopeduncular nucleus, ventral tegmental area, retrorubral area, periaqueductal central gray, interpeduncular nucleus and lateral parabrachial nucleus. Groups of numerous labeled perikarya were localized in the magnocellular hypothalamic nuclei, Ar and in the central superior and incertus nuclei in the metencephalon. Moreover, less densely packed fibers and cells appeared widely distributed throughout many nuclei in the region studied. These results are discussed with regard to the pattern described in other species. In addition, the present results were compared with the distribution of met-enkephalin immunoreactivity in the diencephalon and the brainstem of the dog that we have recently described. Although the distributions of these two peptides overlap in many areas, the existence of numerous differences suggest that they form separate opiate systems in the dog.

Immunohistochemical distribution of enkephalin, substance P, and somatostatin in the brainstem of the leopard frog, Rana pipiens

The brainstems of frogs contain many of the neurochemicals that are found in mammals. However, the clustering of nuclei near the ventricles makes it difficult to distinguish individual cell groups. We addressed this problem by combining immunohistochemistry with tract tracing and an analysis of cell morphology to localize neuropeptides within the brainstem of Rana pipiens. We injected a retrograde tracer, Fluoro-Gold, into the spinal cord, and, in the same frog, processed adjacent sections for immunohistochemical location of antibodies to the neuropeptides enkephalin (ENK), substance P (SP), and somatostatin (SOM). SOM+ cells were more widespread than cells containing immunoreactivity (ir) to the other substances. Most reticular nuclei in frog brainstem contained ir to at least one of these chemicals. Cells with SOM ir were found in nucleus (n.) reticularis pontis oralis, n. reticularis magnocellularis, n. reticularis paragigantocellularis, n. reticularis dorsalis, the optic tectum, n. interpeduncularis, and n. solitarius. ENK-containing cell bodies were found in n. reticularis pontis oralis, n. reticularis dorsalis, the nucleus of the solitary tract, and the tectum. The midbrain contained most of the SP+ cells. Six nonreticular nuclei (griseum centrale rhombencephali, n. isthmi, n. profundus mesencephali, n. interpeduncularis, torus semicircularis laminaris, and the tectum) contained ir to one or more of the substances but did not project to the spinal cord. The descending tract of V, and the rubrospinal, reticulospinal, and solitary tracts contained all three peptides as did the n. profundus mesencephali, n. isthmi, and specific tectal layers. Because the distribution of neurochemicals within the frog brainstem is similar to that of amniotes, our results emphasize the large amount of conservation of structure, biochemistry, and possibly function that has occurred in the brainstem, and especially in the phylogenetically old reticular formation.

Role of endogenous opioid system in the regulation of the stress response

Numerous studies and reviews support an important contribution of endogenous opioid peptide systems in the mediation, modulation, and regulation of stress responses including endocrine (hypothalamopituitary-adrenal, HPA axis), autonomic nervous system (ANS axis), and behavioral responses. Although several discrepancies exist, the most consistent finding among such studies using different species and stressors is that opioids not only diminish stress-induced neuroendocrine and autonomic responses, but also stimulate these effector systems in the non-stressed state. A distinctive feature of the analgesic action of opioids is the blunting of the distressing, affective component of pain without dulling the sensation itself. Therefore, opioid peptides may diminish the impact of stress by attenuating an array of physiologic responses including emotional and affective states. The widespread distribution of enkephalin (ENK) throughout the limbic system (including the extended amygdala, cingulate cortex, entorhinal cortex, septum, hippocampus, and the hypothalamus) is consistent with a direct role in the modulation the stress responses. The predictability of stressful events reduces the impact of a wide range of stressors and ENK appears to play an important role in this process. Therefore, ENK and its receptors could represent a major modulatory system in the adaptation of an organism to stress, balancing the response that the stressor places on the central stress system with the potentially detrimental effects that a sustained stress may produce. Chronic neurogenic stressors will induce changes in specific components of the stress-induced ENKergic system, including ENK, delta- and mu-opioid receptors. This review presents evidences for adaptive cellular mechanisms underlying the response of the central stress system when assaulted by repeated psychogenic stress, and the involvement of ENK in these processes.

Distribution of met-enkephalin immunoreactivity in the diencephalon and the brainstem of the dog

The endogenous opioid system, in particular the enkephalins, has been implicated in a vast array of neurological functions. The dog could be a suitable model for the study of complex interactions between behavioral state and regulatory physiology in which the opioid system appeared to be implicated. Moreover, opiate derivatives are currently used in veterinary clinic and sometimes pharmacologically tested in the dog. However, there are no anatomical data regarding the organization of the opioid system in this species. The present work represents the first attempt to map the distribution of Met(5)-enkephalin-like-immunoreactive (Met-enk-li) cell bodies and fibers in the diencephalon and the brainstem of the dog. In the diencephalon, labeled cells were present in all the mid-line and intralaminar thalamic nuclei; the lateral posterior, pulvinar and suprageniculate nuclei; the ventral nucleus of the lateral geniculate body and the medial geniculate body. Additionally, Met-enk-li cells were seen in every hypothalamic nucleus except in the supraoptic. Variable densities of labeled fibers were also seen in all these nuclei except in the medial geniculate body and in most areas of the lateral posterior and pulvinar nuclei. In the mesencephalon, positive cells were found in the periaqueductal gray, the Edinger-Westphal and interpeduncular nuclei, delimited areas of the superior and inferior colliculi and the ventral tegmental area. In the rhombencephalon, labeled cells were seen in the majority of the nuclei in the latero-dorsal pontine tegmentum, the nuclei of the lateral lemniscus, the trapezoid, vestibular medial, vestibular inferior and cochlear nuclei, the prepositus hypoglossal, the nucleus of the solitary tract and the dorsal motor nucleus of the vagus, the infratrigeminal nucleus and the caudal part of the spinal trigeminal nucleus and in the rhombencephalic reticular formation. The distribution of fibers included additionally the substantia nigra, all the trigeminal nerve nuclei, the facial nucleus and a restricted portion of the inferior olive. These results are discussed with regard to previous reports on the distribution of Met-enk in other species.

Serotonergic input to cholinergic neurons in the substantia innominata and nucleus basalis magnocellularis in the rat

The aim of the present study was to determine, at the light microscopic level, whether the serotonergic fibers originating from the dorsal raphe nucleus (B7), median raphe nucleus (B8) and ventral tegmentum (B9) make putative synaptic contacts with cholinergic neurons of the nucleus basalis magnocellularis and substantia innominata. For this purpose, we utilized: (i) the anterograde transport of Phaseolus vulgaris leucoagglutinin combined with choline acetyltransferase immunohistochemistry; (ii) choline acetyltransferase/tryptophan hydroxylase double immunohistochemistry; and (iii) the FluoroGold retrograde tracer technique combined with tryptophan hydroxylase immunohistochemistry. Following iontophoretic injections of Phaseolus vulgaris leucoagglutinin in the dorsal raphe nucleus, labeling was observed primarily in the ventral aspects of the nucleus basalis magnocellularis and in the intermediate region of the substantia innominata. When Phaseolus vulgaris leucoagglutinin was combined with choline acetyltransferase immunohistochemistry, a close association between the Phaseolus vulgaris leucoagglutinin-positive fibers and cholinergic neurons was observed, even though the majority of the Phaseolus vulgaris leucoagglutinin-immunoreactive terminals seemed to establish contact with non-cholinergic elements. Following Phaseolus vulgaris leucoagglutinin injection in the median raphe nucleus, very few labeled fibers with no evident close contact with nucleus basalis magnocellularis and substantia innominata cholinergic neurons were observed. After tryptophan hydroxylase/choline acetyltransferase double immunohistochemistry, a plexus of serotonergic (tryptophan hydroxylase-positive) fibers in the vicinity of choline acetyltransferase-immunoreactive neurons of the substantia innominata and nucleus basalis magnocellularis was observed, and some serotonergic terminals have been shown to come into very close contact with the cholinergic cells. Most of the tryptophan hydroxylase-immunoreactive terminals seem to establish contacts with non-cholinergic cells. Following FluoroGold injection in the nucleus basalis magnocellularis and substantia innominata, the majority of retrogradely labeled neurons was observed mainly in the ventromedial cell group of the dorsal raphe nucleus. In this area, a minority of the FluoroGold-positive neurons was tryptophan hydroxylase immunoreactive. These findings show that serotonergic terminals, identified in very close association with the cholinergic neurons in the substantia innominata and nucleus basalis magnocellularis, derive primarily from the B7 serotonergic cell group of the dorsal raphe nucleus, and provide the neuroanatomical evidence for a direct functional interaction between these two neurotransmitter systems in the basal forebrain.

Localization of mu-opioid receptors on amygdaloid projection neurons in the parabrachial nucleus of the rat

The parabrachial nucleus (PB) is a major relay of noxious and non-noxious visceral sensory information from the nucleus of the solitary tract, spinal cord, and spinal trigeminal nucleus to the forebrain. The nucleus of the solitary tract, spinal cord, and trigeminal dorsal horns contain many enkephalin- and dynorphin-immunoreactive neurons that project to the PB. To study the role of mu-opioid receptors in relaying these inputs, we examined the distribution of mu-opioid receptor immunoreactivity in the PB. The most intense staining was in the external lateral parabrachial subnucleus (PBel), including dendrites extending from the PBel into the lateral crescent subnucleus. Because the Pbel is a major source of projections to the amygdala, we combined retrograde tracing from the central nucleus of the amygdala with immunohistochemistry for mu-opioid receptors. These experiments showed that mu-opioid receptors are expressed by Pbel neurons that project to the amygdala, including those Pbel neurons whose dendrites extend into the lateral crescent subnucleus. These results indicate that mu-opioid receptors in the PB may mediate or modulate nociceptive information relayed to the amygdala from medullary or spinal cord neurons that terminate not only in the Pbel, but also in the adjacent lateral crescent parabrachial subnucleus.

Localization of amino acids, neuropeptides and cholinergic neurotransmitter markers in identified projections from the mesencephalic tegmentum to the mammillary nuclei of the rat

Retrograde labelling has been combined with immunohistochemistry to localize neurons containing GABA, glutamate, choline acetyltransferase, leu-enkephalin, neurotensin and substance P-like immunoreactivity in the projection pathways from the midbrain tegmental nuclei to the mammillary nuclei in the rat. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the medial mammillary nucleus resulted in retrogradely labelled neurons in the ventral tegmental nucleus of Gudden, whereas injections into the lateral mammillary nucleus resulted in large numbers of retrogradely labelled neurons in the ipsilateral dorsal tegmental nucleus of Gudden and in the laterodorsal tegmental nucleus. In the ventral tegmental nucleus, moderate to small numbers of retrogradely labelled neurons were also immunolabelled for GABA and approximately ten to 18 WGA-HRP-labelled neurons per section were immunoreactive for leu-enkephalin. In addition, small numbers of WGA-HRP-labelled neurons in the principal subnucleus of the ventral tegmental nucleus were immunoreactive for Glu whereas small numbers of retrogradely labelled neurons in the compact subnucleus of the central superior nucleus displayed neurotensin-like immunoreactivity. In the ventral subnucleus of the dorsal tegmental nucleus, moderate to small numbers of retrogradely labelled neurons were also GABA-immunoreactive and approximately ten to 14 WGA-HRP labelled neurons per section were immunoreactive for leu-enkephalin. The ventral subnucleus of the dorsal tegmental nucleus also contained small numbers of retrogradely labelled neurons that displayed either glutamate or substance P-like immunoreactivity. In addition, moderate to small numbers of WGA-HRP-labelled neurons (five to 20 per section) in the laterodorsal tegmental nucleus were immunoreactive for choline acetyltransferase. These results are compatible with the possibility that tegmentomammillary projection neurons use several different neurochemicals as neurotransmitter(s) and/or neuromodulator(s).

Morphological substrates underlying opioid, epinephrine and gamma-aminobutyric acid inhibitory actions in the rat locus coeruleus

The locus coeruleus (LC) has been implicated in attentional processes related to orienting behaviors, learning and memory, anxiety, stress, the sleep-wake cycle, and autonomic control, as well as to contributing to the affective state. Direct activation of LC neurons causes desynchronization of the electroencephalogram, suggesting that the LC is an important modulator of the behavioral state. The LC has been an intensely studied neuronal system, as the physiology and pharmacology of this nucleus is well understood. This is mainly because of the similarity in neurochemical composition of LC cells which all contain norepinephrine in the rat. However, the homogeneity in neurotransmitter content in LC neurons is sharply contrasted by the heterogeneity of neurochemicals found in its afferent processes. Among these are axon terminals that contain inhibitory and excitatory amino acids, monoamines, and neuropeptides, many of which have been shown to exert differential physiological effects on LC discharge activity. Although much attention has focused on physiological activation of LC neurons, substantial evidence indicates that diverse afferents prominently inhibit noradrenergic cellular activity. Such inhibitory neurochemicals, which arise from local and extrinsic sources, include gamma-aminobutyric acid (GABA) and epinephrine as well as the neuropeptides methionine5-enkephalin and leucine5-enkephalin. Inhibitory transmission in the LC has widespread implications for norepinephrine release at diverse postsynaptic targets, and clinically useful pharmacological agents such as clonidine, an alpha2 adrenergic receptor agonist that potently inhibits the firing of LC neurons, alleviate some negative physical symptoms observed following withdrawal from opiates. In the present review, the synaptic and functional organization of selected inhibitory-type neurotransmitters in the LC obtained from immunoelectron microscopic data will be discussed.

Morphine administered in the substantia gelatinosa of the spinal trigeminal nucleus caudalis inhibits nociceptive activities in the spinal trigeminal nucleus oralis

The present study investigates the effects of morphine microinjection into the spinal trigeminal nucleus caudalis (Sp5C) or the spinal trigeminal nucleus oralis (Sp5O) on C-fiber-evoked activities of Sp5O convergent neurons, after supramaximal percutaneous electrical stimulation in halothane-anesthetized rats. When it was microinjected into the Sp5O, morphine (2.5 microg in 0. 25 microl) never depressed the C-fiber-evoked responses of Sp5O convergent neurons (n = 13), whereas these neurons were responsive to the inhibitory effects of systemic morphine (6 mg/kg, i.v.) in a naloxone-reversible manner. On the contrary, morphine microinjected into the Sp5C produced a naloxone-reversible inhibition of the C-fiber-evoked responses of Sp5O neurons (n = 14). The magnitude and the time course of this effect varied according to the location of the injection sites. After microinjection into the superficial laminae (n = 7), a strong depressive effect of morphine (7 +/- 5% of control) on the C-fiber-evoked responses was apparent as soon as 5 min after the injection and could always be reversed by naloxone, administered either intravenously (0.4 mg/kg) or locally (2.5 microg in 0.6 microl) at the same site as morphine. After microinjection into deeper laminae (V-VI), a significant depressive effect (34 +/- 5% of control) of morphine could be detected only 20 min after the injection and was reversed only by intravenous administration of naloxone. These results suggest that morphine exerts its antinociceptive action on Sp5O convergent neurons by blocking the C-fiber inputs that relay in the Sp5C substantia gelatinosa. The mechanisms that underlie the activation of Sp5O convergent neurons by C-fibers and the inhibition of C-fiber-evoked responses of Sp5O convergent neurons by morphine microinjected into the Sp5C are discussed.

Enkephalin neurons that project to the A7 catecholamine cell group are located in nuclei that modulate nociception: ventromedial medulla

The location of methionine enkephalin neurons in the medulla oblongata that project to the dorsolateral pontine tegmentum was investigated using anterograde and retrograde tract tracing combined with immunocytochemical neurotransmitter identification. The results of these experiments demonstrate that enkephalinergic neurons from areas known to modulate nociception project to the region of the A7 catecholamine cell group in the dorsolateral pontine tegmentum. The medullary nuclei that contain these enkephalinergic neurons include the nucleus raphe magnus and the nucleus reticularis gigantocellularis pars alpha in the ventromedial medulla. While some of these enkephalinergic axons appose the somata and dendrites of A7 neurons, the majority of these axons appear to contact non-catecholamine neurons in the dorsolateral pontine tegmentum. Unidentified neurons located in the nucleus raphe magnus, the nucleus reticularis gigantocellularis pars alpha, and the nucleus reticularis gigantocellularis also project to the A7 area. Many of the neurons in the nucleus reticularis gigantocellularis pars alpha appear to contact both noradrenergic A7 neurons and non-catecholamine neurons in the dorsolateral pontine tegmentum, whereas most of those in the nucleus raphe magnus appear to contact non-catecholamine neurons. The anatomical findings described in this report and the results of preliminary behavioral studies provide evidence to support a model in which activation of the enkephalin-containing neurons in the ventromedial medulla facilitates nociception, while the non-enkephalin neurons mediate part of the antinociception produced by stimulating sites in the ventromedial medulla.