Preproenkephalin mRNA expression in rat parabrachial neurons: relation to cells activated by systemic immune challenge

Molecular and anatomical characterization of parabrachial neurons and their axonal projections

The parabrachial nucleus (PBN) is a major hub that receives sensory information from both internal and external environments. Specific populations of PBN neurons are involved in behaviors including food and water intake, nociceptive responses, breathing regulation, as well as learning and responding appropriately to threatening stimuli. However, it is unclear how many PBN neuron populations exist and how different behaviors may be encoded by unique signaling molecules or receptors. Here we provide a repository of data on the molecular identity, spatial location, and projection patterns of dozens of PBN neuron subclusters. Using single-cell RNA sequencing, we identified 21 subclusters of neurons in the PBN and neighboring regions. Multiplexed in situ hybridization showed many of these subclusters are enriched within specific PBN subregions with scattered cells in several other regions. We also provide detailed visualization of the axonal projections from 21 Cre-driver lines of mice. These results are all publicly available for download and provide a foundation for further interrogation of PBN functions and connections.

Neuronal mechanisms underlying opioid-induced respiratory depression: our current understanding

Opioid-induced respiratory depression (OIRD) represents the primary cause of death associated with therapeutic and recreational opioid use. Within the United States, the rate of death from opioid abuse since the early 1990s has grown disproportionally, prompting the classification as a nationwide "epidemic." Since this time, we have begun to unravel many fundamental cellular and systems-level mechanisms associated with opioid-related death. However, factors such as individual vulnerability, neuromodulatory compensation, and redundancy of opioid effects across central and peripheral nervous systems have created a barrier to a concise, integrative view of OIRD. Within this review, we bring together multiple perspectives in the field of OIRD to create an overarching viewpoint of what we know, and where we view this essential topic of research going forward into the future.

Parabrachial opioidergic projections to preoptic hypothalamus mediate behavioral and physiological thermal defenses

Maintaining stable body temperature through environmental thermal stressors requires detection of temperature changes, relay of information, and coordination of physiological and behavioral responses. Studies have implicated areas in the preoptic area of the hypothalamus (POA) and the parabrachial nucleus (PBN) as nodes in the thermosensory neural circuitry and indicate that the opioid system within the POA is vital in regulating body temperature. In the present study we identify neurons projecting to the POA from PBN expressing the opioid peptides dynorphin and enkephalin. Using mouse models, we determine that warm-activated PBN neuronal populations overlap with both prodynorphin (Pdyn) and proenkephalin (Penk) expressing PBN populations. Here we report that in the PBN Prodynorphin (Pdyn) and Proenkephalin (Penk) mRNA expressing neurons are partially overlapping subsets of a glutamatergic population expressing Solute carrier family 17 (Slc17a6) (VGLUT2). Using optogenetic approaches we selectively activate projections in the POA from PBN Pdyn, Penk, and VGLUT2 expressing neurons. Our findings demonstrate that Pdyn, Penk, and VGLUT2 expressing PBN neurons are critical for physiological and behavioral heat defense.

Neurochemistry of the Kölliker-Fuse nucleus from a respiratory perspective

The Kölliker-Fuse nucleus (KF) is a functionally distinct component of the parabrachial complex, located in the dorsolateral pons of mammals. The KF has a major role in respiration and upper airway control. A comprehensive understanding of the KF and its contributions to respiratory function and dysfunction requires an appreciation for its neurochemical characteristics. The goal of this review is to summarize the diverse neurochemical composition of the KF, focusing on the neurotransmitters, neuromodulators, and neuropeptides present. We also include a description of the receptors expressed on KF neurons and transporters involved in each system, as well as their putative roles in respiratory physiology. Finally, we provide a short section reviewing the literature regarding neurochemical changes in the KF in the context of respiratory dysfunction observed in SIDS and Rett syndrome. By over-viewing the current literature on the neurochemical composition of the KF, this review will serve to aid a wide range of topics in the future research into the neural control of respiration in health and disease.

Place preferences induced by electrical stimulation of the external lateral parabrachial subnucleus in a sequential learning task: Place preferences induced by NLPBe stimulation

It is known that electrical stimulation of the external lateral parabrachial nucleus (NLPBe) can sustain concurrent taste and place learning. Place preferences can be learned through different procedures. Previous studies demonstrated that electrical stimulation of the PBNLe can generate aversive and preference place learning using concurrent procedures. In the concurrent procedure, the animals can move freely in the maze, and intracranial electrical stimulation is associated with their voluntary stay in one of the two maze compartments. However, the rewarding properties of most stimuli, whether natural or drugs of abuse, have usually been investigated using the sequential procedure, in which animals are confined while receiving the unconditioned stimulus and then undergo a choice test without stimulation in a later phase. This study examined whether this stimulation can sustain place preference learning in sequential tasks. Results demonstrated that place preferences can also be induced by the electrical stimulation of the NLBe using sequential procedures. These findings suggest that the NLPBe may form part of a brain reward axis that shares certain characteristics with those observed in the processing of natural rewarding agents and especially of drugs of abuse.

Interleukin-1 reduces food intake and body weight in rat by acting in the arcuate hypothalamus

A reduction in food intake is commonly observed after bacterial infection, a phenomenon that can be reproduced by peripheral administration of Gram-negative bacterial lipopolysaccharide (LPS) or interleukin-1beta (IL-1β), a pro-inflammatory cytokine released by LPS-activated macrophages. The arcuate nucleus of the hypothalamus (ARH) plays a major role in food intake regulation and expresses IL-1 type 1 receptor (IL-1R1) mRNA. In the present work, we tested the hypothesis that IL-1R1 expressing cells in the ARH mediate IL-1β and/or LPS-induced hypophagia in the rat. To do so, we developed an IL-1β-saporin conjugate, which eliminated IL-R1-expressing neurons in the hippocampus, and micro-injected it into the ARH prior to systemic IL-1β and LPS administration. ARH IL-1β-saporin injection resulted in loss of neuropeptide Y-containing cells and attenuated hypophagia and weight loss after intraperitoneal IL-1β, but not LPS, administration. In conclusion, the present study shows that ARH NPY-containing neurons express functional IL-1R1s that mediate peripheral IL-1β-, but not LPS-, induced hypophagia. Our present and previous findings indicate that the reduction of food intake after IL-1β and LPS are mediated by different neural pathways.

Sodium deprivation and salt intake activate separate neuronal subpopulations in the nucleus of the solitary tract and the parabrachial complex

Salt intake is an established response to sodium deficiency, but the brain circuits that regulate this behavior remain poorly understood. We studied the activation of neurons in the nucleus of the solitary tract (NTS) and their efferent target nuclei in the pontine parabrachial complex (PB) in rats during sodium deprivation and after salt intake. After 8-day dietary sodium deprivation, immunoreactivity for c-Fos (a neuronal activity marker) increased markedly within the aldosterone-sensitive neurons of the NTS, which express the enzyme 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2). In the PB, c-Fos labeling increased specifically within two sites that relay signals from the HSD2 neurons to the forebrain--the pre-locus coeruleus and the innermost region of the external lateral parabrachial nucleus. Then, 1-2 hours after sodium-deprived rats ingested salt (a hypertonic 3% solution of NaCl), c-Fos immunoreactivity within the HSD2 neurons was virtually eliminated, despite a large increase in c-Fos activation in the surrounding NTS (including the A2 noradrenergic neurons) and area postrema. Also after salt intake, c-Fos activation increased within pontine nuclei that relay gustatory (caudal medial PB) and viscerosensory (rostral lateral PB) information from the NTS to the forebrain. Thus, sodium deficiency and salt intake stimulate separate subpopulations of neurons in the NTS, which then transmit this information to the forebrain via largely separate relay nuclei in the PB complex. These findings offer new perspectives on the roles of sensory information from the brainstem in the regulation of sodium appetite.

Learned preferences induced by electrical stimulation of a food-related area of the parabrachial complex: Effects of naloxone

Electrical stimulation of the External Lateral Parabrachial Subnucleus (LPBe), a food-related area, induced behavioral preferences for associated stimuli in a taste discrimination learning task. Although this stimulation appeared to be ineffective to elicit standard lever press self-stimulation, it induced place preference for one of two training compartments of a rectangular maze in which animals (adult male Wistar rats) received concurrent electrical brain stimulation. In subjects that consistently showed a preference behavior in different trials, administration of the opioid antagonist naloxone (4 mg/ml/kg) blocked concurrent learning when the test was made in a new maze but not in the same maze in which animals had learned the task. These results are discussed in terms of the possible participation of the LPBe subnucleus in different natural and artificial brain reward systems.

Expression of melanocortin-4 receptor by rat parabrachial neurons responsive to immune and aversive stimuli

The pontine parabrachial nucleus is a major relay area for visceral and other interoceptive information, and has been implicated in mechanisms underlying anorexia and food aversion during disease. Thus, physiological studies have shown that peripheral immune stimuli, as well as the administration of aversive substances such as lithium chloride, evoke a prominent Fos-expression in the lateral parabrachial nucleus and behavioral experiments have demonstrated that this structure is critical for the acquisition of conditioned taste aversion. The present study examined in rats the relationship between parabrachial neurons activated by systemic administration of bacterial cell-wall lipopolysaccharide or lithium chloride and the melanocortin system, a major regulator of feeding and energy homeostasis that also has been implicated in aversive behavior. Dual-labeling in situ hybridization showed melanocortin-4 receptor expression on neurons in the external lateral parabrachial subnucleus that displayed lipopolysaccharide- or lithium chloride-induced expression of c-fos mRNA. Melanocortin-4 receptor mRNA was also co-expressed with mRNA for calcitonin gene-related peptide in this subnucleus. Taken together with previous observations showing that calcitonin gene-related peptide expressing neurons in the external lateral parabrachial subnucleus are activated by peripheral immune challenge, that lipopolysaccharide-activated external lateral parabrachial subnucleus neurons project to the amygdala, and that the amygdala-projecting neurons in the external lateral parabrachial subnucleus are calcitonin gene-related peptide-positive, the present findings suggest the presence of a melanocortin-regulated calcitonin gene-related peptide-positive pathway from the external lateral parabrachial subnucleus to the amygdala that relays information of importance to forebrain responses to certain aspects of sickness behavior. These observations may thus help explain how melanocortins can reduce feeding and influence conditioned taste aversion during inflammation and other disease conditions.

Involvement of prostaglandin E2 released from leptomeningeal cells in increased expression of transforming growth factor-β in glial cells and cortical neurons during systemic inflammation

The leptomeninges play a central role in the antiinflammatory response through the glia-neuron interaction during systemic inflammation. In the present study, we examined the possible production of two potent antiinflammatory mediators, prostaglandin E(2) (PGE(2)) and transforming growth factor-beta1 (TGF-beta1) by leptomeningeal cells during systemic inflammation. After immunization with the complete Freund's adjuvant (CFA), cyclooxygenase (COX)-2 and membrane-bound PGE synthase-1 (mPGES-1) were induced in the leptomeninges. Primary cultured leptomeningeal cells secreted PGE(2) after treatment with lipopolysaccharide (LPS) or proinflammatory cytokines. The LPS-induced release of PGE(2) was depressed by a selective COX-2 inhibitor, NS-398. On the other hand, TGF-beta1 and TGF-beta receptor II (TGF-betaRII) both markedly increased in the leptomeninges and the parenchymal cells after the CFA injection. Double-staining immunohistochemistry demonstrated TGF-beta1 to be induced in both glial cells and cortical neurons, whereas TGF-betaRII was induced only in cortical neurons. Furthermore, the conditioned medium prepared from the leptomeningeal cells after LPS stimulation was able to induce an increased expression of TGF-beta1 and TGF-betaRII in the primary cultured glial cells and cortical neurons. This increased expression was suppressed by NS-398. PGE(2) was found to increase directly the production of TGF-beta1 and TGF-betaRII in the primary cultured cells. These observations strongly suggest that PGE(2), which is biosynthesized by the leptomeninges, mainly regulates the production of TGF-beta1 by glial cells and cortical neuron, thus playing a protective role in the cortical neurons during systemic inflammation. Furthermore, TGF-beta1 may also exert a protective effect directly on the cortical neurons.

EP3 and EP4 receptor mRNA expression in peptidergic cell groups of the rat parabrachial nucleus

This study examines the distribution of prostaglandin E2 receptors of subtype EP3 and EP4 among brain stem parabrachial neurons that were characterized with respect to their neuropeptide expression. By using a dual-labeling in situ hybridization method, we show that preprodynorphin mRNA expressing neurons in the dorsal and central lateral subnuclei express EP3 receptor mRNA. Such receptors are also expressed in preproenkephalin, calcitonin gene related peptide and preprotachykinin mRNA positive neurons in the external lateral subnucleus, whereas preprodynorphin mRNA expressing neurons in this subnucleus are EP receptor negative. In addition, EP3 receptor expression is seen among some enkephalinergic neurons in the Kölliker-Fuse nucleus. Neurons in the central part of the cholecystokininergic population in the regions of the superior lateral subnucleus express EP4 receptor mRNA, whereas those located more peripherally express EP3 receptors. Taken together with previous findings showing that discrete peptidergic cell groups mediate nociceptive and/or visceral afferent information to distinct brain stem and forebrain regions, the present results suggest that the processing of this information in the parabrachial nucleus is influenced by prostaglandin E2. Recent work has shown that prostaglandin E2 is released into the brain following peripheral immune challenge; hence, the parabrachial nucleus may be a region where humoral signaling of peripheral inflammatory events may interact with neuronal signaling elicited by the same peripheral processes.

Systemic immune challenge induces preproenkephalin gene transcription in distinct autonomic structures of the rat brain

The involvement of enkephalins in the immune response was investigated in rats injected intravenously with interleukin-1beta (2 microg/kg). In situ hybridization with a riboprobe complementary to intron A of the preproenkephalin (ppENK) gene showed distinct transcriptional activation within several brain regions known to be activated by immune stimuli, including the nucleus of the solitary tract, the area postrema, the paraventricular hypothalamic nucleus, and the oval nucleus of the bed nucleus of the stria terminalis, and dual labeling confirmed that a large proportion of the intron expressing neurons co-expressed c-fos mRNA. Rats injected with saline (controls) showed little or no heteronuclear transcript in these structures. The induced signal was strongest after 1 hour but was present in some structures 30 minutes after interleukin-1beta injection. At 3 hours, transcriptional activity returned to basal levels. High basal expression of the heteronuclear transcript that appeared unchanged by the immune stimulus was seen in regions not primarily involved in the immune response, such as the striatum, the olfactory tubercle, and the islands of Calleja and in the immune activated central nucleus of the amygdala. The heteronuclear transcript colocalized with ppENK mRNA, demonstrating that it occurred in enkephalinergic neurons and was not the result of alternative transcription from the ppENK gene in other cells. These results demonstrated that enkephalin transcription is induced in central autonomic neurons during immune challenge, suggesting that enkephalins are involved in the centrally orchestrated response to such stimuli.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (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).