micro-Opioid receptor stimulation in the medial subnucleus of the tractus solitarius inhibits gastric tone and motility by reducing local GABA activity

Brainstem activation of GABAB receptors in the nucleus tractus solitarius increases gastric motility

Background and aim

Local GABAergic signaling in the dorsal vagal complex (DVC) is essential to control gastric function. While the inhibitory GABA_A receptor action on motility in the DVC is well-documented, the role of the GABA_B receptor on gastric function is less well-established. Microinjection of baclofen, a selective GABA_B receptor agonist, in the dorsal motor nucleus of the vagus (DMV) increases gastric tone and motility, while the effect on motility in the nucleus tractus solitarius (NTS) needs to be investigated. Previous in vitro studies showed that GABA_B receptors exert a local inhibitory effect in unidentified NTS neurons. Since the NTS and DMV nuclei have differential control of gastric motility, we compared GABA_B receptor activation in the NTS to that reported in the DMV. We microinjected baclofen unilaterally in the NTS while monitoring intragastric pressure and compared its action to optogenetic activation of somatostatin (SST) neurons in transgenic sst-Cre::channelrhodopsin-2 (ChR2) mice. We also performed patch-clamp recordings from SST and DMV neurons in brainstem slices from these mice.

Methods

In vivo drug injections and optogenetic stimulation were performed in fasted urethane/α-chloralose anesthetized male mice. Gastric tone and motility were monitored by an intragastric balloon inserted in the antrum and inflated with warm water to provide a baseline intragastric pressure (IGP). Coronal brainstem slices were obtained from the sst-Cre::ChR2 mice for interrogation with optogenetics and pharmacology using electrophysiology.

Results

The unilateral microinjection of baclofen into the NTS caused a robust increase in gastric tone and motility that was not affected by ipsilateral vagotomy. Optogenetic activation of SST neurons that followed baclofen effectively suppresses the gastric motility in vivo. In brain slices, baclofen suppressed spontaneous and light-activated inhibitory postsynaptic currents in SST and gastrointestinal-projection DMV neurons and produced outward currents.

Conclusion

Our results show that GABA_B receptors in the NTS strongly increase gastric tone and motility. Optogenetic stimulation in vivo and in vitro suggests that these receptors activated by baclofen suppress the glutamatergic sensory vagal afferents in the NTS and also inhibit the interneurons and the inhibitory neurons that project to the DMV, which, in turn, increase motility via a cholinergic excitatory pathway to the stomach.

Interactions between Brainstem Neurons That Regulate the Motility to the Stomach

Activity in the dorsal vagal complex (DVC) is essential to gastric motility regulation. We and others have previously shown that this activity is greatly influenced by local GABAergic signaling, primarily because of somatostatin (SST)-expressing GABAergic neurons. To further understand the network dynamics associated with gastric motility control in the DVC, we focused on another neuron prominently distributed in this complex, neuropeptide-Y (NPY) neurons. However, the effect of these neurons on gastric motility remains unknown. Here, we investigate the anatomic and functional characteristics of the NPY neurons in the nucleus tractus solitarius (NTS) and their interactions with SST neurons using transgenic mice of both sexes. We sought to determine whether NPY neurons influence the activity of gastric-projecting neurons, synaptically interact with SST neurons, and affect end-organ function. Our results using combined neuroanatomy and optogenetic in vitro and in vivo show that NPY neurons are part of the gastric vagal circuit as they are trans-synaptically labeled by a viral tracer from the gastric antrum, are primarily excitatory as optogenetic activation of these neurons evoke EPSCs in gastric-antrum-projecting neurons, are functionally coupled to each other and reciprocally connected to SST neurons, whose stimulation has a potent inhibitory effect on the action potential firing of the NPY neurons, and affect gastric tone and motility as reflected by their robust optogenetic response in vivo. These findings indicate that interacting NPY and SST neurons are integral to the network that controls vagal transmission to the stomach.SIGNIFICANCE STATEMENT The brainstem neurons in the dorsal nuclear complex are essential for regulating vagus nerve activity that affects the stomach via tone and motility. Two distinct nonoverlapping populations of predominantly excitatory NPY neurons and predominantly inhibitory SST neurons form reciprocal connections with each other in the NTS and with premotor neurons in the dorsal motor nucleus of the vagus to control gastric mechanics. Light activation and inhibition of NTS NPY neurons increased and decreased gastric motility, respectively, whereas both activation and inhibition of NTS SST neurons enhanced gastric motility.

Brainstem Neuronal Circuitries Controlling Gastric Tonic and Phasic Contractions: A Review

This review is on how current knowledge of brainstem control of gastric mechanical function unfolded over nearly four decades from the perspective of our research group. It describes data from a multitude of different types of studies involving retrograde neuronal tracing, microinjection of drugs, whole-cell recordings from rodent brain slices, receptive relaxation reflex, accommodation reflex, c-Fos experiments, immunohistochemical methods, electron microscopy, transgenic mice, optogenetics, and GABAergic signaling. Data obtained indicate the following: (1) nucleus tractus solitarius (NTS)-dorsal motor nucleus of the vagus (DMV) noradrenergic connection is required for reflex control of the fundus; (2) second-order nitrergic neurons in the NTS are also required for reflex control of the fundus; (3) a NTS GABAergic connection is required for reflex control of the antrum; (4) a single DMV efferent pathway is involved in brainstem control of gastric mechanical function under most experimental conditions excluding the accommodation reflex. Dual-vagal effectors controlling cholinergic and non-adrenergic and non-cholinergic (NANC) input to the stomach may be part of the circuitry of this reflex. (5) GABAergic signaling within the NTS via Sst-GABA interneurons determine the basal (resting) state of gastric tone and phasic contractions. (6) For the vagal-vagal reflex to become operational, an endogenous opioid in the NTS is released and the activity of Sst-GABA interneurons is suppressed. From the data, we suggest that the CNS has the capacity to provide region-specific control over the proximal (fundus) and distal (antrum) stomach through engaging phenotypically different efferent inputs to the DMV.

Current overview of opioids in progression of inflammatory bowel disease; pharmacological and clinical considerations

Inflammatory bowel diseases (IBD) belong to a subgroup of persistent, long-term, progressive, and relapsing inflammatory conditions. IBD may spontaneously develop in the colon, resulting in tumor lesions in inflamed regions of the intestine, such as invasive carcinoma. The benefit of opioids for IBD treatment is still questionable, thereby we investigated databases to provide an overview in this context. This review demonstrates the controversial role of opioids in IBD therapy, their physiological and pharmacological functions in attenuating the IBD symptoms, and in improving inflammatory, oxidative stress, and the quality of life factors in IBD subjects. Data were extracted from clinical, in vitro, and in vivo studies in English, between 1995 and 2019, from PubMed, Google Scholar, Scopus, and Cochrane library. Based on recent reports, there are promising opportunities to target the opioid system and control the IBD symptoms. This study suggests a novel approach for future treatment of functional and inflammatory disorders such as IBD.

GABAB Receptor Signaling in the Dorsal Motor Nucleus of the Vagus Stimulates Gastric Motility via a Cholinergic Pathway

Central nervous system regulation of the gastric tone and motility is primarily mediated via preganglionic neurons of the dorsal motor nucleus of the vagus (DMV). This is thought to occur by simultaneous engagement of both independent excitatory and inhibitory pathways from the DMV and has been proposed to underlie the opposing effects seen on gastric tone and motility in a number of in vivo models. Contrary to this view, we have been unable to find any evidence for this "dual effector" pathway. Since this possibility is so fundamental to how the brain-gut axis may interact in light of both peripheral and central demands, we decided to explore it further in two separate animal models previously used in conjunction with GABAB signaling to report the existence of a "dual effector" pathway. Using anesthetized rats or ferrets, we microinjected baclofen (7.5 pmol; n = 6), a GABAB agonist into the DMV of rats or intravenously administered it (0.5 mg/kg; n = 4) in ferrets. In rats, unilateral microinjection of baclofen into the DMV caused a robust dose-dependent increase in gastric tone and motility that was abolished by ipsilateral vagotomy and counteracted by pretreatment with atropine (0.1 mg/kg; IV). Similarly, as microinjection in the rats, IV administration of baclofen (0.5 mg/kg) in the ferrets induced its characteristic excitatory effects on gastric tone and motility, which were blocked by either pre- or post-treatment with atropine (0.1 mg/kg; IV). Altogether, our data provide evidence that the gastric musculature (other than the gastric sphincters) is regulated by a "single effector" DMV pathway using acetylcholine.

Subdiaphragmatic Vagotomy With Pyloroplasty Ameliorates the Obesity Caused by Genetic Deletion of the Melanocortin 4 Receptor in the Mouse

Background/Objectives: We tested the hypothesis that abolishing vagal nerve activity will reverse the obesity phenotype of melanocortin 4 receptor knockout mice (Mc4r^−/−).

Subjects/Methods: In two separate studies, we examined the efficacy of bilateral subdiaphragmatic vagotomy (SDV) with pyloroplasty in the prevention and treatment of obesity in Mc4r^−/− mice.

Results: In the first study, SDV prevented >20% increase in body weight (BW) associated with this genotype. This was correlated with a transient reduction in overall food intake (FI) in the preventative arm of the study. Initially, SDV mice had reduced weekly FI; however, FI normalized to that of controls and baseline FI within the 8-week study period. In the second study, the severe obesity that is characteristic of the adult Mc4r^−/− genotype was significantly improved by SDV with a magnitude of 30% loss in excess BW over a 4-week period. Consistent with the first preventative study, within the treatment arm, SDV mice also demonstrated a transient reduction in FI relative to control and baseline levels that normalized over subsequent weeks. In addition to the accompanying loss in weight, mice subjected to SDV showed a decrease in respiratory exchange ratio (RER), and an increase in locomotor activity (LA). Analysis of the white fat-pad deposits of these mice showed that they were significantly less than the control groups.

Conclusions: Altogether, our data demonstrates that SDV both prevents gain in BW and causes weight loss in severely obese Mc4r^−/− mice. Moreover, it suggests that an important aspect of weight reduction for this type of monogenic obesity involves loss of signaling in vagal motor neurons.

Optogenetic and pharmacological evidence that somatostatin-GABA neurons are important regulators of parasympathetic outflow to the stomach

KEY POINTS

The dorsal motor nucleus of the vagus (DMV) in the brainstem consists primarily of vagal preganglionic neurons that innervate postganglionic neurons of the upper gastrointestinal tract. The activity of the vagal preganglionic neurons is predominantly regulated by GABAergic transmission in the DMV. The present findings indicate that the overwhelming GABAergic drive present at the DMV is primarily from somatostatin positive GABA (Sst-GABA) DMV neurons. Activation of both melanocortin and μ-opioid receptors at the DMV inhibits Sst-GABA DMV neurons. Sst-GABA DMV neurons may serve as integrative targets for modulating vagal output activity to the stomach.

ABSTRACT

We have previously shown that local GABA signalling in the brainstem is an important determinant of vagally-mediated gastric activity. However, the neural identity of this GABA source is currently unknown. To determine this, we focused on the somatostatin positive GABA (Sst-GABA) interneuron in the dorsal motor nucleus of the vagus (DMV), a nucleus that is intimately involved in regulating gastric activity. Also of particular interest was the effect of melanocortin and μ-opioid agonists on neural activity of Sst-GABA DMV neurons because their in vivo administration in the DMV mimics GABA blockade in the nucleus. Experiments were conducted in brain slice preparation of transgenic adult Sst-IRES-Cre mice expressing tdTomato fluorescence, channelrhodopsin-2, archaerhodopsin or GCaMP3. Electrophysiological recordings were obtained from Sst-GABA DMV neurons or DiI labelled gastric-antrum projecting DMV neurons. Our results show that optogenetic stimulation of Sst-GABA neurons results in a robust inhibition of action potentials of labelled premotor DMV neurons to the gastric-antrum through an increase in inhibitory post-synaptic currents. The activity of the Sst-GABA neurons in the DMV is inhibited by both melanocortin and μ-opioid agonists. These agonists counteract the pronounced inhibitory effect of Sst-GABA neurons on vagal pre-motor neurons in the DMV that control gastric motility. These observations demonstrate that Sst-GABA neurons in the brainstem are crucial for regulating the activity of gastric output neurons in the DMV. Additionally, they suggest that these neurons serve as targets for converging CNS signals to regulate parasympathetic gastric function.

Autonomic consequences of spinal cord injury

Spinal cord injury (SCI) results not only in motor and sensory deficits but also in autonomic dysfunctions. The disruption of connections between higher brain centers and the spinal cord, or the impaired autonomic nervous system itself, manifests a broad range of autonomic abnormalities. This includes compromised cardiovascular, respiratory, urinary, gastrointestinal, thermoregulatory, and sexual activities. These disabilities evoke potentially life-threatening symptoms that severely interfere with the daily living of those with SCI. In particular, high thoracic or cervical SCI often causes disordered hemodynamics due to deregulated sympathetic outflow. Episodic hypertension associated with autonomic dysreflexia develops as a result of massive sympathetic discharge often triggered by unpleasant visceral or sensory stimuli below the injury level. In the pelvic floor, bladder and urethral dysfunctions are classified according to upper motor neuron versus lower motor neuron injuries; this is dependent on the level of lesion. Most impairments of the lower urinary tract manifest in two interrelated complications: bladder storage and emptying. Inadequate or excessive detrusor and sphincter functions as well as detrusor-sphincter dyssynergia are examples of micturition abnormalities stemming from SCI. Gastrointestinal motility disorders in spinal cord injured-individuals are comprised of gastric dilation, delayed gastric emptying, and diminished propulsive transit along the entire gastrointestinal tract. As a critical consequence of SCI, neurogenic bowel dysfunction exhibits constipation and/or incontinence. Thus, it is essential to recognize neural mechanisms and pathophysiology underlying various complications of autonomic dysfunctions after SCI. This overview provides both vital information for better understanding these disorders and guides to pursue novel therapeutic approaches to alleviate secondary complications.

SLITRK3 expression correlation to gastrointestinal stromal tumor risk rating and prognosis

AIM: To assess the influence of SLIT and NTRK-like family member 3 (SLITRK3) on the prognosis of gastrointestinal stromal tumor (GIST) and determine whether SLITRK3 can help improve current risk stratification systems.

METHODS: We hypothesized that SLITRK3 could be used as a prognostic molecular biomarker for GIST. 35 fresh tumor samples and 417 paraffin-embedded specimens from GIST patients were utilized. SLITRK3 mRNA expression in GIST tumor tissue was detected by real-time polymerase chain reaction, and SLITRK3 protein levels were estimated by immunohistochemistry. The correlation of SLITRK3 expression with various tumor clinicopathological characteristics and follow-up data were analyzed.

RESULTS: GIST tumors had high expression of SLITRK3 compared with adjacent normal tissues and the expression level gradually increased with risk grade. SLITRK3 protein expression was closely associated with gastrointestinal bleeding, tumor site, tumor size, mitotic index, and National Institutes of Health (NIH) classification. Survival analysis showed that SLITRK3 expression was closely correlated with overall survival and disease-free survival of GIST patients. Multivariate analysis also identified SLITRK3 expression, mitotic index, and NIH stage as significant risk factors of GIST recurrence.

CONCLUSION: SLITRK3 expression is a highly significant predictor of GIST recurrence and metastasis. Combinations of SLITRK3 and NIH stage have strong predictive and prognostic value, and are feasible markers for clinical practice in gastrointestinal stromal tumor.

Ghrelin increases vagally mediated gastric activity by central sites of action

BACKGROUND

Vagally dependent gastric reflexes are mediated through vagal afferent fibers synapsing upon neurons of the nucleus tractus solitarius (NTS) which, in turn modulate the preganglionic parasympathetic dorsal motor nucleus of the vagus (DMV) neurons within the medullary dorsal vagal complex (DVC). The expression and transport of ghrelin receptors has been documented for the afferent vagus nerve, and functional studies have confirmed that vagal pathways are integral to ghrelin-induced stimulation of gastric motility. However, the central actions of ghrelin within the DVC have not been explored fully.

METHODS

We assessed the responses to ghrelin in fasted rats using: (i) in vivo measurements of gastric tone and motility following IVth ventricle application or unilateral microinjection of ghrelin into the DVC and (ii) whole cell recordings from gastric-projecting neurons of the DMV.

KEY RESULTS

(i) IVth ventricle application or unilateral microinjection of ghrelin into the DVC-elicited contractions of the gastric corpus via excitation of a vagal cholinergic efferent pathway and (ii) ghrelin facilitates excitatory, but not inhibitory, presynaptic transmission to DMV neurons.

CONCLUSIONS & INFERENCES

Our data indicate that ghrelin acts centrally by activating excitatory synaptic inputs onto DMV neurons, resulting in increased cholinergic drive by way of vagal motor innervation to the stomach.

Physiology, signaling, and pharmacology of opioid receptors and their ligands in the gastrointestinal tract: current concepts and future perspectives

Opioid receptors are widely distributed in the human body and are crucially involved in numerous physiological processes. These include pain signaling in the central and the peripheral nervous system, reproduction, growth, respiration, and immunological response. Opioid receptors additionally play a major role in the gastrointestinal (GI) tract in physiological and pathophysiological conditions. This review discusses the physiology and pharmacology of the opioid system in the GI tract. We additionally focus on GI disorders and malfunctions, where pathophysiology involves the endogenous opioid system, such as opioid-induced bowel dysfunction, opioid-induced constipation or abdominal pain. Based on recent reports in the field of pharmacology and medicinal chemistry, we will also discuss the opportunities of targeting the opioid system, suggesting future treatment options for functional disorders and inflammatory states of the GI tract.

Opioid induced nausea and vomiting

Opioids are broad spectrum analgesics that are an integral part of the therapeutic armamentarium to combat pain in the palliative care population. Unfortunately, among the adverse effects of opioids that may be experienced along with analgesia is nausea, vomiting, and/or retching. Although it is conceivable that in the future, using combination agents (opioids combined with agents which may nullify emetic effects), currently nausea/vomiting remains a significant issue for certain patients. However, there exists potential current strategies that may be useful in efforts to diminish the frequency and/or intensity of opioid-induced nausea/vomiting (OINV).

Melanocortin signaling in the brainstem influences vagal outflow to the stomach

Activation of melanocortin 4 receptors (MC4-Rs) in brain nuclei associated with food intake profoundly influences consummatory behavior. Of these nuclei, the dorsal motor vagal nucleus (DMV), which has a dense concentration of MC4-Rs, is an important regulator of gastric tone and motility. Hence, the present study sought to examine the role of MC4-Rs in this nucleus on these activities. Using an in vivo approach, MC4-R agonists, melanotan-II (MT-II) or α-melanocyte stimulating hormone (α-MSH), were unilaterally microinjected into the DMV of rats, and their effects were noted on gastric activity. MT-II decreased phasic contractions, whereas α-MSH increased their amplitude. Both effects were blocked by the MC4-R antagonist SHU9119 or by ipsilateral vagotomy. Microinjection of the agonists (MT-II and α-MSH) into the overlying nucleus of the solitary tract (NTS), an important component of "vago-vagal" gastric circuitry, decreased phasic contractions. In addition, α-MSH reduced gastric tone and mean arterial blood pressure. To study the underlying mechanisms of the effect of MC4-R stimulation on gastric activity, electrophysiological recordings were made from labeled DMV antrum neurons in rat pups and MC4-R(-/-) mice. Bath application of MT-II or α-MSH significantly reduced spontaneous action potentials (but not in MC4-R(-/-) mice). However, in low-calcium ACSF, MT-II decreased neuronal firing, whereas α-MSH increased it. These effects mirror those of our in vivo DMV studies. Altogether, our novel findings show that activation of MC4-Rs in the brainstem, particularly in the medial NTS by the endogenous peptide α-MSH, modulates gastric activity, which may have physiological relevance for food intake and gastric function.

The μ-opioid receptor agonist DAMGO presynaptically suppresses solitary tract-evoked input to neurons in the rostral solitary nucleus

Taste processing in the rostral nucleus of the solitary tract (rNST) is subject to modulatory influences including opioid peptides. Behavioral pharmacological studies suggest an influence of μ-opioid receptors in rNST, but the underlying mechanism is unknown. To determine the cellular site of action, we tested the effects of the μ-opioid receptor agonist DAMGO in vitro. Whole cell patch-clamp recordings were made in brain stem slices from GAD67-GFP knockin mice expressing enhanced green fluorescent protein (EGFP) under the control of the endogenous promoter for GAD67, a synthetic enzyme for GABA. Neuron counts showed that ∼36% of rNST neurons express GABA. We recorded monosynaptic solitary tract (ST)-evoked currents (jitter ≤ 300 μs) in both GAD67-EGFP-positive (GAD67+) and GAD67-EGFP-negative (GAD67-) neurons with equal frequency (25/31; 22/28), but the inputs to the GAD67+ neurons had significantly smaller paired-pulse ratios compared with GAD67- neurons. DAMGO (0.3 μM) significantly suppressed ST-evoked currents in both cell types (mean suppression = 46 ± 3.3% SE), significantly increased the paired-pulse ratio of these currents, and reduced the frequency of spontaneous miniature excitatory postsynaptic currents but did not diminish their amplitude, indicating a presynaptic site of action. Under inhibitory amino acid receptor blockade, DAMGO was significantly more suppressive in GAD67+ neurons (59% reduction) compared with GAD67- neurons (35% reduction), while the reverse was true in normal artificial cerebrospinal fluid (GAD67+: 35% reduction; GAD67-: 57% reduction). These findings suggest that DAMGO suppresses activity in rNST neurons predominantly via a presynaptic mechanism, and that this effect may interact significantly with tonic or evoked inhibitory activity.

Upper gastrointestinal dysmotility after spinal cord injury: is diminished vagal sensory processing one culprit?

Despite the widely recognized prevalence of gastric, colonic, and anorectal dysfunction after spinal cord injury (SCI), significant knowledge gaps persist regarding the mechanisms leading to post-SCI gastrointestinal (GI) impairments. Briefly, the regulation of GI function is governed by a mix of parasympathetic, sympathetic, and enteric neurocircuitry. Unlike the intestines, the stomach is dominated by parasympathetic (vagal) control whereby gastric sensory information is transmitted via the afferent vagus nerve to neurons of the nucleus tractus solitarius (NTS). The NTS integrates this sensory information with signals from throughout the central nervous system. Glutamatergic and GABAergic NTS neurons project to other nuclei, including the preganglionic parasympathetic neurons of the dorsal motor nucleus of the vagus (DMV). Finally, axons from the DMV project to gastric myenteric neurons, again, through the efferent vagus nerve. SCI interrupts descending input to the lumbosacral spinal cord neurons that modulate colonic motility and evacuation reflexes. In contrast, vagal neurocircuitry remains anatomically intact after injury. This review presents evidence that unlike the post-SCI loss of supraspinal control which leads to colonic and anorectal dysfunction, gastric dysmotility occurs as an indirect or secondary pathology following SCI. Specifically, emerging data points toward diminished sensitivity of vagal afferents to GI neuroactive peptides, neurotransmitters and, possibly, macronutrients. The neurophysiological properties of rat vagal afferent neurons are highly plastic and can be altered by injury or energy balance. A reduction of vagal afferent signaling to NTS neurons may ultimately bias NTS output toward unregulated GABAergic transmission onto gastric-projecting DMV neurons. The resulting gastroinhibitory signal may be one mechanism leading to upper GI dysmotility following SCI.

Ghrelin prevents levodopa-induced inhibition of gastric emptying and increases circulating levodopa in fasted rats

BACKGROUND

Levodopa (L-dopa) is the most commonly used treatment for alleviating symptoms of Parkinson's disease. However, L-dopa delays gastric emptying, which dampens its absorption. We investigated whether ghrelin prevents L-dopa action on gastric emptying and enhances circulating L-dopa in rats.

METHODS

Gastric emptying of non-nutrient methylcellulose/phenol red viscous solution was determined in fasted rats treated with orogastric or intraperitoneal (i.p.) L-dopa, or intravenous (i.v.) ghrelin 10 min before orogastric L-dopa. Plasma L-dopa and dopamine levels were determined by high pressure liquid chromatography. Plasma acyl ghrelin levels were assessed by radioimmunoassay. Fos expression in the brain was immunostained after i.v. ghrelin (30 μg kg(-1)) 10 min before i.p. L-dopa.

KEY RESULTS

Levodopa (5 and 15 mg kg(-1)) decreased significantly gastric emptying by 32% and 62%, respectively, when administered orally, and by 91% and 83% when injected i.p. Ghrelin (30 or 100 μg kg(-1), i.v.) completely prevented L-dopa's (15 mg kg(-1), orogastrically) inhibitory action on gastric emptying and enhanced plasma L-dopa and dopamine levels compared with vehicle 15 min after orogastric L-dopa. Levodopa (5 mg kg(-1)) did not modify plasma acyl ghrelin levels at 30 min, 1, and 2 h after i.v. injection. Levodopa (15 mg kg(-1), i.p.) induced Fos in brain autonomic centers, which was not modified by i.v. ghrelin.

CONCLUSIONS & INFERENCES

Ghrelin counteracts L-dopa-induced delayed gastric emptying but not Fos induction in the brain and enhances circulating L-dopa levels. Potential therapeutic benefits of ghrelin agonists in Parkinson's disease patients treated with L-dopa remain to be investigated.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 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 (Section 16); and immunological responses (Section 17).

Tonic GABAA receptor conductance in medial subnucleus of the tractus solitarius neurons is inhibited by activation of μ-opioid receptors

Our laboratory previously reported that gastric activity is controlled by a robust GABA(A) receptor-mediated inhibition in the medial nucleus of the tractus solitarius (mNTS) (Herman et al. 2009), and that μ-opioid receptor activation inhibits gastric tone by suppression of this GABA signaling (Herman et al. 2010). These data raised two questions: 1) whether any of this inhibition was due to tonic GABA(A) receptor-mediated conductance in the mNTS; and 2) whether μ-opioid receptor activation suppressed both tonic and phasic GABA signaling. In whole cell recordings from rat mNTS neurons, application of three GABA(A) receptor antagonists (gabazine, bicuculline, and picrotoxin) produced a persistent reduction in holding current and decrease in population variance or root mean square (RMS) noise, suggesting a blockade of tonic GABA signaling. Application of gabazine at a lower concentration abolished phasic currents, but had no effect on tonic currents or RMS noise. Application of the δ-subunit preferring agonist gaboxadol (THIP) produced a dose-dependent persistent increase in holding current and RMS noise. Pretreatment with tetrodotoxin prevented the action of gabazine, but had no effect on the THIP-induced current. Membrane excitability was unaffected by the selective blockade of phasic inhibition, but was increased by blockade of both phasic and tonic currents. In contrast, activation of tonic currents decreased membrane excitability. Application of the μ-opioid receptor agonist DAMGO produced a persistent reduction in holding current that was not observed following pretreatment with a GABA(A) receptor antagonist and was not evident in mice lacking the δ-subunit. These data suggest that mNTS neurons possess a robust tonic inhibition that is mediated by GABA(A) receptors containing the δ-subunit, that determines membrane excitability, and that is partially regulated by μ-opioid receptors.

μ-Opioid modulation in the rostral solitary nucleus and reticular formation alters taste reactivity: evidence for a suppressive effect on consummatory behavior

The neural control of feeding involves many neuromodulators, including the endogenous opioids that bind μ-opioid receptors (MORs). Injections of the MOR agonist, Damgo, into limbic and hypothalamic forebrain sites increase intake, particularly of palatable foods. Indeed, forebrain Damgo injections increase sucrose-elicited licking but reduce aversive responding (gaping) to quinine, suggesting that MOR activation may enhance taste palatability. A μ-opioid influence on taste reactivity has not been assessed in the brain stem. However, MORs are present in the first-order taste relay, the rostral nucleus of the solitary tract (rNST), and in the immediately subjacent reticular formation (RF), a region known to be essential for consummatory responses. Thus, to evaluate the consequences of rNST/dorsal RF Damgo in this region, we implanted rats with intraoral cannulas, electromyographic electrodes, and brain cannulas aimed at the ventral border of the rNST. Licking and gaping elicited with sucrose, water, and quinine were assessed before and after intramedullary Damgo and saline infusions. Damgo slowed the rate, increased the amplitude, and decreased the size of fluid-induced lick and gape bouts. In addition, the neutral stimulus water, which typically elicits licks, began to evoke gapes. Thus, the current results demonstrate that μ-opioid activation in the rNST/dorsal RF exerts complex effects on oromotor responding that contrast with forebrain effects and are more indicative of a suppressive, rather than a facilitatory effect on ingestion.

Opioidergic consequences of dietary-induced binge eating

Endogenous opioids are involved in the hedonic aspects of eating. Opioid impairments and alterations have been implicated in the pathophysiology of bulimia nervosa and binge eating disorder. Specific contributions by Bartley G. Hoebel have furthered the understanding how cyclical caloric restriction and intermittent optional access to sugar solutions result in opioid-like forebrain neural alterations and dependency in rodents. The present study sought to investigate caudal brainstem and nodose ganglion mu-opioid receptor mRNA alterations in a rodent model of dietary-induced binge eating of sweetened fat (vegetable shortening blended with 10% sucrose). Five groups (n=7 or 8) of adult female Sprague Dawley rats were exposed to various dietary conditions for 6 weeks. As measured by in situ hybridization, there was reduced (approximately 25% from naive) mu-opioid receptor mRNA in the nucleus of the solitary tract (NTS) in the binge access group, which had intermittent calorie restriction and optional limited access to the sweetened fat. A similar reduction in expression was demonstrated in the continuous access group, which has unlimited optional sweetened fat and an obese phenotype. In the nodose ganglion, mu-opioid receptor mRNA was increased (approximately 30% from groups with sweetened fat access) in rats with intermittent caloric restriction alone. Our findings and the body of work from the Hoebel laboratory suggest that dietary-induced binge eating can consequentially alter opioidergic forebrain and hindbrain feeding-related neural pathways. Future work is needed to determine whether similar alterations are involved in the maintenance and progression of binge eating and other related eating pathologies.

Endogenously released opioids mediate meal-induced gastric relaxation via peripheral mu-opioid receptors

BACKGROUND

The centrally acting mu-opioid receptor antagonist naloxone inhibits meal-induced gastric accommodation.

AIM

To study the role of peripheral mu-opioid receptors in the regulation of gastric tone and food intake by comparing the effects of naloxone with the peripherally restricted mu-opioid receptor antagonist methylnaltrexone.

METHODS

Methylnaltrexone (12 mg s.c.), naloxone (20 μg/kg/h intravenous infusion after 0.4 mg bolus) and placebo were studied in 23 healthy volunteers. Gastric volume was recorded using an intragastric bag held at constant pressure connected to a barostat, with administration of a nutrient drink after 30 min. Pressure in the stomach was measured during intragastric nutrient drink infusion until the volunteers scored maximal satiation.

RESULTS

Methylnaltrexone inhibited significantly the volume increase after food intake as assessed with the barostat (P < 0.01). During nutrient drink infusion the intragastric pressure significantly decreased as compared with the preprandial pressure after placebo treatment. Both methylnaltrexone and naloxone significantly inhibited this intragastric pressure decrease (P < 0.001 and P < 0.05, respectively). Volunteers scored maximal satiation after 979 ± 96, 958 ± 84 and 1124 ± 107 mL nutrient drink infused (for naloxone, methylnaltrexone and placebo treatment, respectively; P < 0.05).

CONCLUSIONS

These results indicate that endogenous opioids mediate gastric accommodation and satiation via peripheral mu-opioid receptors. Effects were less pronounced after naloxone treatment, which indicates that centrally involved mu-opioid receptors mediate an opposing effect.