Mu-opioid receptor is present in dendritic targets of Endomorphin-2 axon terminals in the nuclei of the solitary tract

Nucleus Tractus Solitarius Neurons Activated by Hypercapnia and Hypoxia Lack Mu Opioid Receptor Expression

Impaired chemoreflex responses are a central feature of opioid-induced respiratory depression, however, the mechanism through which mu opioid receptor agonists lead to diminished chemoreflexes is not fully understood. One brainstem structure involved in opioid-induced impairment of chemoreflexes is the nucleus of the solitary tract (NTS), which contains a population of neurons that express mu opioid receptors. Here, we tested whether caudal NTS neurons activated during the chemoreflex challenge express mu opioid receptors and overlap with neurons activated by opioids. Using genetic labeling of mu opioid receptor-expressing neurons and cFos immunohistochemistry as a proxy for neuronal activation, we examined the distribution of activated NTS neurons following hypercapnia, hypoxia, and morphine administration. The main finding was that hypoxia and hypercapnia primarily activated NTS neurons that did not express mu opioid receptors. Furthermore, concurrent administration of morphine with hypercapnia induced cFos expression in non-overlapping populations of neurons. Together these results suggest an indirect effect of opioids within the NTS, which could be mediated through mu opioid receptors on afferents and/or inhibitory interneurons.

Estrogens synthesized and acting within a spinal oligomer suppress spinal endomorphin 2 antinociception: ebb and flow over the rat reproductive cycle

The magnitude of antinociception elicited by intrathecal endomorphin 2 (EM2), an endogenous mu-opioid receptor (MOR) ligand, varies across the rat estrous cycle. We now report that phasic changes in analgesic responsiveness to spinal EM2 result from plastic interactions within a novel membrane-bound oligomer containing estrogen receptors (mERs), aromatase (aka estrogen synthase), metabotropic glutamate receptor 1 (mGluR1), and MOR. During diestrus, spinal mERs, activated by locally synthesized estrogens, act with mGluR1 to suppress spinal EM2/MOR antinociception. The emergence of robust spinal EM2 antinociception during proestrus results from the loss of mER-mGluR1 suppression, a consequence of altered interactions within the oligomer. The chemical pairing of aromatase with mERs within the oligomer containing MOR and mGluR1 allows estrogens to function as intracellular messengers whose synthesis and actions are confined to the same signaling oligomer. This form of estrogenic signaling, which we term "oligocrine," enables discrete, highly compartmentalized estrogen/mER-mGluR1 signaling to regulate MOR-mediated antinociception induced by EM2. Finally, spinal neurons were observed not only to coexpress MOR, mERα, aromatase, and mGluR1 but also be apposed by EM2 varicosities. This suggests that modulation of spinal analgesic responsiveness to exogenous EM2 likely reflects changes in its endogenous analgesic activity. Analogous suppression of spinal EM2 antinociception in women (eg, around menses, comparable with diestrus in rats) as well as the (pathological) inability to transition out of that suppressed state at other menstrual cycle stages could underlie, at least in part, the much greater prevalence and severity of chronic pain in women than men.

Optogenetic Stimulation of Arcuate Nucleus Kiss1 Neurons Reveals a Steroid-Dependent Glutamatergic Input to POMC and AgRP Neurons in Male Mice

Kisspeptin (Kiss1) neurons are essential for reproduction, but their role in the control of energy balance and other homeostatic functions remains unclear. Proopiomelanocortin (POMC) and agouti-related peptide (AgRP) neurons, located in the arcuate nucleus (ARC) of the hypothalamus, integrate numerous excitatory and inhibitory inputs to ultimately regulate energy homeostasis. Given that POMC and AgRP neurons are contacted by Kiss1 neurons in the ARC (Kiss1(ARC)) and they express androgen receptors, Kiss1(ARC) neurons may mediate the orexigenic action of testosterone via POMC and/or AgRP neurons. Quantitative PCR analysis of pooled Kiss1(ARC) neurons revealed that mRNA levels for Kiss1 and vesicular glutamate transporter 2 were higher in castrated male mice compared with gonad-intact males. Single-cell RT-PCR analysis of yellow fluorescent protein (YFP) ARC neurons harvested from males injected with AAV1-EF1α-DIO-ChR2:YFP revealed that 100% and 88% expressed mRNAs for Kiss1 and vesicular glutamate transporter 2, respectively. Whole-cell, voltage-clamp recordings from nonfluorescent postsynaptic ARC neurons showed that low frequency photo-stimulation (0.5 Hz) of Kiss1-ChR2:YFP neurons elicited a fast glutamatergic inward current in POMC and AgRP neurons. Paired-pulse, photo-stimulation revealed paired-pulse depression, which is indicative of greater glutamate release, in the castrated male mice compared with gonad-intact male mice. Group I and group II metabotropic glutamate receptor agonists depolarized and hyperpolarized POMC and AgRP neurons, respectively, which was mimicked by high frequency photo-stimulation (20 Hz) of Kiss1(ARC) neurons. Therefore, POMC and AgRP neurons receive direct steroid- and frequency-dependent glutamatergic synaptic input from Kiss1(ARC) neurons in male mice, which may be a critical pathway for Kiss1 neurons to help coordinate energy homeostasis and reproduction.

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

Descending projections from the rostral ventromedial medulla (RVM) to trigeminal and spinal dorsal horns are morphologically and neurochemically distinct

Neurons in the rostral ventromedial medulla (RVM) are thought to modulate nociceptive transmission via projections to spinal and trigeminal dorsal horns. The cellular substrate for this descending modulation has been studied with regard to projections to spinal dorsal horn, but studies of the projections to trigeminal dorsal horn have been less complete. In this study, we combined anterograde tracing from RVM with immunocytochemical detection of the GABAergic synthetic enzyme, GAD67, to determine if the RVM sends inhibitory projections to trigeminal dorsal horn. We also examined the neuronal targets of this projection using immunocytochemical detection of NeuN. Finally, we used electron microscopy to verify cellular targets. We compared projections to both trigeminal and spinal dorsal horns. We found that RVM projections to both trigeminal and spinal dorsal horn were directed to postsynaptic profiles in the dorsal horn, including somata and dendrites, and not to primary afferent terminals. We found that RVM projections to spinal dorsal horn were more likely to contact neuronal somata and were more likely to contain GAD67 than projections from RVM to trigeminal dorsal horn. These findings suggest that RVM neurons send predominantly GABAergic projections to spinal dorsal horn and provide direct input to postsynaptic neurons such as interneurons or ascending projection neurons. The RVM projection to trigeminal dorsal horn is more heavily targeted to dendrites and is only modestly GABAergic in nature. These anatomical features may underlie differences between trigeminal and spinal dorsal horns with regard to the degree of inhibition or facilitation evoked by RVM stimulation.

Insulin acts in the arcuate nucleus to increase lumbar sympathetic nerve activity and baroreflex function in rats

Although the central effects of insulin to activate the sympathetic nervous system and enhance baroreflex gain are well known, the specific brain site(s) at which insulin acts has not been identified. We tested the hypotheses that (1) the paraventricular nucleus of the hypothalamus (PVN) and the arcuate nucleus (ArcN) are necessary brain sites and (2) insulin initiates its effects directly in the PVN and/or the ArcN. In α-chloralose anaesthetised female Sprague–Dawley rats, mean arterial pressure (MAP), heart rate (HR) and lumbar sympathetic nerve activity (LSNA) were recorded continuously, and baroreflex gain of HR and LSNA were measured before and during a hyperinsulinaemic–euglycaemic clamp. After 60 min, intravenous infusion of insulin (15 mU kg−1 min−1), but not saline, significantly increased (P < 0.05) basal LSNA (to 228 ± 28% control) and gain of baroreflex control of LSNA (from 3.8 ± 1.1 to 7.4 ± 2.4% control mmHg−1). These effects were reversed (P < 0.05) by local inhibition (bilateral microinjection of musimol) of the PVN (LSNA to 124 ± 8.8% control; LSNA gain to 3.9 ± 1.7% control mmHg−1) or of the ArcN (LSNA in % control: from 100 ± 0 to 198 ± 24 (insulin), then 133 ± 23 (muscimol) LSNA gain in % control mmHg−1: from 3.9 ± 0.3 to 8.9 ± 0.9 (insulin), then 5.1 ± 0.5 (muscimol)). While insulin receptor immunoreactivity was identified in neurons in pre-autonomic PVN subnuclei, microinjection of insulin (0.6, 6 and 60 nU) into the PVN failed to alter LSNA or LSNA gain. However, ArcN insulin increased (P < 0.05) basal LSNA (in % control to 162 ± 19, 0.6 nU; 193 ± 19, 6 nU; and 205 ± 28, 60 nU) and LSNA baroreflex gain (in % control mmHg−1 from 4.3 ± 1.2 to 6.9 ± 1.0, 0.6 nU; 7.7 ± 1.2, 6 nU; and 7.8 ± 1.3, 60 nU). None of the treatments altered MAP, HR, or baroreflex control of HR. Our findings identify the ArcN as the site at which insulin acts to activate the sympathetic nervous system and increase baroreflex gain, via a neural pathway that includes the PVN.

Differential localization of vesicular glutamate transporters and peptides in corneal afferents to trigeminal nucleus caudalis

Trigeminal afferents convey nociceptive information from the corneal surface of the eye to the trigeminal subnucleus caudalis (Vc). Trigeminal afferents, like other nociceptors, are thought to use glutamate and neuropeptides as neurotransmitters. The current studies examined whether corneal afferents contain both neuropeptides and vesicular glutamate transporters. Corneal afferents to the Vc were identified by using cholera toxin B (CTb). Corneal afferents project in two clusters to the rostral and caudal borders of the Vc, regions that contain functionally distinct nociceptive neurons. Thus, corneal afferents projecting to these two regions were examined separately. Dual immunocytochemical studies combined CTb with either calcitonin gene-related peptide (CGRP), substance P (SP), vesicular glutamate transporter 1 (VGluT1), or VGluT2. Corneal afferents were more likely to contain CGRP than SP, and corneal afferents projecting to the rostral region were more likely to contain CGRP than afferents projecting caudally. Overall, corneal afferents were equally likely to contain VGluT1 or VGluT2. Together, 61% of corneal afferents contained either VGluT1 or VGluT2, suggesting that some afferents lack a VGluT. Caudal corneal afferents were more likely to contain VGluT2 than VGluT1, whereas rostral corneal afferents were more likely to contain VGluT1 than VGluT2. Triple-labeling studies combining CTb, CGRP, and VGluT2 showed that very few corneal afferents contain both CGRP and VGluT2, caudally (1%) and rostrally (2%). These results suggest that most corneal afferents contain a peptide or a VGluT, but rarely both. Our results are consistent with a growing literature suggesting that glutamatergic and peptidergic sensory afferents may be distinct populations.

Clinically relevant infusion rates of mu-opioid agonist remifentanil cause bradypnea in decerebrate dogs but not via direct effects in the pre-Bötzinger complex region

Systemic administration of mu-opioids at clinical doses for analgesia typically slows respiratory rate. Mu-opioid receptors (MORs) on pre-Bötzinger Complex (pre-BötC) respiratory neurons, the putative kernel of respiratory rhythmogenesis, are potential targets. The purpose of this study was to determine the contribution of pre-BötC MORs to the bradypnea produced in vivo by intravenous administration of clinically relevant infusion rates of remifentanil (remi), a short-acting, potent mu-opioid analgesic. In decerebrate dogs, multibarrel micropipettes were used to record pre-BötC neuronal activity and to eject the opioid antagonist naloxone (NAL, 0.5 mM), the glutamate agonist D-homocysteic acid (DLH, 20 mM), or the MOR agonist [D-Ala(2), N-Me-Phe(4), gly-ol(5)]-enkephalin (DAMGO, 100 microM). Inspiratory and expiratory durations (T(I) and T(E)) and peak phrenic nerve activity (PPA) were measured from the phrenic neurogram. The pre-BötC was functionally identified by its rate altering response (typically tachypnea) to DLH microinjection. During intravenous remi-induced bradypnea (approximately 60% decrease in central breathing frequency, f(B)), bilateral injections of NAL in the pre-BötC did not change T(I), T(E), f(B), and PPA. Also, NAL picoejected onto single pre-BötC neurons depressed by intravenous remi had no effect on their discharge. In contrast, approximately 60 microg/kg of intravenous NAL rapidly reversed all remi-induced effects. In a separate group of dogs, microinjections of DAMGO in the pre-BötC increased f(B) by 44%, while subsequent intravenous remi infusion more than offset this DAMGO induced tachypnea. These results indicate that mu-opioids at plasma concentrations that cause profound analgesia produce their bradypneic effect via MORs located outside the pre-BötC region.

Neurochemical phenotypes of endomorphin-2-containing neurons in vagal nodose neurons of the adult rat

It has been shown that endomorphin-2-like immunoreactive (EM2-LI) neurons in dorsal root ganglion play important roles in regulating somatic information transmission. Although EM2-ergic neurons have been found in nodose ganglion (NG) which is mainly involved in transmitting visceral information into the nucleus tractus solitarii (NTS), the neurochemical phenotypes of EM2-ergic neurons have not yet been investigated. In the present study, immunofluorescent histochemical staining showed that 43.5% of the NG neurons contained EM2 and these neurons were small to medium in size. 15.2%, 27.8%, 74.4% and 25.2% of the EM2-LI NG neurons expressed substance P (SP), calcitonin gene-related peptide (CGRP), nitric oxide synthase (NOS) and vasoactive intestinal peptide (VIP), respectively. In addition, about 90.8% of EM2-LI NG neurons also contained mu-opioid receptor (MOR). EM2/MOR and EM2/SP double-labeled peripheral axons were observed in the vagal trunk. Anterograde tracing combined with immunofluorescent staining showed EM2/MOR and EM2/SP double-labeled vagal afferents in the NTS. EM2/MOR/SP and EM2/MOR/CGRP triple-labeled neurons and axons were observed in the NG. Importantly, at the ultrastructrual level, post-embedding electron microscopy revealed that EM2-LI and SP-LI gold particles coexisted in the same large dense-cored synaptic vesicles in the pre-synaptic button, while MOR-LI gold particles existed on both pre- and post-synaptic membranes in the NTS. These results suggest that EM2 in axon terminals of NG neurons might be involved in visceral information transmission and homeostatic control through modulating the release of other neurotransmitters (such as SP, CGRP, NO, VIP) via pre-synaptic MOR and through post-synaptic mechanisms in the NTS.

Endomorphin-2 is released from newborn rat primary sensory neurons in a frequency- and calcium-dependent manner

Recent evidence indicates that endomorphins, endogenous mu-opioid receptor (MOR) agonists, modulate synaptic transmission in both somatic and visceral sensory pathways. Here we show that endomorphin-2 (END-2) is expressed in newborn rat dorsal root ganglion (DRG) and nodose-petrosal ganglion complex (NPG) neurons, and rarely co-localizes with brain-derived neurotrophic factor (BDNF). In order to examine activity-dependent release of END-2 from neurons, we established a model using dispersed cultures of DRG and NPG cells activated by patterned electrical field stimulation. To detect release of END-2, we developed a novel rapid capture enzyme-linked immunosorbent assay (ELISA), in which END-2 capture antibody was added to neuronal cultures shortly before their electrical stimulation. The conventional assay was effective at reliably detecting END-2 only when the cells were stimulated in the presence of CTAP, a MOR-selective antagonist. This suggests that the strength of the novel assay is related primarily to rapid capture of released END-2 before it binds to endogenous MORs. Using the rapid capture ELISA, we found that stimulation protocols known to induce plastic changes at sensory synapses were highly effective at releasing END-2. Removal of extracellular calcium or blocking voltage-activated calcium channels significantly reduced the release. Together, our data provide the first evidence that END-2 is expressed by newborn DRG neurons of all sizes found in this age group, and can be released from these, as well as from NPG neurons, in an activity-dependent manner. These results point to END-2 as a likely mediator of activity-dependent plasticity in sensory pathways.

Colocalization and shared distribution of endomorphins with substance P, calcitonin gene-related peptide, gamma-aminobutyric acid, and the mu opioid receptor

The endomorphins are endogenous opioids with high affinity and selectivity for the mu opioid receptor (MOR, MOR-1, MOP). Endomorphin-1 (Tyr-Pro-Trp-Phe-NH(2); EM1) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH(2); EM2) have been localized to many regions of the central nervous system (CNS), including those that regulate antinociception, autonomic function, and reward. Colocalization or shared distribution (overlap) of two neurotransmitters, or a transmitter and its cognate receptor, may imply an interaction of these elements in the regulation of functions mediated in that region. For example, previous evidence of colocalization of EM2 with substance P (SP), calcitonin gene-related peptide (CGRP), and MOR in primary afferent neurons suggested an interaction of these peptides in pain modulation. We therefore investigated the colocalization of EM1 and EM2 with SP, CGRP, and MOR in other areas of the CNS. EM2 was colocalized with SP and CGRP in the nucleus of the solitary tract (NTS) and with SP, CGRP and MOR in the parabrachial nucleus. Several areas in which EM1 and EM2 showed extensive shared distributions, but no detectable colocalization with other signaling molecules, are also described.

Endomorphin-2-immunoreactive fibers selectively appose serotonergic neuronal somata in the rostral ventral medial medulla

The rostral portion of the ventral medial medulla (RVM) is a crucial site for the supraspinal antinociceptive actions of opioids. Previous studies have reported that serotonergic antagonists block the analgesia induced by microinjection of morphine into the RVM (Hammond and Yaksh [1984] Brain Res 298:329-337) and that spinally projecting serotonergic RVM neurons express mu-opioid receptors (MOR) (Kalyuzhny et al. [1996] J Neurosci 16:6490-6503; Wang and Wessendorf [1999] J Comp Neurol 404:183-196). In addition, axons immunoreactive for the endogenous MOR ligand endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) (EM-2) have been reported to be in the RVM (Martin-Schild et al. [1999] J Comp Neurol 405:450-471; Pierce and Wessendorf [2000] J Chem Neuroanat 18:181-207). In the present study we examined the relationship of EM-2-immunoreactive (EM-2-ir) axons to serotonergic and nonserotonergic RVM neurons in rats, including neurons projecting to the dorsal spinal cord. We also examined the origins of EM-2-ir in the RVM. Using unbiased methods we estimated the total number of cells in the RVM to be 1.50 x 10(4) and of these up to 70% were retrogradely labeled from the dorsal spinal cord. EM-2-ir fibers apposed both serotonergic and nonserotonergic RVM neuronal profiles. However, serotonergic profiles were significantly more likely to be apposed than nonserotonergic profiles. Thus, although serotonergic neurons comprise a minority of RVM neurons (23% of the total RVM neurons), they appear to be selectively apposed by EM-2-ir fibers. We also found that hypothalamic EM-2-ir neurons, but not EM-2-ir neurons, in the nucleus of the solitary tract projected their axons to the RVM.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Comparison of opioid receptor distributions in the rat central nervous system

The opioid receptors, mu, delta and kappa, conduct the major pharmacological effects of opioid drugs, and exhibit intriguing functional relationships and interactions in the CNS. Previously established hypotheses regarding the mechanisms underlying these phenomena specify theoretical patterns of relative cellular localisation for the different receptor types. In this study, we have used double-label immunohistochemistry to compare the cellular distributions of delta and kappa receptors with those of mu receptors in the rat CNS. Regions of established significance in opioid addiction were examined. Extensive mu/delta co-localisation was observed in neuron-like cells in several regions. mu and kappa receptors were also often co-localised in neuron-like cell bodies in several regions. However, intense kappa immunoreactivity (ir) also appeared in a separate, morphologically distinct population of cells that did not express mu receptors. These small, ovoid cells were often closely apposed against the larger, mu-ir cell bodies. Such cellular appositions were seen in several regions, but were particularly common in the medial thalamus, the periaqueductal grey and brainstem regions. These findings support proposals that functional similarities, synergy and cooperativity between mu and delta receptors arise from widespread co-expression by cells and intracellular molecular interactions. Although co-expression of mu and kappa receptors was also detected, the appearance of a separate population of kappa-expressing cells supports proposals that the contrasting and functionally antagonistic properties of mu and kappa receptors are due to expression in physiologically distinct cell types. Greater understanding of opioid receptor interaction mechanisms may provide possibilities for therapeutic intervention in opioid addiction and other conditions.

Endomorphin-2 in the medial NTS attenuates the responses to baroreflex activation

We have previously reported that microinjections of endomorphin-2 (E-2; an endogenous mu-receptor agonist) into the medial subnucleus of the NTS (mNTS) elicit depressor and bradycardic responses via activation of ionotropic glutamate receptors located on secondary mNTS-neurons. Based on this report, it was hypothesized that activation of secondary mNTS neurons by E-2 may result in an exaggeration of baroreflex responses. In order to test this hypothesis, baroreflex responses were studied in adult, urethane-anesthetized, artificially ventilated, male Wistar rats before and after the microinjections of E-2 into the mNTS. Baroreceptors were stimulated by applying pressure increments (80-100 mm Hg) in the carotid sinus and by electrical stimulation (stimulus intensity: 0.5 V, frequencies 5, 10, and 25 pulses/s, pulse duration: 1 ms) of the aortic nerve for 30-s periods. Baroreceptor stimulation elicited depressor and bradycardic responses. Microinjections (100 nl) of E-2 (0.4 mmol/l) into the mNTS attenuated the baroreflex responses. Microinjections of naloxone (an opioid receptor antagonist) into the mNTS (0.5 mmol/l) did not alter baroreflex responses. Based on these results, it was concluded that activation of mu-opioid receptors in the mNTS attenuates baroreflex responses. Possible mechanisms for excitatory effects of E-2 in the mNTS resulting in depressor and bradycardic responses, on one hand, and inhibitory effects resulting in attenuation of baroreflex responses, on the other, are discussed.