Review: Neurotransmitter receptors V

Opiate receptors

The central nucleus of the amygdala (CeA) is a key hub integrating sensory inputs and modulating behavioural outputs. The CeA is a complex structure with discrete subdivisions, high peptidergic heterogeneity and broad CNS afferent and efferent projections. While several neuropeptide systems within the CeA have been examined in detail, less is known about CeA preproenkephalin (ppENK) cells. Here, we used a recently developed transgenic Penk-Cre mouse line to advance our understanding of the efferent and afferent connectivity of ppENK in the CeA. First, to determine the fidelity of Cre expression in Penk-Cre transgenic mice, we conducted RNAscope in the CeA of Penk-Cre mice. Our analysis revealed that 96.6 % of CeA Cre+ neurons co-expressed pENK mRNA, and 99.7 % of CeA pENK+ neurons co-expressed Cre mRNA, indicating faithful recapitulation of Cre expression in CeA ppENK-expressing cells, supporting the fidelity of the Penk-Cre reporter mouse. Anterograde tracing of CeA^Penk cells showed strong efferent projections to the extended amygdala, midbrain and hindbrain PBN and NTS. Retrograde tracing of Penk afferents to the CeA were more restricted, with primary innervation originating within the amygdala complex and bed nucleus of the stria terminalis, and minor innervation from the parabrachial nucleus and nucleus of the solitary tract. Together, our data provide a comprehensive map of ENKergic efferent and afferent connectivity of the CeA in Penk-Cre mice. Further, we highlight both the utility and limitations of the Penk-Cre mice to study the function of CeA, PBN and NTS ppENK cells.

Diabetes mellitus is one of the diseases that affect nociception. In type 1 diabetes, the insulin release declines. One of the regions that respond to insulin and have insulin receptors is the hypothalamus, especially the arcuate nucleus. This hypothalamic nucleus has proopiomelanocortin (POMC)-containing neurons that affect the pain endogenous modulatory pathways such as dorsal raphe nucleus (DR) via releasing endorphins. So it was tried to investigate the influence of insulin within the arcuate nucleus with/without DR opioid receptors blockade on the nociception in the formalin test paradigm. In the present study, the role of different doses of insulin (2, 10, and 50 mIU/0.5 µl saline) within the arcuate nucleus was investigated via formalin test in type 1 (STZ-induced) diabetic rats. To perform the formalin test, 50 µl of formalin 2.5% was injected subcutaneously (s.c.) into the right palm. The behavior of the animal after the stimulation of pain receptors by s.c. formalin injection was scored from 0 (no distinguished pain) to 3 (the most nociception and highest pain score). Insulin within the arcuate nucleus diminished the nociception in formalin-induced paw in the STZ-induced diabetic rats. Intra-DR naloxone 0.2 µg/0.5 µl saline prevented this analgesia. A possible suggested mechanism for this observation is that insulin reinforces the POMC and endorphin release from the arcuate nucleus and decreases pain through DR.

This review assesses the parallel literature on the role of gamma - aminobutyric acid (GABA) in brain plasticity and GABA elements dysfunction related disorders. I review historical and new data from both animal and human sources which have helped define the key role for this transmitter synthesis, release and reuptake, GABA receptors subtype regulation, and GABAergic neurons function in the adult brain. The role of GABAergic elements in neurological and psychiatric disorders is briefly discussed.

An innovative chemical strategy named peptide welding technology (PWT) has been developed for the facile synthesis of tetrabranched peptides. [Dmt¹]N/OFQ(1–13)-NH₂ acts as a universal agonist for nociceptin/orphanin FQ (N/OFQ) and classical opioid receptors. The present study investigated the pharmacological profile of the PWT derivative of [Dmt¹]N/OFQ(1–13)NH₂ (PWT2-[Dmt¹]) in several assays in vitro and in vivo after spinal administration in monkeys subjected to the tail withdrawal assay. PWT2-[Dmt¹] mimicked the effects of [Dmt¹]N/OFQ(1–13)-NH₂ displaying full agonist activity, similar affinity/potency and selectivity at human recombinant N/OFQ (NOP) and opioid receptors in receptor binding, stimulation of [³⁵S]GTPγS binding, calcium mobilization in cells expressing chimeric G proteins, and BRET studies for measuring receptor/G-protein and receptor/β-arrestin 2 interaction. In vivo in monkeys PWT2-[Dmt¹] elicited dose-dependent and robust antinociceptive effects being more potent and longer lasting than [Dmt¹]N/OFQ(1–13)-NH₂. The analgesic action of PWT2-[Dmt¹] was sensitive to the NOP receptor antagonist J-113397, but not naltrexone. Thus, the present study demonstrated that the tetrabranched derivative of [Dmt¹]N/OFQ(1–13)-NH₂ obtained with the PWT technology maintains the in vitro pharmacological profile of the parent peptide but displays higher potency and longer lasting action in vivo.

Endomorphin-1 (EM-1) and endomorphin-2 (EM-2) are endogenous ligands for μ-opioid receptors. Both EM-1 and EM-2, given supraspinally or spinally, produce potent antinociception (analgesia) in mice and rats, measured by the thermal tail-flick response. The antinociception produced by either EM-1 or EM-2 is mediated by the stimulation of μ-opioid receptors, but not by δ- or κ-opioid receptors. EM-1 or EM-2 given supraspinally stimulates primarily μ-opioid receptors and subsequently releases spinipetal noradrenaline and serotonin, acting on α₂-adrenoceptors and serotonin receptors in the spinal cord for producing antinociception. However, the antinociception produced by EM-2, but not by EM-1, also contains an additional component, which is mediated by the release of dynorphin A_1–17 and Met-enkephalin acting on κ-opioid receptors and δ₂-receptors, respectively, in the spinal cord for producing antinociception. Pretreatment with EM-1 or EM-2, given supraspinally or spinally, attenuates the antinociception (antinociceptive tolerance) produced by EM-1 or EM-2, respectively. Pretreatment with EM-2 attenuates the antinociception produced by EM-1; however, pretreatment with EM-1 does not attenuate the antinociception produced by EM-2 (asymmetric cross-tolerance). The antinociception produced by (–)-morphine given into the ventral periaqueductal gray is attenuated by pretreatment with a subanalgesic dose of EM-1 or EM-2 given into the ventral periaqueductal gray in rats (antianalgesia). The antianalgesia produced by EM-2, but not by EM-1, is mediated by the release of dynorphin A_1–17, which antagonizes the analgesic response to (–)-morphine. EM-2, but not EM-1, given into the centromedial amygdala decreases the tail-flick latencies (hyperalgesia) in rats. The hyperalgesia induced by EM-2 from centromedial amygdala is mediated by the release of dynorphin A_1–17 acting on N-methyl-D-aspartate receptors. It is therefore proposed that there are two separate subtypes of μ-opioid receptors: μ and μ′. The μ-opioid receptors are stimulated by both EM-1 and EM-2, (–)-morphine, and [D-Ala²,NMePhe⁴,Gly⁵-ol]enkephalin, and blocked by D-Pro²-endomorphin-1. The μ′-opioid receptors are stimulated by EM-2 but not by EM-1, and blocked by D-Pro²-endomorphin-2, naloxonazine, and 3-methoxynaltrexone. However, both subtypes of μ-opioid receptors are commonly blocked by β-funaltrexamine, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH₂, and (–)-naloxone.