Coexpression of the mu-opioid receptor splice variant MOR1C and the vesicular glutamate transporter 2 (VGLUT2) in rat central nervous system

Automated Classification of Whole Body Plethysmography Waveforms to Quantify Breathing Patterns

Whole body plethysmography (WBP) monitors respiratory rate and depth but conventional analysis fails to capture the diversity of waveforms. Our first purpose was to develop a waveform cluster analysis method for quantifying dynamic changes in respiratory waveforms. WBP data, from adult Sprague-Dawley rats, were sorted into time domains and principle component analysis was used for hierarchical clustering. The clustering method effectively sorted waveforms into categories including sniffing, tidal breaths of varying duration, and augmented breaths (sighs). We next used this clustering method to quantify breathing after opioid (fentanyl) overdose and treatment with ampakine CX1942, an allosteric modulator of AMPA receptors. Fentanyl caused the expected decrease in breathing, but our cluster analysis revealed changes in the temporal appearance of inspiratory efforts. Ampakine CX1942 treatment shifted respiratory waveforms toward baseline values. We conclude that this method allows for rapid assessment of breathing patterns across extended data recordings. Expanding analyses to include larger portions of recorded WBP data may provide insight on how breathing is affected by disease or therapy.

Inhibition of glutamatergic transmission and neuronal excitability by oxycodone in the rat hippocampal CA3 neurons

Oxycodone, a semisynthetic opioid analgesic with actions similar to morphine, is extensively prescribed for treatment of moderate to severe acute pain. Given that glutamate plays a crucial role in mediating pain transmission, the purpose of this study was to investigate the effect of oxycodone on glutamatergic synaptic transmission in rat hippocampal CA3 area, which is associated with the modulation of nociceptive perception. Whole-cell patch-clamp recordings revealed that oxycodone effectively reduced presynaptic glutamate release, as detected by decreased frequencies of spontaneous excitatory postsynaptic currents (sEPSCs) and miniature EPSCs (mEPSCs), without eliciting significant changes in the amplitudes of sEPSCs and mEPSCs and glutamate-evoked inward currents. The inhibitory effect of oxycodone on the frequency of sEPSCs was blocked by the nonselective opioid receptor antagonist naloxone. In addition, oxycodone suppressed burst firing induced by 4-aminopyridine and tonic repetitive firing evoked by the applied depolarizing current. These results suggest that oxycodone inhibits spontaneous presynaptic glutamate release possibly by activating opioid receptors and consequently suppressing the neuronal excitability of hippocampal CA3 neurons.

Hippocampal µ-opioid receptors on GABAergic neurons mediate stress-induced impairment of memory retrieval

Stressful life events induce abnormalities in emotional and cognitive behaviour. The endogenous opioid system plays an essential role in stress adaptation and coping strategies. In particular, the µ-opioid receptor (μR), one of the major opioid receptors, strongly influences memory processing in that alterations in μR signalling are associated with various neuropsychiatric disorders. However, it remains unclear whether μR signalling contributes to memory impairments induced by acute stress. Here, we utilized pharmacological methods and cell-type-selective/non-cell-type-selective μR depletion approaches combined with behavioural tests, biochemical analyses, and in vitro electrophysiological recordings to investigate the role of hippocampal μR signalling in memory-retrieval impairment induced by acute elevated platform (EP) stress in mice. Biochemical and molecular analyses revealed that hippocampal μRs were significantly activated during acute stress. Blockage of hippocampal μRs, non-selective deletion of μRs or selective deletion of μRs on GABAergic neurons (μR_GABA) reversed EP-stress-induced impairment of memory retrieval, with no effect on the elevation of serum corticosterone after stress. Electrophysiological results demonstrated that stress depressed hippocampal GABAergic synaptic transmission to CA1 pyramidal neurons, thereby leading to excitation/inhibition (E/I) imbalance in a μR_GABA-dependent manner. Pharmaceutically enhancing hippocampal GABA_A receptor-mediated inhibitory currents in stressed mice restored their memory retrieval, whereas inhibiting those currents in the unstressed mice mimicked the stress-induced impairment of memory retrieval. Our findings reveal a novel pathway in which endogenous opioids recruited by acute stress predominantly activate μR_GABA to depress GABAergic inhibitory effects on CA1 pyramidal neurons, which subsequently alters the E/I balance in the hippocampus and results in impairment of memory retrieval.

Differential content of vesicular glutamate transporters in subsets of vagal afferents projecting to the nucleus tractus solitarii in the rat

The vagus nerve contains primary visceral afferents that convey sensory information from cardiovascular, pulmonary, and gastrointestinal tissues to the nucleus tractus solitarii (NTS). The heterogeneity of vagal afferents and their central terminals within the NTS is a common obstacle for evaluating functional groups of afferents. To determine whether different anterograde tracers can be used to identify distinct subpopulations of vagal afferents within NTS, we injected cholera toxin B subunit (CTb) and isolectin B4 (IB4) into the vagus nerve. Confocal analyses of medial NTS following injections of both CTb and IB4 into the same vagus nerve resulted in labeling of two exclusive populations of fibers. The ultrastructural patterns were also distinct. CTb was found in both myelinated and unmyelinated vagal axons and terminals in medial NTS, whereas IB4 was found only in unmyelinated afferents. Both tracers were observed in terminals with asymmetric synapses, suggesting excitatory transmission. Because glutamate is thought to be the neurotransmitter at this first primary afferent synapse in NTS, we determined whether vesicular glutamate transporters (VGLUTs) were differentially distributed among the two distinct populations of vagal afferents. Anterograde tracing from the vagus with CTb or IB4 was combined with immunohistochemistry for VGLUT1 or VGLUT2 in medial NTS and evaluated with confocal microscopy. CTb-labeled afferents contained primarily VGLUT2 (83%), whereas IB4-labeled afferents had low levels of vesicular transporters, VGLUT1 (5%) or VGLUT2 (21%). These findings suggest the possibility that glutamate release from unmyelinated vagal afferents may be regulated by a distinct, non-VGLUT, mechanism.

Effects of μ-opioid receptor modulation on the hippocampal network activity of sharp wave and ripples

BACKGROUND AND PURPOSE

Hippocampus-dependent memory involves the activity of sharp wave ripples (SWRs), which are thought to participate in the process of memory consolidation. The hippocampus contains high levels of endogenous opioids and of μ-opioid receptors (MORs). Here, we have assessed the role of MOR agonists in the modulation of SWRs.

EXPERIMENTAL APPROACH

Using recordings of extracellular potentials from the CA1 field of rat hippocampal slices, we examined the pharmacological actions of morphine, DAMGO and fentanyl on SWRs and on network excitability and paired-pulse inhibition.

KEY RESULTS

All three MOR agonists (1 nM-10 μM) significantly increased the amplitude of sharp waves and the occurrence of SWR sequences, but reduced the initiation of episodes of SWRs. Fentanyl was most potent in producing these effects and morphine the least. Interestingly, although SWRs were reduced by relatively high concentrations (≥100 nM) of all agonists, they were significantly enhanced by very low concentrations of morphine (5-10 nM). Morphine and DAMGO at moderate-to-high concentrations increased network excitability and reduced inhibition. Furthermore, DAMGO suppressed inhibition more readily than it increased excitation, whereas morphine suppressed inhibition only at high concentrations. These drug effects were reversed by the MOR antagonists naloxone and CTOP.

CONCLUSIONS AND IMPLICATIONS

We found that the SWRs were significantly modulated by three MOR agonists and that the SWRs were very sensitive to subtle changes in the excitation/inhibition balance induced by MOR agonists. Such modulation might underlie the effects of these agonists on hippocampus-dependent memory.

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.

Evidence for a role of glutamate as an efferent transmitter in taste buds

Background

Glutamate has been proposed as a transmitter in the peripheral taste system in addition to its well-documented role as an umami taste stimulus. Evidence for a role as a transmitter includes the presence of ionotropic glutamate receptors in nerve fibers and taste cells, as well as the expression of the glutamate transporter GLAST in Type I taste cells. However, the source and targets of glutamate in lingual tissue are unclear. In the present study, we used molecular, physiological and immunohistochemical methods to investigate the origin of glutamate as well as the targeted receptors in taste buds.

Results

Using molecular and immunohistochemical techniques, we show that the vesicular transporters for glutamate, VGLUT 1 and 2, but not VGLUT3, are expressed in the nerve fibers surrounding taste buds but likely not in taste cells themselves. Further, we show that P2X2, a specific marker for gustatory but not trigeminal fibers, co-localizes with VGLUT2, suggesting the VGLUT-expressing nerve fibers are of gustatory origin. Calcium imaging indicates that GAD67-GFP Type III taste cells, but not T1R3-GFP Type II cells, respond to glutamate at concentrations expected for a glutamate transmitter, and further, that these responses are partially blocked by NBQX, a specific AMPA/Kainate receptor antagonist. RT-PCR and immunohistochemistry confirm the presence of the Kainate receptor GluR7 in Type III taste cells, suggesting it may be a target of glutamate released from gustatory nerve fibers.

Conclusions

Taken together, the results suggest that glutamate may be released from gustatory nerve fibers using a vesicular mechanism to modulate Type III taste cells via GluR7.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 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 and thermoregulation (Section 16); and immunological responses (Section 17).