Activation of μ-Opioid Receptors Inhibits Synaptic Inputs to Spinally Projecting Rostral Ventromedial Medulla Neurons

Abstract

The rostral ventromedial medulla (RVM) is a major locus for the descending control of nociception and opioid analgesia. However, it is not clear how opioids affect synaptic inputs to RVM neurons. In this study, we determined the effect of -opioid receptor activation on excitatory and inhibitory synaptic transmission in spinally projecting RVM neurons. RVM neurons were retrogradely labeled with a fluorescent tracer injected into the dorsal horn of the spinal cord in rats. Whole-cell voltage-clamp recordings were performed on labeled RVM neurons in brain slices in vitro. The -receptor agonist [D-Ala 2 ,N-Me-Phe 4 ,Gly 5 -ol]-enkephalin (DAMGO, 1 ⌴) significantly decreased the amplitude of evoked excitatory postsynaptic currents (EPSCs) in 52% (9 of 17) of labeled cells. DAMGO also significantly reduced the amplitude of evoked inhibitory postsynaptic currents (IPSCs) in 69% (11 of 16) of cells examined. Furthermore, DAMGO significantly decreased the frequency of miniature EPSCs in 55% (15 of 27) of cells and significantly decreased the frequency of miniature IPSCs in all 12 cells studied. Although most EPSCs and IPSCs were mediated by glutamate and GABA, the nicotinic and glycine receptor antagonists attenuated EPSCs and IPSCs, respectively, in some labeled RVM neurons. Immunocytochemical labeling revealed that only 35% of recorded RVM neurons were tryptophan hydroxylase-positive, and 15% cells had GABA immunoreactivity. Thus, this study provides important functional evidence that activation of -opioid receptors decreases the release of both excitatory and inhibitory neurotransmitters onto most spinally projecting RVM neurons.The rostral ventromedial medulla (RVM) plays an important role in the descending modulation of nociceptive information and opioid analgesia (Fields et al., 1983a;Jones and Gebhart, 1988;Urban and Smith, 1994). The RVM receives synaptic inputs from the periaqueductal gray (PAG) and relays information to the dorsal horn of the spinal cord through the dorsolateral funiculus. The analgesia produced by microinjection of morphine into the PAG is dependent upon the connection to the RVM (Urban and Smith, 1994;Pan and Fields, 1996). Direct stimulation of the RVM also can produce potent analgesia. For example, medullary electrical stimulation or microinjection of glutamate and morphine into the RVM results in either an increase in the tail-flick latency or a decrease in spinal dorsal horn neuronal activity (Llewelyn et al., 1986;Jones and Gebhart, 1988;McGowan and Hammond, 1993;Urban and Smith, 1994). However, the cellular mechanisms of the action of opioids in the RVM remain unclear.Three classes of RVM neurons have been identified, including ON-, OFF-, and NEUTRAL-cells, that show, respectively, increases, decreases, and no changes in the firing activity in response to noxious stimuli (Fields et al., 1983b(Fields et al., , 1995. Furthermore, activation of -opioid receptors inhibits ON-cells, but stimulates OFF-cells during the application of noxious stimulation (Fields et al., 1983b). Althoug...

Mechanisms Responsible for the Enhanced Antinociceptive Effects of μ-Opioid Receptor Agonists in the Rostral Ventromedial Medulla of Male Rats with Persistent Inflammatory Pain

“…Electrophysiological studies indicate that MOR agonists inhibit synaptic transmission in the RVM by both postsynaptic and presynaptic mechanisms (Pan et al, 1990;Marinelli et al, 2002;Finnegan et al, 2004). In contrast, DOR-like immunoreactivity is predominantly located on presynaptic elements in the RVM (Kalyuzhny et al, 1996;Ma et al, 2006), although the presence of postsynaptic DOR is suggested by the ability of DELT to hyperpolarize a subpopulation of RVM neurons (Marinelli et al, 2005).…”

Effect of the μ Opioid on Excitatory and Inhibitory Synaptic Inputs to Periaqueductal Gray-Projecting Neurons in the Amygdala

“…The G-protein inhibitor, GDP-␤-S (1 mM), was added to the internal solution in the evoked protocol to eliminate the potential postsynaptic effect of opioids (Pan et al, 2002. Also, the internal pipette solution contained 0.2% biocytin (Sigma-Aldrich, St. Louis, MO) to label the recorded neuron for later use in the immunofluorescence labeling (Finnegan et al, 2004). The brain slice was placed in a glass-bottomed chamber (Warner Instrument, Hamden, CT) and fixed with a grid or parallel nylon threads supported by a U-shaped stainless steel weight.…”

“…Miniature inhibitory postsynaptic currents (mIPSCs) and miniature excitatory postsynaptic currents (mEPSCs) were recorded at a holding potential of 0 and Ϫ70 mV, respectively (Pan et al, 2002;Finnegan et al, 2004). All mIPSCs were recorded in the presence of tetrodotoxin (TTX; 1 M) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 M).…”

“…Recordings of miniature and evoked excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) were performed using whole-cell voltage-clamp methods as previously described (Pan et al, 2002Finnegan et al, 2004). The electrode for the whole-cell recordings was pulled with a puller (P-97; Sutter Instrument Company, Novato, CA) using borosilicate glass capillaries (o.d., 1.2 mm; i.d., 0.86 mm; World Precision Instruments, Inc., Sarasota, FL).…”

Modulation of pain transmission by G-protein-coupled receptors

“…For example, activation of presynaptic μ-opioid receptors primarily attenuates GABAergic synaptic input in the amygdala (Finnegan et al, 2005;Finnegan et al, 2006). Opioids also reduce synaptic GABA release to spinally projecting neurons in the rostral ventromedial medulla (Finnegan et al, 2004) and periaqueductal gray (Vaughan et al, 1997). Furthermore, through presynaptic inhibition of GABA release, activation of δ-opioid receptors may disinhibit spinally projecting noradrenergic neurons in the locus coeruleus (Pan et al, 2002a).…”