Lamina-specific alteration of C-fibre evoked activity by morphine in the dorsal horn of the rat spinal cord

Morphine modulation of temporomandibular joint-responsive units in superficial laminae at the spinomedullary junction in female rats depends on estrogen status

The influence of analgesic agents on neurons activated by stimulation of the temporomandibular joint (TMJ) region is not well defined. The spinomedullary junction [trigeminal subnucleus caudalis (Vc)/C(1-2)] is a major site of termination for TMJ sensory afferents. To determine whether estrogen status influences opioid-induced modulation of TMJ units, the classical opioid analgesic, morphine, was given to ovariectomized (OvX) rats and OvX rats treated for 2 days with low-dose (LE2) or high-dose (HE2) 17beta-estradiol-3-benzoate. Under thiopental anesthesia, TMJ units in superficial and deep laminae at the Vc/C(1-2) junction were activated by injection of ATP (1 mm) directly into the joint space. In superficial laminae, morphine inhibited evoked activity in units from OvX and LE2 rats in a dose-related and naloxone-reversible manner, whereas units from HE2 rats were not inhibited. By contrast, in deep laminae, morphine reduced TMJ-evoked unit activity similarly in all groups. Morphine reduced the background activity of units in superficial and deep laminae and resting arterial pressure similarly in all groups. Morphine applied to the dorsal surface of the Vc/C(1-2) junction inhibited all units independently of E2 treatment. Quantitative polymerase chain reaction and immunoblots revealed a similar level of expression for mu-opioid receptors at the Vc/C(1-2) junction in LE2 and HE2 rats. These results indicated that estrogen status differentially affected morphine modulation of TMJ unit activity in superficial, but not deep, laminae at the Vc/C(1-2) junction in female rats. The site(s) for estrogen influence on morphine-induced modulation of TMJ unit activity was probably outside the medullary dorsal horn.

Loss of TRPV1-expressing sensory neurons reduces spinal mu opioid receptors but paradoxically potentiates opioid analgesia

Systemic administration of resiniferatoxin (RTX), an ultrapotent capsaicin analogue, removes transient receptor potential vanilloid type 1 (TRPV1)-expressing afferent neurons and impairs thermal but not mechanical nociception in adult animals. In this study, we determined how loss of TRPV1-expressing sensory neurons alters the antinociceptive effect of mu opioids and mu opioid receptors in the spinal cord. The effect of morphine and (D-Ala2,N-Me-Phe4,Gly-ol5)-enkephalin (DAMGO) was measured by testing the paw mechanical withdrawal threshold in rats treated with RTX or vehicle. RTX treatment deleted TRPV1-immunoreactive dorsal root ganglion neurons and nerve terminals in the spinal dorsal horn. Also the mu opioid receptor immunoreactivity was markedly reduced in the superficial dorsal horn of RTX-treated rats. However, RTX treatment did not affect the dorsal horn neurons labeled with both TRPV1- and mu opioid receptor-immunoreactivity. Surprisingly, intrathecal morphine or DAMGO produced a greater increase in the withdrawal threshold in RTX- than in vehicle-treated rats. The duration of the effect of intrathecal morphine and DAMGO in RTX-treated rats was also profoundly increased. Furthermore, the antinociceptive effect of systemic morphine was significantly potentiated in RTX-treated rats. The B(MAX) (but not K(D)) of [3H]-DAMGO binding and DAMGO-stimulated [35S]GTPgammaS activity in the dorsal spinal cord were significantly reduced in the RTX group. This study provides novel information that loss of TRPV1 afferent neurons eliminates presynaptic mu opioid receptors present on TRPV1-expressing afferent neurons but paradoxically potentiates the analgesic effect of mu opioid agonists. Mechano-nociception, transmitted through non-TRPV1 sensory neurons, is subject to potent modulation by mu opioid agonists.

Effect of morphine on deep dorsal horn projection neurons depends on spinal GABAergic and glycinergic tone: implications for reduced opioid effect in neuropathic pain

The mu opioid agonist morphine has distinct effects on spinal dorsal horn neurons in the superficial and deep laminae. However, it is not clear if the inhibitory effect of morphine on dorsal horn projection neurons is secondary to its potentiating effect on inhibitory interneurons. In this study, we tested the hypothesis that removal of GABAergic and glycinergic inhibitory inputs attenuates the effect of morphine on dorsal horn projection neurons and the reduced spinal GABAergic tone contributes to attenuated morphine effect in neuropathic pain. Single-unit activity of deep dorsal horn projection neurons was recorded in anesthetized normal/sham controls and L(5) and L(6) spinal nerve-ligated rats. Spinal application of 10 microM morphine significantly inhibited the evoked responses of dorsal horn neurons in both normal/sham controls, and this effect was abolished by the specific mu opioid antagonist. However, the effect of morphine on dorsal horn projection neurons was significantly reduced in nerve-injured rats. Furthermore, topical application of the GABA(A) receptor antagonist bicuculline (20 microM) almost abolished the effect of morphine in normal/sham control rats but did not significantly attenuate the morphine effect in nerve-injured rats. On the other hand, the glycine receptor antagonist strychnine (4 microM) significantly decreased the effect of morphine in both nerve-injured and control animals. These data suggest that the inhibitory effect of opioids on dorsal horn projection neurons depends on GABAergic and glycinergic inputs. Furthermore, reduced GABAergic tone probably contributes to diminished analgesic effect of opioids in neuropathic pain.

Dose-dependent effects of morphine on experimentally induced cutaneous pain in healthy volunteers

This study examines the dose dependent analgesic effects of two doses of morphine and a single dose of alfentanil on experimentally induced cutaneous pain. In 16 healthy volunteers pain was induced by a skin burn injury and by continuous electrical skin stimulation. Mechanical pain thresholds (PT, von Frey filament), area of secondary hyperalgesia (SH) and 'wind-up like pain' upon repetitive stimulation (40-g load, 3Hz, 30s) were assessed. Analgesic effects on these pain parameters were tested at steady-state IV infusions of morphine, 50% (plasma concentration 15ng/ml) and 100% (plasma concentration 30ng/ml) of maximal tolerable dose to be given to healthy volunteers, and with an effective dose of alfentanil (plasma concentration 70ng/ml). All effects were compared to active placebo, midazolam infusion (20microg/kg for 10min). Alfentanil significantly diminished the SH area in the burn injury model as well as in the electrical pain model. Additionally, alfentanil increased PT several fold in both models. The high dose of morphine showed a similar analgesic response pattern as alfentanil even though the effects were only statistically significant in the electrical pain model. The low dose of morphine as well as placebo did not affect these pain parameters. 'Wind-up like pain' was not influenced by any of the given drugs. In conclusion, the present study clearly indicates dose dependent effects of morphine on experimentally induced cutaneous pain. The high dose of morphine (30ng/ml) was approximately equianalgesic to the administered alfentanil dose (70ng/ml).

Systemic morphine reduces the wind-up of trigeminal nociceptive neurons

We assessed the effects of intravenous morphine on the wind-up of nociceptive neurons of the spinal trigeminal nucleus oralis (Sp5O). Extracellular recordings of Sp5O nociceptive convergent neurons were performed in intact halothane-anesthetized rats. Wind-up of C-fiber-evoked responses was elicited by repetitive electrical stimulation (train of 16 shocks, 0.66 Hz) of their receptive field at C-fiber intensity (3 times the threshold). Wind-up was tested for its sensitivity to morphine (6 mg/kg,i.v.), and the specificity of the effects was verified with naloxone (0.4 mg/kg, i.v.). Nineteen convergent neurons displaying wind-up were recorded. Morphine reduced the wind-up of all but one. In five cases, notwithstanding a reduced wind-up, the neuronal response evoked by the first stimulus in the train (initial input) was unexpectedly increased. Naloxone always antagonized morphine inhibitory effects on the wind-up. When administered systemically, morphine reduced the wind-up of trigeminal nociceptive neurons. This inhibitory effect occurred independently of morphine's ability to affect the initial C-fiber-evoked input. Our findings support the idea that systemic morphine probably blocks wind-up by acting at opioid receptors located postsynaptically to nociceptive primary afferents.

Cornea-responsive medullary dorsal horn neurons: modulation by local opioids and projections to thalamus and brain stem

Previously, it was determined that microinjection of morphine into the caudal portion of subnucleus caudalis mimicked the facilitatory effects of intravenous morphine on cornea-responsive neurons recorded at the subnucleus interpolaris/caudalis (Vi/Vc) transition region. The aim of the present study was to determine the opioid receptor subtype(s) that mediate modulation of corneal units and to determine whether opioid drugs affected unique classes of units. Pulses of CO(2) gas applied to the cornea were used to excite neurons at the Vi/Vc ("rostral" neurons) and the caudalis/upper cervical spinal cord transition region (Vc/C1, "caudal" neurons) in barbiturate-anesthetized male rats. Microinjection of morphine sulfate (2.9-4.8 nmol) or the selective mu receptor agonist D-Ala, N-Me-Phe, Gly-ol-enkephalin (DAMGO; 1.8-15.0 pmol) into the caudal transition region enhanced the response in 7 of 27 (26%) rostral units to CO(2) pulses and depressed that of 10 units (37%). Microinjection of a selective delta ([D-Pen(2,5)] (DPDPE); 24-30 pmol) or kappa receptor agonist (U50488; 1.8-30.0 pmol) into the caudal transition region did not affect the CO(2)-evoked responses of rostral units. Caudal units were inhibited by local DAMGO or DPDPE but were not affected by U50,488H. The effects of DAMGO and DPDPE were reversed by naloxone (0.2 mg/kg iv). Intravenous morphine altered the CO(2)-evoked activity in a direction opposite to that of local DAMGO in 3 of 15 units, in the same direction as local DAMGO but with greater magnitude in 4 units, and in the same direction with equal magnitude as local DAMGO in 8 units. CO(2)-responsive rostral and caudal units projected to either the thalamic posterior nucleus/zona incerta region (PO/ZI) or the superior salivatory/facial nucleus region (SSN/VII). However, rostral units not responsive to CO(2) pulses projected only to SSN/VII and caudal units not responsive to CO(2) projected only to PO/ZI. It was concluded that the circuitry for opioid analgesia in corneal pain involves multiple sites of action: inhibition of neurons at the caudal transition region, by intersubnuclear connections to modulate rostral units, and by supraspinal sites. Local administration of opioid agonists modulated all classes of corneal units. Corneal stimulus modality was predictive of efferent projection status for rostral and caudal units to sensory thalamus and reflex areas of the brain stem.

Superficial dorsal horn neurons identified by intracutaneous histamine: chemonociceptive responses and modulation by morphine

We have investigated whether neurons in superficial laminae of the spinal dorsal horn respond to intracutaneous (ic) delivery of histamine and other irritant chemicals, and thus might be involved in signaling sensations of itch or chemogenic pain. Single-unit recordings were made from superficial lumbar dorsal horn neurons in pentobarbital sodium-anesthetized rats. Chemoresponsive units were identified using ic microinjection of histamine (3%, 1 microl) into the hindpaw as a search stimulus. All superficial units so identified [9 nociceptive-specific (NS), 26 wide-dynamic-range (WDR)] responded to subsequent ic histamine. A comparison group of histamine-responsive deep dorsal horn neurons (n = 16) was similarly identified. The mean histamine-evoked discharge decayed to 50% of the maximal rate significantly more slowly for the superficial (92.2 s +/- 65.5, mean +/- SD) compared with deep dorsal horn neurons (28. 2 s +/- 11.6). In addition to responding to histamine, most superficial dorsal horn neurons were also excited by ic nicotine (22/25 units), capsaicin (21/22), topical mustard oil (5/6), noxious heat (26/30), and noxious and/or innocuous mechanical stimuli (except for 1 unit that did not have a mechanosensitive receptive field). Application of a brief noxious heat stimulus during the response to ic histamine evoked an additive response in all but two cases, followed by transient depression of firing in 11/20 units. Intrathecal (IT) administration of morphine had mixed effects on superficial dorsal horn neuronal responses to ic histamine and noxious heat. Low morphine concentrations (100 nM to 1 microM) facilitated histamine-evoked responses (to >130% of control) in 9/24 units, depressed the responses (by >70%) in 11/24, and had no effect in 4. Naloxone reversed morphine-induced effects in some but not all cases. A higher morphine concentration (10 microM) had a largely depressant, naloxone-reversible effect on histamine responses. Responses of the same superficial neurons to noxious heat were facilitated (15/25), reduced (8/25), or unaffected (2/25) by low morphine concentrations and were depressed by the higher morphine concentration. In contrast, deep dorsal horn neuronal responses to both histamine and noxious heat were primarily depressed by low concentrations of morphine in a naloxone-reversible manner. These results indicate that superficial dorsal horn neurons respond to both pruritic and algesic chemical stimuli and thus might participate in transmitting sensations of itch and/or chemogenic pain. The facilitation of superficial neuronal responses to histamine by low concentrations of morphine, coupled with inhibition of deep dorsal horn neurons, might underlie the development of pruritus that is often observed after epidural morphine.

Spinal laminae I-II neurons in rat recorded in vivo in whole cell, tight seal configuration: properties and opioid responses

Using the in vivo whole cell recording procedure described previously, we recorded 73 neurons in laminae I and II in the lumbar spinal cord of the rat. Input impedances averaged 332 MOmega, which indicated that prior sharp electrode recordings contained a significant current shunt. Characterization of the adequate stimuli from the excitatory hindlimb receptive field indicated that 39 of 73 neurons were nociceptive, 6 were innocuous cooling cells, 20 responded maximally to brush, and 8 cells were not excited by stimulation of the skin of the hindlimb. The locations of 15 neurons were marked with biocytin. Nociceptive neurons were mostly found in lamina I and outer II, cooling cells in lamina I, and innocuous mechanoreceptive cells were mostly found in inner II or in the overlying white matter. The mu-opioid agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-Enkephalin (DAMGO) hyperpolarized 7 of 19 tested neurons with a conductance increase. This hyperpolarization was reversed by naloxone in the neurons in which it was applied. DAMGO also decreased the frequency of spontaneous PSPs in 13 neurons, 7 of which were also hyperpolarized by DAMGO. Five of the seven hyperpolarized neurons were nociceptive, responding to both heat and mechanically noxious stimuli, whereas two responded to slow, innocuous brush. These results indicate that whole cell, tight seal recordings sample a similar population of lamina I and II neurons in the rat as those found with sharp electrode recordings in cat and monkey. They further indicate that DAMGO hyperpolarizes a subset of the nociceptive neurons that have input from both heat and mechanical nociceptors and that presynaptic DAMGO effects can be observed in nociceptive neurons that are not hyperpolarized by DAMGO.

Spinal opioid analgesia: how critical is the regulation of substance P signaling?

Although opioids can reduce stimulus-evoked efflux of Substance P (SP) from nociceptive primary afferents, the consequences of this reduction on spinal cord nociceptive processing has not been studied. Rather than assaying SP release, in the present study we examined the effect of opioids on two postsynaptic measures of SP release, Fos expression and neurokinin-1 (NK-1) receptor internalization, in the rat. The functional significance of the latter was first established in in vitro studies that showed that SP-induced Ca(2+) mobilization is highly correlated with the magnitude of SP-induced NK-1 receptor internalization in dorsal horn neurons. Using an in vivo analysis, we found that morphine had little effect on noxious stimulus-evoked internalization of the NK-1 receptor in lamina I neurons. However, internalization was reduced when we coadministered morphine with a dose of an NK-1 receptor antagonist that by itself was without effect. Thus, although opioids may modulate SP release, the residual release is sufficient to exert maximal effects on the target NK-1 receptors. Morphine significantly reduced noxious stimulus-induced Fos expression in lamina I, but the Fos inhibition was less pronounced in neurons that expressed the NK-1 receptor. Taken together, these results suggest that opioid analgesia predominantly involves postsynaptic inhibitory mechanisms and/or presynaptic control of non-SP-containing primary afferent nociceptors.

Differential effects of morphine on corneal-responsive neurons in rostral versus caudal regions of spinal trigeminal nucleus in the rat

The initial processing of corneal sensory input in the rat occurs in two distinct regions of the spinal trigeminal nucleus, at the subnucleus interpolaris/caudalis transition (Vi/Vc) and in laminae I-II at the subnucleus caudalis/spinal cord transition (Vc/C1). Extracellular recording was used to compare the effects of morphine on the evoked activity of corneal-responsive neurons located in these two regions. Neurons also were characterized by cutaneous receptive field properties and parabrachial area (PBA) projection status. Electrical corneal stimulation-evoked activity of most (10/13) neurons at the Vi/Vc transition region was increased [146 +/- 16% (mean +/- SE) of control, P < 0.025] after systemic morphine and reduced after naloxone. None of the Vi/Vc corneal units were inhibited by morphine. By contrast, all corneal neurons recorded at the Vc/C1 transition region displayed a naloxone-reversible decrease (55 +/- 10% of control, P < 0.001) in evoked activity after morphine. None of 13 Vi/Vc corneal units and 7 of 8 Vc/C1 corneal units tested projected to the PBA. To determine if the Vc/C1 transition acted as a relay for the effect of intravenous morphine on corneal stimulation-evoked activity of Vi/Vc units, morphine was applied topically to the dorsal brain stem surface overlying the Vc/C1 transition. Local microinjection of morphine at the Vc/C1 transition increased the evoked activity of 4 Vi/Vc neurons, inhibited that of 2 neurons, and did not affect the remaining 12 corneal neurons tested. In conclusion, the distinctive effects of morphine on Vi/Vc and Vc/C1 neurons support the hypothesis that these two neuronal groups contribute to different aspects of corneal sensory processing such as pain sensation, autonomic reflex responses, and recruitment of descending controls.

Effects of systemic morphine on the activity of convergent neurons of spinal trigeminal nucleus oralis in the rat

The spinal trigeminal nucleus oralis has been shown to relay nociceptive inputs mainly from the oral and perioral regions. In this study, we examined the effects of intravenous administration of morphine on C-fiber-evoked activities of spinal trigeminal nucleus oralis convergent neurons in halothane-anesthetized rats. Morphine depressed the C-fiber-evoked responses of spinal trigeminal nucleus oralis convergent neurons in a dose-related (3-12 mg/kg range) and naloxone-reversible fashion. The ED50 was 6.1 mg/kg, a dose similar to that found in the spinal horn. The observed strong depressive action of morphine on noxious-evoked activities of spinal trigeminal nucleus oralis neurons is consistent with our previous statement, based on electrophysiological studies, that this region plays an important role in the transmission of trigeminal nociceptive information. The effect of morphine on the spinal trigeminal nucleus oralis neurons is discussed in relation to its possible site and mechanism of action.

Morphine selectively depresses the slowest, NMDA-independent component of C-fibre-evoked synaptic activity in the rat spinal cord in vitro

The effects of morphine on the depolarizing synaptic responses produced in motoneurons by electrical stimulation of primary sensory neurones have been recorded in hemisected spinal cord preparations (8- to 12-day-old rat pups). Morphine at concentrations of 0.1-20 microM reduced a slow, long-lasting (latency greater than 1 s, duration up to 10 s) component of the ventral root potential (VRP) evoked by C-fibre strength stimulation of the dorsal root. At 2 microM the reduction in area of this slow synaptic potential was 71.7 +/- 0.9% of control values (n = 15). The earliest components of the C-fibre strength VRP (the first 100 ms) and the responses to A beta strength stimuli were unaffected by the opioid even at 10-20 microM. The intermediate, NMDA receptor antagonist (D-AP5, 40 microM)-sensitive component (which lasts 100-1000 ms) was reduced by 34 +/- 2.2% of control (n = 15), which was significantly less than the reduction of the later NMDA-independent component (P < 0.001). Morphine (0.1-20 microM) also depressed the cumulative depolarization generated by the temporal summation of synaptic responses evoked by brief trains of C-fibre strength stimuli (1 or 10 Hz). A significantly greater reduction at the lower frequency of stimulation (56.3 +/- 2.0%) than at the higher (20.3 +/- 1.69%, n = 10, measured at 2 microM morphine) was found (P < 0.005). The effects of morphine were reversible upon wash-out or superfusion with the opioid receptor antagonist naloxone (2 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of systemic morphine on lamina I spinothalamic tract neurons in the cat

Lamina I spinothalamic tract (STT) neurons are an integral component of the central representation of pain and temperature and thus their sensitivity to various analgesics needs to be examined. In the present study, the effects of successive, cumulative doses (0.125-2.0 mg/kg) of intravenous morphine sulfate on the quantitative stimulus-response properties of nociceptive lamina I STT cells have been tested in the intact, barbiturate-anesthetized cat. Both nociceptive-specific (n = 7) and multireceptive (heat, pinch and cold sensitive; n = 7) lamina I STT cells were inhibited in a dose-dependent manner. Parallel dose-dependent effects on responses to noxious heat and pinch were generally observed that reduced ongoing discharge levels and the slopes of the stimulus-response functions. However, non-STT lamina I cells (n = 5) differed significantly; the responses of one multireceptive (heat, pinch and cold-sensitive) cell and the responses to pinch of 3 of 4 wide dynamic range cells were not inhibited. In addition, two-thirds of the nociceptive lamina I STT cells showed enhanced responses at the lowest dose of morphine (0.125 mg/kg). These results contrast with the varied effects of morphine reported for superficial dorsal horn cells with uncharacterized projections and they support the role of lamina I STT cells in pain. Furthermore, these observations are consistent with previous findings indicating that lamina I STT neurons are a distinct subpopulation of lamina I cells. These results support previous evidence that opiatergic modulation of sensory activity in lamina I is functionally organized.

Morphine differentially modulates nociceptive input in the superficial versus the deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in the rat

Morphine enhanced the noxious thermal stimulus-evoked responses in 4/13 (31%) selectively nocireceptive neurons in the superficial dorsal horn, inhibited the responses in 4/13 (31%) neurons and produced a biphasic effect in 2/13 (17%) neurons. Naloxone antagonized these effects in 7/7 neurons. In contrast, morphine produced a naloxone reversible reduction in the nociceptive responses of 4/4 (100%) multireceptive neurons in the deeper dorsal horn of the medulla. The data are interpreted to indicate that opiates may differentially modulate nociceptive input in the superficial versus the deeper dorsal horn.

Morphine enhances the activity of thermoreceptive cold-specific lamina I spinothalamic neurons in the cat

The possibility that morphine might differentially affect spinal neurons responsive to small-diameter thermoreceptive-specific afferents was tested. Systemic morphine enhanced a portion or all of the stimulus-response function of 7 of 9 lamina I spinothalamic tract cells specifically sensitive to cold applied to the glabrous hindpaw in the cat. This result contrasts strongly with the predominant inhibition of nociceptive lamina I neurons by morphine.

Delta-opioid mediated inhibitions of acute and prolonged noxious-evoked responses in rat dorsal horn neurones

1. The effects of a selective delta-opioid agonist Tyr-D-Ser(Otbu)-Gly-Phe-Leu-Thr (DSTBULET) were examined on the C- and A beta-evoked responses of convergent dorsal horn neurones in the halothane anaesthetized, intact rat. 2. Intrathecal DSTBULET produced selective dose-dependent inhibitions of electrically-evoked C fibre responses of both superficial and deep neurones. A near-complete inhibition of 83 +/- 5% followed 100 micrograms of DSTBULET and the ED50 was 9 micrograms (13.5 nmol). Inhibitions were antagonised by intrathecal naloxone and ICI 174,864 but were not antagonised by pretreatment with intrathecal beta-funaltrexamine at a dose that blocked mu-opioid effects. By contrast, DSTBULET produced excitations of electrically-evoked responses of cells recorded in a zone intermediate between the superficial and deep neurones. 3. DSTBULET (50 micrograms) was also tested on the more prolonged noxious neuronal response produced by subcutaneous formalin (5%, 50 microliters) into the receptive field. DSTBULET profoundly inhibited the response to formalin. Pretreatment with ICI 174,864 before DSTBULET antagonised the effects of the delta-agonist on the formalin response. 4. The full peptidase inhibitor kelatorphan, known to protect endogenous enkephalins, was also tested on the formalin response. The intrathecal administration of 50 micrograms kelatorphan has previously been shown to inhibit electrically-evoked C fibre resonses of dorsal horn neurones and to be antagonised by ICI 174,864. The same dose of kelatorphan inhibited the formalin response in the present study. 5. From this study it appears that the delta-opioid agonist DSTBULET can produce profound inhibitions of the responses of convergent neurones to nociceptive afferent inputs. Furthermore, activation of delta-opioid receptors either by DSTBULET, or by protection of endogenous enkephalins with kelatorphan, can inhibit a more prolonged chemically-evoked nociceptive input onto these dorsal horn neurones.

The spinal antinociceptive actions of morphine metabolites morphine-6-glucuronide and normorphine in the rat

The profound and prolonged effects of morphine in patients with renal dysfunction have been associated with high plasma levels of the opiate metabolites morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G) rather than an increased concentration of morphine. We present here electrophysiological evidence to suggest that potent spinal antinociception can be produced by both M6G and normorphine, another metabolite of morphine. Extracellular recordings of A beta- and C-fibre-evoked responses of convergent dorsal horn neurones were made in the halothane anaesthetised rat. M6G elicited dose-dependent, naloxone-reversible inhibitions of C-fibre-evoked responses which were completely suppressed (8% of control) by 2 micrograms M6G whereas A beta-fibre-evoked responses were only reduced to 57% of controls. The ED50 for the effects of M6G on C-fibre-evoked activity was calculated to be 0.53 micrograms. Systemic administration of M6G (2 mg/kg) also profoundly reduced noxious evoked neuronal activity. Intrathecal normorphine was less potent than M6G but complete selective inhibitions of C-fibre-evoked response could be elicited by 25 micrograms and the ED50 was calculated to be 2.68 micrograms. No such inhibitions were observed following administration of M3G. A comparison with intrathecal morphine in the same preparation reveals that normorphine is equipotent with morphine whereas M6G is 13-fold more potent. These results therefore confirm that M6G and normorphine might be significant contributers to opiate analgesia after administration of morphine.

Intrathecal cholecystokinin interacts with morphine but not substance P in modulating the nociceptive flexion reflex in the rat

The effect of intrathecal (IT) cholecystokinin (CCK), substance P (SP) and morphine (MO) on spinal cord excitability was studied in decerebrate, spinalized rats. CCK had a weaker facilitatory effect on the nociceptive flexion reflex than SP. The possible functional significance of the coexistence of CCK and SP in neurons projecting to the spinal cord was tested by coadministration of the two peptides. At the doses tested no synergistic interaction on the reflex was found with CCK and SP. IT MO caused a brief enhancement followed by a prolonged depression of the reflex. A high dose of CCK injected prior to MO increased the facilitatory effect and decreased the depressive effect of the opiate on the reflex. The effect of desulfated (D) CCK was similar to CCK but at a higher dose. Naloxone (NAL) had a similar effect as CCK when administered prior to MO. The MO-induced depression of the reflex was readily reversed by NAL, but not by CCK. The results indicate that CCK may prevent the inhibitory effect of MO on spinal cord excitability to nociceptive stimulation, but does not reverse it. CCK may alter the balance of excitation-inhibition between various types of dorsal horn interneurons that are involved in the transmission of nociceptive information.

Electrophysiological studies on the effects of intrathecal morphine on nociceptive neurones in the rat dorsal horn

We have studied the effects of intrathecal morphine on the responses of 38 dorsal horn neurones in the intact rat under halothane anaesthesia to A and C fibre electrical stimulation and to natural stimuli applied to their receptive fields. Morphine selectively reduced the C fibre and pinch evoked activity in a dose-dependent naloxone-reversible manner with an ED50 of 7 nmoles. The 'wind-up' of neurones to repetitive stimulation was little altered except with the highest doses (50-150 nmoles) tested. By contrast, the A fibre evoked responses of the neurones were only slightly reduced by morphine and both the tactile responses and receptive field size to innocuous stimuli enhanced for certain cells. The results are discussed in relation to the spinal actions of opiates and their clinical applications.

The properties of neurones recorded in the superficial dorsal horn of the rat spinal cord

The physiological properties of neurones in the superficial laminae of the dorsal horn of the fourth and fifth lumbar segments of the rat spinal cord have been investigated in decerebrate spinal animals. Both extracellular recordings with platinum-plated tungsten microelectrodes (n = 72) and intracellular recordings with glass microelectrodes (N = 79) were made. Attempts were made to fill cells intracellularly with horseradish peroxidase or Lucifer Yellow. Thirty-seven percent of the intracellularly injected neurones were recovered after histological processing and their cell bodies found to be in lamina 1 or 2 and in the dorsal white matter overlying lamina 1. The dendritic spread of the stained neurones was maximal in the rostrocaudal plane with a restricted mediolateral spread. The physiological properties of the extracellularly recorded units, the intracellularly unidentified units, and the intracellularly stained units were the same. The neurones were characterized by low background activity and all had excitatory receptive fields on the lower limb. Some neurones responded only to low-threshold mechanical stimulation of the skin or only to noxious skin stimulation but the majority of units (58%) were wide-dynamic-range cells responding to both types of stimuli. Receptive field classification was made questionable, however, by the existence of cells (9%) that exhibited a spontaneous shift in the size of their receptive fields and in the type of stimulus that elicited a response. The neurones in the superficial dorsal horn commonly showed a marked inhibition to repeated cutaneous stimuli (27%) or a prolonged afterdischarge followed a single stimulus (20%). Afferent input from the sural nerve was found to be from A and C fibres in both extra-and intracellular recordings. A delta- and C-mediated excitations were most common although convergent inputs from A beta-fibres occurred in 40% of units. No correlation was found between cell structure or distribution of dendritic fields and physiological properties in our small sample of intracellularly stained cells. The morphology of the cells was highly diverse, as were the different receptive fields. There was, however, some correlation between the location of cell bodies and their responses. Neurones responding only to low-threshold stimuli were distributed either in the dorsal white matter or in inner lamina 2. Wide-dynamic-range cells were distributed throughout the superficial dorsal horn. These results suggest that neurones of different shapes and positions may subserve the same function and, conversely, that neurones of the same shape and position may subserve different functions.

Do convergent neurones in the spinal dorsal horn discriminate nociceptive from non-nociceptive information?

Thirty convergent neurones responding to both noxious and non-noxious cutaneous stimuli were recorded at the lumbar level in either anaesthetized 'intact' rats or unanaesthetized 'spinal' rats. Their responses to radiant heat application and to repetitive innocuous mechanical stimulation of the centre of their receptive fields were analysed, both in terms of the maximal and the mean firing rates. These neurones increased their discharge rates in relation to the temperature applied to their receptive fields with the highest levels being produced by noxious intensities. This finding confirms earlier reports suggesting the capacity of these neurones to encode nociceptive information of thermal origin. However, a very high level of firing could also be evoked by repetitively applied innocuous mechanical stimuli. This was a consistent finding, observed both in intact and spinal animals, which was true for the two subgroups into which we divided the convergent neurones (warming/noxious heat units and noxious heat units). These results are discussed in terms of the role of convergent neurones in nociception. It is suggested that a single channelled signal emanating from these neurones could not be the basis of a clear nociceptive message to the brain; two alternative hypotheses involving multichannelled organizations of impulses are proposed for discussion.