Differential depressive action of two mu and delta opioid ligands on neuronal responses to noxious stimuli in the thalamic ventrobasal complex of rat

Robust age, but limited sex, differences in mu-opioid receptors in the rat brain: relevance for reward and drug-seeking behaviors in juveniles

In the brain, the µ-opioid receptor (MOR) is involved in reward-seeking behaviors and plays a pivotal role in the mediation of opioid use disorders. Furthermore, reward-seeking behaviors and susceptibility to opioid addiction are particularly evident during the juvenile period, with a higher incidence of opioid use in males and higher sensitivity to opioids in females. Despite these age and sex differences in MOR-mediated behaviors, little is known regarding potential age and sex differences in the expression of MORs in the brain. Here, we used receptor autoradiography to compare MOR binding densities between juvenile and adult male and female rats. Age differences were found in MOR binding density in 12 out of 33 brain regions analyzed, with 11 regions showing higher MOR binding density in juveniles than in adults. These include the lateral septum, as well as sub-regions of the bed nucleus of the stria terminalis, hippocampus, and thalamus. Sex differences in MOR binding density were observed in only two brain regions, namely, the lateral septum (higher in males) and the posterior cortical nucleus of the amygdala (higher in females). Overall, these findings provide an important foundation for the generation of hypotheses regarding differential functional roles of MOR activation in juveniles versus adults. Specifically, we discuss the possibility that higher MOR binding densities in juveniles may allow for higher MOR activation, which could facilitate behaviors that are heightened during the juvenile period, such as reward and drug-seeking behaviors.

RFamide-related peptides signal through the neuropeptide FF receptor and regulate pain-related responses in the rat

The mammalian RFamide-related peptide RFRP1 was found to signal through the neuropeptide FF 2 receptor expressed in Xenopus oocytes. The peptide induced a dose-dependent outward current, which was dependent on the simultaneous expression of GIRK1 and GIRK4 potassium channels. In neuropathic rats, RFRP1 administered intrathecally induced tactile antiallodynia and thermal antinociception, whereas in the solitary tract nucleus it produced only mechanical antihyperalgesia. Expression of the RFamide-related peptide mRNA in the rat CNS was distinctly different from that of neuropeptide FF. Most notably, the gene was not expressed in the hindbrain or spinal cord at detectable levels. However, there was a prominent group of RFamide-related peptide mRNA-expressing neurons in the central hypothalamus, in the area in and between the dorsomedial and ventromedial nuclei. The results suggest that RFamide-related peptides are potentially involved in pain regulation through a hypothalamo-medullary projection system, and possibly via action on neuropeptide FF 2 receptors. In neuropathic animals, the pain suppressive effect of RFamide-related peptide varies depending on the submodality of noxious test stimulation and the site of RFamide-related peptide administration.

Antioscillatory effects of nociceptin/orphanin FQ in synaptic networks of the rat thalamus

Postsynaptic and presynaptic effects of nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the opioid-like orphan receptor, were investigated in an in vitro slice preparation of the rat thalamic reticular nucleus (NRT) and ventrobasal complex (VB). In NRT as well as VB, all tested neurons developed an outward current on application of 1 micrometer N/OFQ. Basic properties of the N/OFQ-induced current included inward rectification, dependence on extracellular K(+), reduction by 100 micrometer Ba(+), antagonistic effect of [Nphe(1)]nociceptin(1-13)NH(2), and sensitivity to internal GDP-beta-S. Miniature IPSCs (mIPSCs) mediated by GABA(A) receptors in VB neurons were not affected by 1 micrometer N/OFQ. In addition, paired-pulse depression of evoked IPSCs was unchanged, indicating a lack of presynaptic effects. By comparison, N/OFQ application resulted in a reduction in frequency of miniature EPSCs (mEPSCs) in a subpopulation of NRT neurons, whereas paired-pulse facilitation of evoked EPSCs was not altered. In either nucleus, current-clamp experiments revealed a hyperpolarization and associated decrease in input resistance in response to N/OFQ. Although N/OFQ had no measurable effect on calcium-mediated burst activity evoked by depolarizing steps from hyperpolarized values of the membrane potential, rebound bursts on relief of hyperpolarizing current steps were decreased. Slow thalamic oscillations induced in vitro by extracellular stimulation were dampened by N/OFQ in VB and NRT, as seen by delayed onset of rhythmic multiple-unit activity and reduction in amplitude and duration. We conclude that N/OFQ reduces the excitability of NRT and VB neurons predominantly through an increase of a G-protein-coupled inwardly rectifying K(+) conductance.

mu-Opioid peptides inhibit thalamic neurons

Opioidergic inhibition of neurons in the centrolateral nucleus of the thalamus was investigated using an in vitro thalamic slice preparation from young rats. The mu-opioid receptor agonist D-Ala2,N-Me-Phe4,glycinol5-enkephalin (DAMGO) evoked a hyperpolarization and decrease in input resistance that was reversible, concentration-dependent, and persisted in the presence of tetrodotoxin. Application of the specific mu-receptor antagonist Cys2,Tyr3,Orn5,Pen7-amide blocked this response. The respective delta- and kappa-opioid receptor agonists, (D-Pen2, D-Pen5)-enkephalin and (+/-)-trans-U-50488 methanesulfonate had no effect. Voltage-clamp experiments showed that DAMGO activated an inwardly rectifying potassium conductance (GKIR) characterized by rectification at hyperpolarized potentials that increased in elevated extracellular potassium concentrations, a complete block by Ba2+ (1 mM), and a voltage-dependent block by Cs+. The extent of mu-opioid inhibition in other thalamic nuclei was then investigated. Widespread inhibition similar to that seen in the centrolateral nucleus was observed in a number of sensory, motor, intralaminar, and midline nuclei. Our results suggest that the net action of opioids would depend on their source: exogenous (systemically administered) opiates inhibiting the entire thalamus and favoring the shift of cell firing from tonic to bursting mode; and endogenously released opioids acting on specific thalamic nuclei, their release depending on the origin of the presynaptic input.

Local heat effect on sympathetic skin responses after pain of electrical stimulus

OBJECTIVE

To investigate the analgesic effort of local superficial heating by studying sympathetic skin responses.

DESIGN

Randomized trial.

SETTING

Electromyography laboratory in the department of physical therapy and rehabilitation of a university hospital.

SUBJECTS

Twenty healthy volunteers participated with informed consent.

INTERVENTIONS

Sympathetic skin response (SSR) amplitudes following electrical stimulation of the right peroneal nerve and skin temperatures in both hands were recorded simultaneously. All of the recordings were repeated at 5-minute intervals during local heat application over the right palm and within 15 minutes after heat application was stopped.

RESULTS

SSR amplitudes in both hands decreased significantly during local heating (p < .05) and did not return to their initial levels within 15 minutes of the recovery period; the reductions remained statistically significant (p < .05). Amplitude reductions were statistically more significant on the heated hand compared with those on the contralateral hand (p < .05).

CONCLUSION

Therapeutic local heat application reduces the sudomotor response to a painful stimulus. This analgesic effect may be due to suppression of cortical pain sensation resulting from increased levels of endorphins, and may also be a result of local inhibition of both afferent and efferent C fibres.

The highly selective delta agonist BUBU induces an analgesic effect in normal and arthritic rat and this action is not affected by repeated administration of low doses of morphine

The effect of various doses of the selective delta agonist BUBU (Tyr-D-Ser(O-t-butyl)-Gly-Phe-Leu-Thr(O-t-butyl) on the vocalization threshold to paw pressure were compared in normal and arthritic rats, a suitable clinical model of chronic pain. In both group of rats, the intravenous administration of BUBU (6, 9, 12 mg/kg in normal and 1.5, 3, 6 mg/kg in arthritic rats) led to significant antinociceptive effects. The same dose of BUBU (6 mg/kg i.v.) produced a much more potent antinociceptive effect in arthritic than in normal rats, and a dose as low as 1.5 mg/kg produced a significant analgesic effect in the arthritic animal, whereas at 3 mg/kg BUBU was ineffective in normal rats. The analgesic effects of BUBU (9 mg/kg in normal and 3 mg/kg in arthritic rats) were completely prevented by the selective delta antagonist naltrindole (1 mg/kg i.v. a dose devoid of analgesic potency per se), while they were not affected by the selective mu antagonist naloxone (0.05 mg/kg i.v.). In addition, 3 mg/kg i.v. of BUBU remained effective in morphine tolerant arthritic rats. These results suggest that delta opioid receptor activation can modulate the transmission of cutaneous mechanical nociceptive information in rats, especially in inflammatory pain conditions.

Limitedly selective action of a delta-agonistic leu-enkephalin on the transmission in spinal motor reflex pathways in cats

1. The influence of the delta-opioid receptor agonist (D-Ser2)-leu-enkephalin (Thr6) (DSLET) on different spinal reflex pathways was investigated in anaemically decapitated, high spinal cats. Monosynaptic reflexes were tested to analyse excitatory and inhibitory flexor reflex afferent (FRA) pathways from nociceptive (from the skin of the central pad) and non-nociceptive (from skin, joint or group II muscle) afferents, as well as an excitatory nociceptive non-FRA pathway from the central pad to plantaris and intrinsic foot extensors and the inhibitory pathway from Ib muscle afferents. 2. DSLET suffused over the spinal cord (concentration 10(-3)-10(-6) M) caused a concentration-dependent depression of transmission in nociceptive and non-nociceptive FRA pathways. The excitatory FRA pathways including those from group II muscle afferents were more sensitive than the inhibitory ones. The nociceptive non-FRA pathway from the central pad to plantaris and intrinsic foot extensors was less affected than the FRA pathways. The inhibitory pathway from Ib muscle afferents remained almost unaffected. 3. Intravenous injection of DSLET (0.5-3.6 mg/kg) induced dose-dependent effects similar to those from local spinal application. The main difference was that I.V. injection more readily caused depression of the inhibitory FRA pathways to extensors. 4. The effects of local spinal application and of I.V. injection of DSLET were antagonized by I.V. injection of naloxone (0.1-1 mg/kg). 5. The effects of DSLET on spinal reflex pathways in many respects resemble that of monoamines. Possibly there is an interaction and a co-operation of enkephalins and monoamines in motor control.

Attempted reversal of cocaine-induced antinociceptive effects with naloxone, an opioid antagonist

We attempted to reverse the behavioral and neuronal antinociceptive effects of cocaine with naloxone, an opioid antagonist. Cocaine (20 mg/kg i.p.) produced a strong analgesic effect in the formalin test and in the tail pinch test. These cocaine-induced analgesic effects could be reversed by naloxone at a very high dose (10 mg/kg) but not at a dose (1 mg/kg) which was sufficient to attenuate morphine (10 mg/kg)-induced analgesia. Naloxone alone at a dose of 10 mg/kg did not produce significant effects. In general, nociceptively evoked responses in medial thalamic neurons were suppressed by cocaine (20 mg/kg), and this suppression was attenuated by naloxone (10 mg/kg). The results suggest that opioid receptors which are not involved in mediating morphine-induced analgesia and which have a low sensitivity to naloxone are involved in cocaine-induced central analgesia.

Brain passage of BUBU, a highly selective and potent agonist for delta opioid receptors: in vivo binding and mu versus delta receptors occupancy

The peptidase-resistance and bioavailability of BUBU [H-Tyr-D.Ser(OtBu)-Gly-Phe-Leu-Thr(OtBu)-OH], a highly selective and potent agonist of the delta opioid receptor, have been investigated in vitro and in vivo. In vitro at 37 degrees C, the peptide was fully resistant to degradation by rat serum and strongly resistant to degradation by rat brain membranes. In vivo 0.065% of the dose of [3H]BUBU injected intravenously to the mouse was present 15 min later in the brain. The percentage determined for [3H]DAGO [H-Tyr-D.Ala-Gly-(NMe)Phe-Gly-ol], a selective ligand for mu sites, was 0.038%. Specific binding to mouse brain membranes, determined after intracerebroventricular injection of [3H]BUBU, was saturable and a high affinity (KDapp = 25 pmol) was evaluated for the delta-agonist. Competition experiments showed that BUBU is a selective ligand for delta receptors in vivo. Comparison of the analgesic potency (hot plate test) of ICV or IV administered increasing doses of BUBU and DAGO with their in vivo binding properties supports the preferential involvement of mu receptors in supraspinal analgesia. BUBU also induced an increase in spontaneous locomotion after IV administration at a dose lower than that which produced analgesia. The quantitative results obtained in the present study demonstrate that BUBU and DAGO could be used to characterize the pharmacological responses induced by selective stimulation of delta and mu receptors after systemic administration.

Effect of naloxone on tooth pulp-evoked jaw-opening reflex in the barbiturate-anaesthetized cat

The threshold of the tooth pulp-evoked jaw-opening reflex was uninfluenced by administration of 0.5 or 5.0 mg kg-1, i.v. naloxone, a specific opioid antagonist, in the barbiturate-anaesthetized cat. Furthermore, facilitatory or inhibitory interactions between two successive tooth pulp-evoked jaw reflex responses were not influenced by naloxone. It is concluded that naloxone-sensitive opioid receptor-mediated mechanisms do not contribute to the modulation of tooth pulp-evoked reflexes by conditioning dental stimuli. Also they do not exert a tonic inhibition on the sensory or motor part of the tooth pulp-driven reflexes. Experiments performed for comparison showed that the non-nociceptive polysynaptic reflex discharge in the flexo-motor neurons of the limb was not influenced by naloxone either.

Opioid receptor types and antinociceptive activity in chronic inflammation: both kappa- and mu-opiate agonistic effects are enhanced in arthritic rats

The antinociceptive effects obtained in arthritic rats with morphine, the opioid mu-agonist DAGO [D-Ala2,MePhe4,Gly-ol5]enkephalin, the delta-selective agonist DTLET [D-Thr2, Leu5]enkephalyl-Thr, and the kappa-agonist U-50,488H were compared to their corresponding effects in normal animals and morphine-pretreated arthritic rats, respectively, using a paw pressure test. The effects of the mu- and kappa-agonists were increased in arthritic rats. While morphine-treated rats were cross-tolerant to the mu- and kappa-agonists, no tolerance to the delta-selective agonist was found. The possibility that the potent action of morphine in this model for chronic inflammatory pain is mediated partly through kappa-mechanisms is discussed.