The effects of sham and full spinalization on the systemic potency of mu- and kappa-opioids on spinal nociceptive reflexes in rats.

1. Flexor withdrawal reflexes to noxious mechanical pinch stimuli were recorded as single motor unit activity in alpha-chloralose anaesthetized rats, by means of tungsten bipolar electrodes inserted percutaneously into hindlimb flexor muscles. The relative spinal and supraspinal contributions to mu- and kappa-opioid agonists in inhibiting these spinal reflexes, together with possible potency changes elicited by surgical trauma, were examined by comparing their relative potencies in spinally unoperated, sham spinalized and spinalized rats. 2. The noxious stimuli, which were of comparable intensity in the three groups, elicited similar mean firing rates of the motor units in all groups. This indicates that the excitability levels in the reflex pathway were not greatly affected by either sham or actual spinalization. 3. The mu-agonists morphine and fentanyl, and the kappa-agonist U-50,488H, inhibited the reflexes in a dose-dependent manner, when administered intravenously in a log2 cumulative dose regime. 4. The surgery of sham spinalization had little effect on the potency of morphine and fentanyl, whereas it doubled the potency of U-50,488. 5. Spinalization did not affect the potency of morphine. In contrast it decreased the potency of fentanyl 2-4 fold and that of U-50,488 approximately 6 fold. 6. The effects of all agonists were antagonized by naloxone. Dose-dependence studies indicating that antagonism of U-50,488H required about 5 times the dose of naloxone that antagonized morphine. 7. The data suggest that surgical trauma to the spinal column and/or dura mater triggers supraspinal mechanisms that significantly enhance the potency of kappa- but not mu-agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Spinal antinociceptive actions and naloxone reversibility of intravenous mu- and kappa-opioids in spinalized rats: potency mismatch with values reported for spinal administration

1. The relative spinal effectiveness of mu- and kappa-opioids has been assessed by their intravenous potencies on nociceptive responses (heat and/or pinch) of single motoneurones recorded in alpha-chloralose anaesthetized, spinalized rats. 2. The depressant actions of both mu- and kappa-opioids were reversed by low intravenous doses of naloxone (10 to 100 micrograms kg-1). When tested at a dose of 1 microgram kg-1 i.v., naloxone antagonized the effects of the mu-agonist morphine but had no effect on the kappa-opioid U-50,488. This provides further support for the theory that the actions of mu- and kappa-ligands were mediated at different subclasses of opioid receptor but highlights the difficulties in using antagonists with poor receptor selectivity to differentiate between mu- and kappa-receptor-mediated effects in vivo. 3. The molar potency rations of fentanyl: morphine:U-50,488: tifluadom for thermal and mechanical nociceptive responses were 620: 1.0:0.74:5.7 and 520:1.0:0.56:7.7 respectively. These potency ratios, as well as the absolute potencies, agree well with those reported in several behavioural studies in which systemic administration of agonists was used in non-thermal tests. 4. The agonist potency values obtained in this study contrast with those reported for local spinal administration. By this route, the potency of lipophilic opioids (e.g. fentanyl, U-50,488 and tifluadom) relative to hydrophilic opioids (e.g. morphine) is much reduced, implying that activity of intrathecally administered opioids is more dependent on the physico-chemical properties of the agonists used than on the relative abundance in the spinal cord of functional opioid receptors of the mu- and kappa-subtypes. This conclusion indicates that the results with locally applied opioids should not be used to assess spinal opioid receptor function.     

Spinal antinociceptive actions of mu- and kappa-opioids: the importance of stimulus intensity in determining ''selectivity'' between reflexes to different modalities of noxious stimulus.

1. In electrophysiological experiments in spinalized rats, mu- and kappa-opioids were tested intravenously on the responses of single motoneurones to electronically controlled, alternating noxious heat and noxious pinch stimuli. The effects of mu- and kappa-opioids were compared with those of the general anaesthetic alpha-chloralose and the dissociative anaesthetic/PCP ligand ketamine. 2. The kappa-opioids U-50,488 (0.5-16 mgkg-1 i.v.) and tifluadom (0.05-1.6 mgkg-1 i.v.) had very similar actions to the mu-opioid fentanyl (0.5-16 micrograms kg-1 i.v.). Thus all three agonists reduced thermal and mechanical nociceptive reflexes in parallel and in a dose-dependent manner, but only so long as neuronal responses to the alternating stimuli elicited similar excitability levels in the neurone under study. Ketamine (0.5-16 mgkg-1 i.v.) had similar actions to the opioids whereas alpha-chloralose (20 mgkg-1 i.v.) had very little effect on neuronal responsiveness. 3. Apparently 'selective' depressions by both mu- and kappa-opioids could be orchestrated by a deliberate mismatch of the intensities of alternating noxious heat and pinch stimuli; as measured by neuronal firing rate, the weaker of the responses to either type of stimulus was invariably reduced to a greater degree. 4. Similar 'selectivity' could be demonstrated for both mu- and kappa-ligands when the weaker and stronger responses were of the same modality, being applied by the same pincher device but with alternating applied force. 5. It is concluded that the 'selective' spinal actions of kappa-opioids seen in non-thermal over thermal behavioural models of nociception is likely to be related to the relative intensities, rather than the modalities, of the noxious stimuli used. The validity of the interpretation of results obtained in such behavioural studies is discussed.     

Cannabinoidergic and opioidergic inhibition of spinal reflexes in the decerebrated, spinalized rabbit

The present experiments were designed to investigate the role(s) of cannabinoid receptors in modulating transmission in the sural-medial gastrocnemius withdrawal reflex of the decerebrated, spinalized rabbit and how, if present, cannabinoid-mediated control might interact with opioid-mediated inhibitions known to impinge on this reflex pathway. The selective CB(1) receptor antagonist SR 141716A enhanced reflexes by a factor of two after a cumulative dose of 100 nmol kg(-1) i.v., but had no effect on the endogenous opioid-mediated inhibition generated by repetitive electrical stimulation of the common peroneal nerve, or on the suppression of reflexes caused by i.v. administration of the synthetic opioid fentanyl. Given at a dose of 10 nmol kg(-1) i.v., the potent, CB(1)--CB(2) cannabinoid receptor agonist HU 210 inhibited medial gastrocnemius reflexes to approximately 30% of controls and significantly decreased both heart rate and blood pressure, but did not alter the inhibition of reflexes resulting from common peroneal nerve stimulation or i.v. fentanyl. The effects of HU 210 were reversed by SR 141716A. HU 210 was just as effective in inhibiting reflexes in the presence of the opioid antagonist naloxone (5 micromol kg(-1)) as it was in untreated animals. The data show that cannabinoids, acting through CB(1) receptors, are inhibitory in rabbit spinal cord and that there appears to be some endogenous cannabinoid tone under the conditions of the present experiments. The evidence of this study is that the inhibitory effects of opioids and cannabinoids in rabbit spinal cord are completely independent of each other, and are additive rather than synergistic.     

Functional evidence for multiple receptor activation by kappa-ligands in the inhibition of spinal nociceptive reflexes in the rat.

1. The evidence for kappa-receptor heterogeneity is equivocal. We have now investigated this question by comparing the effects of five putatively selective kappa-agonists. The parameters examined were: the relative potencies in depressing hindlimb flexor muscle reflexes to noxious pinch stimuli in both spinalized and sham-spinalized rats; the reversibility of these effects by naloxone; and the effects on blood pressure. 2. Two types of drug effect was discriminated. One drug group, represented by U-50,488, U-69,593 and PD-117,302, had a potency ratio between sham and spinalized rats approximately 10 fold lower than the other group, which comprised GR103545 and CI-977. 3. Under sham-spinalized conditions, CI-977 and GR103545 at high doses caused only sub-maximal reductions of spinal reflexes. U-50,488 was still active when superimposed on these high doses of GR103545. 4. Naloxone reversed all effects, but different doses were required between compounds, with GR103545 taking some 20 times higher doses of naloxone to cause reversal than did U-50,488. 5. The effects on mean arterial pressure were opposite between groups. 6. The results imply that more than one type of naloxone-sensitive non-mu opioid receptor must be involved in mediating these complex actions of ligands that have been claimed to be selective for kappa-receptors.     

Characterisation of sevoflurane effects on spinal somato-motor nociceptive and non-nociceptive transmission in neonatal rat spinal cord: an electrophysiological study in vitro

Sevoflurane is the latest halogenated ether introduced in clinical anaesthesia, and its effects at the spinal level are not fully characterised. The rat hemisected spinal cord preparation was used to test the effects of sevoflurane on spinal nociceptive and non-nociceptive synaptic transmission as well as on excitations produced by application of glutamate-receptor agonists. Sevoflurane was dissolved in artificial cerebrospinal fluid (ACSF) with a specific vaporiser, and its final concentration was assessed with gas chromatography. Sevoflurane reduced the mono-synaptic reflex (EC(50) approximately 219 microM) and the slow components of the dorsal root-ventral root potentials (EC(50) approximately 72 microM) elicited by single dorsal root stimulation as well as the cumulative depolarisation (CD) elicited by repetitive stimulation (EC(50) approximately 98 microM). AMPA- and NMDA-induced depolarisations were also reduced by sevoflurane (respective EC(50)s were 206 and 127 microM). Inhibition of NMDA-induced depolarisation was TTX resistant. However, complete blockade of NMDA receptors with d-AP5 did not prevent further reduction of the CD by sevoflurane. All the effects reported were concentration-dependent and reversible.We conclude that sevoflurane applied at clinically relevant concentrations induces a strong depression of nociceptive and non-nociceptive spinal systems, which may be partly mediated by interfering with excitatory amino acid transmission.     

Contribution of supraspinal mu- and delta-opioid receptors to antinociception in the rat.

This study evaluated the contribution of supraspinal opioid receptors to the production of antinociception, in the rat. I.c.v. administration of a selective mu- (DAMGO) and a selective delta- (DPDPE), but not a selective kappa- (U50,488H) opioid receptor agonist, produced significant dose-dependent increase in mechanical nociceptive thresholds. ICI 174,864, a delta-opioid receptor antagonist, completely blocked the antinociceptive effects produced by DPDPE ([D-Pen2,D-Pen5]enkephalin) at a dose that had no effect on the increases in nociceptive thresholds produced by DAMGO ([D-Ala2,N-MePhe4,Gly5-ol]enkephalin). The simultaneous i.c.v. administration of a low-antinociceptive dose of DAMGO or DPDPE given in combination with sequentially increasing doses of the other opioid agonist, produced synergy (i.e., a more than additive antinociceptive effect), at the lower doses tested. The results of these experiments provide evidence to support the suggestion that both supraspinal mu- and delta-opioid receptors contribute to the production of antinociception, in the rat.     

The effects of intra-arterial bradykinin, histamine, acetylcholine and prostaglandin E1 on nociceptive and non-nociceptive dorsal horn neurones of the cat.

The effects of peripherally administered algesic agents were investigated on the firing of cat dorsal horn interneurones classified as nociceptive or non-nociceptive according to the peripheral stimuli that excited them. A small amount of bradykinin injected into the blood supplying the receptive fields of cells was a potent specific stimulus causing activation of nociceptive cells and slowly conducting nerve fibres. Larger amounts of bradykinin and large amounts of histamine, 5-hydroxytryptamine and acetylcholine activated both nociceptive and non-nociceptive cells. Prostaglandin E1 enhanced the effects of bradykinin and histamine on nociceptive cells. Prostaglandin E1 also increased the response of these cells to the application of noxious heat whilst aspirin reduced this response. These results support a chemosensitive theory of nociceptor activation and show bradykinin to be the most potent and specific of the suggested endogenous algesic agents in causing activation of CNS nociceptive pathways.     

The contributions of mu-, delta- and kappa-opioid receptors to the actions of endogenous opioids on spinal reflexes in the rabbit.

Spinal reflexes in the rabbit are suppressed tonically by endogenous opioids. The contributions made to this suppression by mu-, delta- and kappa-opioid receptors have been investigated by studying the actions of a range of opioid antagonists and agonists on reflexes evoked by sural nerve stimulation in the ankle extensor gastrocnemius medialis (g.m.), and in the knee flexor semitendinosus (s.t.). When given at a total dose of 88.5 micrograms kg-1 i.v., either of the universal opioid receptor antagonists (-)-naloxone and (-)-quadazocine enhanced the g.m. response to more than 7 times the pre-drug control values, and the s.t. reflex to 1.5 times controls. The effects of quadazocine were stereospecific. The selective delta antagonist ICI 174864 (3.5 mg kg-1 i.v. total) also augmented the g.m. reflex but only to twice pre-drug controls. The mu-agonists fentanyl (100 micrograms kg-1) and morphine (50 mg kg-1) suppressed both g.m. and s.t. reflex responses to less than half control levels by a naloxone-reversible mechanism. The kappa-agonists bremazocine (50 micrograms kg-1 total), tifluadom (100 micrograms kg-1), ethylketocyclazocine (200 micrograms kg-1) and U50488H (1 mg kg-1) potentiated the g.m. reflex and had variable effects on the s.t. response. Naloxone usually added to the facilitatory actions of these drugs. kappa-Opioid receptor agonists also caused a profound, naloxone-reversible depression of arterial blood pressure. It may be concluded that the endogenous opioid-mediated suppression of spinal reflexes in the rabbit is mediated mainly, if not exclusively, through mu-receptors. There are no known endogenous ligands which are specific for the mu-receptor, so in the present case it seems that selectivity is determined by the receptor population rather than by the ligand.     

The effects of intravenous fentanyl,morphine and naloxone on nociceptive responses of neurones in the rat caudal medulla

Fifty-three per cent of 106 spontaneously active single neurones, located in the caudal medullary reticular formation of the urethane or halothane anaesthetized rat, were found to respond to a noxious stimulation, such as immersion of the tail in water at 52–53°C for 30 sec. The evoked responses consisted of either an increase (82%), decrease (7%) or biphasic (11%) alteration in firing rate, and could be produced by other noxious stimuli, such as a pinch, but not by stroking, tapping or joint manipulation. Morphine (0.8–1.0 mg/kg) or fentanyl (5–10 μg/kg) administered intravenously blocked the acceleration in firing rate produced by noxious stimuli, and this effect was antagonized by naloxone administered intravenously, suggesting it to be an action on specific opiate receptors. Naloxone (0.5–10 mg/kg) blocked those decreases in firing rate produced by noxious stimuli, suggesting that an endogenous opioid may be released onto these neurones as a result of noxious stimuli applied to the periphery.     

Cardiac nociceptive reflexes: role of kinins, prostanoids and capsaicin-sensitive afferents

Spinal sympathetic afferent fibres with endings in the heart are essential for signalling cardiac pain. In anesthetized, open-chest dogs, stimulation of these cardiac afferents with the algesic agents, bradykinin or capsaicin, results in reflex increases in arterial pressure and cardiac rate. The reflex responses induced by bradykinin are enhanced by concomitant application of prostaglandins of the E type or prostacyclin, and reduced by indomethacin. In contrast, the reflex effects of capsaicin are not influenced by either indomethacin or prostaglandin E1 treatment. Evidence is presented that reflexogenic effects of bradykinin involve its interaction with specific B2-receptors for kinins and that proteolysis of endogenous kininogen precursor to kinins by epicardially applied tissue kallikrein can initiate reflex cardiovascular effects.     

Role of spinal serotonin1 receptor subtypes in thermally and mechanically elicited nociceptive reflexes

The ability of 5-HT_1A and 5-HT_1B agonists to alter a spinal animal's nociceptive threshold was examined using two analgesiometric tests. In the spinal withdrawal reflex test, administration of the selective 5-HT_1A agonists ipsapirone, gepirone and PAPP resulted in significant dose-dependent increases in receptive field (RF) area for withdrawal reflexes when compared to predrug baseline values, indicating an increase in nociceptive sensitivity. The average overall percent maximal increase in RF area following administration of 5-HT_1A selective compounds was: 80±16% for the ventroflexion reflex, 90±6% for the dorsiflexion reflex and 87±8% for the lateral flexion reflex. Similar to the effects noted with 5-HT_1A agonists, administration of 5-HT_1B agonists RU24969, mCPP and TFMPP resulted in a hyperalgesic response with an overall percent maximal increase of 43±6% for the ventroflexion reflex, 51±6% for the dorsiflexion reflex and 38±9% for the lateral flexion reflex. In the tail-flick analgesiometric test, administration of the 5-HT_1A agonists 8-OH-DPAT and ipsapirone and the 5-HT_1B agonists RU24969 and mCPP resulted in a significant dose-dependent increase in tail-flick latencies when compared to predrug baseline values, indicating a decrease in nociceptive sensitivity to noxious thermal stimuli. No differences in magnitude of the effect of the two receptor subtypes were found, indicating that stimulation of either 5-HT_1A or 5-HT_1B receptors was equipotent in producing the antinociceptive tail-flick response. Administration of the 5-HT_1A antagonist meter-goline completely reversed the increase in TFL produced by RU24969, providing further evidence that the effects seen here are mediated by spinal 5-HT receptors. The results of this study indicate that spinal 5-HT₁ receptor subtypes may either facilitate or inhibit nociceptive input depending upon the type of nociceptor that is activated, as opposed to the type of receptor subtype that is stimulated.     

Effect of mu-, kappa-, and delta-selective opioid agonists on thermoregulation in the rat.

The effect of selective mu-, kappa-, and delta-agonists on brain surface temperature (Tb), oxygen consumption (Vo2), and heat exchange (Q) was studied in unrestrained, male Sprague-Dawley rats using whole-body calorimetry. Hyperthermia, produced by PL-017 (1.86 nM) given ICV, resulted from increased Vo2 and reduced Q during the first 15-45 min postinjection. Tb returned to control levels due to a combination of increased Q and reduced Vo2. PL-017-induced hyperthermia was abolished by the mu-selective antagonist CTAP (0.75 nM). Dynorphin A1-17 (4.65 nM), a kappa-selective agonist, reduced both Vo2 and Q, resulting in hypothermia that was blocked by the kappa-selective antagonist nor-binaltorphimine (25 nM). The delta-selective agonist DPDPE (4.64 nM) caused no significant changes in Tb, Vo2, or Q. The data indicate that central stimulation of the mu- and kappa-opioid receptors affects both oxidative metabolism and heat exchange, which result in a change in Tb. These alterations can be prevented with selective opioid antagonist pretreatment.     

Effects of intravenous mu and kappa opioid receptor agonists on sensory responses of convergent neurones in the dorsal horn of spinalized rats.

1. Electrophysiological experiments have been performed to assess the effects of intravenously administered mu and kappa opioid agonists on the responses to noxious thermal and mechanical and non-noxious tactile stimuli of single convergent neurones in laminae III-VI of the dorsal horn of spinalized rats anaesthetized with alpha-chloralose. 2. The mu receptor agonists tested were fentanyl (1-16 micrograms kg-1) and morphine (0.5-16 mg kg-1) and the kappa-receptor agonists U-50,488 (1-16 mg kg-1) and tifluadom (0.1-1.6 mg kg-1). Multiple drug tests were made on each cell so that compounds could be compared under closely comparable conditions. 3. In one protocol, thermal and mechanical nociceptive responses of matched amplitudes were elicited alternately. Both mu and kappa agonists dose-dependently reduce the neuronal responses. Thermal nociceptive responses were as sensitive to the kappa agents as were the mechanical nociceptive responses; the mu agonists similarly reduced both types of response in parallel. 4. In another protocol, nociceptive and non-nociceptive responses were elicited alternately to permit the degree of selective antinociception to be assessed. The mu agonists were scarcely selective, fentanyl reducing nociceptive only slightly (but significantly at 4-16 micrograms kg-1) more than non-nociceptive responses. The kappa-opioid agonist U50,488 reduced tactile responses somewhat more than nociceptive responses. 5. The spontaneous discharge of these cells with ongoing activity was reduced to a significantly greater degree than the evoked responses; this is likely to have contributed to the non-selectivity of the reduction of the evoked responses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of pharmacological substances on nociceptive vasomotor reflexes in experiments on unanesthetized animals

Experiments on unanesthetized cats (conscious and curarized) were made to analyze the method of studying the effect of pharmacological substances on the pressor vasomotor reflexes produced by nociceptive stimulation. It was shown that the activity profiles of the drugs depend not only on their cardiovascular properties but also on the parameters of stimulation applied to the reflex recording, which is related to the shape of the curve of reflex magnitude dependence on the stimulation intensity. The most precise test in the application of the method to the unanesthetized animals involved the recording of the threshold of reflex occurrence, with the use of curare-like drugs being mandatory. Making of experiments without use of the curare-like agents or without considering the shape of the curve of reflex magnitude dependence on the parameters of stimulation might lead to errors in the appraisal of the results obtained.     

Effect of morphine on the change in the nociceptive and non-nociceptive evoked potentials in the orbitofrontal and secondary somatosensory areas of the cerebral cortex in different functional states

The effect of morphine on alterations in nociceptive and non-nociceptive evoked potentials (EP) elicited in the second somatosensory (S2) and orbito-frontal cortex of the brain was studied in acute experiments on cats. Administration of morphine in a dose of 5 mg/kg to the above-mentioned structures caused an increase in the amplitude of EP to non-nociceptive stimulation and a decrease of EP to nociceptive stimulation. Under alternate functional inactivation of the orbito-frontal cortex and S2 a unidirectional effect of the projection cortex on the associative orbito-frontal cortex was observed, manifested by a decrease of EP in the orbito-frontal cortex. Administration of morphine with the S2 cooled led to further decrease of EP followed by their restoration in the orbito-frontal cortex, in response to both nociceptive and non-nociceptive stimulation. Meanwhile administration of morphine, with the orbito-frontal cortex cooled, elicited the response in the S2 analogous to that observed after administration of morphine with intact orbito-frontal cortex: a decrease of EP to nociceptive stimulation and an increase of EP to non-nociceptive stimulation. It is suggested that the S2 plays an important role in control of nociceptive and non-nociceptive input to the CNS.     

The spinal and peripheral roles of bradykinin and prostaglandins in nociceptive processing in the rat.

The effect of peripheral and intrathecal administration of the bradykinin B2 receptor antagonist HOE140 and the non-steroidal anti-inflammatory drug indomethacin were studied on the response of dorsal horn nociceptive neurones to formalin in the anaesthetized rat. Peripheral pretreatment with HOE140 reduced both phases of the formalin response whereas subcutaneous administration of indomethacin (5 mg/kg) reduced only the second phase. Pre-treatment with intrathecal HOE140 resulted in a dose-dependent reduction of the second phase whereas a high dose (500 micrograms) intrathecal indomethacin reduced both phases of the response. Bradykinin plays a role in peripheral nociception during the first and second phase of the response whereas the prostaglandins are only involved during the second phase. The converse is true centrally, the prostaglandins appear to be involved in both phases of the formalin response whereas bradykinin only plays a central role during the second phase of the response.     

Effect of insulin hypoglycemic stress on nociceptive responses to mu- and kappa-opioid receptor agonists at LH-surge in female rats

In a randomized, prospective, controlled and crossover study, the effects of insulin hypoglycemic stress on nociceptive responses to mu- and kappa-opioid receptor directed drugs during steroid-induced preovulatory LH-surge were seen in ovariectomized female rats. Ovariectomized rats were equally distributed in two groups of 10. In group 1 rats, LH-surge was induced by sequential treatment with estradiol benzoate 7.5 micro g/rat s.c. and progesterone 5 mg/rat s.c., whereas in group 2 rats, vehicles of estradiol benzoate and progesterone were given in a sequential manner. A third group consisted of sham-operated rats, which received no treatment. Rats were exposed to insulin-induced hypoglycemic stress 1 h before the peak of LH-surge. Antinociceptive responses of morphine, buprenorphine and pentazocine were observed at peak LH-surge during hypoglycemic stress. Increased nociceptive responses to noxious stimulus and decreased percent maximal possible effect for morphine, buprenorphine and pentazocine at LH-surge were significantly (p < 0.01) reversed during insulin hypoglycemic stress. There was a significant (p < 0.01) decrease in the ED(50) values of morphine, buprenorphine and pentazocine during hypoglycemic stress. The present study indicates that insulin hypoglycemic stress is responsible for increased antinociceptive activities of morphine, buprenorphine and pentazocine and decreased sensitivity to noxious stimulus in ovariectomized rats with or without steroid-induced LH-surge.     

A microiontophoretic study of the actions of mu-, delta- and kappa-opiate receptor agonists in the rat brain.

The actions of mu-, delta- and kappa-opiate receptor agonists have been compared on the activity of single neurones in the brain stem, caudate nucleus and hippocampus of the rat, using the technique of microiontophoresis. In the brain stem and caudate nucleus the predominant effect of all the opiate agonists tested was depression of neuronal activity which was antagonized by naloxone. The selectivity of naloxone as an opiate receptor antagonist was indicated by its lack of effect on gamma-aminobutyric acid (GABA)-induced responses. In the hippocampus both mu- and delta-agonists mainly caused an increase in neuronal firing rates, though some neurones were depressed. In contrast, all the kappa-agonists, including the proposed endogenous ligand for the kappa-receptor, dynorphin, caused depression of neuronal activity. All of these effects were antagonized by naloxone. There was a clear distinction in the areas within the hippocampus in which the mu- and delta-agonists produced different effects. Neurones in the pyramidal cell layer were always excited by these drugs, whereas neurones in the granule cell layer of the dentate gyrus were always depressed by the same drug.