Effects of chronic infusion of neurotensin and a neurotensin NT1 selective analogue PD149163 on amphetamine-induced hyperlocomotion.

Neurotensin (NT) has been proposed as an endogenous antipsychotic based in part on the similarity in behavioural effects to antipsychotic drugs, for example, attenuation of both amphetamine-induced hyperlocomotion (AH) and amphetamine disrupted pre-pulse inhibition in the rat. However, there is some evidence that repeated administration of NT or an analogue produces behavioural tolerance to such effects. The present experiments sought to confirm and extend these findings by testing the effects on AH of 7 days central administration of NT and the NT1 selective analogue PD 149163 and the effects of 21 days central administration of NT. NT and PD149163 continuously administered for 7 days produced no effect on AH (in contrast to attenuation with a single injection here and previously reported), whereas 21 days of NT administration potentiated AH. Together, these studies report that the effects of NT or a NT analogue on AH depends on the duration of administration of peptide. The results are discussed in comparison with the reported antipsychotic properties of acute administration of NT and possible mechanisms involving NT1 receptors.

Behavioral, central monoaminergic and hypothalamo-pituitary-adrenal axis correlates of fear-conditioned analgesia in rats.

Fear-conditioned analgesia is an important survival response which is expressed upon re-exposure to a context previously paired with a noxious stimulus. The aim of the present study was to characterize further the behavioral, monoaminergic and hypothalamo-pituitary-adrenal axis alterations associated with expression of fear-conditioned analgesia. Rats which had received footshock conditioning 24 h earlier, exhibited reduced formalin-evoked nociceptive behavior upon re-exposure to the footshock chamber, compared with non-footshocked formalin-treated rats. Intra-plantar injection of formalin reduced the duration of contextually-induced freezing and 20-40 kHz ultrasound emission. Intra-plantar injection of formalin to non-footshocked, non-conditioned rats did not induce ultrasonic vocalizations. Intra-plantar injection of formalin to footshock-conditioned rats, significantly increased tissue levels of 3,4-dihydroxyphenylacetic acid and the 3,4-dihydroxyphenylacetic acid:dopamine ratio in the periaqueductal gray and reduced levels of dopamine in the thalamus, compared with saline-treated footshocked controls. Non-footshocked, non-conditioned rats were capable of mounting a robust formalin-evoked increase in plasma corticosterone levels. Moreover, plasma corticosterone levels were significantly higher in saline-treated, footshock conditioned rats compared with saline-treated non-footshocked rats and levels did not differ between saline- and formalin-treated footshock conditioned rats. Assessment of the effects of the intra-plantar injection procedure revealed an attenuation of short-term extinction of contextually-induced freezing in rats anesthetized for intra-plantar injection of saline compared with non-anesthetized, non-injected rats as well as discrete effects on monoamines, their metabolites and plasma corticosterone levels. These data extend behavioral characterization of the phenomenon of fear-conditioned analgesia and suggest that measurement of ultrasound emission may be used as an ethologically relevant index of the defense response during fear-conditioned analgesia. Ultrasonic vocalization may also be a useful behavioral output to aid separation of nociception and aversion. The data provide evidence for discrete alterations in dopaminergic activity in the periaqueductal gray and thalamus and for altered hypothalamo-pituitary-adrenal axis activity following expression of defensive behavior.

Effects of direct periaqueductal grey administration of a cannabinoid receptor agonist on nociceptive and aversive responses in rats.

The analgesic potential of cannabinoids may be hampered by their ability to produce aversive emotion when administered systemically. We investigated the hypothesis that the midbrain periaqueductal grey (PAG) is a common substrate mediating the anti-nociceptive and potential aversive effects of cannabinoids. The rat formalin test was used to model nociceptive behaviour. Intra-PAG microinjection of the excitatory amino acid D,L-homocysteic acid (DLH) was used to induce an aversive, panic-like reaction characteristic of the defensive "fight or flight" response. Administration of the cannabinoid receptor agonist HU210 (5 microg/rat) into the dorsal PAG significantly reduced the second phase of formalin-evoked nociceptive behaviour, an effect which was blocked by co-administration of the CB(1) receptor antagonist SR141716A (50 microg/rat). This anti-nociceptive effect was accompanied by an HU210-induced attenuation of the formalin-evoked increase in Fos protein expression in the caudal lateral PAG. Intra-dorsal PAG administration of HU210 (0.1, 1 or 5 microg/rat) significantly reduced the aversive DLH-induced explosive locomotor response. The anti-nociceptive effect of HU210 is likely to result from activation of the descending inhibitory pain pathway. Mechanisms mediating the anti-aversive effects of cannabinoids in the PAG remain to be elucidated. These data implicate a role for the PAG in both cannabinoid-mediated anti-nociceptive and anti-aversive responses.

Oleamide is a selective endogenous agonist of rat and human CB1 cannabinoid receptors.

1. The ability of the endogenous fatty acid amide, cis-oleamide (ODA), to bind to and activate cannabinoid CB(1) and CB(2) receptors was investigated. 2. ODA competitively inhibited binding of the nonselective cannabinoid agonist [(3)H]CP55,940 and the selective CB(1) antagonist [(3)H]SR141716A to rat whole-brain membranes with K(i) values of 1.14 microm (0.52-2.53 microm, Hill slope=0.80, n=6) and 2.63 microm (0.62-11.20 microm, Hill slope=0.92, n=4), respectively. AEA inhibited [(3)H]CP55,940 binding in rat whole-brain membranes with a K(i) of 428 nm (346-510 nm, Hill slope=-1.33, n=3). 3. ODA competitively inhibited [(3)H]CP55,940 binding in human CB(1) (hCB(1)) cell membranes with a K(i) value of 8.13 microm (4.97-13.32 microm, n=2). In human CB(2) transfected (hCB(2)) HEK-293T cell membranes, 100 microm ODA produced only a partial (42.5+/-7%) inhibition of [(3)H]CP55,940 binding. 4. ODA stimulated [(35)S]GTPgammaS binding in a concentration-dependent manner (EC(50)=1.64 microm (0.29-9.32 microm), R(2)=0.99, n=4-9), with maximal stimulation of 188+/-9% of basal at 100 microm. AEA stimulated [(35)S]GTPgammaS binding with an EC(50) of 10.43 microm (4.45-24.42 microm, R(2)=1.00, n=3, 195+/-4% of basal at 300 microm). Trans-oleamide (trans-ODA) failed to significantly stimulate [(35)S]GTPgammaS binding at concentrations up to 100 microm. 5. ODA (10 microm)-stimulated [(35)S]GTPgammaS binding was reversed by the selective CB(1) antagonist SR141716A (IC(50)=2.11 nm (0.32-13.77 nm), R(2)=1.00, n=6). 6. The anatomical distribution of ODA-stimulated [(35)S]GTPgammaS binding in rat brain sections was indistinguishable from that of HU210. Increases of similar magnitude were observed due to both agonists in the striatum, cortex, hippocampus and cerebellum. 7. ODA (10 microm) significantly inhibited forskolin-stimulated cyclic AMP (cAMP) accumulation in mouse neuroblastoma N1E 115 cells (P=0.02, n=11). ODA-mediated inhibition was completely reversed by 1 microm SR141716A (P<0.001, n=11) and was also reversed by pretreatment with 300 ng ml(-1) pertussis toxin (P<0.001, n=6). 8. These data demonstrate that ODA is a full cannabinoid CB(1) receptor agonist. Therefore, in addition to allosteric modulation of other receptors and possible entourage effects due to fatty acid amide hydrolase inhibition, the effects of ODA may be mediated directly via the CB(1) receptor.

Evidence for differential modulation of conditioned aversion and fear-conditioned analgesia by CB1 receptors.

Fear-conditioned analgesia is an important survival response mediated by substrates controlling nociception and aversion. Cannabinoid(1) (CB(1)) receptors play an important role in nociception and aversion. However, their role in fear-conditioned analgesia has not been investigated. This study investigated the effects of systemic administration of the CB(1) receptor antagonist, SR141716A (1 mg/kg, i.p.), on fear-conditioned analgesia and conditioned aversion in rats. Twenty-four hours after receiving footshock, rats exhibited reduced formalin-evoked nociceptive behaviour, increased freezing and increased defecation when tested in the footshock apparatus, compared with non-footshocked formalin-injected rats. SR141716A attenuated fear-conditioned analgesia, freezing and defecation. Importantly, SR141716A had no effect on formalin-evoked nociceptive behaviour over an equivalent time period in rats not receiving footshock. SR141716A had no effect on contextually induced freezing during the first half of the test trial in rats receiving intra-plantar injection of saline. Administration of SR1417176A did, however, attenuate short-term extinction of contextually induced freezing and ultrasound emission in rats receiving intra-plantar saline, compared with vehicle-treated saline controls. These data suggest an important role for the CB(1) receptor in mediating fear-conditioned analgesia and provide evidence for differential modulation of conditioned aversive behaviour by CB(1) receptors during tonic, persistent pain.

Effects of coadministration of cannabinoids and morphine on nociceptive behaviour, brain monoamines and HPA axis activity in a rat model of persistent pain.

The antinociceptive effects of Delta9-tetrahydrocannabinol (Delta9-THC) have been widely described; however, its therapeutic potential may be limited by secondary effects. We investigated whether coadministration of low doses of cannabinoids or cannabinoids and morphine produced antinociception in the absence of side-effects. Effects of preadministration (i.p.) of Delta9-THC (1 or 2.5 mg/kg), cannabidiol (5 mg/kg), morphine (2 mg/kg), Delta9-THC + morphine, Delta9-THC + cannabidiol or vehicle on formalin-evoked nociceptive behaviour were studied over 60 min. Trunk blood and brains were collected 60 min after formalin injection and assayed for corticosterone and tissue levels of monoamines and metabolites, respectively. Drug effects on locomotor activity, core body temperature and grooming were assessed. Delta9-THC reduced both phases of formalin-evoked nociceptive behaviour, enhanced the formalin-evoked corticosterone response and increased the 4-hydroxy-3-methoxyphenylglycol : noradrenaline ratio in the hypothalamus. Cannabidiol alone had no effect on these indices and did not modulate the effects of Delta9-THC. Morphine reduced both phases of formalin-evoked nociceptive behaviour. Coadministration of Delta9-THC and morphine reduced the second phase of formalin-evoked nociceptive behaviour to a greater extent than either drug alone, and increased levels of thalamic 5-hydroxytryptamine. While the antinociceptive effects of Delta9-THC and morphine alone occurred at doses devoid of effects on locomotor activity, coadministration of Delta9-THC and morphine inhibited locomotor activity. In conclusion, coadministration of a low dose of morphine, but not cannabidiol, with Delta9-THC, increased antinociception and 5-hydroxytryptamine levels in the thalamus in a model of persistent nociception. Nevertheless, these enhanced antinociceptive effects were associated with increased secondary effects on locomotor activity.