The roles of nerve growth factor and cholecystokinin in the enhancement of morphine analgesia in a rodent model of central nervous system inflammation

Abstract

Animal models of inflammatory pain are characterized by the release of inflammatory mediators such as cytokines and neurotrophic factors, and enhanced analgesic sensitivity to opioids. In this study, we examine the mechanisms underlying this effect, in particular the roles of cholecystokinin (CCK) and nerve growth factor (NGF), in an animal model of central nervous system (CNS) inflammation induced by spinal administration of lipopolysaccharide (LPS). Although spinal administration of LY-225910 (25 ng), a CCK-B antagonist, enhanced morphine analgesia in naïve rats, it was unable to do so in LPS-treated animals. Conversely, spinal CCK-8S administration (1 ng) decreased morphine analgesia in LPS-treated rats, but not in naïve animals. Further, spinal anti-NGF (3 microg) was able to reduce morphine analgesia in LPS-treated rats, but not in naïve animals, an effect that was reversed by spinal administration of LY-225910. While CCK-8S concentration was increased in spinal cord extracts of LPS animals as compared to controls, morphine-induced spinal CCK release in the extracellular space, as measured by in-vivo spinal cord microdialysis was inhibited in LPS animals as compared to controls, and this was reversed by anti-NGF pretreatment. Finally, chronic spinal administration of beta-NGF (7 microg/day) for 7 days enhanced spinal morphine analgesia, possibly by mimicking a CNS inflammatory state. We suggest that in intrathecally LPS-treated rats, spinal CCK release is altered resulting in enhanced morphine analgesia, and that this mechanism may be regulated to an important extent by NGF.

Fig. 1

Dose-response curve for the analgesic effect of morphine (0.3 μg, 1 μg, 3 μg, 10 μg, 30 μg) in 4 conditions: 1) non-LPS/vehicle (open circles; n=6); 2) non-LPS/LY225910 (open squares; n=6); 3) LPS/vehicle (dark circles; n=6); and 4) LPS/LY225910 (dark squares; n=6). Morphine analgesia expressed as % maximum possible effect (%MPE) based on thermal withdrawal latencies in the plantar test. LPS treatment enhances morphine analgesia, and LY225910 enhances morphine analgesia in non-LPS animals, but not in LPS animals. (**P < 0.01)

Fig. 2

Effect on pre- and post-morphine thermal latencies in the plantar test of the following treatments: 1) non-LPS/vehicle (n=6); 2) LPS/vehicle (n=6); 3) non-LPS/CCK (n=6); and 4) LPS/CCK (n=6). LPS treatment enhances morphine analgesia, and CCK reduces morphine analgesia in the LPS condition. (*P < 0.05, **P < 0.01)

Fig. 3

Effect on pre- and post-morphine thermal latencies in the plantar test of the following treatments: 1) non-LPS/IgG (n=6); 2) LPS/IgG (n=6); 3) LPS/anti-NGF (n=6); and 4) non-LPS/anti-NGF (n=6). LPS increases morphine analgesia in IgG-treated rats, and anti-NGF reduces morphine analgesia only in the LPS condtion. (**P < 0.01)

Fig. 4

Effect on pre- and post-morphine thermal latencies in the plantar test of the following treatments: 1) non-LPS/LY225910 (n=6) 2) LPS/vehicle (n=6) 3) LPS/anti-NGF (n=6) 4) LPS/anti-NGF/LY225910 (n=6). Anti-NGF again reduces morphine analgesia in LPS-treated rats, and LY-225910 enhances morphine analgesia in LPS animals only in the presence of anti-NGF. (** P < 0.01)

Fig. 5

Effect on pre- and post-morphine thermal latencies in the plantar test of animals treated for 7 days with chronic β-NGF (7 μg/day; n=6) compared to control animals treated for 7 days with artificial CSF (n=6). Morphine analgesia is enhanced in animals treated with chronic β-NGF. (*P < 0.05)

Fig. 6

A) Concentration of CCK-8S in spinal cord extracts in saline control and LPS-treated rats (n=6–7). There is a significant increase in CCK-8S levels in LPS-treated animals as compared to control animals. (*P < 0.05) B) Morphine-induced change from baseline of CCK concentration in saline control, LPS-treated, and after anti-NGF pretreatment inI LPS-treated rats (n=6–9). Morphine-induced CCK release is inhibited in LPS-treated animals, an effect that is reversed with anti-NGF treatment. (*P < 0.05)