Electrophysiological studies on the postnatal development of the spinal antinociceptive effects of the delta opioid receptor agonist DPDPE in the rat

Abstract

1. The antinociceptive effects of the delta opioid receptor selective agonist, DPDPE [(D-Pen2,D-Pen5)-enkephalin] was studied in rats aged postnatal day (P) 14, P21, P28 and P56. 2. Antinociceptive effects of DPDPE were measured as percentage inhibition of the C-fibre evoked response and post-discharge of dorsal horn neurones evoked by peripheral electrical stimulation. DPDPE was administered by topical application, akin to intrathecal injection. 3. DPDPE (0.1-100 microg) produced dose-related inhibitions at all ages; these inhibitions were reversed by 5 microg of the opioid antagonist naloxone. 4. The dose-response curves for C-fibre evoked response and post-discharge of the neurones were not different in rats aged P14 and P21. DPDPE was significantly more potent at P14 and P21 compared with its inhibitory effects on these responses at P28 and P56. 5. DPDPE produced minor inhibitions of the A-fibre evoked response of the neurones at P14, P21, P28 and P56, suggesting that the inhibitory effects of DPDPE are mediated via presynaptic receptors on the terminals of C-fibre afferents. 6. Since spinal delta opioid receptor density changes little over this period, the increased antinociceptive potency of DPDPE in the rat pups compared with the adult is likely to be due to post-receptor events, or in developmental changes in the actions of other transmitter/receptor systems within the spinal cord.

Figure 1

Evoked neuronal responses of convergent dorsal horn neurones in rats at P14, P21, P28 and P56. (A) The thresholds (mA) for the C-fibre evoked responses with age. ANOVA analysis showed there was an overall significant variation with age, F_3,131=3.748; P=0.013, P21 and P28 thresholds were significantly different in comparison with P56 threshold (^*P<0.05), Student-Newmans-Keuls post-hoc test. (B) The neuronal responses evoked by the train of stimuli, expressed as action potentials (AP), for the C-fibre, post-discharge and A-fibre evoked responses. ANOVA analysis showed there was an overall significant variation with age for: C-fibres, F_3,131=40.34, P<0.0001; PD, F_3,131=7.92, P<0.0001; A-fibres, F_3,131=16.6, P<0.0001. P14, P21 and P28 responses were significantly different in comparison with responses at P56 (^*P<0.05), Student-Newmans-Keuls post-hoc test. n=23 at P14, n=35 at P21, n=22 at P28, n=43 at P56. Values are mean±s.e.mean.

Figure 2

Dose response curves showing the inhibitory effects of DPDPE on the C-fibre evoked response at different postnatal ages; P14, P21, P28 and P56 (adult). Comparison of derived ED₅₀ values indicate that DPDPE was more potent at the earlier postnatal ages compared with P28 and adult (see results). The order of potency was: P14=P21>P28=P56 (n=6–8 for each dose at each postnatal age).

Figure 3

Dose-related inhibitions of the post-discharge of convergent neurones were produced by topical application of DPDPE (μg) in rats at P14, P21, P28 and P56 (adult). The potency order for spinal DPDPE was as follows: P14=P21>P28>P56 (n=6–8 for each dose at each postnatal age).

Figure 4

Dose response curves for the inhibition of A-fibre evoked responses by spinal application of DPDPE on dorsal horn neurones at different postnatal ages (n=6–8 for each dose at each postnatal age).

Figure 5

Antagonism of DPDPE (50 μg) effects on the C-fibre evoked response and post-discharge of the neurones by the selective δ receptor antagonist naltrindole (50 μg). ^*P<0.05, Mann-Whitney t-test.

Figure 6

The effect of morphine (2.5 μg) in the presence of the selective δ receptor antagonist naltrindole (50 μg) was unchanged (n=4). The inhibition of the C-fibre evoked response and post-discharge of dorsal horn neurones in P14 rat pups was identical to that previously reported for morphine alone (Rahman et al., 1998). Naloxone (1 μg) significantly reversed the inhibitory effects of morphine produced in the presence of naltrindole (50 μg). ^*P<0.05, Mann-Whitney t-test.