Effect of chronic morphine treatment on alpha(2)-adrenoceptor mediated autoinhibition of transmitter release from sympathetic varicosities of the mouse vas deferens

Abstract

1. The effect of chronic morphine treatment (CMT) on sympathetic innervation of the mouse vas deferens and on alpha(2)-adrenoceptor mediated autoinhibition has been examined using intracellular recording of excitatory junction potentials (EJPs) and histochemistry. 2. In chronically saline treated (CST) preparations, morphine (1 microM) and the alpha(2)-adrenoceptor agonist (clonidine, 1 microM) decreased the mean amplitude of EJPs evoked with 0.03 Hz stimulation by 81+/-8% (n=16) and 92+/-6% (n=7) respectively. In CMT preparations, morphine (1 microM) and clonidine (1 microM) decreased mean EJP amplitude by 68+/-8% (n=7) and 79+/-8% (n=7) respectively. 3. When stimulating the sympathetic axons at 0.03 Hz, the mean EJP amplitude recorded from smooth muscles acutely withdrawn from CMT was four times greater than for CST smooth muscles (40.7+/-3.8 mV, n=7 compared with 9.9+/-0.3 mV, n=7). 4. Part of the increase in mean EJP amplitude following CMT was produced by a 31% increase in the density of sympathetic axons and varicosities innervating the smooth muscle. 5. Results from the present study indicate that the effectiveness of alpha(2)-adrenoceptor mediated autoinhibition is only slightly reduced in CMT preparations. Most of the cross tolerance which develops between morphine, clonidine and alpha(2)-adrenoceptor mediated autoinhibition occurs as a consequence of increased efficacy of neuromuscular transmission which is produced by an increase in the probability of transmitter release and an increase in the density of sympathetic innervation.

Figure 1

Effect of morphine and clonidine on EJP amplitude in CST and CMT preparations. The mean EJP amplitude±s.e.mean (n⩾7) is shown on the ordinate and treatment of the vasa deferentia is indicated by the key. The concentration of morphine and clonidine added to the bathing solution was 1 μM. Nerves were stimulated by trains of impulses, 30 V (strength), 0.1 ms (duration) and 0.03 Hz.

Figure 2

Effect of increasing the stimulus intensity on the mean amplitude of EJPs from CST and CMT preparations. Symbols represent different treatment groups as indicated by the key. Symbols indicate the mean EJP amplitude±s.e.mean (n⩾7) in mV. The nerves were stimulated at 0.03 Hz.

Figure 3

Facilitation of EJP amplitude at different stimulus intensities from CST and CMT preparations. In (A) and (B) no drug was present in the bathing solution while in (C) and (D) idazoxane (1 μM) was added to the bathing solution. (A) and (C) are results obtained from CST preparations while (B) and (D) are results obtained from CMT preparations. The normalized amplitude of EJPs with respect to the first EJP in a train is plotted. The symbols indicate the voltage used to stimulate the sympathetic axons: ▪ 15 V; • 20 V; ▴ 30 V. Symbols with a value greater than 1 on the ordinate indicates facilitation. Dashed lines indicate the predicted level of facilitation assuming a time constant of decay in facilitation of 6 s. The difference between the predicted and experimental level of facilitation was used as a measure of the depression of EJP amplitude (see Figure 4). EJPs were evoked by trains of five stimuli at 1 Hz.

Figure 4

Effect of idazoxane on depression of EJP amplitude in CST and CMT preparations. The level of depression of EJPs during stimulation by trains of impulses (5 at 1 Hz). (A) stimulus intensity of 15 V and (B) stimulus intensity of 20 V. In (A) and (B) symbols indicate the mean level of depression while positive going and negative going lines indicate plus or minus s.e.mean (see key for explanation of each symbol). Dashed line indicates no depression. (C) normalized EJP amplitude of the 5th EJP amplitude with respect to the 1st EJP amplitude is indicated on the ordinate while on the abscissa tissue treatment is indicated. Three levels of stimuli intensities are indicated on the abscissa (15, 20 and 30 V). Bars indicate the mean normalized EJP amplitude while lines on top of bars indicate±s.e.mean (n>7). The different bar patterns in (C) indicate different experimental conditions as indicated by the key.