Cocaine facilitates the effects of alpha-agonists and antagonists on adrenergic neurotransmission

Cocaine synergism with α agonists in rat aorta: computational analysis reveals an action beyond reuptake inhibition

BACKGROUND

Cocaine has long been known to increase blood pressure, but the degree and mechanism of vasoconstricting action remain poorly understood. Here we examine the interaction between cocaine and alpha-adrenoceptor agonists, with the action of reuptake inhibition minimized.

METHODS

Cocaine was administered to isolated rings of rat thoracic aorta, alone and in combination with three different adrenoceptor agonists: phenylephrine, methoxamine, and norepinephrine. Synergy analysis begins with the predicted additive effect of the combination of two agonists, based upon dose equivalence theory. This case where one agonist (cocaine) has no effect when administered alone requires only a t-test to demonstrate that a departure from additivity has occurred.

RESULTS

At doses where cocaine alone produced no vasoconstriction, it potentiated the vasoconstriction produced by all three alpha agonists, a clear indication of synergism between cocaine and these agents. Higher doses of cocaine in combination with alpha adrenoceptor agents gave an inverted-U shaped (hormetic) dose-effect curve, i.e., dose-related relaxation at higher doses. The hormetic dose-effect relation was analyzed using computational methodology based on dose equivalence to derive the unknown second component of action that causes relaxation.

CONCLUSIONS

Cocaine exhibits both vasoconstricting and vasorelaxant effects. This relaxing component, possibly related to activation of myosin light chain phosphatase, was quantified as a dose-effect curve. Most important is the synergism between cocaine and alpha-adrenoceptor stimulation which cannot be explained as an action due to reuptake inhibition, and has not been previously described.

Effect of blockade of noradrenaline re-uptake on evoked tritium overflow from mouse vasa deferentia and rat cortex slices

1. In tissues previously incubated with [3H]-noradrenaline exposure to cocaine (0.1 to 10 microM) or desmethylimipramine (0.01 to 1 microM) produced a concentration-dependent increase (up to 2 fold) in electrically evoked (3 Hz, 2 ms, 20 mA, 120s every 20 min) fractional overflow of tritium from rat brain cortex slices but not from mouse vas deferens (2.5 Hz, 2 ms, 400 mA, for 90s every 14 min). 2. Yohimbine and idazoxan (0.01 to 1 microM) increased fractional evoked overflow of tritium by up to 2 fold; in the presence of these drugs, cocaine (10 microM) produced an increase in both tissues (up to 3.5 fold over control). 3. In brain slice an increase in stimulation frequency (0.1, 0.5, 1, 3 and 6 Hz) decreased fractional evoked overflow of tritium per pulse but cocaine (10 microM) produced a significant enhancement at each frequency except 6 Hz. In vas deferens fractional tritium overflow per pulse changed little with increasing frequency and cocaine produced no effect. 4. In both tissues fractional evoked overflow of tritium was dependent on the stimulation current; cocaine (10 microM) increased fractional evoked overflow from brain slice at every current tested but was without effect in vas deferens. 5. Chromatographic separation of the released tritium showed there was little difference in the proportions of [3H]-noradrenaline and 3H-metabolites overflowing from the tissues. Cocaine increased the proportion of [3H]-noradrenaline and decreased the proportion of [3H]-DOPEG overflowing both at rest and during stimulation. 6. In brain slice an increase in electrically evoked overflow was produced by cocaine (10 microM) whether total tritium overflow (1.8 fold), overflow of [3H]-noradrenaline (1.8 fold) or overflow of unlabelled noradrenaline (1.8 fold) was measured. Evoked overflow from vas deferens was unaffected when assessed by any of these three methods. 7. The mechanism responsible for this differential effect of cocaine is unclear but may involve differences in the physical relationship between release sites, reuptake sites and presynaptic autoreceptors.

Modulation of noradrenaline release from cat cerebral arteries by presynaptic alpha 2-adrenoceptors. Effect of chronic treatment with desipramine and cocaine

1. Field electrical stimulation elicited an increase of the tritium efflux over the basal level from cat cerebral arteries previously incubated with (+/-) [3H]noradrenaline ([3H]NA). 2. This efflux was: (a) reduced by clonidine, NA or B-HT 920; (b) unaffected by methoxamine, prazosin and yohimbine (10(-6) M); (c) reduced by yohimbine (5 x 10(-6) M), and (d) increased by phentolamine. 3. The effect of clonidine was blocked by yohimbine. 4. The daily treatment with the neuronal uptake blockers desipramine (10 mg/kg, i.p.) or cocaine (10 mg/kg, i.p.) [during 12 days], antagonized the inhibitory action of clonidine totally or partially, respectively. 5. These results suggest: (1) the existence of presynaptic alpha 2-adrenoceptors in these arteries, which modulate the NA release, and (2) that chronic treatment with desipramine or cocaine induces a subsensitivity of these alpha 2-receptors, which facilitates the NA release.

Amphetamine antagonizes the presynaptic inhibitory effect of clonidine through an interaction at the level of the alpha 2-adrenoceptors

1. In the whole rat vas deferens 20 microM noradrenaline (NA) and 0.011 microM clonidine decreased (36 +/- 7.4% and 80 +/- 6.0% respectively) the motor response induced by hypogastric nerve stimulation. These effects were reverted by 1 microM yohimbine. Amphetamine 5.4 microM failed to antagonize the inhibitory effect of NA and attenuated clonidine effect. 2. The effect of amphetamine was not altered by preincubation with either cocaine 1 microM, (-)-propranolol 0.3 microM or cocaine plus prazosin 0.028 microM. 3. In reserpine pretreated animals amphetamine 5.4 microM shifted to the right the concentration-response curve (CRC) to clonidine 0.62 +/- 0.05 log units with a KB value of 1.83 +/- 0.30 microM. 4. Binding of [3H]clonidine and [3H]prazosin were inhibited by amphetamine. Amphetamine was 90 times more potent to inhibit [3H]clonidine binding. 5. The results obtained suggest a possible direct interaction between clonidine and amphetamine on alpha-adrenoceptor.

Increase of alpha 2-presynaptic adrenoceptor response after neuronal uptake inhibition in vivo

alpha 2-adrenoceptor antagonists, yohimbine or idazoxan (1 mg kg-1 i.p.), administered alone, did not change noradrenaline content in the central and peripheral tissues of the rat (hypothalamus, brain stem, frontal cortex and heart). The inhibition of neuronal uptake by desipramine (DMI) administered alone or prior to alpha 2-adrenoceptor antagonists did not affect the neurotransmitter content either. alpha-methyl-p-tyrosine (alpha-MT) 6 hr before sacrifice, induced a marked disappearance of the noradrenaline content, greater in central nervous tissues than in heart. When the catecholamine synthesis was inhibited by alpha-MT, neither alpha 2-adrenoceptor antagonists nor DMI at the dose used, significantly changed the disappearance rate of noradrenaline in any of the tissues studied. Under these experimental conditions, however, the combination of DMI plus alpha 2-adrenoceptor antagonists significantly decreased the neurotransmitter content in all tissues when the values were compared with the control or DMI-treated groups. The present results might suggest evidence in favour of a functional coupling between presynaptic alpha 2-autoreceptors and noradrenaline uptake mechanism.

Prolonged treatment with (+/-) propranolol induces supersensitivity to (L)noradrenaline in mesenteric arteries in the rat

In the isolated perfused mesenteric arteries of the rat, neither (+/-) propranolol (0.1 microM) nor (+/-)isoproterenol (0.05 microM) modified the overflow of DL-[3H]noradrenaline (DL-[3H]NA) induced by sympathetic nerve stimulation at either 5 or 10 Hz. The blockade of alpha presynaptic receptors with phentolamine (4.7 microM) increased the 3H-transmitter overflow at 5 and 10 Hz. (+/-)Propranolol (0.1 microM) failed to modify this effect. Vasoconstrictor responses to exogenous NA or sympathetic nerve stimulation were not modified by (+/-)propranolol (0.1 microM). Prolonged treatment with (+/-)propranolol (7 mg/kg) for 15 days potentiates responses to both exogenous NA and sympathetic nerve stimulation; however, the fractional release per pulse of DL-[3H]NA was not modified at either 5 or 10 Hz. These results provide no evidence to support the hypothesis that the release of NA is regulated by presynaptic beta-adrenoreceptors in the mesenteric arteries of the rat. The enhancement of vascular responses after prolonged treatment with propranolol could be caused by postsynaptic supersensitivity.