Factors affecting the action of guanethidine on adrenergic neurones

Effects of sympathectomy in a model of causalgiform pain produced by partial sciatic nerve injury in rats

In a previous report we presented a novel behavioral model of neuropathic pain disorders, produced in rat by a unilateral ligation of about half of the sciatic nerve. The model is characterized by rapid onset of behaviors suggesting spontaneous pain and disordered responses to non-noxious and noxious stimuli. These include reduced withdrawal thresholds to repetitive touch in the partially deafferented skin ('touched-evoked hyperesthesia'), touch-evoked allodynia, reduced withdrawal thresholds to noxious thermal stimuli and exaggerated responses to noxious heat and mechanical stimuli ('thermal hyperalgesia'). Some of these disorders are seen at mirror image sites on the hind limb opposite the lesion. These disorder start within hours after partial nerve injury, last many months and are very similar to causalgia in humans following partial nerve injury. Since sympathetic efferent activity is known to aggravate causalgia in humans and sympathectomy is known to relieve it, we studied the effect of changing sympathetic outflow in the rat model. Reversible sympathectomy was carried out using guanethidine injected intraperitoneally in 3 experiments, each at a different time in relation to the partial nerve injury. We found that: (1) sympathectomy performed several months postoperatively alleviated the sensory disorders bilaterally; (2) sympathectomy prior to nerve injury partially prevented the appearance of thermal hyperalgesia but did not affect hyperesthesia to repetitive touch; and (3) sympathectomy at the time of nerve injury aggravated the sensory disorders during the first few days. As maintenance and production of the sensory disorders in this animal model depended on sympathetic nervous outflow, we conclude that the rats were suffering from a syndrome analogous to sympathetically maintained causalgia in man.

Sympathetically-maintained causalgiform disorders in a model for neuropathic pain: a review

Partial nerve injury is the main cause of sympathetically maintained causalgiform pain disorders in humans. We present here an animal model of this condition, produced in rats by a unilateral ligation of about half of the sciatic nerve. Starting hours after the operation and for several months thereafter, the rats developed signs of spontaneous pain, touch-evoked allodynia and hyperesthesia, and mechanical and thermal hyperalgesia in the partially denervated as well as the intact contralateral foot. These disorders were maintained by the sympathetic outflow and disappeared following postoperative sympathectomy. In neonatally capsaicinated rats we found that touch-evoked allodynia and hyperesthesia were mediated by A-fibers whereas thermal hyperalgesia was mediated by C-fibers. These disorders were not due to receptor sensitization of remaining afferent fibers by prostaglandins. We found strain differences and genetic inheritance of these causalgiform disorders which were correlated with the expression of autotomy to hind-paw denervation.

Drug interactions that matter. A critical reappraisal

Drug interactions are ubiquitous but those with proven clinical relevance are much less common. Only when the combined effects of the interacting drugs are greater or less than the arithmetic sum of their individual actions can the event be considered a true interaction. This eliminates many candidate 'interactions' which in reality merely describe the summation of similar or opposing, but independent, drug effects. An appreciation of those drug interactions that really do matter can be best achieved by combining a practical knowledge of the pharmacological mechanisms involved with awareness of the most vulnerable patients (those with little reserve capacity) and the drugs associated with the greatest risk (those with a narrow therapeutic index). This review follows these guidelines and provides an account of well documented drug interactions categorised according to mechanism.

Presynaptic beta-adrenoceptors

The existence of facilitatory presynaptic beta-adrenoceptors has been shown in approximately 30 tissues of 6 different species including human. A positive feed back loop for further release of the transmitter appears to be activated by an endogenous agonist, epinephrine, taken up and released as a cotransmitter with norepinephrine rather than norepinephrine itself released from peripheral noradrenergic nerve terminals. Presynaptic beta-adrenoceptors are mainly of a beta 2-subtype. Some beta 1-subtype receptors are also suggested. There coexist presynaptic beta 1- and beta 2-adrenoceptors in cat and rat hypothalamus. Higher sensitivity of peripheral presynaptic beta-adrenoceptors to isoproterenol may be implicated in the early development of hypertension in SHR. Epinephrine taken up and released initiates the development of hypertension in rats via activation of these receptors. Increased activation of these receptors by epinephrine may play a role in the development of essential hypertension. The antihypertensive action of beta-antagonists may be in part due to blockade of these facilitatory presynaptic beta-adrenoceptors.

Comparative studies of (--)-, (+/-)-propranolol, atenolol, guanethidine, bretylium and tetracaine on adrenergic transmission

1 Effects of (--)-, (+/-)-, and (+)-propranolol, atenolol, guanethidine, bretylium and tetracaine were studied on relaxation responses of rabbit ileum and contractile responses of rabbit pulmonary artery and guinea-pig vas deferens to electrical nerve stimulation (2 to 50 Hz). 2 In the ileum, inhibition by tetracaine 3.3 x 10(-6) M occurred at high frequencies of stimulation, while bretylium 1.2 x 10(-4) M and guanethidine 2 x 10(-5) M inhibited responses at all frequencies, the latter producing greater inhibition at low frequencies. 3 (+/-)-Propranolol 10(-5) M produced a tetracaine-type inhibition after 1 h and a bretylium-pattern after 2 h in the ilea and pulmonary arteries and a transition from bretylium- to guanethidine-pattern in the vas deferens, while atenolol 2 x 10(-5) to 10(-4) M produced guanethidine-type inhibition in all preparations. 4 (--)-, (+/-)-, and (+)-Propranolol 3 x 10(-6) to 3.3 x 10(-5) M were equipotent in the vas deferens and ileum. However, inhibition by (--)-propranolol 3.3 x 10(-5) M persisted in the ileum, while that by the (+)-isomer was partially restored by washing. 5 (--)- or (+)-Propranolol 3.3 x 10(-5) M or atenolol 2 x 10(-5) M did not inhibit relaxation of the ileum after the bath temperature was maintained at 4 degrees C for 2 h during drug application. 6 In conclusion, propranolol and atenolol both have gradually developing guanethidine-like adrenergic neurone blocking actions.

Further evidence for the prevention by guanethidine of noradrenaline efflux from rabbit ventricular slices induced by a low external sodium concentration

When guanethidine was applied to rabbit ventricular slices at 37 degrees C for an initial 30 min incubation period, doses of 4 X 10(-5) and 8 X 10(-5) M irreversibly inhibited increases in efflux of noradrenaline induced by the subsequent reduction of external sodium concentrations to 18 mM. This inhibition was prevented if the temperature was reduced to 0 degrees C during the drug application period. Guanethidine transported into adrenergic nerve endings appears to increase permeability to Na+, thereby leading to the reduction in efflux.

Selectivity of neuronal degeneration produced by chronic guanethidine treatment

Chronic guanethidine treatment of rats produced extensive damage to sympathetic neurons of the superior cervical ganglion and pelvic plexus. No ultrastuctural changes were observed in parasympathetic cholinergic neurons in the ciliary ganglion and pelvic plexus, nor in sensory neurons in nodose and dorsal root ganglia. A total of only six nerve cell bodies free of degenerative changes were observed in sections of superior cervical ganglia from 20 rats. This suggests either that the earlier estimates of 5% cholinergic neurons in the superior cervical ganglion based on acetylcholinesterase staining are too high, or implies that sympathetic cholinergic neurons, unlike parasympathetic neurons, are damaged by chronic guanethidine treatment.

Some characteristics of the adrenergic neurone blocking action of dehydroemetine

The uptake of dehydroemetine by adrenergic neurones was studied indirectly by testing the ability of various procedures to prevent or reverse adrenergic neurone blockade in the periarterially stimulated rabbit isolated ileum. Adrenergic neurone blockade was prevented but not reversed by equilibration with dehydroemetine at low temperature (0 degrees C), in the absence of sodium or in the presence of tetrodotoxin. Noradrenaline, cocaine, potassium deprivation and potassium excess did not modify the adrenergic neurone blocking action of dehydroemetine.