Jewell W. Sloan

Innervation of the spinal cord by sympathetic fibers

Abstract Pharmacologic and neurochemical studies suggest that catecholamines are still present below the level of transection in the spinal cord of the chronic spinal animal, despite the degeneration of bulbospinal catecholamine pathways. Histofluorescence studies of rat and dog spinal cord revealed noradrenergic fibers and varicosities remaining in the chronically decentralized spinal cord which can account for the low concentrations of norepinephrine (NE) found below the transection. The fibers appear to enter the spinal cord with blood vessels through the anterior median fissure, and are probably of sympathetic origin. In the spinal cord, these fibers can dissociate from blood vessels and continue through the neuropil; they are associated with neurons in the ventral horn and occasionally in the central gray. These peripheral sympathetic fibers may influence motor systems and other nervous functions.

Nature of nicotine binding to rat brain P2 fraction

(-)-Nicotine may bind to as many as 5 sites in the rat brain P2 preparation: A very high affinity site (KD approximately 2.2 X 10(-11) M); a positive cooperativity site; a high affinity site (KD approximately 5.2 X 10(-9) M); a low affinity site (KD approximately 4.5 X 10(-5) M) and a very low affinity site. The curvilinear nature of both Scatchard plots and kinetic curves indicates the presence of multiple binding sites. Evidence for a positive cooperativity site includes: (1) The configuration of Scatchard plots (at low concentrations) of saturation as well as inhibition curves for (-)- and (+)-nicotine. (2) The Hill number of 1.37 for the binding of low concentrations of (+/-)-[3H]nicotine. (3) Selectivity among cholinergic drugs for producing positive cooperativity. (4) Markedly different specificities of drugs for the positive cooperativity site. Thus while only (+)- and (-)-nicotine interacted with the very high affinity site, acetylcholine, atropine, mecamylamine, lobeline, carbachol, (+)-nicotine and (-)-nicotine enhanced the binding of (+/-)-[3H]nicotine and cytisine, anabasine, cotinine and choline selectively inhibited binding at the high affinity site. Several lines of evidence indicate that there is stereospecificity. (+)-Nicotine was more potent than (-)-nicotine in inducing positive cooperativity whereas (-)-nicotine was 80 times more potent than (+)-nicotine in inhibiting binding at the high affinity site. Further, the specificity of the binding sites can be altered by changing the concentration of the buffer which gives additional evidence for the lability of the nicotine binding site. Although the pharmacologic significance of the different binding sites has not been determined, these data taken together indicate that (+/-)-[3H]nicotine binds with specificity to multiple sites in the rat brain P2 preparation with a complexity not addressed heretofore.

Interaction between nicotine and endogenous opioid mechanisms in the unanesthetized dog

Nicotine produced a distinct reproducible syndrome in the conscious dog when injected intravenously or intracerebroventricularly. Intravenously administered nicotine (40 micrograms/kg/min for 20 minutes) increased cardiac and respiratory rates and produced analgesia, miosis, hypothermia, behavioral restlessness and emesis. When microinjected into the third cerebral ventricle, nicotine (100-200 micrograms) similarly increased cardiac and respiratory rates and pupillary diameter; and produced behavioral restlessness, emesis, erratic analgesia and maintained wakefulness and a desynchronized EEG. Microinjection of nicotine (5-25 micrograms) into the periaqueductal gray failed to alter any of the parameters studied. Intravenous pretreatment with the opioid antagonist naltrexone (2 mg/kg) influenced the action of intravenous nicotine on certain physiological systems. While naltrexone alone produced a significant degree of tachycardia, miosis, and analgesia, it potentiated the tachypnea and antagonized the miotic response evoked by nicotine. Methionine-enkephalin was detected in perfusates obtained from the lateral cerebral ventricles of conscious dogs. Nicotine produced a non-significant decrease in enkephalin levels. These observations suggest that there are interactions between endogenous opioid and nicotinic processes. However, they are complex and may differ from one functional system to another.

Precipitated abstinence in the diazepam-dependent rat

Physical dependence was produced in the rat by exposure to continuous release of diazepam from silastic capsule implants (recrystallized diazepam) or by dosing through a gastric fistula. The precipitated abstinence syndrome induced by the IV infusion of flumazenil was characterized by clonic and tonic-clonic seizures, retropulsion, digging, rearing, head, limb and body tremors, twitches and jerks of the body, and ear twitches. This abstinence syndrome differed both qualitatively and quantitatively from the milder syndrome induced in previous experiments by the intragastric administration of flumazenil in the diazepam-dependent gastric fistula rat. Capsule-implanted rats had free plasma and extraneuronal brain levels of diazepam, oxazepam, and nordiazepam in the 10(-3) and 10(-4) mg/ml range, and their brain: plasma ratios were not significantly different from 1. The diazepam capsules had a sustained release of over 28 days. These studies show that the capsule implantation technique is an efficacious way of maintaining plasma levels of diazepam and its metabolites, and producing a high level of physical dependence in the rat.

Morphine tolerance in male and female rats

Several studies indicate greater sensitivity to morphine (MOR) analgesia in male compared to female rats under the acute dosing condition. The present study investigated whether the same sex difference in sensitivity persists in MOR-tolerant rats. MOR was administered chronically (7 mg/kg twice daily) until tolerance developed in each rat. Tolerant rats were treated randomly with higher graded doses of MOR (10-25 mg/kg). Analgesia (tail-flick test) and spontaneous motor activity (total locomotion) were measured. The present data confirmed previous studies showing a greater sensitivity to acute MOR in male than in female rats. However, the sex differences seen in MOR sensitivity were abolished in tolerant rats. The rate of acquisition of tolerance was similar in male and female rats. The analgesic response was not affected by motor depression.

Modification of morphine analgesia and tolerance by flumazenil in male and female rats

This study assessed the effect of the central benzodiazepine receptor antagonist, 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester (flumazenil), on morphine-induced analgesia, locomotor effects, and development of tolerance in rats. The thermally evoked pain (tail flick) response was determined after acute and chronic intraperitoneal (i.p.) administration of morphine and flumazenil, alone and in combination. In acute studies, flumazenil induced weak analgesia unrelated to dose and sex, whereas morphine-induced analgesia was dependent on both dose and sex (male>female). Flumazenil dose-dependently enhanced the analgesic effect of morphine in female but not in male rats. Isobolographic analysis suggested synergism between flumazenil and morphine in female rats, but antagonism in male rats. Flumazenil-induced locomotor changes (alone and with morphine) were related to sex but not dose. Chronic coadministration of flumazenil with morphine enhanced analgesia and attenuated tolerance development in female rats. The findings suggest a possible role for flumazenil as an adjunct with opioids in acute and chronic pain therapy.

Binding characteristics of (−)- and (+)-nicotine to the rat brain P2 fraction☆

Saturation studies employing (-)- and (+)-[3H]nicotine indicate that the isomers bind to different very high and high affinity sites since the binding density for (-)-[3H]nicotine is 10 times that for (+)-[3H]nicotine. Both isomers also bind to a low affinity site (KDS = approximately 10(-5) to 10(-4) M). Competition studies employing unlabelled (-)- and (+)-nicotine reveal greater complexities. The isomers also appear to bind to a separate site which enhances binding at the (-)- and (+)-nicotine high affinity sites. (+)-Nicotine is more effective in increasing the binding of (-)-[3H]nicotine at its high affinity site than (-)-nicotine. Further, (+)-nicotine has a greater specificity for enhancing binding than (-)-nicotine in that it enhances (-)-[3H]nicotine binding at lower concentrations and inhibits binding at higher concentrations than (-)-nicotine.

Advances in cancer pain management.

Control of malignant pain and related symptoms is paramount to clinical success in caring for cancer patients. To achieve the best quality of life for patients and families, oncologists and palliative care clinicians must work together to understand problems related to psychologic, social, and spiritual pain. Pain is the primary problem targeted for control using the World Health Organization’s (WHO) analgesic ladder. This article focuses on increased knowledge of analgesic action that may enable expansion of the WHO analgesic ladder to fulfill the broader objectives of palliative medicine. We discuss clinical experience with several classes of drugs that are currently used to treat cancer pain: 1) nonsteroidal anti-inflammatory drugs, with emphasis on cyclooxygenase-2 inhibitors; 2) opioid analgesics, with specific emphasis on methadone and its newly recognized value in cancer pain; 3) ketamine, an antagonist at N-methyl-D-aspartate receptors; and 4) bisphosphonates, used for pain resulting from bone metastases. New concepts that compare molecular actions of morphine at excitatory opioid receptors, and methadone at nonopioid receptor systems, are presented to underscore the importance of balancing central nervous system excitatory (anti-analgesic) versus inhibitory (analgesic) influences.

Opioid and nicotinic medullary hyperalgesic influences in the decerebrated rat.

The effects of ethylketazocine (EKC) administered intraperitoneally and the nicotinic ligands (-)- and (+)-nicotine, (-)-cytisine, (-)-lobeline, and (+)-2-methylpiperidine administered into the 4th ventricle on the latency of the thermally evoked withdrawal reflex of the decerebrate rat were investigated. EKC administered intraperitoneally produced both hyperalgesia and analgesia. (-)-Nicotine administered into the 4th ventricle produced a biphasic dose related effect on the latency of the withdrawal reflex; low doses produced a dose related analgesia while higher doses produced hyperalgesia. (-)-Cytisine and (-)-lobeline administered into the 4th ventricle produced biphasic effects. (+)-2-Methylpiperidine administered into the 4th ventricle produced a significant degree of hyperalgesia. Both the analgesic and hyperalgesic effects of (-)-nicotine were antagonized by mecamylamine (1 mg/kg) and naltrexone (5 mg/kg). The hyperalgesic action of (+)-2-methylpiperidine was antagonized by naltrexone but not by mecamylamine. These observations suggest that there are both medullary opioidergic and nicotinic cholinergic mechanisms for modulating both analgesic and hyperalgesic processes and that nicotinic ligands have multiple mechanisms of action in the brain.

Interactions of "ultra-low" doses of naltrexone and morphine in mature and young male and female rats.

Sex and age influence morphine analgesia in humans and animals. Mature rats show greater morphine analgesia in males than in females. Ultra-low doses of naltrexone enhance morphine analgesia. In mature rats (18-22 weeks), naltrexone (0.002-2.0 mg/kg)-morphine (2 mg/kg) cotreatment enhanced morphine analgesia in females, an effect inversely related to naltrexone dose. Conversely, in mature male rats, naltrexone tended to decrease morphine analgesia with increasing dose. In young rats (8-10 weeks), morphine analgesia was unrelated to sex and in both sexes the naltrexone-morphine interaction was negligible. These data show that dose, age, and sex alter the naltrexone-morphine interaction in rats.