James R. Weeks

Chapter 6 – Long-term Intravenous Infusion

Publisher Summary This chapter discusses long-term intravenous infusion. Long-term intravenous infusions or frequent intermittent injections require that the animals be un-anesthetized and relatively unrestrained. A study of drug abuse whereby an unrestrained rat could self-administer a morphine sulfate solution intravenously and maintain an experimental addiction combines the experimental techniques of behavioral psychology and pharmacology. Long-term intravenous infusion has several other research applications. Within The Upjohn Company, the technique has been used for the intravenous safety tests of up to 30 days duration to study the effects of prostaglandins in essential fatty acid deficient rats; to demonstrate diuretic activity of ethacrynic acid in rats; combined with an aortic cannula to evaluate blood pressure responses in un-anesthetized rats; to study the effect of chronic morphine administration and addiction on the EEG of rats; and to prepare quantities of urinary drug metabolites by infusing the parent drug for three weeks. The chapter focuses on experimental drug abuse studies of not only opiates but central nervous system stimulants, barbiturates, and tranquilizers.

Prediction of abuse liability of drugs using IV self-administration by rats

A total 31 psychoactive drugs were offered to groups of naive rats for IV self-administration and an injection rate greater than that for rats offered only saline indicated reinforcement. Two protocols were used: in the first, rats were offered drug at a selected dose for 5 days, then the dose was reduced by 1 log unit (to 0.1 the original dose) for an additional 4 days; in the second, rats were offered saline for 3 days as a ‘prescreen’ to eliminate rats with high or low operant-injection rates. Drug was offered to acceptable rats for 5 days, then the dose was reduced 0.5 log unit (to 0.32 the original dose) for 5 more days. A scoring system, based upon the injection rates during the last 3 days of each period, describes the reinforcing action. Scores were dose-related. Tests on both protocols gave similar results. Data from monkey studies have been reported for 27 of the drugs tested. Of these drugs, 18 were reinforcers and six were nonreinforcers in both species, nalorphine and ethylketazocine were reinforcers only in rats, and ethanol was a reinforcer only in monkeys.

Factors affecting voluntary morphine intake in self-maintained addicted rats

SummarySelf-maintained morphine addicted rats were prepared by implanting chronic venous cannulas and fitting the rats with a device permitting relatively free movement and also enabling them to obtain morphine injections at will by pressing a lever. Three factors modifying voluntary morphine intake were studied. 1. Using a continuous reinforcement schedule, progressively decreasing the size of the morphine dose led to a greater number of doses daily. Compensation was incomplete in that the total daily morphine intake decreased. 2. Progressively increasing a fixed ratio reinforcement schedule up to about FR-75, caused rats to continue responding on the lever until the dose was obtained. Above FR-75 responding became intermittent and daily morphine decreased as the time interval between doses increased. 3. Continuous intravenous infusion of a second drug, leaving voluntary access to morphine at FR-10, led to decreased morphine intake following infusion of morphine itself, codeine and meperidine. Nalorphine infusion increased morphine intake. Effectiveness of infusions varied with the infusion rate.

Changes in morphine self-administration in rats induced by prostaglandin E1 and naloxone.

Interactions of prostaglandin E1 (PGE1) with morphine have been reported in several test systems and an hypothesis has been advanced for a role of prostaglandins in morphine analgesia and physical dependence. In rats self-administering morphine intravenously, a simultaneous and continuous infusion of naloxone hydrochloride at 56 to 560 mug/kg/day caused the expected increase in injection rate for morphine. Infusion of PGE1 by itself at 56 or 180 mug/kg/day had no effect on the rate of morphine intake. Likewise the addition of PGE1 at 180 mug/kg/day did not potentiate the increase caused by naloxone (56 or 180 mug/kg/day) when it was added to the naloxone infusion. These results do not support a role for prostaglandins in the behavioral aspects of morphine addiction. However, larger doses of PGE1 (1 and 1.8 mg/kg/day), which were without overt effects in normal rats, caused severe and incapacitating prostration in morphinized rats.

Biological activities of 17-phenyl-18,19,20-trinorprostaglandins.

In a number of assay systems, some 17-phenyl-trinor-prostaglandins were similar in activity and potency to the corresponding parent prostaglandin. In others, the 17-phenyl analogs appeared several times more potent. In the hamster antifertility assay, which is considered to measure luteolytic activity, 17-phenyl-18,19,20-trinor prostaglandin F2alpha was about 90-times PGF2alpha in potency. Rat blood pressure responses to 17-phenyl analogs were significant. The 17-phenyl-trinor PGF2alpha pressor potency was 5 times that of PGF2alpha. The 17-phenyl-trinor PGE2 blood pressure response was atypical since a pressor rebound phenomenon followed the expected depressor response. Lastly, 17-phenyl-trinor PGF2alpha was more potent than PGF2alpha in synchronizing the estrous cycle in beef cows.

Dose and physical dependence as factors in the self-administration of morphine by rats

Groups of naive rats were offered morphine sulfate for self-administration in doses of 0.0032–10 mg/kg for 6 days. On day 7 saline was substituted for morphine. Loss of weight was taken as physiological evidence of dependence. Rats that did not lose weight formed a single population whose mean injection rate did not differe from control rats receiving only saline injections. Injection rates for rats losing weight were log-normally distributed, and the mean of the logarithms of the injection rates was linearly related to the logarithm of the dose. Mean daily injection rates averaged 12 for controls, 23 at 10 mg/kg, and 411 at 0.01 mg/kg. A transient increase in morphine intake after an injection of nalorphine was taken as behavioral evidence of dependence. Nalorphine increased morphine intake when rats were self-injecting 0.32 and 1.0 mg/kg of morphine, but not 0.032 or 0.1 mg/kg. The reinforcing property of morphine may occur without behavioral evidence of dependence.

The cardiovascular pharmacology of prostacyclin (PGI2) in the rat

Physiological roles have been suggested for prostacyclin in the cardiovascular system. Prostacyclin was administered by intravenous infusion to unanesthetized rats. Over a 24 hr period, 0.32 mg/kg/day caused only flushing of the ears. Larger doses (0.56 and 1 mg/kg/day) caused hypothermia, behavioral depression, and swelling of the paws. Cumulative dose-response curves for its depressor action were determined in both unanesthetized and anesthetized, vagotomized, ganglion-blocked rats. In unanesthetized rats, the threshold dose was about 0.1 ug/kg/min. Respiratory depression precluded doses larger than 1 ug/kg/min. In anesthetized rats, the threshold dose was about 0.001 ug/kg/min, and the maximally effective dose was about 0.1 micrograms/kg/min. At 0.032 ug/kg/min, blood pressure first fell and then rose slightly. This compensatory rise did not occur in nephrectomized rats, suggesting renin release as the mechanism. Intravenous infusion of 0.1 but not 0.01 ug/kg/min in unanesthetized rats doubled plasma renin activity. In saline-loaded unanesthetized rats, urine volume and urinary sodium excretion were decreased by 0.1 ug/kg/min of prostacyclin.

Metabolism of prostacyclin. III. Urinary metabolite profile of 6-keto PGF1α in rat

The in vivo metabolism of 6-keto PGF1 alpha was investigated in rats. Following continuous intravenous infusion for 14 days the urinary metabolites were isolated and identified. A substantial amount of unchanged 6-keto PGF1 alpha was recovered in the urine. The metabolic pattern very closely resembles that of PGI2 in rats. Metabolites were found which represented 15-dehydrogenation, beta-oxidation, omega and omega-1-hydroxylation and oxidation. Previous work showed that 6-keto PGF1 alpha is very poorly oxidized by 15-PGDH. We administered 15-[H3]-PGI2 and 15-[H3]-6-keto PGF1 alpha to rats and measured urinary tritiated water as an index for in vivo 15-PGDH activity. The results showed that PGI2 and 6-keto PGF1 alpha were both oxidized to the 15-keto product, although the rate of oxidation of PGI2 was greater than that of 6-keto PGF1 alpha. We concluded that the administered PGI2 was oxidized by 15-PGDH before hydrolysis to 6-keto PGF1 alpha. A portion of the dose is probably hydrolzyed before 15-dehydrogenation.

The pneumatic syringe: A simple apparatus for self-administration of drugs by rats

Drug solution is delivered by a syringe operated by a pneumatic cylinder. Recommended delivery volumes are from 10 to 200 microliter. A solid-state control unit is described which can operate two syringes (drug injection and flush), has outputs for recording responses and injections, and can be programmed to provide several schedules of reinforcement. All components are readily commercially available.

Relative potency of codeine, methadone and dihydromorphinone to morphine in self-maintained addict rats

SummaryA method is described for obtaining potency estimates of narcotic analgesics to morphine in experimental addict rats. Drugs were administered through a chronic right heart cannula and intake was under the rats voluntary control. Animals were offered two doses of morphine (3.2 and 10 mg/kg/injection) and two doses of test compound on a fixed ratio reinforcement schedule of 10:1. Logarithm of the daily number of injections taken was used as the effect metameter. Each potency estimate was based on results in four rats. Rats averaged 137 mg/kg/day of morphine when offered 10 mg/kg/injection. A diurnal variation in opiate intake was observed, the nighttime rate being about one-third greater than the daytime rate. Morphine intake measured before and after the assay period was essentially unchanged. Potency estimates and the 95% fiducial interval were: dihydromorphinone = 10 (5.2−19) × morphine, methadone = 3.4 (2.7−4.6) × morphine and codeine = 0.67 (0.45−1.0) × morphine. Analgesic activity for codeine was not proportional to its ability to substitute for morphine in addict rats.