Tolerance to Morphine Analgesia: Influence of Pain and Method of Morphine Delivery

Whether some kinds of pain can modify the development of tolerance to morphine analgesia is a controversial issue. Clinically, the development of tolerance is often difficult to establish because many factors can contribute to a decline in analgesic coverage, including disease progression. Basic animal research designed to examine tolerance provides the experimental control necessary to differentiate 'real' tolerance from other variables that can influence morphine analgesia. The present study examines the effects of inflammation induced by complete Freund's adjuvant (CFA) on the development of tolerance to morphine analgesia produced by two different methods of morphine delivery - repeated bolus injections and continuous infusion. Male Long-Evans hooded rats were injected with CFA (0.2 mL) into the right hind paw under sodium pentobarbital anesthesia or were given anesthesia alone. Starting 24 h later, rats either received an injection of morphine (20 mg/kg, intraperitoneal) on four consecutive days or were implanted with a 72 h osmotic minipump that delivered 80 mg/kg morphine over a similar time period. Control animals received saline injections or were implanted with empty minipumps. After 24 h, sensitivity to morphine-induced analgesia was measured by the tail-immersion test (water maintained at 52°C) using a cumulative dosing procedure. It was found that CFA attenuated tolerance to the morphine analgesia that was induced by intraperitoneal injections of morphine. In contrast, when morphine was delivered via osmotic minipumps, significant analgesic tolerance was observed in animals that received morphine in the presence of CFA but not in those that received morphine in the absence of CFA. These results show the importance of the method used to deliver morphine in determining the effects of pain on the development of tolerance to morphine analgesia.

Microvascular Oxygen Delivery and Interstitial Oxygenation during Sodium Pentobarbital Anesthesia

Background Anesthesia may represent a considerable bias in experimental medicine, particularly in conditions of stress (such as hemorrhage). Sodium pentobarbital (PB), widely used for cardiovascular investigations, may impair oxygen delivery by hemodynamic and respiratory depression. The critical issue, however, is whether the microcirculation can still maintain tissue oxygenation during anesthesia. To answer this question, the authors studied the effect of PB anesthesia on subcutaneous microvascular oxygen delivery and interstitial oxygenation in Syrian golden hamsters. Methods Sodium pentobarbital anesthesia was induced by intravenous injection (30 mg/kg body weight) and maintained by a 15-min infusion (2 mg.kg-1.min-1), with animals breathing spontaneously (PB-S) or ventilated with air (PB-V). Systemic parameters evaluated were mean arterial pressure (MAP), heart rate, cardiac index (CI), arterial oxygen tension (PaO2), arterial carbon dioxide tension (PaCO2), base excess, and pH. Microvascular and interstitial oxygen tension (PO2), vessel diameter, red blood cell velocity (vRBC), and blood flow (Qb) were measured in a dorsal skinfold preparation. Microcirculatory PO2 values were determined by phosphorescence decay. Results Sodium pentobarbital anesthesia significantly decreased CI, MAP, vRBC, and Qb. During PB infusion, PaO2 values were 56 +/- 12.8 mmHg (PB-S) and 115.9 +/- 14.6 mmHg (PB-V) compared with 69.4 +/- 18.2 mmHg and 61.4 +/- 12.6 mmHg at baseline. However, microvascular PO2 was reduced by 25-55% in both groups, resulting in an interstitial PO2 decrease from 23.9 +/- 5.6 mmHg (control) to 13.1 +/- 9.1 mmHg (PB-S) and 15.2 +/- 7 mmHg (PB-V). Microcirculatory PO2 values were restored 30 min after PB infusion, even though hemodynamic depression and a light anesthetic plane were maintained. Conclusions Sodium pentobarbital anesthesia caused impairment of microvascular oxygen delivery and interstitial oxygenation, effects that were not prevented by mechanical ventilation. Although these effects were restricted to deep anesthetic planes, prolonged hemodynamic depression suggests that caution is warranted when using PB as an anesthetic in cardiovascular investigations.

The effect of orchidectomy on rat pituitary responsiveness to GHRH depends on age

Pituitary responsiveness to GHRH (1-29) NH2 (GHRH, 5 μg/kg iv) was analysed under sodium pentobarbital anesthesia (50 mg/kg ip), on days 30 and 90 in male rats orchidectomized or sham-operated 7 days earlier. Other groups of rats were orchidectomized or shamoperated on day 23 and tested on days 30, 45, 60 and 90. In the sham-operated animals, GHRH stimulated GH secretion on day 90, but not on day 30. GHRH-induced secretion was similar on days 45, 60 and 90 in orchidectomized and sham-operated animals. Orchidectomy on day 83 reduced this GHRH-induced GH secretion on day 90. In contrast, orchidectomy on day 23 enhanced the pituitary responsiveness to GHRH a week later. These results suggest that the increase in pituitary responsiveness to GHRH with age is independent of the testicular function and that the effect of orchidectomy depends on both the age of the rats at orchidectomy and the time elapsed between the orchidectomy and the administration of GHRH.

Assessing the pressure at zero flow in canine epigastric skin flaps

The pressure at zero flow was examined using three independent techniques in island epigastric skin flaps in dogs under sodium pentobarbital anesthesia. (i) The flap was perfused from a reservoir which was varied in height to supply a range of pressures. The corresponding flows were measured by electromagnetic blood flowmetry and the pressure at zero flow was determined by extrapolation to zero flow. (ii) Arterial inflows as low as 2.2 μL/min were delivered by a Harvard pump and corresponding pressures were measured. The pressure at zero flow was determined by inspection. (iii) The artery supplying the flap was occluded with a microclip and the plateau following the decay of pressure was read as the pressure at zero flow. The pressure decay technique was the simplest, most reproducible method and was verified by the other two methods. It gave a pressure at zero flow of 5.8 ± 0.4 mmHg (0.77 ± 0.05 kPa). The critical closing pressure is discussed and related to the pressure at zero flow.

Renal function in rats with innervated and denervated kidneys before and during sodium pentobarbital anesthesia

We have used a model for measuring renal clearances in the undisturbed rat to assess the role of the renal nerves in the depression of renal function during sodium pentobarbital anesthesia. One group of rats was studied with renal nerves intact and a second group was studied 7 – 9 days after bilateral renal denervation. Rats were prepared by placement of cannulae an average of 5 days prior to the clearance experiments. Renal function was measured before and after the injection of saline as the control vehicle and 2 – 3 days later, before, and after the injection of sodium pentobarbital (50 mg/kg) in the same rat. Sodium pentobarbital produced comparable decreases in glomerular filtration rate, para-aminohippuric acid clearance, urine flow rate, and sodium excretion in rats with denervated or innervated kidneys, injection of saline resulted in no differences in measured variables between the rats with intact or sectioned renal nerves. Sodium pentobarbital caused a drop in arterial pressure in the denervated group but not in the innervated group. In a second series of experiments rats with denervated kidneys were implanted with an inflatable occluder around the aorta. This occluder was inflated to limit the drop in arterial pressure during anesthesia. When the blood pressure to the kidneys was maintained, renal function did not decrease during sodium pentobarbital anesthesia. These experiments suggest that the renal nerves are involved in the decrease in renal function during sodium pentobarbital anesthesia.