Studies on the development of tolerance and potential spinal neurotoxicity after chronic intrathecal carbachol-antinociception in the rat

Carbachol interactions with nonsteroidal anti-inflammatory drugs

The inhibition of cyclooxygenase enzymes by nonsteroidal anti-inflammatory drugs (NSAIDs) does not completely explain the antinociceptive efficacy of these agents. It is known that cholinergic agonists are antinociceptive, and this study evaluates the interactions between carbachol and some NSAIDs. Antinociceptive activity was evaluated in mice by the acetic acid writhing test. Dose-response curves were constructed for NSAIDs and carbachol, administered either intraperitoneally (i.p.) or intrathecally (i.t.). The interactions of carbachol with NSAIDs were evaluated by isobolographic analysis after the simultaneous administration of fixed proportions of carbachol with each NSAID. All of the drugs were more potent after spinal than after systemic administration. The combinations of NSAIDs and carbachol administered i.p. were supra-additive; however, the i.t. combinations were only additive. Isobolographic analysis of the coadministration of NSAIDs and carbachol and the fact that atropine antagonized the synergistic effect suggest that carbachol may strongly modulate the antinociceptive activity of NSAIDs; thus, central cholinergic modulation would be an additional mechanism for the antinociceptive action of NSAIDs, unrelated to prostaglandin biosynthesis inhibition.

Preemptive analgesia I: physiological pathways and pharmacological modalities

PURPOSE

This two-part review summarizes the current knowledge of physiological mechanisms, pharmacological modalities and controversial issues surrounding preemptive analgesia.

SOURCE

Articles from 1966 to present were obtained from the MEDLINE databases. Search terms included: analgesia, preemptive; neurotransmitters; pain, postoperative; hyperalgesia; sensitization, central nervous system; pathways, nociception; anesthetic techniques; analgesics, agents.

PRINCIPAL FINDINGS

The physiological basis of preemptive analgesia is complex and involves modification of the pain pathways. The pharmacological modalities available may modify the physiological responses at various levels. Effective preemptive analgesic techniques require multi-modal interception of nociceptive input, increasing threshold for nociception, and blocking or decreasing nociceptor receptor activation. Although the literature is controversial regarding the effectiveness of preemptive analgesia, some general recommendations can be helpful in guiding clinical care. Regional anesthesia induced prior to surgical trauma and continued well into the postoperative period is effective in attenuating peripheral and central sensitization. Pharmacologic agents such as NSAIDs (non-steroidal anti-inflammatory drugs) opioids, and NMDA (N-methyl-D-aspartate) - and alpha-2-receptor antagonists, especially when used in combination, act synergistically to decrease postoperative pain.

CONCLUSION

The variable patient characteristics and timing of preemptive analgesia in relation to surgical noxious input requires individualization of the technique(s) chosen. Multi-modal analgesic techniques appear most effective.

Intrathecal acetyl cholinesterase inhibitors produce analgesia that is synergistic with morphine and clonidine in rats

Intrathecal (IT) administration of acetyl cholinesterase inhibitors produces analgesia through a muscarinic action. The addition of IT cholinergic agonists to IT opioids or alpha 2-adrenergic agonists results in enhanced analgesic effects, but it is not clear whether these interactions are synergistic, additive, or less than additive. Dose-response curves for hot plate and tail immersion tests were established for IT neostigmine, physostigmine, and echothiophate in rats. Dose-response curves for hot plate testing were established for IT morphine and clonidine. The effect of maximally effective doses of each of the three cholinergic drugs on hot plate testing was plotted over time. Isobolographic analysis was performed for hot plate testing using neostigmine-morphine and neostigmine-clonidine combinations. The three cholinesterase inhibitors produced profound analgesia on hot plate testing but incomplete analgesia using the tail immersion test. Duration of analgesia on hot plate testing ranged from 45 min for physostigmine to more than 24 h for echothiophate. IT administration of combinations of neostigmine plus morphine and neostigmine plus clonidine both produced significantly more profound analgesia than the calculated additive effects and are, therefore, synergistic in their actions.

Intrathecal injection of lysine acetylsalicylic acid in the rat: a neurotoxicological study

Lysine acetylsalicylic acid has been reported to induce analgesic effects in humans after intrathecal (i.t.) injection. Before conducting further studies in humans with this drug, it is important to evaluate potential toxicological effects on the spinal cord in animals. In the present study the effects of chronic intrathecal administration of provocative doses of lysine acetylsalicylic acid (L-ASA) on the rat spinal cord were evaluated using light and electron microscopy and a quantitative morphometric method. We also investigated the effects of single doses of the drug on the spinal cord blood flow (SCBF) using the laser-Doppler flowmetry technique. No histopathological changes or differences in number or density of neuronal cells could be seen after chronic administration of L-ASA as compared to controls. The SCBF decreased immediately after i.t. injection of a large dose (4 mg) of L-ASA and returned to predrug levels within 10 min. At the end of the experiment metabolic acidosis was detected, indicating a systemic effect of acetylsalicylic acid. It is concluded that no neurotoxic effects on the spinal cord were seen after chronic i.t. injection of L-ASA. From a neurotoxicological point of view, our findings do not contraindicate the spinal use of L-ASA in humans.

Spinal cord morphology and antinociception after chronic intrathecal administration of excitatory amino acid antagonists in the rat

Drugs that antagonize the action of excitatory amino acids on the NMDA receptor in the spinal cord are of interest in pain treatment. Before such drugs can be applied clinically, their potential toxicity should be studied. This study was performed in rats in order to reveal possible neurotoxicologic side effects following chronic intrathecal (i.t.) application of two NMDA receptor antagonists: 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and kynurenic acid (KYN). Rats equipped with i.t. catheters were injected twice a day for 2 weeks with saline, 2 nmol (0.5 micrograms) CPP or 210 nmol (40 micrograms) KYN, where the doses of CPP and KYN were chosen on the basis of similar analgesic effects after one administration. Antinociception was tested daily using the tail-flick and hot-plate tests. The antinociceptive effect was similar in CPP- and KYN-treated rats on days 1 and 2. The effect of CPP decreased during the following days, whereas that of KYN persisted for the 12-day testing period. The spinal cord was then removed and prepared for light and electron microscopic examination, and a morphometric method using an unbiased stereological estimator of cell number and cell volume was applied as a sensitive variable of spinal cord neurotoxicity. Morphologic and ultrastructural analyses of the spinal cord segment adjacent to the tip of the catheter showed normal appearance with no differences between the groups. Furthermore, no differences in cell number or cell volume in the dorsal horn were found between the groups. In conclusion, chronic i.t. administration of pharmacologically active doses of CPP and KYN in rats did not produce neurotoxic effects in the spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic catheterization of the epidural space in rabbits: a model for behavioural and histopathological studies. Examination of meptazinol neurotoxicity

A technique of epidural catheterization in rabbits is described. Twelve albino rabbits received a totally implanted epidural catheter system. The system was implanted surgically, and the functioning of the system tested for a period of 3 months. X-ray examinations following epidural contrast injections showed a distribution up to Th4 following 1.5 ml and Th8-9 following 1.0 and 1.25 ml. Epidural injection of lidocaine throughout the study period proved the system to be functioning for all 3 months. Another 12 rabbits were included for the neurotoxicological examinations following epidural catheterization, without any injections (three rabbits), epidural injections of saline (four rabbits) and meptazinol (five rabbits) once a day for 14 days. Histopathological examinations showed a fibrous cocoon, at the tip of the catheter, in all rabbits. In the group of rabbits which did not receive any injections, the cocoon was slightly infiltrated with leukocytes and local depression of the spinal cord was observed in one rabbit. In the saline-injected group this infiltration was more pronounced and in one rabbit it extended into the meninges. Three rabbits showed local depression of the spinal cord and local myelopathy of the white matter in the area adjacent to the cocoon. In the group of rabbits receiving meptazinol, three out of five had local depression and myelopathy of the white matter. In this group these findings were more pronounced. In two rabbits the myelopathy extended transversely through the white matter into the grey matter of the spinal cord. The number of pathological changes in the group receiving meptazinol was significantly higher compared to the control and placebo groups.(ABSTRACT TRUNCATED AT 250 WORDS)

The NMDA-receptor antagonist CPP abolishes neurogenic 'wind-up pain' after intrathecal administration in humans

Involvement of the NMDA receptor system in the transmission of nociceptive information, including the development of central sensitization and a wind-up phenomenon, has increased interest in NMDA-receptor antagonists as antinociceptive drugs. This case report describes the use of an NMDA receptor antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) in a carefully selected patient with severe and intractable neurogenic pain in her left leg. The pain syndrome had components of a continuous deep pain, an allodynia, and a wind-up-like component, including afterdischarge and spread of painful sensations outside the territory of the injured nerve. After intrathecal (i.t.) administration of 200 nmol of CPP the continuous deep pain component and allodynia were unchanged, but the following 'wind-up' phenomenon with afterdischarge and spread of the pain sensation in the left half of the body was completely abolished. Another 500 nmol of CPP administered over 2 h did not improve pain relief. Pain thresholds for heat and cold stimulation, measured with a Marstock thermostimulator, did not change. There was no effect on blood pressure, heart rate, sensitivity, reflexes, coordination or motor performance. Psychotomimetic ketamine-like side effects developed 4 h after the last injection of CPP and were probably due to rostral spread of CPP. These early experiences with i.t. administration of NMDA-receptor antagonists to humans indicate that the NMDA-receptor system plays an important role in neurogenic pain and that antagonizing this system may be a useful way to obtain better pain control although psychotomimetic side effects due to rostral spread may be a problem.

Histopathology and evaluation of potentiation of morphine-induced antinociception by intrathecal droperidol in the rat

Clinical studies have indicated that a low dose of the dopamine D2-receptor antagonist droperidol given epidurally potentiate the antinociceptive effect of epidural morphine. The present study was conducted in order to evaluate this drug interaction in a rat model. Rats were given morphine and droperidol intrathecally in several combinations of doses. It was found that droperidol had no antinociceptive effect by itself, nor in combination with morphine. It was also shown that droperidol and morphine exert no histopathological effects on the rat spinal cord. This discrepancy between clinical findings and experimental pain studies suggests different modes of action of droperidol in the two situations.