An analysis of the 'tolerance' which develops to analgetic electrical stimulation of the midbrain periaqueductal grey in freely moving rats

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Intermittent dosing prolongs tolerance to the antinociceptive effect of morphine microinjection into the periaqueductal gray

Tolerance to the antinociceptive effect of microinjecting morphine into the ventrolateral periaqueductal gray (vPAG) develops with repeated administration. The objective of the present experiment was to determine whether the magnitude and duration of tolerance differ depending on the interval between injections. Rats were injected with morphine or saline into the vPAG either once daily for 4 days or twice daily for 2 days. All rats were injected with morphine into the vPAG for the fifth injection to determine whether tolerance had developed. Morphine microinjection produced tolerance in both morphine-pretreated groups regardless of inter-dose interval. One and two weeks later, microinjection of morphine produced an increase in hot plate latency in all groups except rats pretreated with daily morphine microinjections. That is, tolerance was evident 2 weeks following the induction of tolerance produced by daily, but not twice daily injections of morphine. Although a long inter-dose interval has been shown to prolong the duration of tolerance after systemic morphine administration, this is the first report showing a similar effect with direct administration of morphine into the brain. Given that associative learning underlies prolonged tolerance with systemic morphine administration, the present data suggest that associative mechanisms of tolerance are also engaged when morphine administration is restricted to the PAG.

Differential susceptibility of the PAG and RVM to tolerance to the antinociceptive effect of morphine in the rat

The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are part of a nociceptive modulatory system. Microinjection of morphine into either structure produces antinociception. Tolerance develops to ventrolateral PAG mediated antinociception with repeated microinjection of morphine. In contrast, there are no published reports of tolerance to morphine administration into the RVM. Three experiments were conducted to determine whether tolerance develops to morphine microinjections into the RVM. Experiment 1 compared tolerance to the antinociceptive effect of microinjecting morphine (5 microg/0.5 microl) into the PAG and RVM following daily injections for four consecutive days. Experiment 2 assessed tolerance to a range of morphine doses (2.5-20 microg) after injecting morphine into the RVM twice a day for two consecutive days. Experiment 3 followed a similar procedure except twice as many RVM injections were made (8 microinjections in 4 days). The degree to which tolerance developed to the antinociceptive effect of morphine was much greater with microinjections into the PAG compared to the RVM. There was a 64% drop in hot plate latency from the first to the fifth injection of morphine into the PAG, but only a 36% drop in latency following RVM microinjections. Reducing the interdose interval to two injections a day or increasing the total number of injections from 4 to 8 did not enhance the development of tolerance to RVM morphine administration. These data demonstrate that opioid-sensitive neurons in the RVM are relatively resistant to the development of tolerance compared to PAG neurons.

Opioid-NMDA receptor interactions may clarify conditioned (associative) components of opioid analgesic tolerance

Recent evidence suggests that acute administration of opioid analgesic drugs (such as morphine or heroin) produces delayed hyperalgesia. This hyperalgesic response is likely to result from hyperactivation of NMDA receptors triggered by stimulation of opioid receptors and may mediate acute tolerance. In support of this hypothesis, blockade of NMDA receptors attenuates opioid-induced delayed hyperalgesia and prolongs the duration of antinociceptive activity of morphine. Furthermore, the NMDA receptor-induced hyperalgesia is likely an unconditioned response to opioid receptor stimulation that becomes spatiotemporally associated with environmental cues accompanying repeated opioid exposure. This hypothesis conforms to the traditional Pavlovian requirement for conditioned and unconditioned responses to be qualitatively similar. In support of the role of NMDA receptor hyperactivation in morphine tolerance, NMDA receptor antagonists have been shown to block development of analgesic tolerance induced by repeated exposures to morphine. The view of the conditioned nature of opioid tolerance may be significantly extended by assuming that upon repeated drug administration an early-onset effect of a drug may become a predictive stimulus for a later-onset effect and, consequentially, it may become empowered to elicit the later-onset effect itself. Such 'intra-drug' conditioning hypothesis is well in line with the current experimental evidence but further studies will be needed to verify it directly.

Dorsal raphe nucleus efferents: termination in peptidergic fields

This study was performed to identify the efferents of the dorsal raphe nucleus (DRN), particularly in regard to nociception. Phaseolus vulgaris leucoagglutinin (PHA-L) was microiontophoresed into the DRN and visualized immunocytochemically; PHA-L-immunoreactive (-IR) fibers and terminals were identified in the forebrain, hypothalamus, midline and intralaminar thalamus, habenula, periaqueductal gray, locus coeruleus, parabrachial nucleus, medullary raphe and reticular nuclei, and nucleus tractus solitarius. Dual immunocytochemistry was used to identify corticotropin-releasing factor, neurotensin, and enkephalin neurons in DRN terminal fields, in some cases in close proximity to PHA-L-IR terminals. Terminal fields were identified in regions that influence nociception, and the neuroactive substances identified in these terminal fields may play modulatory roles in nociception.

Arcuate nucleus projections to brainstem regions which modulate nociception

Anterograde tracing studies were conducted in order to identify efferents from the arcuate nucleus, which contains the hypothalamic opiocortin neuronal pool. Phaseolus vulgaris leucoagglutinin (PHA-L) was stereotaxically iontophoresed into the arcuate nucleus and the terminal fields emanating from the labelled perikarya were identified immunocytochemically. PHA-L-immunoreactive (-ir) fibers were identified in nucleus accumbens, lateral septal nucleus, bed nucleus of the stria terminalis, medial and lateral preoptic areas, anterior hypothalamus, amygdaloid complex, lateral hypothalamus, paraventricular nucleus, zona incerta, dorsal hypothalamus, periventricular gray, medial thalamus and medial habenula. In the brainstem, arcuate terminals were identified in the periaqueductal gray (PAG), dorsal raphe nucleus (DRN), nucleus raphe magnus (NRM), nucleus raphe pallidus, locus coeruleus, parabrachial nucleus, nucleus reticularis gigantocellularis pars alpha, nucleus tractus solitarius and dorsal motor nucleus of the vagus nerve. Dual immunostaining was used to identify the neurochemical content of neurons in arcuate terminal fields in the brainstem. Arcuate fiber terminals established putative contacts with serotonergic neurons in the ventrolateral PAG, DRN and NRM and with noradrenergic neurons in periventricular gray, PAG and locus coeruleus. In the PAG, arcuate fibers terminated in areas with neurons immunoreactive to substance P, neurotensin, enkephalin and cholecystokinin (CCK) and putative contacts were identified with CCK-ir cells. This study provides neuroanatomical evidence that putative opiocortin neurons in the arcuate nucleus influence a descending system which modulates nociception.