Blockade by naltrexone of analgesia produced by stimulation of the dorsal raphe nucleus

Involvement of the periaqueductal gray in the effect of motor cortex stimulation

Several clinical and animal studies of different pain models reported that motor cortex stimulation (MCS) has an antinociceptive effect. In our previous study, the response of the primary somatosensory cortex (SI) to peripheral stimuli decreased after MCS. The aim of the present study was to investigate involvement of the periaqueductal gray (PAG) in this inhibitory effect of MCS. Responses of the SI to electrical stimuli applied to both forepaws of anesthetized rats were monitored to evaluate the effect of MCS. After sensory-evoked potentials (SEPs) were stable, either saline, opioid, or dopamine receptor antagonists were locally microinjected into the PAG. After drug or saline administration, MCS was applied to the forepaw area of the right motor cortex. SEPs after MCS were compared to those before MCS. In the saline group, SEPs ipsilateral to MCS decreased, but SEPs contralateral to MCS did not. The decrease in SEPs was prevented by pretreatment of the PAG with naloxone. Application of a nonspecific dopamine receptor antagonist (α-flupenthixol) to the PAG also blocked the inhibition of SEPs after MCS. Inhibition of SEPs after MCS was blocked by local application of a D1 antagonist (SCH-23390) in the PAG, but not by a D2 antagonist (eticlopride). These results suggest that the PAG participates in the inhibitory effect of MCS, and this effect of MCS may be mediated by opioid and dopamine D1 receptors within thePAG.

Epidural motor cortex stimulation suppresses somatosensory evoked potentials in the primary somatosensory cortex of the rat

Motor cortex stimulation (MCS) is a promising clinical procedure to help alleviate chronic pain. Animal models demonstrated that MCS is effective in lessening nocifensive behaviors. The present study explored the effects of MCS on cortical somatosensory evoked potentials (SEPs) recorded at the primary somatosensory cortex (SI) of the rat. SEPs were evoked by electrical stimulation applied to the contralateral forepaws. Effects of different intensities, frequencies, and durations of MCS were tested. MCS at ≥2V suppressed SEPs of the ipsilateral SI. Suppression lasted 120 min at an intensity of 5 V. The optimal frequency was 50 Hz, and the duration was 30s. In contrast, MCS did not affect SEPs recorded on the contralateral SI. Cortical stimulation out of the motor cortex did not induce a decrease in the ipsilateral SEPs. We also investigated involvement of the endogenous opioid system in this inhibition of SEPs induced by MCS. The opioid antagonist, naloxone (0.5 mg/kg), was administered 30 min before MCS. Application of naloxone completely prevented the inhibitory effect of MCS on ipsilateral SEPs. These results demonstrate that MCS blocked the transmission of somatosensory information to the primary somatosensory cortex, and this interference was mediated by the endogenous opioid system. This inhibitory effect on sensory transmission induced by MCS may reflect its antinociceptive effect.

The development of stimulation-produced analgesia (SPA) in the rat

The present study studied the development of stimulation produced analgesia (SPA) from the periaqueductal gray (PAG) in rats. A monopolar stimulating electrode was lowered into the dorsal or ventral PAG of animals aged 7, 14, 21, or 90-120 days. Constant current cathodal pulses (100 Hz, 100 microseconds) were delivered, starting 10 s before analgesia was tested by the tail-flick (TF) test and continuing throughout each TF trial or until cut-off (7 s). Current intensity was increased stepwise (3-200 microA). It was found that SPA can be elicited starting at 21 days, but not earlier. However, supraspinal modulation of nociception is still immature at 3 weeks after birth. First, stimulation intensities needed to produce SPA are higher in 21-day-old pups than in adult animals. Second, in 21-day-old pups, but not in adults effective current intensities in the dorsal PAG are higher than in the ventral PAG. Third, naltrexone decreases SPA from the ventral PAG in 21-day-old pups, but not in adult animals. These findings indicate that supraspinal modulation of nociception develops only 3 weeks after birth, with the ventral PAG maturing prior to the dorsal PAG, and that the contribution of endogenous opioids to SPA does not remain constant throughout the ontogeny of rats.

Analysis of neurotransmitter receptors mediating changes in LH release induced by electrical stimulation of the dorsal raphe nucleus in the rat

We report studies of neurotransmitter and receptor species mediating the inhibition of LH release in ovariectomized (OVX) rats and stimulation in OVX estrogen-primed rats induced by electrical stimulation of the dorsal raphe nucleus (DRN). Attempts were made to block effects of stimulation of the DRN in groups of four to nine ovariectomized rats with or without priming with estradiol benzoate (EB) by pretreatment with the 5HT receptor antagonists ketanserin, methysergide, or metergoline; the alpha-adrenergic antagonist phenoxybenzamine; or the opiate antagonist naltrexone. Blood samples were collected via jugular cannulae from the unanesthetized rats at 10-min intervals before and during electrical stimulation of the DRN, and assayed by double antibody radioimmunoassay for LH. To test the effect of stimulation, data from animals in each treatment group were subjected to analysis of variance to obtain the contrast matrix including prestimulation and stimulation time points of interest. The contrast matrix was then used to construct an F ratio and thereby to evaluate the level of significance. In unprimed OVX rats the sustained decrease in plasma LH concentration during stimulation was prevented in rats pretreated with ketanserin or phenoxybenzamine. Methysergide pretreatment delayed the inhibitory effect of DRN electrical stimulation for 30 min, whereas metergoline and naltrexone were ineffective. In EB-primed animals the increase in plasma LH observed during stimulation was prevented in rats pretreated with metergoline, and reversed in those receiving naltrexone. Ketanserin limited the duration of the increase to 10 min, while phenoxybenzamine and methysergide had no significant effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Opiate antagonists and rewarding brain stimulation

This review examines the literature on the effects of opiate antagonists on brain stimulation (ICSS) reward. Antagonists should have predictable effects if endogenous opioids modulate ICSS. Naloxone is the antagonist most often used, and it has produced inconsistent results in some ICSS paradigms. When schedules of intermittent reinforcement are used, however, naloxone reliably reduces the rate of responding. It reverses the effects of opiate agonists on ICSS behavior, and it also attenuates the effects of psychomotor stimulants, such as amphetamine. The results produced by naloxone are consistent with a modulatory effect of endogenous opioid systems on reward, and suggest that the opiate and dopamine systems together exert significant control over ICSS. Further research is needed to characterize better the actions of the antagonists on ICSS behavior, and productive research directions are proposed. Data obtained in future studies might suggest how the endogenous opioid systems modulate both natural and brain stimulation reward.

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

Electrical stimulation of the ventral midbrain periaqueductal grey (PAG) elicits an opioidergic antinociception against noxious heat and pressure in freely moving rats. Recurrent stimulation was associated with a gradual decline and eventual loss of this stimulation-produced antinociception (SPA). This could be reinstated by an increase in current intensity and this reinstatement was preventable by naloxone. The current intensity--antinociception (dose--response) curve was shifted to the right in recurrently stimulated rats and parallel to that in naive animals. The loss of SPA upon repetitive simulation did not represent a conditioning phenomenon. Thus, tolerant rats exposed to all cues which accompanied stimulation revealed no (compensatory) hyperalgesic response--but rather a slight antinociception. Further, SPA recovered spontaneously in tolerant rats. Moreover, 'extinction' by repeated exposure to all cues accompanying stimulation did not restore or accelerate the recovery of SPA in tolerant animals. Tolerant rats showed no depletion in midbrain PAG or other CNS or hypophyseal pools of beta-endorphin, Met-enkephalin or dynorphin indicating that a depletion of endogenous opioid peptides does not underlie the tolerance which develops to stimulation. In fact recurrently stimulated rats did not show any of the pronounced effects upon CNS pools of opioid peptides which are seen with long-term stress. Moreover, repetitively stimulated rats revealed no indications of stress as judged by a diversity of stress-sensitive parameters; basal nociceptive threshold, core temperature, ingestive behaviour, body weight, adrenal weight and hypophyseal secretion of beta-endorphin and prolactin. The data offer two major conclusions. Firstly, the gradual loss of analgesia upon recurrent stimulation of the midbrain PAG does not reflect a generalized debilitation or stress and neither a conditioning phenomenon nor a depletion of pools of endogenous opioid peptides. Rather it closely corresponds to the pharmacological definition of tolerance and may reflect a process occurring at the level of the opioid receptor and coupled processes. This finding explains the cross-tolerance which we observe recurrently stimulated rats to display to morphine. Secondly, this SPA is not a form of stress-induced analgesia and rats undergoing recurrent stimulation reveal no indications of stress as judged by biochemical, physiological and behavioural parameters.

Evidence that mu-opioid receptors mediate midbrain "stimulation-produced analgesia" in the freely moving rat

Electrical stimulation of the ventral midbrain in freely moving rats led to an antinociception against both noxious heat and noxious pressure. Recurrent stimulation was associated with a progressive loss of the antinociceptive efficacy of stimulation. Rats adapted ("tolerant") to stimulation revealed a significant reduction in the antinociceptive potency of a low dose of the systemically applied selective mu-opioid agonist, morphine. In distinction, the antinociceptive effect of the selective kappa-agonist, trans-3,4-dichloro-N-methyl-N[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetam ide (U50488H) was not modified. In the presence of naloxone, delivered subcutaneously via minipumps at a low dose for 7 days, the antinociceptive action of morphine was abolished, whereas that of U50488H was not attenuated: this reflects the selective blockade of mu-receptors. Rats receiving naloxone failed to develop an antinociception upon midbrain electrical stimulation. Removal of the pumps led to a supersensitivity to the antinociceptive effects of morphine but not U50488H. Similarly, midbrain stimulation-produced antinociception was enhanced. These data demonstrate that (1) midbrain stimulation-produced analgesia is selectively cross-tolerant to a mu- as compared to a kappa-agonist; (2) a very low dose of naloxone selective for the mu-receptor blocks midbrain stimulation-produced analgesia, and (3) chronic naloxone treatment leads to a selective supersensitivity to a mu-agonist as compared to a kappa-agonist and an enhancement of midbrain stimulation-produced analgesia. Collectively, the data indicate that a mu-opioid receptor mediates midbrain stimulation-produced analgesia in the rat against both noxious heat and noxious pressure.

Depletion of central beta-endorphin blocks midbrain stimulation produced analgesia in the freely-moving rat

The present study examines the role of central beta-endorphin in the generation of stimulation-induced analgesia from the ventral midbrain periaqueductal gray of freely-moving rats. Electrical stimulation of the ventral midbrain periaqueductal gray led to an antinociception against noxious heat which gradually subsided post-stimulation over a period of about 15 min. Locomotor effects (ipsilateral rotation) were also seen which were not correlated in intensity with the analgesia and which disappeared immediately with termination of stimulation. There was no indication of any aversive effects. Application of the opioid antagonist, naloxone, 10 min pre-stimulation, strongly attenuated the antinociception without changing basal thresholds. It did not influence the locomotor changes. Bilateral, radiofrequency lesions of the mediobasal arcuate hypothalamus greatly depleted immunoreactive beta-endorphin from brain tissues without affecting its levels in plasma. Lesioned rats showed a pronounced reduction of stimulation-produced antinociception in the absence of any change in basal thresholds; the locomotor effects of stimulation were not influenced. The degree of depletion of immunoreactive-beta-endorphin significantly correlated with the degree of attenuation of antinociception. These data suggest: stimulation of the ventral midbrain periaqueductal gray leads both to an antinociception and locomotor effects in freely-moving rats: these can be clearly dissociated from each other; the antinociception (but not locomotor effects) are mediated by an endogenous opioid sensitive to blockade by naloxone; and central beta-endorphin may be the endogenous opioid mediating stimulation-produced antinociception from the ventral midbrain periaqueductal gray in the rat.

A reinvestigation of the analgesic effects induced by stimulation of the periaqueductal gray matter in the rat. II. Differential characteristics of the analgesia induced by ventral and dorsal PAG stimulation

This study consists of a detailed analysis of the analgesic effects induced by stimulation of the various parts of the periaqueductal gray matter (PAG) in the freely moving rat. In order to characterize the analgesia, two criteria are considered: (1) the evaluation of the degree of analgesia and behavioral side effects evoked during central stimulation; and (2) the presence of post-effects. Central stimulation (50 Hz sine waves) was delivered via bipolar concentric electrodes and analgesia was quantified by the change in the vocalization threshold induced by electrical stimulation of the tail. Within the ventral PAG, the vocalization threshold increased gradually with the intensity of the central stimulation, the degree of analgesia generally being powerful. There was no relationship between the strength of the analgesic effects and the motor disturbances also produced by stimulation of this region. Antinociceptive effects generally disappeared when the stimulation ceased. Only when the intensity of the stimulation was strong enough to induce very powerful analgesic effects were post-stimulation analgesic effects noticed. Within the dorsal and dorsolateral PAG as well as in the ventral region just surrounding the aqueduct, analgesia appeared suddenly, was generally less pronounced and was always concomitant with strong aversive reactions. In contrast with the analgesia from the ventral PAG, marked post-effects were observed. These latter characteristics were also obtained from stimulation of regions located outside the PAG (colliculi, intercollicular commissure and tectum adjacent to the dorsolateral PAG) although these zones were not extensively studied. By consideration of various data in the literature, it is concluded from this study, which clearly distinguishes stimulation-produced-analgesia (SPA) from ventral PAG versus dorsal PAG, that analgesia induced from this midbrain area involves at least two different neuronal substrates. Whilst the ventral PAG seems to be more preferentially involved in pain modulation, the authenticity of 'analgesia' triggered by stimulation of aversive regions (which are widely spread over the PAG) is questioned and proposals to explain the simultaneous appearance of analgesic effects and aversion are considered.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.