Monoaminergic mechanisms of stimulation-produced analgesia

Analgesia from the periaqueductal gray in the developing rat: focal injections of morphine or glutamate and effects of intrathecal injection of methysergide or phentolamine

The aim of these experiments was to examine the changes in antinociception elicited by morphine or glutamate stimulation of the periaqueductal gray of the midbrain (PAG) during the postnatal development of the rat. Pups, aged 3, 10, and 14 days, were implanted with cannulas aimed at either the dorsal or the ventral aspect of the PAG, and glutamate (vehicle, 60 mM or 180 mM) or morphine (vehicle, 2 micrograms or 6 micrograms) was microinjected into one of those two sites. Pups were tested for analgesia against noxious thermal and mechanical stimuli. Morphine produced analgesia at 3 and 10 days of age only when administered to the ventral part of the PAG and the thermal noxious stimulus was tested. Conversely, analgesia induced by glutamate was seen at 3 and 10 days of age only when glutamate was given to the dorsal aspect of the PAG and the mechanical stimulus was used. In 14-day-old pups, both drugs produced analgesia against both types of noxious stimuli regardless of their site of administration within the PAG. Systemically administered naloxone attenuated the analgesic effects of both drugs when they were administered to the ventral PAG, but did not consistently attenuate the analgesic effect of either compound given to the dorsal aspect of the PAG. When either morphine or glutamate was injected into the ventral PAG, intrathecal injections of methysergide attenuated analgesia against the thermal stimulus to a significantly greater degree than the mechanical stimulus and intraspinal injection of phentolamine attenuated analgesia against the mechanical stimulus more potently. When glutamate was given to the dorsal PAG, analgesia against both stimulus types was significantly attenuated. These results indicate that the morphine- and glutamate-induced analgesia mediated by the PAG are developmentally differentiated. These ontogenetic differences most likely reflect differences in the mechanism of action by which these drugs produce analgesia when administered to the PAG, as well as neuroanatomical differences within the dorsal and the ventral regions of the PAG.

Influence of some dopaminergic agents on antinociception produced by quinine in mice

1. The analgesic effect of quinine and the influence of some dopaminergic agents on it were studied in mice. 2. Quinine (25-130mg/kg, ip) effectively elicited antinociceptive effect in a dose related manner. 3. D-Amphetamine (2.5-4mg/kg, ip), L-dopa (25mg/kg, sc), L-dopa (25mg/kg, sc) plus benserazide (12.5mg/kg, sc), alpha-methyl-p-tyrosine (50mg/kg, ip) plus L-dopa (25mg/kg, sc) and pargyline (50mg/kg, ip) significantly attenuated the antinociceptive effect of quinine (50mg/kg, ip), while DOPS (4mg/kg, ip) did not affect quinine antinociception. 4. Pimozide (4mg/kg, ip), L-sulpiride (40mg/kg, ip), SCH 23390 (0.2mg/kg, sc) and alpha-methyl-p-tyrosine (50mg/kg, ip) effectively potentiated the antinociceptive effects of quinine (50mg/kg, ip). 5. Pimozide (4mg/kg, ip) also antagonised the antagonistic effect of d-amphetamine (4mg/kg, ip) on the antinociceptive effect of quinine (50mg/kg, ip). 6. These data indicate that quinine elicited antinociception dose dependently. Furthermore, the influence of pimozide, L-sulpiride and SCH 23390 on quinine antinociception suggests the involvement of dopaminergic mechanisms.

Serotonin and substance P afferents to parafascicular and central medial nuclei

Potent analgesia is elicited by electrical stimulation of the periaqueductal gray (PAG), dorsal raphe nucleus (DRN) and intralaminar thalamus. Horseradish peroxidase conjugated wheat germ agglutinin (HRP-WGA) was stereotaxically pressure injected into the parafascicular (PF) or central medial (CM) nucleus to identify brainstem afferents to the intralaminar thalamus. WGA-immunoreactive (-ir) neurons were identified in the DRN, PAG and lateral dorsal tegmentum (LDTg) after PF and CM injections. Many retrogradely labeled cells in the DRN and ventral PAG were also serotonin-ir, and a portion of WGA-ir cells in the LDTg were substance P-ir. These results substantiate previous studies implicating the intralaminar thalamus and periaqueductal region, as well as serotonin and substance P, in antinociception.

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.

Alpha 2-adrenoceptor- and D2-dopamine receptor-mediated analgesic response of B-HT 920

The involvement of D2-dopamine receptors in the antinociceptive action of B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-(4H) thiazolo-(4,5d)-azepine) has been investigated in mice. B-HT 920 (0.1-2.0 mg kg-1) and apomorphine (0.1-2.0 mg kg-1) produced a dose-dependent increase in tail flick latency. Analgesia induced by apomorphine was blocked by the D2-antagonist, haloperidol (1 mg kg-1) but not by the opioid antagonist, naloxone (1 mg kg-1). The antinociceptive action of B-HT 920 was potentiated by the D1-agonist SKF 38393 (5 mg kg-1), an action antagonized by haloperidol. The selective alpha 2-adrenoceptor blocking drug yohimbine (1 mg kg-1) and naloxone (1 mg kg-1) blocked the antinociceptive action of B-HT 920 (1 mg kg-1). Haloperidol, however, failed to modify the B-HT 920-induced increase in tail flick latency. B-HT 920 and apomorphine reversed reserpine (2 mg kg-1) 4 h-induced hyperalgesia. The reversing action of apomorphine was blocked by haloperidol but not by yohimbine. Thus, a role of alpha 2-adrenoceptors and D2-dopamine receptors is postulated in the antinociceptive action of B-HT 920.

Immunohistochemical distribution of corticotropin-like intermediate lobe peptide (CLIP) immunoreactivity in the human brain

The immunocytochemical distribution of CLIP (corticotropin-like intermediate lobe peptide) or ACTH(18-39), a small biologically active peptide, was examined in the human brain, using a monoclonal antibody against this peptide. Groups of CLIP-immunoreactive cell bodies, small to medium size and bipolar or triangular in shape, were found in the basal hypothalamus extending from the retrochiasmatic region to the premammillary nuclei area. Immunoreactive fibers with varicosities, terminals and "pipe shape" structures, were distributed within the hypothalamus, limbic structures, the brainstem and spinal cord nuclei, forming a particularly rich network in the hypothalamus, the preoptic area, the septal region, the amygdala and the upper brainstem periaqueductal gray matter. The above neuroanatomical observations confirm and extend previous findings in animals, strengthening even more the possibility that this peptide may be involved in numerous behavioral, autonomic and physiological functions such as regulation of sleep-waking cycle, pain control and respiratory and cardiovascular regulation.

Efferent projections of the periaqueductal gray in the rabbit

The efferent projections of the periaqueductal gray in the rabbit have been described by anterograde tract-tracing techniques following deposits of tritiated leucine, or horseradish peroxidase, into circumscribed sites within dorsal, lateral or ventral periaqueductal gray. No attempts were made to place labels in the fourth, extremely narrow (medial), region immediately surrounding the aqueduct whose size and disposition did not lend itself to confined placements of label within it. These anatomically distinct regions, defined in Nissl-stained sections, corresponded to the same regions into which deposits of horseradish peroxidase were made in order for us to describe afferent projections to the periaqueductal gray. In this present study distinct ascending and descending fibre projections were found throughout the brain. Terminal labelling was detected in more than 80 sites, depending somewhat upon which of the three regions of the periaqueductal gray received the deposit. Therefore, differential projections with respect to both afferent and efferent connections of these three regions of the periaqueductal gray have now been established. Ventral deposits disclosed a more impressive system of ramifying, efferent fibres than did dorsal or lateral placements of labels. With ventral deposits, ascending fibres were found to follow two major pathways from periaqueductal gray. The periventricular bundle bifurcates at the level of the posterior commissure to form hypothalamic and thalamic components which distribute to the anterior pretectal region, lateral habenulae, and nuclei of the posterior commissure, the majority of the intralaminar and midline thalamic nuclei, and to almost all of the hypothalamus. The other major ascending pathway from the periaqueductal gray takes a ventrolateral course from the deposit site through the reticular formation or, alternatively, through the deep and middle layers of the superior colliculus, to accumulate just medial to the medial geniculate body. This contingent of fibres travels more rostrally above the cerebral peduncle, distributing terminals to the substantia nigra, ventral tegmental area and parabigeminal nucleus before fanning out and turning rostrally to contribute terminals to ventral thalamus, subthalamus and zona incerta, then continuing on to supply amygdala, substantia innominata, lateral preoptic nucleus, the diagonal band of Broca and the lateral septal nucleus. Caudally directed fibres were also observed to follow two major routes. They either leave the periaqueductal gray dorsally and pass through the gray matter in the floor of the fourth ventricle towards the abducens nucleus and ventral medulla, or are directed ventrally after passing through either the inferior colliculus or parabrachial nucleus. These ventrally directed fibres merge just dorsal to the pons on the ventral surface of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Partial involvement of monoamines and opiates in the inhibition of rat spinal nociceptive neurons evoked by stimulation in midbrain periaqueductal gray or lateral reticular formation

In rats anesthetized with sodium pentobarbital, we tested the effects of systemic or intrathecal administration of the opiate antagonist naloxone, the serotonergic antagonist methysergide, or the adrenergic antagonist phentolamine, on inhibition produced by electrical stimulation in midbrain periaqueductal gray (PAG) or lateral reticular formation (LRF) of the responses of single lumbar dorsal horn neurons to noxious heating (50-54 degrees C, 10 s) of hindpaw skin. Systemically administered naloxone (1-10 mg/kg i.v.) reduced (greater than 20% below predrug inhibition) inhibition from PAG and/or LRF in 5/12 units and had no effect in the remainder. Systemic methysergide (2-6 mg/kg i.v.) selectively reduced PAG-evoked inhibition in 6 units, while inhibitions from both PAG and LRF stimulation were reduced in one unit and unaffected in 8 units. Systemic phentolamine (2-4 mg/kg) reduced LRF-evoked inhibition in 4 units, while inhibitions from PAG and LRF were reduced in one unit and unaffected in 5. Intrathecally applied methysergide reduced or abolished PAG-evoked inhibition in 8/16 units and reduced or abolished LRF-evoked inhibition in 6/14 units. Reductions in the level of inhibition were reversible in one-half of the cases, whereas they persisted for over 2 h in the remainder. The mixed effects of both systemically and intrathecally administered drugs suggest that monoamines and opiates may be partly involved in spinal inhibitory mechanisms activated from the midbrain.

Origins and terminations of bulbospinal axons that contain serotonin and either enkephalin or substance-P in the North American opossum

We have shown previously that some enkephalin, substance-P, and serotoninergic neurons in the medullary raphe and adjacent reticular formation project to the spinal cord in the opossum. In the present study we have combined the retrograde transport of True Blue and immunofluorescence histochemistry to determine whether methionine enkephalin or substance-P containing bulbospinal neurons are serotoninergic. Furthermore, we have used the same immunofluorescence protocol to determine whether spinal axons contain the same substances. Neurons that immunostained for both enkephalin and serotonin were observed in many brainstem nuclei. However, those that projected to the spinal cord were limited to the nuclei raphe magnus and obscurus, and the ventral part of nucleus reticularis gigantocellularis, pars ventralis. Neurons that immunostained for both substance P and serotonin were fewer in number, but some of the ones in the above nuclei and within the nucleus raphe pallidus, projected to the spinal cord. Spinal axons exhibiting both enkephalin- and serotonin-like immunoreactivity were observed in the superficial laminae of the dorsal horn, lamina X, and the intermediolateral cell column, whereas those showing both substance-P and serotonin-like immunoreactivity were seen primarily in lamina X, the intermediolateral cell column, and the ventral horn. Some of the axons in the ventral horn were in close apposition to presumed motoneurons. Comparison of the above results with those obtained from previous studies of bulbospinal projections has allowed us to infer the origins of axons that innervate different spinal targets.

Serotonin involvement in analgesia induced by transcranial electrostimulation

The experiments described here were intended to investigate whether serotonin (5HT) may be involved in analgesia induced by low current transcranial electrostimulation (TE). The TE stimulus is a 10 mu-ampere, 10 Hz, pulsed current transmitted via electrodes in the pinnae. Combinations of the following were given as intraperitoneal injections: 300 mg/kg p-chlorophenylalanine (pCPA) 48 hours before testing, 100 mg/kg 5-hydroxytryptophan (5HTP) 30 min before testing and the saline vehicle for these drugs. Rats were tested prior to and 30 minutes after TE or sham TE. Testing for analgesia consisted of putting progressively increasing pressure on the rat tail 1/4 inch from the tip with a pneumatically driven, right angle wedge. The amount of pressure at which the rat moved its tail was measured both before and after TE, or sham TE, and recorded as the difference in tolerated peak pressure (DTPP). TE produced analgesia as manifested by a 613 percent increase in DTPP compared with sham TE treatment values. Among TE treated rats, pretreatment with pCPA decreased DTPP 91.5 percent compared with saline control values, indicating 5HT involvement. 5HTP restored TE induced analgesia in pCPA treated rats to the level of saline treated control animals, confirming 5HT involvement.

Nociceptive responses in nucleus parafascicularis thalami are modulated by dorsal raphe stimulation and microiontophoretic application of morphine and serotonin

Single-cell experiments were undertaken to examine the hypothesis that serotonin (5-HT) and morphine participate in ascending pain suppression phenomena. The observations demonstrate that: 1) dorsal raphe stimulation (DRS) modulates the spontaneous activity and the noxious-evoked responses of parafasciculus (PF) neurons, and the modulating effects of DRS are altered by either naloxone or methysergide; 2) morphine ejection into the PF alters the spontaneous activity and the noxious-evoked responses of PF neurons, and naloxone prevents morphine effects; and 3) serotonin ejection into the PF alters the spontaneous activity and the noxious-evoked responses of PF neurons and methysergide prevents the serotonin effects. These findings support the hypothesis that opioid and serotonin participate, at least in part, in the control of ascending pain mechanisms.

Evaluation of selective actions of dopamine D-1 and D-2 receptor agonists and antagonists on opioid antinociception

The effect of the selective dopamine receptor agonists SKF 38393 (D-1) and quinpirole (D-2) on nociception was studied in the mouse tail immersion test. The D-1 receptor agonist induced mild hyperalgesia whereas the D-2 agonist produced antinociception. Pretreatment with either the selective D-1 receptor antagonist SCH 23390 or the D-2 receptor antagonist (-)-sulpiride converted the hyperalgesia produced by the D-1 agonist into an antinociceptive response whereas the effect of the D-2 receptor agonist was significantly antagonised. The antinociceptive response of selective opioid agonists was also studied in combination with selective dopamine receptor agonists and antagonists. Sufentanil (mu-opioid) antinociception was enhanced in animals pretreated with (-)-sulpiride but not SCH 23390. In animals co-administered sufentanil with SKF 38393 there was a reduced antinociceptive effect whilst quinpirole enhanced the action of sufentanil. Likewise, antinociception induced by the kappa-opioid agonist U50,488H was unaltered in animals pretreated with SCH 23390, increased by (-)-sulpiride, and reduced by SKF 38393. delta-Opioid antinociception induced by [D-Ala2,D-Leu5]enkephaline remained unmodified following pretreatment with either (-)-sulpiride or SCH 23390 but was potentiated in animals which received both the delta-agonist and the D-2 receptor agonist. It is concluded that D-2 receptor agonists not only have intrinsic antinociceptive activity, but can also potentiate opioid-induced antinociception. Similarly, dopamine D-2 receptor antagonists appear to potentiate opioid-induced antinociception in this nociceptive model.

Analgesic activity of morphiceptin, beta-casomorphin-4, and deltakephalin in normotensive Wistar-Glaxo and spontaneously hypertensive rats

The effects of intraventricular injection of beta-casomorphin-4, morphiceptin and deltakephalin (DTLET) on hot water tail flick and tail compression responses were investigated in Wistar Albino Glaxo (WAG) and spontaneously hypertensive rats (SHR). The effects of the mu agonist morphiceptin (20 nmol/rat), as assessed by the tail compression test, were significantly greater in SHR rats but did not differ between both strains when measured by tail flick latency. Opioid agonist deltakephalin (2 nmol/rat) in both tests elicited stronger analgesic effects in SHR as compared to WAG and these effects were blocked by naloxone in both tests used. beta-Casomorphin-4 exhibits moderate activity for mu receptors. In the tail flick test peptide (60 nmol/rat) produced an increase in latencies in SHR rats that was significantly greater than was observed in WAG rats. Naloxone pretreatment abolished the analgesic activity of beta-casomorphin-4 solely in the tail compression test in SHR. Analysis of the slopes of the dose-response curves seems to suggest that differences between the activity of these opioid peptides in SHR and WAG rats are based on a difference in the density and affinity of the subpopulation of the opioid receptors in these strains of rats.

An ultrastructural analysis of serotoninergic neurons in the nucleus raphe magnus of the rat

The synaptic organization of serotonin elements in the nucleus raphe magnus of adult rat was investigated by electron microscopic immunocytochemistry with an antiserum against serotonin itself. Immunoreactive somata in the nucleus raphe magnus were usually of the same size (7-15 microns) and showed similar cytological features as their unlabeled congeners. The serotonin perikarya were contacted by a few unlabeled axon terminals containing round synaptic vesicles, and gave rise to dendrites which often ran perpendicularly to the midline. These dendrites received many contacts from axon terminals containing round or pleomorphic synaptic vesicles, but themselves failed to show vesicular aggregates or membrane differentiations suggestive of synaptic specialization. Immunoreactive axon terminals were numerous and mostly contained round or pleomorphic vesicles. Several exhibited synaptic contacts on dendrites, but there were also others which did not show any synaptic membranous differentiation, even when followed in serial sections. Immunostained myelinated as well as unmyelinated axons could be observed. These results provide a first description of the morphology and synaptic organization of the serotonin neurons in adult rat nucleus raphe magnus.

Prevention of the analgesic consequences of social defeat in male mice by 5-HT1A anxiolytics, buspirone, gepirone and ipsapirone

Behavioural and pharmacological studies have suggested that anxiety may be an important factor in the initiation of non-opioid analgesia in defeated male mice. In the present study, the effects of three 5-HT1A anxiolytics (buspirone, ipsapirone and gepirone) on basal nociception and defeat analgesia were examined. Results show that the analgesic consequences of social defeat were potently blocked by all three compounds, with a rank-order potency (minimum effective doses) of ipsapirone (0.05 mg/kg) greater than gepirone (0.1 mg/kg) greater than buspirone (0.5 mg/kg). These inhibitory effects on defeat analgesia were observed in the absence of intrinsic activity on basal nociception (tail-flick assay). When administered alone, (-)pindolol produced biphasic effects on defeat analgesia with enhancement at 0.5 mg/kg and inhibition at 5.0 mg/kg. Lower doses of (-)pindolol (0.05 and 0.25 mg/kg) which did not affect defeat analgesia when administered alone, totally blocked the inhibitory effects of ipsapirone (0.5 mg/kg). Data are discussed in relation to the involvement of 5-HT1A receptor mechanisms in this adaptive form of pain inhibition.

Endogenous opiates and the pathogenesis of hypertension

Opiates are now known to be important modulators of cardiovascular function in both the normotensive and hypertensive states. There is accumulating evidence that endogenous opiates are elevated in models of hypertension of various etiologies including genetic and renovascular hypertension. Early evidence for elevated opiates in hypertension arose from observations that hypertensive humans and rats with genetic or experimental hypertension exhibited hypoalgesia in various tests of pain sensitivity. Because pain and cardiovascular regulatory systems have in common a number of brain loci, cardiovascular effects of opiates and opiate blockade were studied. These studies have shown that opiate blockade can attenuate the development of hypertension and reduce blood pressure in chronic hypertension possibly via actions on the baroreflexes and/or by modulating the centrally mediated pressor actions of angiotensin II.

Addiction to tranylcypromine (Parnate): a case report

The monoamine oxidase inhibitors are increasingly prescribed for several disorders today, after a hiatus of nonuse lasting a decade. One case of excessive MAOI use and two cases of MAOI withdrawal are reported in the literature. This case report adds a fourth instance and suggests patient characteristics which should alert the clinician to the risk of MAOI abuse or addiction.

Dopaminergic involvement in nociceptive sensitivity/behavioral reactivity regulation during aversive states of different nature in the rat

To investigate the involvement of dopamine (DA) in nociceptive sensitivity-behavioral reactivity regulation in animals during aversive states of different nature, the influence of pharmacologically-induced decrease and increase of DA neurotransmission on vocalization and movement reactivity were studied in rats in free behavior, during restraint stress, after acute trauma of an extremity and under intraperitoneal acetic acid administration. The influence of longterm increase (apomorphine in a high dose) and decrease (haloperidol, apomorphine in a low dose) on suprarenals weight and gastric ulceration in animals exposed by polymodal aversive stimulation was also studied. The data obtained are discussed in relation with; 1. DA involvement in regulation of nociceptive sensitivity and behavioral reactivity in aversive environment; 2. the role of DA and endogenous opioid peptides in endogenous analgesic mechanisms; 3. the functional significance of cerebral DA in organization and realization of various types of an organism's adaptive activity produced by different environmental and homeostatic variables; and 4. the interaction of DA and endogenous opioid peptides in mediation of this activity.

The antinociceptive role of a bulbospinal serotonergic pathway in the rat brain

The antinociceptive role of spinal serotonin (5-HT) neurons descending from 5-HT cells near the ventrolateral surface of the medulla oblongata was investigated by stimulating these cells in normal rats, in rats with generalized or selective chemical ablation of 5-HT nerves, and in rats with postsynaptic blockade of 5-HT receptors. Electrical stimulation of the lateral medulla elicited analgesia in normal rats; the increase in pain threshold was proportional to the intensity and to the frequency of stimulation. In addition, microinjection of kainic acid or L-glutamate at the same sites also produced analgesia. However, generalized destruction of CNS 5-HT nerves produced by intraventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) or selective destruction of spinal 5-HT nerves produced by intraspinal injection of 5,7-DHT reduced the magnitude of the antinociceptive responses to electrical stimulation. Postsynaptic blockade of CNS 5-HT receptors produced by intraventricular injection of cyproheptadine also reduced the stimulation-produced analgesia. The specificity of the lesions for 5-HT nerves is demonstrated by the lack of effect on the levels of noradrenaline in the same brain regions. The results indicate that the activity of 5-HT nerve cells adjacent to the ventrolateral surface of the medulla oblongata and projecting to the spinal cord serves to elevate pain threshold.

Course and mode of action of descending system conveying nucleus raphe magnus induced inhibition of flexion reflex in rats

In lightly anaesthetized rats, the latencies and magnitudes of heat-evoked withdrawal reflexes from the hindlimb were measured electromyographically. Low-intensity (20-50 microA) stimulation of the nucleus raphe magnus (NRM) resulted in an inhibition of the flexion reflex (commonly referred to as analgesia) as evidenced by increased latency and decreased amplitude. The effect lasted for several minutes after the stimulation was terminated. Following lesions of the dorsolateral funiculus (DLF) at the neural thoracic levels 7-8, baseline latencies were reduced and the effect of the NRM stimulation was abolished. Lesions of the DLF at lumbar level 1 resulted in unaltered baseline latencies with persistence of the inhibitory effect of NRM stimulation. The results of the present experiment show that pathways exerting a tonic inhibition of the withdrawal reflex, and mediating the effect of electrical stimulation from the NRM, descend in the DLF at the thoracic level of the spinal cord. At the lumbar level, there is a shift away from the DLF. The antinociceptive effect of 20 microA NRM stimulation was partly reduced by pretreatment with the serotonin synthesis inhibitor para-chlorophenylalanine (PCPA) or the opiate naloxone (1 mg kg-1 i.v.). In animals pretreated with PCPA, naloxone (1 mg kg-1 i.v.) completely abolished the analgesic effect of the stimulation. Thus, both serotonergic and opioid systems may be implicated in mediating the analgesia. With 50 microA stimulation the same treatment only partly attenuated the NRM-induced analgesia, indicating an additional non-serotonergic and non-opioid mechanism which requires a higher current intensity for its activation.

Intraventricular morphine produces pain relief, hypothermia, hyperglycaemia and increased prolactin and growth hormone levels in patients with cancer pain

The effects of analgesic, thermoregulatory and endocrine functions of administering morphine sulphate (0.3 mg) into the lateral cerebral ventricle via an Ommaya catheter were assessed in eight patients with cancer pain. Satisfactory control of intractable pain was obtained in these patients, without any change in other sensory modalities. The delay in the onset of pain relief and the duration of analgesia ranged, respectively, from 20 to 40 min and from 12 to 16 h after drug injection. In addition, intraventricular administration of morphine caused a reduction in rectal temperature in these patients at an ambient temperature of 24 degrees C. The hypothermia in response to the injection of morphine was due to cutaneous vasodilation and sweating. There was no change in metabolism or in respiratory evaporative heat loss after morphine injection. Further, 10 to 20 min after intraventricular administration of morphine, the blood levels of prolactin, growth hormone and glucose were elevated in these patients. The changes in temperature and endocrine levels lasted for 1-3 h. In addition to the pain relief, these side-effects of morphine treatment were short-lasting and disappeared as the morphine treatment continued. The results indicate that activation of opiate receptors in the brain produced pain relief, hypothermia (due to cutaneous vasodilation and sweating), and increased blood levels of prolactin, growth hormone and glucose in patients with cancer pain.

Differential effects of p-chlorophenylalanine on indoleamines in brainstem nuclei and spinal cord of rats. I. Biochemical and behavioral analysis

The involvement of endogenous serotonergic pathways in the mediation of antinociception has been indicated by electrophysiological, pharmacological and behavioral experiments. However, manipulation of the indole pathway, either by lesioning of raphe nuclei or drug intervention, often produces disparate results. In particular, serotonin (5-HT) synthesis inhibition with p-chlorophenylalanine (PCPA) has been reported to produce either hyperalgesia or analgesia, depending upon the type of pain measurement examined. In the present study, we sought to evaluate the effects of PCPA on (1) behavioral responses to noxious stimulation, and (2) levels of serotonin, tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) in raphe nuclei (pallidus, obscurus, magnus and dorsalis) and spinal cord regions by HPLC with electrochemical detection. Treatment of rats with 400 or 600 mg/kg of PCPA for 3 consecutive days resulted in significant elevations in pain thresholds assessed by tail withdrawal from radiant heat as well as vocalization to electric shock of the tail. The effect of PCPA on vocalization threshold was particularly striking, for the majority of animals showed a nociceptive-specific attenuation of this response. Although the PCPA induced changes in indole content of the various raphe nuclei were not unequivocally dose-dependent, differential reductions of serotonin and 5-HIAA were clearly detected in the various raphe regions. Nuclei raphe pallidus and obscurus were depleted of 5-HT and 5-HIAA to the greatest extent, whereas levels detected in nuclei raphe magnus and dorsalis were reduced by 30-40% from control values. Metabolism of 5-HT and 5-HIAA appeared unaffected by PCPA in all regions examined except the dorsal portion of the spinal cord. These findings collectively suggest that the effects of PCPA are not uniform throughout the central nervous system and raise the possibility that discrepancies in the behavior literature may be attributed to drug-induced changes in some, but not all serotonergic pathways.

On the relationship between chronic pain and depression when there is no organic lesion

Patients suffering from pain without evidence of either depression or organic lesions, or of any pathophysiological process to which the pain might be attributed, may be diagnosed as suffering from 'indeterminate pain.' The evidence in the literature which suggests that some of these cases might be due to a 'depressive equivalent' is examined. It is suggested that it is difficult, if not impossible, to formulate a diagnosis of depressive illness in patients who do not have clear symptoms of depression. However, there is evidence in the literature that a relatively high percentage of patients with chronic indeterminate pain appear to have a family history of depression and depressive spectrum disorders. Biological markers of depression also give some indication that certain of these patients may have a link with depressive illness as well as with pain. It is suggested that there is a need to explore the existence of a sub-group of patients with indeterminate pain in whom the mechanism of the pain may be related to the mechanism of depressive illness even though formal depressive symptoms are not found.

Morphine-induced alterations in plasma and tissue cholesterol levels

In rats fed a cholesterol-cholic acid supplemented diet, implantation of a 75 mg morphine pellet elevated total plasma cholesterol, raised low density lipoprotein (LDL) plus very low density lipoprotein (VLDL) cholesterol, and lowered high density lipoprotein (HDL) cholesterol levels. The resultant increase of the atherogenic index was accompanied by enhanced aortic cholesterol deposition. These alterations were prevented by daily administration of naltrexone (1.0 mg/kg, sc), and were not associated with hyperphagic or hepatotoxic actions of morphine. An increase in total plasma cholesterol and in LDL plus VLDL cholesterol was also observed in morphine pelleted rats maintained on a normal diet. The possible implications of opiate-induced hypercholesterolemia are discussed.

L-dopa in uremic patients with the restless legs syndrome

Restless legs syndrome (RLS) is a poorly understood, often distressing condition that is particularly prevalent among patients with chronic renal failure. A wide variety of medications have been used to treat RLS with variable results. In order to evaluate the efficacy of carbidopa/levodopa therapy, eight consecutive uremic patients with RLS on maintenance hemodialysis were treated with doses ranging from 25/100 to 25/250 twice daily. Six of eight patients obtained satisfactory relief which has continued for 3 months follow-up. Carbidopa-levodopa appears to be an effective opinion in management of RLS in patients with chronic rental failure.

Altered local cerebral glucose utilization induced by electrical stimulations of the thalamic sensory and parafascicular nuclei in rats

Alterations in local cerebral glucose utilization (LCGU) induced by electrical stimulation of the sensory relay nucleus (VPL) or parafascicular nucleus (Pf) of the thalamus in conscious rats were measured by the [14C]2-deoxyglucose method, the objective being to assess the mechanism of analgesia induced by electrical stimulations of these structures. Stimulation of the VPL induced an ipsilateral increase in LCGU in the sensory thalamic nucleus itself, the sensory cortex and substantia nigra. Stimulation of the Pf induced bilateral increases in LCGU in the Pf and central medial nucleus of the thalamus, sensory cortex, ventral areas of the striatum and substantia nigra, and ipsilateral increase in LCGU in the periaqueductal gray, parabrachial pontine nucleus and deep layers of the superior colliculus. No significant change in LCGU was detected in the raphe dorsalis, raphe magnus and spinal dorsal horn, in both groups. Our observations coincide with clinical findings that unilateral electrical stimulation of the Pf leads to amelioration of intractable pain bilaterally, while that of the VPL induces an analgesia restricted to the contralateral side.

A peripheral sympathetic component in inflammatory hyperalgesia

Non-steroidal anti-inflammatory drugs (NSAID) only partially inhibit the hyperalgesia in the inflammation induced by carrageenin in the hind rat paw, one of the most frequently used nociceptive tests. We now report that either the guanethidine depletion of peripheral sympathomimetic amines or the treatment with adrenoceptor antagonists (beta-blockers) and a specific dopamine (DA)-1 antagonist (SCH 23390) significantly reduced carrageenin hyperalgesia. These antagonists also abolished the rat paw hyperalgesia induced by several sympathomimetic amines as well as that induced by a selective DA-1 agonist, SKF 38393. Blockade of uptake-1 by cocaine potentiated the hyperalgesia induced by carrageenin and sympathomimetic amines. We conclude that there is a sympathetic component, possibly mediated by a DA-1 type receptor in carrageenin-induced hyperalgesia. This component may predominate in certain types of pain not sensitive to NSAID. If so, selective peripheral DA-1 antagonists could be developed as a novel class of analgesics.

Pain relief by electrical stimulation of the periaqueductal and periventricular gray matter. Evidence for a non-opioid mechanism

Pain relief following stimulation of the periaqueductal gray matter (PAG) or periventricular gray matter (PVG) in man has been ascribed to stimulation-induced release of endogenous opioid substances. Forty-five patients were studied and followed for at least 1 year after placement of chronic stimulating electrodes in the PAG or PVG to determine if pain relief due to stimulation could be ascribed to an endogenous opioid mechanism. Three criteria were assessed: the development of tolerance to stimulation; the possibility of cross-tolerance to morphine; and reversibility of stimulation-induced pain relief by the opiate antagonist naloxone. Sixteen patients (35.6%) developed tolerance to stimulation, that is, they obtained progressively less effective pain relief. Twelve (44.4%) of 27 patients undergoing stimulation of the thalamic sensory relay nuclei for treatment of chronic pain (a presumably non-opioid mechanism) also developed tolerance. Morphine sulfate was administered in a blind, placebo-controlled protocol to 10 patients who had become tolerant to PAG-PVG stimulation and none showed evidence of cross-tolerance. Fifteen of 19 patients, already tolerant to morphine at the time of PAG-PVG electrode implantation, experienced excellent pain relief by stimulation, also indicating a lack of cross-tolerance. Twenty-two patients who experienced excellent pain relief from chronic PAG-PVG stimulation received intravenous naloxone in a double-blind, placebo-controlled protocol. Pain intensity as assessed by the visual analog scale was increased to the same degree by both placebo and naloxone. Eight patients showed no increase in pain intensity with either placebo or naloxone. Although tolerance to PAG-PVG stimulation developed in these patients, the frequency of tolerance was similar to that seen in patients undergoing thalamic sensory nuclear stimulation. Since the latter technique presumably relieves pain by a non-opioid mechanism, the development of tolerance to PAG-PVG stimulation does not, in itself, confirm an opioid mechanism. Cross-tolerance between PAG-PVG stimulation and morphine was not seen and cross-tolerance to PAG-PVG stimulation in patients already tolerant to morphine was rare. The pain-relieving effect of PAG-PVG stimulation was reversed to an approximately equal degree by naloxone and placebo. The authors do not believe that, in most patients, pain relief elicited by PAG-PVG stimulation depends on an endogenous opioid mechanism. It appears that other, non-opioid mechanisms are primarily responsible for such pain relief.

Spinal dopamine mechanisms and primary sensory symptoms in Parkinson's disease

Primary sensory symptoms occur in approximately 40% of patients with Parkinson's disease. Although the pathophysiology of these symptoms is poorly understood, the recent discovery of diencephalospinal dopaminergic neuronal pathways potentially implicates this system in the etiology of these symptoms. The possible contribution of the hypothalamus with its connections to descending midbrain structures and the spinal cord to the etiology of primary sensory symptoms in Parkinson's disease are discussed.

The nociceptive jaw-opening reflex: evidence for alpha 2 adrenoceptor involvement

The ability of alpha adrenoceptor agonists to modulate the tooth pulp stimulation evoked (TPS) jaw-opening reflex (JOR) was investigated in rats and rabbits. Low doses of clonidine (6.25-50 micrograms/kg, IV) significantly increased dEMG thresholds. These effects were antagonized by alpha 2 adrenoceptor antagonists (e.g., yohimbine), but not by alpha 1 adrenoceptor antagonists (e.g., prazosin) or mu receptor antagonists (e.g., naloxone). Polar alpha 2 adrenoceptor agonists (e.g., ST-91 and 4-hydroxyclonidine) that cross the blood brain barrier (BBB) poorly and lipophilic alpha 1 adrenoceptor agonists (e.g., ST-587) that cross the BBB easily were without affect on the TPS-JOR. Structures of the peripheral efferent neurocircuitry of the JOR (e.g., the digastric muscle and the neuromuscular junction of the digastric muscle and its motor nerve, the mylohyoid) were shown not to be active sites of clonidine's effect on the TPS-JOR. Treatment with phentolamine (an alpha adrenoceptor antagonist that poorly crosses the BBB) completely poorly crosses the BBB) completely antagonized clonidine's initial transient cardiovascular (pressor) effect without altering its TPS-JOR effects. Pretreatment with reserpine (a catecholamine depleting agent) failed to alter clonidine's affects on the TPS-JOR. Our studies suggest that alpha 2 adrenoceptors potently modulate the TPS-JOR and such modulation may be important in understanding trigeminal neuronal circuitries that partake in pain processing.

Central gray modulation of affective defense is differentially sensitive to naloxone

Specific areas of feline central gray (CG) modulate affective aggression elicited by hypothalamic stimulation and produce analgesia, possibly mediated by enkephalins. Despite correlations between opiates and aggressive behavior suggested previously, their relationship has not been clearly demonstrated. The goal of this study was to examine the possible role of endogenous opiate mechanisms in CG modulation of affective aggression. Electrodes were placed in the ventromedial hypothalamus (VM) which, when stimulated, elicited affective attack. Control latency to VM-elicited hiss was established. CG cannula-electrodes were then implanted. Response modulation was determined by dual stimulation of CG and VM electrodes, alternated with VM stimulation alone. Naloxone was introduced through the CG electrodes and VM-elicited hiss latency was again determined, followed by paired trials of dual and single stimulation. Six ventral facilitatory and eight dorsal inhibitory sites were identified. Naloxone had no effect on CG facilitation but completely blocked CG inhibition of VM-elicited hissing. In support of these findings, it was observed that D-A1A2-MET5-enkephalin injected directly into CG-inhibitory sites suppressed VM-elicited hissing. These results demonstrate that opiate mechanisms are operative in the inhibitory modulation of intra-specific aggression.

Inhibition of dorsal-horn cell responses by stimulation of the Kölliker-Fuse nucleus

The Kölliker-Fuse nucleus (KF) in the dorsolateral pons has been shown to be the major source of catecholamine innervation of the spinal cord. This has important implications in terms of pain control mechanisms, since catecholamine-mediated mechanisms are essential for the expression of opiate and other varieties of antinociception. This study examines the effects of KF stimulation on responses of dorsal-horn cells to innocuous and noxious cutaneous stimuli in anesthetized cats. Stimulation of the KF potently inhibits the responses of dorsal-horn cells to both noxious and innocuous stimuli. The threshold for the inhibitory effect is significantly lower for responses to noxious stimuli as opposed to innocuous stimuli. The inhibitory effect is specific to the stimulus site, as evidenced by a marked decrease in the effect following small changes in the position of the stimulating electrode in the brain stem. The latency of the effects indicates a bulbospinal conduction velocity of 4 to 5 m/sec, which is much slower than usual reticulospinal effects and is consistent with a catecholamine-mediated system. The dependence of KF-spinal inhibition on intact biogenic amines was tested by depleting the animals of these amines with reserpine pretreatment. Depletion of biogenic amines resulted in a significant decrease in the KF spinal inhibitory effects, suggesting their dependence on intact noradrenergic stores. The results of these studies are consistent with the idea that the KF-spinal system plays an important noradrenergic-dependent role in the brain-stem modulation of spinal processing of noxious, potentially painful stimuli.

Evidence of the involvement of dopamine in the analgesic effect of nefopam

The involvement of brain monoamines in the mechanism of action of nefopam, a new analgesic, was investigated in rats. The study was designed to evaluate the effect of various means of impairing monoaminergic transmission on nefopam analgesia as measured with the hot plate method. Pretreatment with reserpine (2 mg/kg) significantly reduced the antinociceptive action of nefopam (40 mg/kg), indicating that the interaction of this drug with the monoaminergic systems is important for its effects. A role for serotonin (5-HT) or norepinephrine (NE) was ruled out by the fact that selective depletion of 5-HT (using 5,7-dihydroxytryptamine) or NE (using DSP-4 or FLA-63) did not affect nefopam analgesia. A significant reduction of the effect of nefopam was found in rats pretreated with 6-hydroxydopamine (6-OHDA). Also 6-OHDA plus desipramine, which selectively depleted brain DA, markedly reduced the antinociceptive effect of nefopam. The data strongly suggest that a critical dopaminergic synapse is involved in the mechanism by which nefopam inhibits nociceptive responses in rats.

Anticholinergic-induced analgesia: possible role for the cholinergic system in abnormal sensory symptoms in Parkinson's disease

Sensory symptoms related to pain perception have been reported to occur in 30-50% of parkinsonian patients. Two patients with Parkinson's disease are reported, in whom painful sensory phenomena preceded or accompanied the disease process. In both patients the sensory phenomena were unresponsive to therapy with oral narcotics, anti-inflammatory drugs or administration of levodopa/carbidopa. Benzhexol (4-6 mg/day) produced dramatic amelioration of symptoms, indicating a role for the cholinergic system in the pathophysiology of abnormal sensory symptoms in Parkinson's disease and possibly in human analgesia in general.

The relationship between cardiovascular and pain regulatory systems

An increasing amount of anatomical, physiological, and pharmacological evidence suggest that pain inhibitory circuitry is linked with cardiovascular regulatory systems in man and laboratory animals. Induction of hypertension in rats by different methods (mineralocorticoid treatment, stenosis of renal artery, or social deprivation) is associated with reduced responsiveness to noxious thermal stimuli (hot-plate) or to noxious mechanical stimuli (paw pressure). Genetically hypertension-prone rats derived from the SABRA strain and spontaneously hypertensive rats derived from Wistar/Kyoto strain also display a similar hypoalgesia. Acute increases in blood pressure are associated with reduced sensitivity to painful stimuli. Additionally, the interaction between blood pressure and pain perception has also been supported by the demonstration that various experimental interventions that diminish the magnitude of hypertension also attenuate the hypoalgesia. Recent clinical findings are also in agreement with the laboratory animal findings since sensory and pain thresholds have been shown to be significantly higher in unmedicated essential hypertensive subjects compared to normotensive controls. Thus, the human data corroborate animal data and suggest that a relation between blood pressure and pain sensitivity is likely to be a general phenomenon. It is unlikely that damage to peripheral pain fibers caused by a change in blood pressure contributes to the observed hypoalgesia. Naloxone, which has no effect on blood pressure, returns the pain sensitivity to normal levels. Behavioral tests (open field and motor activity cage) of normotensive and of renal and genetically (SBH and SHR) hypertensive rats exclude the possibility of a general motor deficit in hypertensive rats. Endogenous opioid peptides in central and peripheral nervous systems as well as in endocrine organs are implicated, although non-opioid mechanisms are also evident. Activation of baroreceptor afferents by acute or chronic increases in arterial or venous blood pressure may play an important role in the somatosensory responses associated with the increase in blood pressure. Coordinated cardiovascular-pain regulatory responses may be part of an adaptive mechanism that helps the body to face stressful events.

Alpha 2 adrenoceptors modulate the nociceptive jaw-opening reflex in rats

The jaw-opening reflex in lightly anesthetized rats induced by intrapulpal (left maxillary) electrical tooth pulp stimulation and quantified by the electromyograms (threshold values) recorded from the ipsilateral digastric muscle was used as the experimental pain index. The threshold for the jaw-opening reflex was significantly elevated by clonidine (12.5 to 50 micrograms/kg, i.v.) and was inversely correlated with the frequency of stimulation. The analgesia elicited by clonidine was antagonized by pretreatment with the alpha 2-adrenoceptor antagonist yohimbine (1 mg/kg, i.v.). All doses of clonidine produced an initial transient pressor response followed by a sustained hypotension and bradycardia. However, there was no direct correlation between the antinociceptive and cardiovascular effects of clonidine. It is proposed that clonidine modulates jaw-opening reflex analgesia by specifically activating alpha 2-adrenoceptors.

The site and the mode of analgesic actions exerted by clonidine in monkeys

We used intracerebral administration of clonidine in monkeys to map effective sites for analgesia. The jaw opening reflex elicited by tooth pulp stimulation was used for analgesia testing. We found that the most consistently effective sites for analgesia in monkeys are in at least three brain regions: the diencephalic periventricular gray, the dorsal raphe nuclei, and the periaqueductal gray. In addition, the analgesia induced by intracerebral administration of clonidine was effectively antagonized by pretreatment of animals with either naloxone (a narcotic antagonist) or yohimbine (an alpha-adrenergic antagonist). These results suggest the existence of an opiate and an adrenergic antinociceptive mechanism in the diencephalic periventricular gray, the dorsal raphe nuclei, and the periaqueductal gray activated by clonidine in primates.

A fictive tail flick reflex in the rat

The discharges of motor axons participating in the tail flick reflex were recorded from nerve filaments innervating the medial longissimus muscles of anesthetized rats. The reflex discharges evoked by stimulation of the tail with either noxious radiant heat or pinch were recorded before and after paralysis of the animals. Nociceptive discharges recorded from motor axons in the paralyzed state showed a strong correlation with those observed in the absence of the paralytic agent. For this reason, the electrophysiological response triggered by noxious input was termed a 'fictive tail flick reflex'. To evaluate the potential applicability of this model in the analysis of pain blocking mechanisms, vaginal stimulation was found to produce a profound reduction to the nociceptive discharges of the fictive tail flick. By eliminating movement artifacts from the experimental paradigm, this model expands our ability to study a basic nociceptive response which was previously limited to behavioral observation.

Sources of the catecholaminergic innervation of the trigeminal nucleus caudalis in cat

Injections of the retrogradely transported fluorescent dye, Evans blue, into the trigeminal nucleus caudalis were combined with the glyoxylic acid histofluorescence technique to determine the sources of catecholamine-containing varicosities innervating nucleus caudalis. Results indicate that the sources of this catecholamine innervation are widespread, originating from cell bodies throughout the brain stem including the medullary catecholamine cell groups as well as the noradrenergic nuclei of the dorsolateral pons, including locus ceruleus, subceruleus, Kölliker-Fuse, and the parabrachial nuclei. A small projection from the presumably dopaminergic neurons of the hypothalamus was also noted. The catecholamine innervation of n. caudalis in the cat is from widespread brain stem sources, a pattern different from the catecholamine innervation of the spinal cord, which receives its major catecholamine input from the Kölliker-Fuse nucleus.

Immunohistochemistry of synaptic input and functional characterizations of neurons near the spinal central canal

Neurons surrounding the central canal in sacral spinal segments were functionally characterized on the basis of somatic and/or visceral afferent input, then intracellularly marked with horseradish peroxidase (HRP). Tissue sections containing portions of HRP-stained neurons were subsequently immunohistochemically examined for the presence of contacts made by axonal enlargements containing vasoactive intestinal polypeptide (VIP), substance P (SP), somatostatin (SS), Leu-enkephalin (ENK), or serotonin (5-HT). ENK-and 5-HT-containing enlargements were found to contact all neurons examined. SP and SS terminals contacted fewer neurons, and were not associated with specific functional classes. On the other hand, VIP-containing fibers contacted only those neurons receiving visceral afferent input, thus supporting the contention that VIP is contained in a population of visceral afferent fibers projecting to the gray matter surrounding the central canal at sacral levels.

Narcotic antagonist-induced hypotension in the spontaneously hypertensive rat

Intravenous naloxone or naltrexone produced transient, dose-related reductions in the mean arterial pressure (MAP) and heart rate (HR) of urethane-anesthetized spontaneously hypertensive rats (SHRs). Yet these same doses of narcotic antagonists reduced HR but not MAP of normotensive Wistar-Kyoto rats (WKYs). Such effects were not observed upon administration to SHRs of increasing doses of methylnaltrexone, which possesses no central activity. (+)-Naloxone, which does not block opiate receptors, reduced HR but not MAP of both SHRs and WKYs. These findings indicate that SHRs and WKYs differ in their MAP and HR responses to narcotic antagonists. The high doses required for effect plus the brevity of the responses suggest that these drug effects are perhaps not mu-opiate receptor-mediated; however, the methylnaltrexone and (+)-naloxone findings clearly implicate a central specificity of action. We conclude that narcotic antagonist-induced changes in MAP and HR in SHRs are possibly specific and central in origin yet not mediated by mu-opiate receptors.