Ventrolateral medullary lesions block the antinociceptive and cardiovascular responses elicited by stimulating the dorsal periaqueductal grey matter in rats

The antinociception evoked by anterior pretectal nucleus stimulation is partially dependent upon ventrolateral medullary neurones

Electrical stimulation (35 microA rms/15 s) of the anterior pretectal nucleus (APtN) inhibits the spinal reflex of the tail-flick (TF) to noxious heat in unanaesthetised rats. APtN stimulation also reduces the nociceptive response of spinal dorsal horn neurones in halothane-anaesthetised rats. This study determined if the antinociceptive effects of APtN stimulation depended on neurones in the ventral medulla. Bilateral electrolytic lesions of the ventrolateral medulla, but not the nucleus raphe magnus, reduced by 70% the antinociceptive effect of APtN stimulation in the TF test. In rats anaesthetised with halothane, electrical stimulation of the APtN (single square wave 0.1 msec pulses, 2-20 microA, 1 Hz) excited cells in the ventrolateral medulla. These data suggest a connection between both areas. This connection is further confirmed by neuroanatomical tract tracing studies in which the retrograde dye Fast Blue was injected into the ventrolateral medulla. Fluorescent cell bodies were found in the APtN. We therefore conclude that the ventrolateral medulla is part of a descending antinociceptive pathway from the APtN.

Midbrain central grey: regional haemodynamic control and excitatory amino acidergic mechanisms

The haemodynamic responses to electrical and chemical stimulation of the periaqueductal or central grey (CG) was investigated in urethane-anaesthetized rats. CG stimulation resulted in a characteristics pattern of mesenteric and renal vasoconstriction accompanied by modest hindquarter vasodilatation. This haemodynamic response was also accompanied by widening of the palpebral fissure, tachycardia and by twitching of the vibrissae. This constellation of physiological responses constitutes the 'defence reaction' and indicates that the CG area under investigation is involved in these phenomena. Both electrical and chemical (kainic acid) lesions of the pressor area of the rostral ventrolateral medulla (RVLM) attenuated the pressor responses to CG stimulation. Intrathecal administration of the excitatory amino acid receptor antagonist kynurenic acid (0.5 mumole/10 microliter) markedly reduced the pressor responses produced by stimulation of both the CG and the RVLM. These results provide additional evidence in support of the notion that neurons arising in the CG relay in the RVLM where they may, in turn, communicate with a descending excitatory amino-acidergic pathway involved in cardiovascular control.

Periaqueductal gray stimulation produces a spinally mediated, opioid antinociception for the inflamed hindpaw of the rat

The objective of the present study was to characterize stimulation-produced antinociception from the periaqueductal gray matter (PAG) in rats with unilateral hindlimb inflammation induced by an intraplantar injection of Freund's complete adjuvant. Rats were chronically implanted with a bipolar stimulating electrode in the PAG. Nociception was assessed using a paw pressure test. Prior to inflammation, PAG stimulation significantly increased paw pressure threshold in both paws compared to non-stimulated controls. Following inflammation, PAG stimulation inhibited nociception in the inflamed, but not the non-inflamed paw. Systemic administration of naloxone blocked antinociception from ventral, but not dorsal PAG stimulation sites. Intrathecal, but not subcutaneous, administration of quaternary naltrexone completely blocked stimulation-produced antinociception from the PAG. The known increased levels of endogenous opioids occurring in the spinal cord ipsilateral to the site of inflammation suggest a mechanism for the selective antinociceptive effect of ventral PAG stimulation seen for the inflamed paw.

Hypothalamic, midbrain and bulbar areas involved in the defense reaction in rabbits

The present study mapped neuroanatomical sites in the hypothalamus and periaqueductal gray (PAG) of the rabbit which, when stimulated electrically, evoked the cardiorespiratory components of the defense reaction (CRDR). This included increases in heart rate, blood pressure, hindlimb blood flow and respiration rate. All of the components of the CRDR were elicited by electrical stimulation of the posterior hypothalamus, at sites dorsal and medial to the fornix. Although there were regions throughout the PAG in which electrical stimulation elicited concomitant increases in blood pressure, hindlimb blood flow and respiration rate, only stimulation of the dorsal PAG evoked tachycardia. Injection of horseradish peroxidase into the rostral ventrolateral medulla (RVLM) led to heavy retrograde and anterograde labeling in the region of the hypothalamus that yielded the CRDR when stimulated electrically. Heavy labeling was also observed in the dorsal and ventral PAG. The results of this study provide evidence that the posterior hypothalamus and the dorsal PAG are nodal structures in the mediation of the CRDR and that cells in posterior hypothalamus, dorsal PAG and ventral PAG make monosynaptic connections with the RVLM.

GABAergic circuitry in the rostral ventral medulla of the rat and its relationship to descending antinociceptive controls

This study used postembedding immunocytochemistry to examine the organization of GABA-immunoreactive synapse in the rostral ventral medulla (RVM) of the rat. To determine whether the outflow neurons of the RVM are under GABAergic control, we examined the distribution of GABA-immunoreactive synapses upon bulbospinal projection neurons that were labelled by retrograde transport of wheatgerm agglutinin-HRP from the cervical spinal cord. To study the possible convergence of GABAergic and periaqueductal gray (PAG) synaptic inputs to RVM neurons, we also made lesions in the PAG and examined the relationship between degenerating PAG axons and GABA-immunoreactive terminals. Approximately 45% of all synapses in the RVM, which includes the midline nucleus raphe magnus and the nucleus reticularis paragigantocellularis lateralis, were GABA-immunoreactive. The vast majority of GABA-immunoreactive terminals contained round, clear, and pleomorphic vesicles and made symmetrical axodendritic synapses; axoaxonic synapses were not found. Almost 50% of the retrogradely labeled dendrites in the NRM were postsynaptic to GABA-immunoreactive terminals. Several examples of convergence of degenerating PAG terminals and GABAergic terminals onto the same unlabelled dendrite were also found. These data indicate that the projection neurons of the RVM are under profound GABAergic inhibitory control. The results are discussed with regard to the hypothesis that the analgesic action of narcotics and electrical stimulation of the midbrain PAG involves the regulation of tonic GABAergic inhibitory controls that are exerted upon spinally-projecting neurons of the nucleus raphe magnus.

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)

Contribution of brainstem GABAergic circuitry to descending antinociceptive controls: I. GABA-immunoreactive projection neurons in the periaqueductal gray and nucleus raphe magnus

The fact that GABA receptor agonists and antagonists influence nociceptive thresholds when microinjected into the rostroventral medulla or in the spinal cord may reflect the involvement of GABAergic neuronal elements in endogenous antinociceptive pathways. In the present study we used immunocytochemistry and retrograde tract tracing to investigate the contribution of GABAergic projection neurons to the antinociceptive network linking the midbrain periaqueductal gray matter (PAG), the nucleus raphe magnus (NRM), and the spinal cord dorsal horn. The tracer, WGAapoHRP-Au was injected into either the NRM or the spinal cord and the distribution of labeled neurons in sections of the PAG and medulla, respectively, was studied. The same sections were immunostained to demonstrate GABA-immunoreactive neurons. Although GABA-immunoreactive neurons were abundant in the PAG, only 1.5% were retrogradely labeled from the NRM. Similarly, very few GABA-immunoreactive neurons within the cytoarchitectural boundaries of the NRM were retrogradely labeled from the spinal cord. A much higher proportion of GABA-immunoreactive neurons in the region lateral to the NRM, however, were retrogradely labeled from the spinal cord. Eighteen percent of GABA-immunoreactive neurons were retrogradely labeled in the nucleus reticularis paragigantocellularis; conversely, 15% of the retrogradely labeled neurons in this region were GABA-immunoreactive. These results indicate that GABAergic projections constitute a very minor component of the PAG-NRM-spinal cord pathway; however, there is a significant contribution of GABAergic neurons to the spinal projections that originate lateral to the NRM. The majority of GABAergic neurons in the PAG and NRM are presumed to be inhibitory interneurons that directly or indirectly regulate activity in efferent pathways from these regions.

Contribution of brainstem GABAergic circuitry to descending antinociceptive controls: II. Electron microscopic immunocytochemical evidence of GABAergic control over the projection from the periaqueductal gray to the nucleus raphe magnus in the rat

Pharmacological, physiological, and behavioral studies suggest that inhibitory GABAergic neurons influence the projection from the midbrain periaqueductal gray matter to the medullary nucleus raphe magnus. The present study used electron microscopic immunocytochemical techniques to examine the morphology and synaptic relationships of GABA-immunoreactive terminals in the ventrolateral periaqueductal gray. These putative GABAergic terminals comprise almost 40% of all axon terminals in the periaqueductal gray. GABA-immunoreactive terminals contain small, clear, pleomorphic or round, vesicles, and 46% also contain some dense-cored vesicles. In some experiments we also used a colloidal gold-conjugated retrograde tracer to label periaqueductal gray neurons that project to the nucleus raphe magnus. About half of the synaptic inputs onto the cell bodies and proximal dendrites of retrogradely labeled neurons are GABA-immunoreactive; these putative GABAergic synapses, which directly control activity in neurons projecting from the periaqueductal gray to the nucleus raphe magnus, might mediate the antinociception-related effects of exogenous GABAA receptor ligands.

Selective modulation of the cardiovascular response but not the antinociception evoked from the dorsal PAG, by 5-HT in the ventrolateral medulla

In lightly ananesthetised rats, electrical stimulation in the dorsal part of the periaqueductal grey matter (dPAG) produced an increase in the latency of the tail flick response accompanied by an increase in blood pressure and heart rate. Microinjection of 40-50nmol 5-HT bilaterally into the rostral part of nucleus paragigantocellularis lateralis (PGL) at the level of the facial nucleus attenuated the cardiovascular response to stimulation of the dPAG but had no effect on the analgesia. It is suggested that cardiovascular control neurones, but not the pain control neurones in the rostral part of PGL are subject to inhibitory serotonergic modulation.

Involvement of solitary tract nucleus in control of nociceptive transmission in cat spinal cord neurons

In cats anesthetized with Nembutal and immobilized with Flaxedil, extracellular recordings were made from dorsal horn neurons and lamina X neurons in the lumbar spinal cord. The nociceptive responses of these neurons elicited by peripheral nerve stimulation were significantly inhibited by stimulation of the nucleus tractus solitarius (NTS) at low intensity without any noticeable cardiovascular reaction. As usual, the late response or C-response was found to be preferentially inhibited by NTS stimulation as compared with the early response or A-response. The effective current intensity for NTS stimulation-produced inhibition ranged from 80 microA to 200 microA. Stronger inhibition was induced when the stimulating site was within or in the immediate vicinity of the NTS. There was no significant difference in the efficacy of the NTS stimulation-produced inhibition of nociceptive response between dorsal horn neurons and lamina X neurons. A similar inhibitory effect was elicited by microinjection of monosodium glutamate into the NTS area. The results demonstrate that the NTS may be involved in the control of nociceptive transmission at the spinal cord level.

High activity neurons in the reticular formation of the medulla oblongata: a high-resolution autoradiographic 2-deoxyglucose study

Accumulation of radioactivity in the medulla oblongata of the rat was assessed with high resolution autoradiography after injection of [3H]2-deoxyglucose or [14C]2-deoxyglucose. The autoradiographic approach employed allowed analysis at the cellular level. A salient feature of autoradiograms was the presence of foci of very high silver grain density in the reticular formation. These heavily labeled foci were shown to be associated with neurons by combined Acridine Orange staining and autoradiography. The high activity neurons in the ventral lateral medulla were predominantly located in the caudal portion of the paragigantocellular reticular nucleus. In addition, a group of high activity neurons was present in the dorsal reticular formation. The potential involvement of the high activity neurons in cardiovascular and respiratory regulation is discussed. In dry-mount autoradiograms produced after injection of [3H]2-deoxyglucose, certain small cells, presumably glial cells, were observed throughout the brainstem to accumulate radioactivity to a greater extent than the surrounding tissue. High activity neurons and high activity glial cells were observed for 5 min and 45 min survival intervals after intravenous injection of [3H]2-deoxyglucose. The similar appearance of autoradiograms at 5 min and 45 min after injection of [3H]2-deoxyglucose indicates that the utility of a 5-min survival period deserves further evaluation for assessment of functional activity patterns in brain.

Axonal trajectories and terminations of on- and off-cells in the cat lower brainstem

Two physiologically defined classes of pontomedullary raphe neurons were intracellularly labeled in order to determine the target nuclei of their axonal projections. In the lightly anesthetized cat, cells either increased (on-cells) or decreased (off-cells) their discharge rate during the paw withdrawal reflex evoked by noxious pinch or heat. On- and off-cells were injected with horseradish peroxidase and the initial course of labeled axons through the lower brainstem was reconstructed. On-cell projections to the pontomedullary raphe and medial reticular nuclei were sparse. On-cells projected densely to regions of the lateral reticular formation and the ventrolateral medulla at both rostral and caudal medullary levels. In general, on-cells had few collaterals and spare axonal swellings. In contrast to on-cells, most off-cells had axons that collateralized densely within the brainstem raphe and adjacent reticular formation. Such collaterals were either local, within the neuron's dendritic field, or distant, involving a projection of 1-8 mm. One off-cell had a dense terminal field within the sensory trigeminal complex, a projection that may subserve the inhibition of trigeminal sensory neurons produced by raphe magnus stimulation. Well-labeled off-cells had numerous collaterals and dense regions of axonal swellings. In summary, off-cells terminated densely in the raphe magnus and adjacent reticular formation whereas on-cells projected predominantly to the ventrolateral medulla, a region implicated in autonomic control. Local off-cell collaterals provide an anatomical substrate that would enable off-cells to coordinate the activity of on- and off-cells through synaptic contacts.

Relationship between cardiovascular neurones and descending antinociceptive pathways in the rostral ventrolateral medulla of the cat

It has been previously reported that injection of neuroexcitatory compounds into the rostral ventrolateral medulla (RVLM) can produce an inhibition of nociceptive reflexes, often associated with a rise in arterial blood pressure. The aim of this study was to determine whether the subretrofacial (SRF) nucleus, which is a highly circumscribed group of cells within the RVLM known to play a major role in cardiovascular regulation also has an antinociceptive function. In barbiturate-anaesthetised and paralysed cats, unilateral microinjections of the neuroexcitatory compound sodium glutamate (8-20 nl of 0.5 M solution) into the SRF nucleus produced large increases in mean arterial pressure but had only small and inconsistent effects on the simultaneously measured ventral root responses to stimulation of primary afferent C-fibres. On the other hand, glutamate microinjections into RVLM sites closely adjacent to the SRF nucleus, or into the nucleus raphe magnus, produced powerful inhibition of the C-fibre evoked response in the ventral root which was accompanied by no or only small changes in arterial pressure. It is concluded that the SRF pressor cells do not exert any control over nociceptive spinal reflexes, but that such a function may be served by cells in closely adjacent parts of the RVLM. Moreover, the method of recording C-fibre evoked responses in ventral roots as a measure of the magnitude of nociceptive spinal reflexes, combined with the glutamate microinjection procedure, was shown to have a sufficient resolution to allow an accurate mapping of the location of antinociceptive cell groups within the ventrolateral medulla.

Putative nociceptive modulating neurons in the rostral ventromedial medulla of the rat: firing of on- and off-cells is related to nociceptive responsiveness

In the unstimulated, lightly anesthetized rat, both on- and off-cells exhibit alternating periods of silence and activity lasting from several seconds to a few minutes. In the preceding paper, we showed that the active periods of all cells of the same class are always in phase, whereas the firing of cells of different classes is invariably out of phase. Thus, the pattern of firing of any single on- or off-cell provides a useful indication of the excitability of all on- and off-cells in the rostral ventromedial medulla (RVM). In this study, we measured the latency of the tail flick response (TF) at set intervals while recording from TF-related neurons in RVM, and were able to demonstrate a significant relationship between the spontaneous firing of both on- and off-cells and the latency of the TF response. If noxious heat is applied at a time when an off-cell is spontaneously active (or an on-cell is silent), the TF latency is longer than if the TF trial falls during a period in which the off-cell is silent (or the on-cell is active). This correlation between on- and off-cell firing and changes in TF latency is consistent with a nociceptive modulatory role for either or both cell classes. These findings support the hypothesis that off-cells inhibit and on-cells facilitate spinal nociceptive transmission and reflexes.

GABA-mediated inhibition in nucleus paragigantocellularis lateralis in the cat

In decerebrate cats electrical stimulation (10-20 ms trains, 35-450 microA, 0.1 ms pulses, 450 Hz) in nucleus tractus solitarius (NTS, medial or lateral divisions) or nucleus raphe magnus (NRM) inhibited spontaneous or amino acid-induced neuronal activity in nucleus paragigantocellularis lateralis (PGL) for 8-140 (mean 51) ms. Iontophoretically applied GABA (2-50 nA) also inhibited these cells. Iontophoretically applied bicuculline methiodide (10-80 nA) blocked the effects of GABA and reduced the duration of the inhibition evoked from NRM by greater than 50% (5/6 cells) but had no effect on the inhibition evoked from NTS (6/7 cells). The results are discussed in relation to the role of GABA in mediating inhibitory afferent input to PGL.

Convergent afferent inputs to neurones in nucleus paragigantocellularis lateralis in the cat

Neuronal activity was recorded in nucleus paragigantocellularis lateralis (PGL) of the cat in response to stimulation of the dorsal periaqueductal grey matter (dPAG), parabrachial nucleus (PBN) nucleus raphe magnus (NRM), and nucleus tractus solitarii (NTS). Stimulation in dPAG or PBN evoked excitatory responses whilst the prevalent response to stimulation in NRM or NTS was inhibition of neuronal activity. 45/55 (82%) of cells tested received convergent inputs from 2 or more sites of stimulation. These findings are discussed in relation to the role of PGL in cardiovascular control and descending control of noxious input.

Relationship between analgesia and cardiovascular changes induced by electrical stimulation of the mesencephalic periaqueductal gray matter in the rat

Analgesia and cardiovascular changes produced by electrical stimulation of the midbrain periaqueductal gray matter were examined in the lightly pentobarbital-anesthetized rat. The current intensity required to elicit analgesia was first determined, using the tail-flick test, after which the effects on arterial pressure and heart rate were recorded from stimulating at the same intensity. Intensity thresholds for decreases and/or increases in arterial pressure were also ascertained at the same sites. Although stimulation at the analgesia threshold produced increases in arterial pressure at more than 60% of the sites within the periaqueductal gray, decreases, no change, and mixed responses were also observed. Below the periaqueductal gray, increases in arterial pressure occurred at analgesia threshold for more than 70% of the sites studied, and no cardiovascular changes were found for 20% of the sites. Above the periaqueductal gray, no change and mixed responses were the predominant effects at analgesia threshold. A correlation across sites within the periaqueductal gray was found between the threshold for stimulation-produced analgesia and the threshold for a change in arterial pressure. No reliable alterations in heart rate were observed at any stimulation site. These results are in agreement with the existence of a common midbrain substrate for both the regulation of pain inhibition and cardiovascular function. However, they indicate that analgesia resulting from stimulation of the periaqueductal gray matter does not necessarily occur concurrently with an increase in arterial pressure.

Cardiovascular and respiratory changes elicited by stimulation of rat superior colliculus

Stimulation of the rat superior colliculus can produce either orienting or defensive movements, which if elicited by natural stimuli would be accompanied by cardiovascular changes. To assess whether cardiovascular changes might also be mediated by the superior colliculus, blood pressure and heart rate were measured in Saffan-anaesthetised rats while the dorsal midbrain was systematically explored with electrical and chemical stimulation. Electrical stimulation (10 sec trains of 0.3 msec 100 Hz cathodal pulses, 50 microA) within the superficial and intermediate layers of the rostral superior colliculus transiently lowered blood pressure without affecting heart rate. In contrast sites within the deep layers, and in adjacent periaqueductal grey and midbrain tegmentum, gave pressor responses accompanied by a variety of heart-rate changes, that usually included a period of bradycardia. A roughly similar distribution was obtained with the cell-stimulant bicuculline (200 or 500 nl, 490 microM), though sodium L-glutamate (200 nl, 0.05 or 1.0 M) was ineffective. These results suggest that (a) cardiovascular responses can be produced by stimulation of the rat superior colliculus; (b) their nature depends on the location of the stimulation; and (c) they may be mediated in part by cells differentially sensitive to glutamate and to bicuculline. In addition, in some animals respiratory responses were measured stethographically. Short-latency increases in thoracic girth, often accompanied by increases in respiratory rate and depth, were elicited by electrical stimulation from 61% of the collicular sites examined, and by microinjection of glutamate from 56% of collicular sites. These data suggest that (a) cells within the superior colliculus are capable of influencing respiration; (b) given the widespread distribution of responsive sites within the superior colliculus, the respiratory changes may be preparatory for both approach and defensive movements; (c) the collicular cells that affect respiration may be different from those that influence blood pressure, because the latter are relatively insensitive to microinjection of glutamate.

Tonic GABAergic and cholinergic influences on pain control and cardiovascular control neurones in nucleus paragigantocellularis lateralis in the rat

In rats anaesthetised with alphaxalone/alphadolone (Saffan), bilateral microinjections of the GABA antagonist, bicuculline, into a restricted region of nucleus paragigantocellularis lateralis (PGL), ventromedial to the caudal pole of the facial nucleus, produced an increase in the latency of the tail flick response to noxious heat. The analgesia was always accompanied by a rise in mean arterial blood pressure but the time course of the cardiovascular and antinociceptive changes was different. Guanethidine (7 mg/kg i.v.) blocked the pressor response but had no effect on the magnitude or time course of the analgesia. In contrast, microinjection of physostigmine into PGL produced a pressor response but no change in the latency of the tail flick response. It is concluded that there are functionally distinct pools of neurones within PGL which respectively produce antinociception and changes in vasomotor activity. Ongoing activity in both types of neurone is regulated by a tonic inhibitory GABAergic influence. In addition, the cardiovascular neurones receive a tonic excitatory cholinergic input.

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.

Site specificity in the development of tolerance to stimulation-produced analgesia from the periaqueductal gray matter of the rat

Pentobarbital-anesthetized rats were subjected to 21 min of continuous electrical stimulation of the caudal periaqueductal gray matter (PAG) at the current threshold for analgesia. Stimulation at ventral PAG sites supported analgesia for only 1 or 2 min in most animals. Stimulation at more dorsal PAG sites supported analgesia for the entire 21 min of stimulation. This demonstration of acute tolerance with continuous ventral, but not more dorsal, PAG stimulation corresponds well with previous evidence suggesting opioid mediation of analgesia from this brain region.

The distribution of substance P-, enkephalin- and dynorphin-immunoreactive neurons in the medulla of the rat and their contribution to bulbospinal pathways

This study examined the medullary distribution of peptide-containing neurons at the origin of bulbospinal pathways in the rat. Antisera directed against substance P, methionine-enkephalin-arg-gly-leu and dynorphin B were used on sections in which spinally projecting brainstem neurons had been identified by the retrograde transport of a protein-gold complex that was injected into the spinal cord. Both the relative numbers and distribution of the different peptide-immunoreactive spinally projecting neurons differed. Methionine-enkephalin-immunoreactive neurons were twice as numerous as the substance P-immunoreactive cells and seven times more numerous than the dynorphin B-positive neurons. The methionine-enkephalin cells were found in all medullary raphé nuclei, and in the ventromedial and ventrolateral medullary reticular formation. Caudally, the methionine-enkephalin cells were concentrated laterally; more rostrally they were located more medially. Three major loci of methionine-enkephalin-immunoreactive cells were found: (1) the nucleus reticularis paragigantocellularis lateralis, at levels caudal to the facial nucleus, (2) the B3 cell group (nucleus raphé magnus and the nucleus reticularis magnocellularis, pars alpha) and the most rostral part of the B1 and B2 cell groups (nuclei raphé pallidus and obscurus), (3) a dense cluster of cells that flanks the dorsal surface of the dorsal accessory olive (referred to as the nucleus interfascicularis hypoglossi, pars dorsalis). Substance P-like cells were seen in all raphé nuclei except for the most anterior portion of the B3 cell group. Substance P-immunoreactive cells were also seen in both the ventromedial (nuclei reticularis ventralis and magnocellularis) and ventrolateral medulla (nucleus reticularis paragigantocellularis lateralis). Finally there was a dense concentration of substance P neurons in the nucleus interfascicularis hypoglossi, pars ventralis. The distribution of dynorphin-immunoreactive neurons differed significantly from that of methionine-enkephalin and substance P. Dynorphin cells were almost exclusively found in the ventrolateral medulla (nucleus reticularis paragigantocellularis lateralis), at all levels between the lateral reticular nucleus and the caudal pole of the facial nucleus. The proportion of each of these peptidergic-immunoreactive cells at the origin of bulbospinal pathways differed considerably. Substance P spinally projecting neurons were more numerous than methionine-enkephalin spinally projecting neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Dissociation of antinociceptive from cardiovascular effects of stimulation in the lateral reticular nucleus in the rat

The lateral reticular nucleus (LRN) in the caudal ventrolateral medulla has been implicated in the regulation of spinal nociceptive transmission and hemodynamics. Experiments were undertaken to examine the relationship between inhibition of the tail flick reflex and cardiovascular effects produced by electrical stimulation in the LRN in rats lightly anesthetized with pentobarbital. Intensity- and frequency-dependent increases in mean arterial pressure and vascular resistance in the hindquarter, mesenteric, renal and caudal arterial beds were observed. Inhibition of the tail flick reflex, however, occurred at intensities of electrical stimulation which produced no significant changes in mean arterial pressure or vascular resistance in any of the arterial beds studied. Selective stimulation of cell bodies in the LRN by microinjection of glutamate similarly inhibited the tail flick reflex but produced significant reductions in mean arterial pressure, without substantially affecting regional vascular resistances. These results suggest that the antinociceptive and depressor effects of stimulation in the LRN are mediated by activation of cell bodies, while pressor effects produced by focal electrical stimulation are mediated by activation of fibers of passage. The descending inhibition produced by stimulation in the LRN is independent of stimulation-produced cardiovascular responses.

Suppression of the jaw-opening reflex by periaqueductal gray stimulation is decreased by paramedian brainstem lesions

Electrical stimulation of the midbrain periaqueductal gray region (PAG) suppresses the tooth pulp-evoked jaw-opening reflex (TP-JOR). In the present study the pathways that mediate this suppression were investigated by placing brainstem lesions in lightly anesthetized cats. Parasagittal lesions that interrupted the afferent and efferent connections of the medullary and pontine raphe nuclei attenuated (but did not abolish) suppression of the TP-JOR evoked by PAG stimulation. This result provides further evidence that medial brainstem structures partially mediate the effects of PAG stimulation in the trigeminal system.

Brain-stem areas tonically inhibiting dorsal horn neurones: studies with microinjection of the GABA analogue piperidine-4-sulphonic acid

In barbiturate anaesthetized cats, tonic inhibition of the excitation of lumbar dorsal horn neurones by impulses in unmyelinated primary afferents was measured by reversibly cooling the spinal cord at the thoraco-lumbar junction. Tonic inhibition was reduced by microinjection of the GABA analogue, piperidine-4-sulphonic acid (2.5 nM in 0.5 microliter) mainly at AP -7, L 2-5 and V -8 to -10. This area in the ventrolateral medulla is just ventral to the facial nucleus and has been shown to be important in cardiovascular control, particularly in relation to fear-defence reactions. It is proposed that tonic inhibition of the nociceptive responses of spinal neurones is part of such a reaction in response to the trauma of surgery. Since previous experiments had shown that the ventrolateral medulla was important in spinal inhibition produced by PAG stimulation, these experiments support the proposal that analgesia does not occur in isolation but is part of a complex behavioural response of an animal in a potentially injurious environment.

Opioid, cholinergic and alpha-adrenergic influences on the modulation of nociception from the lateral reticular nucleus of the rat

The lateral reticular nucleus (LRN) has been identified as an area in the caudal medulla involved in the centrifugal modulation of spinal nociceptive transmission and withdrawal reflexes. The data presented in this report further support a role for the LRN in the modulation of nociceptive responses. It was confirmed in the present study that focal electrical stimulation in the LRN inhibits the nociceptive tail-flick (TF) reflex at low intensities of stimulation in lightly pentobarbital-anesthetized rats. Aversive effects, however, were typically produced at similar and higher intensities of stimulation in the LRN in the same rats in the awake state. It was also determined that an inhibitory modulation of nociceptive responses organized both spinally and supraspinally could be activated independently by muscarinic cholinergic or opioid mechanisms in the LRN. Microinjection of morphine into the LRN in conscious rats produced an antinociception in both TF and hot plate (HP) tests which could be attenuated significantly by naloxone, but not atropine, previously microinjected into the same site in the LRN. Carbachol microinjected into the LRN also produced an antinociception which was attenuated significantly by atropine but not naloxone previously microinjected into the same site in the LRN. In contrast, the microinjection of clonidine or norepinephrine into the LRN either did not affect or shortened significantly response latencies in the TF and HP tests. These results further establish that the LRN contributes to the modulation of nociception. Opioid and cholinergic influences in the LRN appear to independently activate inhibition of responding to nociceptive stimuli organized either spinally or supraspinally, although descending inhibition was most clearly activated. An action at alpha 2 adrenoceptors in the LRN, conversely, produces an hyperalgesia as reflected by shortened latencies to respond in TF and HP tests.

Inhibition of spinal nociceptive transmission accompanies cardiovascular changes from stimulation in diencephalic 'defence' regions of cats

In anaesthetized cats, diencephalic regions were electrically stimulated while recording evoked responses from lumbar dorsal horn neurones and cardiovascular parameters. Neuronal responses to impulses evoked electrically in unmyelinated primary afferents were inhibited by stimulation in many diencephalic regions. Responses to non-noxious cutaneous stimulation (hair deflection) were inhibited at relatively few sites. Indirect circulatory measurements showed that this selective spinal inhibition was accompanied by increases in cardiac output and muscle blood flow but reduced cutaneous perfusion. This association between selective inhibition of nociceptive spinal neuronal responses and a cardiovascular response pattern associated with the defence reaction supports the proposal that analgesia could be an important component of defensive behaviour.

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.

Analgesia and the cardiovascular changes evoked by stimulating neurones in the ventrolateral medulla in rats

In rats anaesthetised with Saffan, selective excitation of neurones in the ventrolateral medulla in the region of nucleus paragigantocellularis lateralis (PGL) by microinjection of the synaptic excitant, D,L-homocysteic acid, produced an increase in the latency of the tail flick response to noxious heat usually accompanied by an increase in blood pressure and heart rate. These findings are discussed in relation to the role of neurones in PGL in generating both tonic descending inhibition in the dorsal horn and sympathetic vasomotor tone as well as their involvement in a descending pathway that mediates the hypoalgesia which is a feature of certain stress-induced hypertensive states.

Excitatory projections from hypothalamic and midbrain defense regions to nucleus paragigantocellularis lateralis in the rat

Electrical stimulation of the hypothalamic and midbrain defense regions evoked convergent excitatory responses in neurons in nucleus paragigantocellularis lateralis (PGL) in rats anesthetized with urethane. Stimulation in PGL activated neurons in the periaqueductal gray matter (PAG) and in the hypothalamus antidromically. In the PAG the projecting neurons were localized in the defense region but in the hypothalamus they were more widely distributed lateral and dorsal to it, mainly in the lateral hypothalamic region and the zona incerta. We conclude that there are independent excitatory pathways from the midbrain and hypothalamic defense regions which relay on neurons in PGL. Stimulation in the midbrain defense region may activate the monosynaptic projection to PGL as well as polysynaptic pathways. In contrast, stimulation of the hypothalamic defense region probably excites mainly fibers of passage: the integrated pattern of autonomic changes produced by electrical stimulation may be due to activation of the efferent outflow from widely dispersed cell bodies whose axons run through the defense region.