Stimulation Of PAG Research Articles

Stimulation of the dorsal periaqueductal gray enhances spontaneous recovery of a conditioned taste aversion

Due to its relevance to clinical practice, extinction of learned fears has been a major focus of recent research. However, less is known about the means by which conditioned fears re-emerge (i.e., spontaneously recover) as time passes or contexts change following extinction. The periaqueductal gray represents the final common pathway mediating defensive reactions to fear and we have reported previously that the dorsolateral PAG (dlPAG) exhibits a small but reliable increase in neural activity (as measured by c-fos protein immunoreactivity) when spontaneous recovery (SR) of a conditioned taste aversion (CTA) is reduced. Here we extend these correlational studies to determine if inducing dlPAG c-fos expression through electrical brain stimulation could cause a reduction in SR of a CTA. Male Sprague-Dawley rats acquired a strong aversion to saccharin (conditioned stimulus; CS) and then underwent CTA extinction through multiple non-reinforced exposures to the CS. Following a 30-day latency period after asymptotic extinction was achieved; rats either received stimulation of the dorsal PAG (dPAG) or stimulation of closely adjacent structures. Sixty minutes following the stimulation, rats were again presented with the saccharin solution as we tested for SR of the CTA. The brain stimulation evoked c-fos expression around the tip of the electrodes. However, stimulation of the dPAG failed to reduce SR of the previously extinguished CTA. In fact, dPAG stimulation caused rats to significantly suppress their saccharin drinking (relative to controls) – indicating an enhanced SR. These data refute a cause-and-effect relationship between enhanced dPAG c-fos expression and a reduction in SR. However, they highlight a role for the dPAG in modulating SR of extinguished CTAs.

Acute long term stimulation of the ventral periaqueductal grey (vPAG) evokes persistent hypotensive responses in spontaneously hypertensive (SH) rats

Deep brain stimulation of the ventral PAG has been used for the clinical treatment of chronic neuropathic pain. However, it also causes sustained reductions in arterial pressure (AP) in hypertensive patients. To better understand the latter, we have developed an animal model in the anaesthetised SH rat and applied long term electrical stimulation of the vPAG (30–60 min, 30Hz, 2V, 200–400μs pulse width) while measuring AP and heart rate (HR) responses. vPAG stimulation reduced AP by 14±1mmHg (basal: 139±10mmHg) in SH rats, which persisted throughout the stimulation period, whereas smaller depressor responses were evoked in Wistar rats (5±2mmHg; basal 101±9mmHg) that were poorly maintained. In the SH rat, HR decreased by 33±12bpm and was stimulus locked; a poorly maintained bradycardia of 15±3bpm was seen in Wistar rats. Our observations indicate that the vPAG of the SH rat is a more effective depressor site compared to Wistar rats, lending itself as a potential therapeutic target. Future studies will determine the mechanisms for the enhanced hypotensive effect of the vPAG in the SH versus Wistar rat.

Endogenous Opioids Inhibit Early-Stage Pancreatic Pain in a Mouse Model of Pancreatic Cancer

The endogenous opioid system is involved in modulating the experience of pain, the response to stress, and the action of analgesic therapies. Recent human imaging studies have shown a significant tonic modulation of visceral pain, raising the question of whether endogenous opioids tonically modulate the pain of visceral cancer. Transgenic mice expressing the first 127 amino acids of simian virus 40 large T antigen, under the control of the rat elastase-1 promoter, that spontaneously develop pancreatic cancer were used to investigate the role of endogenous opioids in the modulation of pancreatic cancer pain. Visceral pain behaviors were assessed as degree of hunching and vocalization. Although mice with late-stage pancreatic cancer displayed spontaneous, morphine-reversible, visceral pain-related behaviors such as hunching and vocalization, these behaviors were absent in mice with early-stage pancreatic cancer. After systemic administration of the central nervous system (CNS)-penetrant opioid receptor antagonists naloxone or naltrexone, mice with early-stage pancreatic cancer displayed significant visceral pain-related behaviors, whereas systemic administration of the CNS-nonpenetrant opioid antagonist naloxone-methiodide did not induce an increase in visceral pain behaviors. Our findings suggest that a CNS opioid-dependent mechanism tonically modulates early and late-stage pancreatic cancer pain. Understanding the mechanisms that mask this pain in early stage disease and drive this pain in late-stage disease may allow improved diagnosis, treatment, and care of patients with pancreatic cancer.

Stimulating the human midbrain to reveal the link between pain and blood pressure

The periaqueductal grey area (PAG) in the midbrain is an important area for both cardiovascular control and modulation of pain. However, the precise relationship between pain and blood pressure is unknown. We prospectively studied 16 patients undergoing deep brain stimulation of the rostral PAG for chronic pain. Pre-operatively, post-operatively, and at 1 year, pain scores were assessed using both visual analogue scores and the McGill Pain Questionnaire. Patients were tested post-operatively to determine whether electrical stimulation of the PAG would modulate blood pressure. We found that the degree of analgesia induced by deep brain stimulation of the rostral PAG in man is related to the magnitude of reduction in arterial blood pressure. We found that this relationship is linear and is related to reduced activity of the sympathetic nervous system. Thus stimulation of the PAG may partly control pain by reducing sympathetic activity as predicted by William James over a century ago.

Excitatory convergence of periaqueductal gray and somatic afferents in the solitary tract nucleus: role for neurokinin 1 receptors.

Our previous studies (Boscan P, Kasparov S, and Paton JF. Eur J Neurosci 16: 907-920, 2002) showed that activation of somatic afferents attenuated the baroreceptor reflex via neurokinin type 1 (NK(1)) and GABA(A) receptors within the nucleus of the solitary tract (NTS). The periaqueductal gray matter (PAG) can also depress baroreceptor reflex function and project to the NTS. In the present study, we have tested the possibility that the dorsolateral (dl)-PAG projects to the NTS neurons that also respond to somatic afferent input. In an in situ, arterially perfused, unanesthetized decerebrate rat preparation, somatic afferents (brachial plexus), cervical spinal cord, and dl-PAG were stimulated electrically, whereas NTS neurons were recorded extracellularly. From 45 NTS neurons excited by either brachial plexus or dl-PAG stimulation, 41 received convergence excitatory inputs from both afferents. Onset latency and evoked peak discharge frequency from brachial plexus afferents were 39.4 +/- 4.7 ms and 10.7 +/- 1.1 Hz, whereas this was 43.9 +/- 6.4 ms and 7.9 +/- 1 Hz, respectively, following dl-PAG stimulation. As revealed by using a paired pulse stimulation protocol, monosynaptic connections were found in 9 of 36 neurons tested from both spinal cord and dl-PAG. We tested NK(1)-receptor sensitivity in 38 neurons that received convergent inputs from brachial plexus/PAG. Fifteen neurons were sensitive to selective antagonism of NK(1) receptors. CP-99994, the NK(1) antagonist, failed to alter ongoing firing activity but reduced the evoked peak discharge frequency following stimulation of both brachial plexus (from 12.3 +/- 1.8 to 7.2 +/- 1.3 Hz; P < 0.01) and PAG (from 7.8 +/- 1.5 to 4.5 +/- 1 Hz; P < 0.01). We conclude that 1) somatic brachial and PAG afferents can converge onto single NTS neurons; 2) this convergence occurs via either direct or indirect pathways; and 3) NK(1) receptors are activated by some of these inputs.

Organization of single components of defensive behaviors within distinct columns of periaqueductal gray matter of the rat: role of N-METHYL- d-aspartic acid glutamate receptors

The periaqueductal gray matter (PAG) is functionally organized in longitudinal columns arranged along the aqueduct. Stimulation of lateral and dorsal columns produces a complex set of unconditioned behaviors named the ‘defense reaction.’ Overt responses in rats comprise a tense immobile display, fully opened eyes (herein named exophthalmus), trotting, galloping, jumping, micturition and defecation. Besides, the PAG is rich in glutamate and respective receptors, including the N-methyl- d-aspartic acid (NMDA) type. Therefore, the present study employed regression analysis to map out electrically and NMDA-induced single components of defensive behaviors produced by stepwise increasing stimulation of PAG. Data confirmed the defensive nature of PAG-evoked responses. Neither the appetitive, nor offensive, mouse-killing or male reproductive behaviors were produced by stimulation of PAG in presence of appropriate targets. Threshold and dose-response logistic analyses largely corroborated the columnar organization of PAG-evoked responses. Thus, whereas the defecation was restricted to PAG lateral column, exophthalmus, micturition and somatic defensive responses were similarly organized in dorsolateral and lateral, but not in the ventrolateral column. Moreover, thresholds of dorsolateral and lateral repertoires were strictly hierarchical, with exophthalmus, immobility, trotting, galloping and jumping appearing in this very order. However, the defensive responses of PAG dorsolateral column required NMDA doses significantly lower than those of lateral PAG. Accordingly, NMDA receptors within the dorsolateral PAG are likely to play a major role in the initiation of PAG-evoked defensive responses. In contrast, the present data do not support the organization of unconditioned defensive behaviors in ventrolateral PAG. The neuroanatomical substrate of each response and the role of PAG and NMDA receptors are discussed in relation to the present data. Further, this is the first report on PAG columnar organization of single components of defensive behaviors.

Antiallodynic effects produced by stimulation of the periaqueductal gray matter in a rat model of neuropathic pain

It has been well documented that there is opioid resistance in neuropathic pain. This indicates that the endogenous opioid system may not be involved effectively in modulating neuropathic pain. The present study sought to determine if activation of the descending pain inhibition system might produce analgesia in the animal neuropathic model we developed. Under ketamine anesthesia, male Sprague–Dawley rats were chronically implanted with stimulating electrodes in the ventral periaqueductal gray matter (PAG) and both the tibial and sural nerves of the sciatic nerve branches were severed. Pain sensitivity was measured with a von Frey filament and acetone applied to the sensitive area for 1 week postoperatively. Rats with neuropathic pain syndrome after transection of the tibial and sural nerves were tested as to the analgesic effects of ventral PAG stimulation for an additional two weeks. Electrical stimulation of the ventral PAG turned out to be highly effective in alleviating neuropathic pain. Mechanical allodynia and cold allodynia were reduced by PAG stimulation. Naloxone reversed the antiallodynic effects of ventral PAG stimulation. These results suggest that activation of the descending pain inhibition system including the ventral PAG reduces neuropathic pain syndrome and that opiates are involved in this system.

Ventrolateral periaqueductal gray matter and the control of tonic immobility

Tonic immobility is an inborn defensive behavior characterized by a temporary state of profound and reversible motor inhibition elicited by some forms of physical restraint. The periaqueductal gray matter (PAG) contains neural circuits involved in descending pain modulation, as well as in the modulation of TI. We have reported previously that the cholinergic stimulation of the ventrolateral PAG increases the duration of TI in guinea pigs. In the present study, we attempted to characterize further the modulation of TI by pharmacological alteration of the neurochemistry of the ventrolateral PAG circuitry. We observed that both cholinergic (carbachol, 5.4 nmol/0.2 μl) and opioidergic stimulations (morphine, 4.48 nmol/0.2 μl) of the ventrolateral PAG increase the duration of TI and that these effects can be reversed by pre-treatment with naloxone (2.74 nmol/0.2 μl). Our results also showed that microinjection of the GABAergic agonist muscimol (1, 0.5, and 0.26 nmol/0.2 μl) decreased the duration of TI episodes, while microinjection of the GABAergic antagonist bicuculline (1 nmol/μl) increased it. Moreover, we observed that preadministration of muscimol (0.13 nmol/0.2 μl) at a dose that had no effect per se at this site antagonized the potentiating effect of morphine. Our results suggest that this modulation of TI from the ventrolateral PAG circuitry is accomplished by a complex interaction of cholinergic, opioidergic, and GABAergic mechanisms, similar to that proposed for descending antinociceptive circuits.

B-8 視床歯髄ニューロン活動に及ぼすモルヒネ全身投与および中脳中心灰白質電気刺激の影響

B-8 視床歯髄ニューロン活動に及ぼすモルヒネ全身投与および中脳中心灰白質電気刺激の影響

Differential effects of neurokinin-1 receptor activation in subregions of the periaqueductal gray matter on conditional and unconditional fear behaviors in rats.

Central neurokinin-1 (NK-1) receptors are thought to modulate aversion, whereas the periaqueductal gray matter (PAG) is a common pathway for the integration of fear behaviors. The authors determined whether injection of an NK-1 agonist (GR73632) into subregions of the PAG would alter fear-related behaviors. Behavioral inactivity was increased by GR73632 injected into the caudodorsal PAG or the dorsal raphe. Flight behavior induced by stimulation of the dorsal PAG or by a footshock was decreased after injection of GR73632 into the dorsal PAG. Rats that had 6 pairings of a tone with a footshock after injection of GR73632 into the dorsal PAG displayed more freezing behavior than controls at the beginning of the session. However, there was no change in the shock- or the tone-induced freezing because some GR73632-treated rats, but no controls, froze during the baseline period. It is concluded that NK-1 receptors in the dorsal PAG modulate the unconditional but not the mnemonic aspects of fear behaviors.

Differential effects of neurokinin-1 receptor activation in subregions of the periaqueductal gray matter on conditional and unconditional fear behaviors in rats.

Central neurokinin-1 (NK-1) receptors are thought to modulate aversion, whereas the periaqueductal gray matter (PAG) is a common pathway for the integration of fear behaviors. The authors determined whether injection of an NK-1 agonist (GR73632) into subregions of the PAG would alter fear-related behaviors. Behavioral inactivity was increased by GR73632 injected into the caudodorsal PAG or the dorsal raphe. Flight behavior induced by stimulation of the dorsal PAG or by a footshock was decreased after injection of GR73632 into the dorsal PAG. Rats that had 6 pairings of a tone with a footshock after injection of GR73632 into the dorsal PAG displayed more freezing behavior than controls at the beginning of the session. However, there was no change in the shock- or the tone-induced freezing because some GR73632-treated rats, but no controls, froze during the baseline period. It is concluded that NK-1 receptors in the dorsal PAG modulate the unconditional but not the mnemonic aspects of fear behaviors.

Periaqueductal gray matter projection to vagal preganglionic neurons and the nucleus tractus solitarius

The periaqueductal gray matter (PAG) has been implicated in a variety of different functions, including autonomic regulation. Chemical stimulation of the lateral PAG produces hypertension and tachycardia while activation of the ventrolateral PAG produces the opposite effect. While these effects are the result of alterations in sympathetic activity, little is known about whether the PAG can modulate vagal functions as well. The anterograde axonal tracing method using the plant lectin Phaseolus vulgaris leucoagglutinin (PHA-L) was used to determine whether both of the lateral and ventrolateral PAG columns project to vagal preganglionic neurons and/or to the nucleus tractus solitarius (NTS). Highly restricted PHA-L injections were made in all four PAG columns throughout their rostrocaudal extent in rats. Labeled fibers were visualized by immunohistochemistry and studied in relationship with choline acetyltransferase (ChAT) immunostained parasympathetic preganglionic neurons of the dorsal motor vagal nucleus (DMV) and nucleus ambiguus (NA). The lateral PAG projects to the lateral DMV and to the caudal part of the external NA. The ventrolateral PAG innervates the same regions and also projects to the rostral part of the external NA – a site that contains cardiac parasympathetic preganglionic neurons. Both the lateral and ventrolateral PAG project to the NTS in a similar fashion innervating the medial, ventrolateral and commissural subnuclei. In summary, the lateral and ventrolateral PAG have similar patterns of innervation of the NTS and DMV, but their projection to the NA is different: the rostral external NA receives innervation only from the ventrolateral PAG and the lateral PAG innervates the caudal part.

Facilitation of the arterial baroreflex by the ventrolateral part of the midbrain periaqueductal grey matter in rats.

1. The effects of stimulation of the ventrolateral part of the midbrain periaqueductal grey matter (PAG) on the arterial baroreflex were investigated in urethane-chloralose anaesthetized and artificially ventilated rats. 2. Both electrical and chemical stimulation of the ventrolateral PAG provoked hypotension, vagal bradycardia and marked facilitation of baroreflex vagal bradycardia (BVB), which was induced by stimulation of the aortic depressor nerve. The magnitude of ventrolateral PAG facilitation of BVB was 328 +/- 193% (n = 34) for electrical stimulation and 243 +/- 224% (n = 13) for chemical stimulation. Baroreflex hypotension was slightly augmented during either electrical or chemical stimulation of the ventrolateral PAG in vagotomized rats. 3. Stimulation of the nucleus raphe magnus (NRM) also provoked hypotension, vagal bradycardia and facilitation of BVB. The magnitude of BVB facilitation was 234 +/- 132% (n = 8) for electrical stimulation and 328 +/- 170% (n = 7) for chemical stimulation. After microinjection of kainic acid into the NRM region, baroreflex facilitation, as well as hypotension and vagal bradycardia, produced by ventrolateral PAG stimulation, was almost abolished. 4. In conclusion, the ventrolateral PAG, besides producing hypotension and bradycardia, facilitates arterial baroreflexes. These effects are exerted via the NRM, sharply contrasting with effects of the dorsal PAG.

Attenuation of chemically induced defence response by 5-HT1 receptor agonists administered into the periaqueductal gray.

The ability of 5-HT1 receptor agonists to modulate a chemically induced defence response has been studied in Lister hooded rats. Microinjections of the excitatory amino acid D,L-homocysteic acid (DLH) in both rostral and caudal dorsal periaqueductal gray matter (PAG) caused explosive motor behaviour characteristic of defence. This behaviour was quantified in terms of response duration, arena revolutions and number of defensive jumps. Direct administration into the PAG of either 5-carboxamidotryptamine (5-CT) or 8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) produced behaviours (decreased exploratory rearing, dose related onset of flat body posture) indicative of 5-HT1A receptor activation. Pretreatment with either 5-CT or 8-OHDPAT directly in the PAG caused a significant attenuation, and in some cases a complete abolition, of the DLH evoked response. These agonists share high affinity in vitro for the 5-HT1A receptor. Thus the results suggest that in vivo activation of 5-HT1A receptors mediates an antiaversive response with respect to defensive behaviour elicited by specific chemical stimulation of the dorsal PAG.

Suppressive influences from periaqueductal gray and nucleus raphe magnus on respiration and related reflex activities and on solitary tract neurons, and effect of naloxone

Possible opiate-related descending influences from the periaqueductal gray matter (PAG) and nucleus raphe magnus (NRM) were tested on the activity of neural systems involved in respiration and related reflex functions in cats. Stimulation of PAG and NRM could powerfully suppress the simple buccopharyngeal reflexes of jaw-opening and tongue-protrusion and the more complex reflexes of coughing and swallowing; respiration in contrast appeared to be only weakly influenced. The reflexly induced responses of 57 single reflex interneurons recorded in the solidary tract nucleus (NST) could also be markedly suppressed by PAG and NRM conditioning stimulation. In contrast, the rhythmic activity of 30 respiratory neurons in NST was not abolished by PAG and NRM stimuli but most did show a decrease in the peak firing frequency of each rhythmic burst. The suppressive effect of PAG and NRM stimulation on the reflexes and the NST reflex interneurons could be reduced by the intravenous administration of naloxone. These studies indicate that neuronal functions associated with respiration and respiratory-related activities can be suppressed by descending influences from PAG and NRM that are in part opiate-related. The observations add to the accumulating evidence that the raphe system is implicated in functions other than pain and its control, and they may also be relevant to clinical observations of opiate-induced effects on respiration and the cough reflex.

Dual alterations of thalamic nociceptive activity by stimulation of the periaqueductal gray matter

Effects produced by electrical stimulation of the mesencephalic periaqueductal gray matter (PAG) on single nociceptive neurons of the somesthetic thalamus were studied in rats anesthetized with α-chloralose and urethane. Stimulation of the PAG excited the nociceptive neurons, and, although the probability of firing them with a single pulse was low, trains of pulses increased their preexisting activity considerably and lastingly. Poststimulus spike density histograms of the activity evoked by stimulation of the sciatic nerve displayed several peaks which have been analyzed previously. In brief, a short-latency, high-count peak was related to the intensity of stimulation ( I peak); three to four long-latency, low-count peaks were characteristic of nociception (modality, or M peaks). PAG stimulation modified mainly the M peaks. With stimulation of the caudal portion of the PAG, fusion of the M peaks was observed and persisted. With stimulation of its rostral portion each M peak was augmented. At several PAG sites, however, increasing the stimulus frequency and pulse-train duration converted the augmentation effect into fusion of the M peaks. Thalamic neurons that relay tactile afferents were not fired by stimulation of the PAG, although after such a stimulation they began to fire spontaneously. These results parallel the dual behavioral effects previously observed with PAG stimulation. When repeated trains of pulses were used, analgesia was induced that outlasted the period of stimulation. However, with stimulation of the rostral PAG, behavioral agitation and lowering of the nociceptive thresholds were observed. It is likely that augmentation of the M peaks represents central sensitization to noxious stimulation, and fusion of the M peaks represents a conversion of pain into less pain-like sensations. These effects may be mediated via a pathway that connects the PAG with the ventrobasal and the parafascicular nuclei of the thalamus.

An evaluation of stimulation-produced analgesia in the cat

The analgesia produced by focal electrical brain stimulation of the periaqueductal central gray matter (PAG) was examined in 24 cats. The cats were chronically implanted with PAG and radial nerve bipolar stimulating electrodes. After the cats were trained to escape noxious radial nerve stimulation by crossing a low barrier in a shuttle box, the effects of focal electrical stimulation of the PAG, morphine, and chlorpromazine were evaluated on the escape threshold and latency. It was found that analgesic brain stimulation loci in the cat PAG were localized in the caudal ventrolateral PAG. The most efficacious analgesia-producing loci were observed to be at the ventrolateral edge of the PAG, suggesting a catecholaminergic involvement. Tolerance to repeated analgesia-producing electrical brain stimulation was apparent. Naloxone was not found to attenuate or antagonize the analgesia produced by focal electrical stimulation of the cat PAG.