Galanin microinjection into the dorsal periaqueductal gray matter produces paradigm-dependent anxiolytic effects.

Galanin is a peptide that is present in the central nervous system in mammals, including rodents and humans. The actions of galanin are mediated by three types of metabotropic receptors: GAL1, GAL2, and GAL3. GAL1 and GAL3 increase K(+) efflux, and GAL2 increases intracellular Ca(2+) levels. The distribution of galanin and its receptors suggests its involvement in fear and/or anxiety. The periaqueductal gray matter (PAG) is a key mediator of defensive behaviors that is both targeted by galaninergic projections and supplied with GAL1 receptors and, less markedly, GAL2 receptors. We examined the effects of galanin microinjections in the dorsal PAG (dPAG) on the performance of rats in different models of anxiety. Male Wistar rats (n=7-12) were implanted with guide cannulae in the dPAG. They received microinjections of either galanin (0.3, 1.0, and 3.0 nmol) or vehicle and were tested in the Vogel conflict test (VCT), elevated plus maze (EPM), and elevated T-maze (ETM). Rats that were tested in the ETM were further evaluated for exploratory activity in the open field test (OFT). Galanin microinjections had no effects on anxiety-like behavior in the EPM or VCT or exploratory activity in the EPM or OFT. In the ETM, however, microinjections of 3 nmol galanin impaired learned anxiety (i.e., avoidance of the open arms) without changing unconditioned fear (i.e., escape from the open arms). The present data suggest that galanin transmission in the dPAG inhibits the acquisition of anxiety-like responses in the ETM.

Galanin subtype 1 and subtype 2 receptors mediate opposite anxiety-like effects in the rat dorsal raphe nucleus.

About 40% of the dorsal raphe nucleus (DRN) neurons co-express serotonin (5-HT) and galanin. Serotonergic pathways from the DRN to the amygdala facilitate learned anxiety, while those from the DRN to the dorsal periaqueductal grey matter (DPAG) impair innate anxiety. Previously, we showed that galanin infusion in the DRN of rats induces anxiolytic effect by impairing inhibitory avoidance without changing escape behaviour in the elevated T-maze (ETM). Here, we evaluated: (1) which galanin receptors would be involved in the anxiolytic effect of galanin in the DRN of rats tested in the ETM; (2) the effects of galanin intra-DRN on panic-like behaviours evoked by electrical stimulation of the DPAG. The activation of DRN GAL1 receptors by M617 (1.0 and 3.0nmol) facilitated inhibitory avoidance, whereas the activation of GAL2 receptors by AR-M1896 (3.0nmol) impaired the inhibitory avoidance in the ETM, suggesting an anxiogenic and an anxiolytic-like effect respectively. Both agonists did not change escape behaviour in the ETM or locomotor activity in the open field. The anxiolytic effect of AR-M1896 was attenuated by the prior administration of WAY100635 (0.18nmol), a 5-HT1A antagonist. Galanin (0.3nmol) administered in the DRN increased discreetly flight behaviours induced by electrical stimulation of the DPAG, suggesting a panicolytic effect. Together, our results showed that galanin mediates opposite anxiety responses in the DRN by activation of GAL1 and GAL2 receptors. The anxiolytic effect induced by activation of Gal2 receptors may depend on serotonergic tone. Finally, the role of galanin in panic related behaviours remains uncertain.

Functional specializations within the tectum defense systems of the rat.

Here we review the differential contribution of the periaqueductal gray matter (PAG) and superior colliculus (SC) to the generation of rat defensive behaviors. The results of studies involving sine-wave and rectangular pulse electrical stimulation and chemical (NMDA) stimulation are summarized. Stimulation of SC and PAG produced freezing and flight behaviors along with exophthalmus (fully opened bulged eyes), micturition and defecation. The columnar organization of the PAG was evident in the results obtained. Defecation was elicited primarily by lateral PAG stimulation, while the remaining defensive behaviors were similarly elicited by lateral and dorsolateral PAG stimulation, although with the lowest thresholds in the dorsolateral column. Conversely, the ventrolateral PAG did not appear to participate in unconditioned defensive behaviors, which were only elicited by high intensity stimulation likely to encroach on adjacent regions. In the SC, the most important differences relative to the PAG were the lack of stimulation-evoked jumping in both intermediate and deep layers, and of NMDA-evoked galloping in intermediate layers. Therefore, we conclude that the SC may be only involved in the increased attentiveness (exophthalmus, immobility) and restlessness (trotting) of prey species exposed to the cues of a nearby predator. These responses may be distinct from the full-blown flight reaction that is mediated by the dorsolateral and lateral PAG. However, other evidences suggest the possible influences of stimulation schedule, environment dimensions and rat strain in determining outcomes. Overall our results suggest a dynamically organized representation of defensive behaviors in the midbrain tectum.

Organization of electrically and chemically evoked defensive behaviors within the deeper collicular layers as compared to the periaqueductal gray matter of the rat.

Stimulation of the periaqueductal gray matter (PAG) and the deeper layers of superior colliculus (SC) produces both freezing (tense immobility) and flight (trotting, galloping and jumping) behaviors along with exophthalmus (fully opened bulging eyes) and, less often, micturition and defecation. The topography of these behaviors within the distinct layers of SC remains unclear. Therefore, this study compared the defensive repertoire of intermediate (ILSC) and deep (DLSC) layers of SC to those of dorsolateral periaqueductal gray matter (DLPAG) and lateral periaqueductal gray matter (LPAG) [Neuroscience 125 (2004) 71]. Electrical stimulation was carried out through intensity- (0-70 microA) and frequency-varying (0-130 Hz) pulses. Chemical stimulation employed a slow microinfusion of N-methyl-d-aspartic acid (NMDA, 0-2.3 nmol, 0.5 nmol/min). Probability curves of intensity-, frequency- and NMDA-evoked behaviors, as well as the unbiased estimates of median stimuli, were obtained by threshold logistic analysis. Compared with the PAG, the most important differences were the lack of frequency-evoked jumping in both layers of SC and the lack of NMDA-evoked galloping in the ILSC. Moreover, although galloping and jumping were also elicited by NMDA stimulation of DLSC, effective doses were about three times higher than those of DLPAG, suggesting the spreading of the injectate to the latter structure. In contrast, exophthalmus, immobility and trotting were evoked throughout the tectum structures. However, whatever the response and kind of stimulus, the lowest thresholds were always found in the DLPAG and the highest ones in the ILSC. Besides, neither the appetitive, nor the offensive, muricide or male reproductive behaviors were produced by any kind of stimulus in the presence of appropriate targets. Accordingly, the present data suggest that the deeper layers of SC are most likely involved in the increased attentiveness (exophthalmus, immobility) or restlessness (trotting) behaviors that herald a full-blown flight reaction (galloping, jumping) mediated in the PAG.

Serotonin in the dorsal periaqueductal gray inhibits panic-like defensive behaviors in rats exposed to acute hypoxia.

It has been proposed that spontaneous panic attacks are the outcome of the misfiring of an evolved suffocation alarm system. Evidence gathered in the last years is suggestive that the dorsal periaqueductal gray (dPAG) in the midbrain harbors a hypoxia-sensitive suffocation alarm system. We here investigated whether facilitation of 5-HT-mediated neurotransmission within the dPAG changes panic-like defensive reactions expressed by male Wistar rats submitted to a hypoxia challenge (7% O2), as observed in other animal models of panic. Intra-dPAG injection of 5-HT (20 nmol), (±)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT) (8 nmol), a 5-HT1A receptor agonist, or (±)-2,5-dimethoxy-4-iodo amphetamine hydrochloride (DOI) (16 nmol), a preferential 5-HT2A agonist, reduced the number of upward jumps directed to the border of the experimental chamber during hypoxia, interpreted as escape attempts, without affecting the rats' locomotion. These effects were similar to those caused by chronic, but not acute, intraperitoneal administration of the antidepressant fluoxetine (5-15 mg/kg), or acute systemic administration of the benzodiazepine receptor agonist alprazolam (1-4 mg/kg), both drugs clinically used in the treatment of panic disorder. Our findings strengthen the view that the dPAG is a key encephalic area involved in the defensive behaviors triggered by activation of the suffocation alarm system. They also support the use of hypoxia-evoked escape as a model of respiratory-type panic attacks.

Logistic analysis of the defense reaction induced by electrical stimulation of the rat mesencephalic tectum.

Subliminal and threshold functions of behavioral output to electrical stimulation of the rat mesencephalic tectum were fitted using the logistic model. The results suggest the existence of isotopic albeit anisotropic freezing and flight mechanisms in the dorsal periaqueductal gray and deep layers of the superior colliculus. Moreover, the marked parallelism of immobility and running threshold functions indicates the probable coupling of these mechanisms through a kind of negative feedback. Finally, the good fitting to the model suggests that the behavioral output to electrical stimulation of these areas follows a logistic function of the logarithm of the stimulus intensity.

Modeling panic attacks.

The isomorphism of dorsal periaqueductal gray-evoked defensive behaviors and panic attacks was appraised in the present study. Thresholds of electrically induced immobility, trotting, galloping, jumping, exophthalmus, micturition and defecation were recorded before and after acute injections of anxiolytic, anxiogenic and antidepressant drugs. Antidepressant effects were further assessed 24h after injections of 7-14- and 21-day treatments. Chronic administration of clomipramine (CLM, 5-10mg/kg) a clinically effective antipanic drug increased the thresholds of immobility (24%), trotting (138%) galloping (75%), jumping (45%) and micturition (85%). The 21-day treatment with fluoxetine (FLX, 1mg/kg) virtually abolished galloping without changing the remaining responses. Galloping thresholds were also increased by 5mg/kg acute injections of CLM (19%) and FLX (25%). In contrast, chronically administered maprotiline (10mg/kg), a noradrenaline (NE) selective reuptake inhibitor, selectively increased the thresholds of immobility (118%). Diazepam (1.8mg/kg) and midazolam (MDZ, 2.5mg/kg) failed in attenuating the somatic defensive responses. Yet, the sedative dose of MDZ (5mg/kg) attenuated immobility. The panicogenic drug, pentylenetetrazole (50mg/kg), markedly decreased the thresholds of galloping (-51%) and micturition (-66%). These results suggest that whereas immobility is a NE-mediated attentional response, galloping is the panic-like behavior best candidate.

Role of periaqueductal gray matter in hypertension in spontaneously hypertensive rats.

We performed experiments to study the effects of electrolytic lesions of periaqueductal gray matter on mean blood pressure, heart rate, and cardiac baroreflex in adult male spontaneously hypertensive rats. Cardiac baroreflex was assessed by the administration of randomly assigned doses of phenylephrine (0.3 to 5.0 micrograms/kg i.v.) or sodium nitroprusside (1.5 to 5.0 micrograms/kg i.v.) to unanesthetized rats. Bilateral lesions of the periaqueductal gray matter (0.5 mA/5 s) were then performed with rats under sodium pentobarbital anesthesia (35 mg/kg i.p.). Twenty hours after lesion, cardiac baroreflex was retested. Baroreflex data were analyzed by sigmoidal curve fitting. Lesion rats (n = 12) showed a significant decrease in both the gain (delta = -0.89 +/- 0.38 beats per minute [bpm]/mm Hg, P < .05) and curve midpoint (delta = -15 +/- 6 mm Hg, P < .05) of the cardiac baroreflex. Moreover, despite a moderate increase in heart rate (delta = 34 +/- 10 bpm, P < .01), resting mean blood pressure was significantly decreased 24 hours after the lesions (delta = -19 +/- 5 mm Hg, P < 01). No significant changes in cardiac baroreflex were observed in sham-lesion rats (n = 12). Histological examination showed circumscribed bilateral damage of dorsolateral periaqueductal gray matter. Dorsolateral periaqueductal gray matter is an area of the brain putatively related to fear and anxiety. It also projects onto premotor sympathetic neurons in the medulla. Although electrolytic lesions damage neurons as well as fibers of passage, these data suggest that dorsolateral periaqueductal gray matter has a far greater influence on resting cardiovascular control in spontaneously hypertensive rats than was previously suspected.

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.

Long-term effects of clomipramine and fluoxetine on dorsal periaqueductal grey-evoked innate defensive behaviours of the rat.

RATIONALE: The defensive responses induced by electrical stimulation of dorsal periaqueductal grey (DPAG) of the rat have been proposed as a model of panic attacks in humans. OBJECTIVE: The present experiments were carried out to evaluate the long-term effects of clinically effective panicolytics on these responses. METHODS: Rats that had electrodes implanted into the DPAG were treated for 21 days with clomipramine (CLM; 5, 10 and 20 mg/kg), fluoxetine (FLX; 1 and 5 mg/kg) or saline. Long-term effects were assessed prior to the treatment to avoid acute drug effects. Threshold logistic functions of defensive responses were compared by likelihood ratio coincidence tests. RESULTS: CLM attenuated DPAG-evoked defensive behaviours at a time-course similar to that observed in panic therapy. Administration of 10 mg/kg CLM for 21 days produced significant increases in the thresholds of immobility (24%), running (49%) and jumping (45%). Splitting of running into single responses disclosed selective threshold increases of galloping (75%) and trotting (138%) with 5 and 10 mg/kg, respectively. Thresholds of micturition were markedly increased (87%) by 5 mg/kg CLM. In turn, FLX (1 mg/kg) virtually abolished the galloping response. No threshold changes were observed following the long-term administration of the higher dose of either CLM or FLX. Saline-treated rats had a significant increase (35%) in galloping thresholds only. CONCLUSIONS: The present data partially validate the DPAG-evoked defence reaction of the rat as a model of panic attacks in humans. Attenuation of galloping by lower doses of FLX and CLM also suggests the prominent modulation of this response by serotonin.

NMDA-coupled periaqueductal gray glycine receptors modulate anxioselective drug effects on plus-maze performance.

The present study was carried out to investigate a possible interaction between the effects of anxiety modulating drugs which act at the GABA-A receptor complex and selective N-methyl-D-aspartic acid (NMDA) coupled glycine receptor (GLY-B receptor) ligands within the dorsal periaqueductal gray (DPAG). The plus-maze performance of rats pretreated with diazepam (0.37 and 0.75 mg/kg, i.p.) or pentylenetetrazole (15 and 30 mg/kg, i.p.), standard anxiolytic and anxiogenic drugs respectively, was assessed following intra-periaqueductal injections of either glycine (0.2 M, 0.4 microl/30 s, i.c.) or its competitive antagonist, 7-chlorokynurenic acid (7ClKYN, 0.02 M, 0.4 microl/30 s, i.c.). Whilst diazepam produced a typical anxiolytic effect in intracranially-injected CSF rats, increasing open arm exploration, pentylenetetrazole displayed an opposite anxiogenic profile. Either anxiogenic or anxiolytic effects were seen in peripherally-injected vehicle rats following intra-periaqueductal injections of glycine or 7ClKYN, respectively. Intra-periaqueductal injection of glycine markedly attenuated the anxiolytic effect of diazepam. Moreover, while the anxiogenic effects of pentylenetetrazole were barely changed by glycine, they were markedly attenuated by intra-periaqueductal injection of 7ClKYN. Interaction of diazepam and 7ClKYN produced non-selective sedative-like effects which masked any possible anxiolytic action. Accordingly, the present results suggest that the NMDA-coupled glycine receptors located in the DPAG interfere with anxioselective effects of GABA-A acting drugs on the elevated plus-maze. In spite of the prevailing notion that the NMDA coupled glycine receptor is saturated at in vivo brain concentrations of glycine, our results also suggest that either unoccupied or low-affinity GLY-B receptors are likely to be activated by glycine injection into DPAG.

Thresholds of electrically induced defence reaction of the rat: short- and long-term adaptation mechanisms.

The thresholds of electrically induced defence reaction of the rat were studied through the logistic fitting of the response output. When stepwise increasing stimuli were applied at the dorsal midbrain, hierarchically organized mean thresholds, spaced 10 microA apart, were observed for immobility, running and jumping defensive behaviours. The parallel threshold functions of these responses, ranked in the above order, denote that they have distinct output probabilities when induced with sequential stepwise increasing stimuli. In contrast, when single daily stimuli were given in a random order, virtually superimposed threshold functions were obtained for these defensive behaviours. In this case, since the same output probabilities would be expected for immobility, running and jumping behaviours, the defence system seems to operate in a state of maximum entropy. The above data suggest that the dorsal midbrain, including the deep collicular layers and the periaqueductal gray, may encode hierarchical or non-hierarchical defensive patterns which, respectively, mimic either the attentive behaviour of the prey watching the approaching predator or its chaotic behaviour when cornered by a sudden attack. On the other hand, whereas quite stable thresholds were observed for the somatic defensive responses when 5 stimulation sessions were repeated over 15 days, the defecation and micturition output underwent a marked and progressive lessening. Since these autonomic responses have long been considered as reliable indexes of fear, their attenuation throughout the repeated sessions could express the rat adaptation to fear by the recurrence of the aversive experience. Taken together, these data suggest that while short-term neuronal adaptation could be responsible for the hierarchical threshold structure of the short interval stepwise stimulation, long-term neuronal adaptation could underlie the selective decrease of defecation and micturition responses over repeated sessions of intracranial stimulation.

L-type calcium channels selectively control the defensive behaviors induced by electrical stimulation of dorsal periaqueductal gray and overlying collicular layers.

The present study reports the involvement of L-type calcium channels in the control of defensive behaviors produced by electrical stimulation of dorsal periaqueductal gray and overlying collicular layers. Rats that had chemitrodes in the dorsal midbrain and which stimulation produced freezing or flight behaviors with less than 55 microA were selected for drug experiments. Stimulation was repeated the day after the screening session 20 min following the microinjection into the dorsal periaqueductal gray of 15 nmol of either verapamil, a selective L-type calcium channel antagonist, or cobalt chloride (CoCl(2)), a calcium-specific channel modulator. Post-drug sessions were performed 48 h after. Threshold functions were obtained by logistic fitting of accumulated response frequencies. Verapamil and CoCl(2) significantly attenuated the output of immobility, exophthalmus, running and jumping. Although to a lesser degree, verapamil also attenuated defecation. Because CoCl(2) had no effect on defecation, the attenuation of this response by verapamil suggests a non-specific action of this drug. Neither verapamil nor CoCl(2) changed the output of micturition. Finally, whereas there was a complete recovery of defensive thresholds following the microinjection of verapamil, the attenuating effects of CoCl(2) were still present 48 h after. These results support an important role of L-type calcium channels in the neurogenesis of dorsal periaqueductal gray-evoked immobility, exophthalmus, running and jumping, but not defecation and micturition responses.

Neurones in the medullary raphe nuclei attenuate the cardiovascular responses evoked from the dorsolateral periaqueductal grey matter.

In rats anaesthetised with alphaxalone/alphadolone, electrical stimulation in the dorsolateral part of the periaqueductal grey matter (PAG; 10 s trains of 1 ms pulses at 80 Hz, 40-80 microA) evoked a pressor response accompanied by tachycardia. Both components of the response were attenuated following microinjection of 200 nl 0.1 M D,L-homocysteic acid into the caudal pole of the nucleus raphe magnus (NRM; n = 12) and into the nucleus raphe obscurus (NRO; n = 22) to selectively activate neuronal perikarya. Microinjection of 200 nl 165 mM NaCl into the same region (n = 15) had no effect. The attenuation of the midbrain-evoked cardiovascular responses lasted for 10-20 min and was independent of changes in resting blood pressure and heart rate. The maximum reduction in the pressor component of the midbrain-evoked responses was similar following stimulation in NRM (-35.4%) and NRO (-36.7%). However, the reduction in the midbrain-evoked tachycardia was greater following stimulation in NRM (-62.8%) compared to NRO (-27.2%). These results indicate that neurones in NRM and NRO may be involved in modulating the level of excitability of the midbrain defence area in the PAG and/or its efferent pathway.

Cardiac baroreflex dynamics during the defence reaction in freely moving rats.

To determine the extent of baroreceptor reflex involvement in the cardiovascular changes observed during electrically induced defence reaction, the mean arterial blood pressure (MBP) and heart rate (HR) of conscious intact or sinoaortic baroreceptor denervated (SAD) rats were continuously recorded from indwelling cannulae during a 1-min period of electrical stimulation of the mesencephalic tectum. Electrical stimulation produced stimulus intensity-dependent behaviours including freezing at lower intensities and flight at higher intensities. The cardiovascular responses in intact rats were dependent on both the intensity and duration of the stimulus. A linear increase in MBP was observed with increasing stimulus intensities. However, while a slight bradycardia was observed during the freezing behaviour, a marked tachycardia occurred during flight. Simultaneous increases of MBP and HR were seen throughout the first 15 s of the flight response, after which the HR rapidly fell to baseline levels, whereas the MBP remained at a hypertensive plateau until the end of the stimulus. The baroreflex HR curve showed a parallel shift to the left during the first half of the freezing period, being fully reset 40 s after that. So, while the baroreflex gain remained unchanged, the reflex set point was lowered during the freezing stage of the defence reaction. The experiments with SAD rats corroborated the above data. The baroreceptor denervation reversed the freezing bradycardia to tachycardia. Moreover, the denervation potentiated the flight tachycardia and prevented its later reset. MBP responses of baroreceptor denervated rats did not differ from the sham-operated group. The sustained hypertension, thus, appears to be mediated by mechanisms other than the mere baroreceptor reflex deactivation. (ABSTRACT TRUNCATED AT 250 WORDS)

Micturition and defensive behaviors are controlled by distinct neural networks within the dorsal periaqueductal gray and deep gray layer of the superior colliculus of the rat.

Electrical stimulation of the dorsal periaqueductal gray (DPAG) or the deep gray layer of the superior colliculus (DGSC) of rats placed in an open-field elicited either a display of tense immobility, accompanied by exophthalmus and/or defecation and micturition, or running and jumping responses. Threshold curves of each response were obtained for each structure by the logistic fitting of accumulated response frequencies. DPAG and DGSC threshold curves were compared by likelihood-ratio coincidence tests. The output of micturition was significantly higher following the stimulation of DPAG (P < 0.0005). In contrast, no differences were found for the remaining responses. These data support previous studies in anaesthetized cats suggesting the critical involvement of DPAG in the control of micturition. Furthermore, they also suggest that topographically distinct neural networks within the DPAG and DGSC control micturition and the other defensive behaviors.

GABA modulation of the defense reaction induced by brain electrical stimulation.

Earlier behavioral results led to the suggestion that GABA exerts a tonic inhibitory influence in the dorsal periaqueductal gray (DPAG) matter of the rat integrating defensive behavior. In the present experiments, the role of GABAergic mechanisms in the modulation of the autonomic component of the defense reaction was studied. Thus, the effects of intravenous (IV) injections of chlordiazepoxide as well as of intracerebral (IC) injections of midazolam in the dorsal midbrain, on the blood pressure (BP), heart rate (HR) and respiratory increases induced by electrical stimulation of the DPAG were measured in rats anesthetized with urethane. Chlordiazepoxide (10 mg/kg, IV) as well as midazolam (40 and 160 nmol, IC) attenuated the centrally-induced hypertension, without affecting basal BP. The tachycardia induced by aversive brain stimulation was similarly decreased by the benzodiazepines. In addition, the HR baseline was significantly raised by chlordiazepoxide and by the highest dose of midazolam. The tachypnea induced by brain electrical stimulation was also reduced by both benzodiazepines. Basal respiratory rate was slightly, but significantly decreased by chlordiazepoxide as well as by the two doses of midazolam used and to a lesser extent by the vehicle alone. Chlordiazepoxide attenuated the increase in respiratory depth caused by brain stimulation, while basal respiratory amplitude was not affected. The effects of midazolam on this parameter were unclear. Microinjection of bicuculline (5 and 10 nmol) or picrotoxin (0.3 and 1 nmol) into the DPAG increased the BP, HR and respiration, like the electrical stimulation. The latency and duration of bucuculline effects were shorter than those of picrotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuroeffector mechanisms of the defense reaction in the rat.

Electrical stimulation of the dorsal periaqueductal gray matter (DPAG) eliciting flight behavior in awake rats caused an increase in arterial blood pressure (BP), heart rate (HR) and respiration in rats anesthetized with urethane. The hypertension was markedly reduced by 5 mg/kg of intravenously injected hexamethonium or bretylium, virtually abolished by 5 mg/kg of phentolamine and partially antagonized by 0.1 mg/kg of the alpha 1-adrenoceptor blocker, prazosin. The tachycardia induced by DPAG stimulation was partially antagonized by hexamethonium or bretylium and abolished by propranolol (5 mg/kg, IV) or practolol (5 mg/kg, IV), but not affected by N-butylscopolamine (10 mg/kg, IV). Phentolamine increased basal HR and abolished the tachycardic response caused by either brain stimulation or intravenous noradrenaline. Prazosin moderately decreased the response to noradrenaline, but did not affect basal HR or the tachycardia induced by brain stimulation. The increase in respiratory amplitude occurring during brain stimulation was abolished by phentolamine as well as by prazosin, while the increase in respiratory rate was moderately reduced by phentolamine and propranolol. These results demonstrate that the cardiovascular component of the defense reaction of the rat is almost entirely due to a sharp increase in sympathetic tone. They also suggest that the hyperventilation induced by aversive brain stimulation is modulated by central and peripheral adrenergic mechanisms.

Role of the periaqueductal gray substance in the antianxiety action of benzodiazepines.

In order to study the interactions between serotonergic mechanism and electrical stimulation of the mesencephalic central gray substance, rats were trained to lever-press for terminating aversive electric stimuli applied at the Periaqueductal gray and adjoining tectum of the mesencephalon. Experimental sessions consisted of 40 discrete escape trials of a maximum of 30 sec duration, separated by 30 sec intervals. Dose-effect curves of two tryptamine antagonists, cyproheptadine and methysergide, as well as of the benzodiazepine minor tranquilizer, chlordiazepoxide, on average escape latencies and on frequency distribution of individual latencies were determined. Doses of 3 to 10 mg/kg of cyproheptadine decreased average latencies of escape responding in six of eight rats studied. Doses of 10 and 30 mg/kg of methysergide also facilitated escape responding in one of three rats. In contrast, doses from 1 to 10 mg/kg of chlordiazepoxide, that cause little sedation or ataxia, produced dose-dependent increases in escape latencies. Furthermore, doses of 5.6 and 10 mg/kg of chlordiazepoxide partially blocked escape responding. The facilitatory effects of the tryptamine antagonists suggest that escape behavior is inhibited by brain tryptaminergic mechanisms, whereas the specific depressant effect of chlordiazepoxide on escape from Periaqueductal gray electrical stimulation suggest that this region may be involved in the antianxiety action of benzodiazepines.

Cardiovascular change in response to prolonged defense area stimulation in freely moving rats.

The mean blood pressure and heart rate of freely moving rats were directly recorded over a 1-min period of electrical stimulation of the periaqueductal gray with intensities that induced freezing behavior, intense flight or no behavioral changes. Blood pressure and heart rate increased only when flight was induced and only during the first 15 s of stimulation. These cardiovascular changes suggest that homeostatic mechanisms act during the defense reaction and are markedly inhibited only at the beginning of the stimuli that induce the flight response, this inhibition quickly undergoing attenuation. These data do not suggest that activation of defense area, per se, contributes to the development of primary hypertension.