Cardiovascular and behavioral responses to conditioned fear after medullary raphe neuronal blockade.

Conditioned fear to context in the rat leads to a host of sympathetically mediated physiological changes, including a marked rise in mean arterial pressure, a delayed rise in heart rate and a marked cutaneous vasoconstriction, along with the behavioral responses of freezing and ultrasonic vocalization. In this study we examine the role of the rostral ventromedial medulla (RVM), which includes raphe nuclei pallidus and magnus, in the expression of these changes. RVM is a major premotor sympathetic and somatic center and an important integrating center in the descending emotional motor system. To evaluate its role, conditioned fear was tested after temporary blockade with microinjections (0.4 microl) of the GABA-A receptor agonist muscimol (0.2 mM) or the glutamate receptor antagonist kynurenic acid (0.1 M). Changes in mean arterial pressure, heart rate and activity were recorded by radio-telemetry. Cutaneous vasoconstriction in the tail was recorded indirectly by infrared thermography. Muscimol and kynurenic acid had different, almost complementary effects. Muscimol abolished the skin vasoconstrictor response and significantly reduced the tachycardic response, but did not reduce the pressor response significantly and had little effect on the somatic motor components, freezing and ultrasonic vocalization. In contrast, kynurenic acid abolished ultrasonic vocalization and significantly reduced freezing but had no effect on the cardiovascular components. The results show that neurons in the rostral ventromedial medulla are implicated in the expression of some of the cardiac, vascular and somatic motor components of conditioned fear. Most importantly, these cardiovascular components are not under local glutamatergic control whereas the somatic motor components are.

Cortical and subcortical distribution of ionotropic purinergic receptor subunit type 1 (P2X(1)R) immunoreactive neurons in the rat forebrain.

Ionotropic purinergic receptors (P2XR) are ATP-gated cationic channels composed of seven known subunits (P2X(1-7)R) and involved in different functions in neural tissue. Although their presence has been demonstrated in the brain, few studies have investigated their expression pattern. In particular, ionotropic purinergic receptor subunit type 1 (P2X(1)R) has been observed in the cerebellum and in brainstem nuclei. The present study investigates the P2X(1)R expression pattern in the rat forebrain using immunohistochemistry. The specificity of the immunolabeling has been verified by Western blotting and in situ hybridization methods. P2X(1)R immunoreactivity was specifically localized in neurons, dendrites and axons throughout the forebrain. Characteristic differences in the distribution of P2X(1)R were observed in different cortical areas. In prefrontal, cingulate and perirhinal cortices, very intense labeling was present in neuronal bodies. In frontal, parietal, temporal and occipital cortices, immunostaining was lighter and mainly found in dendrites and axons. The hippocampal formation was intensely labeled. Labeling was present almost exclusively in dendrites and axons and never in neuronal bodies. The diencephalon was devoid of P2X(1)R positive neurons or fibers except for the medial habenular nucleus, which showed very intense P2X(1)R immunostaining. Furthermore, two subcortical regions, namely, the nucleus centralis of the amygdala and the bed nucleus of the stria terminalis, showed intense P2X(1)R neuronal labeling. Present data indicate that P2X(1)R are prevalent in forebrain areas involved in the integration of cognitive, limbic and autonomic functions.

Changes in formalin-evoked spinal Fos expression and nociceptive behaviour after oral administration of Bufferin A (aspirin) and L-5409709 (ibuprofen + caffeine + paracetamol).

This study compares the effects of two non-steroidal anti-inflammatory drugs, Bufferin A (BA) and L-5409709 (L-54), on nociceptive behaviour and spinal Fos expression induced by subcutaneous formalin in the rat. BA contains aspirin. L-54 contains ibuprofen, caffeine and paracetamol. Doses based on the human posology were administered orally 30 or 40 min before subcutaneous intraplantar injection of formalin (1.5%, 50 microl) in the right hindpaw. Low doses (BA, 24 mg/kg; L-54, 21.5 mg/kg) did not significantly affect the behavioural pain response. High doses (BA, 480 mg/kg; L-54, 430 mg/kg) reduced the late phase of the response by 42% and 62% respectively, but did not affect the early phase of the response. No sedative side-effects were observed. The two drugs had different effects on the number of spinal Fos-like immunoreactive neurones 2 h after the formalin injection. Fos expression was reduced after BA treatment, and this reduction was correlated to and matched the reduction of the pain response. In contrast, Fos expression after L-54 treatment was not reduced and was not correlated to the reduction in the pain response. The Fos results reveal clear differences in the way that BA (aspirin) and L-54 (ibuprofen + caffeine + paracetamol) affected transmission of the noxious signal. They suggest that BA did not act beyond the spinal cord and that L-54 had more central sites of action than BA.

Role of ventrolateral periaqueductal gray neurons in the behavioral and cardiovascular responses to contextual conditioned fear and poststress recovery.

We have previously shown that conditioned fear to context increases Fos expression in the caudal ventrolateral region of the periaqueductal gray in the rat. To understand the reason for this activation and its role in the expression of the contextual fear response, the ventrolateral periaqueductal gray was temporarily blocked with bilateral microinjections (0.4 microl) of the GABA agonist muscimol (0.2 mM) or the glutamate antagonist kynurenic acid (0.1 M). Cardiovascular changes and activity were recorded by radio-telemetry and the microinjections were made immediately before testing the conditioned response in the aversive context. Muscimol and kynurenic acid had the same effects: when compared to saline controls, freezing immobility and ultrasonic vocalizations were reduced and replaced by marked locomotor activity, and the increase in heart rate was enhanced; however, the increase in arterial blood pressure remained the same. Interesting changes were also observed when animals were returned to the safe context of their home box after fear (recovery). Basically, the recovery response was either prevented or delayed: instead of returning to resting immobility, the rats remained agitated in their home box with a moderately elevated activity, heart rate and blood pressure. However, the effect of ventrolateral periaqueductal gray blockade on heart rate, arterial pressure and activity did not appear to be specific to the fear response or its recovery because they were also observed in animals returned to the safe context of their home box immediately after injection. The later response was also a recovery response from the milder stress of handling and the injection procedure.We discuss the results by arguing that the ventrolateral periaqueductal gray is involved in the immobility component of both the fear response and poststress recovery responses. To explain our interpretation we consider the findings in relation to the classic descending defence-arousal system and the hyporeactive-hypotensive immobility pattern that has been attributed to the ventrolateral periaqueductal gray. We propose that there is a dual activation of the defence-arousal system and of the ventrolateral periaqueductal gray during fear, with the ventrolateral periaqueductal gray acting as a brake on the defence-arousal system. The role of this brake is to impose immobility and hold off active defence responses such as fight and flight. The result of this combination of arousal and immobility is a hyperreactive freezing immobility associated with ultrasonic vocalizations, and a pressor response accompanied with a slow rise in heart rate. Basically, the animal is tense and ready for action but temporarily immobilised. The ventrolateral periaqueductal gray also acts to impose immobility during recovery; however, this is without coactivation of the defence-arousal system. The result is a return to resting immobility, associated with a return to baseline blood pressure and heart rate. This is an active process that insures a faster and complete return to rest. We conclude that the ventrolateral periaqueductal gray is an immobility center involved not only in the fear response but also in poststress recovery responses.

Activation of the ventrolateral periaqueductal gray reduces locomotion but not mean arterial pressure in awake, freely moving rats.

Activation of the ventrolateral periaqueductal gray produces immobility and antinociception. It has been argued that these behaviors are part of either a defensive fear response to threat or a recuperative quiescence response to deep tissue injury. Data collected in anesthetized animals showing that activation of the ventrolateral periaqueductal gray has a hypotensive effect supports the quiescence hypothesis. Our objective was to determine whether activation of the ventrolateral periaqueductal gray in awake, freely moving rats results in a decrease in blood pressure as it does in anesthetized animals. Changes in blood pressure produced by microinjection of the neuroexcitant D,L-homocysteic acid were measured using radio telemetry while rats were awake and while anesthetized with pentobarbital. Consistent with earlier reports, microinjection of D,L-homocysteic acid into the ventrolateral periaqueductal gray caused a decrease in blood pressure in anesthetized rats. In contrast, microinjection at the same ventrolateral periaqueductal gray sites while rats were awake had no effect on blood pressure, even though the animals became immobile and heart rate decreased. Thus, the immobility evoked from ventrolateral periaqueductal gray is not associated with a fall in mean arterial pressure. Two conclusions can be drawn from these data. (1) Caution must be used in generalizing from data collected in anesthetized animals. (2) The ventrolateral periaqueductal gray is as likely to contribute to defensive fear as to recuperative quiescence.

Lidocaine blockade of amygdala output in fear-conditioned rats reduces Fos expression in the ventrolateral periaqueductal gray.

We showed recently that conditioned fear to context induces Fos expression in the ventrolateral periaqueductal gray [Neuroscience (1997) 78, 165-177]. Neurons in this region are thought to play an important role in the expression of freezing during conditioned fear. To test the possibility that this activation comes directly from the amygdala, we looked at changes in Fos expression after a unilateral blockade of the ventral amygdalofugal pathway with lidocaine. The pathway contains fibres originating from the central nucleus of the amygdala that project directly and mainly ipsilaterally to the ventrolateral periaqueductal gray. Conditioned fear was evoked by re-exposing rats to the same box in which they had previously received electric footshocks. The test re-exposure was preceded by a unilateral microinjection of lidocaine (2%, 0.5-1 microl; n = 20) or saline (n = 14). Lidocaine was also tested in non-conditioned animals (n = 13). The results show that, when lidocaine was microinjected in the medial part of the central nucleus of the amygdala or along the ventral amygdalofugal pathway of conditioned rats, fear-induced Fos expression in the ventrolateral periaqueductal gray was reduced on the side ipsilateral to the injection (up to 37% reduction in comparison to the contralateral side). Ipsilateral reductions were also observed with saline, but they were weaker (maximum of 27% reduction). Fos expression remained low on both sides in the non-fear-conditioned animals injected with lidocaine. Finally, although freezing was only partly reduced in the conditioned animals unilaterally injected with lidocaine, it was significantly correlated to the ipsilateral reduction in Fos expression. This study provides direct evidence that the projection from the central nucleus of the amygdala to the ventrolateral periaqueductal gray is activated during fear and that it contributes to the Fos response of the ventrolateral periaqueductal gray.

Conditioned fear to context is associated with increased Fos expression in the caudal ventrolateral region of the midbrain periaqueductal gray.

Immunohistochemical detection of Fos was used to determine which regions of the periaqueductal gray are activated during conditioned fear to a context in the rat. More specifically, the aim of the study was to test the role of its lateral and ventrolateral columns in freezing behaviour during fear. Conditioned fear was evoked by re-exposing rats to the same footshock chamber in which they had received footshocks 4 h earlier. Conditioned Re-exposed rats were compared to Not Conditioned Re-exposed and Conditioned Not Re-exposed rats. Freezing was observed in the Conditioned-Re-exposed group only. It was associated with an overall increase in Fos expression in the entire periaqueductal gray that was significantly greater than in the two other groups. The largest and most significant increase in Fos immunoreactivity was found in the ventrolateral column (especially in its caudal part), whereas only a moderate increase was found in the lateral column. The present results argue in favour of the ventrolateral column as the region of the periaqueductal gray that is preferentially involved in expression of conditioned fear. As previous lesion studies suggested, the ventrolateral periaqueductal gray may play a role in mediating the immobility component of freezing induced by fear. Other lines of evidence suggest that it may also play a role in mediating the quiescence immobility associated with deep pain. We propose that the ventrolateral column of the periaqueductal gray acts as an integrating centre mediating behavioural inhibition.

The supraoculomotor cap: a region revealed by NADPH diaphorase histochemistry.

The distribution of the enzyme NADPH diaphorase in the region of the periaqueductal gray (PAG) was studied in the rat, rabbit, cat, monkey and human. In each species, the enzyme labelled a small band of neurones located below the level of the aqueduct and directly above the supraoculomotor gray in the middle of the antero-posterior extent of the PAG. The chemical specificity of this region suggests that it is distinct from the lateral PAG above and the supraoculomotor nucleus (Su3) below. We have named it the supraoculomotor cap (Su3C).

The periaqueductal gray and defensive behavior: functional representation and neuronal organization.

Recent findings suggest that the periaqueductal gray (PAG) can be subdivided on the basis of its anatomical connections and functional representation of cardiovascular and behavioral functions. This new scheme of subdivision postulates the existence of 4 major longitudinal columns located dorsomedial, dorsolateral, lateral and ventrolateral to the aqueduct. Attention has focussed on the lateral and ventrolateral columns, because they contain topographically distinct groups of neurons whose activation results in different forms of defensive or protective reactions. Reactions evoked from the lateral PAG column are associated with somatomotor and autonomic activation and are characteristic of an organism's response to superficial or cutaneous noxious stimuli, whereas reactions evoked from the ventrolateral PAG column are associated with somatomotor and autonomic inhibition and appear to correspond to an organism's response to deep or visceral noxious stimuli. Furthermore, the neurons of these two columns possess some degree of somatotopic and viscerotopic organization and send axon collaterals to multiple targets in the medulla. This model of PAG neuronal organization outlines the basic architectural features of a network involved in the coordinated expression of certain types of defensive/protective reactions.

Flight induced by microinjection of D-tubocurarine or alpha-bungarotoxin into medial hypothalamus or periaqueductal gray matter: cholinergic or GABAergic mediation?

Microinjections of various doses (50-300 ng) of the nicotinic antagonist D-tubocurarine (TUBO) into the rat's medial hypothalamus (MH) or dorsal periaqueductal gray (PAG) produced flight reactions characterized by jumps. Two different types of flight reactions were produced depending on whether the drug was injected into the MH or into the PAG. MH injections provoked an increase in both locomotor activity and rearing together with well-oriented jumps. PAG injections provoked either freezing reactions or running with explosive jumps, but no increase in rearing. In addition, the rat exhibited an asymmetry in responsiveness to tactile stimulation. These reactions also differed depending on whether the drug was injected into the dorsal or ventral PAG. Behavioral reactions similar to those produced by TUBO were also produced by microinjection of the GABA receptor antagonist bicuculline into the same brain sites. Among the 4 putative cholinergic antagonists tested under the same conditions only alpha-bungarotoxin produced effects that were qualitatively similar to those induced by D-tubocurarine or bicuculline. Gallamine and hemicholinium produced tremor when injected into sites located near the ventricular system at either the MH or the PAG level, while vocalizations were only produced by PAG injections. Hexamethonium produced no marked effect. The hypothesis that flight reactions induced by D-tubocurarine or alpha-bungarotoxin do not result from their antinicotinic action but rather from a direct effect on GABAergic transmission is discussed.

Effect of dorsal periaqueductal gray lesions on cardiovascular and behavioral responses to cat odor exposure in rats.

Previously we demonstrated Fos expression in the dorsal periaqueductal gray (DPAG) of the rat following cat odor exposure. Further work correlated the response to cat odor with a sustained blood pressure increase and deployment of defense behavior. It was therefore of interest to determine whether lesions of the DPAG would abolish these two effects of cat odor exposure. Male Wistar rats were given excitotoxic NMDA (N-methyl-D-aspartate) lesions of the DPAG and anterior tectum under halothane, then implanted with blood pressure telemetric probes. Sham lesions were made with saline. Rats were then exposed to cat odor with a hide option, followed 2 weeks later by re-exposure to cat odor without a hide option. Controls were exposed to rat odor in the same way. Trends toward attenuation in defense and cardiovascular indices were found in lesioned rats for cat odor exposure with a hide option, but these were not significant. Re-exposure to cat fur without a hide option enhanced the cardiovascular response and under these conditions, lesioned rats showed a significant change of the heart rate and locomotor activity response to cat fur. However, the blood pressure response was not significantly attenuated. Thus, the present results support the Fos data and indicate that the DPAG is involved in the expression of some but not all of the cardiovascular and behavioral components of the response to cat odor.

Effect of dorsal periaqueductal gray lesion on cardiovascular and behavioural responses to contextual conditioned fear in rats.

Contextual conditioned fear in the rat is characterized by a freezing immobility associated with a marked increase in blood pressure, a slow increase in heart rate, and ultrasonic vocalizations. A previous Fos study also revealed a marked activation of the ventrolateral part of the periaqueductal gray (VLPAG) and a much smaller activation of its dorsal part (DPAG). Recent chemical blockade experiments indicate that the main role of the VLPAG in the response is to impose the immobility necessary for the expression of the freezing component. We now test the role of the DPAG to see if its small activation (as revealed by Fos) is of any functional significance in the contextual fear response. Large N-methyl-D-aspartate (NMDA) excitotoxic lesions that destroyed most of the DPAG were made in 10 rats. Another group of 10 rats had sham lesions with saline. The animals were then implanted with blood pressure telemetric probes, fear conditioned, and finally tested. There was no significant difference in the amount of freezing and in the blood pressure response between the two groups. However, there was a complete abolition of ultrasonic vocalizations and a significantly greater increase in heart rate in the DPAG-lesioned group. The effect on vocalization and heart rate may be explained by lesion of adjacent structures: the lateral PAG and the superior colliculus (baroreflex alteration), respectively. Thus, most of DPAG appears to play little role in the expression of the contextual fear response.

A neuropharmacological study of the periventricular neural substrate involved in flight.

This paper reviews results obtained in experiments concerning the neurochemical characteristics of the substrate involved in the control of flight reactions and the induction of aversive effects in the rat. These experiments investigated the behavioural effects produced by microinjecting into the periaqueductal grey matter (PAG) or the medial hypothalamus (MH) compounds known to interfere with the functioning of some neurotransmitter systems known to exist in these structures. The data obtained show that: the activity of the substrate involved in the production of flight reactions is tonically inhibited by the release of GABA (gamma-aminobutyric acid); the behavioural reactions produced by microinjecting GABA antagonists can be clearly distinguished, depending on whether such drugs were injected into the PAG or the MH, despite the fact that jumps were produced from either level; behavioural effects, comparable to some extent to those produced by microinjections of GABA antagonists, can be obtained by injecting drugs which act on non-GABAergic neurochemical substrates, namely opioidergic or cholinergic systems; and behavioural effects, comparable to those produced by injecting GABA antagonists into the PAG, can be obtained by injecting such drugs into various sites located in other parts of the tectum such as the inferior colliculus or adjacent structures.

Conditioned fear to environmental context: cardiovascular and behavioral components in the rat.

This study compares the time course of the cardiovascular changes (mean arterial blood pressure, heart rate) and behavioral changes (freezing, rearing, grooming and activity) evoked by 30 min long exposures to a footshock chamber before and after conditioning with footshocks. The main finding is that the conditioned fear evoked by re-exposure to the footshock chamber after conditioning is associated with a prolonged freezing response, a marked rise in mean arterial pressure (+35 mm Hg above a resting baseline of 105 mm Hg) and a delayed rise in heart rate. The pattern of behavioral and cardiovascular changes is the same as with conditioned fear to a discrete stimulus but the effect is a lot longer.

Viscerotopic organization of neurons subserving hypotensive reactions within the midbrain periaqueductal grey: a correlative functional and anatomical study.

Microinjection of the excitatory amino acid D,L-homocysteic acid (40 nmol, in 200 nl) made into the ventrolateral part of the caudal half (A2.5-P1.5) of the midbrain periaqueductal gray (PAG) of the decerebrate cat evoked a hypotensive reaction associated with a slowing of the heart and a decrease in either external iliac or renal vascular resistance. The decrease in iliac vascular resistance was elicited from the pretentorial portion (A2.5-A0.6) of the PAG hypotensive area, whereas the decrease in renal vascular resistance was elicited from the subtentorial portion (A0.6-P1.5). Anatomical experiments using the method of retrograde transport of rhodamine-labelled microspheres or wheat germ agglutinin-horseradish peroxidase demonstrated topographically organized projections from the ventrolateral PAG to the subretrofacial (SRF) pressor nucleus in the rostral ventrolateral medulla. The pretentorial part of the ventrolateral PAG projected mainly to the caudal part of the SRF nucleus, which preferentially controls iliac vascular resistance. The subtentorial part of the ventrolateral PAG projected mainly to the rostral part of the SRF nucleus, which preferentially controls renal vascular resistance. Taken together, these findings suggest: (i) that neurons within the ventrolateral PAG are viscerotopically organized; and (ii) that their hypotensive function may be mediated by an inhibition of SRF pressor neurons. The results are discussed in relation to the recently described PAG hypertensive area which also is viscerotopically organized and projects to the SRF nucleus.

Integrated defence reaction elicited by excitatory amino acid microinjection in the midbrain periaqueductal grey region of the unrestrained cat.

Unilateral microinjections (0.20 microliter) of the excitatory amino acids (EAA), L-aspartate (ASP), D,L-homocysteate (DLH) or kainate (KA) were made into the midbrain of freely moving cats. Injections of DLH (20 nmol) or ASP (200 nmol) made within the midbrain periaqueductal grey matter (PAG) consistently elicited a threat display characteristic of defensive behaviour (i.e., pupillary dilatation, piloerection, retraction of the ears, sideways backing, arching of the back, hissing, howling, growling), whereas injections of DLH or ASP made in the tegmentum bordering the PAG did not elicit such behaviour. Injections of KA (940 pmol) made within the PAG, but not the tegmentum, elicited not only a threat display but also directed attack (striking with unsheathed claws and biting). As injections of EAA depolarize cell bodies, but not axons, the results suggest that a population of neurones whose excitation elicits all of the behavioural signs of defence, including directed attack, is found within the PAG. Histology indicated that the region of the PAG from which the defence reaction was elicited was not confined to any PAG subnucleus. Rather, the 'defence region' of the PAG formed a cylindrical column lateral to the midbrain aqueduct, approximately 1.5 mm in diameter and 5.0 mm in length, the rostral end of which lay dorsal to the caudal end. Further, it was found that EAA microinjections made in different portions of the defence region of the PAG elicited defence reactions characterised by different patterns of vocalization and differing intensities of display. It was also observed following unilateral injection of KA into the PAG that defence reactions, including attack, were elicited by approach in the visual hemifield or touch of the body contralateral, but not ipsilateral, to the injection site. The asymmetry of the defence reaction was not due to any obvious ipsilateral motor impairment and thus suggests that the PAG mediation of the defence reaction, in addition to controlling the outflow to the somatic and autonomic motor systems, also affects sensory processing.

Flight and immobility evoked by excitatory amino acid microinjection within distinct parts of the subtentorial midbrain periaqueductal gray of the cat.

Unilateral microinjections of the excitatory amino acid, D,L-homocysteic acid (DLH) made in the lateral and ventrolateral parts of the subtentorial (A 1.0-P 1.5) midbrain periaqueductal gray (PAG) of the freely moving cat evoked two distinct patterns of coordinated somatic changes. When DLH injection (80 nmol) was made within the lateral part of the subtentorial PAG it evoked a flight reaction, characterized by strong locomotion (running) and multiple jumps. This flight reaction was quite distinct from the defensive threat display previously described following DLH microinjection in the lateral part of the pretentorial PAG. When DLH injection (80 nmol) was made in the subtentorial PAG region, ventrolateral to the aqueduct, it elicited a cessation of both spontaneous locomotion and general movements (e.g. licking, scratching, grooming, head and limb movements), a reaction termed immobility. The subtentorial PAG regions from which flight and immobility were evoked are seemingly identical to the lateral and ventrolateral subtentorial PAG regions in which hypertensive and hypotensive reactions have been evoked previously by DLH microinjection. The present results together with our previous studies suggest that: (1) the lateral PAG of the cat contains at least two, topographically separable neuronal pools, which mediate different types of defense reactions (i.e. threat display--lateral part of the pretentorial PAG; flight reaction--lateral part of the subtentorial PAG); and (2) excitation of neurons in the ventrolateral PAG alters autonomic and somatic functions, but in a direction opposite to that of lateral PAG neurons, namely decreased somatomotor activity and hypotension.

Anatomical evidence that hypertension associated with the defence reaction in the cat is mediated by a direct projection from a restricted portion of the midbrain periaqueductal grey to the subretrofacial nucleus of the medulla.

Wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections were made at sites within a restricted portion of the midbrain periaqueductal grey region (PAG) of the cat at which microinjection of the excitant amino acid, D.L-homocysteic acid, elicits the strongest form of a defence reaction, including a hypertensive response. Among the revealed projections, significant anterograde labelling was found in a discrete region of the rostral ventrolateral medulla, the subretrofacial nucleus (SRF). In the cat, the SRF contains pressor neurones which project to the spinal preganglionic sympathetic outflow. The labelling was most marked ipsilaterally, although substantial contralateral labelling was also observed. To verify that the projection to the SRF originated from the restricted 'defence region' of the PAG, WGA-HRP or rhodamine-labelled microspheres were injected into physiologically-identified sites in the SRF. In all experiments, labelled neurones were found in the same restricted region of the PAG at which DLH injection evokes hypertension and behavioural signs of the defence reaction. The results are consistent with the hypothesis that a discrete cell group within the PAG mediates both somatic and autonomic components of the defence reaction and that the characteristic hypertensive response is mediated by a direct pathway from these PAG cells to pressor neurones in the SRF.

Somatic and autonomic integration in the midbrain of the unanesthetized decerebrate cat: a distinctive pattern evoked by excitation of neurones in the subtentorial portion of the midbrain periaqueductal grey.

Microinjections of the excitant amino acid D,L-homocysteic acid (DLH) made in a restricted part of the subtentorial (P0.2-P0.9) midbrain periaqueductal grey (PAG) of the unanesthetized decerebrate cat evoked a distinctive pattern of coordinated somatic and autonomic changes which was characterized by strenuous hindlimb movement and a concomitant vasodilation in the hindlimb vascular bed. The vasodilation was not secondary to movement as it could still be evoked in the paralyzed preparation. The autonomic changes also included pupillary dilation, increases in arterial pressure and heart rate, and vasoconstriction in renal and mesenteric vascular beds. This evoked response is quite different from that elicited by DLH microinjections made in a restricted part of the pretentorial PAG of the unanesthetized cat (Carrive et al., Neurosci. Lett., 81 (1987) 273-278). This latter response is characterized by a threat display which includes strong facial and vocal changes, but no strenuous hindlimb movement, and skeletal muscle vasoconstriction. The present results together with our previous research suggest that two distinct sets of neurons located in different midbrain PAG regions mediate coordinated patterns of somatic and autonomic change characteristics of different aspects of defensive behavior.

The cardiovascular and behavioral response to cat odor in rats: unconditioned and conditioned effects.

Cardiovascular and behavioral responses were recorded in rats during exposure to cat odor. Rats were habituated to an open rectangular arena that contained a small enclosed wooden box in which they could hide. On day 1 of the experiment, after 30 min in the apparatus, rats were presented with a piece of fabric collar for 60 min. On day 2, rats were presented with an identical piece of fabric collar, except that it had been worn by a cat and therefore exuded cat odor. On day 3, rats were again presented with an unworn cat collar, to determine any conditioned responses to the environment or stimulus (collar) previously associated with cat odor. Results showed significantly increased blood pressure and decreased activity during exposure to cat odor as well as avoidance of the odor stimulus and an increase in vigilance and risk-assessment measures. No significant change in heart rate was found during cat odor exposure. On day 3, a transient increase in blood pressure was seen as well as reduced activity and a range of defensive behaviors. This suggests some conditioning of fear to a context in which cat odor had previously been experienced. Heart rate was also significantly decreased on day 3. A transient rise in blood pressure was also seen when the unworn cat collar was placed into the apparatus on day 3, suggesting a conditioned response to a stimulus that has been previously associated with cat odor. This study demonstrates that a natural stressful stimulus can induce both unconditioned and conditioned autonomic and behavioral responses.