Pascal Carrive

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 organisms 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 organisms 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.

Chapter 13 Integration of somatic and autonomic reactions within the midbrain periaqueductal grey: Viscerotopic, somatotopic and functional organization

Publisher Summary Considerable evidence suggests that the cell dense region surrounding the midbrain aqueduct, the midbrain periaqueductal gray region (PAG), is a crucial neural substrate for the integration of an animals reactions to threatening or stressful stimuli. This chapter describes the great diversity of PAG output functions that make an understanding of its functional organization prerequisite to the study of its neural organization. It considers the results of studies from laboratory that have employed the technique of intracerebral stimulation with excitatory amino acids (EAA) to map the diverse functional output zones that exist within the PAG. Results of anatomical studies of the PAG efferents, which may mediate certain outputs, have been considered in the chapter. Data pertaining to the afferent regulation of the functionally diverse PAG regions have also been discussed.

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.

Changes in cutaneous and body temperature during and after conditioned fear to context in the rat

Infrared thermography was used to image changes in cutaneous temperature during a conditioned fear response to context. Changes in heart rate, arterial pressure, activity and body (i.p.) temperature were recorded at the same time by radio‐telemetry, in addition to freezing immobility. A marked drop in tail and paws temperature (−5.3 and −7.5 °C, respectively, down to room temperature), which lasted for the entire duration of the response (30 min), was observed in fear‐conditioned rats. In sham‐conditioned rats, the drop was on average half the magnitude and duration. In contrast, temperature of the eye, head and back increased (between + 0.8 and + 1.5 °C), with no difference between the two groups of rats. There was a similar increase in body temperature although it was slightly higher and delayed in the fear‐conditioned animals. Finally, ending of the fear response was associated with a gradual decrease in body temperature and a rebound increase in the temperature of the tail (+ 3.3 °C above baseline). This study shows that fear, and to some extent arousal, evokes a strong cutaneous vasoconstriction that is restricted to the tail and paws. This regionally specific reduction in blood flow may be part of a preparatory response to a possible fight and flight to reduce blood loss in the most exposed parts of the rats body in case of injury. The data also show that the tail is the main part of the body used for dissipating internal heat accumulated during fear once the animal has returned to a safe environment.

Excitation of neurones in a restricted portion of the midbrain periaqueductal grey elicits both behavioural and cardiovascular components of the defence reaction in the unanaesthetised decerebrate cat

Microinjections of the excitant amino acid D,L-homocysteic acid (DLH) into a restricted part of the midbrain periaqueductal grey (PAG) of unanaesthetized decerebrate cats evoked a distinctive pattern of facio-vocal and cardiovascular changes characteristic of a defence reaction, including pupillary dilatation, howling vocalization, an increase in arterial pressure and heart rate, and skeletal muscle vasoconstriction. These facio-vocal and cardiovascular responses always occurred together, and thus may arise from excitation of a common population of neurones. DLH injections within a greater extent of the PAG elicited other facio-vocal changes characteristic of defence, such as hissing or growling, but these were not accompanied by significant cardiovascular changes.

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 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.

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.

Viscerotopic control of regional vascular beds by discrete groups of neurons within the midbrain periaqueductal gray

It is well established that a group of bulbospinal neurons within the rostral ventrolateral medulla plays a crucial role in the tonic and phasic control of arterial pressure. In the cat, these neurons are confined to a discrete region which has been termed the subretrofacial (SRF) nucleus. Recent evidence suggests that this nucleus is viscerotopically organized with respect to its control over different vascular beds. These observations raise the question as to whether functionally different subgroups of SRF pressor neurons receive inputs from supramedullary cell groups that also exert a specific control over particular vascular beds. To answer this question retrogradely transported tracers (i.e. rhodamine or fluorescein-labelled microspheres, wheat germ agglutinin-horseradish peroxidase) were injected into physiologically identified sites within the rostral or caudal parts of the SRF nucleus of the cat. Separate groups of neurons in the midbrain periaqueductal gray region (PAG) were found to project specifically to subgroups of cells within the rostral and caudal parts of the SRF nucleus. These findings, together with the results of recent functional studies of the PAG suggest that these distinct projections from the PAG to the SRF nucleus are involved in the expression of different patterns of emotionally coupled cardiovascular responses.

Hypocretin /orexin contributes to the expression of some but not all forms of stress and arousal

Hypocretin/orexin has a well‐established role in wakefulness and in the maintenance of arousal. Because stress is associated with arousal, it has been proposed that hypocretin is also involved in stress. However, it is not clear if this is true for all forms of stress. To clarify this issue, we compared four conditions combining high arousal with no or low stress (wakefulness and exploration) or high stress (contextual fear and restraint) in the rat. We looked at Fos expression in hypocretin neurons, hypocretin‐1 levels in cerebrospinal fluid and cardiovascular and behavioural changes after pharmacological blockade with the dual hypocretin receptor antagonist, almorexant. Fos expression in hypocretin neurons was highest with wakefulness and exploration, also high with fear but not significant with restraint. Hypocretin‐1 levels were consistent with this pattern, although the differences were not as marked. Hypocretin receptor blockade with almorexant reduced the pressor, tachycardic and locomotor responses of wakefulness and exploration as well as the pressor and sympathetic component of the tachycardic response of fear. In contrast, almorexant did not reduce the pressor and tachycardic responses of restraint and nor did it reduce the pressor, tachycardic and locomotor responses of another stressor, i.e. cold exposure. Thus, hypocretin is not involved in all forms of stress. Comparison of the different conditions suggests that, regardless of stress, hypocretin involvement occurs when the arousal associated with the response includes increased attention to environmental cues. When it does, hypocretin will at least contribute to the cardiovascular response. The findings are of clinical relevance to some forms of psychological stress.