Kevin A. Keay

Parallel circuits mediating distinct emotional coping reactions to different types of stress

All animals, including humans, react with distinct emotional coping strategies to different types of stress. Active coping strategies (e.g. confrontation, fight, escape) are evoked if the stressor is controllable or escapable. Passive coping strategies (e.g. quiescence, immobility, decreased responsiveness to the environment) are usually elicited if the stressor is inescapable and help to facilitate recovery and healing. Neural substrates mediating active versus passive emotional coping have been identified within distinct, longitudinal neuronal columns of the midbrain periaqueductal gray (PAG) region. Active coping is evoked by activation of either the dorsolateral or lateral columns of the PAG; whereas passive coping is triggered by activation of the ventrolateral PAG. Recent anatomical studies indicate that each PAG column receives a distinctive set of ascending (spinal and medullary) and descending (prefrontal cortical and hypothalamic) afferents. Consistent with the anatomy, functional studies using immediate early gene expression (c-fos) as a marker of neuronal activation have revealed that the preferential activation of a specific PAG column reflects (i) the type of emotional coping reaction triggered, and (ii) whether a physical or psychological stressor was used.

Orbitomedial prefrontal cortical projections to distinct longitudinal columns of the periaqueductal gray in the rat.

We utilised retrograde and anterograde tracing procedures to study the origin and termination of prefrontal cortical (PFC) projections to the periaqueductal gray (PAG) in the rat. A previous study, in the primate, had demonstrated that distinct subgroups of PFC areas project to specific PAG columns. Retrograde tracing experiments revealed that projections to dorsolateral (dlPAG) and ventrolateral (vlPAG) periaqueductal gray columns arose from medial PFC, specifically prelimbic, infralimbic, and anterior cingulate cortices. Injections made in the vlPAG also labeled cells in medial, ventral, and dorsolateral orbital cortex and dorsal and posterior agranular insular cortex. Other orbital and insular regions, including lateral and ventrolateral orbital, ventral agranular insular, and dysgranular and granular insular cortex did not give rise to appreciable projections to the PAG. Anterograde tracing experiments revealed that the projections to different PAG columns arose from specific PFC areas. Projections from the caudodorsal medial PFC (caudal prelimbic and anterior cingulate cortices) terminated predominantly in dlPAG, whereas projections from the rostroventral medial PFC (rostral prelimbic cortex) innervated predominantly the vlPAG. As well, consistent with the retrograde data, projections arising from select orbital and agranular insular cortical areas terminated selectively in the vlPAG. The results indicate: (1) that rat orbital and medial PFC possesses an organisation broadly similar to that of the primate; and (2) that subdivisions within the rat orbital and medial PFC can be recognised on the basis of projections to distinct PAG columns. J. Comp. Neurol. 422:556–578, 2000.

Chapter 17 Columnar organization in the midbrain periaqueductal gray and the integration of emotional expression

Publisher Summary Recently, it has become clear that discrete longitudinal columns of neurons within the periaqueductal gray region (PAG) play special roles in coordinating, distinct emotional strategies for coping with different types of stress or threat. This chapter reviews the evidence in favor of columnar organization within the PAG and considers the mechanisms by which different PAG columns coordinate distinct patterns of behavioral and physiological reactions critical for survival. Anatomically, the PAG lies at a crossroads for a multitude of “emotional motor systems”. The PAG (i) receives substantial projections from limbic cortical and subcortical structures critical for the evaluation of the emotional significance of the environment; (ii) projects to somatic and autonomic pre-motor neural pools in the ventrolateral medulla; and (iii) massively innervates midline raphe and paramedian medullary neural pools; these latter projections providing major routes by which limbic cortical and subcortical structures influence neural activity within the medulla. The working hypothesis guiding the present research is that different, longitudinally organized PAG columns play potentially important roles in coordinating different emotional strategies for coping with different sets of environmental demands.

Expression of c-Fos-like immunoreactivity in the caudal medulla and upper cervical spinal cord following stimulation of the superior sagittal sinus in the cat

Migraine is an episodic vascular headache with a well-recognized clinical picture but a poorly understood pathogenesis. Stimulation of a pain-sensitive trigeminally innervated intracranial structure, the superior sagittal sinus (SSS), was undertaken to map the higher-order neurons potentially involved in the processing of vascular head pain. The animals were prepared for stimulation by exposure of the sinus and then maintained under alpha-chloralose anaesthesia for 24 h before SSS stimulation, perfusion and immunohistochemical processing for the detection of Fos protein. Examination of the medulla and upper cervical cord revealed marked increases in Fos-like immunoreactivity in laminae I and IIo of the trigeminal nucleus caudalis and the dorsal horn of the upper cervical spinal cord. In addition, Fos-like immunoreactivity was observed in lamina X of the upper cervical spinal cord, in the commissural and medial nuclei of the solitary tract and in the nucleus retroambigualis. The use of immunohistochemical detection of Fos has allowed visualization of several populations of neurons likely to be involved in the central neural processing of vascular headache syndromes, particularly migraine.

Longitudinal neuronal organization of defensive reactions in the midbrain periaqueductal gray region of the rat

SummaryIn a previous study we investigated the intraspecific defensive reactions evoked by excitation of neurons in the intermediate third of the midbrain periaqueductal gray matter (PAG) of the rat. Experiments revealed that activation of neurons in this region of the PAG mediated: (i) backward defensive behavior, characterized by upright postures and backward movements, and (ii) reactive immobility (“freezing”), in which the rat remained immobile, but reacted with backward defensive behavior to investigative, non-aggressive contact initiated by the partner. In the present study, we aimed to extend our understanding of PAG mediation of defensive behavior by observing: (i) in a non-aggressive social interaction test, the behavioral effects of microinjections of low doses of kainic acid (40 pmol in 200 nl) made in the caudal third of the PAG; and (ii) the behavioral and cardiovascular effects of microinjections of d, l-homocysteic acid (5–10 nmol in 50–100 nl) made in the PAG of the unanesthetized decerebrate rat. Kainic acid injections into the area lateral to the midbrain aqueduct in the caudal third of the PAG evoked: (i) forward avoidance behavior, characterized by forward locomotion and occasional hop/jumps; (ii) reactive immobility (“freezing”), in which the rat remained immobile, but reacted with forward avoidance behavior to investigative, non-aggressive contact initiated by the partner; and (iii) 22–28 kHz ultrasonic vocalizations. These injections also evoked a dramatic increase in defensive responsiveness to tactile stimuli on the half of the body contralateral, but not ipsilateral, to the site of injection. Electroencephalographic measurements indicated that none of these effects were secondary to seizure activity. In the decerebrate rat, d, l-homocysteic acid injections in the caudal third of the PAG evoked forward running movements along with increased blood pressure and heart rate, the strongest effects being evoked from the region lateral to the midbrain aqueduct. More rostrally, sites in the intermediate PAG evoked backward “defensive” movements, which were also associated with increased blood pressure and heart rate. These data, along with those from our previous study in the rat indicate that: (i) defensive reactions are integrated within a longitudinal neuronal column which spans the caudal two thirds of the lateral PAG; (ii) the lateral PAG “defensive behavior” column contains two distinct populations of neurons, one within the intermediate lateral PAG which integrates defensive behavior characterized by facing towards and backing away from a “threatening” stimulus, and a second in the caudal lateral PAG which integrates defensive behavior characterized by forward avoidance behavior; and (iii) neurons within the lateral PAG couple strong cardiovascular changes with each distinctive defensive behavior pattern. The emerging view from this and recent studies of this midbrain region in other species, suggests that similar rostrocaudal differences within a longitudinally oriented lateral PAG neuronal column represent a fundamental principle underlying the PAG organization of defensive behavior.

Immune cell involvement in dorsal root ganglia and spinal cord after chronic constriction or transection of the rat sciatic nerve.

Chronic constriction injury (CCI) of the sciatic nerve in rodents produces mechanical and thermal hyperalgesia and is a common model of neuropathic pain. Here we compare the inflammatory responses in L4/5 dorsal root ganglia (DRGs) and spinal segments after CCI with those after transection and ligation at the same site. Expression of ATF3 after one week implied that 75% of sensory and 100% of motor neurones had been axotomized after CCI. Macrophage invasion of DRGs and microglial and astrocytic activation in the spinal cord were qualitatively similar but quantitatively distinct between the lesions. The macrophage and glial reactions around neurone somata in DRGs and ventral horn were slightly greater after transection than CCI while, in the dorsal horn, microglial activation (using markers OX-42(for CD11b) and ED1(for CD68)) was greater after CCI. In DRGs, macrophages positive for OX-42(CD11b), CD4, MHC II and ED1(CD68) more frequently formed perineuronal rings beneath the glial sheath of ATF3+ medium to large neurone somata after CCI. There were more invading MHC II+ macrophages lacking OX-42(CD11b)/CD4/ED1(CD68) after transection. MHC I was expressed in DRGs and in spinal sciatic territories to a similar extent after both lesions. CD8+ T-lymphocytes aggregated to a greater extent both in DRGs and the dorsal horn after CCI, but in the ventral horn after transection. This occurred mainly by migration, additional T-cells being recruited only after CCI. Some of these were probably CD4+. It appears that inflammation of the peripheral nerve trunk after CCI triggers an adaptive immune response not seen after axotomy.

Deep and superficial noxious stimulation increases Fos-like immunoreactivity in different regions of the midbrain periaqueductal grey of the rat

UNLABELLED We have reported that the lateral region of the caudal third of the midbrain periaqueductal grey (PAG), which mediates flight and hypertension, receives inputs from lamina I and IIo and the lateral cervical nucleus (LCN) of the upper cervical spinal cord (UCC); whereas the ventrolateral PAG region, which mediates hypotension, quiescence and immobility, is targeted by cells in laminae VII, VIII and X. In the UCC the cells of laminae VII and VIII receive a significant afferent input from the deep neck muscles, whereas cells of laminae I and IIo and the LCN receive a large input from cutaneous nociceptors. Thus we investigated the hypothesis that nociceptive activation of the deep neck muscles would activate the spinal-ventrolateral PAG projection, whereas cutaneous nociceptive stimulation would activate the spinal-lateral PAG projection, by examining the expression of Fos protein. We found that deep noxious stimulation led to Fos-positive cells predominantly in the ventrolateral PAG and superficial noxious stimulation led to Fos-positive cells predominantly in the lateral PAG. THE RESULTS (i) indicate that the UCC afferent regulation of the PAG arises from topographically separable and functionally dissociable populations of neurons and (ii) raise the possibility that the ventrolateral and lateral PAG play important but different roles in mediating the distinctive affective, emotional and autonomic responses evoked by pain arising from deep or superficial structures.

Quiescence and hyporeactivity evoked by activation of cell bodies in the ventrolateral midbrain periaqueductal gray of the rat

Much evidence suggests that the midbrain periaqueductal gray region (PAG) plays a pivotal role in mediating an animals responses to threatening, stressful, or painful stimuli. Active defensive reactions, hypertension, tachycardia and tachypnea are coordinated by a longitudinally oriented column of cells, found lateral to the midbrain aqueduct, in the caudal two-thirds of the PAG. In contrast, microinjections of excitatory amino acid (EAA) made in the ventrolateral region of the PAG in anesthetized or isolated animals evoke hypotension, bradycardia, and behavioral arrest. The aim of the present study was to examine further the effects of activation of neurons in the ventrolateral PAG. By injecting into this region low doses (40 pmol) of kainic acid (KA), a long-acting EAA, it was possible to observe a freely moving rats behavior in a social situation (i.e., paired with a weight-matched, untreated partner). Such injected rats become quiescent, i.e., there was a cessation of all ongoing spontaneous activity. These rats were also hyporeactive: the investigative approaches of the partner failed to evoke orientation, startle reactions, or vocalization. Electroencephalographic measurements indicated that the effects of injections of KA in the ventrolateral PAG were not secondary to seizure activity. In addition to the quiescence and hyporeactivity reported here, and the hypotension and bradycardia reported previously, the ventrolateral PAG is a part of the brain from which analgesia has been readily evoked by electrical stimulation, or microinjections of either EAA or morphine. As a reaction to “deep” or “inescapable” pain, chronic injury, or defeat, animals often reduce their somatomotor activity, become more solitary, and are generally much less responsive to their environment. These data, and those from other recent studies, suggest that neurons in the ventrolateral PAG may play an important role in integrating such a passive behavioral response of which quiescence and hyporeactivity are the major components.

Orbitomedial prefrontal cortical projections to hypothalamus in the rat

A previous study in the rat revealed that distinct orbital and medial prefrontal cortical (OMPFC) areas projected to specific columns of the midbrain periaqueductal gray region (PAG). This study used anterograde tracing techniques to define projections to the hypothalamus arising from the same OMPFC regions. In addition, injections of anterograde and retrograde tracers were made into different PAG columns to examine connections between hypothalamic regions and PAG columns projected upon by the same OMPFC regions. The most extensive patterns of hypothalamic termination were seen after injection of anterograde tracer in prelimbic and infralimbic (PL/IL) and the ventral and medial orbital (VO/MO) cortices. Projections from rostral PL/IL and VO/MO targeted the rostrocaudal extent of the lateral hypothalamus, as well as lateral perifornical, and dorsal and posterior hypothalamic areas. Projections arising from caudal PL/IL terminated within the dorsal hypothalamus, including the dorsomedial nucleus and dorsal and posterior hypothalamic areas. There were also projections to medial perifornical and lateral hypothalamic areas. In contrast, it was found that anterior cingulate (AC), dorsolateral orbital (DLO), and agranular insular (AId) cortices projected to distinct and restricted hypothalamic regions. Projections arising from AC terminated within dorsal and posterior hypothalamic areas, whereas DLO and AId projected to the lateral hypothalamus. The same OMPFC regions also projected indirectly, by means of specific PAG columns, to many of the same hypothalamic fields. In the context of our previous findings, these data indicate that, in both rat and macaque, parallel but distinct circuits interconnect OMPFC areas with specific hypothalamic regions, as well as PAG columns. J. Comp. Neurol. 432:307–328, 2001.

Spinal afferents to functionally distinct periaqueductal gray columns in the rat: an anterograde and retrograde tracing study.

The segmental and laminar organization of spinal projections to the functionally distinct ventrolateral (vlPAG) and lateral periaqueductal gray (lPAG) columns was examined by using retrograde and anterograde tracing techniques. It was found 1) that spinal input to both vlPAG and lPAG columns arose predominantly from neurons in the upper cervical (C1–4) and sacral spinal cord; 2) that there was a topographical separation of vl‐PAG projecting and lPAG‐projecting neurons within the upper cervical spinal cord; but 3) that below spinal segment C4, vlPAG‐projecting and lPAG‐projecting spinal neurons were similarly distributed, predominantly within contralateral lamina I, the nucleus of the dorsolateral fasciculus (the lateral spinal nucleus) and the lateral (reticular) part of lamina V. Consistent with the retrograde results, the greatest density of anterograde label, within both the vlPAG and lPAG, was found after tracer injections made either in the superficial or deep dorsal horn of the upper cervical spinal cord. Tracer injections made within the thoraco‐lumbar spinal cord revealed that the vlPAG column received a convergent input from both the superficial and deep dorsal horn. However, thoraco‐lumbar input to the lPAG was found to arise uniquely from the superficial dorsal horn; whereas the deep dorsal horn was found to innervate the “juxta‐aqueductal” PAG region rather than projecting to the IPAG.