Thackery S. Gray

Direct Projections from the Central Amygdaloid Nucleus to the Hypothalamic Paraventricular Nucleus: Possible Role in Stress-Induced Adrenocorticotropin Release

The amygdala, particularly the central amygdaloid nucleus, is important for the expression of adrenocorticotropin and corticosterone responses during stress. The aim of the present study was to determine if the central amygdaloid nucleus directly innervated the hypothalamic paraventricular nucleus. To accomplish this aim, the Phaseolus vulgaris leucoagglutinin lectin anterograde tracing method was used. Injections of the tracer into the medial central amygdaloid nucleus resulted in axonal and terminal labeling within the medial and lateral parvocellular parts of the caudal paraventricular nucleus. A dense patch of labeling was observed within the lateral wing of the lateral part of the parvocellular paraventricular nucleus. Only a few labeled axons were observed within the paraventricular nucleus of animals that had lectin injections localized to the lateral part of the central nucleus. Tracer injections localized to the medial amygdaloid nucleus resulted in axonal and terminal labeling primarily within the anterior parvocellular and periventricular regions of the paraventricular hypothalamic nucleus. Sparse to moderate axonal and terminal labeling was observed within the magnocellular parts of the paraventricular nucleus in animals that had injections of tracer into either the medial central nucleus or the medial nucleus. No labeling was observed within the paraventricular nucleus of animals that had injections of lectin within other amygdaloid nuclei or adjacent regions of the striatum. The results demonstrated a topographically organized projection from the amygdala to the hypothalamic paraventricular nucleus. The central nucleus mainly innervates the caudal lateral and medial parvocellular paraventricular nucleus. The medial nucleus innervates the rostral parvocellular parts of the paraventricular nucleus. These pathways could form the anatomical substrates of amygdaloid modulation of neuroendocrine responses to stressors.

Amygdaloid CRF pathways. Role in autonomic, neuroendocrine, and behavioral responses to stress.

The results of numerous studies have provided compelling evidence that CRF plays an important function in the amygdala. Stimulation of the amygdala produces physiological changes similar those observed after central injections of CRF. Central injections of CRF activate neurons in the amygdala as measured by increases in c-fos protein expression. Destruction of cells or injections of CRF antagonist in the amygdala can attenuate some of the central effects of CRF. The amygdala is the origin of major CRF-containing pathways in the brain. Amygdaloid CRF neurons project to widespread regions of the basal forebrain and brain stem. These amygdaloid pathways mainly arise from the central amygdaloid nucleus where there are a large number of CRF immunoreactive neuronal perikarya. Glucocorticoid and CRF-binding protein are located in cells of the central amygdaloid nucleus. CRF neurons in the central nucleus send their axons to the bed nucleus of the stria terminalis, lateral hypothalamus, midbrain central gray, raphe nuclei, parabrachial region, and the nucleus of the solitary tract. Tract tracing studies have suggested that amygdaloid CRF neurons also innervate CRF neurons in some of these regions and, furthermore, that CRF neurons in some of these areas project back to the CRF neurons in the amygdala. Thus, the amygdala is part of a network of brain nuclei interconnected by CRF pathways. In addition, amygdaloid CRF neurons may project directly to dopaminergic, noradrenergic, and serotonergic neurons, which have widespread projections throughout the neuroaxis.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptide immunoreactive neurons in the amygdala and the bed nucleus of the stria terminalis project to the midbrain central gray in the rat

The central nucleus of the amygdala, bed nucleus of the stria terminalis, and central gray are important components of the neural circuitry responsible for autonomic and behavioral responses to threatening or stressful stimuli. Neurons of the amygdala and bed nucleus of the stria terminalis that project to the midbrain central gray were tested for the presence of peptide immunoreactivity. To accomplish this aim, a combined immunohistochemical and retrograde tracing technique was used. Maximal retrograde labeling was observed in the amygdala and bed nucleus of the stria terminalis after injections of retrograde tracer into the caudal ventrolateral midbrain central gray. The majority of the retrogradely labeled neurons in the amygdala were located in the medial central nucleus, although many neurons were also observed in the lateral subdivision of the central nucleus. Most of the retrogradely labeled neurons in the BST were located in the ventral and posterior lateral subdivisions, although cells were also observed in most other subdivisions. Retrogradely labeled neurotensin, corticotropin releasing factor (CRF), and somatostatin neurons were mainly observed in the lateral central nucleus and the dorsal lateral BST. Retrogradely labeled substance P-immunoreactive cells were found in the medial central nucleus and the posterior and ventral lateral BST. Enkephalin-immunoreactive retrogradely labeled cells were not observed in the amygdala or bed nucleus of the stria terminalis. A few cells in the hypothalamus (paraventricular and lateral hypothalamic nuclei) that project to the central gray also contained CRF and neurotensin immunoreactivity. The results suggest the amygdala and the bed nucleus of the stria terminalis are a major forebrain source of CRF, neurotensin, somatostatin, and substance P terminals in the midbrain central gray.

Organization of amygdaloid projections to brainstem dopaminergic, noradrenergic, and adrenergic cell groups in the rat.

The distribution of amygdaloid axons in the various brainstem dopaminergic, noradrenergic, and adrenergic cell groups was examined. This was accomplished by means of the Phaseolus vulgaris leucoagglutinin lectin (PHA-L) anterograde tracing technique combined with glucose-oxidase immunocytochemistry to catecholamine markers (i.e., tyrosine hydroxylase, dopamine beta hydroxylase, and phenylethanolamine N-methyltransferase). Injections of PHA-L in the medial part of the central amygdaloid nucleus resulted in axonal and terminal labeling in most catecholamine cell groups in the brainstem. Amygdaloid terminals appeared to contract catecholaminergic cells in several brainstem regions. The most heavily innervated catecholaminergic cells were the A9 (lateral) and A8 dopaminergic cell groups and the C2/A2 adrenergic/noradrenergic cell groups in the nucleus of the solitary tract. The medial part of the A9 and adjacent A10 dopaminergic cell groups was moderately innervated. A moderate innervation by amygdaloid terminals was observed on rostral locus coeruleus noradrenergic cells (A6 rostral) and adrenergic cells of the rostral ventrolateral medulla (C1). Noradrenergic cells of the A5, main body of the locus coeruleus (A6), A7, and subcoeruleus were sparsely innervated. Amygdaloid axons were not observed on noradrenergic neurons of the A4 cell group, area postrema, and A1 cells of the ventrolateral medulla. The results demonstrate that the amygdala primarily innervates the dopaminergic cells of midbrain (i.e., A8 and lateral A9 cells) and the adrenergic cells (C2) and noradrenergic (A2) cells in the nucleus of the solitary tract. The possible functional significance of amygdaloid innervation of catecholaminergic cells is discussed.

Amygdaloid lesions : differential effect on conditioned stress and immobilization-induced increases in corticosterone and renin secretion

The purpose of this study was to examine the contribution of the central nucleus of the amygdala to the expression of stress-induced increase in corticosterone and renin secretion. Neurons in the central amygdaloid nucleus of male rats were destroyed by bilateral injections of ibotenic acid, a neurotoxin that destroys cells but leaves fibers of passage intact. Two weeks later, the rats were subjected to immobilization for 20 min or to a conditioned stress (conditioned emotional response) procedure. Central amygdala lesions inhibited the increases in plasma corticosterone after exposure to both conditioned stress and immobilization. Lesions in the lateral amygdala had no effect on the corticosterone response to either stressor. Lesions in the central amygdala attenuated the renin response to conditioned stress but not to immobilization. In contrast, lateral amygdala lesions potentiated the renin response to immobilization but did not affect the renin response to conditioned stress. The results confirm previous studies that demonstrate the importance of the central amygdaloid nucleus in the expression of corticosterone to immobilization stress. In addition, the results show that neurons within the central amygdaloid nucleus are necessary for the full expression of conditioned stress-induced increase in corticosterone and renin secretion. The results are discussed with respect to the potential pathways that mediate stress-induced increases in corticosterone and renin secretion.

Ibotenic Acid Lesions in the Bed Nucleus of the Stria terminalis Attenuate Conditioned Stress-Induced Increases in Prolactin, ACTH and Corticosterone

The contribution of the bed nucleus of the stria terminalis (BST) to the expression of stress-induced increases in ACTH/corticosterone, prolactin and renin secretion was assessed. Neurons in the lateral part of the BST were destroyed with bilateral injections of the cell-selective neurotoxin ibotenic acid (1.5 micrograms in 0.1 microliter of solution per side). Two weeks later, the rats were stressed using an immobilization or conditioned stress paradigm. Rats with lesions in the lateral part of the BST showed attenuated ACTH and corticosterone responses to conditioned stress. Bilateral ablation of lateral BST significantly reduced the prolactin secretory response to conditioned stress. The same lesions had no effect upon plasma increases in renin that occur in response to conditioned stress. Also, destruction of neurons in the BST did not affect immobilization-induced increases in ACTH, corticosterone, prolactin or renin. Previous studies have demonstrated that ibotenic acid lesions in the central amygdala reduce corticosterone and renin response to conditioned stress. Thus, both the BST and central amygdala are important for the adrenocortical response to conditioned stress. Neurons in the central nucleus of the amygdala are part of the circuitry that mediates renin responses to conditioned stress. Neurons in the BST are important for the full expression of prolactin responses to conditioned stress. The neuronal circuitry and stressor specificity in the mediation of prolactin, renin and ACTH/corticosterone responses are discussed.

Androgen inhibits the increases in hypothalamic corticotropin-releasing hormone (CRH) and CRH-immunoreactivity following gonadectomy

To characterize the effect of androgens on the hypothalamo-pituitary-adrenal (HPA) axis we examined the regulation of corticotropin-releasing hormone (CRH) following gonadectomy and hormone replacement. Three-month-old male Fischer 344 (F344) rats were gonadectomized (GDX) or sham GDX. Control animals remained intact. Animals were sacrificed 1, 4, 7, 10, or 21 days following surgery. GDX rats had significantly elevated (p < 0.05) levels of hypothalamic CRH 21 days after surgery compared to intact and sham-operated rats. In a second study, 3-month-old male F344 rats were GDX and treated with the non-aromatizable androgen, dihydrotestosterone (DHT), using a Silastic capsule containing crystalline DHT propionate subcutaneously implanted in each animals back. Control animals were GDX and sham-treated or left intact (INT). Three weeks following gonadectomy, CRH levels in the hypothalamus of GDX rats showed a significant increase (p < 0.05) compared to intact animals. DHT treatment, beginning at the time of gonadectomy prevented this increase. CRH or arginine vasopressin (AVP) immunoreactivity was examined using immunocytochemistry. The number of CRH-immunoreactive (IR) cells in the paraventricular nucleus (PVN) of GDX, DHT-treated animals was significantly decreased (p < 0.05) compared to GDX rats. No differences were seen between treatment groups in CRH-IR cell numbers in the bed nucleus of the stria terminalis or the central amygdaloid nucleus or in AVP-IR cell numbers in the PVN. These data demonstrate that long-term castration increases hypothalamic CRH content and CRH-IR cell numbers in the PVN by removal of an androgen-dependent repression.

Organization of peptidergic and catecholaminergic efferents from the nucleus of the solitary tract to the rat amygdala

Previous studies have focused on the role of the central nucleus of the amygdala (CeA) in cardiovascular and other amygdaloid functions. The combined retrograde tracing/immunohistochemical method was used to test for the presence of enkephalin, neurotensin, neuropeptide Y, and catecholamine neurons within the nucleus of the solitary tract that send efferents to the CeA. After injections of retrograde tracer into the CeA, retrogradely labeled neurons were observed within the caudal, medial nucleus of the solitary tract. Most CeA-projecting neurons were located ipsilaterally within the medial nucleus of the solitary tract at the level of the area postrema. Retrogradely labeled enkephalin- and neurotensin-immunoreactive neurons were found within the medial nucleus of the solitary tract at this level, while retrogradely labeled neuropeptide Y-immunoreactive neurons were found within the medial nucleus of the solitary tract rostral to the area postrema. About 60-74% of CeA-projecting cells were also immunoreactive for tyrosine hydroxylase. Approximately 9% of retrogradely neurons were phenylethanolamine-N-methyltransferase immunoreactive. The results provide evidence that within the nucleus of the solitary tract, peptidergic CeA-projecting neurons have a topographic distribution. In addition, noradrenergic neurons within the A2 group, rather than adrenergic neurons of the C2 group, provide the bulk of catecholaminergic input to the CeA from the nucleus of the solitary tract. Cell counts indicate that each of these peptides may be colocalized (to varying extents) within catecholamine-producing neurons. Also the catecholaminergic and enkephalinergic contribution to the ascending pathway from the nucleus of the solitary tract to the CeA distinguishes it neurochemically from the descending pathway. Thus, although there are afferent and efferent connections between the nucleus of the solitary tract and CeA, their peptidergic/neurotransmitter connections are not necessarily reciprocal. Input from nucleus of the solitary tract peptidergic and catecholaminergic neurons to the CeA may be important in the etiology of a number of pathophysiological conditions including hypertension, gastric ulcers, and schizophrenia.

The central amygdaloid nucleus innervation of the dorsal vagal complex in rat: a Phaseolus vulgaris leucoagglutinin lectin anterograde tracing study

The central nucleus of amygdala (Ce) participates in expression of autonomic responses associated with fear or stress-related behaviors. The Ce can alter autonomic activity through its direct projection to the dorsal vagal complex [i.e., nucleus of the solitary tract (nTS) and the dorsal vagal nucleus]. In order to more precisely define the anatomical organization of the neurons within the Ce and their terminal fields within the dorsal vagal complex, the Phaseolus vulgaris leucoagglutinin lectin (PHA-L) anterograde tracing method was employed in rats. In cases where injections of PHA-L were centered within the medial Ce, dense axon terminal labeling was observed within the medial nTS at rostral levels. Terminal boutons were also observed within the ventral part of the lateral nTS, the dorsal vagal nucleus and contralateral medial nTS. At and just rostral to the obex, numerous axonal boutons were seen within the medial and commissural parts of the nTS and adjacent parts of the dorsal vagal nucleus. Contralateral axon terminal labeling was present within the medial and commissural parts of the nTS. Caudal to the obex, PHA-L immunoreactive boutons were concentrated bilaterally within the medial and commissural nTS and dorsal vagal nucleus. In cases where injections of PHA-L were centered within the lateral Ce moderate axon terminal labeling was observed throughout the rostrocaudal extent of the medial and commissural part of the nTS. Very few PHA-L immunoreactive terminals were observed within the ventral part of the lateral nTS, dorsal vagal nucleus and contralateral medial nTS. The results demonstrate that the medial Ce projects bilaterally to the medial and commissural subnuclei of the nTS and the dorsal vagal nucleus. The lateral Ce projects mainly to the ipsilateral medial and commissural nTS. Thus, both the medial and lateral Ce can directly influence regions of the nTS where peripheral cardiovascular, cardiopulmonary and gastric afferents terminate. The medial Ce can also directly affect vagal nerve outflow through its projection to neurons within the dorsal motor nucleus.

Peptide injections into the amygdala of conscious rats: effects on blood pressure, heart rate and plasma catecholamines

The central nucleus of the amygdala (Ce) mediates cardiovascular and autonomic changes associated with defense or fear responses. At least 16 different neuropeptides have been identified within nerve terminals within the Ce. The role that these peptides play in the Ce regulation of cardiovascular and autonomic function has been assessed. Neuropeptides were microinjected into the region of the Ce and mean arterial pressure (MAP), heart rate (HR) and plasma catecholamine concentrations were measured. Five of the 16 peptides caused changes of MAP and HR. Thyrotropin releasing factor (TRF) and calcitonin gene-related peptide (CGRP) induced increases of MAP and HR. Angiotensin-II (A-II) and somatostatin-28 (SS-28) injection produced increases of MAP and decreases of HR. Bombesin (Bom) injections into the Ce induced an increase of MAP but did not alter HR. Corticotropin releasing factor (CRF), TRF and CGRP were the only peptides found to increase plasma catecholamine concentrations. These results support the conclusion that the Ce contains several peptides that could be involved in the regulation of cardiovascular and autonomic nervous system function. A role of the amygdala in mediating the observed effects of CRF, TRF, CGRP, A-II, SS-28, and Bom is suggested by these studies.