Kirk W. Barron

Corticosterone delivery to the amygdala increases corticotropin-releasing factor mRNA in the central amygdaloid nucleus and anxiety-like behavior

The present study examined the effects of stereotaxic delivery of corticosterone to the amygdala on anxiety-like behavior and corticotropin-releasing factor (CRF) mRNA level in the central nucleus of the amygdala (CeA). Micropellets (30 microg) of crystalline corticosterone or cholesterol (control) were implanted bilaterally at the dorsal margin of the CeA in Wistar rats. Seven days post-implantation, anxiety-like behavior was accessed using an elevated plus-maze. CRF mRNA level in the CeA was determined by in situ hybridization 4 h after being tested on the elevated plus-maze. Corticosterone implants increased indices of anxiety on the elevated plus-maze and produced a concomitant increase in both basal level of CRF mRNA per neuron and the number of neurons with CRF hybridization signal in the CeA. The plus-maze increased CRF mRNA levels in the CeA of cholesterol implanted rats to the elevated basal levels observed in corticosterone treated animals. Exposure to the plus-maze did not increase CRF mRNA level in the CeA of corticosterone implanted rats beyond elevated basal levels. Taken together, these findings support the involvement of the amygdala in anxiety-like behaviors in response to chronically elevated corticosterone and suggests that elevated glucocorticoids may increase anxiety by inducing CRF expression in the CeA.

Modulation of intrinsic cardiac neurons by spinal cord stimulation : implications for its therapeutic use in angina pectoris

OBJECTIVE Electrical stimulation of the dorsal aspect of the upper thoracic spinal cord is used increasingly to treat patients with severe angina pectoris refractory to conventional therapeutic strategies. Clinical studies show that spinal cord stimulation (SCS) is a safe adjunct therapy for cardiac patients, producing anti-anginal as well as anti-ischemic effects. However, little information is yet available about the underlying mechanisms involved. METHODS In order to determine its mechanism of action, the effects of SCS on the final common integrator of cardiac function, the intrinsic cardiac nervous system, was studied during basal states as well as during transient (2 min) myocardial ischemia. Activity generated by intrinsic cardiac neurons was recorded in 9 anesthetized dogs in the absence and presence of myocardial ischemia before, during and after stimulating the dorsal T1-T2 segments of the spinal cord at 66 and 90% of motor threshold using epidural bipolar electrodes (50 Hz; 0.2 ms; parameters within the therapeutic range used in humans). RESULTS The SCS suppressed activity generated by intrinsic cardiac neurons. No concomitant change in monitored cardiovascular indices was detected. Neuronal activity increased during transient ventricular ischemia (46%), as well as during the early reperfusion period (68% compared to control). Despite that, activity was suppressed during both states by SCS. CONCLUSIONS SCS modifies the capacity of intrinsic cardiac neurons to generate activity. SCS also acts to suppress the excitatory effects that local myocardial ischemia exerts on such neurons. Since no significant changes in monitored cardiovascular indices were observed during SCS, it is concluded that modulation of the intrinsic cardiac nervous system might contribute to the therapeutic effects of SCS in patients with angina pectoris.

Stereotaxic localization of corticosterone to the amygdala enhances hypothalamo-pituitary–adrenal responses to behavioral stress

The amygdala is involved in behavioral, autonomic, and neuroendocrine responses to stressful stimuli. The goal of the current study was to determine the effect of directly elevating glucocorticoids in the amygdala on hypothalamo-pituitary-adrenocortical (HPA) responses to the elevated plus maze, a behavioral stressor known to activate the amygdala. Micropellets (30 microg) of crystalline corticosterone or cholesterol (control) were implanted bilaterally at the dorsal margin of the CeA in male Wistar rats; vascular catheters were also placed at this time. Five days post-surgery, blood samples were drawn at 07:00 and 19:00 h to assess diurnal rhythm of plasma corticosterone. At 7 days post-implantation, rats were subjected to behavioral stress using an elevated plus maze and blood was collected 15 min prior to stress, and at 15, 45, and 90 min after the initiation of the stressor. Corticotropin releasing factor (CRF) and arginine vasopressin (AVP) mRNA levels were analyzed by in situ hybridization in the medial parvocellular division of the hypothalamic paraventricular nucleus (mpPVN) in corticosterone- and cholesterol-implanted rats either not exposed to the elevated plus maze (control) or 4 h post-behavioral stress. Localization of corticosterone to the amygdala had no effect on diurnal rhythm of corticosterone secretion. Behavioral stress significantly increased peak plasma corticosterone levels in both groups to a similar level. However, in the corticosterone implanted rats, plasma corticosterone concentrations at 45 and 90 min post-stress were significantly greater compared to control rats indicating a prolonged corticosterone response to behavioral stress. In non-stressed rats, corticosterone delivery to the amygdala elevated basal CRF mRNA in the mpPVN to levels similar to those observed post-stress in control animals; no further increase was observed in CRF mRNA following stress. Behavioral stress resulted in a significant elevation in CRF mRNA in cholesterol controls. Basal AVP mRNA levels were unaffected by corticosterone implants. AVP mRNA did not increase in cholesterol implanted rats in response to behavioral stress. However, AVP mRNA levels were higher in corticosterone implanted rats post stress compared to cholesterol treated controls. In conclusion, direct administration of corticosterone to the amygdala increases plasma corticosterone in response to a behavioral stressor without altering the diurnal rhythm in plasma corticosterone. Elevated basal levels of mpPVN CRF mRNA, and the induction of a mpPVN AVP mRNA response to the behavioral stressor implicate enhanced ACTH secretagogue expression in the increased HPA response to corticosterone modulation of amygdala function.

Cardiovascular effects ofl-glutamate and tetrodotoxin microinjected into the rostral and caudal ventrolateral medulla in normotensive and spontaneously hypertensive rats

Abstract The purpose of this study was to compare the responsiveness of the rostral and caudal ventrolateral medulla in spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats to microinjection of l -glutamate, and to estimate tonic output of these areas by microinjecting the neurotoxin tetrodotoxin. Rats were anesthetized with 1.25 g/kg urethane s.c., implanted with arterial (femoral) and venous (femoral) catheters, artificially ventilated and paralyzed with gallamine triethiodide (10 mg/kg). Using a ventral approach to the brainstem, the mean arterial pressure and heart rate responses to microinjection (30 nl) of l -glutamate (1, 10 and 100 mM) and tetrodotoxin (10 μM) into the rostral and caudal ventrolateral medulla were compared in SH ( n = 7) and WKY ( n = 7) groups. Microinjection of l -glutamate into the rostral ventrolateral medulla produced equivalent increases in mean arterial pressure (maximum+33 ± 3and+36 ± 6 mm Hg, SH and WKY groups respectively) and minimal changes in heart rate. Similar administration of l -glutamate into the caudal ventrolateral medulla caused decreases in mean arterial pressure and heart rate; changes in mean arterial pressure were significantly greater in the SH group than in the WKY group (−52.3 ± 2.9 mm Hg for SH,−22.6 ± 2.6 mm Hg for WKY). Bilateral microinjection of tetrodotoxin into the caudal ventrolateral medulla produced significantly larger increases of mean arterial pressure in WKY rats (+8 ± 4vs+46 ± 8 mm Hg for SH vs WKY). These data indicate that SH rats may have a lower tonic activity of neurons in the caudal ventrolateral medulla, resulting in a lower restraining influence on sympathetic outflow in the SH rat. (Supported by: HL 36552 and KY-THRI 5-42078).

Low intensity spinal cord stimulation may induce cutaneous vasodilation via CGRP release

This study examined whether spinal cord stimulation (SCS) at intensities below motor threshold (MT) produces cutaneous vasodilation through sympathetic inhibition and/or antidromic activation of sensory fibers. SCS was applied to anesthetized rats with stimulus parameters used clinically, i.e. 50 Hz, 0.2 ms and stimulus intensities at 30, 60 or 90% of MT. SCS-induced vasodilation was not attenuated by hexamethonium, an autonomic ganglion blocking agent, but was abolished by CGRP-(8-37), an antagonist of the calcitonin gene-related peptide (CGRP) receptor. We concluded that SCS-induced vasodilation under the conditions of this study was mediated by peripheral release of CGRP via antidromic activation of sensory fibers.

Mechanisms of sustained cutaneous vasodilation induced by spinal cord stimulation

This study was performed to investigate whether spinal cord stimulation (SCS) at intensities below motor threshold prolongs cutaneous vasodilation and whether sustained vasodilation by SCS is mediated through sympathetic inhibition and/or antidromic activation of sensory fibers. SCS was applied to the dorsal surface of the L2-L3 spinal cord of anesthesized rats with stimulus parameters used clinically (i.e., 50 Hz, 0.2 ms duration, and stimulus intensity at 30%, 60%, or 90% of motor threshold). Peripheral vasodilation induced by 5-min SCS was not attenuated by hexamethonium, an autonomic ganglion-blocking agent, but was abolished by dorsal rhizotomy. SCS at < or = 60% of motor threshold increased cutaneous blood flow to the level similar to that obtained at 90% of motor threshold, but the vasodilation did not last for 5 min. SCS-induced vasodilation at 90% of motor threshold persisted for the entire stimulation period up to 30 min, and the vasodilation was not attenuated by hexamethonium. It is concluded that sustained vasodilation, which is induced by SCS at only 90% of motor threshold, in this study was mediated via antidromic activation of sensory fibers.

Role of primary afferents in spinal cord stimulation-induced vasodilation: characterization of fiber types

Selected patients with peripheral vascular disease can be treated with spinal cord stimulation (SCS) to improve blood flow in the limbs. However, the mechanisms producing these effects remain unclear. The present study was designed to investigate if SCS produces cutaneous vasodilation via antidromic activation of the unmyelinated C-fibers and/or the small myelinated fibers. SCS was applied to anesthetized rats with a ball electrode at the L2-L3 spinal level. In Protocol 1, effects of capsaicin were examined. Blood flow changes in the hindpaw induced by SCS were measured in the footpad with laser Doppler flowmeters. Topical application of capsaicin (1%) on the tibial nerve did not affect SCS-induced vasodilation at 30 and 60% of motor threshold (MT). However, the duration of vasodilation induced by SCS at 90% MT and at 10 times MT was significantly reduced after capsaicin application on the tibial nerve. In Protocol 2, antidromic compound action potentials (CAPs) of the tibial nerve were recorded in response to SCS. CAPs of the large and the small myelinated afferent fibers were observed in response to SCS at all intensities. However, even with SCS at ten times MT, CAPs of C-fibers could not be detected in the tibial nerve. In Protocol 3, antidromic CAPs of the dorsal root were measured in response to SCS. Antidromic CAPs of C-fibers in dorsal roots were evoked by SCS at >or=90% of MT. It is concluded that SCS-induced vasodilation at <or=60% of MT may be mediated via only the myelinated fibers, whereas vasodilation at >or=90% of MT may also involve antidromic activation of some unmyelinated C-fibers.

Reevaluation of the Role of the Sympathetic Nervous System in Cutaneous Vasodilation during Dorsal Spinal Cord Stimulation: Are Multiple Mechanisms Active?

Objective. In addition to treatment of refractory chronic pain in patients with peripheral vascular disease, dorsal spinal cord stimulation (DCS) increases cutaneous blood flow to the extremities and may have a limb‐saving effect. The purpose of this study was to examine the role of the sympathetic nervous system in the cutaneous vasodilation due to DCS.

α1-adrenoceptor subtypes and the regulation of peripheral hemodynamics in the conscious rat

The peripheral hemodynamic effects of SZL-49, a prazosin analog capable of selectively inactivating the alpha 1a-adrenoceptor subtype, was evaluated in the conscious rat. One hour after SZL-49 administration, total peripheral vascular resistance and arterial blood pressure significantly decreased and cardiac output and heart rate increased. Twenty-four hours after SZL-49, blood pressure returned to control preinjection levels while peripheral resistance remained decreased and cardiac output and heart rate were elevated. The phenylephrine dose-response curves for mean arterial blood pressure and total peripheral vascular resistance were shifted to the right but the maximal responses were not decreased. These data show that the alpha 1a receptor plays a role in the tonic maintenance of arterial blood pressure. The alpha 1b receptor appears to participate in the response to exogenously administered agonists.

Local cooling alters neural mechanisms producing changes in peripheral blood flow by spinal cord stimulation.

This study was performed to investigate the respective role of sensory afferent and sympathetic fibers in peripheral vasodilatation induced by spinal cord stimulation at different hindpaw skin temperatures. Cooling the skin was used as a strategy to enhance sympathetic activity [Am. J. Physiol.: Heart Circ. Physiol. 263 (1992) H1197]. Cutaneous blood flow in the footpad of anesthetized rats was recorded using laser Doppler flowmetry. Local cooling (<25 degrees C) or moderate local cooling (25-28 degrees C) of the hindpaw was produced with a cooling copper coil. Spinal cord stimulation delivered at clinically relevant parameters and with 30%, 60%, and 90% of motor threshold induced the early phase of vasodilatation in the cooled and the moderately cooled hindpaw. In addition, spinal cord stimulation at 90% of motor threshold produced the late phase of vasodilatation only in the cooled hindpaw, which was possible to block by the autonomic ganglion-blocking agent, hexamethonium. The early responses to spinal cord stimulation in the moderately cooled hindpaw were not affected by hexamethonium. In contrast, both the early and the late phase responses were eliminated by CGRP (8-37), an antagonist of the calcitonin gene-related peptide receptor. After dorsal rhizotomy, spinal cord stimulation at 90% of motor threshold elicited hexamethonium-sensitive vasodilatation in the cooled hindpaw (late phase). These results suggest that spinal cord stimulation-induced vasodilatation in the cooled hindpaw (<25 degrees C) is mediated via both the sensory afferent (early phase of vasodilatation) and via suppression of the sympathetic efferent activity (late phase) although the threshold for vasodilatation via the sympathetic efferent fibers is higher than that via sensory nerves. In contrast, vasodilatation via sensory afferent fibers may predominate with moderate temperatures (25-28 degrees C). Thus, two complementary mechanisms for spinal cord stimulation-induced vasodilatation may exist depending on the basal sympathetic tone.