Stress-induced activation of nitric oxide-producing neurons in the rat brain

Abstract

Nitric oxide (NO) is a gaseous neurotransmitter that may mediate a decrease in sympathetic output to the periphery. This implication predicts that NO-producing neurons in the brain are activated in animals experiencing increased levels of sympathetic activity. To test this prediction, we subjected three groups of experimental rats to differing levels of environmental stimulation for 1 hour: minimal stimulation, moderate stimulation, and restraint stress. NO-producing neurons were histochemically visualized in sections of the brain, and activation of these neurons was assessed according to the neuronal expression of the immediate early gene c-fos. Constitutive activation of NO-producing neurons was found in the hypothalamus (paraventricular and supraoptic nuclei), dorsal raphe nuclei, and spinal nucleus of the trigeminal nerve of minimally stimulated rats. When animals were subjected to a novel environment (moderate stimulation), additional NO-producing neurons were activated in the medial septum, medial amygdala, hypothalamic nuclei (lateral, periventricular, and posterior), colliculi, nucleus raphe obscurus, medial vestibular nucleus, nucleus of the tractus solitarius, and several components of the ventrolateral medulla. Restraint stress caused the activation of NO-producing neurons in all of these areas, often in increasing numbers, and the activation of additional NO-producing neurons in the diagonal band of Broca, lateral and medial preoptic areas, basomedial and basolateral amygdalar nuclei, hypothalamic nuclei (dorsomedial, retrochiasmatic supraoptic, and circularis), nucleus raphe pontus, lateral parabrachial nucleus, and pontine nuclei. Expressed as a proportion of NO-producing neurons per section, the largest percentages (>20%) of double-stained neurons were found in the basolateral amygdala (46%), hypothalamic paraventricular nucleus (35%), corpora quadrigemina (estimated at 40%), dorsal raphe (45%), nuclei raphe pontus (33%) and obscurus (63%), lateral parabrachial nucleus (22%), medial vestibular nucleus (25%), lateral division of the nucleus paragigantocellularis (26%), and lateral reticular nucleus (35%). Evidence from other studies increasingly supports the concept that NO plays a generalized role in autonomic regulation by decreasing sympathetic output. Our results show that more NO-producing neurons were activated during stress than during minimal or moderate levels of stimulation. Together, the evidence suggests that NO is a neurochemical messenger that is utilized by individual autonomic neurons as the organism responds to increased levels of sympathetic activity.

Activation of spinally projecting and nitrergic neurons in the PVN following heat exposure

“…In the heated group of animals, the total number (unilateral) of Fos-positive cell nuclei (1,336 Ϯ 43) in the PVN was significantly elevated by tenfold compared with the control group (133 Ϯ 23; P Ͻ 0.0001) [which is similar to the numbers found in handled animals (31)]. This increase in the production of Fos occurred throughout the rostral-caudal extent of the PVN, with the maximum number of Fos-positive cell nuclei found predominantly in the middle to caudal levels of the PVN (Figs.…”

Decrease in arterial pressure induced by adrenomedullin in the hypothalamic paraventricular nucleus is mediated by nitric oxide and GABA

“…described previously [36][37][38]. Briefly, the sections were incubated for 20 min at 45 °C in a solution containing 1 mg/ml β-NADPH (Sigma), 0.3 mg/ml nitroblue tetrazolium (Sigma) dissolved in dimethyl sulfoxide (Sigma), and 0.6% Triton X-100/PBS.…”

Nitric oxide and hypertension: not just an endothelium derived relaxing factor!

“…This mechanism may be important in limiting sympathetic activation during stress as it has been observed that the number of NO producing neurons at key sites of autonomic processing in the brains of conscious rats increases in response to experimentally induced stress. 64 Some in vivo evidence does exist to suggest that endogenous NO also results in tonic inhibition of efferent sympathetic nerve activity in humans. The systemic infusion of L-NMMA to healthy volunteers at a dose designed to minimise baroreflex loading resulted in higher levels of directly recorded muscle sympathetic nerve activity than an equipressor doses of phenylephrine, given as a non-NO dependent control vasoconstrictor.…”