Effects of footshock stress upon spleen and peripheral blood lymphocyte mitogenic responses in rats with lesions of the paraventricular nuclei.

To assess the role of the hypothalamic paraventricular nucleus (PVN) in mediating stressor-induced immune alterations, male Lewis rats were subjected to a 1-h session of intermittent footshock stress or home cage conditions 6 days after receiving bilateral or sham PVN lesions. Splenic and peripheral blood lymphocyte proliferative responses to the non-specific mitogens, concanavalin A (ConA) and phytohemagglutinin (PHA), were subsequently measured as were plasma corticosterone levels. In sham-operated rats, footshock markedly elevated plasma corticosterone levels and concurrently suppressed the proliferative responses of peripheral blood and splenic lymphocytes. In PVN-lesioned rats, however, the shock-induced suppression of lymphocyte proliferation in the peripheral blood and the elevation of plasma corticosterone were significantly attenuated, while lymphocyte proliferation in the spleen was suppressed below the level of the sham-treated animals. Thus, by utilizing ablation studies, we have determined that the PVN may play a direct role in the alteration of lymphocyte function during stress, and an intact PVN buffers the effect of stress on the responsiveness of spleen lymphocytes to non-specific mitogens.

Activation of brainstem catecholaminergic neurons by conditioned and unconditioned aversive stimuli as revealed by c-Fos immunoreactivity.

In an attempt to define areas of the brain that respond to stressors and influence immune function, we have previously identified stress-induced, c-Fos-immunoreactive areas of the diencephalon. We found that c-Fos was strongly expressed in cells of the paraventricular nuclei (some of which contain corticotropin-releasing hormone (CRH)) and other hypothalamic areas directly associated with autonomic function. To further characterize the presumptive pathways mediating stress-induced immune alterations, including the assessment of brainstem catecholaminergic neuron involvement, the induction of c-Fos immunoreactivity was examined in the brainstem of rats exposed to conditioned and unconditioned, immunomodulating stimuli. In response to electric footshock (the unconditioned stimulus (US)), c-Fos immunoreactivity was strongly induced in the noradrenergic neurons of the locus ceruleus (A6), the nucleus of the solitary tract (A2/C2), the ventral lateral medulla (A1/C1), A5, and A7, as well as in unidentified neurons of the dorsal and ventral subdivisions of the periaqueductal gray (PAG), and in the serotonergic neurons of the dorsal raphe nuclei. Conditioned animals re-exposed to the conditioned stimulus showed c-Fos induction in these same areas but to a lesser degree. Control animals exposed only to the conditioning stimulus (CS) (electronic tone) in the absence of the US, expressed very little, if any, c-Fos activity in the above loci except for a small degree of baseline expression in the PAG. These results further confirm the role of autonomic and endocrine pathways as mediators of the stress response and will help to more fully characterize the pathways of stress-induced immune alteration.

Induction of c-Fos immunoreactivity in the rat forebrain by conditioned and unconditioned aversive stimuli.

The protein product of the c-fos proto-oncogene was immunocytochemically localized in forebrain regions of adult male Lewis rats subjected to a physically aversive footshock stimulus or a Pavlovian-conditioned, non-aversive, auditory stimulus. Animals receiving the conditioned stimulus were first conditioned by repeatedly pairing electric footshock, the unconditioned stimulus (US), with an auditory cue, the conditioned stimulus (CS). These animals were later tested with the CS in the absence of the US, a procedure which, like footshock itself, suppresses immune function. In animals exposed to the conditioned or unconditioned stressor, c-Fos was strongly expressed in cells of the paraventricular nuclei (PVN) of the hypothalamus, some of which contain corticotropin-releasing hormone (CRH), and other forebrain areas directly associated with autonomic function, the ventral lateral septal nuclei (LSV), the medial amygdaloid nuclei (AME), the sensorimotor cortex, the basal ganglia and thalamic nuclei. Control animals exhibited very little or no c-Fos in the above areas. The identified forebrain nuclei can now be targeted for further study aimed at elucidating their role in stress-induced immune alteration.

Corticosterone-independent alteration of lymphocyte mitogenic function by amphetamine.

Amphetamine, a neural stimulatory agent with acute effects mimicking those of stress, is shown here to elevate plasma corticosterone levels and suppress spleen and peripheral blood lymphocyte (PBL) mitogenic responses to concanavalin A (Con A) and phytohemagglutinin (PHA) when administered to rats. Pretreatment of the rats with propranolol, a nonselective beta-adrenergic receptor antagonist, totally prevented the amphetamine-induced suppression of lymphocyte mitogenic reactivity to Con A and PHA in the spleen and to PHA in the peripheral blood; however, the PBL mitogenic response to Con A was only partially restored. Although the amphetamine-induced alterations in immune function were prevented by propranolol pretreatment, the elevated plasma corticosterone response was not. This suggests that corticosterone is not modulating the mitogenic activity of splenic lymphocytes or PHA-reactive PBLs. On the other hand, Con A-reactive PBLs may be affected by corticosterone and/or other mechanisms, which may include the catecholamines.