Regulation by nitric oxide (NO) of gender based differences in stress-induced neurobehavioral and immunological responses in rats

A Ray, K Gulati, A Chakraborti. V.P. Chest Institute, University of Delhi, Delhi, Department of Pharmacology, 110007, India

Aversive stimuli like stress can disrupt the physiological milieu of the organism and initiates a series of adaptive responses to cope with the demands of such homeostatic challenge. Complex neurochemical pathways are reported to be involved in the regulation of such stress responses. Gender related differences in stress induced biological responses are known but the mechanisms for such discrepancies are not clearly defined. Nitric oxide (NO) is a ubiquitious, multidimensional messenger molecule which regulates a plethora of physiological processes with a clear role in signal transduction in the CNS and as a neuromodulator. Initial studies have shown that NO may influence stress effects and hence, the aim of the present study was to evaluate the role of NO in the possible gender based differences in neurobehavioral and immunological responses to stress in Wistar rats (n=6 per group). Restraint stress (RS, for 1h in Plexiglas restrainers) induced anxiogenic responses in the elevated plus maze (EPM) test, and suppression of both open arm entries and time spent were greater in males as compared to females (males: control 29.3±3.2, RS 10.2±2.5; Females: control 33.8±4.8, RS 22.7±3.6 for open arm entries, p< 0.05 in each case, ANOVA test). These were accompanied by reductions in NO metabolites (NOx) (by 64% in males and 23% in females)and increases in malondialdehyde (MDA) levels(by 46% in males and 29% in females over respective controls) in brain homogenates. Pretreatments with L-arginine attenuated both neurobehavioral and biochemical changes after RS. Exposure of female rats to RS resulted in elevated levels of 17-β oestradiol (by 46 %) and MDA (by 16%) and lowered NOx levels( by 18%) as compared to controls, and these effects were antagonized by formestane (aromatase inhibitor and estrogen synthesis blocker) pretreatment. In rats immunized with KLH, RS-induced suppression of both humoral and cell mediated immune responses. The plaque forming cell (PFC) count of the spleen were lowered in males(by 35%) as compared to females(by 21%) when compared to their respective controls. Similarly, the delayed type hypersensitivity (DTH) response showed greater inhibition in response to RS in males(by 51%) as compared to females (by 24%) as compared to controls. Pretreatment with L-arginine attenuated such immune suppression whereas, L-NAME induced opposite effects. Stress induced suppression of immune responses were associated with lower levels of blood and brain NOx and GSH and higher levels of MDA in males as compared to female rats. Further, prior administration of formestane aggravated RS-induced immunosuppression in females which was associated with further decrease in NOx and increase in MDA levels in both brain and blood. These changes were very similar to those seen in stress exposed male rats. Gender based differences are known to exist in a variety of neuropsychiatric disorders and the present results show that NO plays an important regulatory role in the gender based differences in stress induced neurobehavioral and immunological responses. Further, estrogen-RNS-ROS interactions may be playing an important role in lesser susceptibility of females to such stress effects.