RESUMEN
Substance P (SP) is involved in the pathophysiology of several psychiatric disorders and is considered a central stress neurotransmitter. Endogenous SP does not inhibit the initial extent of the HPA axis response to restraint stress, but reduces the duration of the stress suggesting that SP plays an important role in the transition between acute and chronic stress. Stress hormones can alter metabolic functions in white adipose tissue and liver. The HPA axis is the endocrine pathway that promotes lipolysis elevating free fatty acid levels (FFA) in blood, besides indirectly causing hyperglycemia. In the present study, changes in the blood levels of stress markers in the anxiogenic-like effects of SP, as evaluated on the elevated plus-maze (EPM), were studied in adult male rats. Serum corticosterone was used as the traditional stress marker, while the plasma FFA and glucose were used as alternative anxiety/stress markers. Our findings show: (a) elevated corticosterone levels, confirming the aversive situation induced by SP (behaviorally assessed in the EPM) and indicating SP as a "chemical" stressor; (b) elevated levels of FFA and glucose, indicators of stress-induced mobilization of energy substrates, confirming the stressor effect of SP; (c) FFA levels can be used as an accurate, sensitive and reliable index of acute stress situations, including in the anxiogenic-like effect of SP, with the FFA response being as good as corticosterone as a stress marker in this case; (d) NK1 receptors involvement in the underlying mechanisms of the behavioral and metabolic effects of SP. Finally, our study indicates that some of these physiological variables are positively related to the stressor intensity.