Effect of isolation on classical conditioned histamine release in guinea pigs

To investigate the effect of abrupt or non-abrupt isolation stress on the classical conditioned histamine release, socially isolated or paired guinea pigs underwent conditioning procedures in which ovalbumin (OA) as an unconditioned stimulus (US) and dimethylsulfide (sulfur smelling) as a conditioned stimulus (CS) were simultaneously inhaled, and the plasma histamine levels after the exposure to the CS in a state of isolation or pairing were measured. The plasma histamine levels significantly increased from baseline in response to the CS (P < 0.05), except in the animals which were abruptly isolated during the exposure to the CS. The guinea pigs which were isolated during either the conditioning procedures or the exposure to the CS displayed significantly lower levels of plasma histamine than did the paired animals during both periods (P < 0.01, respectively). The plasma histamine levels in the guinea pigs which were isolated or paired during both periods were significantly higher than those of the control animals which had received the CS and the US separately (P < 0.05 and P < 0.01). A change of social relations, particularly isolation during the presentation of the CS, may have a suppressive effect on immediate asthmatic responses due to the conditioning mechanism.