Effects of inescapable shock and conditioned fear on the release of excitatory and inhibitory amino acids in the locus coeruleus

We investigated the importance of endogenous amino acids in the locus coeru leus in inescapable electric shock and conditioned fear. In naive rats and in rats exposed to noise (N), light (L) and electric shock (S) or to N + L only, the locus coeruleus was superfused with artificial cerebrospinal flui d through a push-pull cannula and the release of GABA, taurine, glutamate, aspartate, serine and glutamine was determined in the superfusate by HPLC a fter derivatization with o-phthaldialdehyde. Locomotor activity, arterial b lood pressure and heart rate were telemetrically monitored. The placement of naive rats or conditioned rats from their home cage to a c hamber provided with a grid-floor for shock virtually did not change the re lease rates of the amino acids in the locus coeruleus. Motility was enhance d in naive and conditioned rats to a similar extent. flood pressure and hea rt rate were enhanced in conditioned rats only. Exposure to N + L + S for 5 min greatly enhanced the release rates of all determined amino acids in th e locus coeruleus. In conditioned rats the increase in release of most amin o acids lasted longer than in naive rats. Electric shock also enhanced moti lity, blood pressure and heart rate. In conditioned rats, motility and card iovascular changes were more pronounced and/or lasted longer than in naive rats. Exposure of conditioned rats to the conditioned stimuli N + L for 5 m in led to an increased release of taurine and aspartate. The enhanced relea se of taurine lasted 30 min. Exposure to N + L did not affect the release r ates of amino acids in naive rats. N + L did not influence motility but art erial blood pressure and heart rate were elevated in conditioned rats. The findings show that inescapable electric shock enhances the release of s everal amino acids in the locus coeruleus, while conditioned fear selective ly increases the outflow of taurine and aspartate. Moreover, conditioned fe ar prolongs the response of excitatory and inhibitory amino acids to electr ic shock. The results suggest that an excitatory amino acid (aspartate) and an inhibitory amino acid (taurine) of the locus coeruleus are implicated i n conditioned fear.