Effects of posttraining treatments in the posterior cingulate cortex on short- and long-term memory for inhibitory...

Adult male Wistar rats were bilaterally implanted with indwelling cannulae in the caudal region of the posterior cingulate cortex. After recovery, animals were trained in a step-down inhibitory avoidance task (3.0-s, 0.4-mA foot shock) and received, right after training, a 0.5-microl infusion of vehicle (phosphate-buffered saline, pH 7.4), of the GABA(A) receptor agonist muscimol (0.1 or 0.5 microg), of the cAMP-dependent protein kinase (PKA) stimulant Sp-cAMPS (0.1 or 0.5 microg), or of the PKA inhibitor Rp-cAMPS (0.1 or 0.5 microg). Animals were tested twice, 1.5 h and, again, 24 h after training, in order to examine the effects of these agents on short- and long-term memory, respectively. Muscimol (0.5 but not 0.1 microg) hindered retention for both short- and long-term memory (p <.05). Rp-cAMPS (0.1 or 0.5 microg) hindered retention for short-term memory (p <.05). In addition, these animals showed lower, but not significantly lower, latencies than controls in the test session for long-term memory (p >.10). A trend toward an amnesic effect on long-term memory was also observed after Sp-cAMPS infusion at 0.1 microg (p <.10). These results show that strong stimulation of GABAergic synapses in the caudal region of the rat posterior cingulate cortex right after training impairs short- and long-term memory (the latter less dramatically). The same occurs by inhibiting PKA activity with regard to STM and possibly to LTM.