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The piriform cortex (PCx) and related structures such as hippocampus and frontal cortex could play an important role in olfactory memory. We investigated their involvement in learning the biological value of an odor cue, i.e. predicting reward or non-reward in a two-odor discrimination task. Rats were sacrificed after stimulation by either rewarded or non-rewarded odor and Fos immunocytochemistry was performed. The different experimental groups of rats did not show strongly differentiated Fos expression pattern in either the PCx or the hippocampus. A few differences were noted in frontal areas. In the ventro-lateral orbital cortex, rats, ramdomly rewarded during the conditionning had a higher Fos level in comparison with other groups. In infralimbic cortex, rats, which learned the reward value of the olfactory cue and were water-reinforced the day of sacrifice, showed a higher Fos expression. Data are discussed in view of the olfactory learning paradigm and of the accuracy of the control groups used in the present experimental design. The behavioural conditions leading to Fos expression are further discussed since Fos is a marker of learning-induced plasticity as well as a general activity marker which can be activated by a wide range of stimuli not directly linked to memory.

Little is known of the influence of gustatory, particularly salt, input on neurons of the forebrain and if the same neurons are sensitive to hydromineral balance humoral stimuli. In awake, nonpremedicated rats we recorded the activity of spontaneously active neurons in the preoptic/anterior hypothalamic area of dehydrated and euhydrated rats while allowing them to ingest water or a hypertonic salt solution (1.6% NaCl) administered to the tongue. The hormones angiotensin and aldosterone, both implicated in hydromineral balance, were applied by iontophoresis to the same neurons. In the dehydrated rats, 27% (15/55) of the spontaneously active neurons responded to a liquid (either water or the NaCl) applied to the tongue; in the euhydrated rats 23% (18/78) responded to the same stimuli. In the dehydrated rats, however, 33% (5/15) of the responding neurons were inhibited when the NaCl solution was applied to the tongue compared with only 5% (1/18) in the euhydrated rats. Iontophoretic application of angiotensin increased the spontaneous activity in 21% of those neurons tested that responded to taste. These results suggest that the state of hydration of an animal is able to change the neuronal response to substances applied to the tongue. Furthermore, it appears that these gustatory-sensitive neurons may also be related to hydromineral balance regulation since they are able to respond to angiotensin.