John Apergis-Schoute | PubMed Health

PubMed2006

Ultrastructural organization of medial prefrontal inputs to the rhinal cortices.

Accumulating evidence suggests that the medial prefrontal cortex (mPFC) plays a critical role in the formation, retrieval and long-term storage of hippocampal-dependent memories. Consistent with this, there are direct hippocampal projections to the mPFC. Moreover, the mPFC sends robust projections to the perirhinal and entorhinal cortices, two interconnected cortical fields that funnel information into and out of the hippocampus. However, the significance of the latter projection remains unclear because no data are available regarding the rhinal targets of mPFC axons. This question was examined in the present study using a combination of anterograde tracing with Phaseolus vulgaris leucoagglutinin and pre-embedding gamma-aminobutyric acid (GABA) immunocytochemistry in guinea pigs. Following Phaseolus vulgaris leucoagglutinin injections in the mPFC, anterogradely labeled axons were seen in the perirhinal (mainly superficial layers) and lateral entorhinal (mainly deep layers) cortices. In the electron microscope, the synaptic articulation of anterogradely labeled mPFC axon terminals with perirhinal and entorhinal neurons was found to be nearly identical. In these two rhinal fields, mPFC axon terminals only formed asymmetric synapses, typically with GABA-immunonegative spines ( approximately 70%) but occasionally with dendritic profiles ( approximately 30%), half of which were GABA immunopositive. In the light of earlier observations, these findings indicate that mPFC inputs exert mainly excitatory effects in the rhinal cortices, prevalently on principal neurons. Thus, these results suggest that the mPFC may affect hippocampal-dependent memories by enhancing impulse traffic into and out of the hippocampus at the level of the rhinal cortices.

Animals↗

PubMed2005

Interaction between amygdala and neocortical inputs in the perirhinal cortex.

The rhinal cortices play a critical role in high-order perceptual/mnemonic functions and constitute the main route for impulse traffic to and from the hippocampus. However, previous work has revealed that neocortical stimuli that activate a large proportion of perirhinal neurons are unable to discharge entorhinal cells. In search of mechanisms that might facilitate impulse transfer from the neocortex to the entorhinal cortex, we have examined changes in excitability produced by activation of the lateral amygdala (LA) in isoflurane-anesthetized animals. LA stimulation activated a large proportion of peri- and entorhinal neurons. However, conditioning LA stimuli did not increase the ability of neocortical inputs to activate entorhinal cells even though such pairing produced marked increases in neocortically evoked field potentials and orthodromic firing in the perirhinal cortex. Moreover, increased neocortically evoked perirhinal field potentials and unit responses persisted when the conditioning LA shock was replaced by another neocortical stimulus at the same or at a different site as the testing shock. This perirhinal paired-pulse facilitation (PPF) was maximal with interstimulus intervals of approximately 100 ms. Intracellular recordings of perirhinal neurons revealed that the PPF was generally associated with a rapid shift in the balance between inhibition and excitation, leading to an overall increase in perirhinal responsiveness. The significance of these findings for the role of the perirhinal cortex is discussed.

Action Potentials↗