Hippocampal gamma rhythms during Y-maze navigation in the juvenile rat.

The neurobiology of postnatal hippocampal development in rodents is receiving increased attention as a means to address neurodevelopmental questions and to better understand the neural code(s) for spatial navigation in adulthood. We previously showed that spontaneous alternation (SA) in a Y-maze, which emerges at the end of the third postnatal week, was related to changes in fast glutamatergic synaptic transmission. In adults, oscillations in the hippocampal local field potential (LFP) (i.e., theta, 4-12 Hz; slow gamma, 25-55 Hz; and fast gamma, 65-100 Hz) have been shown to coordinate the activity of spatially tuned cell types in the hippocampus during route planning and execution. Other investigators have shown that theta activity matures during the first month of life. However, relationships between developmental alterations in gamma oscillations and cognitive function have not been investigated in juveniles, and the impact of developmental changes in excitatory synaptic transmission on network activity remains unclear. We implanted rats at postnatal day 14 to record LFPs from the synaptic layer of area CA1 during Y-maze exploration at postnatal Days 18, 19, 23, and 24. The positive allosteric modulator of AMPA receptors, CX614, or vehicle was administered prior to each test. We found that slow gamma peak frequency, but not peak power, remained constant across this age range. Fast gamma event rate increased with increasing age, while peak frequency decreased. AMPA receptor modulation impacted slow and fast gamma differently at different ages. Furthermore, gamma events showed relationships with novelty and movement speed that differed based on gamma sub-band (slow vs. fast). These results support the presence of environmentally-driven gamma oscillations in hippocampal network activity prior to the end of the third postnatal week of development. Differential refinement of slow and fast gamma occurring across the subsequent week supports involvement in the emergence of spatial navigation ability.