Earlier work on freely moving rats classified neurons in Ammon's horn as pyramidal
cells (including place cells) or interneurons (previously called "theta cells") based
on temporal discharge correlates and waveform configurations, but the anatomical and
biochemical diversity of interneurons suggests they may have other distinguishing
characteristics. To explore this possibility, we made extracellular recordings as
rats foraged for food in an open space, used accepted criteria to identify interneurons,
and found two additional categorization methods. First, interneurons were separated
into theta-modulated and theta-independent groups using spike autocorrelograms. Second,
theta-modulated interneurons were further separated into four groups by the phase
of the approximately 8 Hz theta rhythm at which firing was most rapid. These phase
groups resemble the four phase peak groups of five anatomically identified interneuron
types (two with the same preferred phase) recorded during the slow ( approximately
4 Hz) theta rhythm in urethane-anesthetized rats. We suggest that the similar number
of peak phase groups in walking rats and urethane-anesthetized rats and the partial
agreement between peak phase values reflect a similar organization of theta rhythm
in both states, so that the discharge properties of anatomically identified interneurons
can be described in freely moving rats. Interestingly, the average spatial firing
precision of the interneuron classes does not differ significantly, suggesting that
the strong location-specific firing of place cells may be due to segregated high-
and low-precision interneuron ensembles rather than to one or more dedicated high-precision
classes.
