Dopaminergic modulation of superficial layer projection neuron excitability in the lateral entorhinal cortex

Abstract

Projection neurons located in the superficial layers of the entorhinal cortex provide the hippocampus with nearly all its sensory input. This innervation occurs via multiple discrete pathways to different targets within the hippocampus, including the perforant and temporoammonic paths, as well as via direct projections to area CA2. This segregation of sensory information suggests that these input streams each play a separate role in declarative memory. The actions of modulatory neurotransmitters on the excitability of projection neurons in the entorhinal cortex may be instrumental in filtering the content of sensory signals destined for processing by different hippocampal subfields and, the lateral division of the entorhinal cortex receives dense dopaminergic innervation from the ventral tegmental area. But little is known about potential dopamine-mediated effects on the propagation of sensory information from the entorhinal cortex to the hippocampus. To determine this, multiple intracellular recording methods were used to assess the intrinsic excitability of principal neurons located in superficial layers II and III of the lateral entorhinal cortex in juvenile rat brain slices maintained in vitro before, during and after treatment with dopamine. Initial immunolabelling experiments using antibodies against the enzyme tyrosine hydroxylase confirmed the presence of catecholaminergic fibres in the superficial layers of the lateral entorhinal cortex, and that these fibres overlapped considerably with both the somata and neuropil of principal neurons in layers II and III. Moreover, there was a significant difference in the expression pattern of tyrosine hydroxylase-positive fibres across each cortical layer with fluorescent labelling highest in layers I, V and VI, followed next by layer III and then by layer II. This differential innervation pattern supports the notion that dopamine may modulate network activity within each layer of the lateral entorhinal cortex in a slightly different way. Although initial whole-cell recording experiments were somewhat inconclusive, there was a reliable suppression of spiking activity in superficial layer neurons following the addition of 100 µM dopamine to the bathing medium. More focal application of dopamine delivered via pressure injection directly to layer II and layer III neurons individually, however, suggested that neurons in layer III were more sensitive to the suppressive effects of dopamine on excitability. Most layer III neurons stopped spiking in response to ‘puffs’ of dopamine whereas layer II neurons were only mildly affected by the same treatment. The perforated patch clamp method was then used to assess the effects of dopamine on neuronal excitability in layer II and layer III projection neurons separately over an extended period (60 min). Bath-application of 100 µM dopamine for 10-min affected layer II and III neurons differently, with layer II neurons responding more strongly to treatment. In both layers, dopamine caused a significant attenuation of spiking activity, but only layer II neurons showed coincident changes in membrane potential and membrane resistance linked to the suppression of cell firing. Taken together, these findings show that the strength of dopaminergic projections to each layer of the lateral entorhinal cortex differs, and that this distinct pattern of innervation affects the excitability of layer II and layer III projection neurons in unique ways. As such, dopaminergic inputs to the lateral entorhinal cortex may serve to regulate the flow of sensory signals to different targets in the hippocampus depending on the salience of ongoing motivational, appetitive or affective factors known to recruit the mesocortical dopamine system.