Pharmacological analysis of dopaminergic synaptic plasticity in rodent lateral entorhinal cortex

Abstract

Despite receiving the second largest dopaminergic projection from the ventral tegmental area (VTA) and having a pivotal role in conveying non-spatial information received from sensory cortices, the physiological functions of the lateral entorhinal cortex (LEC) are still yet to be fully identified. The LEC is comprised of six distinct layers, with the deep layers (layers V-VI) receiving projections from throughout the hippocampal formation and projecting gammaaminobutyric acid (GABA) rich neurones to the superficial layers (layers I-III). Preliminary experiments showed that superficial layers also receive the aforementioned dopaminergic input from the VTA, a projection which is highlighted by tyrosine hydroxylase staining. This staining also showing that dopamine (DA) is deposited into pools (referred to as islands) which span these superficial layers. Field excitatory postsynaptic potential (fEPSP) experiments under ex vivo conditions studied changes in amplitude, that is the size of postsynaptic responses from localised neurones, and whether they increase (facilitate) or decrease (depress) in size when introduced to various stimuli. Previous studies have shown that application of high concentrations of DA causes significant depression in amplitudes. However, low concentrations of DA show facilitations during bath application of 10 nM DA, or during the washout following 100 nM DA application. Pharmacological analysis of D1-like and D2-like DA receptor function, using the receptor antagonists SCH-23390 and sulpiride respectively, which indicated that both receptors play comparable roles on peak synaptic depression as their co-application with 100 µM DA showed no statistically significant difference from their individual applications. Gradient analysis during DA application, after exertion of the maximal DA effect, suggests that DA desensitisation, a neurological phenomenon during which receptors lose their sensitivity to a neurotransmitter over time, may be D1-receptor mediated within the LEC. This was shown during SCH-23390 application experiments, sequestering D1-like receptor activity (leaving only D2-like receptor activity) showed an absence of desensitisation, however this was not statistically significantly different from other experiments. Although this effect was seen with 100 µM DA, co-application with 1000 µM DA showed no statistical significance from the controls which suggests the agonists competitively bind to the same sites of the receptor as DA. Application of (2R)-Amino-5- Phosphonopentanoate (AP5), an N-Methyl-D-Aspartic Acid receptor (NMDAR) antagonist, showed a greater depression during DA and AP5 co-application. This indicated that recorded amplitudes possess a glutamatergic component to them but are not NMDA dependent. This glutamatergic component was further shown through paired-pulse stimulation experiments, in which two amplitudes are generated over short interpulse intervals (ranging from 10-1000 ms). Paired-pulse stimulations during 100 µM DA application induced a facilitation over all intervals, which indicated DA affects presynaptic glutamate release probabilities. The combination of these components confirms that recorded amplitudes within the LEC arise predominantly from glutamatergic activity, with DA affecting these amplitudes indirectly by interacting with both presynaptic and postsynaptic glutamatergic receptors.