Enhanced actions of adenosine in medial entorhinal cortex layer II stellate neurons in temporal lobe epilepsy are mediated via A(1)-receptor activation

Purpose: The adenosinergic system is known to exert an inhibitory affect in the brain, and as such adenosine has been considered an endogenous anticonvulsant. Entorhinal cortex (EC) layer II neurons, which serve as the primary input to the hippocampus, are spared in temporal lobe epilepsy (TLE) and become hyperexcitable. Because these neurons also express adenosine receptors, the activity of these neurons may be controlled by adenosine, specifically during seizure activity when adenosine levels are thought to rise. In light of this, we determined if the actions of adenosine on medial EC (mEC) layer II stellate neurons are augmented in TLE and by which receptor subtype.

Methods: Horizontal brain slices were prepared from rats exhibiting spontaneous seizures (TLE) induced by electrical stimulation and compared with age-matched control rats. mEC layer II stellate neurons were visually identified, and action potentials (APs) were evoked either by a series of depolarizing current injection steps or via presynaptic stimulation of mEC deep layers. The effects of adenosine were compared with actions of adenosine A(1) and A(2A) receptor-specific agonists (CPA and CGS-21680) and antagonists (DPCPX and ZM-241385), respectively. Immunohistochemical and qPCR techniques were also employed to assess relative adenosine A(1)-receptor message and expression.

Key findings: mEC layer II stellate neurons were hyperexcitable in TLE, evoking a higher frequency of APs when depolarized and generating bursts of APs when synaptically stimulated. Adenosine reduced AP frequency and synaptically evoked APs in a dose-dependent manner (500 nM-100 μM); however, in TLE, the inhibitory actions of adenosine occurred at concentrations that were without affect in control neurons. In both cases, the inhibitory actions of adenosine were mediated via activation of the A(1)- and not the A(2A)-receptor subtype. Quantitative polymerase chain reaction (qPCR) and immunohistochemical experiments revealed an upregulation of the adenosine A(1) mRNA and an increase in A(1)-receptor staining in TLE neurons compared to control.

Significance: Our data indicate that the actions of adenosine on mEC layer II stellate neurons is accentuated in TLE due to an upregulation of adenosine A(1)-receptors. Because adenosine levels are thought to rise during seizure activity, activation of adenosine A(1)-receptors could provide a possible endogenous mechanism to suppress seizure activity and spread within the temporal lobe.