Thalamocortical connectivity in a rat brain slice preparation: participation of the ventrobasal complex to synchronous activities.

We studied the synchronous cortical and thalamic activities induced by low (0.5–1 mM) and high (50–100 mM) concentrations of theK+ channel blocker 4-aminopyridine (4AP) in a rat thalamocortical preparation. The presence of reciprocal thalamocortical connectivitywas documented by diffusion of the fluorescent tracer Di-IC18 between the somatosensory cortex and the ventrobasal complex (VB) of thethalamus in vitro. Functional reciprocal connectivity was also demonstrated by stimulating the cortical middle-deep layers (which elicitedorthodromic responses in VB) or the VB (which induced orthodromic and antidromic responses in the cortex). Spontaneous field potentialswere not recorded in either the thalamus or cortex in control medium. Low concentrations of 4AP produced local spindle-like rhythmicoscillations in cortex and VB (duration = 0.4–3.5 s; frequency = 9–16 Hz). In contrast, high concentrations of 4AP induced widespreadictal-like epileptiform discharges (duration = 8–45 s) characterised by a ‘tonic’ component followed by a period of ‘clonic’ discharges inboth cortex and VB. Spindle-like activity was abolished in cortex and thalamus by applying the excitatory amino acid receptor antagonistkynurenic acid in VB. In contrast, the same procedure exacerbated ictal-like discharges, while depressing VB activity. Our results indicatethat thalamus and cortex can produce synchronous activities in this in vitro thalamocortical network: spindle-like rhythmic oscillationsare generated at the thalamic level and imposed upon the cortical network whereas ictal-like discharges have a cortical origin and aremodulated by the thalamic network activity. In addition, we have shown that it is possible to preserve reciprocal projections between cortexand thalamus in an in vitro rat slice preparation that could be a valuable tool to study epileptic-prone rat strains.