Network-based activity induced by 4-aminopyridine in rat dorsal horn in vitro is mediated by both chemical and electrical synapses

This study investigated the role of electrical and chemical synapses in sustaining 4-aminopyridine (4-AP)-evoked network activity recorded extracellularly from substantia gelatinosa (SG) of young rat spinal cord in vitro. Superfusion of 4-AP (50 mu m) induced two types of activity, the first was observed as large amplitude field population spiking activity and the second manifested within the inter-spike interval as low amplitude rhythmic oscillations in the 4-12 Hz frequency range (mean peak of 8.0 +/- 0.1 Hz). The AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 mu m) abolished field population spiking and disrupted 4-12 Hz rhythmic oscillatory activity whereas the NMDA receptor antagonist d-AP5 (50 mu m) had no significant effect on either activity component. The glycine receptor antagonist strychnine (4 mu m) and the GABA(A) receptor antagonist bicuculline (10 mu m) diminished and abolished, respectively, field population spiking and both antagonists reduced the power of 4-12 Hz oscillations. The non-specific gap junction blockers carbenoxolone (100 mu m) and octanol (1 mm) attenuated both types of 4-AP-induced activity. By comparison, the neuronal-specific gap junction uncouplers quinine (250 mu m) and mefloquine (500 nm) both disrupted 4-12 Hz oscillations but only quinine reduced the frequency of field population spiking. These data demonstrate the existence of 4-AP-sensitive neuronal networks within SG that can generate rhythmic activity, are differentially modulated by excitatory and inhibitory ionotropic neurotransmission and are at least partly reliant on neuronal and/or glial-mediated electrical connectivity. The physiological significance of these putative intrinsic SG networks and the implications in the context of processing of nociceptive inputs are discussed.