Phasic bursting activity of rat paraventricular neurones in the absence of synaptic transmission

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Abstract
Whether the phasic bursting activity, characteristic of certain magnocellular neuropeptidergic neurons in rat hypothalamus, is dependent on chemical synaptic input was studied. Slices of hypothalamus were placed in an in vitro chamber with hippocampal slices. The synaptic response in the CA1 cell layer from Schaffer collateral stimulation was monitored before, during and after synaptic transmission was blocked by superfusion of medium containing high Mg2+ (either 18.7 or 9.3 mM) and low Ca2+ (0.05 mM). This well studied pathway was chosen as an assay of synaptic blockade because hypothalamic circuitry is relatively unknown. The electrical activity of 22 phasic phasic bursting neurons in the lateral portion of the paraventricular nucleus (PVN) was recorded. Of 22 phasic PVN neurons, 19 were recorded only after synaptic transmission was blocked. The remaining 3 cells were firing phasically in standard medium when first encountered and continued to display phasic bursting activity for up to 1.25 h after synaptic blockade. Active cells in nearby hypothalamic areas did not show phasic bursting patterns either before or after synaptic transmission was blocked. The phasic bursting activity of the PVN neurons in this study and that of previously reported PVN cells in vivo were similar in firing rate within bursts, burst length and silent period duration. Phasic bursting in PVN magnocellular neuropeptidergic cells is not dependent upon synaptically mediated excitation or recurrent inhibition as was hypothesized earlier. Alternative hypotheses, based upon acute changes in [K+]o, [extracellular concentration of K+], endogenous membrane currents and electrotonic coupling are discussed as possible explanations of phasic bursting in these magnocellular neuropeptidergic cells.