1. Immunocytochemically identified magnocellular neurosecretory cells (MNCs) in the guinea‐pig supraoptic nucleus (SON) were studied using the in vitro intracellular recording technique. Cells were identified as containing arginine vasopressin (AVP) or oxytocin (OT) following recordings made with biocytin‐filled electrodes. Both AVP and OT MNCs demonstrated a fusiform or pyramidal shape (15‐20 microns by 26‐39 microns), with two to three processes. There were no significant differences in the proportion of AVP and OT cells in the retrochiasmatic (caudal) versus the rostral slices. 2. No significant differences in passive membrane properties were observed between AVP and OT cells, except that AVP cells exhibited a significantly broader action potential width (1.51 +/‐ 0.1 ms, n = 11) than did OT cells (1.01 +/‐ 0.08 ms, n = 7). 3. Firing patterns were recorded for 100 MNCs, 41% of which fired in a phasic manner (repeated clustering of action potentials into bursts). Of the seventy‐seven cells which were immunocytochemically identified, only AVP‐containing MNCs displayed phasic firing. Phasic firing occurred only in MNCs demonstrating a depolarizing potential which followed hyperpolarizing after‐potentials (HAPs). The presence of the depolarizing potential was not always associated with phasic firing, however, as both OT cells and non‐phasic AVP cells sometimes exhibited a depolarizing potential. 4. In 160 MNCs examined for the presence of the time‐dependent inward rectification (TDR in current clamp, or Ih in voltage clamp), a significant difference in the proportion of cells expressing the Ih was observed in the two cell types. The Ih was expressed in forty‐five of fifty‐four AVP MNCs (83%) and in six of fifteen OT MNCs (40%). No significant association was found with firing pattern. 5. The Ih exhibited properties similar to those found in other CNS and peripheral tissues. It appeared on steps to potentials more hyperpolarized than ‐65 mV. It was augmented by raising the extracellular potassium concentration, blocked by 2 mM CsCl, and insensitive to 100‐500 microM BaCl2. Activation followed a single exponential, and the time constant of activation was voltage dependent. 6. The adenylate cyclase activator forskolin increased the Ih and shifted its activation curve to more depolarized levels. In cells recorded for several hours, the Ih varied in amplitude, suggesting intrinsic modulation, possibly by intracellular second messenger systems. The Ih in guinea‐pig SON MNCs appears to serve an excitatory role, bringing cells closer to firing threshold.
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