1. The presubiculum and parasubiculum are retrohippocampal structures bordered by the subiculum and medial entorhinal cortex. Deep layer (IV-VI) neurons from this region exhibit stable synaptically triggered burst behaviour which distinguishes them from superficial layer (I-III) cells. This functional separation was examined with intracellular and field potential recordings from horizontal slices of rat brain. Neurobiotin labelling and rapid Golgi techniques were used to obtain anatomical evidence of axonal trajectories. 2. Extracellular stimulation of the subiculum, deep medial entorhinal cortex or superficial preor parasubiculum caused, in deep layer cells only, a short latency burst discharge which could be followed by one or more after-discharges. Bursts appeared after repetitive stimulation and were stable for the life of the slice. Each event was supported by giant excitatory postsynaptic potentials (EPSPs). Events were similar whether they were evoked in horizontal slices or slices cut perpendicular to the horizontal plane. 3. Bath application of the NMDA receptor antagonist 3-[2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP; 5 ìÒ) elevated the threshold for evoking the giant EPSP. Higher concentrations (10-15 ìÒ) reduced the amplitude and duration of the giant EPSP. Bath application of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5 ìÒ) eliminated the evoked EPSP. 4. In intact slices, superficial layer neurons of pre-and parasubiculum could exhibit EPSPs coincident with bursts recorded in the deep layers. However, in isolated subsections of horizontal slices or in 'vertical slices', both of which contained only pre-andÏor parasubiculum, evoked or picrotoxin-induced bursts occurred only in deep layer cells. Superficial layer cells in these subsections showed no response to deep layer events. 5. Antidromic population spikes confirmed projections from superficial cell layers of pre-and parasubiculum down to their deep cell layers. Reciprocal antidromic responses were absent. 6. Axons of superficial layer stellate and pyramidal cells had horizontal collaterals and at least one ascending and one descending collateral. Branches of the descending collaterals were given off in layer V and some axons were found to reach the angular bundle. Axons of deep layer stellate and pyramidal cells also had horizontal collaterals and descending collaterals which could be traced to the angular bundle. One ascending axon collateral was found among the thirty-one deep layer cells examined morphologically. 7. We conclude that the deep layer cells of the presubiculum and parasubiculum are richly interconnected with excitatory synapses. These interconnections can generate giant excitatory synaptic potentials that support the bursting behaviour exhibited by these neurons. Any of the excitatory inputs to deep layer cells can trigger the population bursts and specific inputs from entorhinal cortex produce the after-discharges. Further, connections between superficial and deep layer cells appear to b...

Area Postrema: The influence on feeding behavior caused by ablation of the area postrema (AP) in rodents indicates the participation of this structure in the control of ingestion. Two types of glucose responsive neurons were identified in the AP: one is characterized by increasing the discharge rate in response to glucose (glucoreceptor type) and the other by decreasing the discharge rates in response to glucose (glucose sensitive type). These glucose responsive neurons may participate in glycemic homeostasis. Nucleus of Solitary Tract:The glucose responsive neurons exist within the caudal portion of nucleus of the solitary tract (NTS), a relay station in visceral afferents. Two types similar to the A P were also recognized. It is confirmed that hepatic glucose sensitive afferents terminate on some of the glucose sensitive neurons. This convergence may serve as a fail-safe mechanism. In addition, the NTS involving complex neural networks of excitatory a n d inhibitory interneurons may be concerned with integration of glycemic information. Dorsal Motor Nucleus of Vagus: Some neurons within the dorsal motor nucleus of the vagus (DMV) were identified as the glucose responsive ones. Both types were also recognized. It is confirmed by antidromic activation that these glucose responsive DMV neurons send their axons toward the gastric or coeliac branch that innervates either the stomach, 5):7358-7403.

To clarify the functional properties of the hyperpolarization‐activated cation current (Ih) as a pacemaker current in area postrema neurons, whole‐cell recordings were made in visually identified cells in rat brain slices. The activation of Ih was identified in approximately 62 % of area postrema neurons tested. The cells displaying Ih showed a depolarizing ‘sag’ in responses to hyperpolarizing current injection in current‐clamp mode. The reversal potential for the Ih was −36 mV, and this was shown to depend on the external concentration of Na+ and K+ ions. Extracellular Cs+ ions (2 mM) and ZD7288 (100 μm), a potent selective Ih channel antagonist, blocked Ih and induced a membrane potential hyperpolarization, suggesting the sustained activation of Ih near the resting potential and a contribution from Ih to membrane potential maintenance at more depolarized levels. In contrast, extracellular Ba2+ ions caused a depolarization of the membrane potential, suggesting the blockade of inward rectifier K+ currents. ZD7288 decreased the spontaneous discharge rate by prolonging the slow depolarization between two spikes, with minimal effect on the amplitude of the afterhyperpolarization or action potential waveforms. Ih stabilized the latency of rebound action potentials. Ih was weakly activated by external 8‐bromoadenosine 3′,5′ cyclic monophosphate (1 mM) or forskolin (50‐100 μM), indicating that the Ih channel subtypes in area postrema cells could be modulated by intracellular cAMP. Our findings indicate that Ih contributes to the subthreshold membrane and firing properties of rat area postrema neurons and may regulate their resting membrane potential and firing patterns.

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