Conduction velocities and membrane properties of different classes of rat septohippocampal neurons recorded in vitro

1. The membrane properties and conduction velocities of antidromically acti vated medial septum-diagonal band (MS-DB) neurons were examined using whole -cell recordings in a longitudinally cut rat brain slice preparation contai ning the MX-DB and the dorsal fornix. 2. MS-DB neurons were divided into three groups according to their action p otential characteristics and firing properties. Slow firing neurons display ed a broad action potential followed by a prominent after-hyperpolarization . Burst firing neurons, when depolarized from hyperpolarized holding potent ials, exhibited a high-frequency burst of spikes on the crest of a slow dep olarizing potential. Fast firing neurons did not fire bursts of spikes when depolarized from hyperpolarized holding potentials. 3. Eighteen MS-DB neurons were identified as septohippocampal by antidromic activation. Of the septohippocampal neurons, four were slow filing neurons , five were burst firing neurons and nine were fast filing: neurons. The me an axon conduction velocities of these neurons fell into two significant gr oups, termed slow conducting and fast conducting. Slow firing septohippocam pal neurons had significantly slower conduction velocities than either fast firing or burst firing neurons (P < 0.05), being 0.7 +/- 0.5 ms(-1) for sl ow firing neurons and 2.9 +/- 2.0 and 2.0 +/- 1.4 ms(-1) for burst firing a nd fast firing neurons, respectively. 4. On the basis of previous evidence which has linked firing properties wit h the neurochemical identities of the neurons, we propose that the slow fir ing septohippocampal neurons are cholinergic whereas the burst firing and f ast firing septohippocampal neurons are GABAergic.