Distinct responses of osphradial neurons to chemical stimuli and neurotransmitters in Lymnaea stagnalis L.

1. In Lymnaea stagnalis L. (Pulmonata, Basommatophora) the neurons in the o sphradium were visualized by staining through the inner right parietal nerv e by 5,6-carboxyfluorescein (5,6-CF). Three types of neurons were identifie d: three large ganglionic cells (GC1-3; 80-100 mu m), the small putative se nsory neurons (SC; 20 mu m) and very small sensory cells (3-5 mu m). 2. The ganglionic and putative sensory neurons were investigated by whole c ell patch-clamp method in current-clamp condition. The three giant ganglion ic neurons (GC1-3) located closely to the root of osphradial nerve, had a m embrane potential (MP) between -30 and -70 mV and showed tonic or bursting activities. The small putative sensory cells (SCs) scattered throughout the osphradial ganglion, possessed a MP between -25 and -55 mV and showed an i rregular firing pattern with membrane oscillations. At resting MP the GC1-3 cells were depolarized and increased the frequency of their firing, while the SCs were hyperpolarized and inhibited by NaCl. (10(-2) M) and L-asparta te (10(-5) M) applied to the osphradium. 3. 5-Hydroxytryptamine (5HT, 10(-6) M), gamma-aminobutyric acid (GABA; 10(- 6) M) and the GABAB agonist baclofen (10(-6) M) depolarized the neurons GC1 -3 and increased their firing frequency. In contrast, on the GC1-3 neurons, acetylcholine (Ach; 10(-6) M) and FMRFamide (10(-6) M) caused hyperpolariz ation and cessation of the firing activity. The 5HT effect was blocked by m ianserin (10(-6) M) but picrotoxin (10(-5) M) failed to block the GABA-indu ced effect on the GC1-3 cells. 4. The small putative sensory neurons (SCs) were excited by Ach (10(-6) M) and 5HT (10(-6) M) but were inhibited by GABA (10(-6) M). FMRFamide (10(-6) M) had a biphasic response. The Ach effect was blocked by hexamethonium (1 0(-6) M) and tetraethylammonium (10(-6) M), indicating the involvement of n icotinic cholinergic receptors. 5. The distinct responses of the two populations of osphradial neurons to c hemical stimuli and neurotransmitters suggest that they can differently per ceive signals from environment and hemolymph.