The islands of Calleja (IC) are groupings of small granule cells (GCs) in the olfactory tubercles (OTs). The GCs have short local axons and are reportedly GABAergic (see Halliwell & Horne, 1998). They appear to envelop larger neurones that give rise to tubercle efferents. This study addresses the properties and activity of these larger cells associated with the IC, which I term satellite cells (SCs).

Coronal slices (200 µm) of OT from humanely killed rats aged 12-21 days were maintained conventionally in vitro at 25 °C. I made whole-cell recordings from SCs, visually identified with Nomarski optics. Eighty SCs were studied using pipettes including CsCl and 2-5 mM QX-314. Stimulating electrodes, which were broken patch electrodes (10 µm tip) filled with the bath solution and delivering cathodal current pulses, were placed amongst the GCs.

Stimulating in the IC at low current strength (< 20 µA, 200 µs) evoked synaptic activity that was predominantly blocked by 20 µM bicuculline (Bic) or gabazine (Gab, 0.5-10 µM). SCs loaded with CsCl and QX 314 displayed a high level of spontaneous input, which was blocked (> 90 %) by Bic or Gab. GABA_A activity was isolated by adding 10 µM 6,7-dinitro-quinoxaline-2,3-dione (DNQX) and 10 µM D(-)-2-amino-5-phosphonopentanoic acid (D-AP5). All spontaneous activity was now blocked reversibly by Bic or Gab. Concomitantly, in 74 % of cells (n = 47) a significant outward current or hyperpolarization (according to voltage- or current-clamp conditions) developed following GABA_A receptor blockade. Three kinds of spontaneous activity (at -60 mV) were observed in Cs⁺-loaded SCs: (i) 38 % showed infrequent large currents (0.25-1 nA) or depolarizations (to ~0 mV) that lasted between 3 and 10 s – termed ‘swoops’; (ii) 46 % had inward currents (150-500 pA) of up to 1 s duration (t{special}_decay >100 ms); (iii) all displayed shorter events (< 10-100 pA, t{special}_decay < 30 ms) – ‘minis’. All events were blocked by Bic or Gab. Swoops were not seen when 0.5-1 µM TTX was added to the bathing medium, but shorter events were resistant (n = 12). In medium containing only 50 µM Ca²⁺, 6.25 mM Mg²⁺ and 1 mM Mn²⁺ as well as TTX, the intermediate spontaneous events were reduced in frequency but not totally abolished (n = 4). TTX, or an 83 % reduction of [Na⁺]_o by replacing Na⁺ with choline, stopped IPSCs from being evoked by standard electrical stimuli; however, by increasing the amplitude and duration of the stimulus in the vicinity of the IC, Bic-sensitive IPSCs could once more be recruited in SCs (n = 5, each treatment).

From these results I conclude that (a) SCs receive a powerful local GABAergic input; (b) both desensitizing and non-desensitizing GABA receptors are present on SCs; (c) GABA release can be mobilized precipitately without the need for transmembrane Ca²⁺ flux; and (d) local GABA release sites can be recruited by purely electrotonic means to generate IPSCs in SCs. The most likely GABA source is a coupled network of IC granule cells (cf. Halliwell & Horne, 1998).

This work was supported by the Wellcome Trust.