The synchronisation of neuronal firing through electrotonic coupling via gap junctions is thought to play an important role in the coordination of sympathetic outflow. Sympathetic preganglionic neurones (SPN) within the intermediolateral cell column (IML) have previously been demonstrated to produce spontaneous activity in the absence of supraspinal innervations (1). Rhythmic population activity in spinal sympathetic nuclei which include these SPN can be abolished by gap junction blockers (2). The primary candidate for the formation of gap junctions within the IML is the connexin36 (Cx36) subunit (3). Using a Cx36-KO mouse we investigated the role of Cx36 in the electrotonic properties of SPN in the IML. Experiments were performed using Cx36-CFP mice bred on a C57/Bl56 template and their wild-type litter mates (4). Neonatal mice (P7-14 days) were anaesthetised with i.p. sodium pentobarbital (60 mg/kg) before transcardial perfusion with ice-chilled, oxygenated sucrose ACSF prior to decapitation. Whole cell recordings from SPN were performed in current clamp mode. Intracellular solution contained the sodium channel blocker, QX-314 bromide (2mM) to reveal the underlying coupled activity and Neurobiotin to allow anatomical identification of the neurone post-experimentally. Data was analysed by Fisher’s exact test, one-way and two-way ANOVA as appropriate. All values are given as mean ± S.E.M. Spontaneous coupled activity was observed in n=19/54 SPN from wild-type animals (Cx36WT/WT) and in proportionally fewer SPN from heterozygote (Cx36WT/CFP; n=16/78) and homozygote (Cx36CFP/CFP; n=2/56) animals. There was a significant difference in the prevalence of spontaneous coupled activity between SPN from Cx36WT/WT and Cx36CFP/CFP animals (P<0.0001). Depolarisation by 5-HT (10μM) was observed in SPN from Cx36WT/WT (n=17/28), Cx36WT/CFP (n=22/42) and Cx36CFP/CFP (n=15/34) animals with no significant difference between wild-type and homozygotes (P=0.213). Induction or augmentation of coupled activity by 5-HT (10μM) was significantly different between Cx36WT/WT (n=16/28) and Cx36CFP/CFP (n=6/34) SPN (P=0.0016). Quantification of the coupled activity showed a lower spontaneous frequency for Cx36CFP/CFP SPN (0.1±0.09 Hz, n=2) compared to Cx36WT/WT (0.235±0.05 Hz, n=12) and Cx36WT/CFP (0.231±0.067 Hz, n=15) SPN. Coupled activity in response to 5-HT (10μM) was induced or increased in SPN with a frequency of 1.12±0.237 Hz (Cx36WT/WT, n=15), 0.761±0.136 Hz (Cx36WT/CFP, n=21) and 0.298±0.133 Hz (CX36CFP/CFP, n=6). Both 5-HT and genotype were significant contributors to these differences (P<0.0001 and P=0.0297 respectively). We present here data showing the important contribution of Cx36 in both the prevalence and magnitude of rhythmogenesis in SPN in the IML.