The wide CNS distribution, neuronal co-localisation and profound actions following administration indicate that cocaine and amphetamine regulated transcript peptides (CART) play an important neuromodulatory role in the CNS. However the cellular actions of CART peptides remain largely unknown. Therefore we have begun to investigate the actions of CART peptides on sympathetic preganglionic neurones (SPNs) which receive direct inputs from both brainstem (Dun et al. 2002) and hypothalamic (Elias et al. 1998) neurones co-expressing CART.

Whole cell patch clamp recordings were made from neurones in transverse spinal cord slices from humanely killed neonatal Wistar rats (P7-14). SPNs were identified by their position in the lateral horn and characteristic electrical properties. The mean resting potential of SPNs was -54.5 ± 1.7 mV and the mean input resistance was 753 ± 39 mV, n = 44). In 4 SPNs, application of 200 nM rlCART (55-102) depolarised the membrane potential (from -48 ± 3.5 to -40.6 ± 4.1 mV) and reduced the neuronal input resistance. In a further 4 cells, 200 nM rlCART (55-102) produced a hyperpolarisation of the membrane potential (from -50.8 ± 3.5 to -55.3 ± 3 mV) and a reduction in input resistance (by 21.3 ± 5 %). The hyperpolarisation and decrease in input resistance were blocked by bicuculline (10 ÁM; n = 2) suggesting rlCART (55-102) activated GABA_A receptors. In a further 30 SPNs, rlCART (55-102) had no action on the membrane potential or input resistance. The heterogeneous actions of CART peptides on SPNs may reflect the different CART inputs that these neurones receive: from the arcuate nucleus in the hypothalamus colocalised with POMC (α-MSH) and from the brainstem colocalised with noradrenaline. Thus we have compared the actions of α-MSH with those of rlCART (55-102). In 6 SPNs, 200 nM α-MSH depolarised the membrane potential (from -57.5 ± 2.7 to -47.5 ± 1.1 mV). However, unlike rlCART (55-102), the α-MSH induced membrane depolarisation was associated with an increase in input resistance (by 27.5 ± 7.5 %) suggesting the closure of resting conductances. In cells in which rlCART (55-102) induced membrane hyperpolarisation, α-MSH was without effect (n = 3) or produced a membrane depolarisation (n = 1).

These data suggest CART peptides differentially regulate electrical excitability of SPN. Further studies are required to clarify the functional significance of these observations and if these effects reflect differential CART inputs arising from the brainstem and hypothalamus.