The cation-chloride cotransporter (CCC) family comprises seven known Na⁺-Cl^– (NCC), Na⁺-K⁺-2Cl^– (NKCC) and K⁺-Cl^– (KCC) cotransporters. Neurones expressing KCC have low intracellular Cl^– such that GABA-gated Cl^– channel activation results in Cl^– influx, generating an outward (inhibitory) current. In contrast, Cl^– channel activation in cells expressing NKCC (or NCC), which maintain high intracellular [Cl^–], will generate an inward current due to Cl^– efflux. We have evidence that the regulation by glucose of pancreatic islet cell function involves activation of an anion channel (Best, 2002), and that differential expression of CCCs determines whether glucose is stimulatory (β-cell) or inhibitory (α-cell). It is likely that glucose-sensing neurones utilise mechanisms similar to those in islet cells. We have therefore studied the distribution of known CCCs by RT-PCR.

The animals used for the experiments were adult male Sprague-Dawley rats, which were humanely killed. KCC1, KCC3a and KCC4 were expressed in all the tissues tested (cerebellum, hypothalamus, kidney, liver and pancreas). Other CCCs had a more limited distribution: NKCC1 in the brain (and more weakly in the kidney and pancreas); NKCC2 in the kidney only; NCC1 and KCC3b in the kidney and pancreas; KCC2 in the brain only. Interestingly, for KCC3a, two amplification products were found in the hypothalamus, cerebellum, and other areas of the brain.

In situ hybridisation of rat forebrain sections demonstrated that while the expression of NKCC1 and KCC1 was restricted to the choroid plexus, KCC3a was expressed in the hippocampus and piriform cortex. KCC2 was widely expressed in the brain, but with higher concentrations in the piriform cortex, hippocampus, thalamus, the lateral hypothalamic area (LHA) and the ventromedial hypothalamic nucleus (VMH). KCC4 was present in choroid plexus and in the suprachiasmatic nucleus of the hypothalamus. Since the LHA and VMH are regions known to be involved in energy homeostasis we examined the expression of KCC2 in rats fasted for 48 h (n = 12). There was a significant decrease in KCC2 mRNA levels following fasting in the VMH and LHA (32 %, P < 0.05 and 69 %, P < 0.001; Student’s unpaired t test), but no significant change in the thalamus. There were no differences in KCC4 in fed and fasted rats. Since the LHA and VMH areas of the brain are rich in glucose-inhibited neurones, regulation of KCC2 by fasting would be consistent with a role in glucose sensing. However, the change in KCC2 may reflect altered functioning of GABA receptors related to energy balance. In contrast, there was no evidence for the predicted expression of either known NKCC isoform in the VMH, which is also rich in glucose-stimulated neurones.

This work was funded by the BBSRC.