RAPID COMMUNICATION Effects of Hyposmolar Solutions on Membrane Currents of Hippocampal Interneurons and Mossy Cells In Vitro

Baraban, Scott C. and Philip A. Schwartzkroin. Effects of hycampal excitability because each interneuron synapses posmolar solutions on membrane currents of hippocampal interneuonto Ç200 pyramidal neurons (Traub and Miles 1991) . rons and mossy cells in vitro. J. Neurophysiol. 79: 1108–1112, Hippocampal ‘‘mossy’’ cells are another subtype of non1998. Whole cell voltage-clamp recordings in rat hippocampal pyramidal hippocampal neuron that play a critical role in slices were used to investigate the effect of changes in extracellular regulating hippocampal excitability (Scharfman and osmolarity on voltage-activated potassium currents. Currents were Schwartzkroin 1988) . The glutamatergic mossy cells synevoked from oriens/alveus (O/A) interneurons, hilar interneurons, apse with both interneurons and granule cells in the denand mossy cells. Hyposmolar external solutions produced a sigtate gyrus (Ribak et al. 1985; Wenzel et al. 1997) . nificant potentiation of K current recorded from O/A and hilar We recently presented evidence to support the hypotheinterneurons, but not from mossy cells. Hyposmolar solutions also dramatically potentiated the spontaneous excitatory postsynaptic sis that a subtype of hippocampal neuron is osmosensitive. currents recorded from mossy cells. These results suggest that hipSpecifically, osmolarity was shown to modulate the neupocampal excitability can be modulated by the complex actions ronal K channel function of L/M interneurons but not exerted by changes in extracellular osmolarity. of CA1 or subicular pyramidal neurons (Baraban et al. 1997) . To test whether other subtypes of hippocampal interneurons (O/A and hilar ) or mossy cells exhibit osmoI N T R O D U C T I O N sensitivity, we investigated the effect of hyposmotic challenge on voltage-activated K currents and spontaneous Osmolarity may play a critical role in regulating excitEPSCs recorded from these neurons. ability in the central nervous system (CNS) . Hyposmolar clinical conditions, such as inappropriate antidiuretic hor-