European Journal of Neuroscience, Vol. 10, pp. 204–212, 1998 © European Neuroscience Association Somatostatin receptor subtypes sst1 and sst2 elicit opposite effects on the response to glutamate of mouse hypothalamic neurones: an electrophysiological and single cell RT-PCR study Christophe Lanneau, Ce ´cile Viollet, Annie Faivre-Bauman, Catherine Loudes, Claude Kordon, Jacques Epelbaum and Robert Gardette INSERM U159, 2ter rue d’Ale ´sia, 75014 Paris, France Keywords: AMPA/KA responses, CH-275, octreotide Abstract We have previously shown that somatostatin can either enhance or decrease AMPA/kainate receptor-mediated responses to glutamate in mouse-dissociated hypothalamic neurones grown in vitro. To investigate whether this effect is due to differential activation of somatostatin (SRIF) receptor subtypes, we compared modulation of the response to glutamate by SRIF with that induced by CH-275 and octreotide, two selective agonists of sst1 and sst2/sst5 receptors, respectively. Somatostatin either significantly decreased (49%) or increased (30%) peak currents induced by glutamate, and was ineffective in the remaining cells. Only the decreased response was obtained with octreotide, whereas only increased responses were elicited by CH-275 (47 and 35% of the tested cells, respectively). Mean amplitude variations under somatostatin or octreotide on the one hand, and under somatostatin or CH-275 on the other hand, were equivalent. Pertussis toxin pretreatment significantly decreased the number of cells inhibited by somatostatin or octreotide, but had no effect on the frequency of neurones showing increased sensitivity to glutamate during somatostatin or CH-275 application. About half of the neurones tested by single cell reverse transcriptase polymerase chain reaction (RT-PCR) expressed only one sst receptor (sst1 in 26% and sst2 in 22% of studied cells). Out of the remaining neurones, 34% displayed neither sst1 nor sst2 mRNAs, whereas 18% showed a simultaneous expression of both mRNA subtypes. Expression of sst1 or sst2 mRNA subtypes matched totally with the effects of somatostatin on sensitivity to glutamate in 79% of the neurones processed for PCR after recordings. These data show that pertussis toxin-insensitive activation of the sst1 receptor subtype mediates somatostatin-induced increase in sensitivity to glutamate, whereas decrease in the response to glutamate is linked to pertussis toxin-sensitive activation of the sst2 receptor subtype. Introduction Glutamate (GLU) is the major hypothalamic endogenous excitatory neurotransmitter acting mostly through aminohydroxy-5-methyl-4- isoazolepropionate/kainate (AMPA/KA) receptors (Van den Pol & Trombley, 1993). GLU also increases gene expression (Rage et al., 1993) and release (Tapia-Arancibia & Astier, 1988) of somatostatin (SRIF), an abundant neuropeptide which can block the release of numerous transmitters from hypothalamic cells (Epelbaum, 1986). SRIF itself can elicit opposite effects, both inhibitory and excitatory, on the response to different neurotransmitters (Mancillas et al., 1986; Wang et al., 1991), neuronal membrane excitability (Delfs & Dichter, 1983; Scharfman & Schwartzkroin, 1988), ionic conductances (Wang et al., 1989; Kurenny et al., 1992; Ribalet & Eddlestone, 1995) or neuronal responses to GLU (Havlicek et al., 1976; Ioffe et al., 1978; Dichter & Delfs, 1981; Mancillas et al., 1986; Wang et al., 1991, 1993; Gardette et al., 1995). Such discrepancies have been attributed to coupling of SRIF receptors to different second messenger systems Correspondence: Dr Jacques Epelbaum, as above. E-mail: epelbaum@broca.inserm.fr Received 2 May 1997, revised 24 July 1997, accepted 28 July 1997 (Scharfman & Schwartzkroin, 1988; Ribalet & Eddlestone, 1995) or to activation of different receptor subtypes (Delfs & Dichter, 1983; Scharfman & Schwartzkroin, 1988; Wang et al., 1989; Kurenny et al., 1992; Gardette et al., 1995). The recent cloning of five different SRIF receptor subtypes (reviewed in Hoyer et al., 1995; Patel et al., 1995; Reisine & Bell, 1995; Viollet et al., 1995b; Schindler et al., 1996), differentially coupled to various heterotrimeric G proteins (reviewed in Bruns et al., 1996; Hofland et al., 1995; Law et al., 1995), may explain the diversity of SRIF physiological effects (reviewed in Coy & Rossowski, 1995). However, direct demonstration of such an hypothesis requires testing of subtype selective agonists that were not available until recently. In the present study, we took advantage of the largely predominant expression of sst1 and sst2 in mouse hypothalamic neurones in vitro (Viollet et al., 1997b), two receptor subtypes for which relatively