D 1 / D 5 receptor agonists induce a protein synthesis-dependent late potentiation in the CAl region of the hippocampus

Agonists of the dopamine D1/D5 receptors that are positively coupled to adenylyl cyclase specifically induce a slowly developing long-lasting potentiation of the field excitatory postsynaptic potential in the CAl region ofthe hippocampus that lasts for >6 hr. This potentiation is blocked by the specific D1/D5 receptor antagonist SCH 23390 and is occluded by the potentiation induced by cAMP agonists. An agonist of the D2 receptor, which is negatively coupled to adenylyl cyclase through Gai, did not induce potentiation. Although this slow D1/D5 agonist-induced potentiation is partially independent of N-methyl-D-aspartate receptors, it seems to share some steps with and is occluded by the late phase of long-term potentiation (LTP) produced by three repeated trains of nerve stimuli applied to the Schaffer collateral pathway. Similarly, the D1/D5 antagonist SCH 23390 attenuates the late phase of the LTP induced by repeated trains, and the D1/D5 agonist-induced potentiation is blocked by the protein synthesis inhibitor anisomycin. These results suggest that the D1/D5 receptor may be involved in the late, protein synthesis-dependent component of LTP in the hippocampal CAI region, either as an ancillary component or as a mediator directly contributing to the late phase. field (13, 14). At the moment, the Dl and D5 receptors (also designated Dla and Dlb) cannot be distinguished pharmacologically. Both are linked to adenylyl cyclase and have a similar pharmacological profile (15). We therefore refer to them collectively as the D1/D5 receptors. One clue that D1/D5 receptors might be involved in LTP has come from the finding that the late stage of LTP can be blocked by the D1/D5 antagonist SCH 23390 (16). To explore further the possible role of D1/D5 receptor activation in L-LTP, we have addressed five questions: (i) Can a specific D1/D5 agonist, when given alone, simulate the late phase of LTP in the CAl region? (ii) If so, is this potentiation mediated byNMDA receptors or by D1/D5 receptors? (iii) Does D1/D5 receptor potentiation occlude LTP induced by tetanization of Schaffer collaterals? (iv) Is this potentiation mediated by an increase in cAMP (consistent with the coupling of D1/D5 receptors to adenylyl cyclase)? (v) Finally, does this potentiation depend on protein synthesis, consistent with its being critical for the late phase? Our experiments suggest that the late phase of LTP is mediated heterosynaptically, at least in part, by a dopaminergic modulatory action mediated through D1/D5 receptors. Repeated, brief, high-frequency trains of stimuli applied to the Schaffer collateral pathway of the CAl region of the hippocampus produce a long-lasting synaptic potentiation (LTP; long-term potentiation) that lasts several hours and even days to weeks in the intact animal (1, 2). LTP has recently been dissected into two components, an early transient component (E-LTP) that requires the influx of calcium into the postsynaptic cell through N-methyl-D-aspartate (NMDA) receptor channels and the subsequent activation of several serinethreonine and tyrosine kinases (2, 3), and a later more persistent component (L-LTP) that requires new protein and RNA synthesis (4-8). The late component ofLTP is mediated, at least in part, by cAMP and by the cAMP-dependent protein kinase, protein kinase A (PKA) (6-9). How does this late cAMPand protein synthesis-dependent phase arise? The early phase of LTP is mediated homosynaptically and requires activity only in this monosynaptic Schaffer collateral pathway. Does the late phase depend only on activity in this pathway? Or does the late phase also require a heterosynaptic modulatory component that recruits additional receptors? One modulatory system known to innervate the CAl region of the hippocampus is the mesolimbic dopaminergic pathway that ends on both the Dl and D5 dopaminergic receptors coupled to adenylyl cyclase (10, 11). Immunohistochemical localization of Dl and D5 receptors reveals a heavy staining along pyramidal cells and the stratum radiatum (12). Retrograde fluorescent tracers show that dopaminergic fibers that project from the ventral tegmental area innervate the CAl MATERIALS AND METHODS Transversely cut hippocampal slices (400 jim) were prepared from 5to 6-week-old Sprague-Dawley rats and maintained in an interface chamber with continuous superfusion at 1.5-2 ml/min. The solution composition was 124 mM NaCl, 1.3 mM MgCl2, 4 mM KCl, 1.2 mM KH2PO4, 2.0-2.5 mM CaCl2, 25.6 mM NaHCO3, and 10 mM D-glucose, bubbled with 95% 02/5% CO2. The temperature of the bath was maintained at 28-29°C. After equilibration of the slices in the chamber for 2 hr, extracellular recordings were performed as described in a previous paper (7). Baseline responses were recorded for 30-60 min before drug application or LTP-inducing tetanization. Four biphasic constant current pulses (0.2 Hz; duration, 0.1-ms pulse) were used to generate test excitatory postsynaptic potentials (EPSPs) at 10-min intervals during the first hour after drug application or LTP induction and at 30-min intervals during the following hours. In LTP experiments an additional test was performed 1 min after tetanization to induce LTP. Three stimulus trains at 100 Hz, 1 s of 0.2-ms pulse duration were used, a tetanization protocol that produces long-lasting potentiation for up to 8 hr (6). The average waveform from four successive responses was saved and the EPSP slope (mV/ms) was measured. The following drugs were made and stored as concentrated stock solutions and dissolved in the superfusing medium to achieve the final concentrations used: D1/D5 agonist, SKF Abbreviations: LTP, long-term potentiation; E-LTP and L-LTP, early and late components of LTP; NMDA, N-methyl-D-aspartate; PKA, protein kinase A; EPSP, excitatory postsynaptic potential; APV, (±)-2-amino-5-phosphonopentanoic acid; PPHT, (±)-2-(N-phenylethyl-N-propyl)amino-5-hydroxytetralin hydrochloride; Sp-cAMPS, adenosine 3',5'-cyclic monophosphorothioate. 2446 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. Proc. NatL Acad ScL USA 92 (1995) 2447 R(+) 38393 HCl (50-100 mM) (Research Biochemicals International, Natich, MA); D1/D5 agonist, 6-bromo-ApB hydrobromide (50-100 mM) (Research Biochemicals International) in dimethyl sulfoxide (DMSO; 0.1% of the final solution); D2 agonist (±)PPHT hydrochloride [(±)-2-(N-phenylethyl-N-propyl)amino-5-hydroxytetralin hydrochloride] (100 mM) (in DMSO); Dl antagonist, R(+) SCH 23390 (1 mM) (Research Biochemicals International); adenosine 3',5'cyclic monophosphorothioate (Sp-cAMPS) Na salt (100 mM) (BioLog Life Sciences Institute, Bremen, Germany); anisomycin (20 mM) (Sigma); (±)-2-amino-5-phosphonopentanoic acid (APV) (50 mM) (Research Biochemicals International).