Dopamine enhances spatiotemporal spread of activity in rat prefrontal cortex

Dopaminergic modulation of prefrontal cortex (PFC) is important for neuronal integration in this brain region known to be involved in cognition and working memory. Because of the complexity and heterogeneity of the effect of dopamine on synaptic transmission across layers of the neocortex, dopamine’s net effect on local circuits in PFC is difficult to predict. We have combined whole-cell patch clamp recording and voltagesensitive dye imaging to examine the effect of dopamine on the excitability of local excitatory circuits in rat PFC in vitro. Whole cell voltage clamp recording from visually identified layer II/III pyramidal neurons in rat brain slices revealed that, in the presence of bicuculline (10 μM), bath-applied dopamine (30-60 μM) increased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by weak intracortical stimulus. The effect was mimicked by the selective D1-receptor agonist SKF 81297 (1 μM). Increasing stimulation resulted in epileptiform discharges. SKF 81297 (1 μM) significantly lowered the threshold stimulus required for generating epileptiform discharges to 83% of control. In the imaging experiments, bath application of dopamine or SKF 81297 enhanced the spatiotemporal spread of activity in response to weak stimulation and previously subthreshold stimulation resulted in epileptiform activity that spread across the whole cortex. These effects could be blocked by the selective D1-receptor antagonist SCH 23390 (10 μM) but not by the D2-receptor antagonist eticlopride (5 μM). These results indicate that dopamine, by a D1 receptor mediated mechanism, enhances spatiotemporal spread of synaptic activity and lowers the threshold for epileptiform activity in local excitatory circuits within PFC. JN-0092-2004.R1 pg 3 Introduction The mammalian prefrontal cortex (PFC) receives a substantial dopaminergic innervation from the midbrain ventral tegmental area (VTA) (Bjorklund and Lindvall 1984). Dopamine is an endogenous neuromodulator in the cerebral cortex and is believed to be important for normal brain processes (Bjorklund and Lindvall 1986; Williams and Goldman-Rakic 1995). There is strong evidence that alterations in dopamine function may play a role in pathogenesis of a number of neuropsychiatric diseases including epilepsy (Starr 1996) and schizophrenia (see Andreason 1996; Egan and Weinberger 1997; Grace et al., 1997; and Jaskiw and Weinberger 1992 for recent reviews). Our understanding of dopamine’s action in the brain at the cellular and network level is still emerging. In vivo studies have shown that dopamine can both increase and decrease spontaneous firing of neocortical neurons (Bassant et al., 1990; Bradshaw et al., 1985; Bunney and Aghajanian 1976; Ferron et al., 1984; Pirot et al., 1992; Sesack and Bunney 1989; Reader et al., 1979; Thierry et al., 1992; Yang and Mogenson 1990). Dopamine may also favor long-lasting transitions of PFC neurons to a more excitable up state (Lewis and O’Donnell 2000). During short-term working memory processing, dopamine-dependent increases in firing of groups of PFC neurons have been observed (Fuster 1995; Goldman-Rakic 1995). In vitro electrophysiological experiments suggest that dopamine has multiple effects on PFC neurons. Both increases (Ceci et al., 1999; Gonzalez-Burgos et al., 2002; Gorelova and Yang 2000; Henze et al., 2000; Penit-Soria et al., 1987; Tseng and O’Donnell 2004; Yang and Seamans 1996; Wang and O’Donnell 2001) and decreases (Geijo-Barrientos and Pastore 1995) in postsynaptic excitability of pyramidal neurons have been reported following D1 receptor activation. In addition, changes in excitability mediated by D2 JN-0092-2004.R1 pg 4 receptors have been reported (Gulledge and Jaffe 1998; 2001; Tseng and O’Donnell 2004). The effects of dopamine on synaptic responses are also complex and may be layer and species specific. AMPA receptor mediated excitatory postsynaptic currents (EPSCs) in layer V pyramidal cells are depressed by a D1 receptor mediated effect of dopamine (LawTho et al., 1994; Seamans et al., 2001) whereas NMDA responses have been reported to be both enhanced (Seamans et al., 2001) and depressed (Law-Tho et al., 1994). EPSCs in layers II/III are enhanced by dopamine in rats (Gonzalez-Islas and Hablitz 2003) but decreased in primates (Urban et al., 2002). Given this complexity and micro-heterogeneity, the net effect of dopamine modulation of local circuits in PFC is not easily predicted. The cerebral cortex contains interconnected local and distant networks of excitatory and inhibitory neurons. Stability of activity in such networks depends on the balance between recurrent excitation and inhibition (Durstewitz et al., 2000a; Shu et al., 2003). A shift of the balance towards excitation may lead to the generation of epileptiform activity. The presence of massive recurrent excitatory connections that depend on inhibition for regulation has been implicated in the susceptibility of the neocortex and the hippocampus to develop epileptiform activity and seizures (McCormick and Conteras 2001). Modulatory influences strongly influence activity in thalamocortical (McCormick and Pape 1990; McCormick 1992) and neocortical circuits (McCormick et al., 1993). Dopamine is known to modulate epileptiform discharges both in vivo (Alam and Starr 1992; 1993b; 1994a; George and Kulkarni 1997) and in vitro (Alam and Starr 1993b; 1994b; Cepeda et al., 1999; Siniscalchi et al., 1997; Suppes et al., 1985). In vivo studies in different models of epilepsy have suggested that dopamine may have a pro-convulsant effect mediated by D1 receptors and an anti-convulsant effect via D2 receptors (see Starr JN-0092-2004.R1 pg 5 1996 for review). A dopamine-mediated recruitment of neurons in local excitatory circuits and synchronization of activity in these neurons may underlie these effects of dopamine in neocortex. Local excitatory neocortical networks are complexes of interconnected pyramidal neurons. Since dopamine has diverse effects on these neurons, the action of dopamine in such networks is difficult to predict based on recordings from individual pyramidal neurons or unitary excitatory connections. The aim of this work was to investigate dopamine’s ability to modulate activity in the local excitatory circuits in PFC. Using a combination of whole-cell recording and voltage-sensitive dye imaging, we have shown that dopamine, via D1 receptors, enhances local spatiotemporal spread of synaptic activity and lowers the threshold for evoking epileptiform discharges in neocortex. A preliminary account of some of these findings has been published (Hablitz and GonzalezIslas, 2002).