The hyperpolarization-activated cation current (I_h) is a determinant of intrinsic excitability in dopaminergic neurons (DA) of the ventral tegmental area (VTA). I_h is a slowly activating cation inward current triggered by hyperpolarization. When elicited I_h depolarizes the membrane to threshold for the generation of action potentials. Our laboratory has previously demonstrated that cocaine sensitization, a chronic cocaine administration model, significantly reduces I_h amplitude in VTA DA neurons (Arencibia-Albite et al., 2012). Despite this current reduction, the spontaneous firing of VTA DA cells remained similar to control animals. The role of I_h in controlling VTA DA excitability is poorly understood. Our hypothesis was that I_h reduction could play a role as a homeostatic controller which compensates for cocaine-induced change in excitability. Our main goal was to understand how I_h contributes to VTA DA neuronal excitability. In our first aim, we tried to elucidate if I_h reduction can alter basal VTA DA neuronal excitability. We blocked I_h and evaluated firing properties, such as rebound spiking, using whole-cell patch-clamp electrophysiology from na&iuml;ve rats. Rebound spiking is defined as the production of one or more action potentials in response to the cessation of a prolonged hyperpolarizing current step. We also determined the effect of I_h blockade on in vitro spontaneous excitability using cell-attached configuration from na&iuml;ve rats. We hypothesized that I_h blockade will reduce rebound and spontaneous firing of VTA DA cells. It was found that I_h blockade can decrease evoked and spontaneous firing activity of VTA DA neurons. In addition, we evaluated the effect of I_h blockade on spontaneous firing patterns using single-unit extracellular recordings from na&iuml;ve rats. We found that I_h blockade can decrease evoked and spontaneous firing activity, while increasing the interspike interval of VTA DA cells.<br /> <br /> To investigate how this current modulates cocaine-dependent excitability, we postulated that I_h reduction serves as a homeostatic regulator (or controller) to oppose cocaine-induced excitability. We measured I_h current using whole-cell patch-clamp electrophysiology, 2 and 24 hours after an acute cocaine injection. We found a progressive reduction of I_h starting 24 hours after acute cocaine administration. Thus, this progressive decline observed from acute to chronic cocaine administration could serve as a homeostatic mechanism to reduce cocaine enhanced excitability. In addition, we quantified the increase in rebound action potentials, 2 and 24 hours after an acute cocaine injection. We found that after 2, 24 hours or seven days of cocaine administration, there is a significant increase in rebound action potentials of VTA DA neurons. Interestingly, there is a higher number of rebound action potentials after 2 hours of cocaine in comparison to 24 hours. When ZD, an I_h blocker was perfused, there was a significant reduction in rebound action potentials. The progressive I_h reduction may be a key factor in the decrease of rebound action potentials found from 2 to 24 hours after cocaine injections. The effect of I_h blockade was also evaluated on in vitro spontaneous excitability using cell-attached configuration, 2 hours after an acute cocaine injection. When perfused with ZD, the spontaneous firing activity was also reduced, while significantly increasing the interspike interval providing further evidence in favor of an I_h acting as a homeostatic regulator of VTA DA cell excitability.<br /> <br /> The overall cocaine effect can be influenced by the environmental context in which the drug is experienced (Caprioli et al., 2007). One question we addressed is how exposure to cocaine in a novel context interferes with chronic alterations on synaptic potentiation and neuronal excitability in the mesolimbic system (MLS). To evaluate this question an acute cocaine injection was administered to subjects in both a novel context or their home-cage. We found a significant increase in the AMPA-to-NMDA receptor-mediated EPSC ratio of VTA DA neurons in both the novelty and the homecage groups. Cocaine experienced in a novel environment produced a higher degree of synaptic potentiation. In addition, we found that I_h blockade can also diminish the acute cocaine-induced increase in spontaneous firing activity in in vivo anesthetized rats. Again, the reduction in firing was correlated with an increase in the inter-spike interval.<br /> <br /> Lastly, we evaluated if I_h can function as a homeostatic regulator of intrinsic excitability and can be modulated when an altered firing activity in VTA DA cells is present. We enhanced VTA DA excitability pharmacologically and recorded changes in evoked rebound action potentials and I_h. VTA brain slices from naive rats were incubated with bicuculline methiodide (BIC) as a mechanism to enhance excitability and promote rebound excitation through GABAergic disinhibition. Our hypothesis was that I_h will be decreased as a consequence of the pharmacological enhancement of firing activity. We found that BIC-induced excitability facilitates a reduction in I_h conductance. Furthermore, I_h blockade can significantly reduce this enhanced BIC-induced excitability.<br /> <br /> It has been reported that high frequency stimulation of the ventral subiculum (HFSvSUB) elicits in vivo persistent hyperactivity of VTA DA neurons (Glangetas et al. 2015). We elucidated the effect of I_h blockade on HFSvSUB induced potentiation on in vivo spontaneous excitability. We hypothesized that I_h blockade will significantly decrease HFSvSUB-induced firing patterns on VTA DA cells. We demonstrated that I_h blockade significantly reduces firing activity after HFSvSUB. Additionally, I_h blockade also significantly increased the interspike interval. These results suggest that I_h is an intrinsic mechanism that can modulate the excitability of VTA DA neurons after an hyperexcitable state is present. <br /> <br /> The study of the regulatory role of this current can provide novel insights on how changes in intrinsic neuronal properties might establish cellular homeostasis in the VTA DA system and in conditions of altered excitability such as substance use disorder (SUD)....