Decreased hyperpolarization-activated cation current (Ih): a response mechanism to reduce cocaine-induced excitability in VTA DA neurons.
Download
Advisor
Jiménez-Rivera, Carlos A.Type
DissertationDegree Level
Ph.D.Date
2022-06-09Metadata
Show full item recordAbstract
The hyperpolarization-activated cation current (Ih) is a determinant of intrinsic excitability in dopaminergic neurons (DA) of the ventral tegmental area (VTA). Ih is a slowly activating cation inward current triggered by hyperpolarization. When elicited Ih 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 Ih 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 Ih in controlling VTA DA excitability is poorly understood. Our hypothesis was that Ih reduction could play a role as a homeostatic controller which compensates for cocaine-induced change in excitability. Our main goal was to understand how Ih contributes to VTA DA neuronal excitability. In our first aim, we tried to elucidate if Ih reduction can alter basal VTA DA neuronal excitability. We blocked Ih and evaluated firing properties, such as rebound spiking, using whole-cell patch-clamp electrophysiology from naïve 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 Ih blockade on in vitro spontaneous excitability using cell-attached configuration from naïve rats. We hypothesized that Ih blockade will reduce rebound and spontaneous firing of VTA DA cells. It was found that Ih blockade can decrease evoked and spontaneous firing activity of VTA DA neurons. In addition, we evaluated the effect of Ih blockade on spontaneous firing patterns using single-unit extracellular recordings from naïve rats. We found that Ih blockade can decrease evoked and spontaneous firing activity, while increasing the interspike interval of VTA DA cells.
To investigate how this current modulates cocaine-dependent excitability, we postulated that Ih reduction serves as a homeostatic regulator (or controller) to oppose cocaine-induced excitability. We measured Ih current using whole-cell patch-clamp electrophysiology, 2 and 24 hours after an acute cocaine injection. We found a progressive reduction of Ih 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 Ih blocker was perfused, there was a significant reduction in rebound action potentials. The progressive Ih 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 Ih 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 Ih acting as a homeostatic regulator of VTA DA cell excitability.
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 Ih 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.
Lastly, we evaluated if Ih 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 Ih. 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 Ih will be decreased as a consequence of the pharmacological enhancement of firing activity. We found that BIC-induced excitability facilitates a reduction in Ih conductance. Furthermore, Ih blockade can significantly reduce this enhanced BIC-induced excitability.
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 Ih blockade on HFSvSUB induced potentiation on in vivo spontaneous excitability. We hypothesized that Ih blockade will significantly decrease HFSvSUB-induced firing patterns on VTA DA cells. We demonstrated that Ih blockade significantly reduces firing activity after HFSvSUB. Additionally, Ih blockade also significantly increased the interspike interval. These results suggest that Ih is an intrinsic mechanism that can modulate the excitability of VTA DA neurons after an hyperexcitable state is present.
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).
To investigate how this current modulates cocaine-dependent excitability, we postulated that Ih reduction serves as a homeostatic regulator (or controller) to oppose cocaine-induced excitability. We measured Ih current using whole-cell patch-clamp electrophysiology, 2 and 24 hours after an acute cocaine injection. We found a progressive reduction of Ih 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 Ih blocker was perfused, there was a significant reduction in rebound action potentials. The progressive Ih 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 Ih 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 Ih acting as a homeostatic regulator of VTA DA cell excitability.
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 Ih 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.
Lastly, we evaluated if Ih 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 Ih. 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 Ih will be decreased as a consequence of the pharmacological enhancement of firing activity. We found that BIC-induced excitability facilitates a reduction in Ih conductance. Furthermore, Ih blockade can significantly reduce this enhanced BIC-induced excitability.
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 Ih blockade on HFSvSUB induced potentiation on in vivo spontaneous excitability. We hypothesized that Ih blockade will significantly decrease HFSvSUB-induced firing patterns on VTA DA cells. We demonstrated that Ih blockade significantly reduces firing activity after HFSvSUB. Additionally, Ih blockade also significantly increased the interspike interval. These results suggest that Ih is an intrinsic mechanism that can modulate the excitability of VTA DA neurons after an hyperexcitable state is present.
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).
Rights
© 2022 Karl Y. Bosque-CorderoCollections
Except where otherwise noted, this item's license is described as Attribution-NonCommercial-NoDerivs 3.0 United States