Electrophysiological changes in laterodorsal tegmental neurons associated with prenatal nicotine exposure: implications for heightened susceptibility to addict to drugs of abuse

Abstract

Prenatal nicotine exposure (PNE) is a risk factor for developing an addiction to nicotine at a later stage in life. Understanding the neurobiological changes in reward related circuitry induced by exposure to nicotine prenatally is vital if we are to combat the heightened addiction liability in these vulnerable individuals. The laterodorsal tegmental nucleus (LDT), which is comprised of cholinergic, GABAergic and glutamatergic neurons, is importantly involved in reward mediation via demonstrated excitatory projections to dopamine-containing ventral tegmental neurons. PNE could lead to alterations in LDT neurons that would be expected to alter responses to later-life nicotine exposure. To examine this issue, we monitored nicotine-induced responses of LDT neurons in brain slices of PNE and drug naive mice using calcium imaging and whole-cell patch clamping. Nicotine was found to induce rises in calcium in a smaller proportion of LDT cells in PNE mice aged 7-15 days and smaller rises in calcium in PNE animals from postnatal ages 11-21 days when compared with age-matched control animals. While inward currents induced by nicotine were not found to be different, nicotine did induce larger amplitude excitatory postsynaptic currents in PNE animals in the oldest age group when compared with amplitudes induced in similar-aged control animals. Immunohistochemically identified cholinergic LDT cells from PNE animals exhibited slower spike rise and decay slopes, which likely contributed to the wider action potential observed. Further, PNE was associated with a more negative action potential afterhyperpolarization in cholinergic cells. Interestingly, the changes found in these parameters in animals exposed prenatally to nicotine were age related, in that they were not apparent in animals from the oldest age group examined. Taken together, our data suggest that PNE induces changes in cholinergic LDT cells that would be expected to alter cellular excitability. As the changes are age related, these PNE-associated alterations could contribute differentially across ontogeny to nicotine-mediated reward and may contribute to the particular susceptibility of in utero nicotine exposed individuals to addict to nicotine upon nicotine exposure in the juvenile period.

Plasticity in the Brainstem: Prenatal and Postnatal Experience Can Alter Laterodorsal Tegmental (LDT) Structure and Function

“…Further, PNE treatment was associated with lower activity-dependent intracellular calcium rises in putative cholinergic LDT cells of juvenile mice, assessed by single-photon calcium imaging of neurons activated by high current injection (Polli and Kohlmeier, 2018), which was an effect attributed to a reduced functionality of voltage-gated calcium channels (VGCCs). This finding is consistent with kinetic alterations seen in the action potential of the PNE LDT found in two independent studies from our laboratory (Christensen et al, 2015;Polli and Kohlmeier, 2018), as lower calcium rises mediated by VGCCs predict smaller activation of calcium-dependent potassium channels, with subsequent broadening of the action potential half-width (Bean, 2007), which were characteristics of the kinetics of the action potential observed in PNE LDT cells. Changes in VGCCs leading to reduced activity-dependent calcium predict a reduced excitability of these neurons, which parallels previous studies showing that PNE LDT neurons displayed a higher rheobase which is the amount of current necessary to induce an action potential (Christensen et al, 2015).…”

“…This finding is consistent with kinetic alterations seen in the action potential of the PNE LDT found in two independent studies from our laboratory (Christensen et al, 2015;Polli and Kohlmeier, 2018), as lower calcium rises mediated by VGCCs predict smaller activation of calcium-dependent potassium channels, with subsequent broadening of the action potential half-width (Bean, 2007), which were characteristics of the kinetics of the action potential observed in PNE LDT cells. Changes in VGCCs leading to reduced activity-dependent calcium predict a reduced excitability of these neurons, which parallels previous studies showing that PNE LDT neurons displayed a higher rheobase which is the amount of current necessary to induce an action potential (Christensen et al, 2015). Consistent with this, in unpublished observations, we noted that while PNE LDT neurons displayed higher inward currents upon AMPA application in the absence of TTX, a significantly smaller percentage of neurons fired action potentials following AMPAR activation in the PNE group relative to the number of cells in which AMPA induced action potential firing in control.…”

Calcium rises induced by AMPA and nicotine receptors in the ventral tegmental area show differences in mouse brain slices prenatally exposed to nicotine

“…In addition, although reductions in cellular excitability in the VTA have been reported following PNE, as well as inhibition of release of DA to target structures (Alkam et al, 2013a, 2013b), other studies have found enhanced DA neural activity, suggestive of heightened excitability in PNE rodent offspring (Zhang et al, ). We have shown that cholinergic signaling in a brain stem cholinergic group which provides excitatory glutamatergic and the majority of cholinergic input to the VTA is altered by PNE, suggesting a role for this alteration in behavioral outcomes associated with PNE (Christensen et al, ; McNair and Kohlmier, ). In our continuing goal of elucidating neural changes associated with early life exposure to nicotine, we wished to test our hypothesis that AMPAR‐ and nAChR‐based excitability is altered in the VTA in PNE animals, reflected as alterations in AMPAR and nAChR‐stimulated rises in intracellular calcium.…”

Sex‐specific alterations in GABA receptor‐mediated responses in laterodorsal tegmentum are associated with prenatal exposure to nicotine

“…By bulk loading LDT cells with Fura-2-AM and conducting dynamic fluorescent imaging to indirectly assess changes in calcium, muscimol, and baclofen were used to activate cellular activity via actions at GABA A Rs and GABA B Rs, and a novel ligand, 1, was used to assay activity at GABA Aρ R, a subclass of GABA A R, in female and male brain slices prenatally exposed to nicotine or saccharine. As age-related differences in PNE effects are known to be present in the LDT, we conducted our study in cells collected within two age groups (Christensen et al, 2015). All three ligands were associated with changes in fluorescence indicative of decreases and increases in intracellular calcium levels in the majority of recorded cells in both treatment groups.…”

“…Given its connectivity, changes induced by PNE in the LDT could influence behaviors regulated by mesolimbic, mesocortical, and thalamic pathways. PNE-associated LDT functional changes have been demonstrated for nAChRs, glutamatergic receptors, voltage-operated calcium channels (VOCCs), and neuronal membrane properties (Christensen, Nielsen, & Kohlmeier, 2015;McNair & Kohlmeier, 2015;. Changes in excitability were shown to be age-dependent, which would be layered upon ontologically-appropriate developmental shifts in cellular distribution (Ninomiya, Kayama, & Koyama, 2005).…”