Impact of prefrontal cortex in nicotine-induced excitation of ventral tegmental area dopamine neurons in anesthetized rats

Abstract

Systemic administration of nicotine increases dopaminergic (DA) neuron firing in the ventral tegmental area (VTA), which is thought to underlie nicotine reward. Here, we report that the medial prefrontal cortex (mPFC) plays a critical role in nicotine-induced excitation of VTA DA neurons. In chloral hydrate-anesthetized rats, extracellular single-unit recordings showed that VTA DA neurons exhibited two types of firing responses to systemic nicotine. After nicotine injection, the neurons with type-I response showed a biphasic early inhibition and later excitation, whereas the neurons with type-II response showed a monophasic excitation. The neurons with type-I, but not type-II, response exhibited pronounced slow oscillations (SOs) in firing. Pharmacological or structural mPFC inactivation abolished SOs and prevented systemic nicotine-induced excitation in the neurons with type-I, but not type-II, response, suggesting that these VTA DA neurons are functionally coupled to the mPFC and nicotine increases firing rate in these neurons in part through the mPFC. Systemic nicotine also increased the firing rate and SOs in mPFC pyramidal neurons. mPFC infusion of a non-α7 nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine blocked the excitatory effect of systemic nicotine on the VTA DA neurons with type-I response, but mPFC infusion of nicotine failed to excite these neurons. These results suggest that nAChR activation in the mPFC is necessary, but not sufficient, for systemic nicotine-induced excitation of VTA neurons. Finally, systemic injection of bicuculline prevented nicotine-induced firing alterations in the neurons with type-I response. We propose that the mPFC plays a critical role in systemic nicotine-induced excitation of VTA DA neurons.

Figure 1.

The recording sites of VTA DA neurons and mPFC pyramidal neurons. A, A phase-contrast image illustrates electrode entry and track sites (black arrows) and the general recording site within the VTA (red arrow). B, Schematic diagram of coronal sections through the VTA shows the recording sites of 24 neurons labeled after electrophysiological and pharmacological experimentation. Black dots are type-I neurons (n = 17 from 17 rats), and white circles are type-II neurons (n = 7 from 7 rats). IF, Intrafascicular nucleus; PN, paranigral nucleus; PIF, paraintrafascicular nucleus; PBP, parabrachial pigmented nucleus; RN, red nucleus; tth, trigeminothalamic tract; IP, interpeduncular nucleus; ml, medial lemniscus; SN, substantia nigra. C, A phase-contrast image illustrates electrode general recording site within the mPFC (red arrow). D, A coronal section through the rat brain illustrating the mPFC (shaded area). Neurons (n = 6 from 6 rats) were filled in coronal sections at AP, 3.0 mm; ML, 0.7 mm; DV, 3.5 mm from bregma. ACd, Dorsal anterior cingulated cortex; PL, prelimbic cortex; IL, infralimbic cortex; OFC, orbitofrontal cortex.

Figure 2.

Nicotine alters the firing of VTA DA neurons. A, Systemic injection of nicotine (0.5 mg/kg, i.v., duration is indicated by the solid horizontal bar labeled “Nic”) induced different responses in VTA DA neurons. B, One group of neurons showed a biphasic response (type-I response, n = 17 from 17 rats) in which the FR was initially reduced (∼1 min after nicotine) and then increased (peak increase ∼5 min after nicotine; Ba). In the second group of neurons, nicotine-induced monophasic response (type-II response, n = 7 from 7 rats), in which FR was transiently increased, peaking ∼1 min after nicotine, and returns to baseline levels ∼3 min after nicotine (Bb). C, Statistical analysis indicated that, 1 min after nicotine injection, VTA DA neurons with type-I (Ca) and type-II (Cb) responses showed opposite direction in FR, BF, and P_SO (at ∼1 Hz). However, 5 min after injection of nicotine, FR and BF were increased, whereas P_SO remained significantly reduced in the neurons with type-I (Ca) but not type-II (Cb) response. In this and the following figures, error bars indicate SEM, and *p < 0.05 and **p < 0.01.

Figure 3.

Firing patterns of VTA DA neurons. A, VTA DA neurons with two types of responses to systemic nicotine exhibited different firing patterns. Segments of spike trains recorded and corresponding, smoothed rate histograms from a type-I(Aa) and a type-II(Ab) responded neuron, respectively. B, Comparison of P_SO between type-I (n = 17 from 17 rats) and type-II (n = 7 from 7 rats) responded neurons. SOs with a peak frequency close to 1 Hz were observed in the VTA DA neurons with type-I (indicated by a horizontal arrow), but not type-II, response. C, Bar graph summarizes the pool data to compare neuronal firing patterns between type-I and type-II neurons.

Figure 4.

Effects of mPFC inactivation on nicotine-induced responses in VTA DA neurons. A, Local infusion of TTX into the mPFC (over the duration indicated by the larger, horizontal, solid bar) prevented the increase in FR induced by nicotine in type-I (Aa, n = 5 from 5 rats) but not type-II (Ba, n = 5 from 5 rats) responded DA neurons. The insets indicate P_SO levels and differences also help to distinguish VTA DA neurons with type-I (Aa) and type-II (Ba) responses. Statistical analysis showed that TTX infusion into the mPFC significantly reduced BF and P_SO in type-I (Ab) but not type-II (Bb) neurons. Subsequent injection of nicotine failed to increase, but persistently decreases, FRs in type-I responded neurons (Ab), while increasing the FR in type-II responded neurons.

Figure 5.

Effects of mPFC transection on nicotine-induced responses in VTA DA neurons. A, In animals with mPFC transections, nicotine (0.5 mg/kg, i.v.) decreased the FR in type-I responded neurons (Aa), but it increased the FR in type-II responded neurons (Ab). B, Bar graphs show the effects of mPFC transection on nicotine-induced changes of firing patterns in type-I (Ba) and type-II (Bb) neurons. C, After mPFC transaction, the SO was very low in both type-I and type-I responded neurons (n = 18 from 18 rats, Ca). Nicotine had no significant effects on BF and P_SO in these VTA cells (Cb).

Figure 6.

Effects of PFC infusion of a nicotinic antagonist or agonist on DA neuron firing. A, Local infusion of MEC (100 μm; duration of infusion shown by open horizontal bar) into the mPFC prevented the increase in firing in the VTA neurons with type-I response (Aa, n = 9 from 9 rats) usually seen after systemic nicotine exposure. Statistical analysis showed that the infusion inhibited all effects of nicotine on FR, BF, and P_SO in these neurons (Ab). B, Local infusion of nicotine (500 nm) into the mPFC did not alter the FR (Ba), but it significantly enhances P_SO in the VTA DA neurons with type-I response (Bb, n = 6 from 6 rats, *p < 0.05).

Figure 7.

Nicotine alters mPFC neuron firing. A, Systemic nicotine exposure produces a biphasic change in mPFC pyramidal neuron firing, an increase in FR, followed by a decrease 5 min after nicotine (n = 6 from 6 rats). B, P_SO is elevated 1 min after nicotine exposure and then reduced 5 min after the injection (left). C, Analysis suggests that all changes induced by nicotine are statistically significant.

Figure 8.

Effects of BMI. Aa, Systemic injection of the GABA_A receptor antagonist BMI (2.5 mg/kg, i.v.; delivery duration indicated by first, solid horizontal bar) had no effect on the early excitation, but it suppressed the late inhibition of mPFC neurons induced by subsequent nicotine (Nic) injection. Ab, Bar graph summarized effects of BMI. BMI had no effect alone and did not affect the early, nicotine-induced increase in FR and BF in mPFC neurons, but it abolished the later inhibition of mPFC neurons (n = 6 from 6 rats). There was a reduction in P_SO after BMI exposure alone and 5 min after subsequent delivery of nicotine, but BMI did not alter the early increase in P_SO induced by nicotine. Ba, After BMI injection (2.5 mg/kg, i.v.), which had no effect on FR alone, systemic injection of nicotine did not significantly alter FR in DA neurons with type-I response (n = 6 from 6 rats). Bb, Statistical analysis showed that BMI exposure alone had no effect on FR and BF but prevented effects of subsequently delivered nicotine on FR and BF measured at 1 or 5 min after nicotine injection. In addition, BMI delivery with or without nicotine reduced P_SO in the VTA DA neurons with type-I response (Bb).