Electrophysiological characteristics of paraventricular thalamic (PVT) neurons in response to cocaine and cocaine- and amphetamine-regulated transcript (CART)

Abstract

Recent work has established that the paraventricular thalamus (PVT) is a central node in the brain reward-seeking pathway. This role is mediated in part through projections from hypothalamic peptide transmitter systems such as cocaine- and amphetamine-regulated transcript (CART). Consistent with this proposition, we previously found that inactivation of the PVT or infusions of CART into the PVT suppressed drug-seeking behavior in an animal model of contingent cocaine self-administration. Despite this work, few studies have assessed how the basic physiological properties of PVT neurons are influenced by exposure to drugs such as cocaine. Further, our previous work did not assess how infusions of CART, which we found to decrease cocaine-seeking, altered the activity of PVT neurons. In the current study we address these issues by recording from anterior PVT (aPVT) neurons in acutely prepared brain slices from cocaine-treated (15 mg/ml, n = 8) and saline-treated (control) animals (n = 8). The excitability of aPVT neurons was assessed by injecting a series of depolarizing and hyperpolarizing current steps and characterizing the resulting action potential (AP) discharge properties. This analysis indicated that the majority of aPVT neurons exhibit tonic firing (TF), and initial bursting (IB) consistent with previous studies. However, we also identified PVT neurons that exhibited delayed firing (DF), single spiking (SS) and reluctant firing (RF) patterns. Interestingly, cocaine exposure significantly increased the proportion of aPVT neurons that exhibited TF. We then investigated the effects of CART on excitatory synaptic inputs to aPVT neurons. Application of CART significantly suppressed excitatory synaptic drive to PVT neurons in both cocaine-treated and control recordings. This finding is consistent with our previous behavioral data, which showed that CART signaling in the PVT negatively regulates drug-seeking behavior. Together, these studies suggest that cocaine exposure shifts aPVT neurons to a more excitable state (TF). We propose that the capacity of CART to reduce excitatory drive to this population balances the enhanced aPVT excitability to restore the net output of this region in the reward-seeking pathway. This is in line with previous anatomical evidence that the PVT can integrate reward-relevant information and provides a putative mechanism through which drugs of abuse can dysregulate this system in addiction.

Keywords: CART; PVT; addiction; cocaine; electrophysiology; midline thalamus; reward-seeking.

Figure 1

Action potential discharge and current-discharge frequency of anterior PVT neurons. Five distinct patterns of AP discharge were observed in aPVT neurons during response to current step injections of increasing amplitude; tonic firing (TF), delayed firing (DF), initial burst (IB), single spike (SS), and reluctant firing (RF) (A). All five AP firing patterns were observed in recordings from saline-exposed animals, with 44% exhibiting TF, 11% DF, 24% IB, 18% SS and 3% RF (B, left). In contrast, cocaine-exposed animals only showed four of the five described AP firing patterns, with 85% exhibiting TF, 3% RF, 6% SS and 6% IB (B, right). A chi-squared analysis revealed a significant interaction between cocaine treatment and firing pattern type, χ² (4, n = 73) = 13.53, p = 0.009. Recordings from cocaine-exposed animals reached higher mean AP discharge frequency at all current step injections above 20 pA (C, upper). In contrast, the same comparison of AP discharge frequency, specifically in tonic firing PVT neurons from both treatment groups, did not differ (C, lower).

Figure 2

Spontaneous excitatory synaptic transmission in saline- and cocaine-exposed animals. Traces show 3 s of continuous recordings from aPVT neurons in saline- (A, upper) and cocaine-exposed animals (A, middle), with corresponding amplitude histograms (including baseline noise distributions) below and arrows indicating the mean (saline, mean amplitude = −22 pA, A, lower left; cocaine, mean amplitude = −18 pA A, lower right). Cumulative probability distributions (from representative recordings) and bar graphs showed no differences in both instantaneous frequency (B, upper) and amplitude (B, lower) of aPVT neurons in response to saline or cocaine. Group data plots (C) show rise time and decay time constant remained similar in both saline and cocaine treatment groups (n = 25 and n = 24, with an average of 1387 events and 1192 events analyzed respectively), with representative traces of rise and decay.

Figure 3

The effect of CART on spontaneous excitatory synaptic transmission in saline-treated animals. Traces show 3 s of continuous recording from an aPVT neuron prior to (A, upper), and then during bath application of 10 nmol CART (A, middle), with corresponding amplitude histograms (including baseline noise distributions) below and arrows indicating the mean (saline pre-CART, mean amplitude = −22 pA, A, lower left; saline post-CART, mean amplitude = −17 pA, A, lower right). Cumulative probability distributions (from representative recordings) showed a significant reduction in both instantaneous frequency (B, upper) and amplitude (B, lower) of aPVT neurons in response to saline or cocaine. Group data plots (C) show sEPSC instantaneous frequency was reduced by CART (* p = 0.002), as was amplitude (* p = 0.008). In contrast, rise time was not affected by CART. Finally, decay time constant was significantly increased following CART (* p = 0.007, an average of 1387 events and 799 events were analyzed respectively) with representative traces of rise and decay time pre- and post-CART.

Figure 4

The effect of CART on spontaneous excitatory synaptic transmission in cocaine-treated animals. Traces show 3 s of continuous recording from an aPVT neuron prior to (A, upper), and then during bath application of 10 nmol CART (A, middle), with corresponding amplitude histograms (including baseline noise distributions) below and arrows indicating the mean (cocaine pre-CART, mean amplitude = −18 pA, A, lower left; cocaine post-CART, mean amplitude = −16 pA, A, lower right). Cumulative probability distributions (from representative recordings) showed a significant reduction in both instantaneous frequency (B, upper) and amplitude (B, lower) of aPVT neurons in response to saline or cocaine. Group data plots (C) show sEPSC instantaneous frequency (* p < 0.001), and amplitude (* p < 0.001) were both reduced after CART application. We did not observe a significant effect of CART on rise time, however CART application resulted in a significant increase in decay time constant (* p = 0.11, an average of 1192 events and 669 events were analyzed respectively), with representative traces of rise and decay time pre- and post-CART.