Norepinephrine Secretion in the Hypothalamic Paraventricular Nucleus of Rats during Unlimited Access to Self-Administered Nicotine: An In Vivo Microdialysis Study

NE levels in PVN microdialysates obtained during day 1 of the acquisition period of self-administration. NE levels (picograms per 10 μl), in dialysis samples collected every 7 min for 9 hr, are shown for two representative rats acquiring self-administration behavior with 23 hr unlimited access. Rats self-administered a bolus injection of nicotine (A; 0.03 mg/kg body weight in 50 μl of heparinized saline over 0.81 sec) or heparinized saline (B) after pressing the active lever in the operant chamber. The filled circlesindicate dialysis samples collected during the brief daily interruption in availability of nicotine or saline (for animal husbandry and resetting the computer clock); the open circles indicate consecutive samples taken once nicotine became available after lights out (0 min). + marks the time when the active lever was pressed to deliver an injection. The inverted triangles, indicating an SAE, point to the first of two consecutive samples used to calculate the NE response to lever presses (see Materials and Methods, Data analysis). The capped triangles indicate an SAE associated with NE levels at least 10% above baseline levels (i.e., an iSAE). In a rat acquiring nicotine SA, three of three SAEs were iSAEs compared with only one of five in a rat receiving saline in response to a lever press.

iSAEs increase during the three phases of self-administration. Nicotine SA resulted in an increase in both the mean number of iSAEs (A1, B1,C1) and the percentage of all SAEs associated with an increase in NE secretion (iSAEs/SAEs; A2,B2). **p < 0.01 compared with saline SA controls (for the nicotine SA groups, n = 6 for acquisition, 12 for early maintenance, and 10 for late maintenance; for saline, n = 7). The exception to this increased percentage during late maintenance (C2) may be indicative of the progression of partial desensitization to the effect of chronic nicotine SA (during acquisition,t = 3.99, p < 0.01 for iSAE;t = 7.58, p < 0.01 for iSAE/SAEs; in maintenance, F_(3,31) = 10.47, p < 0.01 for iSAE;F_(3,31) = 4.77, p< 0.01 for iSAE/SAEs).

NE peak incremental response: day 1 of the acquisition period. The mean incremental response of all iSAEs throughout the entire 9 hr dialysis session (A) was 3.5-fold higher in rats self-administering nicotine (t = 5.39; p < 0.01), as was the level of NE in response to the first nicotine injection(s) (B; t = 2.69; p< 0.025). However, there was no difference in the NE response after 9 hr (C; t = 1.38;p > 0.05), indicating a decline in NE response over time. In addition, the overall mean NE levels, calculated from all of the samples collected during the 9 hr microdialysis session, were similar in both the nicotine and saline groups (D;t = 0.57; p > 0.05).n = 6 in each group. *p < 0.05, **p < 0.01 compared with saline control; #p < 0.05 compared with the last iSAE peak.

SAEs and NE levels in PVN microdialysates obtained during the early maintenance period. As in Figure 1, SAEs and NE levels are shown for two representative rats, nicotine SA (A) and saline SA (B). These rats were dialyzed ∼9.2 ± 0.6 d after nicotine or saline initially became available for SA. In contrast to acquisition, early maintenance was associated with a twofold increase in the mean number of SAEs and a fourfold increase in iSAEs.

NE peak incremental response: early maintenance period. During this phase of SA, the mean incremental NE peak response of all iSAEs was 3.2-fold higher with nicotine SA than saline (A; F_(3,31) = 3.57; p < 0.05), and the NE incremental response to the first SAE after lights-out was almost fourfold higher (B; F_(3,31) = 2.60;p < 0.05). Again, no difference was found in the NE response during an iSAE occurring 9 hr later (C;F_(3,28) = 0.75; p> 0.05). However, despite the decline in NE responsiveness over time, the overall NE release was higher in nicotine SA rats (D; F_(3,31) = 2.97;p < 0.05). *p < 0.05, **p < 0.01 compared with controls; #p < 0.05 compared with the last nicotine iSAE peak (saline, 7 rats; nicotine, 12 rats).

SAEs and NE levels in PVN microdialysates obtained during the late maintenance period. SAEs and NE secretion in two representative rats during a 9 hr microdialysis session (described in Fig. 2) performed ∼18.6 ± 0.8 d after nicotine or saline initially became available for SA: nicotine SA (A) and saline SA (B). The total number of both SAEs and ISAEs remained elevated with nicotine SA.

NE peak incremental response: late maintenance period. The average increment of all iSAEs over the 9 hr microdialysis session was still higher with nicotine SA than saline SA (A; F_(3,31) = 3.57;p < 0.05) during this stage, as was the NE response to the first SAE after lights out (B;F_(3,31) = 2.60; p< 0.05)). Although NE levels showed an overall increase of 79% (D; F_(3,31) = 2.97;p < 0.05), the decline in NE responsiveness during an iSAE 9 hr later was still apparent (C). *p < 0.05 compared with controls (saline, 6 rats; nicotine, 10 rats).

Effect of yohimbine on NE release in self-administering rats. To determine whether a depletion of axonal stores might underlie the reduction in NE release at the end of a 9 hr dialysis session (see C in Figs. 4, 6, 8), 6 mg/kg yohimbine was administered intraperitoneally 8 hr into the session (at 0 min) and dialysis continued. NE levels in nicotine SA rats were comparable with those in saline controls, demonstrating that the secretory pools were comparable (F_(13,102) = 0.94;p > 0.05). Therefore, the reduction in NE response was specific to nicotine SA. n = 4 per group.