Our results demonstrated that neonatal 6-OHDA lesioning of the frontal cortex significantly reduced dopamine levels in the frontal cortex while significantly increasing serotonin, 5-HIAA and norepinephrine concentrations in the nucleus accumbens. This neurochemical effect of neonatal 6-OHDA lesioning was absent in the adult rats indicating a recovery of the initial neurochemical defect. Furthermore, our findings showed that neonatal lesions of frontal cortical dopamine innervation by 6-OHDA had lasting impacts on neurobehavioral function. The effects were seen in locomotor activity, learning and memory, and nicotine self-administration during adulthood.
Our findings showed that male rats treated with neonatal 6-OHDA were significantly hyperactive compared to sham lesioned males. Dopaminergic lesioning via 6-OHDA treatment have been shown to cause locomotor defects in a number of animal models. Neonatal ICV 6-OHDA infusion causes considerable and long-lasting locomotor activity in juvenile and adult rats (Shaywitz et al., 1976
, Miller et al., 1981
, Archer et al., 1988
). Lesioning of dopaminergic systems by ICV injection of 6-OHDA or MPTP also causes both catalepsia and bradykinesia in adult rats and monkeys (Zigmond and Stricker, 1973
, Deumens et al., 2002
). These studies along with the current data suggest an essential role for dopaminergic innervation in locomotor activity (Zigmond and Stricker, 1973
, Archer et al., 1988
, Deumens et al., 2002
). Serotonin also has been implicated in locomotion (Geyer, 1996
, Jacobs and Fornal, 1997
). Increased levels of serotonin in the brain have been associated with hyperactivity in mice (Chia et al., 1999
, Brocco et al., 2002
). Recently, it has been shown that increased striatal 5-HT that follows neonatal DA depletion is involved in hyperlocomotor behavior in mice (Avale et al., 2004
). In the current study we also observed increased ventral striatal 5-HT and increased hyperlocomotion. These studies highlight the importance of frontal DA and striatal 5-HT systems in locomotion.
In the present study only male rats exhibited hyperactivity after 6-OHDA lesioning. It is possible that male sex hormones exacerbate the effect of 6-OHDA lesioning. The lack of effect of 6-OHDA lesion on the locomotor activity in female rats may be the result of sex-specific effects of 6-OHDA lesioning. However, it is important to note that we did not observe significant differences in frontal DA levels between neonatal male and female rats treated with 6-OHDA.
Neonatal 6-OHDA lesioning of the frontal cortex cause a moderate but significant improvement in 8-arm radial maze performance. This effect was not sex specific in that both adult male and female rats showed increased maze acquisition in the later sessions. This finding contradicts data from other studies. Other investigators have shown that 6-OHDA-treated rats exhibit a retarded acquisition of running responses to acquire food pellets in a modified version of the Olton radial arm maze (Archer et al., 1988
). Furthermore, 6-OHDA-treated rats also have been shown to be drastically impaired in the swim maze task compared with controls (Archer et al., 1988
). There might be several methodological reasons for the discrepancy between our findings and these results. First, we used local infusions, which were limited to the frontal cortex while Archer’s group used an anatomically-nonspecific bilateral ICV infusion. ICV infusions are likely to have pronounced effects on dopaminergic functioning in several regions of the brain including the frontal cortex, nucleus accumbens, septum, striatum and the amygdala. Secondly, our rats were treated once on Day 7 while Archer et al.
treated their animals twice, on Day 3 and Day 6. Lastly, we used a standard 8-arm radial maze task with 18 sessions of training while Archer’s group used a modified version of the Olton radial arm maze which required one session to acquire food pellets. We did observed a general trend toward retarded maze acquisition in the early training sessions which may coincide with the impairments observed in other studies. It is also plausible that decreases in frontal cortical dopamine activity actually improve memory related processes. Some antipsychotics, which block DA receptors and lower dopaminergic activity, actually improve deficits in cognition and executive functioning (Harvey et al., 2005
, Wagner et al., 2005
Neonatal 6-OHDA lesions of the frontal cortex significantly affected nicotine self-administration during adulthood. The 6-OHDA lesion × sex interaction was also significant. There was a significant sex difference in the sham lesioned rats, with the female self-administering more nicotine than their male counterparts. Neonatal 6-OHDA lesioned rats exhibited the opposite effect with lesioned male rats self-administering significantly more nicotine than lesioned female rats. It is difficult speculate on mechanisms that account for the sex-difference in nicotine self-administration and the possible role of estrogen in mediating these effects in that there were no significant differences in nicotine self-administration and estrous cycle. We can only hypothesize that the 6-OHDA lesioning has sex-specific effects in the rodent brain and that female rat brain may be is protected from the 6-OHDA insult (Dluzen, 1996
, Dluzen et al., 1996b
, Dluzen et al., 1996a
, Disshon and Dluzen, 2000
, Dluzen and Horstink, 2003
The data showed a direct link between the rate of nicotine self-administration and dopamine and DOPAC in 6-OHDA treated rats. This effect was absent in sham lesioned rats. Opposite to cortical DA, frontal serotonin levels were nearly significantly correlated with the rate of nicotine self-administration in 6-OHDA lesioned rats. Serotonin’s primary metabolite, 5-HIAA, was found to be significantly correlated with nicotine self-administration in 6-OHDA lesioned rats. This finding may suggest a role for the cortical serotonin in nicotine addiction. Even though 6-OHDA rats were not more prone to increased nicotine self-administration, there was still a significant interaction between increased dopamine levels in the frontal cortex and the number of nicotine infusion per session. It is possible that the lesion disrupted the reward and reinforcing behavior of nicotine in the animals but the underlying mechanism is preserved.
It is also possible that the rat frontal cortex is not as important for mediating addictive behavior. There was no significant interaction between frontal DA levels and nicotine self-administration in the control animals. However, it is important to note that we are sampling transit levels of DA and not actual levels at the time of nicotine self-administration. Determining DA levels in response to nicotine exposure would give us a much better indication of nicotine responsiveness and the contribution of frontal DA to nicotine seeking behavior in control and 6-OHDA lesioned animals.
Evidence from both laboratory animals and human studies implicates dopamine as one of the major neurotransmitter being involved in drug self-administration and addiction (Piazza et al., 1991
, Rezvani et al., 1992
, Mason et al., 1997
, Di Chiara, 1999
, Volkow et al., 1999
, Rezvani et al., 2000
, Goldstein and Volkow, 2002
, Volkow et al., 2002b
, Kalivas and Volkow, 2005
, Nader and Czoty, 2005
). In addition to the mesolimbic dopaminergic system, several other neurotransmitter systems including the serotonergic system have been suggested to be involved in drug self-administration (Rezvani et al., 1990
, Rezvani and Grady, 1994
, Volkow and Li, 2004
, Koob, 2000
). It appears that lesioning of the neonatal cortical dopamine innervation makes nicotine more aversive and/or less rewarding during adulthood in female rats. The important role of serotonin within the limbic system and its functional interaction with dopaminergic systems in the brain should also be considered in interpretation of these data. It has been shown previously that neonatal DA depletion in the rat is followed by an increase of striatal serotonin (Snyder et al., 1986
) that appears as a consequence of an extensive serotonergic hyperinnervation in this area (Descarries et al., 1992
, Molina-Holgado et al., 1994
, Zhang et al., 2002
). Our data demonstrate an increase in serotonin levels in the nucleus accumbens in neonatal rats following frontal 6-OHDA lesioning. The nucleus accumbens plays a paramount role in limbic neuronal circuits that are responsible for behaviors such as compulsive drug seeking (Morgane et al., 2005
). Thus, it is possible that increase in serotonin in this part of the reward pathway may lead to a decline in nicotine self-administration. However, functionally, there is considerable overlap within and interactions between dopamine, serotonin, glutamate, GABA and other neurotransmitters in relation to cognition and drug seeking behavior. Considering the complexity of the system and the fact that drug seeking behavior and cognition are not unitary phenomena more extensive research is needed to better understand these functions.
In the present study, we observed behavioral defect in adult rats that received 6-OHDA lesions as neonates. Several mechanisms could explain these long-term behavioral changes. One explanation could be that neonatal lesioning has profound effects on receptor profiles in the frontal cortex. Studies have shown the one consequence of neuronal lesioning is the expression of supersensitive receptors. Depleting dopamine levels in the striatum leads to the expression of supersensitive D1 dopamine receptors (Gerfen et al., 2002). The presence of supersensitive D1 receptors in adult rats could explain long-term behavioral defect even though there was a complete recovery of cortical dopamine levels. If this recovery of dopaminergic innervation happened in the presence of the expression of hypersensitive receptors, then behavioral changes may be expected.
Alternatively, in addition to depleting dopamine levels, 6-OHDA lesions have been shown to effect the expression of genes that are important for proper neuron structure and morphogenesis. Most important was the up-regulation of genes involved in the repulsion of axon guidance and genes that inhibit the growth of dendrites and microtubule formation (Krasnova et al 2007
). In addition, genes necessary for tubulin and actin folding were found to be down-regulated (Krasnova et al. 2007
). In the context of this model, it is possible that while there is a full recovery of relative dopamine levels, there are altered morphological and structural defects in adulthood. It is possible that while there are relatively the same number of dopaminergic neurons in control and 6-OHDA-treated rats, 6-OHDA-treated rats do not have the proper synapse structure for normal functioning. This is supported by anatomical changes observed in the adult mouse brain after neonatal 6-OHDA lesioning (Krasnova et al. 2007
). Lastly, a combination of these two models could account for the long-term effects of 6-OHDA lesioning. However, much more research is needed to distinguish between the two possibilities.
In summary, the neonatal 6-OHDA lesion of the frontal cortex caused a long-lasting neurobehavioral effects during adulthood in rats suggesting the critical role of the neonatal cortical dopamine in locomotor, learning and memory and nicotine addiction. Some of these effects are sex-specific. These data suggest that neonatal frontal cortical dopamine lesion can cause overstimulation of limbic sites and cause long lasting impairments in neurobehavioral functioning.