In the present experiments, our main question was how the sulpiride microinjected into the VP can influence motivational and learning processes in rats. In the allocentric MWM test [24–26], animals can find the invisible platform in two ways: they can learn its exact spatial location, and/or that they have to swim away from the edge of the pool to the inner zone, where they can find the platform with higher probability. Our results demonstrated that only the controls spent significantly more time in the zone surrounding the place of the platform compared to that of the virtual platform revealing that only the controls could learn the specific spatial location of it. Nevertheless, all groups spent more time in the inner zone of the pool in the test compared to the habituation trial, which means they learned that the platform is in the inner zone, though the largest doses of sulpiride also impaired this learning process. Even so, the largest dose of sulpiride induced CPP in the traditional CPP paradigm. How is it possible that the same dose of sulpiride reduced necessary learning skills [22, 23] in one paradigm, while it seemed to spare learning in the other? To solve this discrepancy, we elaborated the spatial version of the CPP paradigm. Applying this, we could observe that the rats could not associate the rewarding effect of the drug with that area which could be recognized only based on its spatial location. Thus, we could solve the apparent paradox: the formation of the traditional CPP requires only the partial intactness of learning processes, the spatial learning skills are not essential for it.
Data obtained from the traditional CPP test are considered as being measures of the rewarding, reinforcing effect of drugs [27]. According to the central theory of reinforcement, reinforcer agents facilitate memory and learning processes [28]. Our present results demonstrated the existence of an important counterexample when a reinforcer agent is not a universal enhancer of learning.
It is well-known that the hippocampus, strongly involved in the control of spatial learning [5, 25, 29], regulates the VTA DAergic population activity via the NAC-VP axis [30], which, doing so, modulates the formation of long-term memory [6]. Furthermore, it has been shown that the 3-day systemic administration of the D2R antagonists induces synaptic degeneration likely involving inhibitory synapses exclusively in the VP [31]. The main inhibitory input to the VP originates in the NAC, supplying it with rich GABAergic innervation [32]. Based on these findings we can hypothesize that the intra-VP sulpiride probably affects the GABAergic fibers originating in the NAC, consequently, leads to an enhanced inhibition of the VTA by the VP and results in the impairment of learning processes.
The DA, and mainly the DA released in the NAC, is considered as a rewarding neurotransmitter [33]. In the NAC shell region, the D2R agonist quinpirole induces CPP [34], but interestingly, the sulpiride cannot induce CPA [35]. In the medial septum, blocking D2Rs leads to CPA and it also suppresses locomotor activity [36]. In contrast, in the lateral hypothalamus, sulpiride has rewarding effect and enhances locomotor activity [37]. It has been demonstrated that these latter effects are realized via the VTA activation and the resulting NAC D2R activation [37, 38]. Recently, we have shown that the largest dose of the D2R agonist quinpirole in the VP leads to CPA and reduces the VTA DAergic activity [14]. These facts make it plausible that also the rewarding effect of intra-VP sulpiride prevails via the VTA DAergic activation. Indeed, in a recent paper, it has been shown that the activation of the D3R expressing (D3R+) VP neurons facilitates VTA DAergic activity and drives DA release in the NAC evoking real-time place preference [39]. It is well-known that the DA can inhibit neurons via the D2Rs [40]. Thus, it can be assumed that the sulpiride, microinjected into the VP, can eliminate the inhibitory DA tone on these D3R + VP neurons, and consequently, it can increase the VTA DAergic activity, releasing DA in the NAC shell region and inducing CPP.
In addition to its rewarding effect, DA in the NAC increases locomotor activity as well [41, 42]. If we suppose that the intra-VP sulpiride did induce CPP via the VTA-NAC axis, then also the locomotor activity should have been increased. Nevertheless, the opposite can be seen, the sulpiride gradually decreased the locomotor activity. This effect was relatively durable because it could be observed also one day after the last sulpiride treatment. All this means, that the locomotor effect cannot be an acute effect of the intra-VP sulpiride. Moreover, our results suggest that it is likely not to be a simple motor effect. In the MWM paradigm, when the sulpiride was administered after the trials, locomotion was not influenced by the drug, demonstrating that the gradually decreasing locomotion was associated with the treatment-environment. It has been shown that the VP is a central element of the circuit, which plays an important role in the initiation of adaptive behavioral responses to environmental stimuli [43] and novelty [44]. If so, then we can assume that the animals’ reactivity to the environment is reduced by the intra-VP sulpiride treatment, and probably a faster habituation can be observed compared to the controls.
The synaptic degeneration, induced by the intra-VP D2R antagonists is not an acute, but a gradually developing effect. It has been shown that stimulation of D2R-expressing medium spiny neurons in the NAC increases VTA DAergic population activity via the VP [45]. If we hypothesize that the synaptic degeneration involves these DAergic fibers, then we get a consistent explanation for our present findings concerning the spatial learning impairing and the locomotion reducing effect of intra-VP sulpiride. Following this thread, we can state more. Yao et al. found that D2R expressing neurons of the dorsomedial NAC shell region innervate glutamatergic neurons of the VP, whereas D2R expressing neurons of the ventral NAC shell region terminate on VP GABAergic cells [46]. It is known that the stimulation of the VP glutamatergic cells decreases VTA DAergic neuron activity [47], while that of the VP GABAergic cells rather increases it [48–50]. In this respect, we can better define the NAC GABAergic fibers affected by the intra-VP sulpiride treatment: likely they originate in the dorsomedial NAC shell region. This is also supported by the fact that the D2R expressing neurons of this NAC region receive input mainly from the hippocampus, compared to the other shell regions [46].
In summary, the sulpiride, microinjected into the VP, induces CPP in a dose-dependent fashion, presumably via the prompt effect on the D3R expressing VP neurons, indirectly activating the VTA DAergic neurons. This is an acute effect, which is necessary to establish CPP since it requires the temporal overlap between the effect of the drug and the conditioning environment [22, 23]. In contrast, the spatial learning impairment and the locomotion decreasing effect probably can be due to the degeneration of the D2R expressing GABAergic fibers originating in the NAC shell region, leading to a decreased VTA DAergic population activity. This is a gradually developing, long-run effect, and it might have a great clinical relevance since one of the main etiological factors underlying the positive symptoms of schizophrenia is the overactivity of the VTA DAergic neurons [51]. In this way, we have revealed a potential mechanism for how the D2R antagonist treatment can exert its effect in the limbic system.