Animals rely on a variety of internal and external cues to guide their behaviour. Internal cues such as hunger or thirst may direct an animal towards outcomes that would serve to fulfil homeostatic need. Cues from the environment can also gain significance through learning (e.g., Pavlovian conditioning) when they reliably predict biologically significant outcomes such as food. Such cues can promote approach and other behaviours that help procure a desired outcome. One way to investigate how such cues impact behaviour is with a task called Pavlovian-instrumental transfer (PIT).

PIT is commonly used to assess the ability of Pavlovian cues to augment instrumental responding for food or other outcomes. In a simple version of this task, subjects are trained during a Pavlovian phase to associate the presentation of a conditioned stimulus (CS; e.g., a tone) with the delivery of an unconditioned stimulus (US; e.g., a food pellet). During a separate instrumental training phase, subjects learn to perform a response such as a lever press to earn the same food. Then, at test, subjects’ instrumental responding is assessed in the presence and absence of the Pavlovian CS. Typically, responding is invigorated by the CS and this is considered the PIT effect (see Corbit and Balleine, 2016, Holmes et al., 2010).

Ghrelin is an orexigenic peptide that is a key regulator of appetite and meal initiation (Klok et al., 2007). While ghrelin is known to serve multiple peripheral metabolic functions (Pradhan et al., 2013), there is growing interest in its role within the brain. Ghrelin’s receptor, growth hormone secretagogue receptor 1A (GHS-R1A; Howard et al., 1996, Kojima et al., 1999, McKee et al., 1997, Pong et al., 1996) is expressed in multiple brain regions including the ventral tegmental area (VTA) and nucleus accumbens (NAc; Finger et al., 2012, Guan et al., 1997; Skibicka, Hansson, Alvarez-Crespo, et al., 2011), two regions involved in the expression of PIT (Corbit et al., 2001, Corbit et al., 2007, Corbit and Balleine, 2011). Ghrelin acts within both structures to enhance dopamine activity (Abizaid et al., 2006, Jerlhag, 2008, Jerlhag et al., 2006, Jerlhag et al., 2007, Jerlhag et al., 2012, Jiang et al., 2006, Skibicka et al., 2011, Weinberg et al., 2011), suggesting a functional interaction between ghrelin and the dopamine reward circuitry. Further, studies have shown ghrelin activity within the VTA increases motivation for food reward (Skibicka et al., 2013; Skibicka, Hansson, Alvarez-Crespo, et al., 2011) while inhibition of ghrelin signaling suppresses food-motivated behaviours including conditioned place preference (Disse et al., 2011, Egecioglu et al., 2010; Landgren, Simms, Thelle, et al., 2011; Perello et al., 2010), cue-potentiated feeding (Kanoski et al., 2013, Walker et al., 2012) and instrumental responding for sucrose (Landgren, Simms, Thelle, et al., 2011).

There are several theoretical reasons to believe that ghrelin plays a role in the expression of PIT. First, satiety is known to reduce the magnitude of PIT (Corbit et al., 2007); given ghrelin’s role in food-motivated behaviours and feeding, it is likely that blocking ghrelin signalling would produce a similar effect as satiety. Second, the VTA and NAc are areas that are necessary for the expression of PIT (Corbit et al., 2007, Corbit and Balleine, 2011, Murschall and Hauber, 2006), and express a high concentration of GHS-R1A receptors (Finger et al., 2012, Guan et al., 1997; Skibicka, Hansson, Alvarez-Crespo, et al., 2011). Finally, PIT is sensitive to modulation by dopamine both generally (Dickinson et al., 2000, Wassum et al., 2011) and within the NAc (Lex and Hauber, 2008, Wyvell and Berridge, 2000), suggesting that dopamine release from the VTA may be critical for PIT expression (see Corbit & Balleine, 2016). As GHS-R1A activation in the VTA is known to increase dopamine release (Abizaid et al., 2006), it is hypothesized that ghrelin activity contributes to regulation of PIT.

While several studies have looked directly at the role of ghrelin in PIT, they have produced conflicting results. Two studies have found that systemic ghrelin receptor antagonism using the drug GHRP-6 [D-Lys3] enhanced PIT within a single-reinforcer paradigm (Dailey et al., 2016, Johnson et al., 2009). These findings suggest that the motivational significance of a Pavlovian CS is enhanced in the absence of ghrelin signalling. However, this is paradoxical for several reasons. Firstly, inactivation of the VTA has been shown to reduce PIT (Corbit et al., 2007, Murschall and Hauber, 2006). As ghrelin acts in the VTA to increase its dopaminergic activity (Abizaid et al., 2006), ghrelin receptor antagonism would therefore be expected to decrease VTA dopamine activity and attenuate the expression of PIT. Similarly, satiety reduces general PIT (Corbit et al., 2007) and to the extent that ghrelin is linked to hunger, blocking ghrelin should be akin to satiety and on this basis would also be expected to reduce PIT. More in line with results expected based on the physiological function of ghrelin, Sommer et al. (2022) found that systemic GHS-R1A antagonism with a different antagonist, JMV-2959, attenuated expression of PIT. However, this study differed from the others as they utilized a two-stimulus version of PIT which allowed them to look at outcome-specificity and choice.

To reconcile previous conflicting findings, the current set of experiments aimed to clarify the role of ghrelin in PIT. Given the relationship between the motivational states of hunger and satiety and changes in ghrelin signalling (Cummings et al., 2001, 2004), we hypothesized that the effects of systemic GHS-R1A antagonism would parallel those of satiety, with both attenuating PIT. Since JMV-2959 has previously been shown to attenuate outcome-specific PIT (Sommer et al., 2022), we first sought to examine its effects in a single-stimulus PIT design similar to that used by Johnson et al. (2009) and Dailey et al. (2016). Next, we examined the impact of satiety in the single-stimulus paradigm, allowing us to compare the effects of satiety to those of systemic GHS-R1A antagonism. Finally, to explore the neural locus of ghrelin’s effect on PIT, we examined the effects of intra-VTA administration of ghrelin and JMV-2959 in the same single-stimulus paradigm.