Effects of melanin-concentrating hormone on licking microstructure and brief-access taste responses

Abstract

The effects of intracerebroventricular application of melanin-concentrating hormone (MCH) on licking for sucrose, quinine hydrochloride (QHCl), and water solutions were evaluated in two experiments. In experiment 1, rats received 90-min access to sucrose and water solutions after MCH or vehicle microinjection to the third ventricle (3V). MCH increased intake largely through increases in the rate of licking early in the meal and in the mean duration of lick bursts, suggesting an effect on gustatory evaluation. Therefore, in experiment 2, brief access tests were used with a series of sucrose and QHCl concentrations to behaviorally isolate the effects of intracerebroventricular MCH on gustatory evaluation. MCH uniformly increased licking for all sucrose solutions, water, and weak concentrations of QHCl; however, it had no effect on licking for the strongest concentrations of QHCl, which were generally avoided under control conditions. Thus MCH did not produce nonspecific increases in oromotor activity, nor did it change the perceived intensity of the tastants. We conclude that MCH enhanced the gain of responses to normally accepted stimuli at a phase of processing after initial gustatory detection and after the decision to accept or reject the taste stimulus. A comparison of 3V NPY and MCH effects on licking microstructure indicated that these two peptides increased intake via dichotomous behavioral processes; although NPY suppressed measures associated with inhibitory feedback from the gut, MCH appeared instead to enhance measures associated with hedonic taste evaluation.

Effects of hindbrain melanin-concentrating hormone and neuropeptide Y administration on licking for water, saccharin, and sucrose solutions.

“…However, there was no significant main effect of drug for either treatment group and no significant interaction term (Table 2). While the failure of NPY to increase burst size is consistent with previous reports after forebrain ICV NPY stimulation, the failure of MCH to affect burst size observed here stands in contrast to findings noted after 3V administration (7,9,50,78). This failure of MCH was also observed when bursts were expressed in terms of their mean duration (s/burst) rather than mean size [F(1,9) = 0.00, P = 0.99], while the monotonic increase in burst duration across tastants was conserved [F(3,27) = 4.54, P = 0.01; data not shown].…”

“…Furthermore, the increases in initial ingestion rate after 3V NPY treatment (where observed) were not accompanied by increases in the mean size of licking bursts (7, 50, 78): these two measures normally vary in tandem when rats are offered tastants of increasing palatability (e.g., Fig. 4) or those that are naturally or conditioned to be aversive (7,9,10,25,74,75). …”

“…Licking microstructure analyses provide advantages for the study of ingestive behavior: detailed analyses of the temporal dynamics of spout licking can be performed for a range of tastants that may be systematically varied in chemosensory and/or caloric properties across tests. Such evaluations also permit assessment of the relative contributions of orosensory and inhibitory postingestive feedback signals that conjointly influence the size of a given meal (7,9,24,25,73).Recently, we characterized the effects of NPY and MCH delivery to the third ventricle (3V) on intake and licking microstructure (described below) for a range of taste solutions (water, saccharin, and sucrose solutions) that varied in palatability and caloric content (7,9). If the MCH system at least partially mediates the influences of NPY, one may hypothesize that the patterns of licking responses to these two hyperphagic compounds would share some similarities, but this does not appear to be the case.…”

“…To measure licking progress over the course of the meal, the number of licks for each minute of the intake test was analyzed. Meals were also divided temporally into thirds, and the mean lick rate (licks/s converted to licks/min) and the number of bursts were calculated for each meal third (7,9,10,74,75).Responses for each measure in experiments 1 and 3 across the 2 drug days (MCH/aCSF) for each of the two (experiment 3) or four (experiment 1) tastants were compared with two-way (drug × tastant) or three-way (drug × tastant × minute) repeated-measures ANOVAs using SPSS 11.5 software. Least significant difference pairwise comparisons and paired T-tests were used to explore significant main effects and interactions.…”

“…4B; Refs. 7,9,25,74). It would be worthwhile in future studies to evaluate whether progressive ratio or other instrumental responses for food access are increased after 4V NPY delivery.…”