PEGylated cholecystokinin prolongs satiation in rats: dose dependency and receptor involvement

Abstract

Background and purpose: Acute intraperitoneal (i.p.) administration of cholecystokinin (CCK) is known to induce a significant, but short-lasting, reduction in food intake, followed by recovery within hours. Therefore, we had covalently coupled CCK to a 10 kDa polyethylene glycol and showed that this conjugate, PEG-CCK(9), produced a significantly longer anorectic effect than unmodified CCK(9). The present study assessed the dose-dependency of this response and the effect of two selective CCK(1) receptor antagonists, with different abilities to cross the blood-brain barrier (BBB), on PEG-CCK(9)-induced anorexia.

Experimental approach: Food intake was measured, for up to 23 h, after i.p. administration of different doses (2, 4, 8, 16 and 32 microg kg(-1)) of CCK(9) or PEG-CCK(9) in male Wistar rats. Devazepide (100 microg kg(-1)), which penetrates the BBB or 2-NAP (3 mg kg(-1)), which does not cross the BBB, were coadministered i.p. with PEG-CCK(9) (6 microg kg(-1)) and food intake was monitored.

Key results: In PEG-CCK(9)-treated rats, a clear dose-dependency was seen for both the duration and initial intensity of the anorexia whereas, for CCK(9), only the initial intensity was dose-dependent. Intraperitoneal administration of devazepide or 2-NAP, injected immediately prior to PEG-CCK(9), completely abolished the anorectic effect of PEG-CCK(9).

Conclusions and implications: The duration of the anorexia for PEG-CCK(9) was dose-dependent, suggesting that PEGylation of CCK(9) increases its circulation time. Both devazepide and 2-NAP completely abolished the anorectic effect of i.p. PEG-CCK(9) indicating that its anorectic effect was solely due to stimulation of peripheral CCK(1) receptors.

Figure 1

Dose–response curves for the intraperitoneal injection of unmodified CCK₉ (a) or PEGylated CCK₉ (b) on food intake in rats over the dose range of 2–32 μg kg⁻¹. The results are expressed as a percentage of the control food intake (100%). CCK, cholecystokinin; PEG, polyethylene glycol.

Figure 2

Dose dependency of the initial anorexic intensity in rats treated with unmodified CCK₉ or PEGylated CCK₉. The relationship between the log dose (X) and food intake measured at 0.5 h (Y) for unmodified CCK₉ and PEG-CCK₉ was approximated by linear regression. Both slopes were significantly different from zero (CCK₉: slope=−0.4526, P=0.0304; PEG-CCK₉: slope=−0.8670, P=0.0002). There was no significant difference between the two slopes (P=0.1665). CCK, cholecystokinin; PEG, polyethylene glycol.

Figure 3

Dose response over time in rats treated with unmodified CCK₉ or PEGylated CCK₉. The relationship between dose (X) and τ₅₀ (Y) for unmodified CCK₉ and PEG-CCK₉ was approximated by linear regression. The response curves as shown in Figures 1a and 1b were fitted with a sigmoidal curve (four-parameter logistic model), which provided τ₅₀ estimates for every concentration used. τ₅₀ is defined as the time for the half-maximal response. The slope of the line from the PEG-CCK₉ results is significantly different from zero, indicating a clear correlation between dose and τ₅₀, whereas that for CCK₉ is not (CCK₉: P=0.1285; PEG-CCK₉: slope=0.2357, P<0.0001, r²=0.9970). CCK, cholecystokinin; PEG, polyethylene glycol.