Intra-accumbens injections of ACTH induce excessive grooming in rats

Blockade of melanocortin transmission inhibits cocaine reward

Melanocortins and the melanocortin-4 receptor (MC4-R) are enriched in the nucleus accumbens, a brain region that has been implicated in the rewarding action of cocaine and other drugs of abuse. In the present study we use a number of rat behavioral models to show that infusion of a melanocortin peptide antagonist into the nucleus accumbens blocks the reinforcing, incentive motivational, and locomotor sensitizing effects of cocaine. We also show that locomotor responses to repeated cocaine exposure are completely blocked in MC4-R null mutant mice and reduced in Agouti mice that overexpress an endogenous inhibitor of melanocortins in the brain. The results also demonstrate that cocaine administration increases the expression of MC4-R in the nucleus accumbens and striatum, and that MC4-R is co-localized with prodynorphin in medium spiny neurons in the nucleus accumbens. Together, these findings indicate that the behavioral actions of cocaine are dependent on activation of MC4-R, and suggest that upregulation of this receptor by drug exposure may contribute to sensitization of these behavioral responses. Modulation of cocaine reward is a novel action of the melanocortin-MC4-R system and could be targeted for the development of new medications for cocaine addiction.

Molecular and behavioral interactions between central melanocortins and cocaine

Behavioral and molecular studies have established a link between drugs of abuse and the central melanocortin system, particularly the melanocortin 4 receptor (MC4-R). The present study expands this line of investigation to characterization of the neurochemical and behavioral interactions between MC4-R and the psychomotor stimulant, cocaine. The results demonstrate that repeated, but not acute, cocaine administration up-regulates MC4-R mRNA expression in the striatum and hippocampus, but not cerebral cortex. Pharmacological studies indicate that the up-regulation of MC4-R expression occurs via dopamine D1 and D2 receptor-dependent mechanisms. The D1/D2 antagonist haloperidol and the D2-selective antagonist eticlopride mimic the effect of cocaine on MC4-R expression. In addition, coadministration of a D1-selective antagonist, SCH 23390 [R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine], completely blocks the up-regulation of MC4 mRNA by cocaine, demonstrating that D1 receptor activation is necessary for this response. Moreover, the results demonstrate that cocaine treatment increases behavioral responses (grooming and locomotor activity) to infusions of a melanocortin agonist, indicating that up-regulation of MC4-R expression results in functional consequences. These data further support a role for the melanocortin-MC4-R neuropeptide system in the biochemical and behavioral effects of cocaine.

Melanotropin receptors in the brain are differentially distributed and recognize both corticotropin and alpha-melanocyte stimulating hormone

Melanotropinergic neurons in the brain may mediate the known modulatory effects of alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH) on brain functions including thermoregulation, pituitary regulation, learning and behavior. To determine the distribution of brain melanotropin receptors, we used biologically active [125I]Nle4, D-Phe7-alpha-MSH ([125I]NDP-MSH) for in situ binding and autoradiography in frozen rat brain sections. Specific (alpha-MSH-inhibitable) [125I]NDP-MSH binding sites were distributed in a region-specific pattern, and were present in numerous structures within the septal area, hypothalamus, thalamus, epithalamus, olfactostriatal complex, and midbrain. Each brain structure studied showed a characteristic, reproducible distribution and relative intensity of binding. Receptor peptide selectivity was assessed by comparing the dose-response relationships for inhibition of binding by alpha-MSH, NDP-MSH and ACTH. In all brain structures studied, the 3 peptides gave comparable maximal inhibition of tracer binding, indicating that all detectable binding sites recognized all 3 melanotropins. The respective relative potencies were: NDP-MSH (EC50 = 1.7 +/- 0.6 nM) greater than alpha-MSH (EC50 = 46.9 +/- 11.7 nM) = ACTH. These results provide a preliminary neuroanatomic map of potential target sites for melanotropin actions, and indicate that these sites are capable of recognizing multiple products of the intrinsic melanotropinergic system of the brain.

ACTH acts via an anterior ventral third ventricular site to elicit grooming behavior

Intracerebroventricular but not parenteral application of ACTH has been shown to elicit excessive grooming behavior in rats and mice. This behavior is elicited by administration of ACTH into the lateral, third, or fourth ventricles. Plugging of the cerebral aqueduct with cold cream fails to prevent grooming in response to lateral ventricle injection of ACTH. However, cold cream plugs in the third ventricle can prevent the subsequent induction of grooming behavior by lateral ventricle injection of ACTH, but only when the plugs are located in the anterior ventral third ventricle in the region of the organum vasculosum laminae terminalis (OVLT) and median eminence. These data suggest the anterior ventral third ventricle as the periventricular site of action of ACTH in eliciting excessive grooming, although it is possible that peptides taken up in this area are transported to other regions to elicit the behavioral response.

The effects of Org 2766 on the performance of sham, neocortical, and hippocampal-lesioned rats in a food search task

The behavioral effects of an ACTH4-9 variant, Org 2766, given for one week postoperatively at a dose of 1 microgram/rat daily, were evaluated in animals given hippocampal, neocortical, or "sham" lesions. After the week during which the injections were given, the animals were tested for 5 days in a food-search task in which food was hidden in two recessed holes in the floor. On the next day the ability of the rats to find food in these same two baited holes was tested in the presence of 14 additional holes that were not baited. On the following day, the animals were tested again, this time with all 16 holes baited. To assess the long-term effects of Org 2766 treatment, the animals were tested once again 2-3 months later in the same apparatus with 16 empty holes. In general, rats with lesions restricted to the neocortex were severely impaired in the task and were unaffected by prior treatment with Org 2766. Animals with hippocampal damage quickly learned the task and were hyperactive. During the test session with 16 baited holes they showed differential behavioral changes suggesting attentional deficits not seen in "sham" operated rats. These deficits were attenuated by prior Org 2766 treatment, whereas the lesion-induced hyperactivity was not. Treatment with Org 2766 impaired all aspects of performance of "sham" operated animals.

Dopamine denervation of frontal cortex or nucleus accumbens does not affect ACTH-induced grooming behaviour

Studies with dopamine (DA) receptor antagonists have implicated brain DA systems in the increased grooming behaviour elicited by intracerebroventricular (i.c.v.) administration of ACTH or exposure to a novel environment. To evaluate the potential contributions of DA terminals innervating frontal cortex and nucleus accumbens, stereotaxically guided injections of 6-hydroxydopamine (6-OHDA) were made into these regions of rat brain following peripheral administration of desmethylimipramine and pargyline. Despite an 88% mean reduction of DA, and 68% mean reduction of noradrenaline in frontal cortex, the amount of grooming behaviour observed either in a novel cage, or after i.c.v. injection of ACTH1-24 was not detectably altered. The lesions also did not affect locomotor activity measured during the scoring of grooming, in an open field, in photocell cages, or in a running wheel, even in the animals most severely depleted of DA. Following nucleus accumbens 6-OHDA infusions, depletions of DA as great as 99% were obtained. Animals with accumbens depletions greater than 90% showed the expected attenuation of amphetamine-stimulated locomotor activity. Nevertheless, they exhibited normal grooming scores in a novel cage, and in response to i.c.v. ACTH1-24. It is concluded that dopaminergic terminals in nucleus accumbens and probably those in frontal cortex are not necessary for the expression of novelty- or ACTH-induced grooming.

Hippocampal damage: effects on dopaminergic systems of the basal ganglia

Although the research topics discussed in this chapter cover a substantial range of areas, methods, and behaviors, there is a consistent central issue: the alterations of brain and behavior subsequent to hippocampal lesions. The initial hypothesis that important secondary changes occur in the basal ganglia after hippocampal lesions has been sustained, at least in part, but other and unexpected results have been obtained that both clarify and mystify at the same time. The restoration of normal locomotion to animals with hippocampal damage by the application of a DAi agonist, DPI, to n. accumbens certainly supports our general idea, but the fact that membrane and transmitter changes in accumbens are transitory needs to be explained and perhaps related to alterations in DA receptor populations. The fact that intra-accumbens DPI can restore ACTH-induced excessive grooming at 28 days after hippocampal damage supports the idea that progressive changes are occurring in DA receptors in n. accumbens, but we need to know their nature more precisely. To do this, however, will probably require advances in our general knowledge of central DA receptors. One of the more interesting aspects of our observations is that the intra-accumbens administration of DPI restores normal behavioral patterns in several types of behaviors, especially locomotion and the excessive grooming response. These two behaviors are not closely linked in nature because large changes can occur in them independently, given appropriate conditions for testing. On the basis of available evidence both of these behaviors seem linked to activities in forebrain DA systems. This would further support the idea that the hippocampus modulates DA activities in basal ganglia systems. The results that suggest the presence of multiple sites for the central induction of grooming were also unexpected. As evidence mounts in support of this idea, it is likely that the one located in n. accumbens is more influenced by hippocampal destruction than the region(s) affected by icv neuropeptide administration. The interactions that take place in the basal ganglia can be extensive. Afferents from the entirety of the neocortical surface, the limbic system, and the projections from the brainstem DA cell groups meet in the striatum and ventral striatum. The efferents from this region not only reach motor cortical and subcortical sites but also project back onto diffusely projecting monoaminergic cells of the brainstem.(ABSTRACT TRUNCATED AT 400 WORDS)