Evidence for co-existence of CCK-8 and GnRH in neurons of the mesencephalic tegmentum in the chameleon.

A double-label immunofluorescence technique was used to demonstrate the co-localization of cholecystokinin-8 (CCK-8) and gonadotrophin-releasing hormone (GnRH) in individual neurons and processes of the chameleon brain. Co-localization was limited to a small population of cells in the dorsomedial tegmentum; in other regions of the brain, neurons were observed to be either CCK-8-immunopositive or GnRH-immunopositive but never both. However, double-labeled fibers and terminals were found to be distributed at a low density throughout the thalamus, the medial hypothalamus, the tegmentum and the spinal cord. These data provide the first indication for the co-localization of CCK-8 and GnRH, whose functional significance remains to be established. ON

Regional characteristics of [125I]cholecystokinin octapeptide specific binding to rat brain membranes following 6-hydroxydopamine treatment.

The relative changes in the amount of specific [125I]cholecystokinin octapeptide (CCK8) bound to regional brain membrane preparations after 6-hydroxydopamine (6-OHDA) treatment were examined. The specific binding in the frontal cortex and striatum decreased and reached a minimum on the 3rd day after 6-OHDA treatment. Thereafter, the specific binding recovered to 60% and 65% of control values in the frontal cortex and striatum respectively on the 28th day. On the other hand, in the nucleus accumbens, where CCK8 co-exists in the dopamine neuron, the specific binding decreased gradually, and its recovery was delayed compared with that of the frontal cortex and striatum. In the hippocampus, 6-OHDA treatment had no effect on the specific binding throughout the experimental period. The decrease of [125I]CCK8-specific binding could be caused by enhanced release of CCK8 in the frontal cortex, striatum, and nucleus accumbens, and the recovery of specific binding could be induced by depletion of CCK8 in nerve terminals. Particularly in the nucleus accumbens, the delayed recovery of specific binding suggests the loss of the pre-synaptic binding site, which exists in the CCK8/DA co-existing neuron, together with a change in CCK8 release.

Reversal of electroacupuncture tolerance by CCK-8 antiserum: an electrophysiological study on pain-related neurons in nucleus parafascicularis of the rat.

Two varieties of neurons were found in nucleus parafascicularis (pf) of the rat: one responds to noxious stimuli with an increase in firing (pain-excited neuron, PEN), the other with a decrease in firing (pain-inhibited neuron, PIN). Electroacupuncture (EA) has been shown to suppress PEN and excite PIN, which can be taken as an electrophysiological index for EA analgesia. This effect of EA subsided after prolonged (6 h) EA stimulation, suggesting the development of tolerance to EA. Intracerebroventricular (icv) injection of CCK-8 antiserum aiming at neutralizing endogenously released CCK-8 resulted in a complete restoration of the EA effect. Normal rabbit serum was not effective. CCK-8 antiserum per se did not affect the firing pattern of the PEN or PIN in nontolerant rat. The results obtained from single neuron recording in anesthetized animals thus confirmed those obtained in intact animals using the tail flick as the end point, implying that an excess of endogenously released CCK-8 may constitute one of the mechanisms for the development of EA tolerance.

Cholecystokinin innervation of rat thalamus, including fibers to ventroposterolateral nucleus from dorsal column nuclei.

The distribution of cholecystokinin octapeptide immunoreactive fibers and puncta in the adult rat thalamus was studied using immunocytochemical methods. Small to moderate numbers of immunoreactive fibers were present in the lateral habenular nucleus, ventral lateral geniculate nucleus, zona incerta, parataenial, mediodorsal, medioventral, and submedial nuclei, the rhomboid, paracentral, central lateral and parafascicular nuclei, and in the medial geniculate and dorsal lateral geniculate nuclei. Moderate to large numbers of cholecystokinin (CCK)-positive fibers were present in the paraventricular nuclei, the reticular nucleus, the anteroventral, anteromedial, and central medial nuclei, and in the rostral extension of the internal medullary lamina between the parataenial and anteroventral nuclei. Dense concentrations of immunoreactive fibers were also found in a principal sensory relay nucleus, the ventroposterolateral nucleus (VPL), of the ventrobasal complex. The number of CCK-positive fibers in VPL showed a marked unilateral decrease in rats which had received lesions of the contralateral gracile and cuneate nuclei. The results of this study demonstrate that CCK-immunoreactive fibers and puncta are widely distributed in the rat thalamus, and that the source of these fibers in VPL is probably the dorsal column nuclei.

Reversal of tolerance to morphine but no potentiation of morphine-induced analgesia by antiserum against cholecystokinin octapeptide.

Tolerance to morphine analgesia was induced in rats by chronic treatment with morphine (5-30 mg/kg, t.i.d. for 6 days). Intracerebroventricular (i.c.v.) injection of antiserum against cholecystokinin octapeptide (CCK-8) reversed tolerance to morphine by 50% (P less than 0.001). Intrathecal (ith) injection of the CCK-8 antiserum produced a similar, although less marked, reversal of tolerance to morphine. Rats made tolerant to analgesia induced by morphine developed a cross tolerance to electroacupuncture-induced analgesia. This cross tolerance was also reversed by the CCK-8 antiserum by more than 50% (P less than 0.001). Intracerebroventricular or intrathecal injection of the CCK-8 antiserum per se produced no significant changes in the basal level of the latency of the tail flick response, nor did it affect the analgesia induced by morphine in naive rats. The results suggest that prolonged activation of opioid receptors may trigger the CCK-8 system in the central nervous system to exert a negative feedback control, which may constitute one of the mechanisms for the development of tolerance to opioids.