Cortical cooling induces conditioned consumption reduction in male rats

Abstract

Previous studies have shown that male rats acquire a conditioned reduction in consumption of a sucrose solution when consumption of that taste solution is followed by cooling the caudate putamen. Because the shaft of the cold probe was not insulated, this cooling also included the cortex and meninges overlying the caudate putamen. When cooling the meninges was eliminated as a factor, the conditioned consumption reduction was weaker but it was not abolished. This suggests that meninges cooling contribute to the conditioned consumption reduction induced when all three structures are cooled, but it is not sufficient. Five experiments were designed to determine whether cooling the cortex also contributes. In the first experiment, the temperature of the cortex and meninges overlying the caudate putamen was measured during cooling. In the following three experiments the ability of male rats to acquire a conditioned consumption reduction was determined after pairing a sucrose solution with cooling the cortex and meninges overlying the caudate putamen, cooling the cortex with and without cooling the dura meninges membrane, and cooling the cortex with and without cooling the entire meninges. When the cortex was cooled without cooling the caudate putamen, dura, or entire meninges, a conditioned consumption reduction was acquired. The last experiment demonstrated that contingent pairing of sucrose and cortical cooling was required to obtain consumption reduction. These results clearly indicate that cortical cooling contributes to the acquisition of conditioned consumption reduction induced when the caudate putamen and overlying cortex and meninges are cooled. Two hypotheses are suggested to account for the ability of neural cooling to act as an unconditioned stimulus in the conditioned consumption reduction paradigm: (1) neuronal inactivation produces physiological changes that can serve as unconditioned stimuli and (2) cooling itself produces physiological changes that can serve as unconditioned stimuli.

Introduction

Male rats acquire a conditioned reduction in consumption of a sucrose solution when consumption of that taste solution is followed by neural cooling. This learned behavior has been found when the tip of the cooling probe was dorsal–posterior to the area postrema, dorsal to the lateral parabrachial nucleus, on the medial posterior cerebellar cortex, and within the caudate putamen [54], [55], [56], [57]. In these studies, the shaft of the cold probe was not insulated, which means the tissue around the shaft as well as the tip of the cold probe was cooled. For all of the neural areas mentioned above, this tissue included the meninges. The meninges are a sequence of three membranes, the dura mater, arachnoid, and pia mater, and the subarachnoid space, which separates the arachnoid and pia mater and contains cerebrospinal fluid. These membranes constitute a distinct organ with multiple functions such as mechanical protection, metabolic processes, immunological processes, and thermoregulation [6], [16], [60]. The dura, which is the thickest and hardest layer of the meninges, is richly vascularized by external carotid arteries and is innervated by the trigeminal nerve. It contains nociceptive sensory innervation of the intracranial blood vessels and painful sensations can be evoked in humans by thermal stimulation of these vessels [1], [19], [20], [28], [30], [40], [42], [45], [52]. In a series of studies, we cut out a circle of dura so that the area postrema and medial posterior cerebellar cortex could be cooled without cooling the overlying dura [54]. Conditioned consumption reduction was still induced and the strength of the reduction was no different than when both neural area and dura were cooled. In addition, when the underlying arachnoid, pia mater, and medial posterior cerebellar cortex were removed so that only the dura was cooled, animals did not acquire conditioned consumption reduction after pairing dura cooling with consumption of a sucrose solution [54]. These data suggest that cooling the dura does not contribute to conditioned consumption reduction induced by neural cooling.

The subdural meninges are also richly vascularized. The subarachnoid space contains cerebrospinal fluid and the blood vessels that traverse through this space as well as the dense network of vessels within the pia mater are thought to interface with the cerebrospinal fluid. These blood vessels are innervated by sympathetic, parasympathetic, and sensory nerves [2], [13], [17], [23], [27], [33], [34], [50], [51], [53]. When the circle of dura was removed, the pia mater was left intact. Recent evidence has shown that there is a decrease in the strength of a conditioned consumption reduction when the caudate putamen and overlying neural tissue is cooled without cooling the entire meninges [55]. Taken together with the results of the dura studies, these data suggest that either the pia mater or the entire meninges as a unit contribute to conditioned consumption reduction induced by neural cooling.

Although the strength of the conditioned reduction was decreased when cooling the meninges was eliminated as a factor, it was not abolished. This suggests that cooling the caudate putamen itself and/or the overlying cortex can induce a conditioned consumption reduction. The following experiments were designed to determine whether cooling the cortex and meninges overlying the caudate putamen induces conditioned consumption reduction and if so, whether cooling the cortex without cooling the dura or the meninges reduces the strength of the conditioned consumption reduction.